The Lower Jaws of Baenid Turtles

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AMERICAN MUSEUM

Novitates PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, N.Y. 10024 Number 2749, pp. 1-10, figs. 1-4, table 1 September 27, 1982 The Lower Jaws of Baenid Turtles

EUGENE S. GAFFNEY'

ABSTRACT The baenid lowerjaw has a well-developed pro- Chisternon and Baena are the only baenids defi- cessus coronoideus, no ridges or pits on the trit- nitely known to lack splenials; the anteroventro- urating surface, a relatively small dorsal opening medial wall of the fossa meckelii is open in these of the fossa meckelii, and, except in Chisternon genera. An associated skull and jaws of Plesio- and Baena, a large splenial bone. None of these baena putorius show that previous identification features is unique to baenids nor is the combi- ofjaws with expanded triturating surfaces as per- nation unique. Distinctly expanded triturating sur- taining to this species was in error, the two known faces are found in Eubaena and Palatobaena. species of this genus have narrow jaws.

INTRODUCTION Previous work on baenids includes Gaff- ABBREVIATIONS ney (1972), a review of the systematics with INSTITUTIONAL literature references up to that date; Gaffney AMNH, American Museum of Natural History (1 975a) discusses relationships of baenids to NMC, National Museum of Canada other turtles; Archibald and Hutchison (1979) PU, Princeton University describe new material of Palatobaena; and ROM, Royal Ontario Museum Gaffney (in press) deals with the cranial mor- UCM, University of Colorado Museum phology ofbaenids. An introduction to turtle UCMP, University of California, Museum of Pa- lower jaw morphology can be found in Gaff- leontology ney (1979), and baenid lowerjaws have been UMMP, University of Michigan, Museum of Pa- described and in and Ar- leontology figured Hay (1908) USNM, National Museum of Natural History, chibald and Hutchison (1979). Smithsonian Institution Figures 1 and 2 are the work of Ms. Lor- UW, University of Wyoming raine Meeker, and I thank her and Mr. Ches- ter Tarka for improving the quality of all the ANATOMICAL figures in this paper. I am grateful to Dr. ang, angular pra, prearticular David Archibald, Yale University, for allow- art, articular sp, splenial ing me to use new baenid material collected cor, coronoid sur, surangular as part of his thesis work. den, dentary

' Curator, Department of Vertebrate Paleontology, American Museum of Natural History.

cor ,sur

imeckeii pra

/oramendentofacialemajus surangI foramen dentofaciale majus sur ang

area articularis mandibularis

foramen intermandibularis caudalis FIG. 1. Palatobaena bairdi (AMNH 8277), Hell Creek Formation, late Cretaceous, Bug Creek Ant- hills, McCone County, Montana. Upper, dorsal view; middle, lateral view of left ramus; bottom, medial view of right ramus. Note partially healed fracture near symphysis. Length of right ramus (measured parallel to axis of ramus), 56.7 mm.

DESCRIPTION baenids, nor is the combination unique being Baenid lower jaws (figs. 1 and 2) typically found in Plesiochelys (Gaffney, 1976). As far have a relatively large splenial (except in as I can determine, the only way to identify Baena and Chisternon), a well-developed a baenid lower jaw is by using features di- processus coronoideus, an absence of ridges agnostic for a particular genus, and even this or pits on the triturating surface, and a rel- might be difficult for Plesiobaena. atively small dorsal opening of the fossa DENTARY: The dentaries of baenids are meckelii. None of these features is unique to comparable to the dentaries of other cryp- 1 982 GAFFNEY: BAENID TURTLES 3

, den

fi fossa meckelii sur pra -

a foramen posterius chorda tympani FIG. 2. Eubaena cephalica (AMNH 2604), Hell Creek Formation, late Cretaceous, 26 miles south of Lismas, Montana. Length of left ramus (measured parallel to axis of ramus), 64.7 mm. todires. There is a broad medial symphysis ally as it reaches the symphysis, narrowing with a variably developed dorsal projection the triturating surface, whereas in Plesioche- or hook, best seen in Baena and Plesiobaena. lys the lingual ridge trends more medially, No trace of a symphyseal suture has been widening the triturating surface. The coro- observed. noid process ofPlesiochelys is slightly higher The triturating surface of Plesiobaena an- and the entire jaw ramus is a bit more mas- tiqua (fig. 3) appears to represent the primi- sive than in Plesiobaena but these are rela- tive condition compared with the lower jaws tively minor features. of other baenids. Plesiobaena antiqua has a The discovery of an associated skull and narrow triturating surface with the labial and jaws of Plesiobaena putorius (UW 3348, see lingual ridges approximately parallel to one notes on Plesiobaena putorius below) re- another as in Baena and Chisternon. The two moves the earlier presumed disparity in ridges are slightly raised, forming a shallow lower jaw morphology between Plesiobaena trough between them. The labial ridge ex- antiqua and Plesiobaena putorius. The lower tends dorsally in the anterior section of the jaw ofPlesiobaena putorius closely resembles jaw to form a well-developed symphyseal that of P. antiqua, differing only in a slight "hook." The triturating surfaces, as in other medial expansion of the triturating surface. baenid genera, closely approximate the max- Stygiochelys (fig. 3) is now known from illary triturating surfaces, indicating that in associated skulls and jaws, one of which life the horny covering ofthe upper and lower (UCMP 113316) is particularly well pre- jaws closely followed the bony morphology. served with the jaws removed from the skull. A comparison ofPlesiobaena antiqua with These jaws are quite similar to Plesiobaena Plesiochelys (Gaffney, 1976) shows a close antiqua, and if the dorsal surface of the jaws similarity between the two forms. The lingual in Plesiobaena putorius were available there ridge of Plesiobaena, however, trends later- might be some difficulty differentiating them. 4 AMERICAN MUSEUM NOVITATES NO. 2749

Although symphyseal breakage and some meet at a more obtuse angle than in Plesio- bone loss prevent a completely confident baena and Chisternon. Also, Chisternon has restoration it appears that the rami met at a a symphyseal development ofthe labial ridge slightly more obtuse angle than in Plesio- that approaches the symphyseal hook ofPle- baena but less obtuse than in Palatobaena. siobaena. Fully adult specimens of Baena The labial ridge in Stygiochelys is the same have an upturned symphyseal "hook" that as in Plesiobaena antiqua but the lingual differs from Chisternon and Plesiobaena in ridge runs more medially; however, this dif- the presence ofa low midline ridge extending ference is not great and may be obscured by across the triturating surface. age or individual variation. Both labial and Age variation in Baena arenosa apparently lingual ridges of Stygiochelys are slightly involves the mandible. The lower jaws of a more pronounced than in Plesiobaena anti- presumed juvenile (AMNH 5977, see Gaff- qua but not to the extent seen in Baena. ney, 1972, for a discussion of possible age Eubaena (fig. 2) has one of the more dis- variation in the skull) differs from the lower tinctive feeding surfaces, with an incipient jaws of what is here concluded to be older secondary palate in the skull. The lower jaw individuals of the same species. These dif- reflects this condition and has characteristic ferences involve the triturating surface and, lingual ridges that curve medially and dor- in general, suggest that juveniles of Baena sally to match the upper jaws. A shallow have certain similarities to Chisternon and trough parallels the lower labial ridge. The Plesiobaena. That is, the juvenile jaw resem- lingual ridges do not meet in the symphysis, bles Chisternon in lacking the more massive instead they become parallel and form a proportions of adult Baena (AMNH 5971) trough along the symphysis. and in having the jaw rami meet at a more Lower jaws with associated skulls of Pal- acute angle as in Chisternon. atobaena bairdi were announced by Archi- Most ofthe contacts ofthe dentary are the bald and Hutchison (1979) substantiating the same in known baenid mandibles as well as identification of Gaffney (1979, fig. 161; in most cryptodires. Posterodorsomedially AMNH 8277) of an isolated but very well- there is a deep suture with the coronoid. Pos- preserved specimen. Palatobaena (figs. 1, 3) teroventromedially a long squamous suture has a very distinctive lower jaw triturating with the angular extends about halfthe length surface morphology unique among baenids of the dentary. Just below the sulcus carti- (assuming my re-identification of jaws pre- laginis meckelii and above the suture with viously ascribed to Plesiobaena putorius is the angular is a fairly restricted suture with correct). The triturating surface is triangular the splenial in Palatobaena and Plesiobaena. with labial and lingual ridges close together In Eubaena and Stygiochelys a splenial seems anteriorly and relatively far apart posteriorly. to be present but none ofthe specimens show The triturating surface is relatively flat with its contacts clearly. In Baena and Chisternon low lingual and labial ridges but the surface the angular extends dorsally to the ventral does not lie horizontal, rather it is tilted to edge of the sulcus cartilaginis meckelii. face anterolaterally. The rami are relatively As in most other turtles, baenids (figs. 1, thick and massive, particularly near the well- 4) have a foramen dentofaciale majus (see developed processus coronoideus. Gaffney, 1979, figs. 111, 112) on the lateral The triturating surfaces of Baena and Chi- surface of the dentary near the margin of the sternon (fig. 3) are most similar to those of rugose bone surface, marking the attachment Plesiobaena. The labial and lingual ridges are of the horny rhamphotheca. On the medial parallel to each other and expanded surfaces surface, toward the posterior end ofthe sulcus are not developed; a shallow trough is formed cartilaginis meckelii is the foramen alveolare between the ridges. The trough is deeper in inferius. This foramen leads into the canalis Baena than in Chisternon, whereas the latter alveolaris inferior. These nutritive foramina has a shallow trough just anterior to the pro- and canal can be seen in a number of broken cessus coronoideus. The jaw rami in Baena baenid jaws but I have not been able to dis- 0) C)

i E aE 6 AMERICAN MUSEUM NOVITATES NO. 2749

-WA

FIG. 4. Internal views ofbaenid lowerjaws. A, Plesiobaena antiqua (UCMP 49759), Lance Formation, late Cretaceous, Wyoming. Length, 49.3 mm. B, Eubaena cephalica (AMNH 2606), Hell Creek For- mation, late Cretaceous, Montana. Length, 57.3 mm. C, Baena arenosa (AMNH 2276), Bridger For- mation, Eocene, Wyoming. Length, 43.5 mm. D, Chisternon undatum (USNM 12839), Bridger For- mation, Eocene, Wyoming. Length of left ramus, 72.8 mm. cern any systematic variation either within is also formed by the articular and prear- baenids or between baenids and eucrypto- ticular. dires. SPLENIAL: Large and well-preserved sple- ARTICULAR: The articular (fig. 1) occupies nials (figs. 1, 3) can be seen in Plesiobaena a position between the prearticular and sur- (ROM 674, UCMP 49759) and Palatobaena angular bones, with the angular attaching (AMNH 8277, PU 17108). The splenial in ventrally. The morphology of the bone in these baenids is quite similar to splenials de- baenids is very similar to the articular of scribed for Solnhofia (Parsons and Williams, other cryptodires. It forms the medial two- 1961; Gaffney, 1975b) and Plesiochelys thirds of the area articularis mandibularis (Gaffney, 1976). It is a flat, roughly equidi- with the surangular forming the lateral third. mensional bone forming the medial margin The dorsal surface of the articular is gently of the foramen intermandibularis medius concave. The articular extends posteriorly to and the anteromedial wall ofthe fossa meck- form a retroarticular process for the attach- elii. Fragments of a narrow splenial appear ment of the M. depressor mandibulae. This to be present in Eubaena (AMNH 2602). As process is largest in Eubaena and is small in I interpret the morphology in this specimen, Chisternon, Baena, and Plesiobaena. The the splenial in Eubaena would differ from dorsal surface of the retroarticular process in that bone in Palatobaena and Plesiobaena in the last forms is oriented in a more vertical being lower, having a more extensive coro- plane than in Eubaena. noid contact, and a less extensive prearticular The foramen posterius chorda tympani is contact. In the one useful jaw of Stygiochelys usually in the posteromedial portion of the (UCMP 113316) the splenial was probably articular in the base ofthe retroarticular pro- present, based on the other elements, but the cess, just behind the area articularis mandib- area is so damaged that this is not directly ularis. The canalis chorda tympani mandib- determinable. Baena and Chisternon lack a ularis seems to be formed between the splenial (fig. 4). articular and prearticular. The chorda tym- CORONOID: The coronoid of baenids (fig. pani nerve enters the fossa meckelii through 1) is very similar in shape and position to the foramen anterior chorda tympani which that bone in most eucryptodires. All baenids 1982 GAFFNEY: BAENID TURTLES 7 have a distinct processus coronoideus which prearticular forms the medial edge ofthe dor- is best developed in Baena (subject to indi- sal opening of the fossa meckelii. Just ante- vidual variation, however), and least devel- rior to this opening the prearticular lies lateral oped in Plesiobaena. As in other cryptodires, to the coronoid bone. In Plesiobaena, Eu- there is an anteromedial process of the cor- baena, and Palatobaena the prearticular has onoid that extends anteriorly along the me- a broad anterior contact with the splenial. In dial surface ofthe jaw ramus and forms part Baena and Chisternon the splenial is absent of the lingual ridge of the triturating surface. (fig. 4) but in contrast to most eucryptodires In Plesiobaena, Eubaena, and Palatobaena which lack a splenial (see Gaffney, 1979, fig. the ventral margin of this anterior process 112), the prearticular in Baena and Chister- contacts the splenial but in Baena and Chi- non does not extend anteriorly to occupy the sternon it contacts the dentary (fig. 4). area of the absent splenial. Instead the an- ANGULAR: The angular of baenids (fig. 1) teroventral half of the fossa meckelii is open has the same basic relations as the angular medially in these two baenids. The preartic- of other cryptodires; it covers the postero- ular of these two genera forms the postero- ventral portion of the jaw curving antero- dorsal margin of this opening. The opening medially from the posterolateral jaw margin. extends to and includes the foramen inter- In Plesiobaena (ROM 674, UCMP 49759), mandibularis caudalis, which is therefore ab- Eubaena (AMNH 2606) and Palatobaena sent in Baena and Chisternon as a discrete (AMNH 8277, PU 17108) the anteromedial foramen. portion has a broad dorsal contact with the The prearticular of Baena and Chisternon splenial. In Stygiochelys (UCMP 113316) the has the same relation to the coronoid as the presence of a splenial is likely but not defi- other baenids but the prearticular does have nite, however, the angular appears to be the a well-developed anterior contact with the same as in the preceding three genera in any dentary above the fossa meckelii in Baena case. In Baena (AMNH 2276, 3808, 5971) and Chisternon, which is absent in the other and Chisternon (USNM 12839, AMNH baenids presumably because they have a 5962) the splenial is absent and the angular splenial which fits in this position. forms the ventral edge of the fossa meckelii SURANGULAR: In most eucryptodires the which is now open medially due to the ab- surangular is restricted to the posterodorsal sence of the splenial (fig. 4). The angular in margin of the lower jaw and most of the lat- these two forms extends farther dorsally on eral surface is covered by the dentary. Bae- the medial surface and expands into the re- nids, however, still have the primitive con- gion occupied by the splenial in the other dition ofa large surangular covering relatively baenids. more of the lateral surface of the lower jaw The foramen intermandibularis caudalis (fig. 1). The dentary-surangular suture varies is formed in the suture between the angular somewhat among the baenid genera from and prearticular in Plesiobaena, Eubaena, roughly vertical in Palatobaena to a Z-shape and Palatobaena (it is indeterminate in Sty- in Chisternon, Eubaena, Baena and most of giochelys) with the splenial entering or nearly the baenid genera. The surangular ofbaenids entering the anterior margin of the foramen forms the lateral portion of the area articu- in some specimens. I have not been able to laris mandibularis, as in other cryptodires. determine whether the splenial contribution Archibald and Hutchison (1979, fig. 4) to the foramen is of systematic significance, have argued that Palatobaena bairdi differs however, on the basis ofthe material at hand from Plesiobaena putorius in the presence in it appears to be an individual variation. In the former of a small fossa on the surangular Plesiobaena the foramen is positioned more just posterior to the large adductor fossa ven- anteriorly than in Eubaena and Palatobaena. tral to the processus coronoideus. Below, I PREARTICULAR: As in other cryptodires the argue that the lower jaw (PU 17108) was prearticular ofbaenids (fig. 1) covers the pos- identified as Plesiobaena putorius based on teromedial part of the jaw anterior to the ar- my error in 1972 in fitting these jaws to a ticular, dorsal to the angular, and lateral to skull of Plesiobaena putorius. The discovery the fossa meckelii. The dorsal margin of the ofan associated skull andjaws ofPlesiobaena 8 AMERICAN MUSEUM NOVITATES NO. 2749

TABLE 1 Comparisons of Baenid Lower Jaws Plesiobaena Eubaena Stygiochelys Palatobaena Chisternon Baena Triturating surface Narrow, with Expanded, Slightly Broadly Narrow, with Narrow, with parallel lingual expanded expanded, parallel parallel margins ridge curves triangular margins, margins medially surface trough anterior to coronoid process Splenial Large Small Probably Large Absent Absent present Fossa meckelii with Yes Yes Yes Yes No No anteroventral wall on medial side putorius (UW 3348), however, shows that the putorius (UW 3348, Tiffanian, Wyoming) jaws ofPlesiobaena putorius are very similar shows that the lower jaws I originally iden- to thejaws ofPlesiobaena antiqua. I therefore tified as Plesiobaena putorius (PU 17108) do identify PU 17108 as Palatobaena bairdi be- not belong to this species. The new specimen cause ofthe very close similarity between PU is readily identified as Plesiobaena putorius 17108 and jaws such as AMNH 8277 and because it has the characteristic semicircular 2603. The absence ofa small surangular fossa expansion of the skull roof overlying the in PU 17108 I consider as taxonomically in- crista supraoccipitalis. The degree of tem- significant at present. poral emargination is less than that seen in PU 20600 and PU 14984 (Gaffney, 1972, NOTES ON PLESIOBAENA PUTORIUS figs. 14, 17). The lower jaws have not been removed from the skull due to the extremely In 1972 I named this Paleocene species on hard matrix the basis of a and tight fit but enough of the nearly complete but crushed jaws can be seen to show that they are very skull (PU 14984, the type specimen), and a similar to the jaws of Plesiobaena antiqua series ofpartial skulls along with a lowerjaw. rather than ofPalatobaena. The Plesiobaena The lower jaw (PU 17108) was identified on putorius lower jaws (UW 3348) differ from the basis of fitting it to the type skull and Plesiobaena antiqua in having a slightly hypothesizing that the relatively close fit larger triturating surface due to medial ex- meant that they were the same taxon. Ar- pansion but otherwise they are the same. In chibald and Hutchison (1979) accepted this particular the jaws of the two species of Ple- identification but remarked on the very close siobaena have the following features in com- similarity between this jaw (PU 17108) and mon in lower jaws of Palatobaena bairdi that they contrast to PU 17108. could identify by association with skulls. 1. Jaws less massive. They were able to distinguish the jaws of 2. Narrower triturating surface. Palatobaena and PU 17108 primarily by the 3. Rami meet at more acute angle. presence ofa small fossa on the lateral surface 4. Lower coronoid process. of the jaw in Palatobaena (Archibald and 5. Symphyseal area projects anteriorly to Hutchison, 1979, fig. 4). They speculated that a greater degree. this fossa was related to the origin of a new I conclude from this that PU 17108 belongs muscle in Palatobaena that extended from to Palatobaena, probably P. bairdi, in spite this fossa to an attachment near the cheek of the absence of a small lateral fossa. emargination. A comparison of the skulls involved sug- Unfortunately, the discovery of an asso- gests that UW 3348 is better preserved and ciated skull and lower jaws of Plesiobaena less distorted than the type skull which is 1982 GAFFNEY: BAENID TURTLES 9 somewhat crushed. The referred skull, PU UCMP 113317*, V73023, McCone Co., 20600, is better preserved than the type but Montana, (?)Paleocene. lacks the right halfofthe triturating surfaces. Palatobaena bairdi Due to these circumstances the original res- AMNH 2603 (Archibald and Hutchison, toration (Gaffney, 1972, fig. 15) probably has 1979). an unnaturally widened palate. Similarly, the AMNH 8277 (Archibald and Hutchison, close fitting of the lower jaws, PU 17108, 1979). with the type skull, PU 14984, is probably PU 17108 (Gaffney, 1972; Archibald and due to the crushing and lateral distortion of Hutchison, 1979; identified in both as Ple- the type skull. The new specimen of Plesio- siobaena putorius). baena putorius (UW 3348) has no difference UCMP 114539* (Archibald and Hutchi- in width or other proportions from Plesio- son, 1979). baena antiqua. As far as I can determine with UCMP 114680 (Archibald and Hutchison, the material at hand, P. putorius differs from 1979). P. antiqua in only two features: the semicir- UCMP 117154, V72208, Garfield County, cular projection overlying the crista supraoc- Montana, Hell Creek Formation. cipitalis and the medial expansion of the tri- UCM 37738 (Archibald and Hutchison, turating surface. As more specimens 1979). accumulate the consistency of the features Chisternon undatum remains to be seen. AMNH 5904 (Gaffney, 1972). AMNH 5962* (Gaffney, 1972). USNM 12839* (Gaffney, 1972). APPENDIX I Baena arenosa AMNH 2276*, Cottonwood Creek, Wyo- LOWER JAWS USED IN THIS STUDY ming, Bridger Formation. Literature citations refer to sources of lo- AMNH 3808*, Grizzly Buttes East, Wy- cality and geologic data. Asterisk (*) indicates oming, Bridger Formation (B). skull associated with lower jaws. AMNH 5907*, Wyoming, Bridger For- mation (B). Plesiobaena antiqua AMNH 5971* (Gaffney, 1972). NMC 8599 (Gaffney, 1972). AMNH 5977 (Gaffney, 1972). ROM 674, near Steveville, Red Deer River AMNH 5984 (Gaffney, 1972). Alberta, Oldman Formation (see also Gaff- ney, 1972, for figures). UCMP 113318*, V73023, McCone Co., LITERATURE CITED Montana, (?) Paleocene. Archibald, J. David, and J. Howard Hutchison UCMP 49759* (Gaffney, 1972). 1979. Revision ofthe genus Palatobaena (Tes- UMMP 20490* (Gaffney, 1972). tudines, Baenidae), with the description Plesiobaena putorius of a new species. Postilla, no. 177, pp. UW 3348*, Tiffanian, Wyoming. 1-19. Eubaena cephalica Gaffney, Eugene S. AMNH 2604 (Gaffney, 1972). 1972. The systematics of the North American AMNH 2605 (Gaffney, 1972). family Baenidae (Reptilia, Cryptodira). AMNH 2606 (Gaffney, 1972). Bull. Amer. Mus. Nat. Hist., vol. 147, UCMP 107617*, V73023, McCone Co., art. 5, pp. 241-320. Montana, (?) Paleocene. 1975a. A phylogeny and classification of the McCone higher categories of turtles. Ibid., vol. UCMP 114706, V73023, Co., 155, art. 5, pp. 387-436. Montana, (?) Paleocene. 1975b. Solnhofia parsonsi, a new cryptodiran UCMP 114710, V73023, McCone Co., turtle from the late Jurassic of Europe. Montana, (?) Paleocene. Amer. Mus. Novitates, no. 2576, pp. 1- Stygiochelys estesi 25. UCMP 113316*, V73023, McCone Co., 1976. Cranial morphology of the European Montana, (?)Paleocene. Jurassic turtles Portlandemys and Ple- 10 AMERICAN MUSEUM NOVITATES NO. 2749

siochelys. Bull. Amer. Mus. Nat. Hist., negie Inst. Washington Publ., no. 75, pp. vol. 157, art. 6, pp. 489-542. 1-568. 1979. Comparative cranial morphology ofRe- Parsons, Thomas S., and Ernest Williams cent and fossil turtles. Ibid., vol. 164, 1961. Two Jurassic turtle skulls: a morpholog- art. 2, pp. 65-375. ical study. Bull. Mus. Comp. Zool., vol. 1982. Cranial morphology of the baenid tur- 125, pp. 43-107. tles. Amer. Mus. Novitates, no. 2737, Russell, Loris S. pp. 1-22. 1934 (1935). Fossil turtles from Saskatchewan Hay, Oliver P. and Alberta. Trans. Royal Soc. Canada, 1908. The fossil turtles ofNorth America. Car- ser. 3, vol. 28, pp. 101-1 10.