New Material of Borealosuchus from the Bridger Formation, with Notes on the Paleoecology of Wyoming’S Eocene Crocodylians

Stout, New Material of Borealosuchus PalArch’s Journal of Vertebrate Palaeontology, 9(5) (2012)

NEW MATERIAL OF BOREALOSUCHUS FROM THE BRIDGER FORMATION, WITH NOTES ON THE PALEOECOLOGY OF WYOMING’S EOCENE CROCODYLIANS

Jeremy B. Stout*

* P.O. Box 70357, Department of Geosciences, East Tennessee State University, Johnson City, Tennessee 37614 USA, [email protected]

Jeremy B. Stout. 2012. New Material of Borealosuchus from the Bridger Formation, with Notes on the Paleoecology of Wyoming’s Eocene Crocodylians. – Palarch’s Jour- nal of Vertebrate Palaeontology 9 (5) (2012), 1-7. ISSN 1567-2158. 7 pages + 3 ﬁ gures, 1 table.

Keywords: Crocodile, Borealosuchus, Herpetofauna, Paleoecology

ABSTRACT

The Eocene Green River and Bridger Formations of Wyoming represent lacustrine and fl uvial environments noteworthy for an extremely diverse crocodylian fauna (at least eight species in seven genera). This paper discusses a fragmentary crocodylian jaw from the Bridger For- mation, and also notes possible ecological partitioning among these sympatric crocodylians. The jaw fragment can be assigned confi dently to Borealosuchus based on the exclusion of the splenial from the mandibular symphysis and the presence of occlusal grooves between the alveoli, and it is referred tentatively to Borealosuchus cf. B. wilsoni. To examine the paleoecology of these crocodylians, variables based on habitat, body size, and inferred diet were formula- ted and species placed within respective categories. The research found that while there were more sympatric crocodylians in the early to mid Eocene of Wyoming than in any present-day biota, direct interspecifi c competition for resources is presumed to have been relatively low.

Introduction Borealosuchus wilsoni – Brochu (1997) erected the Borealosuchus genus to revise ear- The early to mid Eocene of Wyoming (rep- lier (incorrect) inclusions of fossil crocodiles resented extensively by the Green River and within the Cretaceous - Paleocene genus Bridger Formations) is exemplifi ed by a diver- Leidyosuchus. Borealosuchus wilsoni, like oth- sity of aquatic reptiles (Bartels, 1993), and es- er members of the genus, has an oddly shaped pecially, crocodylians (Class Eusuchia). These skull, wide at the posterior and very narrow formations are stratigraphically contiguous anteriorly. Brochu (1997) also includes the and roughly represent the same point in geo- Bridger Diplocynodon stuckeri within B. wil- logic history. The primary difference between soni, though this synonymy is questioned by the two is mode of deposition; the Green Riv- Zonneveld et al. (2000). er is characterized by fi nely laminated marls Brachyuranochampsa eversolei – Described indicative of a lacustrine environment (Eug- by Zangerl (1944), B. eversolei is a Bridgerian ster and Surdam, 1973), whereas high-energy taxon known from a nearly complete skull sandstones from a fl uvial habitat typify the without lower jaws. The skull is triangular in Bridger. shape and resembles the general crocodylid At least eight different crocodylians are morphology of extant crocodylians. The holo- known from these formations. Some esti- type is medium size, with a skull length of 39 mates are much greater, though recent work cm. Another species of Brachyuranochampsa, (especially that of Brochu, 1997) has synony- B. zangerli, has since been synonymized with mized several problematic taxa. “C.” affi nis (Brochu & McEachran, 2000). This paper introduces a new fragmentary “Crocodylus” acer – Mook (1921b) fully de- mandibular specimen of Borealosuchus cf. B. scribed the skull of “Crocodilus” acer briefl y wilsoni from the mid-Eocene Bridger Forma- named by Cope in 1882. Originally described tion of Wyoming. In addition, new research from Utah, Bartels (1993) identifi es the spe- and a literature review seek to examine possi- cies from the Green River of Wyoming. The ble ecological relationships among the many skull length of the holotype is 38.7 cm (Mook, sympatric crocodylian of the Green River. 1921b) and the specimen very much resem- The new crocodylian material described bles that of the extant slender-snouted croco- herein (hereafter referred to as Tate 2070) dile, Mecistops (= Crocodylus) cataphractus, was originally donated to the East Tennes- in overall shape. see State University Natural History Museum “Crocodylus” affi nis – Mook (1921a) de- (ETMNH) by a local surgeon who collected scribed a very complete skull of “Crocodilus” the specimen in situ while on a fl y-fi shing affi nis (Marsh) from the Bridger Formation trip in Le Barge, Wyoming. After laboratory of Wyoming. As with “C. acer above, this preparation and with the donor’s permission, species is listed in quotation marks to note the specimen was transferred to the Tate Mu- its withdrawal from the genus Crocodylus. seum for permanent repository. Brochu (2003) noted that the original inclu- sion within this genus was based on plesio- Review of Included Taxa morphic conditions of Crocodylia found in Allognathosuchus cf. A. polydon – Originally the specimens, and not apomorphic states described by Cope in 1873 as Crocodilus poly- found in Crocodylus. Regardless, “C.” affi nis don, Mook (1921a) established the revised ge- is a relatively large crocodylian (skull length nus. Allognathosuchus polydon was originally approximately 45 - 50 cm) that does, at least described from a single lower jaw 21 cm in superfi cially, resemble many members of ex- length that adheres to an alligatoroid form, tant Crocodylus. dentary curvature along the anterior portion Pristichampsus vorax – Arguably the of the ramus and varying tooth shapes (Bro- strangest and most specialized of the Eocene chu, 2004). There is doubt that the Wasatchi- crocodylian, Langston (1975) described a very an Allognathosuchus represents A. polydon, complete skull (and associated remains) of though the presence of the genus is unques- P. vorax. Pristichampsus is interpreted to have tioned (pers. comm. with C.A. Brochu). been a largely terrestrial crocodile genus with a dorsoventrally tall skull and ziphodont den-

© PalArch Foundation 2 Stout, New Material of Borealosuchus PalArch’s Journal of Vertebrate Palaeontology, 9(5) (2012) tition. Zonneveld et al., 2000 describe isolated Clearly, while some modern methods and ziphodont teeth from the Green River Forma- observations should apply to the fossil herpe- tion and assign them to this species. tofaunas, something unique was occurring to Procaimanoidea kayi – Mook (1941) de- facilitate such a high diversity of sympatric scribed Hassiacosuchus kayi (a European genus) Eocene crocodylians. This has been seen sev- from the Bridger beds of Wyoming. Wassersug & eral times in the geologic past such as in Creta- Hecht (1967) referred the material to Procai- ceous crocodyliforms (Sereno & Larsson, 2009) manoidea. The species is small (total length less and among Miocene crown-group crocodylians than 2 m) and shares some characteristics with (Salas-Gismondi et al., 2007), to name a couple. Alligator (Brochu, 1999). In addition to its oc- In each of these cases, some crocodiles were currence in the Bridger, it is also known from extremely specialized to a point far surpassing the Green River Formation. that seen in the modern world. Tsoabichi greenriverensis – Brochu (2010) The following analysis is not meant to be de- reports this newly described small alligatorid fi nitive, but seeks to make general observations from the Wasatchian Green River Formation. regarding the likely paleoecology of Wyoming’s Some fossils previously assigned to Procaiman- sympatric Eocene crocodylian based on inferred oidea have been included in the new genus and diet, habitat choice, and maximum body size. species and the taxon is likely related to the modern caimans. Materials and Methods Sympatric Crocodile Species in the Modern Paleoecological Analysis World To put the Green River and Bridger crocodylian With 23 crocodylian species extant today, some into an ecological perspective, a table was for- regions of the world boast far more diversity mulated to account for the variables used. Gross than others, notably, areas of South America and morphology of relative tooth shape and size, islands in Southeast Asia. In a few of these ar- along with skull shape were used as the basis eas, up to four or fi ve crocodylian species inhabit for presumed dietary habits. More robust teeth roughly sympatric distributions. In each of these and/or brevirostran skull shape were assumed modern cases, however, ecological partitioning is to aid in capture of a more mixed diet, whereas present to the exclusion of regular interspecifi c gracile teeth and longirostran skulls are more competition. Examples of such partitioning in- indicative of a piscivorous lifestyle (Ross, 1989). clude differing habitat preference, prey utiliza- Presumed habitat choice was based on the tion, and life history strategies (Ross, 1989). sediments in which the fossils were found. As Concerning feeding behavior, Marioni et al. previously noted, the Green River Formation is (2008) observed sympatric caimans (one small thought to have been a low-energy, lacustrine and one large species) during the Amazonian habitat and the Bridger was a much more dy- dry season. Despite an expectation for interspe- namic, fl uvial one. cifi c competition, the frequency was observed to Overall size of the species is the most precar- be low due to differing modes of prey capture ious of the variables used, and should be noted and microhabitat utilization during the stressful with caution. In many cases, species are known season. from few or fragmentary specimens, so average Abercrombie et al. (2001) used computer or maximum size of the animal are question- modeling to observe survivability in hypotheti- able. This and other assumptions are further cal sympatric crocodiles during “catastrophic” explained below. events affecting reproduction. They concluded that large species dominate a habitat until a ca- Assumptions tastrophe severely reduces both populations, in As with much paleoecological interpretation, which the smaller species will rebound quicker some assumptions are inescapable. To avoid and dominate the habitat for some time. This some subsequent criticism, these deserve to at has been observed in the wild with overhunted least be stated with remarks: large species of crocodiles and subsequent colo- 1) The fossil specimens represent an average nization of smaller caimans (Platt & Thorbjar- adult size. With some taxa used, a substantial narson, 2000). fossil record is known, and thus the overall body

© PalArch Foundation 3 Stout, New Material of Borealosuchus PalArch’s Journal of Vertebrate Palaeontology, 9(5) (2012) size is reasonably ascertained. While Brochu (pers. comm.) recommends some osteological characters to deduce ontogenetic maturity, this assumption will have to be maintained for fragmentary specimens; 2) Depositional sediments represent a fairly accurate indicator of habitat. While the fl uvial habitat is a bit more tenuous (as high energy environments have the ability to transfer specimens leading to a depositional bias, as is prob- ably the case with the terrestrial Pristichampsus vorax; Langston, 1975), the lacustrine sediments of the Green River Formation should adequate- Figure 2. Tate 2070, labial view. Anterior is to the right. ly represent taxa present (Eugster & Surdam, Photography by the author. Courtesy of Tate Geological 1973). Further, some taxa found more frequent- Museum, Casper College, Wyoming. ly (or exclusively) in the Bridger Formation than in the Green River can be adequately as- Results sumed from that environment; 3) Skull shape and tooth morphology can inter- Tate 2070 pret the crocodylians’ diet. This is not intended Lingual and labial views of Tate 2070 are in to be a blanket assertion, but merely repre- fi gures 1 and 2 (respectively). Though only the sents generalities about the animals’ behavior. anterior right mandible is preserved, this is one Though this generality is contested by C.A. of the most diagnostic portions of the crocodyl- Brochu (pers. comm.), the author follows Pierce ian osteology (Brochu, 1999). Based on Brochu et al., 2008 in that while skull shape may not (2000), the symphysis extends posteriorly ap- always refl ect phylogenetic history, it at least proximately to the 4th dentary alveolus. Also, can provide very basic ecological and dietary though the splenial is not preserved, the scar of information; its anterior portion on the dentary shows that 4) The Green River and Bridger Formations are it did not participate in the mandibular sym- not contemporaneous, but represent a very nar- physis. In addition, deep occlusal grooves are row interval of time with little generic faunal present between the anterior alveoli; these char- interchange. While these two units are not eco- acteristics exclude all North American Eocene logically or chronologically the same, together for the purpose of this study, they do represent a narrow window of geologic time (Wasatchian - Bridgerian) and regional area, while illustrat- ing substantial faunal overlap.

Figure 3. Reconstruction of symphyseal region of lower Figure 1. Tate 2070, lingual view before tooth repair. jaw using Tate 2070 and its reﬂ ected image (blue). Anterior is to the left. Photography by the author. Courtesy Anterior is up. Photography by the author. Courtesy of of Tate Geological Museum, Casper College, Wyoming. Tate Geological Museum, Casper College, Wyoming.

SIZE/DIET Small-mid size HABITAT Large Piscivore Large Mixed Mixed Brachyuranochampsa "Crocodylus" acer, eversolei, "Crocodylus" Fluvial: Borealosuchus Allognathosuchus affi nis wilsoni (?) polyodon Tsoabichi Borealosuchus "Crocodylus" Lacustrine: greenriverensis, wilsoni (?) affi nis Procaimainoidea kayi Table 1. Purported habitat and Pristichampsus Terrestrial: diet of sympatric Eocene Green vorax River and Bridger crocodylians. taxa except Borealosuchus sp. (Brochu, 1997). of more than 35 cm. The anterior width of the Further, the intact teeth are long and gracile rostrum, however, is estimated to have been and possess thin striae oriented longitudinally less than 6 mm (fi gure 3). This extreme less- from base to tip, additional support for the ge- ening of width from posterior to anterior im- neric identifi cation (Zonneveld et al., 2000). plies an ecological role substantially different The anterior reconstruction of the fragment from that seen in any extant crocodile. Based (fi gure 3) shows a restored symphyseal region on the gracile teeth and this “needle-nose pliers” of the mandibles of Borealosuchus sp. This illus- shaped rostrum, it is speculated by this author trates the narrowness of the anterior region of that this animal was some sort of large, highly the snout, but also the sharp angle moving to- specialized piscivore. ward the posterior of the skull, a strange shape not seen in extant crocodiles (pers. obs.). Crocodylian Paleoecology Overall, when observing the likelihood of inter- Paleoecological Analysis specifi c competition between Wyoming’s Eo- Table 1 shows the purported placement of all con- cene crocodylians, it is here presumed to have clusive Green River and Bridger crocodylian taxa, been low under normal circumstances. Even in based on the criteria mention above. Some taxa broad categories where the crocodiles appear are listed among more than one habitat based on to occupy a similar ecological role (i.e. “Cro- their presence in the fossil record of both forma- codylus” acer and Borealosuchus wilsoni), skull tions. The terrestrial habitat is devoid of known morphology suggests that the feeding ecologies crocodylians, except for Pristichampsus vorax. were still quite different (in the unique skull morphology of B. wilsoni mentioned above, to Discussion the more gradually tapering condition in “C.” acer, Mook 1921b). Borealosuchus cf. B. wilsoni Further, the generalized category occupied Tate 2070 is hereby referred to as Borealosuchus by Allognathosuchus polydon, Brachyurano- cf. B. wilsoni, the generic designation based on champsa eversolei, Procaimainoidea kayi and the exclusion of the splenial from the man- Tsoabichi greenriverensis (fl uvial, small – me- dibular symphysis and the presence of occlusal dium body size, mixed diet) still does not ne- grooves between the anterior alveoli, and is be- cessitate the presence of regular interspecifi c ing tentatively conferred to B. wilsoni based on competition. Brachyuranochampsa eversolei its known presence in the Bridger Formation is much more “crocodile-like” (in the familial (“Diplocynodon stuckeri”, Mook 1960). This des- sense), whereas A. polydon and P. kayi are much ignation should be held cautiously, however, as more “alligator-like” in gross morphology. Also, the fragmentary nature of Tate 2070 precludes adult P. kayi and T. greenriverensis were likely absolute species identifi cation. much smaller than A. polydon, thus potentially Based on the proportions of Mook (1959), leading to a dynamic similar to that discussed the animal represented by Tate 2070 can be es- by Marioni et al. (2008) regarding modern cai- timated to have had a skull length of approxi- mans in which the 2 disparately-sized sympat- mately 70 cm, and a maximum skull width (be- ric species simply do pursue the same prey, and tween the distal-most portions of the quadrates) thus avoid competition for resources.

“Crocodylus” affi nis appears to have been Brochu, C.A. & J.D. McEachran. 2000. Phyloge- the top predator in the aquatic Green River and netic relationships and divergence timing Bridger ecosystems, as its remains are found in of Crocodylus based on morphology and the relative completeness in both the Green River fossil record. – Copeia 2000, 3: 657-673. (Zonneveld et al., 2000) and Bridger Formations Brochu, C.A. 2003. Phylogenetic approaches to- (Mook, 1921a), and could be quite large (esti- ward Crocodylian history. – Annual Review mated skull lengths greater than 50 cm). of Earth and Planetary Sciences 31: 357-397. In the absence of direct fossil, a precise and Brochu, C.A. 2004. Alligatorine phylogeny and detailed paleoecological interpretation is sim- the status of Allognathosuchus Mook, 1921. ply not possible at this point. However, given – Journal of Vertebrate Paleontology 24, 4: the relatively substantial fossil material of the 857-873. Green River and Bridger crocodylians, it can be Brochu, C.A. 2010. A new alligatorid from the reasonably surmised that ecological partition- lower Eocene Green River Formation of Wy- ing played an integral role in the evolution of oming and the origin of caimans. – Journal such a diverse crocodylian fauna. of Vertebrate Paleontology 30, 4:1109-1126. Eugster, H.P. & R.C. Surdam. 1973. Depositional Acknowledgments environment of the Green River Formation of Wyoming: a preliminary report. – Geo- Sincerest gratitude goes to C.A. Brochu for ex- logical Society of America Bulletin 84: 1115- tensive advice on an earlier version of this man- 1120. uscript. Thanks also go to B.W. Schubert and Langston Jr., W. 1975. Ziphodont crocodiles: Dan Rush, M.D. for procurement of the primary Pristichampsus vorax (Troxell), new combi- study specimen, along with thanks to J.P. Cavi- nation, from the Eocene of North America. gelli and M. Connely for aid in locality infor- – Fieldiana 33, 16: 291-314. mation. S. Haugrud helped extensively with the Marioni, B., R. Da Silveira, W.E. Magnusson & preparation of Tate 2070. J. Thorbjarnarson. 2008. Feeding behavior of two sympatric caiman species, Melano- Cited Literature suchus niger and Caiman crocodilus, in the Brazilian Amazon. – Journal of Herpetology Abercrombie, C.L., K.G. Rice & C.A. Hope. 2001. 42, 4: 768-772. The great alligator-caiman debate: medita- Mook, C.C. 1921a. Description of a skull of a tions on crocodilian life-history strategies. Bridger Crocodylian. – Bulletin of the Ameri- In: Grigg, G.C., F. Seebacher, & C.E. Franklin. can Museum of Natural History 44: 111-116. Crocodylian biology and evolution. – Surrey, Mook, C.C. 1921b. The skull of Crocodilus acer Beatty and Sons: 409-418. Cope. – Bulletin American Museum of Natu- Bartels, W.S. 1993. Niche separation of fl uvial ral History 44: 117-125. and lacustrine reptiles from the Eocene Mook, C.C. 1941. Hassiacosuchus kayi from the Green River and Bridger formations of Wyo- Bridger Eocene beds of Wyoming. – Annals ming. – Journal of Vertebrate Paleontology, of the Carnegie Museum 28, 12: 207-220. supplement to number 3: 25A. Mook, C.C. 1959. A new species of fossil croc- Brochu, C.A. 1997. A review of Leidyosuchus odile of the genus Leidyosuchus from the (Crocodyliformes: Eusuchia) from the Cre- Green River Beds. – American Museum No- taceous through Eocene of North America. vitates 1933: 8. – Journal of Vertebrate Paleontology 17, 4: Mook, C.C. 1960. Diplocynodon remains from 679-697. the Bridger Beds of Wyoming. – American Brochu, C.A. 1999. Cranial morphology of Alli- Museum Novitates 2007: 4. gator mississippiensis and phylogeny of Al- Pierce, S.E., K.D. Angielczyk & E.J. Rayfi eld. ligatoroidea. – Journal of Vertebrate Paleon- 2008. Patterns of morphospace occupa- tology 19, 2 (Memoir 6, Supplement). tion and mechanical performance in extant Brochu, C.A. 2000. Borealosuchus (Crocodylia) crocodylian skulls: a combined geometric from the Paleocene of Big Bend National morphometric and fi nite element modeling Park, Texas. – Journal of Paleontology 74, 1: approach. – Journal of Morphology 269: 840- 181-187. 864.

Platt, S.J. & J.B. Thorbjarnarson. 2000. Popula- others than the author. The author can be re- tion status and conservation of Morelet’s quested to submit proof of this permission to crocodile, Crocodylus moreletii, in northern the PalArch Foundation. Pdf texts (papers and Belize. – Biological Conservation 96: 21-29. proceedings) are free to download on the condi- Ross, C.A. Ed. 1989. Crocodiles and alligators. tions that each copy is complete and contains Facts on fi le. – New York, Murdoch Books UK. the PalArch copyright statement; no changes Salas-Gismondi, R., P.-O. Antione, P. Baby, are made to the contents and no charge is made. S. Brusset, M. Benammi, N. Espurt, D. de The downloaded (and/or printed) versions of Franseschi, F. Pujos, J. Tejada & M. Urbina. PalArch publications may not be duplicated in 2007. Middle Miocene crocodiles from the hard copy or machine readable form or repro- Fitzcarrald Arch, Amazonian Peru. In: Diaz- duced photographically, nor may they be redis- Martinez, E. & I. Rabano. Eds. Fourth Eu- tributed, transmitted, translated or stored on ropean Meeting on the Palaeontology and microfi lm or in electronic databases other than Stratigraphy of Latin America, Madrid. – for single use by the person that obtained the Madrid, Instituto Geologico y Minero de Es- fi le. Commercial use or redistribution may only pana: 355-360. be realised after consultation with and with Sereno, P.C. & H.C.E. Larsson. 2009. Cretaceous written permission of the PalArch Foundation. crocodyliforms from the Sahara. – ZooKeys 28, Special Issue. ICZN Wassersug, R.J. & M.K. Hecht. 1967. The status To comply with Article 8.6 of the International of the crocodylid genera Procaimanoidea Code of Zoological Nomenclature (4th edition), and Hassiacosuchus in the New World. – this journal is deposited as hard copy in four Herpetologica 23, 1: 30-34. archival libraries and the archive of the PalArch Zangerl, R. 1944. Brachyuranochampsa everso- Foundation. All of PalArch’s publications are lei, gen. et sp. nov., a new crocodylian from stored in the e-depot of The National Library, the Washakie Eocene of Wyoming. – Annals The Hague, The Netherlands (www.kb.nl). of the Carnegie Museum xxx: 77-84. Natural History Museum Rotterdam Zonneveld, J., G.F. Gunnell & W.S. Bartels. 2000. Westzeedijk 345 Early Eocene fossil vertebrates from the 3015 AA Rotterdam southwestern Green River Basin, Lincoln The Netherlands and Uinta Counties, Wyoming. – Journal of Vertebrate Paleontology 20, 2: 369-386. Royal Belgian Institute of Natural Sciences Library Submitted: 31 January 2011 rue Vautier 29 Published: 5 Apri. 2012 B- 1000 Brussels Belgium About www.PalArch.nl (Netherlands scientifi c journal) copyright. Library Naturalis National Museum of Natural History Copyright © 2003-2012 PalArch Foundation P.O. Box 9517 2300 RA Leiden The author retains the copyright, but agrees The Netherlands that the PalArch Foundation has the exclusive right to publish the work in electronic or other PalArch Foundation formats. The author also agrees that the Foun- Mezquitalaan 23 dation has the right to distribute copies (elec- 1064 NS Amsterdam tronic and/or hard copies), to include the work The Netherlands in archives and compile volumes. The Founda- tion will use the original work as fi rst published Vrije Universiteit at www.PalArch.nl. UBVU-Library of Earth Sciences The author is responsible for obtaining the per- De Boelelaan 1079 mission of the use of illustrations (drawings, 1081 HV Amsterdam photographs or other visual images) made by The Netherlands