Osteology and digital reconstruction of the skull of the early tetrapod Whatcheeria deltae

Abstract

The Early Carboniferous stem tetrapod Whatcheeria deltae is among the earliest-branching limbed tetrapods represented by multiple near-complete specimens, making it an important taxon in understanding the vertebrate water-to-land transition. However, all preserved skulls of Whatcheeria suffer from post-mortem crushing and lateral compression, which has made cranial reconstruction problematic. In this study, computed tomography data of three Whatcheeria specimens were segmented using visualization software to digitally separate each individual skull bone from matrix. Digital methods were used to repair and retrodeform the bones and produce the first complete three-dimensional skull reconstruction of Whatcheeria. We provide a revised description of the cranial and lower jaw anatomy of Whatcheeria based on CT data, focusing on sutural morphology and previously unknown anatomical details. Our findings suggest that Whatcheeria had one of the narrowest skulls of any known early tetrapod, a gap between the nasals, and significant overlap of the lacrimal onto the nasal and prefrontal. Sutural morphology is used to infer loading regime in the skull during feeding and suggests the skull of Whatcheeria was well adapted to resist stresses induced by biting large prey with its enlarged anterior fangs.

Early tetrapod cranial evolution is characterized by increased complexity, constraint, and an offset from fin-limb evolution

“…2) using literature descriptions and available retrodeformed three-dimensional (3D) models (data S1). Our study was enhanced by the use of new 3D datasets of key taxa across the transition, captured via x-ray computed tomography (CT) scans ( 17 , 18 , 32 , 33 ).…”

Skull Sutures and Cranial Mechanics in the Permian Reptile Captorhinus aguti and the Evolution of the Temporal Region in Early Amniotes

“…The slot and tongue-andgroove joints (2 and 3 in Figure 10) only occur in the preorbital region and adjacent palatal contacts. Tongue-and-groove joints had been previously interpreted as an adaptation toward tensile strains (Herring and Mucci, 1991;Porro et al, 2015;Rawson et al, 2021). Based on their horizontally intercalated geometry, we would tentatively argue that slot joints are more resistant to compressive than to tensile strains.…”

Fossil bone histology reveals ancient origins for rapid juvenile growth in tetrapods

“…The elevated juvenile growth rates associated with reticular fibrolamellar bone may have served as an adaptation to the particular ecology and life history of Whatcheeria : individuals could have reached maximum body size and sexual maturity more quickly 5 , allowing them to occupy a large-bodied, predatory niche in their paleoecosystem 44 , 59 . Within its lowland terrestrial lake system, Whatcheeria is recovered as the largest-bodied tetrapod (1.5–2 m) with cranial adaptations indicating that it consumed other large-bodied vertebrates via forceful biting 45 . Further, geologic data suggest that populations of Whatcheeria lived during times of global climate change 60 and local increases in wet-dry seasonality 59 – 61 .…”

“…Abbreviations: eer endosteal erosion, flb fibrolamellar bone, ob osteonal bone, pfb parallel-fibered bone, ser secondary remodeling erosion, so secondary osteon, tr trabecular bone, wfb woven-fibered bone. lowland terrestrial lake system, Whatcheeria is recovered as the largest-bodied tetrapod (1.5-2 m) with cranial adaptations indicating that it consumed other large-bodied vertebrates via forceful biting 45 . Further, geologic data suggest that populations of Whatcheeria lived during times of global climate change 60 and local increases in wet-dry seasonality [59][60][61] .…”

“…Recent anatomical investigations have revealed that Whatcheeria is a highly unusual animal with cranial mechanosensory lines as well as large, robust limbs and a short tail (Fig. 1), blurring the ostensible divide between aquatic and terrestrial adaptations [43][44][45] . This body plan is also present in the Mississippian stem tetrapod Pederpes 44,46 , and reinterpretations of 'whatcheeriid' material suggest that rather than an anatomically plesiomorphic grade, the whatcheeriids were a Carboniferous clade with a distinct body plan.…”