New basal amphisbaenid: Slavoia

An update here, September 28, 2015, looks at the holotype of Slavoia, which does not nest with the specimen described below. 

The amphisbaenids
are typically small burrowing skinks (according to the large reptile tree) with roots deeply preceding the very derived Tamaulipasaurus (Clark and Hernandez 1994, Figs. 1, 4) in the Early Jurassic. Because Tamaulipasaurus reacquired a quadratojugal it was considered a diapsid incerta sedis and is not included in any amphiisbaenid phylogenetic studies. This is a mistake rectified here.

Figure 1. Amphisbaenids recovered by the large reptile tree.

Figure 1. Amphisbaenids recovered by the large reptile tree. Click to enlarge.

With the erroneous deletion
of Tamaulipasaurus, the fossil record of amphisbaneids does not go deeper than the Cretaceous. Many taxa are extant. That is why Talanda (2015) titled his recent paper, “Cretaceous roots of the amphisbaenian lizards.”

Talanda reports on
Slavoia darevskii (ZPAL McR-I.112, from the Late Cretaceous of Mongolia, Fig. 2). Slavoia does indeed nest as a basal amphisbaenid, but it is clear that it must be a late-surviving taxon of an earlier radiation with roots that must go back to the Triassic or Late Permian based on the presence of Tamaulipasaurus and other lepidosaurs.

Unfortunately
Talanda used the squamate cladogram of Gauthier et al. (2012) which nested amphisbaenids between a clade that included the burrowing helodermatid, Cryptolacerta, and the basal scleroglossan, Tupinambis, and a clade that included the amphisbaenid, Dibamus (Fig. 1) and snakes with highly derived burrowing snakes, like Leptotyphlops, nesting with the very basal snake Dinilysia.

Missing from the Gauthier et al./Talanda cladogram
are the large reptile tree sisters to Slavoia, the extant Sirenoscincus and the extinct and odd, Sineoamphisbaena.

Furthermore
in the Gauthier et al./Talanda cladogram the basal gekko, Tchingisaurus nests as a sister to the aquatic pre-snake, Pontosaurus, which nests as a sister to the basal mosasaur, Aigialosaurus. Unfortunately, these taxa are all separated by several other taxa, often in widely separate clades, in the large reptile tree.

There is also an odd mix In the Gauthier et al./Talanda cladogram of proto-squamates mixed in with squamates. For instance, Carusi wrongly nests with the scerloglossan, Shinisaurus. Unfortunately Gauthier et al.and Talanda tree do not yet recognize or distinguish tritosaurs or protosquamates. Nor is there a recognition of the relationship between skinks and amphisbaenids or of geckos and snakes, all as recovered in the large reptile tree.

DGS
found a bone pattern distinct from that interpreted by Talanda who did not identify several bones found using the colorizing technique (Fig. 1).

Figure 1. basal amphisbaenid Slavoia from Talanda 2015, showing in situ fossil, tracing by Talanda and colorizing added here. Several bones, like the lacrimal and prefrontal, are missing in the Talanda tracing, which evidently was not traced from this photograph.

Figure 2. Click to enlarge. Basal amphisbaenid Slavoia from Talanda 2015, showing in situ fossil, tracing by Talanda and colorizing added here. Several bones, like the lacrimal and prefrontal, are missing in the Talanda tracing, which evidently was not traced from this photograph. Every attempt was made to line it up, but only certain elements actually do line up with the Talanda tracing. Can you see the new quadratojgual there?

Given all these problems,
Slavoia is correctly nested as a basal amphisbaenid in the Talanda study. Slavoia provides good clues to the the evolution and morphology of its strange sister, Sineoamphisbaenia (Fig. 3), which it greatly resembles.

Figure 6. Sineoamphisbaena is a sister to Sirenoscincus in which the prefrontal contacts the postfrontal.

Figure 3. Sineoamphisbaena is a sister to Slavoia in the large reptile tree and shares a short rostrum and upper temporal bar with this taxon. The quadrate is prone in this taxon.

Getting back to Tamaulipasaurus…
Traditional paleontologists are loathe to consider Tamaulipasaurus an amphisbaenid, a scincoid, a scleroglossan or even a squamate because it has a complete lower temporal bar created in the usual way with a quadratojugal between a jugal and quadrate. However in this taxon the jugal lacks a postorbital process. A quadratojugal is typically not seen in amphisbaenids, scincoids, scleroglossans and squamates. Clark and Hernandez (1994) write: “The skull is superficially similar to that of burrowing squamates, especially amphisbaenians and dibamids, but the presence of plesiomorphic characters, such as a complete lower temporal bar, contradict a phylogenetic relationship within Squamata.”

The simple solution, of course, 
is to toss Tamaulipasaurus (Figs. 1, 4) into a large gamut cladogram to see where it nests most parsimoniously. And, to no surprise, it nests with derived amphisbaenids. No other of the the 580 tested taxa are closer (more similar). Despite the autapomorphy of the long lost reappearance of the quadratojugal in this one isolated taxon (and also perhaps in Slavoia (Fig. 2), all the rest of the traits of Tamaulipasaurus are amphisbaenid, very similar to Bipes (Fig. 1).

Figure 3. Tamaulipasaurus, distinct from nearly all squamates, has a quadratojugal and the jugal lacks a postorbital process. It is still an amphisbaenid.

Figure 4. Tamaulipasaurus, distinct from nearly all squamates, has a quadratojugal and the jugal lacks a postorbital process. It is still an amphisbaenid.

Upon seeing the quadratojugal in Tamaulipasaurus,
Clark and Hernandez froze in their tracks and didn’t want to venture a solution more specific than “Diapsida”. That QJ bone wasn’t supposed to be there. And yet it was. Clark and Herenandez followed their textbooks instead of testing for parsimony in a large gamut cladogram.

It’s no big deal
We looked at the reappearance of the quadratojugal in other taxa here. So it happens every so often, and it happened again with Tamaulipasaurus.

Along the same lines,
we looked at the reappearance of digit zero in Limusaurus, the bird-like theropod, here. Extra phalanges and extra digits appear in certain plesiosaurs and ichthyosaurs. An extra skull bone appears anterior to the pineal foramen in dicynodonts. Extra vertebrae appear in a amphisbaenids and snakes. So an unexpected quadratojugal is nothing to freak out about.

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.
Talanda M 2015.
Cretaceous roots of the amphisbaenian lizards. Zoologica Scripta. doi:10.1111/zsc.12138

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