Ontogenetic changes in the presacral axial skeleton of Europasaurus holgeri, from the Late Jurassic of Europe.

CARBALLIDO J. L.; SANDER P. M.

Jornada; XXVI Jornadas Argentinas de Paleontología de Vertebrados; 2012

Despite the supremacy of sauropods in the fossil record, early juvenile specimens are extremely rare, limiting the knowledge on sauropod ontogeny. The dwarf camarasauromorph sauropod Europasaurus holgeri Mateus, Laven and Knötschke from the Late Jurassic of Europe, is represented by both, adult and juvenile specimens. Therefore providing an excellent opportunity to bring some light on earlier ontogenetic changes, especially in the presacral axial skeleton (the most represented region). The postcranial axial skeleton of sauropods is extremely modified with respect to the anatomy observed in its ancestors, the ?prosauropods?, proving to be one of the most informative regions of the body. Based in the presence/absence of discrete characters, and in the comparison of juvenile elements with adult specimens, four different Morphological Ontogenetic Stages (MOS) were recognized in Europasaurus. Three of them (MOS1-MOS3) are represented cervical and dorsal elements with open neurocentral suture (considered as immature elements), and the fourth (MOS4) by completely closed sutures in the vertebrae (mature elements). Whereas in early stages (MOS1 and MOS2) derived characters are absents (e.g., prespinal lamina in cervical and dorsal vertebrae, spinodiapophyseal lamina in dorsal vertebrae, lateral expansion in dorsal neural spines, pleurocoels in dorsal centra), all the derived characters (including autapomorphies) can be recognized in what we called late immature specimens (MOS3). This implies that the ?phlygenetic maturity? is reached before neurocentral closure. The link of these MOS with other maturity criteria (e.g. neurocentral closure, sexual maturity) is discussed, in a search of a wider criterion for maturity.