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LEPIDOSAUROMORPHA |--SQUAMATA `--o Rhynchocephalia |--Gephyrosaurus `--Sphenodontidae |--Clevosaurinae `--Stem Group Sphenodontinae |--Pleurosauridae `--+--Sphenodontinae `--+--Sphenodontini `--Eilenodontini
Dethe confrounteth Everyman (and Sphenodon). From Everyman
The Rhynchocephalia, or Sphenodontia, are the sister group of the hugely successful Squamata. The diversity of these poor relations to the snakes and lizards has always been somewhat limited. Now, they are represented only by the genus Sphenodon -- two closely related species of tuataras, small numbers of which still eke out a living among the bird hatcheries on a few islands off the coast of New Zealand.
Anatomically, Sphenodon is rather plain vanilla; and, paradoxically, therein lies its interest. It lies just downstream in phylospace from the split between archosauromorphs and lepidosauromorphs. It is rather unspecialized and, consequently, is about as close to the basic reptile morphotype as one might hope to find. When Romer (1956) wrote his incomparable Osteology of the Reptiles, his constant anatomical point of reference was Sphenodon. Carroll's (1988) Vertebrate Paleontology and Evolution continued this tradition; and, when the next landmark text on vertebrate paleontology is produced, it will doubtless carry on the tradition. There really is no alternative. No reptile more basal than Sphenodon survives. Its closest relatives are the iguanid lizards -- products of the Cretaceous squamate radiation. Iguanids are fairly basic, but have the loose, specialized squamate skull: the very feature which has permitted the squamates to radiate into thousands of separate specialized forms. Sphenodon has a classic, solid skull construction which doesn't permit a great deal of variation. Think, for example, of crocodylomorphs, temnospondyls or Mesozoic mammals. That we are here proves that a solid, akinetic skull doesn't necessarily doom a taxon to morphological stagnation. However, it is certainly a consideration.
So, while we wait patiently for some scholar to pen the successor to Williston, Romer and Carroll, we may as well take a brief look at the phylogenetic neighborhood of paleontology's Everyman. ATW020406.
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