Alligator mississippiensis Modern

Hastings, Alexander K., Schubert, Blaine W., Bourque, Jason R. & Hulbert, Richard C., 2022, Oldest record of Alligator in southeastern North America, Palaeontologia Electronica (a 6) 26 (1), pp. 1-19 : 10-13

Alligator mississippiensis Modern 271

Wyoming Miller-Camp, 2016 YPM-PU 8357

Wyoming Miller-Camp, 2016 YPM-PU 8449

Wyoming Miller-Camp, 2016 MCZ 4367

Wyoming Mook, 1961 AMNH 60491

Wyoming Mook, 1941 CM 9600

Utah Gilmore, 1946 USNM 159962

North Dakota This Study SMM P76.28.247

South Dakota This Study MCZ 1014

South Dakota This Study MCZ 1015

South Dakota Miller-Camp, 2016 YPM 13799

South Dakota Miller-Camp, 2016 YPM 14063

South Dakota Miller-Camp, 2016 YPM 16273

South Dakota Miller-Camp, 2016 SDSM 243

Florida This Study MCZ 1899

Florida This Study MCZ 1887 [Type]

Florida This Study MCZ 101578

Nebraska Miller-Camp, 2016 AMNH 7905

Nebraska Miller-Camp, 2016 FMNH P 26242

No data This Study ETMNH-Z 10937

No data This Study ETMNH-Z 17916

No data This Study ETMNH-Z 5289

No data This Study ETMNH-Z 507

Georgia This Study ETMNH-Z 5025

Georgia This Study ETMNH-Z 7144

Georgia This Study ETMNH-Z 7217

occurs approximately 1 year after hatching, in the area of the presumptive nuchal shield” (Vickaryous and Hall, 2008). Even at this stage, osteoderms begin development along the center, where the median keel eventually forms, then grow outward to the lateral edges (Vickaryous and Hall, 2008). Development of additional features such as lateral sutures and imbricating shelves occurs even later in ontogeny, further indicating growth well past the yearling stage for the Oligocene individuals. However, without morphological studies that track osteoderm features with maturity, there is not currently a way to be more definitive about how old these individuals were outside osteohistological analyses, which are beyond the scope of this paper.

Premaxillary Measurement R 2 Value Overall Width 0.9125

Length of premaxillary-maxillary suture 0.9917

Length of left-right premaxillary suture 0.7712

Width of 4th Premaxillary Alveolus 0.9221

Length of 4 th Premaxillary Alveolus 0.9149

Length of 5 th Premaxillary Alveolus 0.9101

Shortly after Alligator appears in the fossil record of the Great Plains, the region experienced a massive drop in mean annual temperature (8.2±3.1°C), at the beginning of the Oligocene (Zanazzi et al., 2007). Following this, temperature generally warmed through the Oligocene, particularly at the time of the Brooksville 2 fauna (Zachos et al., 2001), which has been further supported by pollen analysis on the eastern coast of North America (Kotthoff et al., 2015; Figure 7 ). Body size in ectothermic vertebrates often relates to mean annual temperature (Makarieva et al., 2005 a,b; Head et al., 2009). Paleotemperature data are currently lacking for the Oligocene of Florida, and thus temperature differences between the Great Plains and Florida at this time remain unknown. Given our current knowledge, Alligator may have originated in the Great Plains during the middle Paleocene–late Eocene and dispersed to the southeastern region of North America sometime prior to the late Oligocene. Large temperature drops in the Great Plains could have been a factor in this hypothetical dispersal. However, despite potentially being a warmer climate in Florida, small body size may have been retained, as no large-bodied individuals have yet been recovered at any Oligocene Florida site. This may indicate a lag between the relationships of body size and latitudinal migration. Figure 7 shows dorsal skull length (as a proxy for body size) across the ancestral Alligator taxa and Oligocene–Miocene species against global temperatures. The occurrence of UF 422816 is actually smaller than the known Alligator prenasalis adults recorded from higher latitudes, but again, sub-adult status cannot be fully ruled out. The position of UF 422816 in time; however, is prior to a significant increase in temperature that precedes the occurrence of the larger Alligator olseni , which may have been able to grow larger in these generally warmer temperatures. Meanwhile, Alligator mcgrewi retained small size in the presumably cooler latitude of Nebraska at the time. The morphological similarity and potential body size similarity between the Oligocene Alligator of Florida and those of the Great Plains indicates a possible faunal affinity between the two regions. Florida has been suggested as a post-Eocene refugium for the turtle Xenochelys , which has its latest fossil occurrence at Brooksville 2 and was previously known from the Eocene of the midwestern United States (Bourque, 2012).

Alternatively, the records of Alligator in the Great Plains prior to the new material, combined with the sparse record of Eocene–Oligocene terrestrial deposits in eastern North America, may give an illusion of migration towards the southeast. Thus, it is possible that Alligator had a distribution that expanded to coastal areas when they occurred in the Great Plains, and over time climate change resulted in the extirpation of more northern populations. Further fossil material from the southeastern United States may clarify the origin and paleogeography of Alligator .

Regardless, the presence of Alligator in Florida during the Oligocene increases the antiquity of the genus in southeastern North America by an additional 10–8 million years, so it now has a ca. 26 million-year history in the region. During much of this time, Alligator has been the largest predator in its aquatic environment, but this fossil occurrence suggests the possibility that this niche may not have been the ancestral condition for southeastern populations.

Figures

FIGURE 7. Box and whisker plots of dorsal skull length of published fossil Alligator specimens and extant Alligator mississippiensis (this study). For data, see Table 3. Time shown in millions of years ago (MYA). UF 422816, an isolated right premaxilla, was estimated following methods outlined in the main document. Most notable is that the estimated dorsal skull length for UF 422816 is small, similar to ancestral forms such as Allognathosuchus, Procaimanoidea, and Wannaganosuchus. Global temperature curve from Zachos et al. (2001).