New early Pleistocene Alligator (Eusuchia: Crocodylia) from Florida bridges a gap in Alligator evolution

Abstract

The American Alligator (Alligator mississippiensis) is one of two species of Alligator in the modern world. It is only distantly related to the other extant species (A. sinensis), with much closer relatives known from the geologic past of North America. A disparity exists, though, in the fossil record between A. mississippiensis and its close relative, the late Miocene (?)—early Pliocene A. mefferdi. While A. mississippiensis is known from the mid-Pleistocene and later, few Alligator remains were known from the earliest Pleistocene of North America until the discovery of the Haile 7C and 7G early Pleistocene (Blancan Land Mammal Age) sites from Alachua County, Florida. The Haile alligators exhibit a suite of characters from both A. mississippiensis and A. mefferdi, displaying intermediate morphology in time. The Haile alligators are distinct from either of the aforementioned taxa, and a new species, Alligator hailensis is suggested, bridging an important gap in the evolution of the American Alligator.

Some Thoughts on Interspecific Mandibular Morphology in Fossil and Modern Alligator

“…The missing splenial in each is shadow outlined. The inferred shape of the splenial in A. hailensis is based on Stout (2020). Table 2.…”

“…As suggested by Stout (2020), the relative height of the splenial is detectable without preservation of its anterior extent and can offer some insight into the character state housed there. The range of observed relative splenial height in Alligator mississippiensis (across diverse sizes of individuals) does not overlap with the closely-related A. hailensis (Figure 4).…”

“…Most modern and fossil crocodylians exhibit slight curvature along this length, though some (such as the early Miocene Alligator olseni) are scored as possessing deep curvature, while others (such as the longirostrine gharial Gavialis gangeticus) are interpreted as linear (Brochu, 1999). Despite the obvious problem of subjectivity in evaluating "slight" versus "deep" there also appears to be a large degree of variability within individuals of the extinct Alligator hailensis (Stout, 2020) and modern A. mississippiensis (pers. obs.).…”

“…Additionally, some characters that are phylogenetically and taxonomically informative might only rarely be preserved in the fossil record due to their fragile nature. This is especially true of the anterior portion of the splenial in derived taxa (Stout, 2020). The splenial is a bone found in the lower jaw of many tetrapods (Romer, 1956), and occurs in crocodylians as a laterally thin flange making up much of the lingual margin of the Meckelian canal (Iordansky, 1973).…”

“…In derived Alligator, the splenial does not participate in the mandibular symphysis and shows either presence or absence of the anterior foramen intermandibularis oralis (an informative diagnostic feature in Alligator species, Brochu, 1999). Stout (2020) described Alligator hailensis partially on the basis of an enlarged and autapomorphic anterior portion of the splenial preserved in a single individual, but suggested that the expanded splenial might be detectable as a scar on the dentary at the 5 th alveolus (the approximate location of the preserved autapomorphic foramen), even if not preserved directly.…”