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

Phoebeannaia mossae

The occipital region is well preserved barring minor damage on the posterior right wall of the aortic canal. The otico-occipital fissure (ot.oc.f; Figs 7A, 9A, B) delimits the dorsal portion of the occipital unit from the otic region. The fissure spans an ovoid posterior dorsal fontanelle (pos.d.fon; Fig. 9B) at the dorsal midline and extends ventrolaterally, expanding to accommodate the exit of the vagus nerve (X; Figs 7A, B, 9A) and terminates in the large vestibular fontanelles ( v.fon ; Figs 7A, 8A, 9A). A narrow bridge of bone separates the vestibular fontanelles from the ventral fissure (v.f; Figs 7B, 9A). The ventral fissure bounds the anterior extent of the basioccipital and is curved with an anterior-facing concavity, allowing it to skirt around but not intersect with the posterior myodome. The perichondrally-lined ventral fissure is laterally fringed by, but not contiguous with, foramina through which the palatine branch of the facial nerve and/or branches of the orbital artery (VII.pal,o.a; Figs 9A, 10A, B) might have communicated with the posterior myodome.

In posterior view ( Fig. 8), the occipital plate is taller than it is wide, with lateral boundaries gently curving out from a narrow base to small craniospinal processes (csp.p; Figs 7A, 8A, 9A, B) level with the top of the foramen magnum (f.m; Figs 7B, 8A). Internally the foramen magnum is pierced by two ventral foramina for the occipital nerves (oc.n; Fig. 7B), which exit the braincase laterally. The notochord canal (nc.c; Figs 7B, 8A) is separated from the foramen magnum dorsally and the aortic canal ventrally, and terminates anteriorly before the otico-occipital fissure. An intermuscular septum is marked by a faint ridge that runs dorsoventrally through the craniospinal fossa (csp.f; Fig. 8A, E): the broad, posterior-facing shallow depression for the attachment of epaxial musculature flanking the foramen magnum. No prominent horizontal ridge or midline crest edging the fossa is present. The left craniospinal process bulges slightly anteriorly around the opening for the vagus nerve, but the right-side equivalent shows no evidence of this protrusion. The occipital plate is narrow relative to the rest of the braincase, approximately 60% as wide as the distance between the post-orbital processes. The occiput is not as wide as in Kansasiella or other contemporaneous species; the narrower dimensions in Phoebeannaia reveal much of the otic wall in posterior view including the parampullary fossa, jugal canal, and articular surfaces for the pharyngobranchials and hyomandibula.

The basioccipital is dominated by the median dorsal aortic canal (ao.c; Figs 7B, 8A). There is a midline foramen approximately level with the exit of the vagus nerve, which must have transmitted some efferent branchial arteries (e.br.a; Figs 7B, 9A). The enclosed canal ends where the grooves for the lateral dorsal aorta diverge (lat.d.ao.c; Fig. 9A), level with the glossopharyngeal nerve exit (IX, Fig. 7A). No associated indentations mark the reception of further efferent branchial arteries, although they might have joined ventral to the neurocranium. A prominent articular surface for the first infrapharyngobranchial (art. 1st.i; Figs 7A, 9A) projects from the ventrolateral extremity of the basioccipital. Anteriorly, this surface is delimited by a deep groove marking the passage of the efferent hyoid artery up into the jugal canal.