| Not sure if you can access this article through your uni, but there is a sentence in there that might be useful:
"The seemingly primitive characteristics of Aspidites may be better interpreted as reversals or specializations that have accompanied a switch to burrowing in this genus."
So basically, an evolutionary loss.
and for referencing:
L. H. Rawlings, D. L. Rabosky, S. C. Donnellan & M. N. Hutchinson (2008 )
Python phylogenetics: inference from morphology and mitochondrial DNA
Biological Journal of the Linnean Society 93 (3) , 603–619 | 
