Reevaluation of the cranial osteology and phylogenetic position of the early crocodyliform Eopneumatosuchus colberti, with an emphasis on its endocranial anatomy

Abstract

Eopneumatosuchus colberti Crompton and Smith, 1980, known from a single partial skull, is an enigmatic crocodylomorph from the Lower Jurassic Kayenta Formation. In spite of its unique morphology, an exceptionally pneumatic braincase, and presence during a critical time period of crocodylomorph evolution, relatively little is known about this taxon. Here, we redescribe the external cranial morphology of E. colberti, present novel information on its endocranial anatomy, evaluate its phylogenetic position among early crocodylomorphs, and seek to better characterize its ecology. Our examination clarifies key aspects of cranial suture paths and braincase anatomy. Comparisons with related taxa (e.g., Protosuchus haughtoni) demonstrate that extreme pneumaticity of the braincase may be more widespread in protosuchids than previously appreciated. Computed tomography scans reveal an endocranial morphology that resembles that of other early crocodylomorphs, in particular the noncrocodyliform crocodylomorph Almadasuchus figarii. There are, however, key differences in olfactory bulb and cerebral hemisphere morphology, which demonstrate the endocranium of crocodylomorphs is not as conserved as previously hypothesized. Our phylogenetic analysis recovers E. colberti as a close relative of Protosuchus richardsoni and Edentosuchus tienshanensis, contrasting with previous hypotheses of a sister group relationship with Thalattosuchia. Previous work suggested the inner ear has some similarities to semi-aquatic crocodyliforms, but the phylogenetic placement of E. colberti among protosuchids with a terrestrial postcranial skeletal morphology complicates paleoecological interpretation.

Neuroanatomy of the mekosuchine crocodylianTrilophosuchus rackhamiWillis, 1993

“…Generally, the brain endocast of T. rackhami follows a pattern that is common among crocodylomorphs. The endocast of T. rackhami is relatively linear and elongated, although not to the extreme seen in the basal crocodylomorph Almadasuchus figarii Pol et al, 2013 , the basal crocodyliform Eopneumatosuchus colberti Crompton & Smith, 1980 , thalattosuchians, or the dyrosaurid tethysuchian Rhabdognathus aslerensis Jouve, 2007 , where the divisions on their endocasts are arranged in a somewhat more linear fashion than most other crocodylomorphs (Brusatte, Muir, et al, 2016 ; Erb & Turner, 2021 ; Leardi et al, 2020 ; Melstrom et al, 2021 ; Pierce et al, 2017 ; Wilberg et al, 2021 ). The hypophyseal fossa of T. rackhami , although incomplete, is clearly inclined anterodorsally and not sub‐horizontal like that of some thalattosuchians (though note that Macrospondylus bollensis (von Jäger, 1828 ) has an inclined fossa; Herrera et al, 2018 ; Wilberg et al, 2021 ).…”

“…A postinfundibular recess is hardly distinguishable, although it is present as a less pronounced secondary expansion of the infundibular recess that is located ventromedially to its parent recess and is largely confluent with the pharyngotympanic cavity. Stretching posteriorly within the quadrate, the infundibular recess narrows for a relatively brief length of ~2.5 mm before it begins to expand again as it becomes continuous with the quadrate re- Erb & Turner, 2021;Leardi et al, 2020;Melstrom et al, 2021;Pierce et al, 2017;Wilberg et al, 2021). The hypophyseal fossa of T. rackhami, although incomplete, is clearly inclined anterodorsally and not sub-horizontal like that of some thalattosuchians (though note that Macrospondylus bollensis (von Jäger, 1828) has an inclined fossa; Herrera et al, 2018;Wilberg et al, 2021).…”

“…Ratios that could not be estimated have omitted data (indicated with a dash). Abbreviations: CCRH, common crus height; ELH, endosseous labyrinth height; ELL, endosseous labyrinth anteroposterior length; VEH, vestibular apparatus height within Crocodylomorpha (e.g., Busbey III & Gow, 1984;Crompton & Smith, 1980;Dufeau, 2011;Kuzmin et al, 2021;Leardi et al, 2017Leardi et al, , 2020Melstrom et al, 2021;Pierce et al, 2017;Walker, 1990;Wu & Chatterjee, 1993). Many studies have surveyed the developmental patterns and morphological diversity of the paratympanic pneumaticity in extant crocodylians (e.g., Bona et al, 2017;Colbert, 1946b;Dufeau & Witmer, 2015;Dufeau, 2011;Gold et al, 2014;Iordansky, 1973;Kuzmin et al, 2021;Owen, 1850;Pierce et al, 2017;Serrano-Martínez, 2018;Serrano-Martínez et al, 2019a;Tarsitano, 1985;.…”

“…Aspects of crocodylomorph palaeoneurology were first investigated in the 19th century (Owen, 1842, 1850) and continued sporadically throughout the 20th century (e.g., Colbert, 1946a, 1946b; Edinger, 1938; Hopson, 1979; Yeh, 1958), though the frequency of palaeoneurological publications has increased drastically in the 21st century. With the advent of computed tomography and its increasing widespread availability to researchers, the past 14 years have witnessed a surge in papers that are dedicated either exclusively or partially on the palaeoneurology of crocodylomorphs (e.g., Blanco et al, 2015; Bona & Paulina Carabajal, 2013; Bona et al, 2013, 2017; Bowman et al, 2021; Brusatte, Muir, et al, 2016; Cowgill et al, 2021; Dumont Jr et al, 2020; Erb & Turner, 2021; Fernández et al, 2011; Fonseca et al, 2020; George & Holliday, 2013; Herrera et al, 2018; Herrera, 2015; Holliday & Gardner, 2012; Kley et al, 2010; Leardi et al, 2020; Melstrom et al, 2021; Pierce et al, 2017; Pochat‐Cottilloux et al, 2021; Puértolas‐Pascual et al, 2022; Ristevski et al, 2020a, 2021; Schwab et al, 2020; Schwab, Young, Herrera, et al, 2021, Schwab, Young, Walsh, et al, 2022; Schwab et al, 2022; Sereno & Larsson, 2009; Serrano‐Martínez et al, 2019a, 2019b, 2021; Sertich & O'Connor, 2014; Wilberg et al, 2021; Witmer & Ridgely, 2008; Witmer et al, 2008). Despite this, our understanding of crocodylomorph palaeoneurology is still in its relative infancy considering that the neuroanatomy for the vast majority of crocodylomorph taxa is yet to be investigated, including for the majority of Australian taxa.…”

Dispatches from the age of crocodiles: New discoveries from ancient lineages

“…These medium‐sized, terrestrial predators possessed relatively long faces, narrow skulls, large eyes, sharp teeth, and demonstrate several key features found in crocodylians including an early occurrence of a fossa for a laterally‐wrapping pterygoideus ventralis muscle (Clark et al, 2004). Melstrom et al (2022) continue into the Jurassic, re‐evaluating the anatomy of the early crocodyliform Eopneumatosuchus colberti from the Kayenta formation of Southwest North America. The authors find that Eopneumatosuchus is most likely a protosuchian, but has features, such as a flattened semicircular canals and a relatively flat skull, that suggests it may have lived a more aquatic lifestyle than its close, terrestrial relatives.…”