Contributions to Zoology, 85 (2) – 2016Frederik H. Mollen; Barry W.M. van Bakel; John W.M. Jagt: A partial braincase and other skeletal remains of Oligocene angel sharks (Chondrichthyes, Squatiniformes) from northwest Belgium, with comments on squatinoid taxonomy
Description of fossil chondrocranium

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Orbital region (i.e., orbito-temporal or sphenoid region)

Together with the ethmoidal region, the anteriormost part of the orbital region is missing, including the supraorbital and suborbital crests. Although most of the paired medial orbital walls (OW) is lacking as well, the posteriormost portion of the right orbital wall is present, albeit covered with matrix (see Fig. 1A, D). In contrast, other parts of determinant structures of the orbit are well preserved, i.e., the left lateral commissure, both upper postorbital processes and orbital grooves.

The left lateral commissure (LC; ‘Fascialis-Spange’ of Iselstöger, 1937) is robust and well developed over its entire length. Its surface clearly consists of tessellated, prismatic cartilage. The distal margin of the lateral commissure is slightly concave in anterior view (Fig. 2A). In lateral view, the lateral commissure is oriented obliquely (Fig. 1C), inclining ventrally in posterior direction, and its anterior margin is rectilinear. In cross section, the distal portion of the lateral commissure is rounded. Dorsally, both lateral commissures were bounded by an upper postorbital process (UPOP; ‘Processus postorbitalis’ of Iselstöger, 1937). Although both upper postorbital processes have broken off, each leaving a clear fracture, their bases are well preserved, especially the left one (Fig. 2A). Unfortunately, because of this, the exact length of the upper postorbital process cannot be determined. A tessellated cartilage structure, well flattened laterally, is pressed into the base of the remaining right upper postorbital process. In lateral view, this structure is relatively tall and has a rounded anterior margin (see Fig. 1D). Because of its unique position and general shape, it appears to be the distalmost portion of the right upper postorbital proces. As suggested by the fracture near the ventral portion of the left lateral commissure, lower postorbital processes (LPOP) were most likely present as well, although they are not preserved. The lateral commissure, together with at least the upper postorbital process, and most likely also the lower ones, forms a robust postorbital wall (POW) (Fig. 2A). In dorsal view, the preserved parts of orbital and postorbital wall have a concave outline anteriorly, forming the posterior part of a large, well-marked orbital groove (OG; ‘Schädellücke’ of Iselstöger, 1937). In lateral view, the orbital groove is oriented obliquely, inclining ventrally in anterior direction, i.e., opposite to the inclination of the lateral commissure. In dorsal view, the posterior margin of the orbital groove almost reaches as far as the anterior margin of the endolymphatic foramen (see otic region). The surface of the orbital grooves was smooth (as seen in the left OG in our specimen); they would have accommodated the orbital processes of the palatoquadrates, moving through the orbital groove when opening the mouth.