Cranial Morphology of the Brachystelechid 'Microsaur' Quasicaecilia texana Carroll Provides New Insights into the Diversity and Evolution of Braincase Morphology in Recumbirostran 'Microsaurs'

Abstract

Recumbirostran 'microsaurs,' a group of early tetrapods from the Late Carboniferous and Early Permian, are the earliest known example of adaptation to head-first burrowing in the tetrapod fossil record. However, understanding of the diversity of fossorial adaptation within the Recumbirostra has been hindered by poor anatomical knowledge of the more divergent forms within the group. Here we report the results of μCT study of Quasicaecilia texana, a poorly-known recumbirostran with a unique, broad, shovel-like snout. The organization of the skull roof and braincase of Quasicaecilia is found to be more in line with that of other recumbirostrans than previously described, despite differences in overall shape. The braincase is found to be broadly comparable to Carrolla craddocki, with a large presphenoid that encompasses much of the interorbital septum and the columella ethmoidalis, and a single compound ossification encompassing the sphenoid, otic, and occipital regions. The recumbirostran braincase conserves general structure and topology of braincase regions and cranial nerve foramina, but it is highly variable in the number of ossifications and their extent, likely associated with the reliance on braincase ossifications to resist compression during sediment compaction and mechanical manipulation by epaxial and hypaxial musculature. Expansion of the deep ventral neck musculature in Quasicaecilia, autapomorphic among recumbirostrans, may reflect unique biomechanical function, and underscores the importance of future attention to the role of the cervical musculature in contextualizing the origin and evolution of fossoriality in recumbirostrans.

Fig 1. Embryonic morphology of the cartilaginous neurocranium of a generalized tetrapod, Crocodylus porosus, after [26].

A, dorsal view; B, lateral view; C, ventral view. Illustrations not to scale. Abbreviations: ec, ethmoid commissure; en, external naris; fao, foramen antoticum; fep, fenestra epioptica; fm, foramen magnum; fmo, foramen metopticum; fo, foramen ovale; fv, fenestra vestibularis; hyp, hypophyseal fenestra; occ, occipital arch; ot, otic capsule; pa, pila antotica; pme, pila metoptica; pss, planum supraseptale; sin, interorbital septum; sn, nasal septum; st, synotic tectum; tn, tectum nasi.

Fig 2. Skull of Quasicaecilia texana, USNM 22079, 3D renders segmented from μCT.

A, dorsal view; B, anterior view; C, left lateral view; D, right lateral view; E, palatal view; F, occipital view. Scale bar equals 5 mm. Abbreviations: cp, cultriform process; en, dorsal margin of external naris; f, frontal; fcbic, foramen serving cerebral branch of the internal carotid artery; fm, foramen magnum; fme, foramen metoticum; fv, fenestra vestibularis; j, jugal; n, nasal; oc, occipital condyle; os, orbitosphenoid; p, parietal; pa, pila antotica; pal, palatine; palp, palpebral bone; pf, pineal foramen; po, postorbital; pof, postfrontal, pop, paroccipital process; prf, prefrontal; prs, presphenoid; pt, pterygoid; q, quadrate; qsp, stapedial process of quadrate; scf, subcondylar fossa; sn, nasal septum; sq, squamosal.

Fig 3. Reconstruction of skull of Quasicaecilia texana.

A, dorsal view; B, left lateral view; C, ventral view. Dashed lines indicate inferred or hypothetical morphology.

Fig 4. Braincase of Quasicaecilia texana, USNM 22079, 3D renders segmented from μCT.

Anterior and posterior braincase digitally re-assembled. A, dorsal view; B, left anterolateral view; C, left lateral view; D, left posterolateral view; E, ventral view; F, posterior view. Scale bar equals 5 mm. Abbreviations: cp, cultriform process; fcbic, foramen serving cerebral branch of the internal carotid artery; flv, foramen serving lateral head vein; fm, foramen magnum; fme, foramen metoticum; fv, fenestra vestibularis; n I, foramen serving olfactory nerve; n II, foramen serving optic nerve; n IV, foramen serving trochlear nerve; n V, foramen serving trigeminal nerve (undivided); oc, occipital condyle; oct, occipital cotyle; os, orbitosphenoid; pa, pila antotica; pop, paroccipital process; prs, presphenoid; scf, subcondylar fossa; sn, nasal septum; st, tectum synoticum; tn, tectum nasi; vs, vidian sulcus.

Fig 5. Arrangement of braincase ossifications in selected Recumbirostra.

A, diagrammatic illustration of structures and key landmarks in the recumbirostran braincase. B, map of ossified regions in the braincase of Huskerpeton englehorni. C, digital dissection of braincase of Huskerpeton englehorni, UNSM 32144; D, map of ossified regions in the braincase of Dvellecanus carrolli, UCMP 202940; E, digital dissection of braincase of Dvellecanus carrolli, UCMP 202940; F, map of ossified regions in the braincase of Brachydectes newberryi; G, digital dissection of braincase of Brachydectes newberryi (KUVP 49541); H, map of ossified regions in the braincase of Carrolla craddocki; H, digital dissection of braincase of Carrolla craddocki (TMM 40031–54), modified from [9]; I, map of ossified regions in the braincase of Quasicaecilia texana, J, digital dissection of braincase of Quasicaecilia texana, USNM 22079. Scale bars equal 5 mm. Abbreviations: bo, basioccipital; ch, choana; en, external naris; ex, exoccipital; flv, foramen serving the lateral head vein; fme, foramen metoticum; fv, fenestra vestibularis; map, median ascending process of the synotic tectum; mse, mesethmoid; nc, nasal capsule; n I, foramen serving the olfactory nerve; n II, optic foramen; n III, oculomotor nerve foramen; n V, trigeminal nerve foramen; n XII, hypoglossal foramen; ob, otoccipital bone; oc, occipital condyle; oca, occipital arch; occ, occipital; op, opisthotic; orb, orbital cartilages; os, orbitosphenoid; otc, otic capsule; otoc, otoccipital; pbs, parabasisphenoid; pmo, posterior median ossification of the ethmoid trabeculae; pro, prootic; prs, presphenoid; psp, parasphenoid; pss, planum supraseptale; sph, sphenoid; so, supraoccipital.