The phylogenetic relationships of Pachyrhachis problematicus, and the evolution of limblessness in snakes (Lepidosauria, Squamata)

0 1999 Academic PalContologie

des sciences

/ iditions

scientifiques

et m&icales

Elsevier SAS. Tous droits

r&erv&.

/ Palaeonfology

The phylogenetic relationships of Pachyrhachis problematicus, and the evolution of limblessness in snakes (Lepidosauria, Squamata) Les relations pbyloghze’tiques de Pachyrhachis problematicus et l’&olution de la r&duction appendiculaire cbez les serpents (Lepidosawia, Squamata) Hussam

Zahef*,

a Universidade Universitaria, b Department (Received

Olivier

de SZo 05508-900, of Geology, 2 September

Rieppelb

Paula, lnstituto de Bioci&ncias, Sao Paula, SP, Brazil The Field Museum, Roosevelt 1999,

accepted

after

revision

Departamento Road

19 October

8 Lake

de Shore

Zoologia, Drive,

rua

do

Chicago,

Matao,

IL 60605-2496,

Travessa

14,

Cidade

USA

1999)

Abstract-We snake from

have revisited the type material of Pachyrbachis problematicus, a fossil the Cenomanian (Cretaceous) of the Middle East, and found a number of characters in need of revision with respect to its recent re-description. A new interpretation is given for several cranial characters. There is no identifiable regionalization of the presacral-vertebral column beyond that found in booid snakes. The identification of a free sacral rib in Pachyrbachis is more parsimoniously interpreted as the first lymphapophysis. It is questionable that the pelvic rudiment would have been suspended from the axial skeleton. We have also extensively reviewed the evidence that is believed to link Pachyrhachis, and with it all other snakes, to a mosasauroid ancestor. We were not able to corroborate this hypothesis, which involves characters related to the dentition, braincase morphology, and the intramandibular joint. Instead, tionships of Pachyrhachis corroborates the hypothesis macrostomatan snakes, i.e., advanced snakes. Given hierarchy of snakes, it is entirely possible that this

re-analysis of the phylogenetic relathat it represents the sister-group of that Pachyrhachis is nested within the snake re-developed almost complete

hind limbs. A less parsimonious hypothesis would be to consider the hind limbs to have been lost independently in the major basal clades of extant snakes (Scolecophidia, Anilioidea, and Macrostomata). The latter hypothesis is considered a serious possibility given the recently described findings on the mechanisms of limb development in Python, which could account for the highly variable pattern of hindlimb reduction found among basal snakes.

(0

1999

Academic

Squamata / Serpentes limblessness

des sciences

/ Editions

/ fossil / Pachyrhachis

scientifiques

problematicus

et medicales

/ phylogeny

Elsevier

SAS.)

/ evolution

of

R&um~ - Nous avons revu le materiel type de Puchyrhachisproblematicus,un fossile du Cenomanien (par-tie moyenne du C&ace) du Moyen-Orient, et avons trouve un nombre de caracteres meritant revision par rapport P leur recente re-description. Une nouvelle interpretation est fournie pour plusieurs caracteres craniens. 11 n’y a pas de regionalisation identifiable de la region presacree de la colonne vertebrale, au-deli de ce qui se voit chez les bodides. I’identification dune tote sacree libre est plus parcimonieusement interpretee comme la premiere lymphapophyse. I1 est douteux que le rudiment pelvien ait et6 suspendu au squelette axial. Nous avons Note

communicated

* Correspondence [email protected]

by Philippe

Taquet.

and reprints.

C. R. Acad. Sci. Paris, Sciences 1999.329.831-837

de la terre

et des plan&es

/ Earth & Planetary

Sciences

831

H. Zaher,

0. Rieppel

aussi revu les evidences qui suggkent un lien entre Pack~yrhachis, et avec lui tous les autres serpents, et un ancetre mosasauroi’de. Nos rkultats ne corroborent pas cette hypoth?se, qui se base particulikrement sur un certain nombre de caracteres tires de la dentition, de la boite cdnienne et de l’articulation intramandibulaire. Au lieu de cela, tous les ksultats obtenus dans le cadre d’une nouvelle analyse sur les relations phylogkn&iques de Puchyrhachis corroborent l’hypoth&e suivant laquelle celui-ci reprkente le groupe-f&e des serpents Macrostomata, c’est-adire des serpents (GCvolu& a’.&ant don& que Pachyrhachis est enracine au sein de la hkrarchie des serpents, il devient tout 2 fait probable que ce serpent ait re-d&eloppe des pattes postkieures presque compl&es. Toutefois, une hypothke moins parcimonieuse pourrait @tre celle de la perte indgpendante de pattes dans les clades majeurs de serpents actuels (Scolecophidia, Anilioidea, Macrostomata). Cette derniPre hypothese est considkrte comme une alternative vraisemblable en fonction des rkentes dkcouvertes sur le mkanisme de dkeloppement des pattes chez Python, celui-ci pouvant ttre ?I l’origine de la grande variabilik prksente dans les patrons de pattes vestigiales des serpents basaux. (0 1999 Acadkmie des sciences / kditions scientifiques et medicales Elsevier SAS.) Squamata / Serpentes / fossile / Pachyrhachis la ri?duction appendiculaire

problematicus

/ phylogikie

/ evolution

de

Version abregee (voir p. 835) 1. Introduction Whereas the well-corroborated 1951; McDowell

monophyly hypothesis and Bogert,

of snakes (Serpentes) is a (Bellairs and Underwood, 1954; Underwood, 1967;

Rieppel, 1988a, b), the answer to the more general question of the phylogenetic relationships of snakes within Squamata as a whole has proven more elusive (Bellairs and Underwood, 1951; McDowell, 1972; Senn and Northcutt, 1973; Rage, 1982; Rieppel, 1983, 1988a, b; Estes et al., 1988; Wu et al. 1996; Lee, 1997, 1998; Evans and Barbadillo, 1998; Hallermann, 1998; Rieppel and Zaher, in press, a). This may reflect the relatively derived morphology of snakes compared to other, nonophidian squamates. The inclusion of fossils would therefore seem to be an important analysis of snake interrelationships, tential to shorten the branch-length autapomorphic terminals (Gauthier discovery of the Mid-Cretaceous

component of the as they hold the poof otherwise highly et al., 1988). The (Lower Cenomanian)

fossil snake fachyrhachis problematicus (Haas, 1979, 1980a, b) with well developed hind limbs thus promised to shed important new light on the origin of snakes. The fossil record of snakes in general is very poor and, aside from the well-known Cretaceous snake Dinilysia, Pachyrhachis is the only other relevant fossil snake with a well-preserved skull and an articulated postcranium. In their recent re-description of Pachyrhachis, Caldwell and Lee (1997), and Lee and Caldwell (1998) corroborated the ophidian affinity of this taxon, yet placed it as the relatively plesiomorphic sister-taxon of all other snakes next to the root of the snake cladogram. In addition, these authors found support for a monophyletic clade Pythonomorpha, which includes varanoids, mosasauroids

a32

and

snakes

(Caldwell

and

Lee,

1997;

Lee,

1997,

C. R. Acad.

1998; Lee and Caldwell, 1998). This critically on the basal position of snakes, and its sister-group relationship Against this phylogenetic background, nally became ‘an excellent example taxon’ between snakes (Scanlon hypothesis that origin

hypothesis

depends within to mosasauroids.

Pachyrhachis

Pachyrhachk of

fi-

a transitional

mosasauroids and more et al., 1999; see also Carroll, implies a marine, rather than

advanced 1988), terrestrial,

a

of snakes.

However, Pachyrhachis shares with relatively advanced snakes a number of characters that are not present in scolecophidians and anilioids (Haas, 1979, 1980a, b; Rieppel, 1994a). Indeed, macrostomatan characters shared by Pachyrhachis suggested that its basal position (1998) bining (1997),

within snakes required re-evaluation. Zaher performed a preliminary cladistic analysis, comnew information with that of Caldwell and Lee which indicated that Pachyrhachis might well be

the sister-taxon of snakes. Additionally,

phis woodwardi govina,

more advanced, i.e., macrostomatan in their re-description of Pachyofrom the Cretaceous of Bosnia Herze-

Lee

et al. (1999) placed this taxon with in the family Pachyophiidae on the basis of two spurious characters (rib pachyostosis and lateral compression of the body), and again considered this family to be the sister-group of all other snakes. Pachyophis is very incomplete by comparison to fachyrhachis, but Lee et al. (1999) were able to identify a higher num-

fachyrhachis

ber (between taxon, which, snake, would Such

results

20 and 24) of dentary if Pachyophis is correctly be another macrostomatan called

pelled us to examine to test the relationships

Sci. Paris. Sciences

de la terre

for

further

a more of

investigation, complete

Pachyrhachis,

et des planetes

teeth in the first identified as a character.

data

and

com-

set designed and snakes in

/ Earth & Planetary Sciences 1999.329,831-837

Pachyrhachis

and the evolution

of limblessness

in snakes

general, within Squamata as a whole. The present paper summarizes the diverging points of views and presents our most recent results concerning the controversy surrounding a mosasaur-snake sister-group relationship, and in particular the phylogenetic position of the Lower Cretaceous snake Pachyrhachis problematicus.

2. The monophyly of Pythonomorpha and the phylogenetic position of Pachyrhachis problematicus Recently, Lee (1998) presented a global analysis of squamate interrelationships using 230 characters and 22 taxa, including all non-ophidian squamates coded as terminals at the family level. The monophyly of the Pythonomorpha was again corroborated, as was the position of Pachyrhachis as an intermediate between mosasauroids and snakes. Based on this result, Lee (1998) discussed the marine versus fossorial origin of snakes, and evaluated the impact on the analysis of presumably correlated characters associated with the so-called ‘fossorial ecomorph’. In our opinion, three important principles that must guide phylogenetic analysis have not been implemented rigorously by Lee (1998), whose results therefore continue to obscure the debate. (1) The choice of terminal taxa. The families chosen as terminals in some cases fail to recognize monophyletic entities (e.g., Agamidae, Iguanidae: Frost and Etheridge, 1989), and the coding for most families fails to recognize variability (polymorphism) of characters within families. Furthermore, taxa that would have been important for the test of disputed hypotheses of relationships were omitted. For example, such critical taxa as derived pygopodids, acontine skinks, and Annie/la are not represented in the codings of the respective families or as separate terminals, while treatment of all snakes as a single terminal does not allow to test the relationships of Pachyrhachis with respect to macrostomatans. (2) The accuracy of assessments of primary homology is problematical for many characters and calls for a more rigorous application of the test of similarity to character delimitation. The characters in question are those of the morphology of the lower jaw (Rieppel and Zaher, in press, a), of the dentition (Zaher and Rieppel, 1999), and of braincase morphology (Rieppel and Zaher, in press, b). (3) No empirical criteria are specified for the coding of continuously variant characters, and characters that are a priori assumed to be correlated with the ‘fossorial ecomorph’ are downweighted arbitrarily. For example, it remains unclear why the characters putatively correlated with the ‘fossorial ecomorph’ are suspected to yield spurious results in the analysis of snake relationships, yet are accepted in support of the Sineoamphisbaena amphisbaenian - dibamid clade. We have revisited the type material of Pachyrhachis problematicus, and, using the original material (figure 1) as well as X-rays of the skull of the holotype, found a C. R. Acad. Sci. Paris, Sciences 1999.329.831-837

de la terre

et des plan&tes

1Omm Figure 1. Dorsal view of the skull of Pachyrhachis problematicus Haas (specimen HUJ-PAL 3659), from the lower Middle Cretaceous of Ein jabrud. Abbreviations: at, atlas; c, coronoid; corn, compound bone; d, dentary; ect, ectopterygoid; f, frontal; m, maxilla; n, nasal; op, opisthotic; p, parietal; pm, premaxilla; po, postorbital; prf, prefrontal; pt, pterygoid; q, quadrate; st, supratemporal; stp, stapes. Vue dorsale du cr$ne de Pachyrhachis problematicus Haas (specimen HUJ-PAL 3659) de la base du Cr&acC moyen de Ein Jabrud. AbrCviations : at, atlas ; c, corono’ide ; corn, OS composk ; d, dentaire ; ect, ectopt&ygo’ide ; f, frontal ; m, maxillaire ; n, nasal ; op, opisthotique ; p, pariktal ; pm, pkmaxillaire ; po, prefrontal ; pt, ptkrygdide ; q, quadratum ; st, supratemporal ; stp, stapes.

number of characters in need of revision (Zaher and Rieppel, 1999; Rieppel and Zaher, in press, a, b) with respect to its recent re-description (Caldwell and Lee, 1997; Lee and Caldwell, 1998). The postorbital of Pachyrhachis is very similar to that in Python in size, shape, and in the way it articulates with the parietal. The supposed postfrontal, believed to have been fused to the postorbital, is in fact a fragment of the laterally descending parietal flange pushed up against the dorsal margin of the orbit upon dorsoventral compression of the skull. The putative jugal is in fact the anterior end of the ectopterygoid, broken and displaced across the posterior end of the maxilla which it broadly overlaps, as in macrostomatan snakes. The putative squamosal is the distal part of the elongated and blade-like shaft of the stapes, and the putative stapes is the distal end of the opisthotic paroccipital process, both macrostomatan features. The quadrate slants posteroventrally, again as in macrosto-

/ Earth & Planetary

Sciences

H. Zaher,

0. Rieppel

and

matans,

tary

of

there

is a single

mental

foramen

in the

den-

Pachyrhachis.

We have re-analyzed the data matrix put together by Lee (1998), but with Serpentes broken up into multiple terminals such as Scolecophidia, Dinilysia, Anilioidea and Macrostomata, in order to propqly test the phylogenetic relationships of Pachyrhachis. After the minimally necessary corrections of character codings, we subjected the data matrix to various search procedures (deletion versus retention of controversial characters, ordering versus not ordering multistate characters, adding or not adding characters that were not included in the original study, and rooting on the actual outgroups versus rooting on

the

reconstructed ancestor), and consistently found to nest within snakes, as sister-taxon of the Macrostomata (Rieppel and Zaher, in press, a). This corroborates the results obtained by Zaher (1998), but as

Pachyrhachis

emphasized by Lee (1998), seems counterintuitive with respect to the presence of well-developed hind limbs in Pachyrhachis. However, a recent paper published by Cohn and Tickle (1999) on molecular mechanisms controlling the development of limbs, or rather the lack thereof, in Python, this conundrum. Although

early

may hind

offer limb

an unexpected budding

appears

solution

to

to proceed

normal in Python, truncation of its development appears to be due to the absence of the apical ectodermal ridge and the lack of expression of ridge-associated genes. However, transplantation experiments suggest that the bud mesenchyme may not have entirely lost the potential for limb development. This could account for the highly variable pattern of hind limb reduction found among basal snakes such as scolecophidians, anilioids, and booids. Cohn and Tickle (1999) place the loss of limbs, and the correlated thoracalization of the snake body, into a phylogenetic context, accepting the position of Pachyrhachis at the root of Serpentes, as a link between snakes and their putative mosasauroid ancestors. This scenario is believed to best explain the presence of well-developed hind limbs and sacral rudiments in

Path yrhachis. However, there is no identifiable regionalization of the presacral vertebral column of Pachyrhachis beyond that found in booid snakes. The hypapophyses of the anterior (cervical) vertebrae become gradually reduced posteriorly, and the presence of paradiapophyses on all vertebrae posterior to the atlas confirm that ribs were present in the anterior cervical region also. More important, perhaps, is the identification, by Lee and Caldwell (1998; see also Caldwell and Lee, 1997, of free sacral ribs for the suspension of the ilium in Pachyrhachis. The ilium of non-ophidian squamates is typically suspended from pleurapophyses (fused sacral ribs), which are continuous with the vertebral elements at the cartilaginous stage, and which later ossify in continuity with the sacral vertebral neural arches (Rieppel, 1992a, b, 199413). The identification of a free sacral rib in Pachyrhachis therefore requires

a34

the

retention

of a free

sacral

rib,

presumably C. R. Acad.

Figure 2. The phylogenetic interrelationships of snakes. The successive nodes are supported by the unequivocal (ci= 1) and unambiguous (optimizing the same under ACCTRAN and DELTRAN) synapomorphies listed below (see also Rieppel and Zaher, in press, a). Node 1 (Serpentes): premaxilla - maxilla contact non-sutural; ventrolateral processes of parietal sutured to para-basisphenoid; trigeminal foramen (foramina) bordered anteriorly by parietal; crista circumfenestralis present; vomer medial to palatine; palatine-vomer contact mobile; palatine with distinct medial process; single mental foramen in dentary; postdentary bones fused to form compound bone (also present in dibamids and amphisbaenians); replacement teeth erupt horizontally, and rotate into functional position; all traces of anterior girdles and limbs lost. Node 2 (Dinilysia plus Alethinophidia): surangular portion of compound bone extends far into lateral surface of the dentary and terminates in a point. Node 3 (Alethinophidia): medial frontal flanges separating olfactory tracts present; maxillary and mandibular branches of trigeminal nerve separated by laterosphenoid; palatine narrow and elongate; palatine teeth similar in size to marginal teeth. Node 4 (Pachyrhachis plus Macrostomata): supratemporal with free ending posterior process; dentigerous anterior process of palatine present; elongated posterior dentigerous process of dentary present; suprastapedial process of the quadrate absent. Les relations phyloge&tiques des serpents. Les noeuds successifs sont etay& par les synapomorphies non kquivoques (ci = 1) et non ambigu@s (optimisees de la meme faGon sous Acctran et Deltran) comme suit (voir aussi Rieppel et Zaher, sous presse, a). Neud 1 (Serpentes) : contact prf!maxillaire-maxillaire non suture ; processus ventro-latkraux du parietal suturk au para-basisphkno’ide ; bord antkrieur des foramina du trijumeau form6 par le pariktal ; crista circumfenestralis prksente ; vomers mkdiaux par rapport aux palatins ; contact palatin-vomer mobile ; palatins presentant un processus m&dial clairement defini ; un seul foramen mentonien sur le dentaire ; les OS postdentaires forment un OS compos6 (aussi present chez les dibamides et amphisbeniens) ; les dents de remplacement apparaissent en position horizontale, presentant un mouvement de rotation pour trouver leur position fonctionnelle ; perte complete des membres antkrieurs et de la ceinture scapulaire. Nceud 2 (Dinilysia plus Alethinophidia) : la portion surangulaire de 1’0s compos6 se prolonge loin sur la surface latkale du dentaire et se termine en forme de pointe. Noeud 3 (Alethinophidia) : presence d’une lame mediale sur chaque frontal separant les tractus olfactifs ; branches maxillaire et mandibulaire du nerf trijumeau &par& par le latkospheno’ide ; palatin etroit et allong ; dentition palatine similaire en taille ?I la dentition marginale. Noeud 4 (Pachyrhachis plus Macrostomata) : supratemporal presentant un processus post& rieur allong et libre de la surface crdnienne ; processus anterieur dent6 du palatin present ; processus suprastapedial du quadratum absent.

as

a consequence differentiation

Sci. Paris,

Sciences

de

of paedomorphosis, into a distally bifurcated la terre

et des

planetes

I Earth

and its subsequent structure. Indeed, & Planetary 1999.329,03

Sciences I-837

Pachyrhochis

the putative sacral rib is more parsimoniously interpreted as the first distally bifurcated lymphapophysis, and was also identified as such by Lee (1998), contra Lee and Caldwell (1998). It therefore remains questionable whether the pelvic rudiment of Pachyrhachis was at all suspended from the axial skeleton. The hind limb is nearly complete; the pes remains unknown, but the absence of any element distal to the astragalus and calcaneum appears, in part at least, to be due to incomplete preservation.

3. Conclusion Our results (figure2) suggest two possible explanations for the presence of hind limbs in Pachyrhachis. The

and the evolution

of limblessness

in snakes

hind limbs may have re-developed in this taxon, or the hind limbs may have been lost independently in the major basal clades of extant snakes (Scolecophidia, Anilioidea, Macrostomata). Given the recently described findings on the mechanisms of limb development in Python (Cohn and Tickle, 1999), a re-development of limbs from rudiments in basal snakes would not appear to be impossible, and the potential to re-develop limbs has also been previously discussed in a phylogenetic context for anguids (Aug6, 1992). However, in spite of being less parsimonious on the basis of present knowldege, we believe that the second hypothesis of multiple limb loss in snakes should still .be considered a serious possibility in view of the developmental plasticity documented by Cohn and Tickle (1999). The latter hypothesis is also open to the potential test by the fossil record, should more fossil snakes with limbs be found in the future.

Version abreg&e Bien que la monophylie des serpents soit une hypothsse fortement corroborke, la question plus g&kale de leur relation phylog&&ique au sein des squamates demeure irksolue. Ceci est probablement dti au fait que les serpents ant une morphologie d&i&e par rapport aux autres squamates non ophidiens. L’apport des fossiles apparait pourtant comme une composante importante dans le cadre d’une analyse phylog&Gtique des serpents, puisqu’ils offrent nombre d’informations capables de raccourcir la longueur des branches correspondant aux taxons terminaux hautement autapomorphiques. La dkcouverte de Pachyrhachis problematicus, un fossile de serpent de la partie moyenne du Cktack (C& nomanien infkieur) avec des pattes postkieures dkveloppees, promettait de fournir d’importantes informations a mCme d’kclairer la question de l’origine des serpents. Dans leur rCcente re-description de Puchyrhachis, Lee et Caldwell (1998) ont confirme ses &nit& ophidiennes, tout en lui reconnaissant une position basale dans le cladogramme, en tant que groupe frke de tous les autres serpents. De plus, ces auteurs ont fourni des arguments etayant la monophylie du &de Pythonomorpha, qui inclut les varandides, les mosasaurdides et les serpents. Cette hypothese dkpend fortement de la position basale de Puchyrhachis comme groupe f&e des mosasauro’ides. Sur ce fond phylogCn&que, Pachyrhzchis apparait comme un x excellent exemple d’un taxon de transition m (Scanlon et al., 1999) entre les mosasaurdides et les serpents plus Cvolu6s, une hypothPse qui implique une origine marine des serpents, plut6t qu’une origine terrestre (= fouisseuse), gCnCralement admise. Neanmoins, Puchyrbachis et les serpents relativement ~~01~6s ant en commun un bon nombre de caractkes qui manquent aux scolecophidiens et anilioides. De fait, les caractkres de Macrostomata prkents chez Puchyrhachi.s suggerent que sa position basale au sein des serpents doit @tre soumise P une nouvelle haluation. Zaher (1998) a prksente une analyse cladistique pritliminaire, ajoutant de nouvelles C. R. Acad. Sci. Paris, Sciences 1999.329,831-837

de la terre

et des plan&es

donrkes P celles publikes par Caldwell et Lee (1997), dans laquelle Pachyrhachis apparait comme le groupe f&e des Macrostomata, serpents plus &oluks. Ces r&ultats nous ant amen& P examiner un ensemble plus complet de donrkes, dans l’intention de mettre P l’kpreuve les relations de parent6 entre Pachyrhachis et les autres serpents, tout en considkrant les squamates en gCn6ral. Le p&sent article fournit une synt&e des points de vue divergents 2 ce sujet, ainsi que nos plus rkents r&sultats concernant la controverse autour de la relation de groupe f&e entre mosasaures et serpents, et particulikement sur la position phylogknktique de Puchyrhachis problematicus. Nous avons revu le matkiel type de Pachyrhachisproblematicus qui, g&e 2 l’analyse du spkcimen original ainsi qu’aux radiographies du cdne, nous a permis de distinguer un nombre de caracdres pour lesquels les re-descriptions kcentes mk-itaient une r&ision. Le postorbital de Puchyrbathis ressemble de prks g celui de Python par sa taille, par sa forme, ainsi que par la manike dont il s’articule avec le pariktal. Le postfrontal putatif, dkcrit comme &ant fusionn6 au postorbital, est en fait un fragment de l’aile descendante du par&al, qui a 636 press&e dorsalement contre la marge dorsale de l’orbite lors de la compression dorso-ventrale du cdne au tours de la fossilisation. Le jugal putatif est en fait la portion antkieure de l’ectoptiirygdide, qui s’est fracturke 2 la hauteur de l’extrkmitk poskrieure du maxillaire, qu’il recouvre largement comme chez les Macrostomata. Le squamosal putatif reprksente la partie distale de la columelle, formke par un corps allongk et aplati en forme de lame de couteau. En revanche, la columelle putative correspond P la region distale du processus paroccipital de l’opisthotique. Ces deux derniitres carackristiques sent typiquement de Macrostomata. Le car& est incline post&o-ventralement, comme chez les Macrostomata, et il n’y a qu’un seul foramen mentonien present sur le dentaire de Puchyrhachis. Aprils avoir incorpork les corrections et additions nikessaires P la matrice de caract&es fournie rkemment par Lee

/ Earth & Planetary

Sciences

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H. Zaher,

0. Rieppel

(1998), nous avons pro&de B une sCrie d’analyses combinant plusieurs prockdures de recherche, qui ont fourni invariablement une topologie d’arbre dans laquelle Pachyrhachis se trouve enracine au sein des serpents, en tant que groupe f&e des Macrostomata. Ces resultats corroborent ceux obtenus par Zaher (19981, mais sont ” contre-intuitifs s),5 cause de la pr&sence, chez ce fossile, de pattes post6rieures dCvelopp&es. Toutefois, un article publii: recemment par Cohn et Tickle (1999) sur les mkcanismes molitculaires responsables du contr6le du d6veloppement des pattes chez le Python peut offrir une solution impr&ue 3 cette question. Les expkriences de transplantations faites par ces auteurs suggerent que le mesenchyme des bourgeons des membres post&ieurs n’a pas perdu entierement le potentiel de d&elopper des membres. Ceci pourrait rendre compte de la grande variabilitit manifestee dans les divers patrons de r& duction de membres presents chez les groupes majeurs des serpents comme les scol&ophidiens, anilioi’des et bodides. Cohn et Tickle (1999) placent la perte des membres, ainsi que la u thoracalisation bb du corps des serpents dans un contexte phylog&&ique oti ils reprennent une position basale de Puchyrhachis, comme groupe f&e des serpents et representant un lien entre les serpents et leur probable an&tre mosasauro1de. Selon eux, ce scknario explique de la meilleure faGon la presence, chew Puchyrhachis, de rudiments sacraux ainsi que de pattes post&ieures d6velopp6es. Toutefois, il n’y a pas, chez Pacbyrbacbis, de regionalisation identifiable de la region prCsacr&e de la colonne vert& brale, au-deli de ce qui se voit chez les boo’ides. Les hypapophyses des vertgbres anterieures (cervicales) se reduisent graduellement post&ieurement, et la presence de paradiapo-

physes sur toutes les vertgbres post&ieures P I’atlas confirme l’existence de &es dans cette region. 11 faut souligner surtout l’identification, par Lee et Caldwell (19981, d’une c8te sacree libre sur laquelle l’ilion de Pacbyrbacbis serait suspendu. Z’ilion des squamates non-ophidiens est typiquement soutenu par les pleurapophyses, &es sacrees qui sont continues par rapport aux 616ments vertebraux dans leurs stages cartilagineux, s’ossifiant 1atCralement par rapport aux arcs neuraux des vert6bres du sacrum. L’identification d’une c8te sacree libre chez Pucbyrbacbis implique la retention d’une c6te libre, probablement en cons6quence d’une pedomorphose, et une modification post&ieure de celle-ci en une c6te sacrCe bifurquCe. De fait, la supposCe c6te sacree est, plus parcimonieusement, interpr&e comme la premiere lymphapophyse bifurquee. Nous pouvons done douter de l’existence, chez Pacbyrbacbis, d’un quelconque contact entre les rudiments pelviens et le squelette axial. La patte posterieure est presque compl&e ; toutefois, l’autopode demeure inconnu, mais l’absence des Uments distaux 5 l’astragalus et au calcaneum semble @tre due, en partie, 5 une preservation incomplete. Nos rksultats sugg&ent deux scCnarios possibles pour expliquer la presence de pattes post&ieures presque compl&es chez Pucbyrhacbis: (1) les pattes se sont red6velopp6es chez ce taxon, ou bien (2) elles ont && perdues (&duites) independamment chez les clades majeurs de serpents actuels (Scolecophidia, Anilioidea, Macrostomata). Bien que la seconde hypothgse soit moins parcimonieuse au vu de nos connaissances actuelles, nous pensons que celle-ci doit neanmoins @tre consid&t?e s6rieusement. Elle a aussi le merite d’&tre ouverte au test potentiel fourni par le registre fossile.

Acknowledgements. We thank D. Goujet and P. Taquet for their help with the French version of the text, and J.-C. Rage who offered helpful advice and criticism on an earlier draft of this paper. We are indebted to the following individuals and curators who kindly allowed the examination of specimens under their care: E. Tchernov (Hebrew University of Jerusalem); H. Voris and A. Resetar (The Field Museum, Chicago); D. Frost, C.W. Myers, and C. Cole (American Museum of Natural History, Department of Herpetology); M. Norell (American Museum of Natural History, Department of Vertebrate Paleontology); N.E. Arnold and C. McCarthy (The Natural History Museum, London): G. Puorto and F.L. Franc0 (Institute Butantan, SBo Paulo); M.T.U. Rodrigues and P.E. Vanzolini (Museu de Zoologia, Universidade de SBo Paula). This research is supported in part by grant No. 98/00064-7 from FAPESP (FundaqBo de Amparo b Pesquisa de Sslo Paulo) to the senior author.

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