Diapsida: Neodiapsida: Claudiosaurus

Taxa on this Page

1. Claudiosaurus X

Claudiosaurus germaini, an aquatic neodiapsid from the Tangasaur from the Late Permian (Sakamena Formation) of Madagascar.. illustration by Nobu Tamura (Wikipedia)

Claudiosaurus, Anchor Taxa, and the Illusion of Uncertainty

Claudiosaurus was a medium-sized 60 cm) low-slung reptile with a small head, fairly long neck, and a long tail. It lived by the shores of lakes or sheltered seas along a pair of rift valleys in what is now Madagascar in the Late Permian -- just before whatever event(s) it was that killed off most life forms on Earth at the end of the Paleozoic. As discussed elsewhere, many of the basal diapsids are rather scrappy; and this has led to some rather odd phylogenies. Claudiosaurus is an exception. It is well known from a number of good specimens. Even better, it seems rather clear that, as Carroll (1981) pointed out in his original description, Claudiosaurus is closely related to a group of neodiapsids now known as the Younginiforms.

Claudiosaurus was almost certainly amphibious. Caldwell 1994). This is not really obvious from a first glance at the skeleton, but has been quite convincingly shown by Carroll and Caldwell. Its most enigmatic feature is the skull. As in snakes, there seems almost nothing holding the skull together. The lower temporal bar is completely absent, the post-orbital skull is full of large holes, and the pre-orbital skull was weakly bound and may not even be completely ossified. It is hard to make sense of all the data, but at least there is no lack of data to make sense of.

In spite of this wealth of information, it has proven no easier to get an exact placement for Claudiosaurus than it has been for Paliguana. Most of the confusion may be due, in addition to the inclusion of taphonomic detritus, to inconsistencies in nomenclature. The most logical definitional benchmark in this area is clearly that of Jacques Gauthier. The benchmark clade is the crown group Sauria = the last common ancestor of birds and snakes. Then Lepidosauromorpha = all Saurians closer to snakes than birds; and Archosauromorpha = all Saurians closer to birds than snakes. "Closer" in this parlance means "having a more recent common ancestor with." We can't use a "diapsid" benchmark because the diapsid condition is a physical character. It has no phylogenetic meaning. We can't really use Reptilia (= turtles + turtledoves) because of the continuing concern that turtles might turn out to be inside Sauria after all. Thus, if our discussions are to be tied to reality at all, it is quite important that the Saurian anchor be firmly in place. In spite of this logical necessity, many phylogenies -- including, until recently, these Notes -- have allowed each member of this crown-stem triad (see Sereno (1998)) to live a wild, free life of its own. See, e.g. Caldwell 1996); Motani et al. 1998) and (to a far lesser extent) deBraga & Rieppel (1997).

Interestingly, when we apply a consistent nomenclature and simply remove the uncertain bits and pieces, everyone's cladogram looks like this:

The only genuinely unsettled points are (a) whether turtles are in this mix somewhere and (b) the position of the Coelurosauravidae. The latter have variously been placed between Claudiosaurus and the Younginiformes, or as a sister to Ichthyosauria. (I have chosen the former alternative for no particular reason.) In truth, the original results of Caldwell (1994) are not quite consistent with this scheme, but Motani et al. 1998) do obtain this pattern with the same data set when they recode Ichthyosauria to incorporate more recent information. ATW010415

Descriptions

Claudiosaurus:  

Links: Icarito Interactivo - dinosaurios (Spanish); Autapomorphies of diapsid clades, Wikipedia (stubby), UCMP - Basal Diapsids (only a paragraph), .

Note: determinate growth of long bones, but ossification of distal limb and some cranial bones continued throughout life. Carroll (1981) makes a fairly good case that Claudiosaurus was a derived relative of Thadeosaurus, now classified as a younginiform.

Illustration of skeleton of Claudiosaurus germaini, by David Peters, Reptile Evolution.