Embryogenesis and ossification of Emydura subglobosa (Testudines, Pleurodira, Chelidae) and patterns of turtle development

Using the Standard Event System (SES) to study patterns of vertebrate development, we describe a series of 17 embryos of the pleurodire turtle Emydura subglobosa. Based on a sequence heterochrony analysis including 23 tetrapod taxa, we identified autapomorphic developmental shifts that characterise Testudines, Cryptodira, and Pleurodira. The main results are that Testudines are characterised by an autapomorphic late neck development, whereas pleurodires and cryptodires show a different developmental timing of the mandibular process. Additionally, we described the ossification pattern of E. subglobosa and compared the data to those of five other turtles. Pleurodires show the epiplastron to ossify before or simultaneously with maxilla and dentary. In contrast, cryptodires show a later ossification of this bone. Because evolutionary developmental studies on turtles have previously focused only on "model organisms" that all belong to Cryptodira, we underline the necessity to include a pleurodire taxon for a more comprehensive, phylogenetically more informative approach.