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Read any newspaper’s science section long enough and you’ll see articles about scientists arguing over the origins of various groups of animals: birds and humans are two particular favorites. But the fact is, while there are a few particulars to be worked out, these origins of these groups are relatively well-understood. Birds are flying (or secondarily flightless) coelurosaurian theropod dinosaurs and humans are odd, big-headed apes related to chimpanzees and bonobos. The real mystery is turtles.
Some turtles can live for centuries. Their shoulder and hip joints are located inside their ribcages. In winter, some sleep underwater, absorbing oxygen through their anuses. What the heck are these freaks?
Well, they’re reptiles, of course, but that’s about all the consensus there is. Reptiles have traditionally been classified according to the holes in the skull posterior to the orbit (eyehole), or, more technically, the temporal fenestrae. The ancestral condition is to have no temporal fenestrae; this is called “anapsid”. Most reptiles today—lizards, snakes, tuataras, and crocodilians—have two temporal fenestrae; this is called “diapsid”. Various extinct marine reptile lineages (sauropterygians, ichthyosaurs, etc.) had just an upper temporal fenestra: “euryapsid”. Finally, various mammal relatives once classified as reptiles had just a lower fenestra: “synapsid”.
Turtles have no such fenestrae, so for a long time it was assumed that they were part of a basal radiation of anapsid reptiles. There was some disagreement as to exactly which group they were related to—captorhinids (Gauthier et al. 1988), procolophonoids (Reisz and Laurin, 1991), or pareiasaurs (Lee, 1997)?—but researchers agreed on the general neighborhood. They were descendants of primitive, armored, herbivorous reptiles.
Enter molecular analysis. Genetic studies have suggested that, far from being “primitive”, turtles are actually extremely derived diapsid reptiles. So far derived, in fact, that they’ve re-solidified their skulls, making them deceptively similarly to the ancestral condition. But even the molecular studies don’t agree on exactly where they belong. The possibilities look something like this (Testudinata being the scientific name for turtles):
--Reptilia sensu Gauthier, 1986 |--Parareptilia (?=Anapsida) | |--+--Procolophonoidea | | `?-Testudinata (Reisz and Laurin, 1991) | `--+==pareiasaurs (paraphyletic) | `?-Testudinata (Lee, 1997) `--Eureptilia |--+--Captorhinidae | `?-Testudinata (Gauthier et al., 1988) `--Diapsida |--Lepidosauromorpha | |--Lepidosauria (tuataras; lizards, including snakes and mosasaurs; etc.) | `--+?-Sauropterygia (plesiosaurs, placodonts, etc.) | `?-Testudinata (Rieppel and deBraga, 1996) `--Archosauromorpha |?-Testudinata (Zardoya and Meyer, 2001) |--+?-Sauropterygia (plesiosaurs, placodonts, etc.) | `?-Testudinata `--Archosauria |--Ornithodira (pterosaurs; dinosaurs, including birds, etc.) `--+--Crurotarsi (crocodylians, etc.) `?-Testudinata (Hedges and Poling, 1999)
They are tricky little weirdos, hopping about the phylogenetic tree! Why are they so hard to pin down? One problem is that we don’t have many fossils from the stem group. There are a few species known outside the crown group (the last common ancestor of living forms and all of its descendants), but these all have hallmarks of turtledom already: a shell with a carapace and a plastron, no temporal fenestrae, and those weird, intra-ribcage limb joints. Even the moast “basal” one, Proganochelys, is fairly derived.
Somewhere out there is a shell-less pan-testudine waiting to be found, or even just waiting to be recognized for what it is….
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