Pelvic spur

Pelvic spurs (also known as vestigial legs) are external protrusions found around the cloaca in certain superfamilies of snakes belonging to the greater infraorder Alethinophidia.

The species of extinct snake Najash rionegrina was first described in 2006, and has been proposed as the earliest branching taxa of the suborder Serpentes.

[7][6] The paleontologists thus concluded via phylogenetic analysis that N. rionegrina is the most ancient taxa within Serpentes, and the three extinct species previously described were more closely related to modern-day snakes belonging to Alethinophidia.

[6] Recent analysis of numerous fossil records supported these findings and further demonstrated the reduction of pelvic and hind limb structures within these lineages.

[5][2] Further evidence for these structures being plesiomorphic of can be found in the pelvis of some living taxa such as Candoia carinata or Eunectes murinus.

However, spurs are present among the basal clades of Alethinophidia, including in Booidea and Pythonoidea,[8] among Amerophidia, and among one member of Uropeltoidea, the Cylindrophiidae.

A 2019 publication provided evidence for similarly ossified structures in the species Liotyphlops beui of the infraorder Scolecophidia.

[2] Members of this sister group of Scolecophidia are poorly understood due to their cryptic nature and are typically small in size, fossorial, and worm-like.

[10] The authors of this study noted that others in the field had made similar observations before, but had not formally researched the topic specifically.

[14] A separate study found similar results in diamond pythons (Morelia spilota), a species that does not exhibit any bouts of combat between males.

[3] It was observed that males repeatedly alternated the orientation of the spurs between horizontal and vertical positions, which the authors suggest stimulated muscle contractions in the female, which would allow for better alignment between the individuals for mating.