Cranial Polymorphism and Systematics of Miocene and Living Alligator in North America
We examined the osteology of Neogene Alligator, with a focus on fossils from the late Miocene (~8–7 million years ago [Ma]) Moss Acres Racetrack locality in Marion County, Florida, USA. These fossils have been referred previously to Alligator cf. A. mefferdi (early late Miocene, ~12–10 Ma, Nebraska), an extinct species that we and others have found to be lacking autapomorphic characters. Furthermore, numerous cranial polymorphisms, previously regarded as diagnostic autapomorphies or synapomorphies, exist in several species of Alligator, particularly in Alligator prenasalis (late Eocene–early Oligocene, ~36–33 Ma, South Dakota and possibly Nebraska), Alligator olseni (early Miocene, ~18–17 Ma, Florida), and the extant American Alligator (Alligator mississippiensis; southeastern United States). Except for minute differences in two scapular characters, the fossil Alligator from Moss Acres Racetrack is virtually indistinguishable from the A. mississippiensis morphotype, suggesting its referral to that lineage rather than to an extinct species. Cladistic analysis upholds this notion, with A. mississippiensis and the Moss Acres Racetrack Alligator being sister taxa in a unified clade isolated from A. mefferdi. This implies that the A. mississippiensis morphological lineage has existed in North America with very little change for the past 7–8 million years.Abstract
Map of the modern continental United States showing the approximate current geographic range of the extant American Alligator (Alligator mississippiensis) in gray; this range was compiled and modified from those of Malone (1979), Lance (1989), and the United States Geological Survey (Somma, 2015). Nebraska inset (top right) shows approximate localities for the holotype specimens of Alligator thomsoni (1) and Alligator mefferdi (2) in Sioux and Cherry Counties, respectively. Florida inset (bottom right) with enlarged county map of the north-central region of the state shows approximate localities for the holotype specimen of Alligator olseni (3) in Gilchrist County, and the location of the Moss Acres Racetrack Alligator (4) in Marion County; see text for further details.
UF 115627, fossil Alligator skull with articulated, incomplete mandibles from the late Miocene (Hemphillian NALMA) Moss Acres Racetrack vertebrate fossil locality, Marion County, Florida, USA. (A) Dorsal aspect; (B) ventral aspect; (C) right lateral aspect. Scale bar equals 10 cm.
UF 69930, fossil Alligator left anterior dentary fragment from the late Miocene (Hemphillian NALMA) Moss Acres Racetrack vertebrate fossil locality, Marion County, Florida, USA, in medial (lingual) aspect. Note the absence of an anterior foramen intermandibularis oralis perforation of the splenial, contra Snyder (2001, 2007). Scale bar equals 5 cm.
Polymorphism and variation of character state expression for the lateral palatine surfaces in the extant American Alligator (Alligator mississippiensis) from Florida. (A) Lateral palatine shelves present and prominent on the lateral sides of the palatines, projecting into the suborbital fenestrae (UF 39106, Herpetology Collection); (B) lateral palatine shelves not present or prominent, with little or no projections into the suborbital fenestrae (UF 1317, Zooarchaeology Collection).
Polymorphism of the lingual foramen in Alligator olseni from the early Miocene (Hemingfordian NALMA) Thomas Farm vertebrate fossil locality, Gilchrist County, Florida, USA. (A) Right surangular (UF 201642) in medial aspect, displaying the symplesiomorphic condition for Alligatorinae with the lingual foramen positioned exclusively on the surangular bone; (B) right surangular (UF 200726) in medial aspect, displaying the synapomorphic condition for Alligator with the lingual foramen positioned on the surangular–articular suture. Scale bar equals 5 cm.
Polymorphism and variation of the frontoparietal suture in the extant American Alligator (Alligator mississippiensis) from Florida. (A) Frontoparietal suture is “concavo-convex” between the anterior corners of the supratemporal fenestrae (UF 3315, Zooarchaeology Collection); (B) frontoparietal suture is roughly straight between the anterior corners of the supratemporal fenestrae (UF 39168, Herpetology Collection).
Strict consensus cladogram of 54 trees (139 steps) generated from the cladistic analysis of 29 alligatorid ingroup taxa with three nonalligatorid alligatoroid taxa as the outgroup, incorporating new morphological codings for the Moss Acres Racetrack Alligator fossils and recodings for some polymorphic characters in several Alligator species (see text for details). In this resulting cladogram, the Moss Acres Alligator falls out as sister to the extant American Alligator (Alligator mississippiensis), and is more derived than the Nebraska metataxon, Alligator mefferdi; therefore, this most parsimonious topology provides support for the Moss Acres Racetrack Alligator belonging to the A. mississippiensis morphotypic lineage. Extant alligatorid taxa in our strict consensus cladogram are denoted by an asterisk (*), and include the two living Alligator species and several modern caimans.
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