The snake supratemporal (= squamosal?) Yes. And No.
Should the snake quadrate anchoring bone be called the squamosal or the supratemporal?
Figure x. Coubrid snake skull, courtesy of Udo M. Savalli, labeled with a large “supratemporal” anchoring the quadrate. Phylogenetic bracketing throws doubt on this identification. That anchor is homologous with the squamosal in ancestral taxa.
Snake ancestors from the Varanus – Adriosaurus line had a larger squamosal and a tiny supratemporal (Fig. 1). So how did the opposite occur in snakes? Maybe it didn’t.
Figure 1. The diphyletic snakes and their lineages. Here the squamosal is highlighted in tan and the supratemporal is in magenta to illustrate the homologies of these bones. The supratemporal on the right is a relatively tiny bone and it disappears. The squamosal on the left is the tiny bone and it disappears, further differentiating the two main clades of snakes.
In the Heloderma – Lanthanotus line (Fig. 1) the supratemporal (formerly considered the tabular in old literature) is the larger bone, “pasted” against the side of the parietal. The squamosal is tiny by comparison and lateral to it. Without a traditional lack of snake ancestors (with legs) this is probably where the identification problem originated.
In Python, Pachyrhachis and Nerodia (Fig. 1, right) the bone in question is inside (proximal to) the quadrate. And it articulates with the parietal. That’s exactly how the supratemporal articulates in Heloderma, Lanthanotes and the pipe snakes on the left. Perhaps that is why it has been traditionally considered a supratemporal.
However, when we look at the ancestry of the snakes on the right we find the squamosal surmounts the quadrate and, with Adriosaurus, the large squamosal articulates with the parietal and is proximal to the quadrate. This is the same pattern seen in Pachyrhachis. The squamosal retreats from the postorbital in living snakes.
Figure 2. The skull of Adriosaurus, a marine snake ancestral taxon, with bones color coded for identification. That’s the squamosal in yellow at the back of the skull and the supratemporal in brown. The lower one is broken and displaced, so you see two parts. See reconstruction, figure 3. The palpebrals, similar to those in water monitors, are in blue.
The marine pre-snake, Adriosaurus, has a difficult to decipher skull. Here, (fig. 2) using Adobe Photoshop, an interpretation has been attempted with the resulting reconstruction shown in figure 3.
Figure 3. Adriosaurus skull reconstructed. The squamosal, in yellow, and supratemporal, in brown, are highlighted. The squamosal no longer surmounts the quadrate, but articulates proximal to it and articulates with the parietal. Palpebral in grey.
Note the large squamosal in this taxon articulates with the parietal, as in most modern snakes (other than pipe snakes, fig. 1 at left). In Varanus the squamosal surmounts the quadrate. In Adriosaurus the squamosal extends along the proximal surface of the quadrate, as in modern snakes. While this is odd but plausible with regard to ancestors, it is perfectly in line with descendants.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
References
Palci A and Caldwell MW 2007. Vestigial forelimbs and axial elongation in a 95 million-year-old non-snake squamate. Journal of Vertebrate Paleontology 27 (1): 1-7. doi:10.1671/0272-4634(2007)27[1:VFAAEI]2.0.CO;2.
Caldwell MW and Palci A 2010. A new species of marine ophidiomorph lizard, Adriosaurus skrbinensis, from the Upper Cretaceous of Slovenia. Journal of Vertebrate Paleontology 30(3): 747-755. doi:10.1080/02724631003762963.
Seeley HG 1881. On remains of a small lizard from Neocomian rocks of Comen, near Trieste, preserved in the Geological Museum of the University of Vienna. Quarterly Journal of the Geological Society of London 37: 52-56.
2012-10-20 23:42:21
Source: http://pterosaurheresies.wordpress.com/2012/10/21/the-snake-supratemporal-squamosal-yes-and-no/
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