Author(s): Truman Holt, Scott Barton, Sara Harris, Sarah Heinz
Mentor(s): Megen Kepas
Institution UTech
Vestigial structures are remnants of features that were functional in an organism's ancestors but have lost their original purpose over evolutionary time. Because they are observable, vestigial structures offer tangible evidence of evolutionary change, offering students a clear and compelling demonstration of how species adapt over time. The present study focuses on the discovery of vestigial egg tooth retention in The Western Terrestrial Gartersnake (Thamnophis elegans). This species belongs to the subfamily Natricinae which originated in Asia during the late Eocene/early Oligocene and dispersed across the Holarctic and into North America during the late Oligocene/early Miocene1. While the common ancestor of all natricines can be confidently inferred to have been oviparous (egg-laying), all extant North American natricines are viviparous2 (live-bearing), but still develop in vivo with a large yolk. In oviparous snakes, the egg tooth is a single structure that arises from the oral epithelium3 to assist with breaking out of the egg. The egg tooth is an ancestral feature of oviparous squamates, and it has been thought that loss of a macroscopic egg tooth accompanies the evolution of viviparity4. Here, we provide visual evidence of egg tooth persistence in a North American natricine with live birth.