by Carl Strang
I have long had a fondness for turtles. These strange animals were diverse and abundant in the lake below my childhood home. Two studies of this group caught my eye in 2013.
Wang, Zhuo, et al. The draft genomes of soft-shell turtle and green sea turtle yield insights into the development and evolution of the turtle-specific body plan. Nature Genetics, 2013; DOI: 10.1038/ng.2615
As described in a ScienceDaily article. This was a genomic study with an evo-devo component that looked at those two highly different turtle species. They placed the turtles with the archosaurs (the group including dinosaurs, birds and crocodiles) rather than being more primitively reptilian. They place the split of the turtle line at 250 million years ago, coinciding with the great end-Permian extinction. Shell development makes use of genes connected with limb development (perhaps bone-extending elements that expand the ribs and sternum to form the shell? This article did not get that specific). Turtles also are found to have the greatest range of odor detection capability in vertebrates other than mammals.
Tyler R. Lyson, Gabe S. Bever, Torsten M. Scheyer, Allison Y. Hsiang, Jacques A. Gauthier. Evolutionary Origin of the Turtle Shell. Current Biology, 2013; DOI: 10.1016/j.cub.2013.05.003
From a ScienceDaily article. They described a fossil from South Africa, Euntosaurus africanus, which is a transitional turtle. The ribs are widened but not fused. This animal lived 260 million years ago, in the Permian. Turtles had complete shells by 215 million years ago. Odontochelys semitestacea, a previously described Chinese fossil of 220 million years ago, had a complete plastron but incomplete carapace.
The first study used genetic methods, and therefore molecular clock estimation, to place the timing of turtle evolution. The agreement with the fossil aging in the second study is impressive.