Fossil Crocodylomorphs Across the National Park Service

By: Austin B. Shaffer, University of Iowa

Introduction

Crocodylomorpha, a group including crocodylians and their extinct relatives, is known from fossils spanning strata from the Upper Triassic to the Quaternary, with native living representatives of Alligator mississippiensis (American alligator) and Crocodylus acutus (American crocodile) found in the United States today, including Everglades National Park. The preliminary findings of an ongoing thematic inventory of crocodylomorph fossils from National Park Service (NPS) units are provided here. Crocodylomorphs, hereafter occasionally referred to simply as ‘crocs’, are represented within NPS units by a number of body and trace fossils. These fossils document a diverse assortment of species throughout the group’s evolutionary history, from near the base of Crocodylomorpha to the more derived nested groups of Neosuchia, Eusuchia, and Crocodylia, each in turn a smaller group of animals more closely related to living crocs. Most commonly represented by osteoderms (bony plates in the skin) and teeth, alongside key more complete specimens, NPS paleontological resources provide important data for the understanding of this dynamic group of reptiles.

Body Fossils

Body fossils of crocodylomorphs are currently confidently confirmed for eleven NPS units, with seven additional parks containing possible croc body fossil records. While many of these croc fossil records consist primarily of isolated teeth, osteoderms, and bone fragments, more significant fossil material is known from multiple NPS units.

Five species have been named based on crocodylomorph fossils from NPS units, of which three are currently considered valid: one from Dinosaur National Monument (DINO) and two from Big Bend National Park (BIBE). Hoplosuchus kayi Gilmore, 1926 is a protosuchid, a small terrestrial croc, established on a small, largely complete skeleton from the Upper Jurassic Morrison Formation of DINO. Deinosuchus riograndensis (Colbert and Bird, 1954), originally named as a species of Phobosuchus, is based on a holotype from the Upper Cretaceous Aguja Formation of BIBE; much more complete material has also since been described for this species from the same park (Cossette and Brochu, 2020). Also from BIBE, the basal caiman Bottosaurus fustidens Cossette, 2020 has been described from the Paleocene strata of the Black Peaks Formation. Two further taxa, Caimanoidea visheri Mehl, 1916 and Nordenosaurus magnus Holman, 1973, were established based on fossils possibly from Badlands National Park (BADL) and Niobrara National Scenic River (NIOB), respectively. However, C. visheri has since been considered a junior synonym of Alligator prenasalis (Higgins, 1971) while N. magnus (originally described as a lizard) is indistinguishable from fossils of Alligator from the same time and can be considered a nomen dubium.

The Triassic body fossil record for Crocodylomorpha within the NPS is represented only by material collected from Petrified Forest National Park (PEFO). Croc fossils from PEFO consist of a number of fragmentary, partial skeletons. While some specimens (e.g., PEFO 21374/UCMP 129740) have been previously attributed to Sphenosuchus (Parrish, 1991) or Hesperosuchus (Long and Murry, 1995), their fragmentary condition does not support such assignments at present and as such, these Chinle Formation fossils are more appropriately referred to Crocodylomorpha indet. (A. Marsh, pers. comm.). Future work revising Triassic North American crocs could produce more specific taxonomic identifications for certain specimens from PEFO. Regardless, the fossils from PEFO preserve crucial anatomical data and are important for the understanding of early crocodylomorph evolution.

Triassic crocodylomorph body fossils (PEFO 21374) from the Chinle Formation of Petrified Forest National Park (PEFO), Arizona: A, partial right femur; B, vertebrae; C, osteoderms; D, partial dentary. Scale bars = 1 cm.

Original photographs courtesy of Adam Marsh

There is no Lower to Middle Jurassic body fossil record for crocs currently known from within the NPS. The Upper Jurassic strata of the Morrison Formation in Colorado and Utah do, however, produce significant croc material. As mentioned above, the Morrison Formation of DINO produced the holotype and only known specimen of the protosuchid Hoplosuchus kayi, discovered during road work. In addition to this, a number of fossils of Goniopholididae, which broadly resembled modern crocs but with key anatomical differences, are known from the same formation and park. At present, these goniopholidids are broadly attributed to the genera Amphicotylus (formerly attributed to Goniopholis) and Eutretauranosuchus (Foster, 2020); given the well-preserved nature of some specimens, it is likely that additional research could provide more specific taxonomic identification. Assorted goniopholidid fragments are also known from the Morrison Formation of Curecanti National Recreation Area (CURE) (Koch et al., 2006; Foster et al., 2015; M. R. King, pers. comm.). Yet further fragments from the Morrison Formation of Colorado National Monument (COLM) possibly represent Crocodylomorpha (Shaffer et al., 2024), but additional fossil material/assessment would be necessary to confirm this.

The Cretaceous crocodylomorph fossil record from NPS units is largely fragmentary. Fossils attributed to Upper Cretaceous neosuchians are recorded from the Wahweap and Straight Cliffs formations of Bryce Canyon National Park (BRCA) in Utah (Irmis et al., 2013; Tran et al., 2024) and the Menefee Formation of Chaco Culture National Historical Park (CHCU) in New Mexico (Varela et al., 2019). Possible fragmentary records of croc teeth and osteoderms come from the Mowry Shale of DINO (Stewart et al., 1994) and the Muddy Sandstone Member (Thermopolis Shale Formation) of Bighorn Canyon National Recreation Area (BICA), as catalogued in the respective museum collections of DINO and BICA. BIBE has the most significant Cretaceous record for NPS crocodylomorphs with teeth, osteoderms, and substantial skeletal material known from the Aguja and Javelina formations of the park, including a goniopholidid (Lehman, 1985; Rowe et al., 1992; Sankey, 1998; Lehman et al., 2019) and the aforementioned giant Deinosuchus riograndensis (Cossette and Brochu, 2020).

Deinosuchus riograndensis skull (TMM 43620-1) from the Upper Cretaceous Aguja Formation of Big Bend National Park (BIBE), Texas. Scale bar = 5 cm.

Original photograph courtesy of Adam Cossette

Paleocene–Eocene strata of BIBE (i.e., Black Peaks, Hannold Hill, and Canoe formations) also record a number of croc fossils, including the aforementioned holotype of Bottosaurus fustidens, as well as material of Borealosuchus (Brochu, 2000), possible alligatoroids (Wick, 2021), and other indeterminate eusuchians (Maxwell et al., 1967; Stanhardt, 1986). Osteoderms, teeth, and bone fragments attributable to Borealosuchus and Alligatoroidea are also known from the Paleocene Fort Union Group of Theodore Roosevelt National Park (THRO) in North Dakota (Salcido et al., 2022). John Day Fossil Beds National Monument (JODA) in Oregon also preserves fragmentary eusuchian and alligatoroid remains from the Eocene Clarno Formation (Stout et al., 2025).

Right partial mandible of Bottosaurus fustidens holotype (TMM 40148-7) from the Paleocene Black Peaks Formation of Big Bend National Park (BIBE), Texas. Scale bar = 5 cm.

Original photograph courtesy of Adam Cossette.

The Eocene Green River Formation of Fossil Butte National Monument (FOBU) in Wyoming, while almost certainly containing croc fossils within its boundaries, has yet to produce any such material and is unlikely to do so for the foreseeable future, given the small scale of quarrying at the monument (A. Aase, pers. comm.). However, private quarrying efforts of the same strata just beyond the boundaries of FOBU have produced beautiful articulated skeletons of Borealosuchus wilsoni and the caimanine Tsoabichi greenriverensis, with casts of both taxa currently exhibited at the FOBU visitor center. The Wasatch Formation of FOBU has produced fragmentary reptile fossils, but it is currently unknown whether these fragments are croc and/or turtle in identity (A. Aase, pers. comm.).

The mid-late Cenozoic fossil record of crocs within NPS units is dominated by the genus Alligator. The most substantial such material comes from the Eocene–Oligocene White River Group strata of BADL, as represented by multiple partial to nearly complete skeletons of A. prenasalis, alongside various fragments (Whiting and Hastings, 2015; E. Starck and E. Welsh, pers. comm.). The Miocene Valentine Formation of NIOB has produced fragmentary materials that are consistent with other Miocene Alligator fossils of the Great Plains (Whiting and Head 2020), as well as producing the aforementioned holotype of the dubious Nordenosaurus magnus (Holman, 1973) that also likely represents Alligator. A now lost tooth from the Miocene Anderson Ranch Formation of Agate Fossil Beds National Monument (AGFO) could also belong to this genus (Hunt, 1984), but the loss of this specimen makes this impossible to verify for now. The only currently documented Quaternary record for fossil crocodylomorphs within the NPS is a fragmentary mandible of Alligator cf. mississippiensis from Waco Mammoth National Monument (WACO) in Texas (Naryskin, 1986). The limited Neogene and Quaternary NPS croc fossil record reflects the reduction of range and loss of diversity among crocs in North America as the climate became cooler and drier throughout the later portion of the Cenozoic.

Several scrappy records of fossil crocs are additionally known from NPS units along the East Coast. They include a heavily eroded bone fragment from the Paleocene Aquia Formation of Fort Washington Park (FOWA) (Nelson et al., 2019; M. Antonioni, pers. comm.), badly worn, possibly Pliocene dredge material from Cumberland Island National Seashore (CUIS) (McLemore et al., 1981), and a reworked tooth from beach washup of Gateway National Recreation Area (GATE) (Stoffer et al., 1996). These fossils, while for the most part damaged from erosion and not found in situ, are nevertheless important possible records of NPS crocodylomorphs.

Trace Fossils

The currently verified trace fossil record of crocodylomorphs within NPS units consists of tracks, swim traces, and bite marks. Such traces have been identified from six park units across five different states, with these records spanning strata from the Upper Triassic to the Paleocene.

Three NPS units are currently confirmed to preserve crocodylomorph tracks: COLM, CURE, and Glen Canyon National Recreation Area (GLCA). GLCA has the most well-documented record at present, consisting of Batrachopus tracks from the Chinle Formation, Kayenta Formation, and Navajo Sandstone (Lockley et al., 2014; Milner et al., 2023; 2024; Tran et al., 2025). Such traces were likely made by small, terrestrial, basal crocodylomorphs, similar to those recorded by the body fossils from PEFO and Hoplosuchus at DINO. Tracks plausibly made by goniopholidids are known from the Morrison Formation of CURE (M.R. King, pers. comm.). COLM possesses likely croc tracks from both the Upper Triassic–Lower Jurassic Wingate Sandstone (cf. Batrachopus) and from the middle Cretaceous Naturita Formation (cf. Crocodylopodus) (Shaffer et al., 2024; pers. obs.).

Basal crocodylomorph manus-pes (hand-foot) print set of Batrachopus isp. (UCM 180.33/GLCA 23891) from the Lower Jurassic Kayenta Formation of Glen Canyon National Recreation Area (GLCA), Utah. Scale bar = 1 cm.

Original photograph courtesy of Andrew Milner, use courtesy of the University of Colorado Museum of Natural History.

In addition to these records of proper tracks, possible croc swim traces are recorded for three NPS units: two in western Colorado (COLM and CURE) and one in northernmost Wyoming (BICA). All of these traces are from the Jurassic—specifically, the Middle Jurassic Gypsum Spring Formation of Wyoming (Mickelson et al., 2006; Boudreau et al., 2014) and the Upper Jurassic Morrison Formation of Colorado (King, 2023; Shaffer et al., 2024; M.R. King, pers. comm.). These traces were likely made by neosuchian crocodylomorphs, with those from the Morrison Formation plausibly made by goniopholidid neosuchians. However, there does remain the possibility that at least some of the swim traces were made by tridactyl dinosaurs (e.g., the ichnogenus Characichnos).

Bite marks on turtle shell material have been attributed to crocs from the Upper Cretaceous Aguja Formation of BIBE and from the Paleocene Sentinel Butte Formation of THRO (Lehman and Wick, 2010; Hoganson and Campbell, 1997; 2002; Salcido et al., 2022). Such traces were likely left by neosuchian crocodylomorphs and document probable feeding behavior on the corresponding turtles.

Other possible crocodylomorph trace fossils may also be known from FOBU, where a number of large coprolites as well as ‘reptilian’ swim traces are known from the Eocene Green River Formation in the park (A. Aase, pers. comm.). More detailed examination of this material and comparison with modern examples is necessary in order to distinguish between crocs and turtles as possible tracemakers.

Acknowledgements

Thanks to Arvid Aase, Mike Antonioni, Diana Boudreau, Chris Brochu, Adam Cossette, Rudolph Hummel, ReBecca Hunt-Foster, Jason Kenworthy, Ryan King, Adam Marsh, Charles Salcido, Vince Santucci, Ellen Starck, Tut Tran, Justin Tweet, Laura Vietti, Ed Welsh, and Lindsey Yann for their contributions and assistance with this ongoing project. Thanks to the American Geosciences Institute and the NPS Paleontology Program for financial and administrative support.

References

Boudreau, D. B., V. Santucci, and T. Connors. 2014. Bighorn Canyon National Recreation Area paleontological resources inventory. Natural Resource Report NPS/BICA/NRR—2014/885. National Park Service, Fort Collins, Colorado.

Brochu, C. A. 2000. Borealosuchus (Crocodylia) from the Paleocene of Big Bend National Park, Texas. Journal of Paleontology 74(1): 181-187.

Colbert, E. H., and R. T. Bird. 1954. A gigantic crocodile from the Upper Cretaceous beds of Texas. American Museum of Natural History Novitates 1688.

Cossette, A. P. 2020. A new species of Bottosaurus (Alligatoroidea: Caimaninae) from the Black Peaks Formation (Palaeocene) of Texas indicates an early radiation of North American caimanines. Zoological Journal of the Linnean Society 191: 276-301.

Cossette, A. P., and C. A. Brochu. 2020. A systematic review of the giant alligatoroid Deinosuchus from the Campanian of North America and its implications for the relationships at the root of Crocodylia. Journal of Vertebrate Paleontology: e1767638.