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A new genus of Ostracoda of the family Limnocytheridae from the non-marine mid-Cretaceous of Alberta, Canada
REVUE DE MICROPALI~ONTOLOGIE Vol. 44, n ° l , m a r s 2001, pp. 93-102
A NEW GENUS OF OSTRACODA OF THE FAMILY LIMNOCYTHERIDAE FROM THE NON-MARINE MID-CRETACEOUS OF ALBERTA, CANADA. UN NOUVEAU GENRE D'OSTRACODE DE LA FAMILLE LIMNOCYTHERIDAE D U CRETACE MOYEN N O N MARIN DE L 'ALBERTA, CANADA b y S t e p h e n J.
TATMAN*
and Robin WHATLEY*
ABSTRACT. -- The l i m n o c y t h c r l d genus Acantharhinocythere is d e s c r i b e d from the A p t i a n - A l b i a n shales of the Lower Cretaceous B l a i r m o r e G r o u p of A l b e r t a , C a n a d a . These sediments are of n o n - m a r i n e , p r o b a b l y m a i n l y f r e s h w a t e r or slightly b r a c k i s h (oligohaline) l a c u s t r i n e origin a n d the genus occurs in a t y p i c a l C a n a d i a n P r a i r i e P r o v i n c e Lower Cretaceous " W e a l d e n " type o s t r a c o d assemblage, together w i t h species of such g e n e r a as Theriosynoecum a n d Cypridea Acantharhinocythere differs f r o m o t h e r l i m n o c y t h e r i d g e n e r a in its shape a n d outline, b u t p a r t i c u l a r l y in its u n i q u e m i d - a n t e r i o r spinose p r o t u b e r e n c e . The possible function of this s t r u c t u r e is discussed. RESUMI~. - Le n o u v e a u genre Actmtharhiuocythere ( O s t r a c o d a , L i m n o c y t h e r i d a e ) est d 6 c n t ~ p a r t i r de d6p6ts argileux ht6s d u Cr6tac6 i n f 6 r i e u r (Aptien-Albien) de la B l a i r m o r e F o r m a t i o n d ' A l b e r t a , C a n a d a . Ces s6diments sont trbs p r o b a b l e m e n t d'omgine continentale, plus pr6cis6ment de milieux d ' e a u douce ou l a c u s t r e s oligohalins. Ce genre est pr6sent d a n s une association d ' o s t r a c o d e s de type ~, ( P r o v i n c e de la P r a i r i e c a n a d i e n n e ) assoc16 avec d ' a u t r e s genres tels que Theriosynoecum et Cypridea. Acantharhinocythere diffbre des autres L i m n o c y t h e r i d a e p a r sa silhouette et sa forme, mais s u r t o u t p a r l'existence d ' u n e p r o t u b 6 r e n c e 6pineuse bien m a r q u 6 e h l ' a v a n t des valves. La signification fonctionnelle de cette s t r u c t u r e est d~scut6e. Key-words : Ostracoda - Limnocytheridae - non-marine - mid-Cretaceous - Alberta - Canada. Mots-cl6s : O s t r a e o d e s - L i m n o c y t h e r i d a e - D 6 p ~ t s c o n t i n e n t a u x - C r 6 t a c 6 i n f 6 r i e u r - A l b e r t a - C a n a d a .
INTRODUCTION In the course of a study of the Ostracoda of the mid-Cretaceous Aptian/Albian shales of the Calcareous or Ostracod member of the Lower Blairmore Group, a new ostracod was encountered belonging to the entirely non-marine family of the Cytheracea, the Limnocytheridae. In the present paper, the monotypic genus Acantharhinocythere is erected to accommodate this taxon. The Canadian Province of Alberta (Fig. 1) is bounded to the west by the Rocky Mountains. To the east of the Cordillera, the Alberta Foreland Basin was formed during the orogeny, although as early as
the Lower and Middle Jurassic, the basin began to develop. During the Cretaceous, a series of clastic wedges were deposited as the basin deepened. Within these, at about the Aptian/Albian boundary is the Ostracod Member (alternatively referred to as the Calcareous Member and also essentially synonymous with the Ostracod Zone) of the Blairmore Group. This comprises shales with subordinate limestones and fine-grained sandstones. Because it is bounded above and below by essentially arenaceous dominated sediments, it almost uniquely provides a preservational "window" from which ostracods and other fossils can be recovered. A generalized stratigraphical succession for central and southern Alberta is given in Table 1.
* M i c r o p a l a e o n t o l o g y R e s e a r c h , D e p a r t m e n t of Geology, U n i v e r s i t y of Wales, A b e r y s t w y t h , UK.
OSTRACODA OF THE C RETACEOUS OF CANADA
94
TABLE1. Generalized section ol the late Jurassic and Lower Cretaceous of central and southern Alberta. -
Su,ceession des ~brmations du Jurassique termi~tal et du Cr6tae~ in[~rieur de I'Alberta central et m6ridional.
C R E T A C E O U S
J U R A S S I C
A L B I A N A P T I A N
B L A I R M O R E G R O U P
L A T E
Beaver Mines Formation
Marine sands with chloritic cement
Ostracod/ Calcareous Member
Shales, fine sandstones and bioclastic limestones
Gladstone Formation
Quartz and chert rich continental channel sands
Cadomin Formation
Chert clast supported conglomerates fining upwards to sandstones
KOOTENAY GROUP
Sand dominated marine and nonmarine sediments
THE OSTRACOD FAUNA AND THE ENVIRONMENT OF DEPOSITION During the deposition of tile Lower Blairmore Group, an area of open sea is thought to have existed to the north of the basin. The extent to which this intparted a marine influence to the Blairmore Group and the Ostraeod Member in particular, has been the subject of debate for the past 50 years. Loranger (1951) considered the fauna to be "predominantly freshwater rather than braekis|t" on the basis of its ostracods and such bivalves as " U n i o " . WAI,L (in Mclean and Wall, 1981) encountered non-marine ostracods in the south of the basin and a few ostracods associated with agglutinating Foraminifera in the northern foothills, which they interpreted as indicating restricted marine conditions. Hayes (1986) suggested that the Calcareous Member was deposited in a series of lakes on the basis of its restricted fauna and sedimentary facies. More recently, Banerjee (1990) and Banerjee and Kidwell (1991), on the basis
of sequence stratigraphy theory, ]lave argued that the Ostracod Member represents the sottthernmost extension of a marine transgression coming from the north. Tatman and Whatley (1996) demonstrate on the basis of its ostracod fauna that the Calcareous Member shows no trace of marine influence and that it was deposited in non-marine conditions. The ostraeod assemblage recovered from the Ostraeod member is of low incidence and diversity and comprises genera that occur in "Wealden" type environments worldwide, especially in Europe. This is despite the fact that European "Wealden" faunas, being Neocomian in age are rather older. The three major elements of post Middle Jurassic non-marine ostraeod faunas, the Darwinulacea, Limnoeytheridae and Cypridacea (Whatley, 1990, 1992) are all present. The former represented by the genus D a r w i n u la, and the latter by C y p r i d e a while the cytheracean Limnocytheridae are dondnated by L i m n o c y t h e r e and T h e r i o s y n o e c u m (Loranger, 1951 ; Finger, 1983 ; Tatman and Whatley, 1996).
TATMAN and WHATLEY
BRITISH COLUMBIA
95
North Western Utilities Limited Legal #1 O ALBERTA , Edmonton
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Alberta CANADA
FIG. ]. Location map of central and southern Alberta (area shaded on insert), showinglocations of cores from which Acantharhinocythere has been recovered. The Rocky Mountain front is shown as in the Alberta/British Columbm boundary. The regional geologyis intentionally omitted, since it is too complex to depict meaningfullyon a map of this scale, and because the principal extent of the Ostracod Member of the Blairmore Formation is in the subsurface. - -
Carte sch6matique de l'Alberta central et m&idional (aire ombrde sar le carton de gauche) sur htquelle sont indiqu6s les emplacements des sondages qui ont fourni des Acantharhiaocythere. Le froat des Rocheuses est cunstd6r6 co~ncider avec la f r o a t ~ r e entre l'Alberta et la Colonic britannique. Les caract&es g6ologiques sont intentionnellement omi*, car ils seraient trop complexes pour ~tre report6s sur la carte ~t cette 6cheUe et attssi ?t cause de l'extension en profondeur du Membre h Ostracodes de ht formation Blairmore.
THE
AGE OF THE
DEPOSIT
The dating of these deposits is difficult since they c o n t a i n few index fossils. F i n g e r (1983) suggested that because the Ostracod Zone is obviously facies controlled it is possibly d i a c h r o u o u s across the A p t i a n / A l b i a n b o u n d a r y . Because t h e r m a l a l t e r a t i o n is often high, p a l y n o m o r p h s are poorly p r e s e r v e d , being very highly oxidized. Also, m a n y of those recovered are long r a n g i n g and, c o n s e q u e n t l y , of little stratigraphical value. However, B a n e r j e e a n d Davies (1988) p u t f o r w a r d a z o n a t i o n scheme based on dinoflagellates. Using their system, it can be shown t h a t the type locality o f A c a n t h a r h i n o c y t h e r e w a l l i , i n the Fall River borehole core corresponds to a C3/C4. late A p t i a n / e a r l y A l b i a n age.
METHODOLOGY
T h a t we have been able to add to this f a u n a the delicately preserved new genus, is due to the fact that it has b e e n preserved i n the b u l k f l u o r i d a t i o n recovery t e c h n i q u e which we have employed. This techn i q u e is the subject of a later p u b l i c a t i o n ( T a t n m n , in prep.). E a r l i e r techniques to recover ostracods from these intensely h a r d shales involved mechanical c r u s h i n g a n d s u b s e q u e n t t r e a t m e n t with very corrosive chemicals, such as sodium hypochlorite a n d hydrogen peroxide. While these techniques recovered the more r o b u s t ostracods, a n d even some of their late stage i n s t a r s , y o u n g e r juveniles a n d delicate, thin-shelled taxa such as A c a n t h a r h i n o c y t h e r e were destroyed.
96
OSTRACOI)A OF T H E CRETACEOUS OF CANADA
Description : As t'or type species.
SYSTEMATIC DESCRIPTIONS
Remarks : On the basis of its overall morphology, its The type material used in this p a p e r is housed in the collections of the Department of Palaeontolog~y, Natural History Museum (British Museum, Natural History), London, to which the catalogue numbers prefixed OS refer. An additional suite of samples is housed in the Geological Survey of Canada collections in Canada. All dimensions are given in millimetres and the following conventions are employed: RV = right valve, LV = left valve, C = articulated carapace, A = adult, juv. = juvenile. The following descriptive size scale is used : 0.40 very small, 0.40-0.50 small, 0.50-0.70 medium, 0.70-1.00 large, >1.00 very large. Phylum CRUSTACEA P E N N A N T , 1777 Class OSTRACODA L A T R I E L L E , 1806 Order PODOCOP1DA MOLLER, 1894 S u b o r d e r P O D O C O P I N A SARS, 1866 S u p e r f a m i l y CYTHERACEA B A I R D , 1 8 5 0 Family LIMNOCYTHERIDAE K L I E , 1938
Subfamily LIMNOCYTHERINAEKLIE, 1938 A c a n t h a r h i n o c y t h e r e gen. nov.
Type species : Acantharhinocythere walli gen. et sp. nov.
Derivatio nominis : Gr. ¢x~ctvOct thorn, spine + p t vo~ nose. "spiny nose" + cythere. With reference to the spinose anterior projection which characterizes this genus.
Diagnosis : A medium to large, elongate subovate to subrhomboidal, thin-shelled and bisulcate genus of Limnocytheridae. Anterior margin angular, with rounded minutely denticulate antero-dorsal slope and straight to slightly convex, denticulate antero-ventral slope above and below respectively a delicate hemispherical to subquadrate spinose protuberence. Shallowly bisulcatc anterior of mid-length. A backward directed spine occurs posteto-ventrally.
PLATE
hingement and musculature and its association with well-known non-marine Ostracoda anti other organisms, this genus is placed within the non-marine cytherid family, the Linmocytheridae. Within that family, on the strength of its carapace morphology and secondary sexual characteristics, it is placed within the subfamily Limnocytherinae rather than the Timiriaseviinac. It differs from all know limnocytherid genera in shape and outline and internal details and particularly in its anterior spinose protuberence. There is some similarity in general shape with the limnocytherid genus Pampacythere W H A T L E Y and CBOLtCH (1974) from the Quaternary (and subsequently the Recent) of Argentina. However, Pampacythere lacks the anterior protuberence, and has higher and more pronounced cardinal angles, although its musculature is virtually identical to that of Acantharhinocythere. Forester (1991) illustrated a number of unusual limnocytherids that he referred to as "exotic" forms, a term he employed to indicate non-marine species exhibiting characteristics normally associated with marine taxa. While none of the species he illustrated arc at all similar to Acantharhinocythere, Forester's study illustrates the propensity for limnocytherids to evolve unusual morphologies. A recent revision of the origins and early evolution of the Limnocytheridae (Whatley and Moguilevsky, 1998) further emphasizes this. Other unrelated ostracod taxa with prominent anterior structures, such as Aratrocypris WIIATLEY et al. (1985) and Danipussella WOUTERS (1988) are marine and belong to the Cypridacea and Bairdiacea respectively. Szczechura (1978) describes a nunnber of species belonging to d i f ferent genera from non-marine Upper Cretaceous strata in the Nemegt Basin of the Gobi Desert, with curious anterior protuberances. All of these, however, are Cypridacea and the "lip-like extensions", as Szczechura (1978) describes them, are quite unlike the anterior protuberence of Acantharhinocythere. The candoniid Iberacypris B A B I N O T , ( e x Iberocypris Babinot, 1985), from the non-marine Maastrichtian of Central Spain has developed horn-like extensions
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(SP pour stdr6o-paire) l-6. Acantharhinocythere tvalli gen. et sp. nov. 1 : RV, holotype, external lateral view, 1 a-b (SP). (OS 14023), × 103. 2 : LV, paratype, internal view, 2 a-b (SP). (OS 14024), x 114.
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at the cardinal angles of the LV. The functional significance of these structures remains an enigma (Babinot, lit. com. July 2000)
Acantharhinocythere is presently monotypic and is very restricted in both its stratigraphical and its geographical occurrence. It occurs in relatively few samples from only three localities. Although in one sample (sample 91252, front core 7722) A. walli makes up 17 % of the total ostracod fauna, it is normally much less significant, comprising a mean of less than 1 % in other samples where it occurs. A c a n t h a r h i n o c y t h e r e walli sp. nov. (PI. 1, fig. 1-6; P1. 2, fig. 2-8)
Derivatio nominis : Named in honour of Dr. John Wall of the Geological Survey of Canada, in recognition of his signal contribution to our understanding of Canadian biostratigraphy.
Holotype : RV OS 14023. Paratypes : Four specimens (OS 14024 - OS 14027). Material : Forty-five specimens : 41 adults, 4 juvenilCS.
Type locality: Fall Creek Test borehole, Alberta, Canada. Grid reference 22-37-11W5. Type level : Core 7722, Geological Survey of Canada sample reference n u m b e r BBA91252, depth 298m. Aptian/Albian dark grey shales from the Calcareous (Ostracod) member.
Diagnosis : As for genus. Description : Medium to large. Thin-shelled. Elongate subovate to subrhomboidal in lateral view, but lateral outline interrupted by postero-ventral spine and mid-anterior spinose protuberence. Elongate subhastate in dorsal view, with greatest width behind mid-length ; the anterior protuberence is directed laterally at an angle of approximately 15 degrees to the mid axis commisure. Anterior margin angular, sloping downwards from the anterior cardinal angle to about one quarter of the distance from the dorsal margin. At this point, the margin changes from being gently convex to almost straight and almost perpendicular, extending to approximately mid-height, from where the margin is extended anteriorly to form the semicircular protuberence. The antero-ventral slope is slightly convex and unites witi~ the ventral margin at almost ]/3 length from the anterior. The anterior margin is denticulate with usually three small triangular denticles above the protuberence, four around its periphery and two below it. Posterior margin asymmetrically rounded with long convex postero-dorsal slope, apex below mid-height and short, almost straight postero-ventral slope. Dorsal margin straight to very slightly convex. Ventral m a r gin biconvex about a marked oral incurvature and with almost keel-like antero and postcro-ventral parts. Shallowly bisulcate. A short oblique sulcus extends from just behind the anterior cardinal angle antero-ventrally to about mid-height ; a longer, vertical sulcus just anterior of mid-length extends from the dorsal margin to a ventro-lateral position. Orna-
PLATE 2 (SP indicates the use of Stereo-Pairs) 1-7. A c a n t h a r h i n o c y t h e r e tvalli gen. et sp. nov. 1 : LV, lmratype, internal view (OS 14027), x 98. 2 : RV, external lateral view of fragmentary specimen showing well developed postero-ventral spine. (Specimen subsequently destroyed on removal from SEM stub). × 111. 3 : 3 a b (SP), RV, internal v,ew showing lophodont hinge w~th terminal sockets and median groove, (OS 14025), x 190. 4 : 4 a-b (SP), LV, internal view showing lophodont hinge with terminal sockets and denticulate median bar. (Specimen subsequently destroyed on removal from SEM stub). x 190. 5 : 5 a-b (SP), RV, paratype, internal view showing detail of anterior protuberance (OS 14024), x 348. 6 : 6 a-b (SP, RV, holotype, external view sbowmg detail of an~ terior protuberance (OS 14023), x 348. 7 : LV, paratype, internal view showing detail of muscle scars (OS 14024), x 534. 8 : RV, paratype, obhque internal view slmwing anterior pro~ tubcrance and extcnsion of inner lalnella (OS 14024). x 425.
(SP pour st6r6o-paire) 1 : V . G . , p a r a t y p e , r u e i n t e r n e (OS 1"1027), x 98. 2 : V . D . , v u e latdrale e x t e r n e d ' u n sp6cimen f r a g m e n t 6 en c o u r s de m a n i p u l a t i o n et m o n t r a n t une 6~pine post6r o - v e n t r a l e bien d6veloppde, x l 11. 3 : 3 a-b ( S P ) . V . D . , r u e i n t e r n e m o n t r a n t uue c h a r n i ~ r e lop h o d o n t e u v e c alv6oles t e r m t n a l e s et gouti~re m 6 d i u n e ( O S 14025), x 190. 4 : 4 a-b ( S P ) , V . G . , r u e i n t e r n e m o n t r t t n t une c h a r n t $ r e lop h o d o u t e a v e c oh,doles t e r m i n a l e s et b a r r e m d d i a n e dentieulge ( s p d c i m e n cl~truit po.stdrieurement h la p r i s e tie vue), x 190. 5 : 5 a-b ( S P ) , ll.D., l m r n t y p e , vue interne a v e c ddtail de In p r o t u b e r a n c e antdrieure ( O S 14024), x 348. 6 : 6 a-b ( S P , V . D . , holotype, vue e x t e r u e avec d6tail de la p r o t u b d r a u c e ont~rieure ( O S 14023), x 348. 7 : V . G . , p a r a t y p e , r u e i n t e r u e avec ddtail des e m p r e i u t e s m u s c u l a i r e s (OS 14024). x 534. 8 : V . D . , p a r a t y p e , r u e i n t e r n e oblique m o u t r a u t la p r o t u b 6 r u n c e ant~rieure et l'exteusion de la lamelle i n t e r n e ( O S 14024), x 425.
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ment feebly reticulo-punctate but weakest between the two sulci. Most of the valve is densely punctate but in places is coarser and virtually reticulate. On the compressed anterior marginal area the puncta are smaller and arranged in three double rows, forruing a delicate reticulation, with dominant muri parallel to the margin and weaker ones perpendicular to it. A variably developed, backward-directed spine arises postero-ventrally. The adductor muscle scars are manifested in external view as a group of four elongate ridges at mid-height in the median sulcus. Inner lamella broadest at the end margins ; it forms a small "flange" extending around the outer margin of the anterior protuberence and extending a short distance along the anterior margin on either side. Normal pore canals large, circular to ovate and sieve type. In internal view they are set in pits between 20 and 30 gin in diameter. The adductor scars are straight and equidistant. Frontal scar heart-shaped and anterior of the dorsal adductor, ventral to which one, or possibly two mandibular scars occur. Hinge modified lophodont with smooth terminal sockets but denticulate median element in LV.
Dimensions : Holotype : L = 0.699, H = 0.350 ; paratype : LV OS 14024 H = 0.289; paratype RV OS 14025 L = 0.716, H = 0.324 ; paratype : LV OS 14026 L = 0.700, H = 0.335; paratype LV OS 14027 H = 0.32. All paratypes are from the type locality and level.
Remarks : All the occurrences of Acantharhinocythere walli were in ostracod assemblages dominated by Cypridea tilleyi Loranger (1951), Theriosynoecttm spp. and other limnocytherids. Although there has been some reassessment of the palaeoecology of these genera in respect of the British Purbeck-Wealden (Anderson, 1985), there is little doubt that E. walli was entirely freshwater or low oligohaline in habit, particularly since it is found closely associated with the gastropod Viviparus (Paludina). The occurrence of Cytheridea bonacordensis Loranger and Orthonotacythere (?) sp. of McLean and Wall (1981) in a sample from Elbow River has been interpreted as indicating a brackish water environment (Wall in Taylor and Walker, 1984); an interpretation with which we agree, provided that the material was in situ and not derived. However, Acantharhinocythere walli does not occur at this locality and has never been encountered in association with either of the species in question.
Age and distribution : This species has only been recorded from the Aptian/Albian Calcareous Member from sites in the Province of Alberta. It has been recovered from the following localities : Fall Creek test
Hole (type locality) Grid reference 22-27-11-W5. Imperial Home Coal Lake, 15-21-47-23-W4. North Western Utilities Limited legal, 20- 58- 24- W4.
DISCUSSION OF DISTRIBUTION During the deposition of the Calcareous Member, the Alberta Foreland Basin was bounded to the west by the Rocky Mountains, to the east by the Canadian Shield and to the north by the Clearwater Sea with its open marine conditions. To the south, continental conditions prevailed and from this area, rivers flowed northwards into the basin (McLean and Wall, 1981; Taylor and Walker, 1984; Hayes, 1986). Most of the non-marine ostracod genera, which were present in the basin during the deposition of the Calcareous Member, are pandemic in Lower Cretaceous Wealden-type environments. Cypridea, its allies. and to a certain extent other contemporary cyprids, were particularly well suited to these conditions as witnessed by their virtually ubiquitous diverse and abundant occurrence in such environments worldwide. This was achieved largely by their possession of desiccation resistant eggs and ability to reproduce parthenogenetically (Whatley, 1990, 1992). Limnocytherids are almost entirely syngamic and do not possess encystable eggs, which is a limiting factor in their dispersal relative to the cyprids. Also, unlike the cyprids, no cytherid is able to swim and this further restricted the ability of Acantharhinocythere to colonize new localities. These constraints may have contributed to the evolution of such an endemic taxon in this isolated basin, although this may be an artifact due to the very fragile nature of the species. Precedents, however, of bizarre species of cytheracean ostracods evolving in restricted basins are legion. An example is the endemic limnocytherid genus Skopaeocythere (Whatley et al., in press) confined to the restricted saline lake of the Upper Amazon Basin in the Neogene, which also produced an entirely endemic species flock of Cyprideis JONES (Whatley et al., 1998 ; Mufioz-Torres et al., 1998).
MODE OF LIFE It is difficuh to understand what advantage its anterior spinose protuberence bestowed on Acantharhinocythere walli. However, such a structure must have had an important function for it to have evolved. In some cases where the anterior margins of various ostracod taxa are modified, we understand
TATMAN and WHATLEY their purpose. For example, the r o s t r u m and rostral incisure of c y p r i d i n i d myodocopids serve the i m p o r tant function of facilitating egress of the 2 "d ant e n n a e , thereby improving their swimming ability. The antero-ventral beak of Cypridea and its allies is thought to be an integral part of their feeding strategy. Whatley et al. (1985) suggested a number of possible modes of life for the deep-sea genus Aratrocypris, so named for its large plough-like antero-ventral projection. This structure, which is very strongly calcified, probably evolved to allow the animal to plough through the soft oozes of the deep-sea in search of its food. However, in Acantharhinocythere walli, a m u c h thinner-shelled ostracod, this is an unlikely mode of life a n d its a n t e r i o r protuberence is too delicate and is situated too far from the oral appendages to be directly associated with alimentation. However, it could possible have served a secondary alimentary function by moving aside a soft, soupy sediment to give better access to the mouthparts. Because the Limnocytheridae have none of the necessary appendage modifications associated with filter feeding, it could not have been a funnel to channel incurrcnt flow into the carapace. More probably, therefore, since the marginal denticles associated with the protuberence and along the anterior margin immediately above and below it would have borne long setae, it represented a device for enhancing sensory perception anteriorly. An alternative but less probable suggestion is that the structure may have provided protection for the 2 nd antennae at the point of egress from the carapace, ahhough why this should be an advantage to a non-swimming ostracod is not immediately apparent, unless it be associated with balance or possibly the detection of food particles or prey. To extend this argument further is to enter the realm of speculation. Suffice to say that this structure, unique among Ostracoda, is likely to remain enigmatic until the discovery of additional, better-preserved material and even then, it will be difficult to determine its fnnction.
ACKNOWLEDGEMENTS SJT gratefully acknowledges his NERC studentship that allowed him to visit Canada in order to undertake this study. Drs J. Wall and 1. Bannerjee are thanked for all their help in Canada and the invaluable technical assistance of Mrs. L. Morrison in Aberystwyth is gratefully acknowledged. The authors thank the referees for their thoughtful review of the paper and to the one whose identity we know, Dr. B a b i n o t , we extend our gratitude for his useful comments on Iberacypris.
101
BIBLIOGRAPHY
ANDERSON F. W. (1985): Ostraeod faunas in the Purbeek and Wealden of E n g l a n d . . l o u r n . Micropal., vol. 4, p. 1-68. BABINOT J.-F. (1986): On lberocypris cornutas BABINOT. Ste reo-Atlas of Ostracod Shells, vol. 13 (2), p. 129-132. BANERJEE ][. (1990 : Some aspects of Lower Mannwlle sedimentation in southeastern Alberta. Geol. Surv. Paper" Canada, p. 90-111. BANERJEE I. and DAVIES E. H. (1988) : An integrated hthostrat~graphic and palynologic study of" the Ostracod Zone and adjacent s t ra t a in the Edmonton Embayment Ccntral Alberta. lu : James, D. P. and Leckie. D. A. (Eds). Sequences, Stratigraphy : Surface and Subsurface. Canadian Soc. Petroleum Geologists, Memoir 15, p. 261-274. BANERJEE I. and KIDWELL S. M. (1991) : Significance of molluscan shell beds in sequence s t r a u g r a p h y : an example from the Lower Cretaceous Malmville Group of Canada. Sedimentology, vol. 38, n ° 5, p. 913-935. FINGER K. L. (1983) : Observations on the Lower Cretaceous ostracod Zone of Alberta. Ball. Can. Pet. Geol., vol. 31 (4),
p. 321-327. FORESTER R. M. (1991) : Pliorene:climate history of the Western United States derived from lacustrine ostracodes. Quaternary Science Reviews, wd. 10, p. 113-14,6. [L~YES B.J.R. (1986) : Stratigraphy of the Basal Cretaceous Lowcr Manville Formation, Southern Alberta and North-Central Montana. Bull. Cart. Pet. Geol., vol. 34, n ° 1, p. 30-48. LORANGER D.M. (1951 ) : Useful Blab'more microfossil zonc in ten tral and southern Alberta, Canada. Ball. Ant. Ass. Petrol. Geol., vol. 35, n ° 1l , p. 2348-2367. McLEAN J.R. and W ~ E J.H. (1981) : The Ea rl y Cretaceous Moosebar Sea in Alberta. Bull. Can. Pet. Geol.. vol. 29, n" 3, p. 334-377. MUI~OZ-TORRES F., WItATLEY R.C. audVAN HARTEN D : (1998) : The endemic non-marine Mmcene ostracod fauna of the Upper Amazon Basin. Rev. Esp. Micropal., vol. 30, p. 89-105. SZCZECIIURA J. (1978) : Fresh-water Ostracoda from the Upper Cretaceous of the Nemegt Basin, Gobi Desert. Palaeontologta Polomca, Warsaw, vol. 21, p. 107-118. TATlVIAN S.J. (In prep) : The recovery of calcareous microfossils from m d u r a t e d sediments by bul k fluoridation. TATMAN S.J. and WHATLE~' R.C. (1996) : A palaeontological reconstructmn of the Ostraeod Member of the Blmrmore Formation, Alberta, Canada. In : Keen M.C. (Ed.). Proceedings of the 2 nd European Ostrarodologtsts Meeting, University of Glasgow, July, 1993, British Micropal. Soc., London. p. 35-38. TAYLOR D.R. and WALKER R.G. (198/1.) : Deposmonal environmerits and palaeogeography in the Albian Moosebar F o r m a t i o , and adjacent fluvial Gladstone and Beaver Mines Formations, Alberta. Can. Joarn. Earth Sci.. vol. 21, p. 698-714. WHATLEY R.C. (1990) : The relatmnship between extrinsic and intrinsic events in the evolution of Mesozoic non-marine Ostraeo da. ht : Kaufmann E.G. and Walliser O.H. (Eds). Extinction events in E a r t h History. Lecture Notes in Ea rt h Scienres 3(1, Spriuger-Verlag, p. 253-263. WHATLEY II.C. (1992) : The reproductive and d ~ p e r s a l strategtes of Cretaceous nonma rme Ostracoda : the key to pandemism. In : Mateer N.J. and Chen P.-J. (Eds). Aspects of nonmarine Cretaceous Geology. Chine Ocean Press, Beijing, p. 177-193.
102
OSTRACODAOFTHECRETACEOUSOFCANADA
WHATLEY I{.C. and T. del C. CHOLICH (1974) : A new Quaternary ostracod genus from Argentina. Palaeontology, vol. 17 (3), p. 669-684, 2 pl. 1 fig. WHATLEY R.C., AYRESS M., DOWNING S., HARLOW C. and KESLER K. (1985) : Aratrocypris, an enigmatic new cyprid ostracod from the Tertiary of D.S.D.P. sites in the S. W. Pacific. Journ. Micropal., vol. 4, n ° 1, p. 69-79. WHATLEY B.C. and ]VIOGUILEVSKYA. (1998) : The origin and early evolution of the Limnocytheridae (Crustacea, Ostracoda). In : Crasquin-Soleau, S. et al. (Eds). What about Ostracoda !, Proc. 3rd. European Symposium on Ostracoda, Bierville, France, July 1996, EIfEp, Edition~ Pau, M6m. 20, p. 271-288.
WHATLEY R.C., MU~OZ-TORRES F and VAN HARTEN D. (1998) : The Ostracoda of an isolated Neogenc saline lake in the Western Amazon Basin. In CrasquimSoleau S., Braccini E. and Lethiers F. (Eds), What about Ostracoda ! Proc. 3 rd. European Symposium on Ostracoda, Bierville, France, July 1996. E1]'Ep, Editions Pau, M6m. 20, Pau, p. 231-248. WHATLEY R.C., MU~oz-TORRES F. and VAN HARTEN D. (In press) : Skopaeocythere : A minute new limnocytherid (Crustacea, Ostracoda) from the Neogene of the Amazon Basin.
Ameghiniana.
A NEW GENUS OF OSTRACODA OF THE FAMILY LIMNOCYTHERIDAE FROM THE NON-MARINE MID-CRETACEOUS OF ALBERTA, CANADA. UN NOUVEAU GENRE D'OSTRACODE DE LA FAMILLE LIMNOCYTHERIDAE D U CRETACE MOYEN N O N MARIN DE L 'ALBERTA, CANADA b y S t e p h e n J.
TATMAN*
and Robin WHATLEY*
ABSTRACT. -- The l i m n o c y t h c r l d genus Acantharhinocythere is d e s c r i b e d from the A p t i a n - A l b i a n shales of the Lower Cretaceous B l a i r m o r e G r o u p of A l b e r t a , C a n a d a . These sediments are of n o n - m a r i n e , p r o b a b l y m a i n l y f r e s h w a t e r or slightly b r a c k i s h (oligohaline) l a c u s t r i n e origin a n d the genus occurs in a t y p i c a l C a n a d i a n P r a i r i e P r o v i n c e Lower Cretaceous " W e a l d e n " type o s t r a c o d assemblage, together w i t h species of such g e n e r a as Theriosynoecum a n d Cypridea Acantharhinocythere differs f r o m o t h e r l i m n o c y t h e r i d g e n e r a in its shape a n d outline, b u t p a r t i c u l a r l y in its u n i q u e m i d - a n t e r i o r spinose p r o t u b e r e n c e . The possible function of this s t r u c t u r e is discussed. RESUMI~. - Le n o u v e a u genre Actmtharhiuocythere ( O s t r a c o d a , L i m n o c y t h e r i d a e ) est d 6 c n t ~ p a r t i r de d6p6ts argileux ht6s d u Cr6tac6 i n f 6 r i e u r (Aptien-Albien) de la B l a i r m o r e F o r m a t i o n d ' A l b e r t a , C a n a d a . Ces s6diments sont trbs p r o b a b l e m e n t d'omgine continentale, plus pr6cis6ment de milieux d ' e a u douce ou l a c u s t r e s oligohalins. Ce genre est pr6sent d a n s une association d ' o s t r a c o d e s de type ~
INTRODUCTION In the course of a study of the Ostracoda of the mid-Cretaceous Aptian/Albian shales of the Calcareous or Ostracod member of the Lower Blairmore Group, a new ostracod was encountered belonging to the entirely non-marine family of the Cytheracea, the Limnocytheridae. In the present paper, the monotypic genus Acantharhinocythere is erected to accommodate this taxon. The Canadian Province of Alberta (Fig. 1) is bounded to the west by the Rocky Mountains. To the east of the Cordillera, the Alberta Foreland Basin was formed during the orogeny, although as early as
the Lower and Middle Jurassic, the basin began to develop. During the Cretaceous, a series of clastic wedges were deposited as the basin deepened. Within these, at about the Aptian/Albian boundary is the Ostracod Member (alternatively referred to as the Calcareous Member and also essentially synonymous with the Ostracod Zone) of the Blairmore Group. This comprises shales with subordinate limestones and fine-grained sandstones. Because it is bounded above and below by essentially arenaceous dominated sediments, it almost uniquely provides a preservational "window" from which ostracods and other fossils can be recovered. A generalized stratigraphical succession for central and southern Alberta is given in Table 1.
* M i c r o p a l a e o n t o l o g y R e s e a r c h , D e p a r t m e n t of Geology, U n i v e r s i t y of Wales, A b e r y s t w y t h , UK.
OSTRACODA OF THE C RETACEOUS OF CANADA
94
TABLE1. Generalized section ol the late Jurassic and Lower Cretaceous of central and southern Alberta. -
Su,ceession des ~brmations du Jurassique termi~tal et du Cr6tae~ in[~rieur de I'Alberta central et m6ridional.
C R E T A C E O U S
J U R A S S I C
A L B I A N A P T I A N
B L A I R M O R E G R O U P
L A T E
Beaver Mines Formation
Marine sands with chloritic cement
Ostracod/ Calcareous Member
Shales, fine sandstones and bioclastic limestones
Gladstone Formation
Quartz and chert rich continental channel sands
Cadomin Formation
Chert clast supported conglomerates fining upwards to sandstones
KOOTENAY GROUP
Sand dominated marine and nonmarine sediments
THE OSTRACOD FAUNA AND THE ENVIRONMENT OF DEPOSITION During the deposition of tile Lower Blairmore Group, an area of open sea is thought to have existed to the north of the basin. The extent to which this intparted a marine influence to the Blairmore Group and the Ostraeod Member in particular, has been the subject of debate for the past 50 years. Loranger (1951) considered the fauna to be "predominantly freshwater rather than braekis|t" on the basis of its ostracods and such bivalves as " U n i o " . WAI,L (in Mclean and Wall, 1981) encountered non-marine ostracods in the south of the basin and a few ostracods associated with agglutinating Foraminifera in the northern foothills, which they interpreted as indicating restricted marine conditions. Hayes (1986) suggested that the Calcareous Member was deposited in a series of lakes on the basis of its restricted fauna and sedimentary facies. More recently, Banerjee (1990) and Banerjee and Kidwell (1991), on the basis
of sequence stratigraphy theory, ]lave argued that the Ostracod Member represents the sottthernmost extension of a marine transgression coming from the north. Tatman and Whatley (1996) demonstrate on the basis of its ostracod fauna that the Calcareous Member shows no trace of marine influence and that it was deposited in non-marine conditions. The ostraeod assemblage recovered from the Ostraeod member is of low incidence and diversity and comprises genera that occur in "Wealden" type environments worldwide, especially in Europe. This is despite the fact that European "Wealden" faunas, being Neocomian in age are rather older. The three major elements of post Middle Jurassic non-marine ostraeod faunas, the Darwinulacea, Limnoeytheridae and Cypridacea (Whatley, 1990, 1992) are all present. The former represented by the genus D a r w i n u la, and the latter by C y p r i d e a while the cytheracean Limnocytheridae are dondnated by L i m n o c y t h e r e and T h e r i o s y n o e c u m (Loranger, 1951 ; Finger, 1983 ; Tatman and Whatley, 1996).
TATMAN and WHATLEY
BRITISH COLUMBIA
95
North Western Utilities Limited Legal #1 O ALBERTA , Edmonton
°
,~ 0
6
100 miles
160
200km
Imperial Home Coal Lake Fall Creek ,~ -oz\ Test Hole 7722 • Calgary
Alberta CANADA
FIG. ]. Location map of central and southern Alberta (area shaded on insert), showinglocations of cores from which Acantharhinocythere has been recovered. The Rocky Mountain front is shown as in the Alberta/British Columbm boundary. The regional geologyis intentionally omitted, since it is too complex to depict meaningfullyon a map of this scale, and because the principal extent of the Ostracod Member of the Blairmore Formation is in the subsurface. - -
Carte sch6matique de l'Alberta central et m&idional (aire ombrde sar le carton de gauche) sur htquelle sont indiqu6s les emplacements des sondages qui ont fourni des Acantharhiaocythere. Le froat des Rocheuses est cunstd6r6 co~ncider avec la f r o a t ~ r e entre l'Alberta et la Colonic britannique. Les caract&es g6ologiques sont intentionnellement omi*, car ils seraient trop complexes pour ~tre report6s sur la carte ~t cette 6cheUe et attssi ?t cause de l'extension en profondeur du Membre h Ostracodes de ht formation Blairmore.
THE
AGE OF THE
DEPOSIT
The dating of these deposits is difficult since they c o n t a i n few index fossils. F i n g e r (1983) suggested that because the Ostracod Zone is obviously facies controlled it is possibly d i a c h r o u o u s across the A p t i a n / A l b i a n b o u n d a r y . Because t h e r m a l a l t e r a t i o n is often high, p a l y n o m o r p h s are poorly p r e s e r v e d , being very highly oxidized. Also, m a n y of those recovered are long r a n g i n g and, c o n s e q u e n t l y , of little stratigraphical value. However, B a n e r j e e a n d Davies (1988) p u t f o r w a r d a z o n a t i o n scheme based on dinoflagellates. Using their system, it can be shown t h a t the type locality o f A c a n t h a r h i n o c y t h e r e w a l l i , i n the Fall River borehole core corresponds to a C3/C4. late A p t i a n / e a r l y A l b i a n age.
METHODOLOGY
T h a t we have been able to add to this f a u n a the delicately preserved new genus, is due to the fact that it has b e e n preserved i n the b u l k f l u o r i d a t i o n recovery t e c h n i q u e which we have employed. This techn i q u e is the subject of a later p u b l i c a t i o n ( T a t n m n , in prep.). E a r l i e r techniques to recover ostracods from these intensely h a r d shales involved mechanical c r u s h i n g a n d s u b s e q u e n t t r e a t m e n t with very corrosive chemicals, such as sodium hypochlorite a n d hydrogen peroxide. While these techniques recovered the more r o b u s t ostracods, a n d even some of their late stage i n s t a r s , y o u n g e r juveniles a n d delicate, thin-shelled taxa such as A c a n t h a r h i n o c y t h e r e were destroyed.
96
OSTRACOI)A OF T H E CRETACEOUS OF CANADA
Description : As t'or type species.
SYSTEMATIC DESCRIPTIONS
Remarks : On the basis of its overall morphology, its The type material used in this p a p e r is housed in the collections of the Department of Palaeontolog~y, Natural History Museum (British Museum, Natural History), London, to which the catalogue numbers prefixed OS refer. An additional suite of samples is housed in the Geological Survey of Canada collections in Canada. All dimensions are given in millimetres and the following conventions are employed: RV = right valve, LV = left valve, C = articulated carapace, A = adult, juv. = juvenile. The following descriptive size scale is used : 0.40 very small, 0.40-0.50 small, 0.50-0.70 medium, 0.70-1.00 large, >1.00 very large. Phylum CRUSTACEA P E N N A N T , 1777 Class OSTRACODA L A T R I E L L E , 1806 Order PODOCOP1DA MOLLER, 1894 S u b o r d e r P O D O C O P I N A SARS, 1866 S u p e r f a m i l y CYTHERACEA B A I R D , 1 8 5 0 Family LIMNOCYTHERIDAE K L I E , 1938
Subfamily LIMNOCYTHERINAEKLIE, 1938 A c a n t h a r h i n o c y t h e r e gen. nov.
Type species : Acantharhinocythere walli gen. et sp. nov.
Derivatio nominis : Gr. ¢x~ctvOct thorn, spine + p t vo~ nose. "spiny nose" + cythere. With reference to the spinose anterior projection which characterizes this genus.
Diagnosis : A medium to large, elongate subovate to subrhomboidal, thin-shelled and bisulcate genus of Limnocytheridae. Anterior margin angular, with rounded minutely denticulate antero-dorsal slope and straight to slightly convex, denticulate antero-ventral slope above and below respectively a delicate hemispherical to subquadrate spinose protuberence. Shallowly bisulcatc anterior of mid-length. A backward directed spine occurs posteto-ventrally.
PLATE
hingement and musculature and its association with well-known non-marine Ostracoda anti other organisms, this genus is placed within the non-marine cytherid family, the Linmocytheridae. Within that family, on the strength of its carapace morphology and secondary sexual characteristics, it is placed within the subfamily Limnocytherinae rather than the Timiriaseviinac. It differs from all know limnocytherid genera in shape and outline and internal details and particularly in its anterior spinose protuberence. There is some similarity in general shape with the limnocytherid genus Pampacythere W H A T L E Y and CBOLtCH (1974) from the Quaternary (and subsequently the Recent) of Argentina. However, Pampacythere lacks the anterior protuberence, and has higher and more pronounced cardinal angles, although its musculature is virtually identical to that of Acantharhinocythere. Forester (1991) illustrated a number of unusual limnocytherids that he referred to as "exotic" forms, a term he employed to indicate non-marine species exhibiting characteristics normally associated with marine taxa. While none of the species he illustrated arc at all similar to Acantharhinocythere, Forester's study illustrates the propensity for limnocytherids to evolve unusual morphologies. A recent revision of the origins and early evolution of the Limnocytheridae (Whatley and Moguilevsky, 1998) further emphasizes this. Other unrelated ostracod taxa with prominent anterior structures, such as Aratrocypris WIIATLEY et al. (1985) and Danipussella WOUTERS (1988) are marine and belong to the Cypridacea and Bairdiacea respectively. Szczechura (1978) describes a nunnber of species belonging to d i f ferent genera from non-marine Upper Cretaceous strata in the Nemegt Basin of the Gobi Desert, with curious anterior protuberances. All of these, however, are Cypridacea and the "lip-like extensions", as Szczechura (1978) describes them, are quite unlike the anterior protuberence of Acantharhinocythere. The candoniid Iberacypris B A B I N O T , ( e x Iberocypris Babinot, 1985), from the non-marine Maastrichtian of Central Spain has developed horn-like extensions
1
(SP imhcates tile use of Stereo-pairs)
(SP pour stdr6o-paire) l-6. Acantharhinocythere tvalli gen. et sp. nov. 1 : RV, holotype, external lateral view, 1 a-b (SP). (OS 14023), × 103. 2 : LV, paratype, internal view, 2 a-b (SP). (OS 14024), x 114.
3 4 5 6
: : : :
RV, LV, LV, RV,
holotype, paratype, paratypc, paratype,
internal view, (OS 14023), × 104. external latcral view (OS 14024), x 114. external lateral view, (OS 14026), x 102. external lateral view, (OS 14025), x 100.
TATMAN
and WHATLEY
PLATE
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98
OSTRACODA OF THE CRETACEOUS OFCANADA
at the cardinal angles of the LV. The functional significance of these structures remains an enigma (Babinot, lit. com. July 2000)
Acantharhinocythere is presently monotypic and is very restricted in both its stratigraphical and its geographical occurrence. It occurs in relatively few samples from only three localities. Although in one sample (sample 91252, front core 7722) A. walli makes up 17 % of the total ostracod fauna, it is normally much less significant, comprising a mean of less than 1 % in other samples where it occurs. A c a n t h a r h i n o c y t h e r e walli sp. nov. (PI. 1, fig. 1-6; P1. 2, fig. 2-8)
Derivatio nominis : Named in honour of Dr. John Wall of the Geological Survey of Canada, in recognition of his signal contribution to our understanding of Canadian biostratigraphy.
Holotype : RV OS 14023. Paratypes : Four specimens (OS 14024 - OS 14027). Material : Forty-five specimens : 41 adults, 4 juvenilCS.
Type locality: Fall Creek Test borehole, Alberta, Canada. Grid reference 22-37-11W5. Type level : Core 7722, Geological Survey of Canada sample reference n u m b e r BBA91252, depth 298m. Aptian/Albian dark grey shales from the Calcareous (Ostracod) member.
Diagnosis : As for genus. Description : Medium to large. Thin-shelled. Elongate subovate to subrhomboidal in lateral view, but lateral outline interrupted by postero-ventral spine and mid-anterior spinose protuberence. Elongate subhastate in dorsal view, with greatest width behind mid-length ; the anterior protuberence is directed laterally at an angle of approximately 15 degrees to the mid axis commisure. Anterior margin angular, sloping downwards from the anterior cardinal angle to about one quarter of the distance from the dorsal margin. At this point, the margin changes from being gently convex to almost straight and almost perpendicular, extending to approximately mid-height, from where the margin is extended anteriorly to form the semicircular protuberence. The antero-ventral slope is slightly convex and unites witi~ the ventral margin at almost ]/3 length from the anterior. The anterior margin is denticulate with usually three small triangular denticles above the protuberence, four around its periphery and two below it. Posterior margin asymmetrically rounded with long convex postero-dorsal slope, apex below mid-height and short, almost straight postero-ventral slope. Dorsal margin straight to very slightly convex. Ventral m a r gin biconvex about a marked oral incurvature and with almost keel-like antero and postcro-ventral parts. Shallowly bisulcate. A short oblique sulcus extends from just behind the anterior cardinal angle antero-ventrally to about mid-height ; a longer, vertical sulcus just anterior of mid-length extends from the dorsal margin to a ventro-lateral position. Orna-
PLATE 2 (SP indicates the use of Stereo-Pairs) 1-7. A c a n t h a r h i n o c y t h e r e tvalli gen. et sp. nov. 1 : LV, lmratype, internal view (OS 14027), x 98. 2 : RV, external lateral view of fragmentary specimen showing well developed postero-ventral spine. (Specimen subsequently destroyed on removal from SEM stub). × 111. 3 : 3 a b (SP), RV, internal v,ew showing lophodont hinge w~th terminal sockets and median groove, (OS 14025), x 190. 4 : 4 a-b (SP), LV, internal view showing lophodont hinge with terminal sockets and denticulate median bar. (Specimen subsequently destroyed on removal from SEM stub). x 190. 5 : 5 a-b (SP), RV, paratype, internal view showing detail of anterior protuberance (OS 14024), x 348. 6 : 6 a-b (SP, RV, holotype, external view sbowmg detail of an~ terior protuberance (OS 14023), x 348. 7 : LV, paratype, internal view showing detail of muscle scars (OS 14024), x 534. 8 : RV, paratype, obhque internal view slmwing anterior pro~ tubcrance and extcnsion of inner lalnella (OS 14024). x 425.
(SP pour st6r6o-paire) 1 : V . G . , p a r a t y p e , r u e i n t e r n e (OS 1"1027), x 98. 2 : V . D . , v u e latdrale e x t e r n e d ' u n sp6cimen f r a g m e n t 6 en c o u r s de m a n i p u l a t i o n et m o n t r a n t une 6~pine post6r o - v e n t r a l e bien d6veloppde, x l 11. 3 : 3 a-b ( S P ) . V . D . , r u e i n t e r n e m o n t r a n t uue c h a r n i ~ r e lop h o d o n t e u v e c alv6oles t e r m t n a l e s et gouti~re m 6 d i u n e ( O S 14025), x 190. 4 : 4 a-b ( S P ) , V . G . , r u e i n t e r n e m o n t r t t n t une c h a r n t $ r e lop h o d o u t e a v e c oh,doles t e r m i n a l e s et b a r r e m d d i a n e dentieulge ( s p d c i m e n cl~truit po.stdrieurement h la p r i s e tie vue), x 190. 5 : 5 a-b ( S P ) , ll.D., l m r n t y p e , vue interne a v e c ddtail de In p r o t u b e r a n c e antdrieure ( O S 14024), x 348. 6 : 6 a-b ( S P , V . D . , holotype, vue e x t e r u e avec d6tail de la p r o t u b d r a u c e ont~rieure ( O S 14023), x 348. 7 : V . G . , p a r a t y p e , r u e i n t e r u e avec ddtail des e m p r e i u t e s m u s c u l a i r e s (OS 14024). x 534. 8 : V . D . , p a r a t y p e , r u e i n t e r n e oblique m o u t r a u t la p r o t u b 6 r u n c e ant~rieure et l'exteusion de la lamelle i n t e r n e ( O S 14024), x 425.
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OSTRACODAOFTHECRETACEOUSOFCANADA
ment feebly reticulo-punctate but weakest between the two sulci. Most of the valve is densely punctate but in places is coarser and virtually reticulate. On the compressed anterior marginal area the puncta are smaller and arranged in three double rows, forruing a delicate reticulation, with dominant muri parallel to the margin and weaker ones perpendicular to it. A variably developed, backward-directed spine arises postero-ventrally. The adductor muscle scars are manifested in external view as a group of four elongate ridges at mid-height in the median sulcus. Inner lamella broadest at the end margins ; it forms a small "flange" extending around the outer margin of the anterior protuberence and extending a short distance along the anterior margin on either side. Normal pore canals large, circular to ovate and sieve type. In internal view they are set in pits between 20 and 30 gin in diameter. The adductor scars are straight and equidistant. Frontal scar heart-shaped and anterior of the dorsal adductor, ventral to which one, or possibly two mandibular scars occur. Hinge modified lophodont with smooth terminal sockets but denticulate median element in LV.
Dimensions : Holotype : L = 0.699, H = 0.350 ; paratype : LV OS 14024 H = 0.289; paratype RV OS 14025 L = 0.716, H = 0.324 ; paratype : LV OS 14026 L = 0.700, H = 0.335; paratype LV OS 14027 H = 0.32. All paratypes are from the type locality and level.
Remarks : All the occurrences of Acantharhinocythere walli were in ostracod assemblages dominated by Cypridea tilleyi Loranger (1951), Theriosynoecttm spp. and other limnocytherids. Although there has been some reassessment of the palaeoecology of these genera in respect of the British Purbeck-Wealden (Anderson, 1985), there is little doubt that E. walli was entirely freshwater or low oligohaline in habit, particularly since it is found closely associated with the gastropod Viviparus (Paludina). The occurrence of Cytheridea bonacordensis Loranger and Orthonotacythere (?) sp. of McLean and Wall (1981) in a sample from Elbow River has been interpreted as indicating a brackish water environment (Wall in Taylor and Walker, 1984); an interpretation with which we agree, provided that the material was in situ and not derived. However, Acantharhinocythere walli does not occur at this locality and has never been encountered in association with either of the species in question.
Age and distribution : This species has only been recorded from the Aptian/Albian Calcareous Member from sites in the Province of Alberta. It has been recovered from the following localities : Fall Creek test
Hole (type locality) Grid reference 22-27-11-W5. Imperial Home Coal Lake, 15-21-47-23-W4. North Western Utilities Limited legal, 20- 58- 24- W4.
DISCUSSION OF DISTRIBUTION During the deposition of the Calcareous Member, the Alberta Foreland Basin was bounded to the west by the Rocky Mountains, to the east by the Canadian Shield and to the north by the Clearwater Sea with its open marine conditions. To the south, continental conditions prevailed and from this area, rivers flowed northwards into the basin (McLean and Wall, 1981; Taylor and Walker, 1984; Hayes, 1986). Most of the non-marine ostracod genera, which were present in the basin during the deposition of the Calcareous Member, are pandemic in Lower Cretaceous Wealden-type environments. Cypridea, its allies. and to a certain extent other contemporary cyprids, were particularly well suited to these conditions as witnessed by their virtually ubiquitous diverse and abundant occurrence in such environments worldwide. This was achieved largely by their possession of desiccation resistant eggs and ability to reproduce parthenogenetically (Whatley, 1990, 1992). Limnocytherids are almost entirely syngamic and do not possess encystable eggs, which is a limiting factor in their dispersal relative to the cyprids. Also, unlike the cyprids, no cytherid is able to swim and this further restricted the ability of Acantharhinocythere to colonize new localities. These constraints may have contributed to the evolution of such an endemic taxon in this isolated basin, although this may be an artifact due to the very fragile nature of the species. Precedents, however, of bizarre species of cytheracean ostracods evolving in restricted basins are legion. An example is the endemic limnocytherid genus Skopaeocythere (Whatley et al., in press) confined to the restricted saline lake of the Upper Amazon Basin in the Neogene, which also produced an entirely endemic species flock of Cyprideis JONES (Whatley et al., 1998 ; Mufioz-Torres et al., 1998).
MODE OF LIFE It is difficuh to understand what advantage its anterior spinose protuberence bestowed on Acantharhinocythere walli. However, such a structure must have had an important function for it to have evolved. In some cases where the anterior margins of various ostracod taxa are modified, we understand
TATMAN and WHATLEY their purpose. For example, the r o s t r u m and rostral incisure of c y p r i d i n i d myodocopids serve the i m p o r tant function of facilitating egress of the 2 "d ant e n n a e , thereby improving their swimming ability. The antero-ventral beak of Cypridea and its allies is thought to be an integral part of their feeding strategy. Whatley et al. (1985) suggested a number of possible modes of life for the deep-sea genus Aratrocypris, so named for its large plough-like antero-ventral projection. This structure, which is very strongly calcified, probably evolved to allow the animal to plough through the soft oozes of the deep-sea in search of its food. However, in Acantharhinocythere walli, a m u c h thinner-shelled ostracod, this is an unlikely mode of life a n d its a n t e r i o r protuberence is too delicate and is situated too far from the oral appendages to be directly associated with alimentation. However, it could possible have served a secondary alimentary function by moving aside a soft, soupy sediment to give better access to the mouthparts. Because the Limnocytheridae have none of the necessary appendage modifications associated with filter feeding, it could not have been a funnel to channel incurrcnt flow into the carapace. More probably, therefore, since the marginal denticles associated with the protuberence and along the anterior margin immediately above and below it would have borne long setae, it represented a device for enhancing sensory perception anteriorly. An alternative but less probable suggestion is that the structure may have provided protection for the 2 nd antennae at the point of egress from the carapace, ahhough why this should be an advantage to a non-swimming ostracod is not immediately apparent, unless it be associated with balance or possibly the detection of food particles or prey. To extend this argument further is to enter the realm of speculation. Suffice to say that this structure, unique among Ostracoda, is likely to remain enigmatic until the discovery of additional, better-preserved material and even then, it will be difficult to determine its fnnction.
ACKNOWLEDGEMENTS SJT gratefully acknowledges his NERC studentship that allowed him to visit Canada in order to undertake this study. Drs J. Wall and 1. Bannerjee are thanked for all their help in Canada and the invaluable technical assistance of Mrs. L. Morrison in Aberystwyth is gratefully acknowledged. The authors thank the referees for their thoughtful review of the paper and to the one whose identity we know, Dr. B a b i n o t , we extend our gratitude for his useful comments on Iberacypris.
101
BIBLIOGRAPHY
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WHATLEY I{.C. and T. del C. CHOLICH (1974) : A new Quaternary ostracod genus from Argentina. Palaeontology, vol. 17 (3), p. 669-684, 2 pl. 1 fig. WHATLEY R.C., AYRESS M., DOWNING S., HARLOW C. and KESLER K. (1985) : Aratrocypris, an enigmatic new cyprid ostracod from the Tertiary of D.S.D.P. sites in the S. W. Pacific. Journ. Micropal., vol. 4, n ° 1, p. 69-79. WHATLEY B.C. and ]VIOGUILEVSKYA. (1998) : The origin and early evolution of the Limnocytheridae (Crustacea, Ostracoda). In : Crasquin-Soleau, S. et al. (Eds). What about Ostracoda !, Proc. 3rd. European Symposium on Ostracoda, Bierville, France, July 1996, EIfEp, Edition~ Pau, M6m. 20, p. 271-288.
WHATLEY R.C., MU~OZ-TORRES F and VAN HARTEN D. (1998) : The Ostracoda of an isolated Neogenc saline lake in the Western Amazon Basin. In CrasquimSoleau S., Braccini E. and Lethiers F. (Eds), What about Ostracoda ! Proc. 3 rd. European Symposium on Ostracoda, Bierville, France, July 1996. E1]'Ep, Editions Pau, M6m. 20, Pau, p. 231-248. WHATLEY R.C., MU~oz-TORRES F. and VAN HARTEN D. (In press) : Skopaeocythere : A minute new limnocytherid (Crustacea, Ostracoda) from the Neogene of the Amazon Basin.
Ameghiniana.