Cranial functional morphology of the pseudosuchian Effigia and implications for its ecological role in the Triassic

Abstract

Pseudosuchians, archosaurian reptiles more closely related to crocodylians than to birds, exhibited high morphological diversity during the Triassic with numerous examples of morphological convergence described between Triassic pseudosuchians and post‐Triassic dinosaurs. One example is the shuvosaurid Effigia okeeffeae which exhibits an “ostrich‐like” bauplan comprising a gracile skeleton with edentulous jaws and large orbits, similar to ornithomimid dinosaurs and extant palaeognaths. This bauplan is regarded as an adaptation for herbivory, but this hypothesis assumes morphological convergence confers functional convergence, and has received little explicit testing. Here, we restore the skull morphology of Effigia, perform myological reconstructions, and apply finite element analysis to quantitatively investigate skull function. We also perform finite element analysis on the crania of the ornithomimid dinosaur Ornithomimus edmontonicus, the extant palaeognath Struthio camelus and the extant pseudosuchian Alligator mississippiensis to assess the degree of functional convergence with a taxon that exhibit “ostrich‐like” bauplans and its closest extant relatives. We find that Effigia possesses a mosaic of mechanically strong and weak features, including a weak mandible that likely restricted feeding to the anterior portion of the jaws. We find limited functional convergence with Ornithomimus and Struthio and limited evidence of phylogenetic constraints with extant pseudosuchians. We infer that Effigia was a specialist herbivore that likely fed on softer plant material, a niche unique among the study taxa and potentially among contemporaneous Triassic herbivores. This study increases the known functional diversity of pseudosuchians and highlights that superficial morphological similarity between unrelated taxa does not always imply functional and ecological convergence.

Functional morphology of the Triassic apex predator Saurosuchus galilei (Pseudosuchia: Loricata) and convergence with a post‐Triassic theropod dinosaur

“…Furthermore, the Saurosuchus vomers are much thinner than in Allosaurus , which is the likely cause of the higher observed stresses from the palate of the pseudosuchian during both biting simulations. These results provide another example where similar form between distantly related extinct taxa does not equate to the same functional morphologies (Bestwick et al, 2022; Ferry‐Graham et al, 2002; Fisher, 1985; Lautenschlager et al, 2016; Thomason, 1995).…”

“…Muscle origination sites on the Saurosuchus cranium (Figure 2) were identified for each jaw adductor muscle independently based on osteological correlates such as muscle scars, ridges and depressions. Muscle insertion sites on the adjusted mandible (Figure 2) were based on the same criteria, assuming a largely similar jaw adductor arrangement across archosaurs following von Baczko (2018), Bestwick et al (2022), and Holliday (2009). As a first step, the origin and insertion sites of each muscle were connected by individual point‐to‐point connections.…”

“…Adductor muscle force loads were applied across multiple loads at the inferred muscle origination and insertion sites of the crania and mandibles, respectively. This was performed using a custom-built macro (Altair, UK) that loads multiple nodes projected towards a node(s), resulting in a vector equivalent to the line of action (Bestwick et al, 2022).…”

“…Many of the skeletal bauplans exhibited by early pseudosuchians have been described as convergent with distantly related dinosaurs from the Jurassic and Cretaceous (Stocker et al, 2016). Examples include the gracile, bipedal stances and skulls with large orbits and edentulous jaws of shuvosaurid poposauroids, which are similar to those of ornithomimids (Bestwick et al, 2022; Nesbitt, 2007; Nesbitt & Norell, 2006), the robust limbs and armored bodies of aetosaurs which are similar to those of ankylosaurians (Desojo et al, 2013; Parker et al, 2021) and the elongate rostra and expanded neural spines of phytosaurs which are reminiscent of some spinosaurids (Stocker et al, 2016). Pseudosuchians thus likely filled a wide range of ecological roles and are regarded as the dominant tetrapod group in Middle and Late Triassic terrestrial food webs (Brusatte et al, 2008; Stocker et al, 2016).…”

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

“…Sharing the Triassic ecosystems with Aetosaurs and other crocodylomorphs were the beaked, edentulous shuvosaurs like Effigia okeeffeae (Nesbitt, 2007). Here, Bestwick et al (2022) use 3D modeling and engineering approaches to test hypotheses of feeding function in these enigmatic animals. The authors found that Effigia had a rather weakly built skull and was likely a specialist that fed on soft plant material, making it among the most divergently‐adapted species of crocodylomorphs.…”