Some thoughts on Sineoamphisbaena

One of the strangest (= most unlike its sister taxa) reptiles is Sineoamphisbaena, which nests in the large reptile tree at the base of the burrowing skinks that ultimately gave rise to amphisbaenids like Amphisbaena and Bipes.

Wikipedia reports: Sineoamphisbaena is an extinct genus of squamate of uncertain phylogenetic placement. Wu et al. (1993), Wu et al. (1996) and Gao (1997) proposed and argued that it was the oldest known amphisbaenian; this, however, was challenged by other authors, such as Kearney (2003) and Conrad (2008), who instead assignedSineoamphisbaena to the group of squamates variously known as Macrocephalosauridae, Polyglyphanodontidae or Polyglyphanodontia. A large-scale study of fossil and living squamates published in 2012 by Gauthier et al. did not find evidence for a particularly close relationship between amphisbaenians and Sineoamphisbaena; in their primary analysis Sineoamphisbaena was found to be the sister taxon of the clade containing snakes, amphisbaenians, the family Dibamidae and the American legless lizard. The primary analysis of Gauthier et al. did not support a close relationship between Sineoamphisbaena and polyglyphanodontians either; however, the authors noted that when all snake-like squamates and mosasaurs were removed from the analysis, and burrowing squamates were then added individually to it, Sineoamphisbaenagrouped with polyglyphanodontians. Gauthier et al. (2012) considered it possible that Sineoamphisbaena was a burrowing polyglyphanodontian.”

The large reptile tree agrees with the original Wu et al. (1993) nesting, at the base of a clade of burrowing prehistoric lizards, some of which included amphisbaenids. Their analysis, unfortunately used suprageneric taxa and they recovered all legless taxa (including snakes) in one clade.

Figure 1. The lineage of Sineoamphisbaena with Chalcides as more primitive and Crythiosaurus + Spathorhynchus as more derived. The quadrate is orange.

Figure 1. The lineage of Sineoamphisbaena with Chalcides as more primitive and Crythiosaurus + Spathorhynchus as more derived. The quadrate is orange.

The temporal region of Sineoamphisbaena has been difficult to interpret because of its unique character and bone fusion patterns not quite like any other. Unlike most burrowing lizards, Sineoamphisbaena did not lose any temporal bones. It rearranged them, fusing some. Here (Fig. 2) is the original interpretation and some suggested reinterpretations.

Figure 2. The skull of Sineoamphisbaena as originally interpreted and as reinterpreted here with color coding matched to that of a more "normal" sister, Chalcides guentheri. Note the squamosal forms the posterior border of the upper temporal fenestra of both taxa.

Figure 2. The skull of Sineoamphisbaena as originally interpreted and as reinterpreted here with color coding matched to that of a more “normal” sister, Chalcides guentheri. Note the squamosal forms the posterior border of the upper temporal fenestra of both taxa. And the long jugal is really composed of the jugal + postorbital. It was not a stretch for the squamosal to contact the postfrontal. If it did fuse, then a crack in the specimen has put a question to that.

Distinct from the original interpretation,
the old postorbital is the new squamosal, continuing to border the posterior upper temporal fenestra. The old jugal is the new jugal + postorbital, matching Chalcides. The old squamosal is the new supratemporal, a bone considered missing originally. The old lacrimal is fused to the prefrontal from what I can tell by comparison to Crythiosaurus. The prefrontal and lacrimal fuses to the maxilla in Bipes.

Burrowing lizards,
evolved in a wide variety of ways and all, except Sineoamphisbaena, lose skull (temporal) bones. All appear to have evolved from a variety of the genus Chalcides because some retain a long low rostrum. Others, like Bipes, have a short blocky snout, but Bipes does not rotate its upper teeth medially as Sineoamphisbaena does. So that split likely preceded tooth rotation. It’s a little confusing with lots of convergence in a little clade due to their burrowing niche.

Figure 3. Chalcides, Crythiosaurus and Bipes with bones colored. Note, only the quadrate remains in Bipes. Other bones are lost or fused. Sineoamphisbaena lost the epipterygoid. Crythiosaurus nests basal to Bipes in the large reptile tree, but the extreme reduction of the quadrate is an autapomorphy.

Figure 3. Chalcides, Crythiosaurus and Bipes with bones colored. Note, only the quadrate remains in Bipes. Other bones are lost or fused. Sineoamphisbaena lost the epipterygoid. Crythiosaurus nests basal to Bipes in the large reptile tree, but the extreme reduction of the quadrate is an autapomorphy.

There may be another skink closer to Sineoamphisbaena, but I haven’t found it yet.

References
Gao K 1997. Sineoamphisbaena phylogenetic relationships discussed. Canadian Journal of Earth Sciences. 34: 886-889. online article
Kearney M 2003. The Phylogenetic Position of Sineoamphibaena hextabularis reexamined. Journal of Vertebrate Paleontology 23 (2), 394-403
Müller J, Hipsley CA, Head JJ, Kardjilov N, Hilger A, Wuttke M and Reisz RR 2011. Eocene lizard from Germany reveals amphisbaenian origins. Nature 473 (7347): 364–367. doi:10.1038/nature09919
Wu XC., Brinkman DB, Russell AP, Dong Z, Currie PJ, Hou L, and Cui G 1993. Oldest known amphisbaenian from the Upper Cretaceous of Chinese Inner Mongolia. Nature (London), 366: 57 – 59.
Wu X-C Brinkman DB and Russell AP 1996. Sineoamphisbaena hexatabularis, an amphisbaenian (Diapsida: Squamata) from the Upper Cretaceous redbeds at Bayan Mandahu (Inner Mongolia, People’s Republic of China), and comments on the phylogenetic relationships of the Amphisbaenia. Canadian Journal of Earth Sciences, 33: 541-577.

wiki/Sineoamphisbaena

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