Histological analysis of long bone thin sections has in recent decades been broadly applied to infer growth rates and ecology in extinct vertebrates, particularly within non-analogous clades. Meaningful interpretation of bone histology and extrapolation to an extinct organism’s life history requires a robust understanding of the factors influencing bone growth and histological presentation. Archosaurs are commonly the subject of osteohistological studies and, as such, much of our knowledge of their paleohistology is derived from the two extant lineages: avian dinosaurs and crocodilians. The American alligator (A. mississippiensis) is widely available for study in the United States and several osteohistological analyses have been published. These studies focused on intraskeletal variation, histovariability, and skeletochronology in one or a few specimens, and in one study a larger sample of pen-raised captive born specimens. However, no published studies test paleohistological assumptions using large-scale geographic and climatic variation in bone histology among extant wild crocodilians. To fill this gap in our knowledge of archosaurian osteohistology, we prepared a collection of humeral and femoral thin sections of 45 Alligator from North Carolina, Arkansas, Georgia, and South Carolina. Previously prepared thin sections from pen-raised Louisiana Alligator were also included. For this study we began by comparing growth rates inferred from counts and measurements of arrested growth (LAGs), which represent annual cycles, and femoral dimensions which correlate strongly to body length. Comparison of these data revealed that, on average, Alligator specimens subject to shorter growing seasons (i.e., those in cooler climates) tend to show more LAGs when compared to more southerly Alligator specimens of similar size. Bone tissue also varies between specimens, suggesting a variable tempo of bone growth in response to differing climatic and environmental regimes. Finally, histological thin sections of early Pliocene Alligator fossils from the Gray Fossil Site (GFS), Washington Co., Tennessee were prepared to explore the paleobiology of this biogeographically unique Appalachian alligator. Assessing variation in this fossil taxon’s closest living relative (A. mississippiensis) provides insight into the paleoecology and growth rates of the GFS Alligator, as well as the climate of the southern Appalachians during the past. Results suggest that the fossil species may have grown more slowly than extant relatives along the southeastern United States coastal plain, and that some parts of the skeleton reached asymptotic growth at a smaller size.

Histological analysis of long bone thin sections is commonly used to infer growth rates and ecology of extinct vertebrates, particularly within Archosauria. However, most comparative neontological studies have used small samples of captive individuals, limiting the scope of variation. To fill this gap, 44 femoral thin sections of wild Alligator mississippiensis were prepared and analyzed. Comparison of slides revealed that larger individuals from cooler climates tend to show more LAGs compared to southerly A. mississippiensis of similar size, however, there is considerable variation. This pronounced variation in wild specimens emphasizes the need to use caution when interpreting paleohistological data with little modern comparative samples. Finally, thin sections of early Pliocene Alligator sp. fossils from the Gray Fossil Site (GFS), Washington Co., Tennessee were prepared. The GFS Alligator grew more slowly than A. mississippiensis examined and may have reached reproductive maturity at smaller sizes.

Submitted by Fabio Sobreira Campos da Costa (fabio.sobreira@ufpe.br) on 2017-02-16T14:09:38Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) 1.UNIVERSIDADE FEDERAL DE PERNAMBUCO - PDF.pdf: 2290380 bytes, checksum: 779f1a4fa95fe4cc4bf311af3f117d25 (MD5) / Made available in DSpace on 2017-02-16T14:09:38Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) 1.UNIVERSIDADE FEDERAL DE PERNAMBUCO - PDF.pdf: 2290380 bytes, checksum: 779f1a4fa95fe4cc4bf311af3f117d25 (MD5) Previous issue date: 2016-03-04 / CAPES / Apresentamos uma descrição detalhada da morfologia e osteohistologia de pequenos ossos de Arcosauromorpha encontrados da Zona de Associação de Hyperodapedon, referente à Sequência Santa Maria II, Supersequência Santa Maria, Triássico Superior do Rio Grande do Sul, Brasil. O material é composto por ossos de tamanho reduzido, com dimensões menores do que 57mm, encontrados associados in situ. O espécime é composto por um úmero e um metatarsal III direitos, extremidade proximal de uma falange ungueal, um arco neural quase completo de uma vértebra pré-sacral e um centro vertebral pré-sacral. Comparações morfológicas entre UFSM11326 e outros organismos do Triássico mostraram se tratar de um dinossauriano, com possível associação ao grupo Sauropodomorpha. São elas: morfologia da crista deltopeitoral do úmero apresentando prolongamento do cume na crista deltoide; eixo de torção das porções proximal e distal do úmero; morfologia do metatarsal III, apresentando ângulo de torção entre as extremidades proximal e distal superior a 60° e presença do ombro medial; posições da diapófise, parapófise e das infracavidades diapofisiais restantes no arco neural. Este grupo de arcossauros já foi registrado anteriormente em estratos triássicos da Supersequência Santa Maria. As análises do fechamento das suturas do arco neural indicaram um indivíduo jovem, com suturas abertas em forma de zíper. As sessões finas da diáfise do úmero e do metatarsal III, corroboram este estado ontogenético, indicando se tratar de um espécime em fase inicial de desenvolvimento. Exibiu um complexo ósseo fibrolamelar (comum em espécimes dinossaurianos), composto por ósteons primários, sem ocorrência de marcas de crescimento (LAGs ou annuli), áreas extensas de reabsorção ou a presença de lamelas circunferenciais externas (external fundamental system = EFS). Este padrão evidencia uma estratégia de crescimento rápido, sustentado por elevadas taxas metabólicas, superiores a dos répteis modernos, e comparáveis com aquelas já encontradas para este grupo de arcossauros. Definimos portanto que UFSM11326 corresponde a um Sauropodomorpha jovem, sem a conclusão de seu crescimento assintótico, com elevados níveis de deposição óssea e consequente crescimento acelerado até o momento de sua morte. / Here we present a detailed description of the morphology and osteohistology of small bones of Arcosauromorpha found at the Hyperodapedon Association Zone, referring to the Santa Maria II Sequence, Santa Maria Supersequence, Upper Triassic of Rio Grande do Sul States, Brazil. The material consists of small sized bone (dimensions smaller than 57mm) found associated in situ. The specimen consists of a right humerus and metatarsal III, proximal extremity of an ungueal phalanx, an almost complete neural arch of a pre-sacral vertebra, and a pre-sacral vertebral center. Morphological comparisons between UFSM11326 and other organisms from Triassic revealed that it is a dinosaurian, possibly related to Sauropodomorpha group. The similarities include the morphology of the deltopectoral humerus crest that has an extension of the deltoid crest; twist axis from proximal and distal portions of the humerus; morphology of metatarsal III with torsion angle between the proximal and distal extremities greater than 60° and medial shoulder; positions of the infracavities of the apophysis, parapophysis and diapophysal remaining at neural arch. This archosaurs group has already been registered previously in Triassic strata of the Santa Maria Supersequence. The analyses of the closure of the sutures of the neural arch indicate it to be a young individual with open sutures zipper-shaped. The final sessions of the humeral diaphysis and metarsal III corroborate this ontogenetic state, showing it to be a specimen at an early stage of development. A fibrolamellar bone complex that is common in dinosaurian specimens is present, composed of primary osteons, without occurrence of growth marks (LAGs or annuli), extensive reabsorption areas or external circumferential lamellas (external fundamental system = EFS). This pattern shows a rapid growth strategy, supported by high metabolic rates that are greater than in modern reptiles and comparable with those already found to this archosaurs group. We conclude that UFSM11326 corresponds to a young Sauropodomorpha without completing its asymptotic growth and with high levels of bone deposition and subsequent rapid growth until the moment of his death.