Editorial Type:
Article Category: Research Article
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Online Publication Date: 01 Dec 2015

First Fossil Alligator from the Late Eocene of Nebraska and the Late Paleogene Record of Alligators in the Great Plains

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Page Range: 560 – 569
DOI: 10.1670/14-069
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Abstract

Fragmentary fossils have been tentatively attributed to Alligator from the Eocene of Wyoming, North Dakota, Saskatchewan, and Utah. The earliest definitive temporal and geographic range of Alligator has been limited to the late Eocene and early Oligocene Chadron and Brule Formations of the White River Badlands in South Dakota, represented by fossils of Alligator prenasalis. New fossils from the Chadron Formation in northwestern Nebraska expand the definitive geographic range of Alligator during the late Eocene. Results from a cladistic analysis of 28 alligatoroid species using 181 morphological characters support the identification of the new specimens as representing the most basally divergent taxon within Alligator, suggesting that they likely represent A. prenasalis; however, a lack of diagnostic features in the Nebraska specimens prevents a definitive assignment to this species. Limited occurrence data may indicate that Alligator experienced a reduction in biogeographic range following global cooling around the Eocene-Oligocene transition, although this may be an artifact of low sample size for Oligocene alligators in the Great Plains.

Copyright: Copyright 2015 Society for the Study of Amphibians and Reptiles 2015
<sc>Fig</sc> . 1. 
Fig . 1. 

Biogeographic distributions of late Eocene (Chadronian) and early Oligocene (Orellan) Alligator in the north-central region of North America. (A) Potential biogeographic distribution of late Eocene and early Oligocene Alligator in North America gathered from literature (see text) and this study; (B) definitive biogeographic distribution of late Eocene and early Oligocene Alligator, entirely in the White River Badlands of South Dakota and Nebraska.

<sc>Fig</sc> . 2. 
Fig . 2. 

Partial skull and mandibles of UF 270767 from the late Eocene Chadron Formation in Sioux County, Nebraska. Abbreviations: ang, angular; art, articular; b oc, basioccipital; b po, postorbital bar; ect, ectopterygoid; f, frontal; f st, supratemporal fenestra; j, jugal; l d, left dentary; l max, left maxilla; l n, left nasal; l prf, left prefrontal; l sur, left surangular; o, orbits; o c, occipital condyle; p, parietal; po, postorbital; ptd, pterygoid; q, distal quadrate rami; ra p, retroarticular process; r d, right dentary; r e n, right external naris; r max, right maxilla; r n, right nasal; r prf, right prefrontal; r sur, right surangular; sq, squamosal; symph, mandibular symphysis. Scale bar equals 1 cm.

<sc>Fig</sc> . 3. 
Fig . 3. 

Partial skull table of UF 270767 from the late Eocene Chadron Formation in Sioux County, Nebraska. (A) Dorsal view; (B) ventral view; (C) oblique view; (D) edge-on view. Abbreviations: f, frontal; f st, supratemporal fenestra; p, parietal; po, postorbital; s fp, frontoparietal suture; s fpo, frontal-postorbital suture; s ps, postorbital-squamosal suture; sq, squamosal. Scale bar equals 1 cm.

<sc>Fig</sc> . 4. 
Fig . 4. 

Maxillary fragments of UF 270767 from the late Eocene Chadron Formation in Sioux County, Nebraska. (A) Anterior left maxillary fragment; (B) partial maxillary tooth (m4?) in its alveolus in ventral (left) and side (right) views; (C) posterior right maxillary fragment in medial (left) and ventral (right) views. Abbreviations: ect, ectopterygoid; m1, m4, m7, maxillary alveoli; max, maxilla; pu mt, penultimate maxillary tooth; u ma, ultimate (last) maxillary alveolus. Scale bar equals 1 cm.

<sc>Fig</sc> . 5. 
Fig . 5. 

Occipital skull components of UF 270767 from the late Eocene Chadron Formation in Sioux County, Nebraska. (A) Ventral view; (B) posteroventral view and sketch; (C) dorsal view; (D) lateral view. Abbreviations: boc, basioccipital; bsp, basisphenoid; eo, exoccipital; leu, lateral eustachian tubes; meu, median eustachian tube; oc, occipital condyle; ptd, pterygoid. Scale bar equals 1 cm.

<sc>Fig</sc> . 6. 
Fig . 6. 

Partial anterior dentaries of UF 270767 from the late Eocene Chadron Formation in Sioux County, Nebraska. (A) Dorsal view; (B) ventral view; (C) lateral view and sketch; (D) posteromedial view and sketch. Abbreviations: d3, d4, d7–8, d8, d11, d12, dentary teeth; m g, Meckelian groove; s sp, splenial sutures; symph, mandibular symphysis. Scale bar equals 5 cm.

<sc>Fig</sc> . 7. 
Fig . 7. 

Partial mandibular remains of fossil alligators from the late Eocene Chadron Formation in Sioux County, Nebraska. (A) Posterior left mandible of UF 270767 in lateral view; (B) posterior left mandible of UF 270767 in dorsomedial view; (C) right mandibular fragment (partial surangular and articular) of UF 209734. Abbreviations: ang, angular; art, articular; e m f, external mandibular fenestra; f a, foramen aerum; lf, lingual foramen; ra p, retroarticular process; s sar, surangular-articular suture; sur, surangular. Scale bar equals 1 cm.

<sc>Fig</sc> . 8. 
Fig . 8. 

Consensus cladograms generated from cladistic analyses of 181 characters for 28 alligatoroid taxa using Leidyosuchus canadensis as the outgroup taxon. (A) Strict consensus cladogram of 2,802 trees; (B) strict consensus cladogram of 13,905 trees (generated using multistate coding for character 69) highlighting the Alligator clade; (C) Adams consensus cladogram of 13,905 trees (generated using multistate coding for character 69) highlighting the Alligator clade.

Contributor Notes

1 Corresponding Author. Present address: Department of Earth and Atmospheric Sciences and University of Nebraska State Museum, University of Nebraska–Lincoln, Lincoln, Nebraska USA; E-mail: evanwhiting29@gmail.com
Accepted: 14 Oct 2014
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