Modeling Skull Network Integrity at the Dawn of Amniote Diversification With Considerations on Functional Morphology and Fossil Jaw Muscle Reconstructions

Ingmar Werneburg,Pascal Abel

doi:10.3389/fevo.2021.799637

Abstract

One of the major questions in evolutionary vertebrate morphology is the origin and meaning of temporal skull openings in land vertebrates. Partly or fully surrounded by bones, one, two, or even three openings may evolve behind the orbit, within the ancestrally fully roofed anapsid (scutal) skull. At least ten different morphotypes can be distinguished in tetrapods with many modifications and transitions in more crownward representatives. A number of potential factors driving the emergence and differentiation of temporal openings have been proposed in the literature, but only today are proper analytical tools available to conduct traceable tests for the functional morphology underlying temporal skull constructions. In the present study, we examined the anatomical network in the skull of one representative of early amniotes, †Captorhinus aguti, which ancestrally exhibits an anapsid skull. The resulting skull modularity revealed a complex partitioning of the temporal region indicating, in its intersections, the candidate positions for potential infratemporal openings. The framework of †C. agutiwas then taken as a template to model a series of potential temporal skull morphotypes in order to understand how skull openings might influence the modular composition of the amniote skull in general. We show that the original pattern of skull modularity (†C. aguti) experiences comprehensive changes by introducing one or two temporal openings in different combinations and in different places. The resulting modules in each skull model are interpreted in regard to the feeding behavior of amniotes that exhibit(ed) the respective skull morphotypes. An important finding is the alternative incorporation of the jugal and palate to different modules enforcing the importance of an integrated view on skull evolution: the temporal region cannot be understood without considering palatal anatomy. Finally, we discuss how to better reconstruct relative jaw muscle compositions in fossils by considering the modularity of the skull network. These considerations might be relevant for future biomechanical studies on skull evolution.

Highlights

The evolutionary transition from a mostly aquatic to a fully terrestrial life in vertebrates is associated with a number of fundamental anatomical and physiological changes (Sumida and Martin, 1997; Laurin, 2010; Clack, 2012)
We found that compared to the original skull modularity of †C. aguti (Figures 2, 3A), the jugal, squamosal, postorbital, and postfrontal usually change their modular association when different temporal openings are modeled
The used anatomical network theory has a number of limitations due to its simplifying methodology, it allows strategical comparisons among different anatomical models

Summary

Introduction

The evolutionary transition from a mostly aquatic to a fully terrestrial life in vertebrates is associated with a number of fundamental anatomical and physiological changes (Sumida and Martin, 1997; Laurin, 2010; Clack, 2012). The temporal skull region received much attention in the scientific literature, historically resulting in taxonomic groups mainly defined by the anatomy of their temporal skull region (e.g., Synapsida, Diapsida, “Anapsida”; Case, 1898; Williston, 1904; Broom, 1922; Zdansky, 1923; Frazzetta, 1968; Kuhn-Schnyder, 1980; Rieppel and Gronowski, 1981; Smith et al, 1983; Rieppel, 1984; Frey et al, 2001; Tarsitano et al, 2001; Müller, 2003; Cisneros et al, 2004; Werneburg, 2012, 2013a, 2015, 2019; Haridy et al, 2016; Elzanowski and Mayr, 2018; Abel and Werneburg, 2021) with only few of them still used today This is because with the rise of phylogenetic systematics and the inclusion of hundreds of other anatomical characters, along with new fossil finds, a more comprehensive picture on amniote interrelationships has been developed (Abel and Werneburg, 2021). They can still be informative on selected phylogenetic levels and in particular taxonomic groups (Ford, 2018) and are considered as highly relevant to understand morphofunctional relationships within the skull

Methods

Discussion

Conclusion