Deconstructing the Gestalt: New concepts and tests of homology, as exemplified by a re‐conceptualization of “microstomy” in squamates

Abstract

Snakes-a subset of lizards-have traditionally been divided into two major groups based on feeding mechanics: "macrostomy," involving the ingestion of proportionally large prey items; and "microstomy," the lack of this ability. "Microstomy"-considered present in scolecophidian and early-diverging alethinophidian snakes-is generally viewed as a symplesiomorphy shared with non-snake lizards. However, this perspective of "microstomy" as plesiomorphic and morphologically homogenous fails to recognize the complexity of this condition and its evolution across "microstomatan" squamates. To challenge this problematic paradigm, we formalize a new framework for conceptualizing and testing the homology of overall character complexes, or "morphotypes," which underlies our re-assessment of "microstomy." Using micro-computed tomography (micro-CT) scans, we analyze the morphology of the jaws and suspensorium across purported "microstomatan" squamates (scolecophidians, early-diverging alethinophidians, and non-snake lizards) and demonstrate that key components of the jaw complex are not homologous at the level of primary character state identity across these taxa. Therefore, rather than treating "microstomy" as a uniform condition, we instead propose that non-snake lizards, early-diverging alethinophidians, anomalepidids, leptotyphlopids, and typhlopoids each exhibit a unique and nonhomologous jaw morphotype: "minimal-kinesis microstomy," "snout-shifting," "axle-brace maxillary raking," "mandibular raking," and "single-axle maxillary raking," respectively. The lack of synapomorphy among scolecophidians is inconsistent with the notion of scolecophidians representing an ancestral snake condition, and instead reflects a hypothesis of the independent evolution of fossoriality, miniaturization, and "microstomy" in each scolecophidian lineage. We ultimately emphasize that a rigorous approach to comparative anatomy is necessary in constructing evolutionary hypotheses that accurately reflect biological reality.

Burrowing in blindsnakes: A preliminary analysis of burrowing forces and consequences for the evolution of morphology

“…However, in lateral view differences between species were striking. Aside from differences in the tooth-bearing elements associated with their radically different feeding modes (Kley, 2001(Kley, , 2006Kley & Brainerd, 1999;Rieppel, Kley, & Maisano, 2009;Strong, Scherz, & Caldwell, 2021), the shape of the anterior part of the cranium was very different with Anilius and Rhinotyphlops having more pointed shapes. The vertebral morphology was also rather different with the condyle and cotyle of Anilios being wide and tall compared to those in the other species (Figure 4).…”

The status of the anomalepidid snake Liotyphlops albirostris and the revalidation of three taxa based on morphology and ecological niche models

“…Two additional specimens from Colombia-CM 44656 from Arjona, Bolívar, illustrated in Santos (2018); and UMMZ 48173, from a locality near Fundaci on, Bolívar (Morphosource Specimen ID 000S21195)-are also referred to the new species. Other species from the genus (L. argaleus, L. beui, L. sousai, L. taylori, L. ternetzii, and L. wilderi diceCT) are illustrated in Santos (2018) and Strong, Scherz, and Caldwell (2021). Terminology follows Cundall & Irish (2008).…”

“…Nonetheless, some of the recently described species do not fit into any of these groups (Santos & Reis, 2018). In recent years, anomalepidid snakes have been re-studied using X-ray microtomography (microCT) data (Santos, 2018;Santos & Reis, 2018Rieppel, Kley, & Maisano, 2009;Strong, Scherz, & Caldwell, 2021), offering a great opportunity to perform skull anatomy comparisons and increasing the amount of available phenotypic data for taxonomic purposes.…”

Hyperdontia in the Paraguayan Martha's marked gecko (Homonota marthae: Phyllodactylidae: Squamata)

“…In some squamates, edentulism might occur in some of these bones: the premaxilla and maxilla in the gekkotan Aprasia repens (Daza & Bauer, 2015), the premaxilla, and sometimes the dentary in blindsnakes (List, 1966; Strong et al, 2021), and the premaxilla in egg eating colubroid snakes (Cundall & Irish, 2008; Gans, 1952).…”