A joint seminar of the section Systematics, Biodiversity & Evolution of Plants, the Botanische Staatssammlung München and the Botanical Garden München-Nymphenburg
When? Wednesdays at 12:15 pm (during the semester, unless specified otherwise).
Where? Big Lecture Hall Botanical Institute, Menzinger Str. 67, 80638 München.
Dr. Serjoscha Evers, Urwelt-Museum Oberfranken, Bayreuth, Germany.
Drivers and constraints in the evolution of the turtle body plan
Host: Prof. Dr. Gudrun Kadereit
Turtles have a strongly modified body plan with a rigid shell and an akinetic skull. Despite a Late Triassic origin and a respectable repertoire of ecological specializations, turtles are species-poor today and fossils document low variation of body form over time. In my research, I explore patterns of skull and shell shape evolution in the context of turtle diversification and examine if the presence of the shell acts as an evolutionary constraint on turtle evolvability. For this, my team and I use digitized skeletons of 300 extant and fossil species and combine 3D geometric morphometrics, anatomical network analysis, and phylogenetic comparative methods. Variation in body proportions achieves diversity of body form in many amniotes. Strong, isometric relationships in phylogenetic regressions of humerus-femur-shell lengths show that turtles have strict body proportions that already existed in Triassic turtles.
We propose that the presence of the shell limits evolutionary possibilities of proportional variation in turtles. These constraints, paired with low rates of variance accumulation in body proportions over evolutionary timescales are likely explanations why turtles have conservative body plans. Despite constrained body proportions, turtles have ecologically diversified, and we explore cranial innovations as drivers. Early turtle skull evolution is characterized by bone reductions and the establishment of unusual contacts. These strengthen the akinetic skull and form mechanical prerequisites for later innovations, such as neck retraction. Turtle skull shape has strong relations with neck retraction, which evolved in the Middle Jurassic.
We hypothesize that the evolution of higher neck mobility weakens the integration of skull and shell, enabling turtles to evolve novel feeding modes that facilitate the Jurassic diversification in aquatic settings. Several feeding ecologies (suction feeding, durophagy) later evolved convergently in different lineages. Anatomical network analysis shows that turtles also experience cranial constraints, by having limited topologies of skull bone connections. Diversity of skull form is more strongly affected by shape changes than by topological variation. Thus, cranial shape evolution explains diversification patterns of turtles, whereas constraints in cranial topology and body form imposed by the shell contribute to high morphological conservatism and low species diversity in the group.