I've just purchased your turtle sets. They are very nice. I hope you will include a giant Galapagos or Aldabra tortoise in a future set (or maybe even both), and I think you mentioned that you may eventually also do a set on the seven extant marine turtles.
I'd like to post a series of proto-turtles here in the hope that it may inspire you to do a series on these too. I am sure a lot of people will be interested to know how the turtle got its shell.
Turtles haven’t changed much over the past 210 million years. They all have a top shell (carapace) formed from the fusion of their spine and ribs, a bottom shell (plastron) that protects their belly, a sharp beak and a mouth without any teeth. But the group lacks a feature common to most modern reptiles - two pairs of holes in their skull, behind their eyes, where jaw muscles attached (diapsid). The absence of those holes (anapsid) has contributed to a decades-long debate on the exact position of turtles on the reptile family tree. Turtles are now considered diapsids that that have become secondarily anapsid.
A series of proto-turtles and turtles in a developmental sequence from unarmoured to heavily armoured:
1.
Eunotosaurus africanus, a 30 centimetre long fossorial proto-turtle that lived 260 million years ago in South Africa, possessed wide and flat ribs that gave it a distinctly rounded and turtle-like profile. Like turtles, it did not have the intercostal muscles that moves the rib cage. It mixed features of its lizard-like ancestors with emerging turtle-like characteristics. It was first described over a century ago and was for a long time ascribed to the Parareptilia (a primitive anapsid). It was only recently discovered (in 2015) that the skull of
Eunotosaurus grew in such a way that its diapsid nature is obvious in juveniles but almost completely obscured in adults (a ‘cryptic’ diapsid). It was a diapsid reptile in the process of becoming secondarily anapsid - a transitional "turtle". It is the oldest precursor of turtles. It is interesting to note that the broad and flat rib precursor to the turtle's protectective shell did not initially evolve for protection but for burrowing.
Eunotosaurus it seems was a fine-tuned digging machine. Broadening ribs in turtle ancestors eventually evolved into the protective shell that turtles have today.
A reconstruction of
Eunotosaurus africanus:
A reconstruction of the skeleton of
Eunotosaurus africanus:
2.
Pappochelys rosinae (
Pappochelys translates as "grandfather turtle"), was discovered in Germany in 2015. The mystery of how the turtle got its shell has been a long-standing question in evolutionary biology. In the case of
Pappochelys, we see that its belly was protected by an array of rod-like bones, some of which are already fused to each other.
Pappochelys had a wide body, small skull, and a long tail that makes up about half of its total body length of 20 centimetres. The discovery of this species confirms that the belly portion of the turtle shell, the plastron, formed through the fusion of rib-like structures and parts of the shoulder girdle. The physical traits of
Pappochelys make it a clear intermediate between two of the earliest known "turtles",
Eunotosaurus (a turtle precursor) and
Odontochelys (a true turtle). It lived approximately 240 million years ago, also intermediate between the other two. It is a diapsid.
A reconstruction of
Pappochelys rosinae:
A reconstruction of the skeleton of
Pappochelys rosinae showing the beginnings of a plastron (in red):
3.
Odontochelys semitestacea, diccovered in 2008, a half-shell turtle species that swam in China's coastal waters 220 million years ago was the oldest turtle known to date (
Odontochelys semitestacea literally means "toothed turtle with a half-shell"). This turtle had a belly shell, but its back was basically bare of armour although its wide ribs hinted at the beginnings of a top shell. In a way, modern turtles replay the evolutionary history of their ancestors as their embryos mature, for the plates of their plastron harden before those of their carapace.
Odontochelys differed grossly from modern turtles. Modern turtles have a horny beak without teeth in their mouth. In contrast,
Odontochelys fossils were found to have had teeth embedded in their upper and lower jaws. It lacked a beak and has no holes in its skull (an anapsid). It is quite small, approximately 35 centimetres long.
A reconstruction of
Odontochelys semitestacea:
Fossil of
Odontochelys semitestacea showing both dorsal and ventral views:
A diagram showing the evolution of the turtle body plan:
4.
Eorhynchochelys sinensis (
Eorhynchochelys translates as “dawn beaked turtle”) is the latest discovery (it was only discovered in 2018) and fills an evolutionary hole in how the reptiles developed the features of a turtle such as a beak and shell and a lack of holes in its skull. This large, roughly 1.8 metre-long animal, lived about 230 million years ago (intermediate between
Pappochelys and
Odontochelys) in coastal Southern China. This fossil turtle possesses a single pair of holes behind its eyes, suggesting a gradual transition from
Pappochelys (two pairs) to
Odontochelys and modern turtles (none). It had a strange disc-like body with no shell and a long tail (like
Eunotosaurus and different from
Pappochelys and
Odontochelys both of which had plastrons), and the anterior part of its jaws developed into a strange-looking beak. By developing a beak before other turtles, while having no semblance of a shell, this early "turtle" is a prime example of mosaic evolution, in which traits evolve independently and at different times.
A reconstruction of
Eorhynchochelys sinensis (note the small partial beak on the anterior part of its jaw):
An
Eorhynchochelys sinensis fossil:
