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Phylogenetic relationships of the marsupials
PHYLOGENETIC RELATIONSHIPS OF THE MARSUPIALS by FREDERICK S. S Z A L A Y "
ABSTRACT
R0,SUM~-
A phylogeny of the Metatheria. based on the synthesis of cranial, dental, and postcranial hard morphology, and external pedal morphoclines, is presented and discussed. The expanded concept Pediomyiclae includes North American Cretaceous and most Palaeogene marsupials. The Borhyaenoidea and Caenolestoidea are united in the Borhyaeniformes, whereas the Pediomyidae and Didelphidae, sensu stricto, make up the Didelphiformes, both of the order of Didelphida, cohort Ameridelphia. Dromiciopsia is the most primitive order of the cohort Australidelphia which also includes the orders Syndactyla and Dasyurida. All these higher categories are re-diagnosed.
Une phyiog~nie des Metatheria, fond~e sur une synth~se des morphoclines des tissus durs cr~niens, dentaires, postcraniens et de la morphologie externe du pied est proposee et discut~e. Le concept ~largi de Pediomyiclae inclut les marsupiaux du Cr~tac~ nord-am~ricain et la plupart de ceux du Pala~og~ne. Les Borhyaeno'idea et les Caenolestoi'dea sont r~unis dans les Borhyaeniformes, alors que les Pediomyidae et les Didelphidae, sensu stricto, forment les Didelphiformes. Les Borhyaeniformes et les Didelphiformes sont inclus dans l'ordre des Didelphida qui appartient h la cohorte des Ameridelphia. L'ordre des Dromiciopsia est le plus primitif h l'int~rieur de la cohorte des Australidelphia qui inclut aussi les ordres des Syndactyla et des Dasyurida. Une nouvelle dia9nose est donn~e pour toutes ces categories de ran 9 sup~rieur.
KEY-WORDS: PHYI,OGENETIC
RELATIONSHIPS, CLASSIFICATION, MARSUPIALS, ASTRAGALUS, CALCANEUS.
MOTS-CL.~-,S : RELATIONS PHYI,OG,fiA'ql~TIQUES, CLASSIFICATION, MARSUPIAl.IX, ASTRAGALE, CALCANl~UM.
" l tunter College, C.U.N.Y. and The American Museum of Natural History. Geobios, m~moire special 6
p. 177-190, 4 fig., 1 tabl.
Lyon, 1982
12
178
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TABLE OF CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Discussion 1. 2. 3. q. 5. 6. 7.
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Pediomyidae . . . . . . . . . . . . . . . . . . . . . . . Borhyaeniformes . . . . . . . . . . . . . . . . . . . Caenolestidae . . . . . . . . . . . . . . . . . . . . . . Polydolopidae . . . . . . . . . . . . . . . . . . . . . Argyrolagidae . . . . . . . . . . . . . . . . . . . . . Didelphidae, s e n s u s t r i c t o . . . . . . . . . . . . Australidelphia . . . . . . . . . . . . . . . . . . . .
178 182 182 185 185 186 186 186 186
8. Dromiciopsia 9. Dasyurida
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187 187
10. Syndactyla . . . . . . . . . . . . . . . . . . . . . . . . I 1. Perameliformes . . . . . . . . . . . . . . . . . . . . .
188 188
12. Phalangeriformes . . . . . . . . . . . . . . . . . . .
188
Classification . . . . . . . . . . . . . . . . . . . . . . . . . .
189
Acknowledgments
189
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References . . . . . . . . . . . . . . . . . . . . . . . . . . . .
189
INTRODUCTION
The study of the metatherian relationships has continued to progress during the past several decades. Added to the traditional avenues of anatomical and paleontological inquiries, the use of serology and chromosome studies have come to contribute increasingly to both available data and phylogenetic hypotheses. This steady accumulation of evidence and interpretive schemes for marsupial evolution, coupled with a heightened appreciation of the necessity for understanding character polarities, brought to the surface stimulating and fundamental disagreements between several competing phylogenetic hypotheses. The extreme differences between the proposed phylogenies of metatherians (and consequently the disagreements in both character analysis, and the approach to the reconciliation of known character cline polarities) are rarely appreciated by students not involved with marsupial evolutionary studies. In Fig. 1 I have summarized a number of influential marsupial phylogenies merely to illustrate a sample of the range of differences published since the turn of the century. A glance will reveal that most of the disagreements still concern the nineteenth century formulation of the polyprotodontdiprotodont, and diadactyl-syndactyl dichotomies. Views on marsupial relationships, even those published in the last two decades, continue to
differ greatly [see for example Archer (1981). Hoffstetter (1970). Kirsch (1977). Marshall (1977). Ride (196't). Simpson (1970), Tedford (1974). etc.]. In spite of a complete lack of phylogenetic meaning, the concepts polyprotodont (the ubiquitous perseverance of primitive teeth in derived higher taxa) and diprotodont (the notorious convergence of enlarged incisors in the Mammalia) continue to dominate some of the most recent discussions of marsupial phylogeny. The temptation for most systematist.s to pursue the phylogeny of taxa based on one character complex is often great, as expertise is difficult to attain in many areas. Paleomammalogists should not be unjustly accused of this practice, in regards to teeth, if they have exhausted all areas of evidence available for the taxa they study. Very often, for single species or for many genera, only dental material is available. For 9 therefore, the dentition represents the morphology of the taxon. For the vast majority of mammalian families, however, if not cranial, then some postcranial materials are available. The data base, therefore, must be accordingly expanded, and undue emphasis on teeth, when available postcranial features suggest more recent affinity with some group, is methodologically not only unsound, but it is also easily corroborated or rejected if the other cha-
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Fig. I. 9.... Selected published phylogenies of marsupials representing the range of conflicting views. Compare with Fig. 4. Abbreviations: A, Austrah~tan Metatheria : B, Borhyaenoidea; C, Caenolestoidea (sensu lato) ; Da, Dasyuroidea; Di, Didelphoidea; Dp. Diprotodonta; M, Marsupicarnivora (including Di, Da, B); Mi, Microbiothetiidae ; Ne, lVecrolestidae; No, lVotocgctidae; Ph, Phalangeroidea ; P, Perameloidea; T, Thglacinadae. Une s~lection de phylog4nies des Marsupiaux montrant la diversit4 des opinions (comparer avec la fig. 4). Abr~viations: A, Metatheria dAustralasie: B. Borhyaenoidea; C, Caenolestoidea (sensu /ato) ; Da, Dasyuroidea; Di. Didelphoidea; Dp, Diprotodonta; M, Marsupicarnlvora (incluant Di, IDa, B); Mi, Microbiotheriidae ; Ne, Necrolestidae ; No, Notorgctidae, Ph. Phalangeroidea ; P. Perameloidea ; T. Thglacinadae.
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f Fig. 2. - - Selected features of right calcanea and astragali of representative ameridelphians to indicate some primitive metatherian characters (a, b). and some diagnostic attributes (c, d, e. f). A b o v e : Early Eocene pediomyid (sensu lato), U C M P unnumbered (calcaneus). U C M P No. 113305 (astra0alus), Bitter Creek. Middle: Caenolestes sp.. A M N H No. 62915. Recent. Below: representative didelphid, Philander opossum, A M N H No. 176707, Recent. Scales in ram. From left to right : dorsal and plantar views of calcanea, plantar and dorsal views of astragali. T h e following characters are indicated by the arrows: a, separate lower ankle joint pattern, primitive for Metatheria ; b. groove for tendon of M. peroneus brevis, primitive for Metatheria ; c, ribbon-like attenuated sustentacular facet extended into recess above hypertrophied astragalar medial plantar tuberosity, primitive or diagnostic of Borhyaeniformes ; d, hypertrophied astragalar medial plantar tuberosity which locks onto underlying calcaneal sustentacular win o, diagnostic of Borhyaeniformes ; e. forward extension of medial tibial facet, diagnostic of Borhyaeniformes, or possibly primitive for Metatheria; f, distally extended medial cuboid facet extension, sharply angled from distal calcaneocuboid facet, diagnostic of Didelphidae, scnsu stricto. Une selection de caracteres du calcandum et de l'astragale des am(~ridelphes comprenant quelques caract~res primitifs de m~tath~riens (a, b) et quelques caract~res diagnostiques (c, d, e, f). En h a u t : p~diomyid~ (serLsu lato) de l'Eoc~ne inf~rieur, U C M [ non num~rot~ (calcan~um). U C M P N ~ 113305 (astra0ale), Bitter Creek. A u centre: Caenolestes sp., A M N H N ~ 62915, actuel. E n b a s : un membre repr~sentatif des didelphid~s, Philander opossum, A M N H N ~ 176707, actuel. Echelle en ram. De gauche "a droite: rues dorsale et plantaire du calcan~um, vues plantaire et dorsale de l'astraoale. Les flC,ches indiquent les caract~res suivants: a, surfaces articulaires astragalo-calcan~ennes disjointes, primitif pour les Metatheria ; b, sillon pour le tendon du M. peroneus brevis, primitif pour les Metatheria ; c, facette sustentaculaire att~nu~e, en forme de ruban, s'~tendant dans tree ~chancrure au-dessus d'une tub~rosit~ hypertrophi~e m~dio-plantaire de l'astragale, primitif ou diagnostique des Borhyaeniformes; e, extension vers l'avant de ]a facette m~diale du tibia, diaonostique des Borhyaeniformes ou peut-~tre primitive pour les Metather~a ; f, extension distale de ]a facette m~diale du cubo'/de, fortement inclin~e depuis ]a facette calcan(~o-cubo'ide, diagnostique des Didelphidae, sen,~u sfricfo.
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Fig. 3. - - Selected features of right catcanea and astragali of representative australidelphians to indicate some diagnostic attributes. Above:Dromiciops anstralis, A M N H No. 97746, representative of the australidelphian morphotype, Recent. Middle: Neophascogale Iorentzi, A M N H No. 101980, representative of the dasyuridan morphotype, Recent. Below: Notoryctes typhlops, A M N H No. 200241, the only known notoryctid, Recent. Scales in ram. From left to right: dorsal and plantar views of calcanea, plantar and dorsal views of astragali. The following characters are indicated by arrows : a, continuous lower ankle joint pattern and the absence of the distal articular portion of the primitive sustentacular facet, diagnostic of the Australidelphia ; b, descending plantolateral wing of calcaneus flanking the <> type calcaneocuboid articulation, well developed in the australidelphian morphotype and retained in a reduced form in a few dasyurids. Line s61ection de caract~res dn calcan~um et de rastragale des australidelphes comprenant quelques traits diagnostiques. En haut: Dromiciops australis, A M N H N ~ 97746, une esp~ce repr6sentative du morphotype australidel~ phe, Actuel. Au centre: Ne~phascogale loeentzi, A M N H N ~ 101980, une esp~ce representative dn morphotype dasyurid6, Actuel. En bas: Notoeyctes t!tphlops, A M N H N ~ 200241, le seul notoryctid6 connu, Actuel. Echelles ell mm. De gauche ~ droite: rues dorsale et plantaire du calcan6um, rues plantaire et dorsale de l'astragale. Les fl~ches indiquent les caract~res suivants: a, surfaces articulaires astragalocalcan6ennes fusionn6es et absence de la partie articulaire distale de la facette sustentaculaire primitive, diagnostique des Australidelphia; b, aile planto-lat~rale du calcan~um dirifl6e vers le has, bordant l'articulation calcan6ocubo'/de <>, bien d6velopp6e chez le morphotype australidelphe et persistant ~ l'6tat r~duit chez quelques dasyurid6s.
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racter complexes are also considered. M y intention here is to merely point out that if paleomammalogists continue to eschew the practice of sorting and describing all mammalian remains (particularly usable joint complexes) from their painstakingly and expensively collected faunal remains, then many hypotheses of mammal relationships proposed will be only dental character phylogenies. Thus both paleontologists and nonpaleontologists will remain ignorant of the extent of information available from the collected fossil
182
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record of a given group. The emphasis on odontolooy for the reason of an often exclusive prevalence of teeth is unavoidable, but the false assumptions about the ontogeny and phylogenetic significance of bone morphology (see Szalay, 1977, p. 315-32't) are unacceptable excuses for ignoring evidence from the postcranial skeleton. The vast repository of phylogenetic information present on the postcranial anatomy has not, as yet, been adequately exploited either for the marsupials, or for the rest of the Theria.
DISCUSSION Several years ago I began a systematic survey and functional-adaptive analysis (for a recent synopsis of the approach see Bock, 1981) of the tarsal morphology of living and fossil marsupials. Along with these efforts I have also perused the published literature in comparative serology of marsupials, and studied both the literature and extensive series of cranial, dental, and pedal specimens. In my tarsal studies, to be presented as a monograph, I have attempted both to understand the adaptive significance of form-function differences, assess the inherited restraints and chanelling factors on phylogenetic change, all these to facilitate the formulation and testing of polarity hypotheses. I have also paid attention to the fossil record both in the formulation of polarity hypotheses and the testing of polarity schemes derived from morphological evaluations. This procedure, contrary to the numerous published guidelines of neocladistic analysis which have come to require an axiomatic divorce from biological iudgment (e.g. Eldredge ~3 Cracraft, 1980), is considered phylooenetic. Given the limitations of space in this contribution, a summary of my interim phylogenetic conclusions are briefly presented. The considerable amount of raw data, detailed interpretive schemes, and lower level taxonomic discussions are being monographed elsewhere. The following discussions are divided into twelve points in order that the issues raised and diagnoses presented can be made closely relevant to the phylooenetic tree on Fig. `1. The numbers below, therefore, have both topical pertinance to the taxa in the tree, as well as give an estimate of the origin and modification of the characters discussed in time.
1 ) Pediomyidae I11 his major survey of the Cretaceous marsupials W . W . Clemens (1966) raised Simpson's (1927) subfamily designation Pediomyinae to family rank, considered the Didelphidae to be represented in the Cretaceous of North America by the subfamilies Didelphinae and Glasbiinae, and recognized the family Stagodontidae. I do not agree with the assignment of the Cretaceous forms to the Didelphinae, and I subsequently question the value of a broad and plesiomorphy-based use of the concept DMelphidae when the living taxon can be diagnosed. M y current usage of the concept Pediomyidae, therefore, as a poorly known waste basket category requires a brief explanation. On the balance of the dentition the living didelphids (e.g. Crochet, 1978, p. 63) are probably as primitive as are the dental features found in the present broad concept of the Pediomyidae. The dental specializations of the Pediomyinae (see Fox, 1979b), however, are so minor compared to other Cretaceous metatherians, that separation of these from other late Cretaceous and Paleogene species on the family level must be questioned. The livin 0 Viverridae, to cite only one example, displays a broader dental spectrum than the whole known late Cretaceous and North American Paleogene Metatheria. Short of assuming special relationships of some of the Cretaceous and Paleogene taxa with the living Didelphidae (affinities which should be based on derived characters), the fossil taxa do not exceed the morphological diversity commonly accepted for the dentitions of living family concepts. W h y , then, should these disputed fossil taxa not be included in the Didelphidae ?
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Fig. 4. - - A phylogeny of the Metatheria, corroborated or not contradicted by the c~aracter cline polarities of the skull, dentition, and selected postcranial features briefly discussed in the text. Continuous thick vertical lines depict the known stratigraphic range of the taxa, the broken downward extensions represent their estimated duration as a taxon diagnosed by the characters listed in the text. Circles represent unsolved branching or ancestor-descendant relationships. Arrows indicate the derivation and estimated points of origin of the diagnostic traits ; numbers correspond to those points sequenced in the DISCUSSION in the text where the pertinent diagnoses and discussions are found. Time scale on the left border in ten million year intervals; on the right border subdivisions correspond to the Upper Cretaceous and the epochs of the Cenozoic. Line phylogenie des Metatheria, corroboree ou non contredite par la polarite des caracteres du crane, de la dentition et du squelette postcrfiniens bri~vement discutes dans le texte. Les lignes verticales continues representent la repartition stratigra~ phique des taxons, les lignes brisees dirigees vers le bas representent la duree d'existence estimee des taxons dont la diagnose est dtablie ~t partir des caract~res cites dans le texte. Les cercles symbolisent les branchements non rdsolus ou les relations d'anc@tre ~ descendant. Les fl~ches indiquent la derivation et les points d'origine estimes des traits diagnostiques ; les chiffres correspondent aux points discutes dans le texte off 1'o11 trouvera les diagnoses pertinentes et les discussions. L'echelle de temps r gauche indique des intervalles de 10 M.A. ; ~t droite, les subdivisions correspondent au Cretace superieur et au CenozoYque.
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As argued under Didelphidae, the members of that family sensu stricto (subdivisible into subfamilies based on dental and other characters), which are the livin 9 species and the mostly Neogene Neotropical taxa, show unmistakable, derived tarsal attributes inherited from a common didelphid ancestor, and therefore their diagnostic distinction from at least those Cretaceous and early T e r t i a r y taxa which are known by tarsals. N o luther case should be made for the relative uselessness of a primitive molar pattern for allocating taxa to higher categories. Dental systematics, however, is a widely accepted substitute for complete taxon evaluations. Let us assume, for example, that Phascogale, a dasyurid, was known in N o r t h America from the Cretaceous by its molars alone. It is very likely that it would be allocated to a broad concept of the Didelphidae. Similarly, dentitions which closely resemble those of living didelphids (or dasyurids) are referred to the Didelphidae in the literature, althou9h the
INFRACLASS METATHERIA
Cohort Ameridelphia
Order Didelphida Suborder Didelphiformes Suborder Borhyaeniformes Superfamily Boryhaenoidea Superfamily Caenolestoidea Superfamily Polydolopoidea Superfamily Ar9yrolagoidea
Cohort Australidelphia
Order Dromiciopsia Order Syndactyla Suborder Syndactyliformes Superfamily unknown Superfamily Notoryctoidea Suborder Perameliformes Suborder Phalangeriformes Superfamily Phalangeroidea Superfamily Vombatoidea Superfamily Diprotodontoidea Superfamily Macropodoidea
Order Dasyurida
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evidence of marsupial tarsal bones from the same quarries strongly suggests that these early forms share only primitive dental similarities with the
Didelphidae, sensu stricto. Because the stereotyped dental morphology merely labels these forms as dentally primitive marsupials, resemblin 9 the primitive didelphids, microbiotheriids, borhyaenids, caenolestids, caroIoameghiniids, and dasyurids (and probably also the primitive, hitherto unknown, syndactylans), it appears that the allocation of these Cretaceous and Paleogene forms to a family of their own is a more stimulating and less biased systematic practice. On account of the close similarity of the Late Cretaceous and Paleogene dentitions to the primitive dental patterns of the Metatheria, I propose to include these taxa previously considered didelphines, steoodontids, 91asbiines, and pediomyines in the family Pediomyidae. Admittedly, this is a waste basket category of heterogeneous dentitions, with, however, an important expansion of the concept from tarsals. This move has the merit of allowing the definition of the complex and varied, primarily South American Didelphidae with at least some derived combinations. Primitive ameridelphian, and therefore presumably metatherian, features of the Pediomyidae include a polyprotodont dentition with at least a 5,/'t incisor formula, and sectorial molars with stylar cusps. It is assumed that the advanced therian pattern of reduced tooth replacement, present in other marsupials, has already been attained. Pes was probably with a large grasping hallux, although this is unknown, and the tarsal
Table I An abridged evolutionary classification of the Metatheria based on the phylogeny of Pig. 4, As noted in Szalay (1982), the abandonment of the ordinal concept Marsupicarnivora RIDE, 1964 and the use of the cohorts Ameridelphia and Australidelphia are based on the conviction of a) the polyphyly of the concept Marsupicarnivora. and b) the lack of correspondence of the cohorts with Simpson's (1970) undiagnosed subordinal concepts Hesperometatheria and Eometatheria. Une classification evolutionniste sommaire des Metatheria fond~e sur la phylog~nie de la fig. 4. Com.ne il est not~ dans Szalay (1982). labandon du concept ordinal Marsupicarnivora RIDE, 1964 et l'usage des cohortes Ameridelphia et Australidelphia sont fondus sur la conviction a) de la polyphylie du concept Marsupicarnivora, et b) de l'absence de correspondance entre ces cohortes et les concepts sous-ordinaux de Simpson (1970): Hesperometatheria et Eometatheria qui sont d~pourvu,x de diagnose.
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morphology, the most primitive known, with the following attributes : a) separate lower ankle joint pattern on astragali and calcanea: b) a simple ovoid articulation, a shallow ball and socket joint, between the calcaneus and cuboid; c) transversely oriented astragalonavicular joint. The possibility exists that all of the taxa which are known by astragali and calcanea in the early Tertiary have special affinities with the Borhyaeniformes. Uniquely among marsupials, the caenolestoids and borhyaenoids share with the Paleogene specimens an attenuated, ribbon-like proximal sustentacular facet which runs above the medial astragalar plantar tuberosity. A functional explanation for the unique astragalocalcaneal contact that is reflected by the process and facet is attempted by F.S. Szalay (1982). 2
) Borhyaeniformes
The borhyaenoids and the caenolestids have been recently reviewed by L.G. Marshall (1978. 1979, 1981a, b). Postcranial features, however, considerably clarify their affinities. More recent previous views, based on primitive character states, have merely advocated an independent didelphoid origin for both groups. Possession of virtually identical series of modifications on the proximal tarsals of borhyaenoids and caenolestids, however, independent of their body size. are difficult to dismiss either as mere convergence or because of nebulous arguments rooted in allomerry. These special similarities can be related to unique mechanical solutions developed for terrestrial hopping or galloping gait, yet no other groups with similar (or any other) locomotor propensities displays the borhyaeniform combination of characters. The primitive borhyeniform character states include: a) polyprotodont dentition with an incisor formula probably reduced to 4,/4 ; b) sectorial, perhaps pediomyine-like (but see Fox, 1979a, b) molars : c) a foot with a somewhat reduced ha]lux (the antecedent condition is unknown); d) the tarsal morphology, uniquely modified for a primarily terrestrial locomotor repertoire, is characterized by mortise-like upper ankle joint with an anteromedial extension of the astragalar tibial facet: e) locking lower ankle joint with hypertrophied medial astragalar plantar tuberosity and a ribbon-like extension of the proximal sustentacular facet into recess above the tuberosity: [) astragalonavicular joint modified to ensure greatest resistance to deformation and maximum stability at the end of extreme eversion and/or
185
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the push-off phase of the stride; g) unmodified sellar articulation between entocuneiform and Mt I. It is most likely that an early Tertiary small borhyaenid lineage had undergone dental and cranial transformation, retaining the ancestral borhyaeniform tarsal morphology, and thus becoming the protocaenolestoid. Small, caenolestid sized borhyaenids are present in the Paleogene of both South and possibly North America, as this can be ascertained from well represented tarsal remains from Eocene deposits in western North America and the Itaborai fauna of Brazil (Szalay, 1982). It should be emphasized that the mechanically complex tarsus, invariably molded around the requirements of substrate preference and locomotor mechanics, is powerfully constrained by the genetic mechanisms inherited in any given group (Szalay, 1977). No two groups of Australasian marsupials, for example, employ similar formfunction solutions to terrestrial life, often involving similar gait, in spite of their similar ancestry. The astonishing tarsal resemblances found in small caenolestids and borhyaenids and very large thylacosmilids preclude any a d hoc explanation rooted in allometry, and the known data allows no other explanation at present except homology. Arguments that such similarities, or syndactyly among australidetphians, for example, are convergent would seem specious, unless shored up by detailed models showing the inevitability of such similar patterns due to similar setectional forces. On the other hand, the taxon specific, derived dental adaptations of the caenolestids require no other explanation than divergence [rom the primitive borhyaeniform pattern. Hitherto undescribed footbones from the Itaborai fauna (Szalay, in preparation), clearly those of borhyaenids, show a large astragalar canal, widely opening both dorsally and plantarly. The presence of such a primitive therian feature, not known in any didelphid, lends additional support to the notion that the borhyaeniforms did not diverge from the Didelphidae, sensu stricto.
3 ) Caenolestidae The widely ranging views on the affinities of the Caenolestidae were summarized by L.G. Marshall (1980) in his comprehensive revision of the systematics of the family. The diagnostic features of the stem caenolestid, as far as known, have usually been restricted to cranial and dental characters. The lower incisors
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are typically diprotodont, and the upper molars, usually the first two, have well developed hypocones. There is no resemblance between any of the four upper incisors of caenolestids and those of diprotodont australidelphians. Marshall's (1980) revision contains several additional dental attributes which characterize the stem caeuolestid. As noted under Borhyaeniformes, the striking derived similarity of various tarsal attributes of borhyaenoids and caenolestids (see Szalay, 1982: in preparation) is yet the strongest line of evidence regarding caenolestid affinities. Caenolestid dental specializations simply postdate a postulated borhyaenid-caenolestid split, and they present no obstacles for the hypotheses offered here.
4) Polydolopidae Whether or not this family is a close relative of caenolestids is a question still unanswered, in spite of the demonstration by C. de Paula Couto (1952), as noted by L.G. Marshall (1980), that the sectorial first molar of caenolestids and the last premolar of polydolopids are not homologous. The possibility still exists that the hypertrophid incisors of the two families (and possibly of the groeheriids and argyrolagids) are homologous. The most diagnostic features of the Polydolopidae is the plagiaulacoid condition of the last premolars, discussed in detail by G.G. Simpson (1933). Although a yet briefly studied skull is known, polydolopids have been known primarily dentally. The Itaborai tarsal collection currently under study contains remnants of polydolopids. Knowledge of these elements is likely to increase our understanding of the affinities and adaptations of these marsupials.
5
) Argyrolagidae
The most recent ties of the Argyrolagidae within the order Didelphida are unclear (Simpson, 1970). The tubular snout projects anterior to the incisors and the cheek teeth are rootless; the hindlimbs are thoroughly modified for a jumping habitus (Simpson, 1970). Although the tarsus requires careful restudy, at least one feature, specifically the enlarged medial portion of the upper ankle joint (Simpson, 1970, fig. 16F), suggests borhyaeniform rather than didelphiform ties. The enlarged incisors suggest, as noted and discussed by G.G. Simpson (1970), caenolestoid (sensu lato) affinities.
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6)
Didelphidae, sensu s t r i c t o
This extant family, regardless of the diverse habitus displayed by the opossums, can be defined by diagnostic derived characters. It is unlikely that any extinct member of this clade gave rise to the borhyaeniforms. Difficult as it may be, a major effort is needed to establish, if possible, the derived dental characters of this family. The ancestral species of the extant Didelphidae may be characterized b y : a) polyprotodont dentition with 5/4 incisor formula; b) transversely shortened molars and possibly attenuated protolophids; c) pes diadactylous with large grasping hallux, lacking a nail or claw ; d) broad and gently curving upper ankle joint of the pes; e) transversely extensive astragalofibular contact ; f) separate lower ankle joint pattern: 9) extensive, projecting cuboid <>, with characteristic plantar position and with a corresponding and equally diagnostic modification of the calcaneus on its distal plantar portion; h) entocuneiformMt I articulation with a distinctive double-ridged pattern. Whether this latest condition is a didelphid specialization or a retention of the primitive metatherian configuration is not understood at present.
7 ) Australideiphia Assessment of the Andean-Patagonian Dromiclops has been pivotal for the phylogenetic hypothesis offered here and in F.S. Szalay (1982). Although Dromiciops australis is a polyprotodont with a primitive dental formula (hence its traditional desiqnation as a didelphoid) and diadactylous, in possession of a uniquely hypertrophied bulla (along with Microbiotherium), its highly derived tarsal morphology securely ties it to the Australasian radiation. Together they make up the cohort Australidelphia in contrast to the other Metatheria, the cohort Ameridelphia (Szalay, 1982). The tarsal similarity of Dromiciops to the Australasian morphotype condition is undoubted, and it is a complex shared and derived combination of characters, These characters are derived, unique, and allow little doubt of the strict monophyly of the Australidelphia. The australidelphian morphotype had the following combination of characters: a) polyprotodont dentition with 5/4 incisor formula: b) incisors probably slightly hypertrophied as in Dromiciops and primitive dasyurids; c) primitive metatherian sectorial cheek teeth ; d) diadactylous pes with large grasping hallux ; e) calcaneofibular
--
articulation almost completely reduced, with probably little or no contact between fibula and calcaneus; f) continuous lower ankle joint pattern, and the loss of distal sustentacular articulation between astralagus and calcaneus: g) calcaneocuboid joint with a unique <> type articulation which wedges the cuboid into a laterally restricted calcaneocuboid contact; h) entocuneiform-Mt I articulation with diagnostic sellar morphology, allowing extensive rotation and abduction of the hallux. A functional-adaptive analysis which accounts for the morphocline polarity of the tarsal modifications in t,he Metatheria is presented elsewhere (Szalay, 1982 ; in preparation). It appears that the primitive austradelphian tarsus evolved as a result of selectional forces associated with an obligate inversion of the pes, possibly in a terminal branch milieu similar to that in which Burramyidae (pigmy possums) move habitually. It. is likely that the evolution of the relatively enormous calcaneal distal lateral process (see Fi 9. 3) and the subsequent <> of the cuboid were the primary adaptive responses to counteract the forces loaded onto the foot during the inverted grasping stance so characteristic of phalangeroids. The osseous modification (the <~angle iron >>articulation) which brought relief to the soft structures of the habitually inverted foot, and subsequently reduced energy expenditure, probably resulted in the loss of the distal sustentacular contact and the concomittant development of the unique australidelphian continuous lower ankle joint articulation. A detailed survey of the tarsals shows the pervasive channelling influence of the morphotype pattern on all subsequently derived species. 8 )
Dromiciopsia
The characters postulated to be present in the australidelphian morphotype are present in Dromiciops. There can be little doubt about the great independent antiquity of that lineage from the Didelphidae. Although based on the basicranial similarities of Microbiotherium and the living Dromiciops, we can associate the fossil forms with the latter due to the primitive dental morphology of this group, but our knowledge of their radiation, antiquity, and history is virtually nonexistent. The evolutionary morphology of at least one investigated area, the tarsus, of Dromiciops assures their long and distinct history. The most recent common ancestor of Dromiciops and Microbiotherium developed composite and highly inflated bullae which are formed anteriorly
187
by the alisphenoid, and posteriorly what has been reported to be an entotympanic (Se9all, 1969). The latter component, however, is more likely to be the petrosal. The derived dromiciopsian bullar hypertrophy was unlikely to have been present in the australidelphian morphotype.
9 ) Dasyurida The origins of the diagnostic dasyuridan character complex is intimately tied to the modifications of the postcranial anatomy for a primarily terrestrial existence. The combined evidence, so far particularly the tarsal morphology, firmly points to the origin of the protodasyurid not from a didelphid (sensu stricto) or pediomyid (sensu lato), but from an australidelphian source which is perhaps best estimated to be a dromiciopsian. The morphotype adaptations of the Dasyurida appear to be the result of the obligate, rapid, terrestrial progression found today in most dasyurid species. The fact that this habitus occurs together with considerable arboreal abilities does not: detract from the diagnostic value of the clearly superimposed changes, required by the more recently acquired terrestriality. The morphotype dasyurid possessed the following known combination of characters : a) polyprotodont dentition with a 4~3 incisor formula; b) relatively enlarged incisors compared to didelphids; c) greatly reduced entocuneiform and hallux on a diadactylous pes ; d) upper and lower ankle joints essentially as described above, under the Australidelphia; e) calcaneocuboid joint is modified from the restrictive ancestral pattern seen in Dromiciops and phalangeroids, allowing the unobstructed eversion necessary for rapid terrestrial progression; the hallmarks of the angle iron type contact are retained in most small dasyurids (Fig. 3 ) : f) entocuneiform-Mt I articulation as in Dromiciops and phalangeroids, and unlike in didelphids. All previous notions, with the astute exception of Bensley's (1903), concerning the phylogenetic position of the dasyurids were based on the largely primitive dental morphology of the many species of this taxon, and these views predominate even today. The tarsal characters, however, clearly show that not only are the dasyuridans (thylacines included) part of a monophyletic Australidelphia, but that they are derived from a group of arboreal animals with a nearly primitive metatherian dentition and a tarsus not like primitive metatherians or didelphids, but one derivable from an australidelphian morphotype condition described above.
--
The form-function of the entocuneiform-Mt I joint is so far one of the best lines of corroborative evidence supporting the hypothesis above based on the proximal tarsals. W h y would, one might ask, a terrestrial group with the virtual loss of all hallucial biological roles, possess a derived, nonrestricted sellar articulation. In spite of its rudimentation the Mt I and entocuneiform mirror the stasis of this region in dasyurids. The australidelphian attribute is retained unchanged because the requirements placed on the pedal cheridia I-IV did not influence t.he greatly shortened hallux. The tarsal evidence securely ties the thylacinids to the Dasyurida as G.G. Simpson (19Zil) and L.G. Marshall (1977) argued on dental and cranial grounds, and it contradicts the borhyaenoid ties argued for by M. Archer (1976b) and I.A.W. Kitsch (1977). The much less controversial ties of Myrmecobius with dasyurids are also corroborated by the tarsal evidence, and they are also indicated by serology (Kitsch, 1977) and basicranial morphology (Archer, 1976b).
188
--
This character combination, although not found in any living syndactylan, diagnosed the earliest syndactyliform (a hypothetical species of a hypothetical radiation) whose descendants are the perameloids, probably notoryctids, and the phalangeroids. The articular tarsal morphology of the tarsus of Notoryctes removes all doubt concerning the broad mammalian affinities of the marsupial mole (Fig. 3). As in all other australidelphians, the continuous lower ankle joint pattern testifies to its cohort affinities. The presence of a very large Mt I would indicate the derivation of this lineage not from a dasyuridan, but from a polyprotodont australidelphian group with a large grasping hallux. The question remains whether this source was a dromiciopsian or a syndactylan one. Preliminary analysis suggests that the subequal second and third metatarsals may be indicative of a syndactylan heritage.
1 I ) Perameliformes 10) Syndactyla It is perhaps instructive to consider both on methodological and morphological grounds why a character complex like the syndactylous pes would be considered convergent between perameloids and phalangeroids without a morphological case made against their homology. ].A.W. Kirsch (1977) in his serological study of marsupials made no substantial arguments for his assertion of such a convergence. The fact that syndactyly prohibits a dasyurid-paremeloid sister group hypothesis generated by Kirsch's studies is perhaps indicative of the phenetic nature of the serological evidence which cannot be cast into the form of morphocline polarities, rather than evidence against the homology of marsupial syndactyly. The order Syndactyla, until proven to the contrary, is best considered to be monophyletic. The morphotype of the order had: a) a polyprotodont dentition with a 5,/3 incisor formula; b) molars possibly with an incipient hypocone; c) tarsal morphology virtually identical to that of the protoaustralidelphian; d) syndactylous and reduced condition of pedal cheiridia II and III, accompanied by superficial and deep myological changes related to the digits (see W o o d ]ones, 1922~25).
The perameliform ancestor, with its nearly complete metatherian dental formula remodelled the syndactylous grasping foot and tarsus into a highly diagnostic terrestrially suited structure (see Marshall, 1972). Detailed study of this transformation is in preparation. Any hypothesis of a dasyurid-peramelid monophyly, based either on primitive morphological retentions or on phenograms generated by the probably conservative serological evidence, is rejected by the presence of the syndactylous pes in bandicoots.
12 )
Phalangeriformes
The ancestral phalangeriform can be diagnosed by the following combination of features: a) reduced number of incisors, with three above and two or possibly three below; b) enlargement of the central incisors into the diprotodont pattern i c) acquisition of the [asciculus aberrans in the brain. In spite of the cranial and dental modifications, probably related to sap, gum, foliage, seed, and insect eating adaptations (fide Smith, 1980), the australidelphian tarsal adaptations of the ancestral syndactylan are retained virtually unchanged.
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189
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CLASSIFICATION There are a very lar0e number of names available for suprafamilial groupings of marsupials, as summarized by L.G. Marshall (1981b). Suprafamilial nomenclature is made particularly frustrating and confusing by the fact that there are no clear rules to adhere to when attempting to present a new classification which wants to reflect a new phylogenetic hypothesis, while still attempting to retain important adaptational information. Employing old names which still encompass concepts of taxa removed from that group appears to greatly diminish the meaning of the original concept. A similar objection may be raised for later additions to a concept, although in that case, new discoveries of groups may be accommodated
without destroying a classification. Another frustrating aspect of the lack of rules of our taxonomic system above superfamily rank is that the endings of names do not convey any rank information. A great deal of communicative facility is lost in constantly having to designate the rank of a category in question. In this paper I simply follow the practice -a ending for orders, and -[ormes for suborders, when designating them formally, and their appropriate vernacular forms otherwise. Table I presents an abbreviated classification of the Metatheria. emphasizing monophyly and the appropriate use of paraphyletic taxa to express adaptive radiations. Additional comments on this classification are in F.S. Szalay (1982).
Acknowledgments
I am particularly indebted to Dr. James Warren for greatlY/ facilitatin0 all aspects of the work during my 1980 sabbatical at Monash University.
Research for this p a p e r w a s supported by C.U.N.Y. Doctoral Research Award 13454, a John Simon Guggenheim Foundation Fellowship, and the Department of Zoology, Monash University, Australia.
to
For help in the preparation of the manuscript, m y Ellen Woodstock.
thanks
REFERENCES AaChEn M. (1976a). - - The Dasyurid Dentition and its relationships to that of didelphids, thylacinids, borhyaenids (Marsupicarnivora) and peramelids (Peramelina, Marsupialia). Aust. ]. Zool., Melbourne, Supplementary Series, vol. 39, p. 1-34. ARCHER M. (1976b). - - The basicranial region of marsupicarnivores (Marsupalia), interrelationship of carnivorous marsupials, and affinities of the insectivorous marsupial peramelids. J. Linn. Soc. Lond. Zool., London, vol. 59, p. 217-322. Am:tIER M. (1981). - - A review of the origins and radiations of Australian mammals. In KEAST A. (ed.), Ecological Biogeography in Australia, Junk Press, Chap. 52, p. 1437-1488. BENSLEY A. (1903). - - On the evolution of the Australian Marsupialia; with remarks on the relationships of the marsupials in general. Trans. Linn. Soc. Lond. Zool., London, vol. 2. n ~ 9. p. 83-217.
BocK W . ( 1 9 8 1 ) . - Functional Adaptive Analysis in Evolutionary Classification. Amer. Zool., Bloomington, vol. 21, p. 5-20. CLEMENS IR., W . W . (1966). - - Fossil mammals of the type Lance Formation, W y o m i n g Part II. Marsupalia. Univ. Calif. Publ. Geol. Sci., Berkeley, vol. 62, p. 1-22. CROCHET I.-Y. (1978). - - Les marsupiaux du Tertiaire d'Europe. Thdse Llnil,. Sci. et Techn. Languedoc, Montpellier, vol. 1-2, p. 1-360. ELDREDGE N. 6 CRACRAFT I. (1980). - - Phylogenetic patterns and the evolutionary process. Columbia Uni~,ersity Press, New York, 349 p. Fox R.C. (1979a). - - Ancestry of the ~ dog-like ,7 marsupials. ]. Paleon., Tulsa, vol. 53, n" 3, p. 733-735. Fox R.C. (1979b). - - Mammals from the Upper Cretaceous Oldman formation, Alberta. II Pediomys Marsh (Marsupials). Can. [. Earth Sci., Vancouver, vol. 16, p. 103-113.
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GREOORV W . K . (1910). - - T h e orders of mammals. Bull. Amer. Mus. Nat. Hist.. N e w York. vol. 3, p. 1-52'1. GR~OORY W . K . (1951). - - Evolution emerging: a survey of changing patterns from primeval life to man. Macmillan, N e w York. HOFPS'rET'r~R R. ( 1 9 7 0 ) . L'histoire biog~ographique des marsupiaux et la dichotomie marsupiaux-placentaires. C.R. Acad. Sc. Paris, vol. 271, p. 388-391. KmSCH J . A . W . (1977). - - T h e comparative serology of Marsupalia, and a classification of marsupials. Aust. ]. Zool., Melbourne. vol. 52, p. 1-152. MARSHALL L.G. (1972). - - Evolution of the peramelid tarsus. Proc. R. Soc. Victoria, Melbourne. vol. 85, n ~ 1, p. 51-55. MARSHALL L.G. (1977). - - Cladistic analysis of borhyaenoid, dasyurid, didelphoid, and thylacinid (Marsupalia: Mammalia) affinity. Sgst. Zool., W a s h i n g t o n . vol. 26, n ~ '1, p. '110-'125. MARSHALL L.G. (1978). - - Evolution of the Borhyaenidae, extinct South American predaceous marsupials. Univ. Calif. Publ. Geol. Soc., Berkeley, vol. 117, p. 1-89. MARSHALL L.G. (1979). - - Review of the Proth 9lacgninae, an extinct subfamily of South American r dog-like >> Marsupials. Fieldiana (Geol.), N.S., Chicago, vol. 5, n ~ 3, p. 1-50. MARSHALL L.G. 0 9 8 0 ) . - - Systematics of the South American marsupial family Caenolestidae. Fieldiana (Geol.), N.S., Chicago, vol. 5. n ~ 1310. p. 1-1'15. MARSHALL L.G. (1981a). - - Review of the H a t h lyacyninae, an extinct subfamily of South American ~ marsupials. Fieldiana (Geol.), N.S., Chicago, vol. 7. n ~ 1318, p. 1-120. MARSHALL L.G. (1981b). - - T h e families and genera of Marsupialia. Fieldiana (Geol.). N.S., Chicago, vol. 8, n ~ 1320, p. 1-65. PAULA COUTO C. de (1952). - - Fossil mammals from the beginning of the Cenozoic in Brazil. M a r s u p a l i a : Polydolopidae and Borhyaenidae. Amer. Mus. Not, it., N e w York, n ~ 1559. RtDE W . D . L . (1962). - - O n the evolution of Australian mammals. In LErpER G . W . (ed.), T h e Evolution of Living Organisms. Melbourne Uni~,. Press. p. 281-306. RmE W . D . L . (196'1). - - A review of Australian fossil marsupials. ]. Proc. R. Soc. West. Aust., Perth. vol. '17. p, 97-131.
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SEGALL W . (1969). - - T h e middle ear region of Dromiciops. Acta Anat., Basel, vol. 72. p. 't89501. SIMPSON G.G. (1927). - - Mammalian fauna of the Hell Creek Formation of Montana. Amer. Mus. Novit., N e w York, n ~ 267, p. 1-7. SIMPSON G.G. (1933). - - T h e e plagiaulacoid type of mammalian dentition. ]. Mammal., Lawrence, vol. 14, n ~ 2, p. 97-107. StepSON G.G. (1941). - -
T h e affinities of the
Borhgaenidae. Amer. Mus. Novir., N e w York, n ~ 1118, p. 1-6. SrMPSON G.G. (1945). - - T h e principles of classification and a classification of mammals. Bull. Amer. Mus. Nat. Hist., N e w York, vol. 85, n ~ V I - X V I , p. 1-350. SIMPSON G.G. (1970). - - T h e Argyrolagidae. extinct South American marsupials. Bull. Mus. Comp. Zool., Cambridge, vol. 139, n ~ 1, p. 1-86. SMXTH A. (1980). - - T h e diet and ecology of leadbeaters possum and the sugar glider. Ph. D. Thesis, Monash Univ., Dep. of Zoology, Victoria, Australia. SzAt.hV F.S. (1977). - - Phylogenetic relationships and a classification of the eutherian Mammalia. In H r c H ' r M.K., GooDY P.C., HECHT B.M. (eds), M a j o r Patterns of V e r t e b r a t e Evolution. Plenum Press, N e w York, p. 317-37`1. SZ,AI.AY F.S. (1981a). - - Functional analysis and the practice of the phylogenetic method as reflected by some mammalian studies. Amer. Zool., Bloomington, vol. 21, p. 37-'t5. SZALAY F.S. (1981b). - - P h y l o g e n y and the problem of adaptive significance: the case of the earliest primates. Folia primatol., Basel, vol. 36, p. 157-182. SZAt.AY F.S. (1982). - - A new appraisal of marsupial phylogeny and classification. In AmCHER M. (ed.), Carnivorous Marsupials, vol. II, chap.
3. The Ro!lal Zoological Society of New South Wales, Sydney, Australia. TEDFORD R.H. (197't). - - Marsupials and the new paleogeography. In Paleogeographic Provinces and Provinciality. Ross C.A. (ed.). Tulsa, p. 109-126. W o o d JoNrs F. (1923-25). - - T h e mammals of South Australia. Parts I-III. Government Prin-
ter, Adelaide.
ABSTRACT
R0,SUM~-
A phylogeny of the Metatheria. based on the synthesis of cranial, dental, and postcranial hard morphology, and external pedal morphoclines, is presented and discussed. The expanded concept Pediomyiclae includes North American Cretaceous and most Palaeogene marsupials. The Borhyaenoidea and Caenolestoidea are united in the Borhyaeniformes, whereas the Pediomyidae and Didelphidae, sensu stricto, make up the Didelphiformes, both of the order of Didelphida, cohort Ameridelphia. Dromiciopsia is the most primitive order of the cohort Australidelphia which also includes the orders Syndactyla and Dasyurida. All these higher categories are re-diagnosed.
Une phyiog~nie des Metatheria, fond~e sur une synth~se des morphoclines des tissus durs cr~niens, dentaires, postcraniens et de la morphologie externe du pied est proposee et discut~e. Le concept ~largi de Pediomyiclae inclut les marsupiaux du Cr~tac~ nord-am~ricain et la plupart de ceux du Pala~og~ne. Les Borhyaeno'idea et les Caenolestoi'dea sont r~unis dans les Borhyaeniformes, alors que les Pediomyidae et les Didelphidae, sensu stricto, forment les Didelphiformes. Les Borhyaeniformes et les Didelphiformes sont inclus dans l'ordre des Didelphida qui appartient h la cohorte des Ameridelphia. L'ordre des Dromiciopsia est le plus primitif h l'int~rieur de la cohorte des Australidelphia qui inclut aussi les ordres des Syndactyla et des Dasyurida. Une nouvelle dia9nose est donn~e pour toutes ces categories de ran 9 sup~rieur.
KEY-WORDS: PHYI,OGENETIC
RELATIONSHIPS, CLASSIFICATION, MARSUPIALS, ASTRAGALUS, CALCANEUS.
MOTS-CL.~-,S : RELATIONS PHYI,OG,fiA'ql~TIQUES, CLASSIFICATION, MARSUPIAl.IX, ASTRAGALE, CALCANl~UM.
" l tunter College, C.U.N.Y. and The American Museum of Natural History. Geobios, m~moire special 6
p. 177-190, 4 fig., 1 tabl.
Lyon, 1982
12
178
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TABLE OF CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Discussion 1. 2. 3. q. 5. 6. 7.
............................
Pediomyidae . . . . . . . . . . . . . . . . . . . . . . . Borhyaeniformes . . . . . . . . . . . . . . . . . . . Caenolestidae . . . . . . . . . . . . . . . . . . . . . . Polydolopidae . . . . . . . . . . . . . . . . . . . . . Argyrolagidae . . . . . . . . . . . . . . . . . . . . . Didelphidae, s e n s u s t r i c t o . . . . . . . . . . . . Australidelphia . . . . . . . . . . . . . . . . . . . .
178 182 182 185 185 186 186 186 186
8. Dromiciopsia 9. Dasyurida
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187 187
10. Syndactyla . . . . . . . . . . . . . . . . . . . . . . . . I 1. Perameliformes . . . . . . . . . . . . . . . . . . . . .
188 188
12. Phalangeriformes . . . . . . . . . . . . . . . . . . .
188
Classification . . . . . . . . . . . . . . . . . . . . . . . . . .
189
Acknowledgments
189
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References . . . . . . . . . . . . . . . . . . . . . . . . . . . .
189
INTRODUCTION
The study of the metatherian relationships has continued to progress during the past several decades. Added to the traditional avenues of anatomical and paleontological inquiries, the use of serology and chromosome studies have come to contribute increasingly to both available data and phylogenetic hypotheses. This steady accumulation of evidence and interpretive schemes for marsupial evolution, coupled with a heightened appreciation of the necessity for understanding character polarities, brought to the surface stimulating and fundamental disagreements between several competing phylogenetic hypotheses. The extreme differences between the proposed phylogenies of metatherians (and consequently the disagreements in both character analysis, and the approach to the reconciliation of known character cline polarities) are rarely appreciated by students not involved with marsupial evolutionary studies. In Fig. 1 I have summarized a number of influential marsupial phylogenies merely to illustrate a sample of the range of differences published since the turn of the century. A glance will reveal that most of the disagreements still concern the nineteenth century formulation of the polyprotodontdiprotodont, and diadactyl-syndactyl dichotomies. Views on marsupial relationships, even those published in the last two decades, continue to
differ greatly [see for example Archer (1981). Hoffstetter (1970). Kirsch (1977). Marshall (1977). Ride (196't). Simpson (1970), Tedford (1974). etc.]. In spite of a complete lack of phylogenetic meaning, the concepts polyprotodont (the ubiquitous perseverance of primitive teeth in derived higher taxa) and diprotodont (the notorious convergence of enlarged incisors in the Mammalia) continue to dominate some of the most recent discussions of marsupial phylogeny. The temptation for most systematist.s to pursue the phylogeny of taxa based on one character complex is often great, as expertise is difficult to attain in many areas. Paleomammalogists should not be unjustly accused of this practice, in regards to teeth, if they have exhausted all areas of evidence available for the taxa they study. Very often, for single species or for many genera, only dental material is available. For 9 therefore, the dentition represents the morphology of the taxon. For the vast majority of mammalian families, however, if not cranial, then some postcranial materials are available. The data base, therefore, must be accordingly expanded, and undue emphasis on teeth, when available postcranial features suggest more recent affinity with some group, is methodologically not only unsound, but it is also easily corroborated or rejected if the other cha-
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Fig. I. 9.... Selected published phylogenies of marsupials representing the range of conflicting views. Compare with Fig. 4. Abbreviations: A, Austrah~tan Metatheria : B, Borhyaenoidea; C, Caenolestoidea (sensu lato) ; Da, Dasyuroidea; Di, Didelphoidea; Dp. Diprotodonta; M, Marsupicarnivora (including Di, Da, B); Mi, Microbiothetiidae ; Ne, lVecrolestidae; No, lVotocgctidae; Ph, Phalangeroidea ; P, Perameloidea; T, Thglacinadae. Une s~lection de phylog4nies des Marsupiaux montrant la diversit4 des opinions (comparer avec la fig. 4). Abr~viations: A, Metatheria dAustralasie: B. Borhyaenoidea; C, Caenolestoidea (sensu /ato) ; Da, Dasyuroidea; Di. Didelphoidea; Dp, Diprotodonta; M, Marsupicarnlvora (incluant Di, IDa, B); Mi, Microbiotheriidae ; Ne, Necrolestidae ; No, Notorgctidae, Ph. Phalangeroidea ; P. Perameloidea ; T. Thglacinadae.
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f Fig. 2. - - Selected features of right calcanea and astragali of representative ameridelphians to indicate some primitive metatherian characters (a, b). and some diagnostic attributes (c, d, e. f). A b o v e : Early Eocene pediomyid (sensu lato), U C M P unnumbered (calcaneus). U C M P No. 113305 (astra0alus), Bitter Creek. Middle: Caenolestes sp.. A M N H No. 62915. Recent. Below: representative didelphid, Philander opossum, A M N H No. 176707, Recent. Scales in ram. From left to right : dorsal and plantar views of calcanea, plantar and dorsal views of astragali. T h e following characters are indicated by the arrows: a, separate lower ankle joint pattern, primitive for Metatheria ; b. groove for tendon of M. peroneus brevis, primitive for Metatheria ; c, ribbon-like attenuated sustentacular facet extended into recess above hypertrophied astragalar medial plantar tuberosity, primitive or diagnostic of Borhyaeniformes ; d, hypertrophied astragalar medial plantar tuberosity which locks onto underlying calcaneal sustentacular win o, diagnostic of Borhyaeniformes ; e. forward extension of medial tibial facet, diagnostic of Borhyaeniformes, or possibly primitive for Metatheria; f, distally extended medial cuboid facet extension, sharply angled from distal calcaneocuboid facet, diagnostic of Didelphidae, scnsu stricto. Une selection de caracteres du calcandum et de l'astragale des am(~ridelphes comprenant quelques caract~res primitifs de m~tath~riens (a, b) et quelques caract~res diagnostiques (c, d, e, f). En h a u t : p~diomyid~ (serLsu lato) de l'Eoc~ne inf~rieur, U C M [ non num~rot~ (calcan~um). U C M P N ~ 113305 (astra0ale), Bitter Creek. A u centre: Caenolestes sp., A M N H N ~ 62915, actuel. E n b a s : un membre repr~sentatif des didelphid~s, Philander opossum, A M N H N ~ 176707, actuel. Echelle en ram. De gauche "a droite: rues dorsale et plantaire du calcan~um, vues plantaire et dorsale de l'astraoale. Les flC,ches indiquent les caract~res suivants: a, surfaces articulaires astragalo-calcan~ennes disjointes, primitif pour les Metatheria ; b, sillon pour le tendon du M. peroneus brevis, primitif pour les Metatheria ; c, facette sustentaculaire att~nu~e, en forme de ruban, s'~tendant dans tree ~chancrure au-dessus d'une tub~rosit~ hypertrophi~e m~dio-plantaire de l'astragale, primitif ou diagnostique des Borhyaeniformes; e, extension vers l'avant de ]a facette m~diale du tibia, diaonostique des Borhyaeniformes ou peut-~tre primitive pour les Metather~a ; f, extension distale de ]a facette m~diale du cubo'/de, fortement inclin~e depuis ]a facette calcan(~o-cubo'ide, diagnostique des Didelphidae, sen,~u sfricfo.
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Fig. 3. - - Selected features of right catcanea and astragali of representative australidelphians to indicate some diagnostic attributes. Above:Dromiciops anstralis, A M N H No. 97746, representative of the australidelphian morphotype, Recent. Middle: Neophascogale Iorentzi, A M N H No. 101980, representative of the dasyuridan morphotype, Recent. Below: Notoryctes typhlops, A M N H No. 200241, the only known notoryctid, Recent. Scales in ram. From left to right: dorsal and plantar views of calcanea, plantar and dorsal views of astragali. The following characters are indicated by arrows : a, continuous lower ankle joint pattern and the absence of the distal articular portion of the primitive sustentacular facet, diagnostic of the Australidelphia ; b, descending plantolateral wing of calcaneus flanking the <
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racter complexes are also considered. M y intention here is to merely point out that if paleomammalogists continue to eschew the practice of sorting and describing all mammalian remains (particularly usable joint complexes) from their painstakingly and expensively collected faunal remains, then many hypotheses of mammal relationships proposed will be only dental character phylogenies. Thus both paleontologists and nonpaleontologists will remain ignorant of the extent of information available from the collected fossil
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record of a given group. The emphasis on odontolooy for the reason of an often exclusive prevalence of teeth is unavoidable, but the false assumptions about the ontogeny and phylogenetic significance of bone morphology (see Szalay, 1977, p. 315-32't) are unacceptable excuses for ignoring evidence from the postcranial skeleton. The vast repository of phylogenetic information present on the postcranial anatomy has not, as yet, been adequately exploited either for the marsupials, or for the rest of the Theria.
DISCUSSION Several years ago I began a systematic survey and functional-adaptive analysis (for a recent synopsis of the approach see Bock, 1981) of the tarsal morphology of living and fossil marsupials. Along with these efforts I have also perused the published literature in comparative serology of marsupials, and studied both the literature and extensive series of cranial, dental, and pedal specimens. In my tarsal studies, to be presented as a monograph, I have attempted both to understand the adaptive significance of form-function differences, assess the inherited restraints and chanelling factors on phylogenetic change, all these to facilitate the formulation and testing of polarity hypotheses. I have also paid attention to the fossil record both in the formulation of polarity hypotheses and the testing of polarity schemes derived from morphological evaluations. This procedure, contrary to the numerous published guidelines of neocladistic analysis which have come to require an axiomatic divorce from biological iudgment (e.g. Eldredge ~3 Cracraft, 1980), is considered phylooenetic. Given the limitations of space in this contribution, a summary of my interim phylogenetic conclusions are briefly presented. The considerable amount of raw data, detailed interpretive schemes, and lower level taxonomic discussions are being monographed elsewhere. The following discussions are divided into twelve points in order that the issues raised and diagnoses presented can be made closely relevant to the phylooenetic tree on Fig. `1. The numbers below, therefore, have both topical pertinance to the taxa in the tree, as well as give an estimate of the origin and modification of the characters discussed in time.
1 ) Pediomyidae I11 his major survey of the Cretaceous marsupials W . W . Clemens (1966) raised Simpson's (1927) subfamily designation Pediomyinae to family rank, considered the Didelphidae to be represented in the Cretaceous of North America by the subfamilies Didelphinae and Glasbiinae, and recognized the family Stagodontidae. I do not agree with the assignment of the Cretaceous forms to the Didelphinae, and I subsequently question the value of a broad and plesiomorphy-based use of the concept DMelphidae when the living taxon can be diagnosed. M y current usage of the concept Pediomyidae, therefore, as a poorly known waste basket category requires a brief explanation. On the balance of the dentition the living didelphids (e.g. Crochet, 1978, p. 63) are probably as primitive as are the dental features found in the present broad concept of the Pediomyidae. The dental specializations of the Pediomyinae (see Fox, 1979b), however, are so minor compared to other Cretaceous metatherians, that separation of these from other late Cretaceous and Paleogene species on the family level must be questioned. The livin 0 Viverridae, to cite only one example, displays a broader dental spectrum than the whole known late Cretaceous and North American Paleogene Metatheria. Short of assuming special relationships of some of the Cretaceous and Paleogene taxa with the living Didelphidae (affinities which should be based on derived characters), the fossil taxa do not exceed the morphological diversity commonly accepted for the dentitions of living family concepts. W h y , then, should these disputed fossil taxa not be included in the Didelphidae ?
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Fig. 4. - - A phylogeny of the Metatheria, corroborated or not contradicted by the c~aracter cline polarities of the skull, dentition, and selected postcranial features briefly discussed in the text. Continuous thick vertical lines depict the known stratigraphic range of the taxa, the broken downward extensions represent their estimated duration as a taxon diagnosed by the characters listed in the text. Circles represent unsolved branching or ancestor-descendant relationships. Arrows indicate the derivation and estimated points of origin of the diagnostic traits ; numbers correspond to those points sequenced in the DISCUSSION in the text where the pertinent diagnoses and discussions are found. Time scale on the left border in ten million year intervals; on the right border subdivisions correspond to the Upper Cretaceous and the epochs of the Cenozoic. Line phylogenie des Metatheria, corroboree ou non contredite par la polarite des caracteres du crane, de la dentition et du squelette postcrfiniens bri~vement discutes dans le texte. Les lignes verticales continues representent la repartition stratigra~ phique des taxons, les lignes brisees dirigees vers le bas representent la duree d'existence estimee des taxons dont la diagnose est dtablie ~t partir des caract~res cites dans le texte. Les cercles symbolisent les branchements non rdsolus ou les relations d'anc@tre ~ descendant. Les fl~ches indiquent la derivation et les points d'origine estimes des traits diagnostiques ; les chiffres correspondent aux points discutes dans le texte off 1'o11 trouvera les diagnoses pertinentes et les discussions. L'echelle de temps r gauche indique des intervalles de 10 M.A. ; ~t droite, les subdivisions correspondent au Cretace superieur et au CenozoYque.
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As argued under Didelphidae, the members of that family sensu stricto (subdivisible into subfamilies based on dental and other characters), which are the livin 9 species and the mostly Neogene Neotropical taxa, show unmistakable, derived tarsal attributes inherited from a common didelphid ancestor, and therefore their diagnostic distinction from at least those Cretaceous and early T e r t i a r y taxa which are known by tarsals. N o luther case should be made for the relative uselessness of a primitive molar pattern for allocating taxa to higher categories. Dental systematics, however, is a widely accepted substitute for complete taxon evaluations. Let us assume, for example, that Phascogale, a dasyurid, was known in N o r t h America from the Cretaceous by its molars alone. It is very likely that it would be allocated to a broad concept of the Didelphidae. Similarly, dentitions which closely resemble those of living didelphids (or dasyurids) are referred to the Didelphidae in the literature, althou9h the
INFRACLASS METATHERIA
Cohort Ameridelphia
Order Didelphida Suborder Didelphiformes Suborder Borhyaeniformes Superfamily Boryhaenoidea Superfamily Caenolestoidea Superfamily Polydolopoidea Superfamily Ar9yrolagoidea
Cohort Australidelphia
Order Dromiciopsia Order Syndactyla Suborder Syndactyliformes Superfamily unknown Superfamily Notoryctoidea Suborder Perameliformes Suborder Phalangeriformes Superfamily Phalangeroidea Superfamily Vombatoidea Superfamily Diprotodontoidea Superfamily Macropodoidea
Order Dasyurida
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evidence of marsupial tarsal bones from the same quarries strongly suggests that these early forms share only primitive dental similarities with the
Didelphidae, sensu stricto. Because the stereotyped dental morphology merely labels these forms as dentally primitive marsupials, resemblin 9 the primitive didelphids, microbiotheriids, borhyaenids, caenolestids, caroIoameghiniids, and dasyurids (and probably also the primitive, hitherto unknown, syndactylans), it appears that the allocation of these Cretaceous and Paleogene forms to a family of their own is a more stimulating and less biased systematic practice. On account of the close similarity of the Late Cretaceous and Paleogene dentitions to the primitive dental patterns of the Metatheria, I propose to include these taxa previously considered didelphines, steoodontids, 91asbiines, and pediomyines in the family Pediomyidae. Admittedly, this is a waste basket category of heterogeneous dentitions, with, however, an important expansion of the concept from tarsals. This move has the merit of allowing the definition of the complex and varied, primarily South American Didelphidae with at least some derived combinations. Primitive ameridelphian, and therefore presumably metatherian, features of the Pediomyidae include a polyprotodont dentition with at least a 5,/'t incisor formula, and sectorial molars with stylar cusps. It is assumed that the advanced therian pattern of reduced tooth replacement, present in other marsupials, has already been attained. Pes was probably with a large grasping hallux, although this is unknown, and the tarsal
Table I An abridged evolutionary classification of the Metatheria based on the phylogeny of Pig. 4, As noted in Szalay (1982), the abandonment of the ordinal concept Marsupicarnivora RIDE, 1964 and the use of the cohorts Ameridelphia and Australidelphia are based on the conviction of a) the polyphyly of the concept Marsupicarnivora. and b) the lack of correspondence of the cohorts with Simpson's (1970) undiagnosed subordinal concepts Hesperometatheria and Eometatheria. Une classification evolutionniste sommaire des Metatheria fond~e sur la phylog~nie de la fig. 4. Com.ne il est not~ dans Szalay (1982). labandon du concept ordinal Marsupicarnivora RIDE, 1964 et l'usage des cohortes Ameridelphia et Australidelphia sont fondus sur la conviction a) de la polyphylie du concept Marsupicarnivora, et b) de l'absence de correspondance entre ces cohortes et les concepts sous-ordinaux de Simpson (1970): Hesperometatheria et Eometatheria qui sont d~pourvu,x de diagnose.
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morphology, the most primitive known, with the following attributes : a) separate lower ankle joint pattern on astragali and calcanea: b) a simple ovoid articulation, a shallow ball and socket joint, between the calcaneus and cuboid; c) transversely oriented astragalonavicular joint. The possibility exists that all of the taxa which are known by astragali and calcanea in the early Tertiary have special affinities with the Borhyaeniformes. Uniquely among marsupials, the caenolestoids and borhyaenoids share with the Paleogene specimens an attenuated, ribbon-like proximal sustentacular facet which runs above the medial astragalar plantar tuberosity. A functional explanation for the unique astragalocalcaneal contact that is reflected by the process and facet is attempted by F.S. Szalay (1982). 2
) Borhyaeniformes
The borhyaenoids and the caenolestids have been recently reviewed by L.G. Marshall (1978. 1979, 1981a, b). Postcranial features, however, considerably clarify their affinities. More recent previous views, based on primitive character states, have merely advocated an independent didelphoid origin for both groups. Possession of virtually identical series of modifications on the proximal tarsals of borhyaenoids and caenolestids, however, independent of their body size. are difficult to dismiss either as mere convergence or because of nebulous arguments rooted in allomerry. These special similarities can be related to unique mechanical solutions developed for terrestrial hopping or galloping gait, yet no other groups with similar (or any other) locomotor propensities displays the borhyaeniform combination of characters. The primitive borhyeniform character states include: a) polyprotodont dentition with an incisor formula probably reduced to 4,/4 ; b) sectorial, perhaps pediomyine-like (but see Fox, 1979a, b) molars : c) a foot with a somewhat reduced ha]lux (the antecedent condition is unknown); d) the tarsal morphology, uniquely modified for a primarily terrestrial locomotor repertoire, is characterized by mortise-like upper ankle joint with an anteromedial extension of the astragalar tibial facet: e) locking lower ankle joint with hypertrophied medial astragalar plantar tuberosity and a ribbon-like extension of the proximal sustentacular facet into recess above the tuberosity: [) astragalonavicular joint modified to ensure greatest resistance to deformation and maximum stability at the end of extreme eversion and/or
185
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the push-off phase of the stride; g) unmodified sellar articulation between entocuneiform and Mt I. It is most likely that an early Tertiary small borhyaenid lineage had undergone dental and cranial transformation, retaining the ancestral borhyaeniform tarsal morphology, and thus becoming the protocaenolestoid. Small, caenolestid sized borhyaenids are present in the Paleogene of both South and possibly North America, as this can be ascertained from well represented tarsal remains from Eocene deposits in western North America and the Itaborai fauna of Brazil (Szalay, 1982). It should be emphasized that the mechanically complex tarsus, invariably molded around the requirements of substrate preference and locomotor mechanics, is powerfully constrained by the genetic mechanisms inherited in any given group (Szalay, 1977). No two groups of Australasian marsupials, for example, employ similar formfunction solutions to terrestrial life, often involving similar gait, in spite of their similar ancestry. The astonishing tarsal resemblances found in small caenolestids and borhyaenids and very large thylacosmilids preclude any a d hoc explanation rooted in allometry, and the known data allows no other explanation at present except homology. Arguments that such similarities, or syndactyly among australidetphians, for example, are convergent would seem specious, unless shored up by detailed models showing the inevitability of such similar patterns due to similar setectional forces. On the other hand, the taxon specific, derived dental adaptations of the caenolestids require no other explanation than divergence [rom the primitive borhyaeniform pattern. Hitherto undescribed footbones from the Itaborai fauna (Szalay, in preparation), clearly those of borhyaenids, show a large astragalar canal, widely opening both dorsally and plantarly. The presence of such a primitive therian feature, not known in any didelphid, lends additional support to the notion that the borhyaeniforms did not diverge from the Didelphidae, sensu stricto.
3 ) Caenolestidae The widely ranging views on the affinities of the Caenolestidae were summarized by L.G. Marshall (1980) in his comprehensive revision of the systematics of the family. The diagnostic features of the stem caenolestid, as far as known, have usually been restricted to cranial and dental characters. The lower incisors
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are typically diprotodont, and the upper molars, usually the first two, have well developed hypocones. There is no resemblance between any of the four upper incisors of caenolestids and those of diprotodont australidelphians. Marshall's (1980) revision contains several additional dental attributes which characterize the stem caeuolestid. As noted under Borhyaeniformes, the striking derived similarity of various tarsal attributes of borhyaenoids and caenolestids (see Szalay, 1982: in preparation) is yet the strongest line of evidence regarding caenolestid affinities. Caenolestid dental specializations simply postdate a postulated borhyaenid-caenolestid split, and they present no obstacles for the hypotheses offered here.
4) Polydolopidae Whether or not this family is a close relative of caenolestids is a question still unanswered, in spite of the demonstration by C. de Paula Couto (1952), as noted by L.G. Marshall (1980), that the sectorial first molar of caenolestids and the last premolar of polydolopids are not homologous. The possibility still exists that the hypertrophid incisors of the two families (and possibly of the groeheriids and argyrolagids) are homologous. The most diagnostic features of the Polydolopidae is the plagiaulacoid condition of the last premolars, discussed in detail by G.G. Simpson (1933). Although a yet briefly studied skull is known, polydolopids have been known primarily dentally. The Itaborai tarsal collection currently under study contains remnants of polydolopids. Knowledge of these elements is likely to increase our understanding of the affinities and adaptations of these marsupials.
5
) Argyrolagidae
The most recent ties of the Argyrolagidae within the order Didelphida are unclear (Simpson, 1970). The tubular snout projects anterior to the incisors and the cheek teeth are rootless; the hindlimbs are thoroughly modified for a jumping habitus (Simpson, 1970). Although the tarsus requires careful restudy, at least one feature, specifically the enlarged medial portion of the upper ankle joint (Simpson, 1970, fig. 16F), suggests borhyaeniform rather than didelphiform ties. The enlarged incisors suggest, as noted and discussed by G.G. Simpson (1970), caenolestoid (sensu lato) affinities.
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6)
Didelphidae, sensu s t r i c t o
This extant family, regardless of the diverse habitus displayed by the opossums, can be defined by diagnostic derived characters. It is unlikely that any extinct member of this clade gave rise to the borhyaeniforms. Difficult as it may be, a major effort is needed to establish, if possible, the derived dental characters of this family. The ancestral species of the extant Didelphidae may be characterized b y : a) polyprotodont dentition with 5/4 incisor formula; b) transversely shortened molars and possibly attenuated protolophids; c) pes diadactylous with large grasping hallux, lacking a nail or claw ; d) broad and gently curving upper ankle joint of the pes; e) transversely extensive astragalofibular contact ; f) separate lower ankle joint pattern: 9) extensive, projecting cuboid <
7 ) Australideiphia Assessment of the Andean-Patagonian Dromiclops has been pivotal for the phylogenetic hypothesis offered here and in F.S. Szalay (1982). Although Dromiciops australis is a polyprotodont with a primitive dental formula (hence its traditional desiqnation as a didelphoid) and diadactylous, in possession of a uniquely hypertrophied bulla (along with Microbiotherium), its highly derived tarsal morphology securely ties it to the Australasian radiation. Together they make up the cohort Australidelphia in contrast to the other Metatheria, the cohort Ameridelphia (Szalay, 1982). The tarsal similarity of Dromiciops to the Australasian morphotype condition is undoubted, and it is a complex shared and derived combination of characters, These characters are derived, unique, and allow little doubt of the strict monophyly of the Australidelphia. The australidelphian morphotype had the following combination of characters: a) polyprotodont dentition with 5/4 incisor formula: b) incisors probably slightly hypertrophied as in Dromiciops and primitive dasyurids; c) primitive metatherian sectorial cheek teeth ; d) diadactylous pes with large grasping hallux ; e) calcaneofibular
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articulation almost completely reduced, with probably little or no contact between fibula and calcaneus; f) continuous lower ankle joint pattern, and the loss of distal sustentacular articulation between astralagus and calcaneus: g) calcaneocuboid joint with a unique <
Dromiciopsia
The characters postulated to be present in the australidelphian morphotype are present in Dromiciops. There can be little doubt about the great independent antiquity of that lineage from the Didelphidae. Although based on the basicranial similarities of Microbiotherium and the living Dromiciops, we can associate the fossil forms with the latter due to the primitive dental morphology of this group, but our knowledge of their radiation, antiquity, and history is virtually nonexistent. The evolutionary morphology of at least one investigated area, the tarsus, of Dromiciops assures their long and distinct history. The most recent common ancestor of Dromiciops and Microbiotherium developed composite and highly inflated bullae which are formed anteriorly
187
by the alisphenoid, and posteriorly what has been reported to be an entotympanic (Se9all, 1969). The latter component, however, is more likely to be the petrosal. The derived dromiciopsian bullar hypertrophy was unlikely to have been present in the australidelphian morphotype.
9 ) Dasyurida The origins of the diagnostic dasyuridan character complex is intimately tied to the modifications of the postcranial anatomy for a primarily terrestrial existence. The combined evidence, so far particularly the tarsal morphology, firmly points to the origin of the protodasyurid not from a didelphid (sensu stricto) or pediomyid (sensu lato), but from an australidelphian source which is perhaps best estimated to be a dromiciopsian. The morphotype adaptations of the Dasyurida appear to be the result of the obligate, rapid, terrestrial progression found today in most dasyurid species. The fact that this habitus occurs together with considerable arboreal abilities does not: detract from the diagnostic value of the clearly superimposed changes, required by the more recently acquired terrestriality. The morphotype dasyurid possessed the following known combination of characters : a) polyprotodont dentition with a 4~3 incisor formula; b) relatively enlarged incisors compared to didelphids; c) greatly reduced entocuneiform and hallux on a diadactylous pes ; d) upper and lower ankle joints essentially as described above, under the Australidelphia; e) calcaneocuboid joint is modified from the restrictive ancestral pattern seen in Dromiciops and phalangeroids, allowing the unobstructed eversion necessary for rapid terrestrial progression; the hallmarks of the angle iron type contact are retained in most small dasyurids (Fig. 3 ) : f) entocuneiform-Mt I articulation as in Dromiciops and phalangeroids, and unlike in didelphids. All previous notions, with the astute exception of Bensley's (1903), concerning the phylogenetic position of the dasyurids were based on the largely primitive dental morphology of the many species of this taxon, and these views predominate even today. The tarsal characters, however, clearly show that not only are the dasyuridans (thylacines included) part of a monophyletic Australidelphia, but that they are derived from a group of arboreal animals with a nearly primitive metatherian dentition and a tarsus not like primitive metatherians or didelphids, but one derivable from an australidelphian morphotype condition described above.
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The form-function of the entocuneiform-Mt I joint is so far one of the best lines of corroborative evidence supporting the hypothesis above based on the proximal tarsals. W h y would, one might ask, a terrestrial group with the virtual loss of all hallucial biological roles, possess a derived, nonrestricted sellar articulation. In spite of its rudimentation the Mt I and entocuneiform mirror the stasis of this region in dasyurids. The australidelphian attribute is retained unchanged because the requirements placed on the pedal cheridia I-IV did not influence t.he greatly shortened hallux. The tarsal evidence securely ties the thylacinids to the Dasyurida as G.G. Simpson (19Zil) and L.G. Marshall (1977) argued on dental and cranial grounds, and it contradicts the borhyaenoid ties argued for by M. Archer (1976b) and I.A.W. Kitsch (1977). The much less controversial ties of Myrmecobius with dasyurids are also corroborated by the tarsal evidence, and they are also indicated by serology (Kitsch, 1977) and basicranial morphology (Archer, 1976b).
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This character combination, although not found in any living syndactylan, diagnosed the earliest syndactyliform (a hypothetical species of a hypothetical radiation) whose descendants are the perameloids, probably notoryctids, and the phalangeroids. The articular tarsal morphology of the tarsus of Notoryctes removes all doubt concerning the broad mammalian affinities of the marsupial mole (Fig. 3). As in all other australidelphians, the continuous lower ankle joint pattern testifies to its cohort affinities. The presence of a very large Mt I would indicate the derivation of this lineage not from a dasyuridan, but from a polyprotodont australidelphian group with a large grasping hallux. The question remains whether this source was a dromiciopsian or a syndactylan one. Preliminary analysis suggests that the subequal second and third metatarsals may be indicative of a syndactylan heritage.
1 I ) Perameliformes 10) Syndactyla It is perhaps instructive to consider both on methodological and morphological grounds why a character complex like the syndactylous pes would be considered convergent between perameloids and phalangeroids without a morphological case made against their homology. ].A.W. Kirsch (1977) in his serological study of marsupials made no substantial arguments for his assertion of such a convergence. The fact that syndactyly prohibits a dasyurid-paremeloid sister group hypothesis generated by Kirsch's studies is perhaps indicative of the phenetic nature of the serological evidence which cannot be cast into the form of morphocline polarities, rather than evidence against the homology of marsupial syndactyly. The order Syndactyla, until proven to the contrary, is best considered to be monophyletic. The morphotype of the order had: a) a polyprotodont dentition with a 5,/3 incisor formula; b) molars possibly with an incipient hypocone; c) tarsal morphology virtually identical to that of the protoaustralidelphian; d) syndactylous and reduced condition of pedal cheiridia II and III, accompanied by superficial and deep myological changes related to the digits (see W o o d ]ones, 1922~25).
The perameliform ancestor, with its nearly complete metatherian dental formula remodelled the syndactylous grasping foot and tarsus into a highly diagnostic terrestrially suited structure (see Marshall, 1972). Detailed study of this transformation is in preparation. Any hypothesis of a dasyurid-peramelid monophyly, based either on primitive morphological retentions or on phenograms generated by the probably conservative serological evidence, is rejected by the presence of the syndactylous pes in bandicoots.
12 )
Phalangeriformes
The ancestral phalangeriform can be diagnosed by the following combination of features: a) reduced number of incisors, with three above and two or possibly three below; b) enlargement of the central incisors into the diprotodont pattern i c) acquisition of the [asciculus aberrans in the brain. In spite of the cranial and dental modifications, probably related to sap, gum, foliage, seed, and insect eating adaptations (fide Smith, 1980), the australidelphian tarsal adaptations of the ancestral syndactylan are retained virtually unchanged.
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CLASSIFICATION There are a very lar0e number of names available for suprafamilial groupings of marsupials, as summarized by L.G. Marshall (1981b). Suprafamilial nomenclature is made particularly frustrating and confusing by the fact that there are no clear rules to adhere to when attempting to present a new classification which wants to reflect a new phylogenetic hypothesis, while still attempting to retain important adaptational information. Employing old names which still encompass concepts of taxa removed from that group appears to greatly diminish the meaning of the original concept. A similar objection may be raised for later additions to a concept, although in that case, new discoveries of groups may be accommodated
without destroying a classification. Another frustrating aspect of the lack of rules of our taxonomic system above superfamily rank is that the endings of names do not convey any rank information. A great deal of communicative facility is lost in constantly having to designate the rank of a category in question. In this paper I simply follow the practice -a ending for orders, and -[ormes for suborders, when designating them formally, and their appropriate vernacular forms otherwise. Table I presents an abbreviated classification of the Metatheria. emphasizing monophyly and the appropriate use of paraphyletic taxa to express adaptive radiations. Additional comments on this classification are in F.S. Szalay (1982).
Acknowledgments
I am particularly indebted to Dr. James Warren for greatlY/ facilitatin0 all aspects of the work during my 1980 sabbatical at Monash University.
Research for this p a p e r w a s supported by C.U.N.Y. Doctoral Research Award 13454, a John Simon Guggenheim Foundation Fellowship, and the Department of Zoology, Monash University, Australia.
to
For help in the preparation of the manuscript, m y Ellen Woodstock.
thanks
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