Phoebeannaia mossae, Caron & Venkataraman & Tietjen & Fls, 2023

Phoebeannaia mossae

The orbital region is loosely defined here as anterior to the ventral fissure and lateral commissure, and posterior to the olfactory capsules and ethmoid processes. Functionally, this region of the braincase transmits cranial nerves and vasculature, houses the extraocular muscles and forebrain, frames the orbits and supports the palatoquadrate.

The supraorbital shelf extends medio-dorsally from the post-orbital processes (po.p; Figs 7A, 9A, B, 10A, B) and descends at the midline beneath the olfactory nerves and forebrain. There is no ossified orbital septum, and the optic nerves consequently exit the intracranial space together through a large midline foramen (II; Fig. 10A, B). The basisphenoid pillar (bs.p; Fig. 7B) transmits the hypophysis from the cranial cavity ventrally through the floor of the neurocranium (hyp.f; Figs 7B, 9A), and flares out laterally into buttresses (lat.b; Fig. 10A, B) which connect directly with the base of each large, anteriorly directed basipterygoid process (bpt.p; Figs 7A, 8A, 9A, 10A, B). A third, slender, medial buttress extends anteriorly a short distance before merging into the midline crest of the basisphenoid (ant.b; Figs 7B, 10B). Fenestrae between each lateral strut allow passage between the orbital space and the large unpaired posterior myodome. Anterior openings of the jugal canal (j.c; Figs 7A, 8A, 9A, 10A), walled by the lateral commissure (lat.com; Fig. 7A), flank the posterior myodome laterally.

The many foramina and fossae in the orbital region can be understood by comparison to extant and extinct taxa. The post-orbital wall, dominated by the jugular vein canal, bears a medial foramen for the exit of the facial nerve (VII; Fig. 10A, B) and a recess marking the location of the geniculate ganglion. Dorsal to the exit of the jugal canal, another recess likely for the gasserian and lateralis ganglia (V,lat; Fig. 10A, B) is associated with a broad communion with the intracranial space. Within this recess, a lateral passage seems to have guided the otic branch of the trigeminal nerve (V.ot; Fig. 10B) back into the otic wall. A shallow depression above and lateral to the trigeminal recess might have served as a site of attachment for jaw musculature, though it lacks clear boundaries and is not as large as the corresponding fossa in Moythomasia . Small foramina for the oculomotor nerves (III; Fig. 10A, B) flank the optic foramen. The posterior wall of the expansive single posterior myodome is pierced on either side by a foramen for the abducens nerve (VI; Figs 7D, 10B, D) and, more ventrally, by a passage to accommodate the descending palatine branch of the facial nerve and/ or the ascending orbital artery (VII.pal,o.a; Figs 7B, 9A, 10A, B). The roof of the myodome bears a midline opening permitting the hypophysis to descend from the intracranial space into the groove of the basisphenoid pillar before reaching the hypophysial canal ventrally (hyp.f; Figs 7B, 9A).

Paired passages (int.car; Figs 9A, 10A, B) located ventromedially to the basipterygoid processes enclose the internal carotids (after giving off the orbital arteries). These canals run medially and dorsally into the space between the basisphenoid pillar and anterior buttress. Grooves on the central pillar (gr.a.int.car; Fig. 10A, B) mark the ascent of the internal carotids to a midline foramen, ventral to the anterior buttress, that opens to the intracranial space. On the ventral surface of the basisphenoid, narrow grooves mark the passage of the efferent pseudobranchial arteries (ep.a; Fig. 9A) prior to anastomosis with the internal carotids. A narrow midline groove runs anteriorly from the hypophysial canal and is laterally joined by a groove for the palatine artery and nerve (gr.p.a,n; Fig. 9A).