A new caimanine alligatorid from the Middle Eocene of Southwest Texas and implications for spatial and temporal shifts in Paleogene crocodyliform diversity

Author:

Stocker Michelle R.12ORCID,Brochu Christopher A.3,Kirk E. Christopher45

Affiliation:

1. Department of Geosciences, Virginia Polytechnic Institute and State University (Virginia Tech), Blacksburg, VA, USA

2. Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX, USA

3. Department of Earth and Environmental Sciences, University of Iowa, Iowa City, IA, USA

4. Department of Anthropology, The University of Texas at Austin, Austin, TX, USA

5. Jackson School Museum of Earth History, The University of Texas at Austin, Austin, TX, USA

Abstract

Dramatic early Cenozoic climatic shifts resulted in faunal reorganization on a global scale. Among vertebrates, multiple groups of mammals (e.g., adapiform and omomyiform primates, mesonychids, taeniodonts, dichobunid artiodactyls) are well known from the Western Interior of North America in the warm, greenhouse conditions of the early Eocene, but a dramatic drop in the diversity of these groups, along with the introduction of more dry-tolerant taxa, occurred near the Eocene–Oligocene boundary. Crocodyliforms underwent a striking loss of diversity at this time as well. Pre-Uintan crocodyliform assemblages in the central Western Interior are characterized by multiple taxa, whereas Chadronian assemblages are depauperate with onlyAlligator prenasalispreviously known. Crocodyliform diversity through the intervening Uintan and Duchesnean is not well understood. The middle Eocene Devil’s Graveyard Formation (DGF) of southwest Texas provides new data from southern latitudes during that crucial period. A new specimen from the middle member of the DGF (late Uintan–Duchesnean) is the most complete cranial material of an alligatorid known from Paleogene deposits outside the Western Interior. We identify this specimen as a caimanine based on notched descending laminae of the pterygoids posterior to the choanae and long descending processes of the exoccipitals that are in contact with the basioccipital tubera. UnlikeEocaiman cavernensis, the anterior palatine process is rounded rather than quadrangular. The relationships and age of this new taxon support the hypothesis that the modern distribution of caimanines represents a contraction of a more expansive early Cenozoic distribution. We hypothesize that the range of caimanines tracked shifting warm, humid climatic conditions that contracted latitudinally toward the hothouse-icehouse transition later in the Eocene.

Funder

The Jackson School of Geosciences, The University of Texas at Austin

UT College of Liberal Arts, the Society of Vertebrate Paleontology

Department of Geosciences at Virginia Tech

Publisher

PeerJ

Subject

General Agricultural and Biological Sciences,General Biochemistry, Genetics and Molecular Biology,General Medicine,General Neuroscience

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