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.
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.
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.
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.
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.