Anapsid v Diapsid Pt I

As promised sometime ago here's a small series of posts concerning the Anapsida and the Diapsida. A topic I am 'mildly' interested in, and have been known to grumble about occasionally. I'm fully aware that of the readers of this blog only two (other than me) are likely to care deeply, but meh, it's my blog and I'll Diapsida if I want to.
Referencing: is likely to be a bit vague through the text as I can't be bothered, but there will be further reading type stuff at the end.

Pt I: tries to summarise the differences between the Anapsida and the Diapsida
Pt II: is likely to summarise support for various positions
Pt III: will probably be my take on it all

Four extant orders make up the Reptilia: Testudines (turtles), Crocodylia (crocodiles and alligators - the caimans have been subsumed recently see Janke et al 05), Sphenodontia (tuatara) and Squamata (lizards, ie. everything else including skinks, geckos, snakes etc). On the basis of morphology the orders are divided into the Anapsida and the Diapsida (Figure 1). By way of comparison all mammals are members of the Synapsida. Turtles are the sole members of the Anapsida, with the other Reptilian orders members of the Diapsida.



This distinction has been used for many years, and until recently was accepted as defining turtles within the Reptilia. During the 1980s and 1990s when molecular data from the Reptilia began to accumulate the (admittedly limited) base pair phylogenies suggested that rather than outside the rest of the Reptilia, that turtles placed within the Archosauria (a grouping of Crocodylia and aves) or the Lepidosauria (Sphenodontia and Squmata). Both positions cause the Reptilia to become paraphyletic, a _bad thing_ in phylogenetic terms. The limited number of base pairs used in the early molecular analyses meant few morphologists took them seriously, especially when some analyses supported a paraphyletic position, other studies were ambiguous and others supported the morphological status quo.

What began as a molecular argument, largely rubbished by morphologists, took a turn when Rieppel published work suggesting that morphologically turtles were not Anapsid. In itself this was not radical and had been suggested on and off for sometime, but the timing of the argument was important with the growing molecular evidence. Rieppel concluded by suggesting that discussion should shift from art turtles diapsid to where in the Diapsida they fit. Radical stuff. As expected this provoked a strong outburst from morphologists (Wilkinson, Lee etc). Without exploring the stats and matrices, Rieppel's central thesis was that the turtles skull hole (thus anapsid) is in the wrong place.

To explain that, precursors to the turtles were a group of extinct reptiles collectively known as the Parareptilia who lived around 300 MYA. The problems stem from the fact that the most primitive turtle skull Proganochelys doesn't possess an anapsid skull arrangement identical to the Parareptilia. therefore Rieppel reasoned turtles are derived Anapsids. The term 'derived Anapsid' implies that turtles were, at some point, members of the Diapsida and that skeletal changes have led to closing of one hole, causing the current anapsid condition.

Strange as that may sound, there are examples of skeletal changes occurring in other animals. For example the extinct Eurapsids (plesiosaurs and ichthyosaurs) were originally diapsid, then changes in muscle attachment led to bone growth causing an anapsid state.
Given molecular and morphological evidence there are three positions available for turtles within the Reptilia.
The Anapsid Hypothesis : turtles are an outgroup to all other reptiles, reflecting a traditional morphological position (AH)
Lepidosauria hypothesis: turtles are the sistergroup to the Lepidosauria (Sphenodon and Squamata) creating a paraphyletic Reptilia (LH)
Archosauria hypothesis: turtles are the sistergroup to the Archosauria (Crocs and Aves), creating a paraphyletic Reptilia.

these three positions are explored by Lee in more detail for those of you playing at home...

So that, in a very abbreviated form, is the anapsid:diapsid turtle problem.

Any questions, leave a comment. The next blog in this series will explore evidence for these positions.

Love, B.

References of interest:
Benton, M. J. (2000). Vertebrate Palaeontology. Oxford, Blackwell Science Ltd.

Carroll, R. L. (1988). Vertebrate Paleontology and Evolution. New York, Freeman.

deBraga, M. and O. Rieppel (1997). "Reptile phylogeny and the interrelationships of turtles." Zoological Journal of the Linnean Society 120: 281-354.

Lee, M. S. Y. (2001). "Molecules, morphology, and the monophyly of diapsid reptiles." Contributions to Zoology 70(1): 1-22.

Rieppel, O. (1999). "Turtle origins." Science 283(12 February): 945-946.

Rieppel, O. (2000). "Turtles as diapsid reptiles." Zoologica Scripta 29(3 July): 199-212.

Rieppel, O. and M. deBraga (1996). "Turtles as diapsid reptiles." Nature 384: 453-55.

Rieppel, O. and R. R. Reisz (1999). "The origin and early evolution of turtles." Annual Review of Ecology and Systematics 30: 1-22.

Wilkinson, M., J. Thorley, et al. (1997). "Uncertain turtle relationships." Nature 387(29 May): 466.