Variation in the cranial osteology of the amphisbaenian genus Zygaspis based on high‐resolution x‐ray computed tomography

Abstract

Amphisbaenians are a specialized fossorial group of reptiles, having developed head‐first burrowing, a specialized skull architecture, and an elongated body. This group is generally small‐bodied, with some species possessing skulls only a few millimeters long. In this study, we used high‐resolution x‐ray computed tomography to compare the skulls of 15 specimens from seven of the eight species in the amphisbaenian genus Zygaspis (Zygaspis dolichomenta, Zygaspis ferox, Zygaspis quadrifrons, Zygaspis kafuensis, Zygaspis nigra, Zygaspis vandami, and Zygaspis violacea). Both interspecific and intraspecific variation, including asymmetry, is observed among the cranial bones of the specimens. There are unique morphological features on some cranial bones, including the premaxilla and ectopterygoid of Z. quadrifrons, the pterygoid and vomer of Z. kafuensis, and the extracolumella of Z. nigra. Sexual dimorphism has been previously reported for the species Z. quadrifrons and is observed here as well.

Intra‐terrestrial aliens—Visualizing the bizarre cranial anatomy of the worm‐lizard!

“…However, this actually does not give their strangeness justice. Amphisbaenians, as demonstrated in the two detailed papers on the genus Zygaspis in this volume (Bell et al, 2024;Meza et al, 2024), have several strange cranial features. For instance, the papers document and describe the so-called "element X."…”

“…Like those giant fictional creatures, hearing seems to be of utmost importance to worm-lizards. As in fossorial mammals (Mason & Narins, 2015), amphisbaenians have large ear bones that they presumably use as their primary sensors though, as is evident in the skulls described in this volume (Bell et al, 2024;Meza et al, 2024), they also have large bony nostrils. While these are covered almost entirely in soft tissue (see images on the cover), perhaps chemoreception-especially at close range (Gans, 1974)also helps balance the lack of vision as demonstrated by their vestigial eyes which would be of little use in their nearly entirely subterranean existence.…”

“…Years ago, on a trip to Southern Africa, I visited the Natural History Museum of Zimbabwe in Bulawayo with Patrick Lewis, one of the authors of the two amphisbaenian papers in this issue (Bell et al, 2024;Meza et al, 2024), to examine their herpetology collection and meet with the legendary Don Broadley (1932Broadley ( -2016)-a herpetologist so renowned that there are nine species of reptiles named in his honor (Branch & Bates, 2018). After regaling us with tales of some of his harrowing encounters with all manner of scaly things and pointing out many impressive creatures in murky jars, he made his way over to one small container and fished out something rather unimposing for us to see.…”

“…For instance, the papers document and describe the so-called "element X." Although this bone comes with the enigmatic name, it is not much to look at: sitting on the ventral side of the otic region (Zangerl, 1944) and sometimes fused to it (Meza et al, 2024), this simple looking bone seems to most likely be a sesamoid of the cranio-cervical muscles (Montero et al, 2017). Still, throughout vertebrate anatomy, there are not too many cranial bones for which we have no real explanation!…”

“…Still, throughout vertebrate anatomy, there are not too many cranial bones for which we have no real explanation! While "element X" is morphologically fairly simple, the normally mundane nasofrontal and frontoparietal sutures are so exceedingly complex in Zygapsis that their interdigitations nearly meet each other in the middle (Meza et al, 2024;fig. 3).…”