Evolução do tamanho corporal dos amonoides (Mollusca, Cephalopoda) da bacia sedimentar de Sergipe- Alagoas ao longo do cretáceo

Abstract

Macroevolutionary studies consider body size one of the most diagnostic phenotypes to understanding the organisms -environment interaction in front of the influence of physical, ecological and evolutionary principles. To determine the evolutionary patterns of body size, fossils are the main tools that allow visualize the morphological variations over geological time. One of the most significant evolutionary patterns, Cope’s Rule, corresponds to a tendency to increase organisms body size over geologic time, expressed as an adaptive general advantage. Some evolutionary models include punctuated balance, stasis and Ornstein-Uhlenbeck, which have already been documented in different groups of organisms. The Sergipe-Alagoas Basin presents a rich and abundant fossiliferous content of marine organisms, mainly of molds of external shells of the extinct amonoids, in a time interval of at least 25 Ma (Aptian superior to the Campanian). Due to the abundance in the records of the Cretaceous rocks of the Sergipe-Alagoas Basin, the ammonoids were selected to be investigated in this work. Therefore, the objective of this research was to investigate if the size of the ammonia shells varied along the Cretaceous, using the geometric morphometry technique, as well as to determine by what mode the evolution of their body size occurred. A total of 2045 fossils were analyzed, but after analyzing the degree of preservation, 506 ammonites were selected to be photographed. The selected fossils belongs to different species and they are registered in the Laboratory of Paleontology of the Federal University of Sergipe comprising the ages, Late Aptian to the Turonian / Coniacian (between ~ 115 Ma and 90 Ma). The images were treated in the software TPSutil and TPSdig, inserting seven landmarks in each photo, in lateral view, obtaining a centroid value (Cs) for each fossil. The morphometric analysis and the investigation of the evolutionary models were performed in the R platform using respectively the "geomorph" and "PaleoTS" packages. The results showed an increase in the body size of the ammonites along the Cretaceous of the Sergipe-Alagoas Basin and the model that best represents the evolutionary pattern of the ammonite body size was the punctuated equilibrium (~94 Ma). Considering the paleoenvironmental scenario of the SergipeAlagoas Basin during the Cretaceous, the possible causes for the increase in body size are related to geological and/or paleoenvironmental events that occurred at the punctuated moment. Therefore, the expansion of the seas along the Cenomanian and possible extinction events at the end of the Cenomanian could have led to the displacement of the ammonite optimal body size of the Sergipe-Alagoas Basin.