Mike Keesey  

Turtles Are Tricky!

November 21st, 2006 by Mike Keesey :: see related comic

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….

References:

  • Gauthier, J., A. G. Kluge, and T. Rowe. 1988. Amniote phylogeny and the importance of fossils. Cladistics 4:105–209.
  • Hedges, S. B. and L. L. Poling. 1999. A molecular phylogeny of reptiles. Science 283:998–1001.
  • Lee, M. S. Y. 1997. Pareiasaur phylogeny and the origin of turtles. Zoological Journal of the Linnean Society 120:197–28.
  • Reisz, R. R. and M. Laurin. 1991. Owenetta and the origin of turtles. Nature 349:324–326.
  • Rieppel, O. and M. deBraga. 1996. Turtles as diapsid reptiles. Nature 384:453–455.
  • Zardoya, R. and A. Meyer. 2001. The evolutionary position of turtles revisited. Naturwissenschaften 88(5):193–200.
7 Responses to “Turtles Are Tricky!”
Albert (Netherlands) wrote:

Mike, is there any proof that turtles have re-solidified their skulls?

Mike Keesey wrote:

Excellent question. I am actually unaware of any developmental evidence.

Peter Cook wrote:

Great summation, Mike. I’ve been trying to track something like this down for several days, as I rebuild—even roughly—an simple cladogram of Reptilia. My wife and I are reissuing an out-of-print book on animal behaviour on the web. The characters are standing in front of a simplified cladogram, and it has changed a lot from the resources we had in 1992. That said, there is so much confusion on where to put Testudinata that I’ve elected to leave them in Anapsids until the dust settles. If you want to see an advance copy of the page, to advise, I’d be delighted. Thanks again for clairfying the dust cloud!

Mike Keesey wrote:

Awesome–yeah, I’d love to take a look. I’ll e-mail you.

If you’re just showing extant sauropsids, you can neatly sidestep the issue like so:

Sauropsida
__?Testudines
__Archosauria
____Aves
____Crocodylia
__Lepidosauria
____Sphenodon
____Squamata

If you’re using fossil taxa as well … then it gets tricky…. At least I’d recommend sticking a “?” or something in there, wherever you place it.

Tamanna wrote:

They are just as popular as the short necks in South Australia. The short neck we have is close to 20 years old. Our trtule was the size of a 50cent coin when, as a child, we bought it. A further recomendation for potential tutle owners is to be prepared for the growth of the trtule, room for larger tank or pond. Durning moderate sunny days, our tutle likes be in the garden. Interesting enough it still has no name

Aveenash wrote:

you disgust me, the way you treat these analims.. do you have any feelings for these analims? You can’t just put that big snake in a small box! it needs space, it can’t stretch out or even lift its head. These are living creatures not objects that you can collect and show off, I do not respect you.

Benu wrote:

yes, i think they are a bit small too and if none of you realised most of the amilans were kept in what seemed just big plastic boxes do they have holes to provide enough fresh air for them? and the boa constrictor. possibly the biggest animal there with one of the smallest enclosures.


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