And This Week's 'Non-existent' Transitional Fossil Is...

230 million year-old Eorhynchochelys sinensis from Guizhou, China.

Photo Credit: Xiao-Chun Wu

230-million-year-old turtle fossil deepens mystery of reptile's origins | Nature.

This weeks 'non-existent' transitional fossil is... a turtle without a shell!

This 230 million year-old specimen of a stem turtle comes from Guizhou province of south west China was more than 2 metres long. It shows the rib cage expanding but there are no signs of a shell. And it has a beak.

It provides us with an illustration of how science proceeds and how new information can cause us to revise and re-assess what we thought we already knew and how it can even cloud the issue rather than clarify it.

One of the mysteries still be to resolved in the evolution of the turtles is exactly how they relate to the other reptiles and this specimen, although showing us how the early turtles evolved does little to resolve that question. If anything, it complicates things. One of the characteristics of modern turtles is that they lack a pair of pits behind their eyes where jaw muscles are attached. These are found in all other reptiles. The other turtle characteristics are a toothless beak and of course the familiar shell consisting of a top shell or carapace, derived from the ribs, spine, shoulder and pelvic girdle and a bottom shell or plastron.

Artist's impression of Eorhynchochelys sinensis.

Credit: Yu Chen

This specimen has those pits behind the eyes, and so shows that turtles share a common ancestry with the other reptiles but it still doesn't show which group of reptiles turtles are closest to and it has no sign of a shell other than the widened ribs. Most genetic studies have suggested turtles are closer to the crocodiles, dinosaurs and modern birds but other studies of DNA and anatomy have pointed to lizards and even snakes as the closer group.

A 220 million year old species, Odontochelys semitestacea, had teeth but no beak, a plastron but no carapace, and it lacked the skull pits. A 240 million year-old species, Pappochelys rosinae on the other hand had the skull pits, the beginnings of a plastron but again no carapace. The discovery of E. sinensis fills the gap between P. rosinae and modern turtles and in that respect both are stages in the transition between the stem turtles and modern turtles.

But there is on slight complication - the beak of E. sinensis. This characteristic places it in direct ancestry to modern turtles but pushes O. semitestacea, with its teeth and absent beak, out to a side branch of the evolutionary tree.

So, although we have evidence that turtles evolved beaks earlier than was thought and we now have good transitional specimens between early, later and modern turtles, we still don't know exactly what the common ancestor of turtles and the other reptiles was. An illustration of how more information can sometime confuse the emerging picture and maybe cause us to revise and re-evaluate what we thought we knew.

What creationists will need to ignore are the age of the specimen (Cue: all the dating methods are wrong; it's a forgery, etc) and the fact that it is yet another transitional species (Cue: they don't exists so it can't be).

Reference:

Li, C., Fraser, N. C., Rieppel, O., & Wu, X.-C. (2018).
A Triassic stem turtle with an edentulous beak.
Nature, 560(7719), 476–479. DOI: 10.1038/s41586-018-0419-1

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