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What is Tamaulipasaurus?
Tamaulipasaurus morenoi (Clark and Hernandez 1994, Fig. 1) Early Jurassic ~165mya, was a very early burrowing lizard originally considered “similar to living amphisbaenian and dibamid squamates but represents an entirely new lineage related to squamates but outside the group.” That’s because it has a robust quadratojugal, a bone otherwise unknown in squamates (see figure 2).
Evidently that was confusing back then.
Despite it’s strong resemblance to Bipes (Figs. 2, 3), Clark and Hernandez did not know what Tamaulipasaurus was back in 1994, and they still don’t know what it is, based on Clark’s current website (which also has other issues covered here and here). They put too much importance on the presence of the quadratojugal.
It’s not that important.
As was demonstrated earlier, the quadratojugal has a way of disappearing and reappearing in various clades. So it’s not a big deal if it’s there or if it is not in a large phylogenetic analysis that includes hundreds of traits. While we’re on the subject, temporal fenestra also appear and disappear on occasion, so don’t apply hard and fast ‘rules’ to their appearance. Just run the analysis. Let parsimony prevail.
Figure 1. Tamaulipasaurus, a burrowing reptile with an autapomorphic quadratojugal. The big question is not the novel quadratojugal, but that tiny little fenestra in the rostrum. Is it the orbit, as in Spathorhynchus and as Clark and Herenandez identify it? Or is it simply a rostral feenstra with the orbit confluent with the temporal fenestra, as in Bipes?
Phylogenetic analysis
Clark and Hernandez ran an analysis based on Gauthier (1988) as modified by Laurin (1990). They found 108 MPTs (that’s low resolution). They nested Tamaulipasaurus either between kuehneosaurs and lepidosaurs or between rhynchocephalia and squamata. Pretty broad. No indicated sister taxa.
With more taxa
in the large reptile tree that novel quaratojugal does not affect the nesting of Tamaulipasaurus as a sister to Spathorhynchus. Both are derived from a sister to Crythiosaurus, given the Clark and Hernandez data (i.e. with the labeled orbit considered the actual orbit). I find that problematic given the many other morphological differences (Fig. 2).
Earliest known burrowing lizard
The presence of such a derived lizard as early as the Early Jurassic indicates that predecessors to all these taxa must have first appeared even earlier, likely in the Permian with radiation and diversification throughout the early Mesozoic.
Figure 2. Spathorhynchus, Tamaulipasaurus and Bipes to scale. Bones colorized in Tampaulpasaurus as Clark and Hernandez identify them. Note the different ways that the jugal is oriented in Spathorhynchus.
Convergence?
As noted by Clark and Hernandez, Tamaulipasaurus shared several traits and bears a strong general resemblance to the living burrowing lizard with forelimbs, Bipes (Figs. 2,3)
The big question is
not the novel quadratojugal. I’m more interested in that tiny little fenestra in the rostrum. Is it the orbit, as in Spathorhynchus (Fig. 2) and as Clark and Herenandez identify it? Or is it simply a rostral feenstra with the orbit confluent with the temporal fenestra, as in Bipes? In Bipes it appears that the prefrontal and jugal are fused to the maxilla, colorized here (Fig. 3). The only bone missing is the quadratojugal. Even the mandibles look very similar.
Figure 3. Tamaulipasaurus compared to Bipes. There is that tiny foramen in the rostrum in Bipes, exactly where the orbit was identified in Tamaulipasaurus. So, here the eyeballs are added to show where the orbits is and perhaps was in Bipes and Tamaulipasaurus. This is DGS used on a drawing of a 3D specimen, useful here just to readily compare like colored bones.
So which is it?
In phylogenetic analysis, surprisingly it makes little difference whether the foramen is an orbit or not. All the other traits trump the identification either way. No other of the 592 tested taxa are closer in morphological traits. My money is on the confluent orbit hypothesis. After seeing the photo (Fig. 1) from the Clark website,the opening looks more like a foramen than an orbit, and it’s right where a foramen should be in a sister. Until higher rez photos become available, I’m working from the drawings of Clark and Hernandez (1994) for ‘circumorbital sutures’, which may include other errors.
References
Clark JM and Hernandez RR 1994. A new burrowing diapsid from the Jurassic La Boca formation of Tamaulipas, Mexico, Journal of Vertebrate Paleontoogy 14: 180-195.
Source: https://pterosaurheresies.wordpress.com/2015/10/27/what-is-tamaulipasaurus/
