Onto the Land and Back to the Sea: The Amniotes 249They also had relatively large eyes, suggesting that they may have been night predators,
catching insects and other small prey. The best specimens of Hylonomus were found pre-
served inside hollow rotten tree trunks in the famous Joggins locality, suggesting to some
that they had been trapped in these deep holes and died there. More recently, paleontologists
have argued that the long delicate limbs and fingers suggest that they were good climbers
like many modern lizards and probably lived in the hollows of trees, where they were occa-
sionally buried and fossilized.
From early forms like Westlothiana and Hylonomus, the amniotes radiated into many
different groups by the Late Carboniferous (fig. 11.1). One of the earliest lineages to branch
off was the synapsids, which eventually gave rise to mammals. They will be discussed in
chapter 13. The other lineage is the one we will call the true Reptilia. Its most primitive
branch is the group known as Anapsida, which includes the turtles as well as several extinct
groups. The next major branch is the Euryapsida, which includes most of the marine reptiles.
The third branch is the Lepidosauria, which includes not only the living lizards and snakes
but also extinct groups like the marine reptiles known as mosasaurs. The final branch is the
Archosauria, or “ruling reptiles,” and these include a lot of familiar creatures: crocodilians,
pterodactyls, dinosaurs, and birds. For space reasons, we cannot go into detail with each of
these groups or look at every transitional form in every lineage. Instead, we will focus on a
few examples of the many transitional forms found in certain lineages to demonstrate just
how good the fossil record of these creatures has become.
Turtle on the Half Shell
Creationists often mock scientists and proclaim how impossible it is to imagine a creature
that is “half a turtle.” Once again, the fossil record has answered them with a perfect tran-
sitional fossil (Li et al. 2008) that rebukes them in turn. Officially known as Odontochelys
semitestacea, its name literally means “toothed turtle with half a shell.” A number of speci-
mens have been found in Triassic deposits of China, and they are truly remarkable (fig. 11.5).
They have a fully developed shell on their belly (the plastron), but their backs are just broad
expanded ribs without a back shell (the carapace). They are literally “turtles on the half
shell.” In addition, they are the last known turtle to have teeth. All more advanced turtles
have toothless beaks. Thus, they bridge the gap between more lizard-like reptiles and turtles
with fully developed shells on their back and belly.
If a creationist replies by saying, “Where is the transition between this fossil and other
reptiles?,” we have those fossils too. Eunotosaurus is an extinct amniote from the Permian of
South Africa that has the broadly expanded back ribs of Odontochelys and more advanced
turtles, along with some other features of the skull and skeleton, that link it to turtles—yet to
the untrained eye it looks like a large, fat lizard. Then, in 2015, my friend Hans-Dieter Sues
of the Smithsonian Institution announced Pappochelys (“grandfather turtle”), which not only
has the broad back ribs of Eunotosaurus but also the broad flattened belly bones (“gastralia”)
that would eventually fuse to become the belly plate (plastron) of Odontochelys. This makes
it a nice transition from Eunotosaurus to all the rest of the turtles—but it has only broad flat
ribs on its belly and back, not a fused shell.
Thus, from the very lizard-like Eunotosaurus, we now have a complete sequence to Pappo-
chelys, Odontochelys, and then to fossils that even a creationist could recognize as a turtle. These
discoveries were made just in the past decade since the last edition of this book was published.
Just imagine how many more transitional fossils we might have in another 10 years!