HIGH LEVELS OF HISTOVARIABILITY IN WILD ALLIGATORS INFORM THE PALEOBIOLOGY OF AN EXTINCT APPALACHIAN ALLIGATOR

Histological analyses of long bone thin sections are commonly used to infer life history parameters in extinct vertebrates, particularly within Archosauria. Meaningful interpretation of fossil bone histology requires an understanding of the myriad factors influencing bone growth, which is necessarily based on the study of living animals. Much of our knowledge of archosaurian paleohistology is derived from the two extant lineages: avian dinosaurs and crocodilians. The American alligator (Alligator mississippiensis) is widely available for study in the United States and has been the subject of several osteohistological analyses. These used one or a few specimens and, in one study, a sample of 30 pen-raised captive-hatched specimens. However, few have explored histovariability in a statistically robust sample of wild individuals. To help fill this gap in our knowledge of archosaurian osteohistology we prepared and analyzed humeral and femoral thin sections from 87 wild A. mississippiensis from across their natural range. We collected data on bone tissue type and compared counts and measurements of circumferential growth marks (CGMs) with femoral length, which is strongly correlated with body size. These data revealed weak but statistically significant correlation of inferred growth rate with various bioclimatic variables. However, there is a high level of variation in bone tissue type and preserved CGMs among specimens suggesting heretofore unidentified sources of variation in growth rate, which may profoundly influence the histological interpretation of a single individual. We compared the modern data with histological thin sections of early Pliocene Alligator sp. femora from the Gray Fossil Site (GFS), Washington Co., Tennessee, an isolated lacustrine deposit of karst origin. Results show that the GFS Alligator grew more slowly than A. mississippiensis and may have reached reproductive maturity at a smaller size. Most species of Alligator are smaller than A. mississippiensis, and our findings suggest a reduced growth rate may be plesiomorphic for the genus. Unfavorable conditions for alligators at the GFS sinkhole pond could also account for slow growth and earlier maturity, but similarity in GFS paleoclimate to coastal North Carolina and abundant prey animals make this unlikely.