The snake supratemporal takes over for the pre-snake squamosal.

Updated February 24, 2015 with new data by Pontosaurus, a snake ancestor.

The snake quadrate anchoring bone is correctly termed the supratemporal because it enlarges as the squamosal shrinks in snake origins, particularly notable in Pontosaurus.

Coubrid snake skull

Figure 1. 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.

In the HelodermaLanthanotus 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.

Snake ancestors from the Adriosaurus (Fig. 2) line had a larger squamosal and a tiny supratemporal. So how did the opposite occur in snakes?

When we look at Adriosaurus in the ancestry of the snakes we find the squamosal surmounts the quadrate and articulates with the postorbital. The squamosal retreats from the postorbital in legless snakes.

Figure 2. Adriosaurus, a snake ancestor sister.

Figure 2. Adriosaurus, a snake ancestor sister.

In Pontosaurus (Fig. 3) the the squamosal no longer contacts the tiny postorbital (also on the way out) nor the supratemporal, which is much larger in a process convergent with that of Heloderma and kin. The supratemporal is taking over for the squamosal as the quadrate anchor.

Figure 2. Pontosaurus and its parts. Data from Caldwell 2006. This is one of the last taxa we know in the snake lineage that still had a pectoral girdle.

Figure 3. Pontosaurus and its parts. Data from Caldwell 2006. This is one of the last taxa we know in the snake lineage that still had a pectoral girdle.

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.

wiki/Adriosaurus

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