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

October 03, 2021

Catherine R. C. Strong (1), Mark D. Scherz (2), Michael W. Caldwell (1)
The Anatomical Record, 304, Issue 10, October 2021: 2303-2351. DOI: 10.1002/ar.24630


ancestral state reconstruction; functional morphology; homology; skull anatomy; snake evolution


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.

How Our Software Was Used

Dragonfly was used to digitally remove soft tissues and to digitally isolate skull elements in CT scans of various squamate skulls.

Author Affiliation

(1) Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada.
(2) Institute for Biochemistry and Biology, University of Potsdam, Potsdam, Germany.
(3)Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada.