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

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Description of fossil chondrocranium

General outline

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The specimen is a partial, albeit exceptionally well-preserved chondrocranium, its surface consisting mostly of prismatic, tessellated cartilage (Figs 1-2). The ethmoid region, which would have included the rostrum and the nasal capsules, together with the anterior half of the cranial roof and basal plate (BP, or basicranium), are missing, exposing the cranial cavity (CC) in anterior view (Fig. 2A). None of the broken surfaces are fresh; fracturing most probably occurred at an early stage of fossilisation. Although some structures are missing, the posteriormost part of the orbital region, together with both otic and occipital regions, are preserved three dimensionally. All together, the structures preserved have a more or less rectangular to trapezoidal outline in dorsal and ventral views, and appear flattened in latero-occipital view. A blackish phosphatic matrix fills the inner structures of the chondrocranium, obscuring some details of the external structures as well. Maximum chondrocranial width (as preserved), situated near the glossopharyngeal bases (GB), slightly exceeds the preserved length, and doubles the chondrocranial height. However, standard measurements for elasmobranch chondrocrania (see Compagno, 1988, Fig. 6.13), cannot be employed for incomplete specimens and matrix-covered structures. Therefore, preliminary measurements, relevant for this specimen, are depicted in Fig. 1B, D.

FIG2

Fig. 1. Chondrocranium of Squatina sp. (IRScNB P.9485), Sint Niklaas Phosphorite Bed (Rupelian, Upper Oligocene), SVK clay pit (Oost-Vlaanderen, Belgium) in dorsal (A), ventral (B) and lateral (C-D) views. Abbreviations: BP, basal plate; CC, cranial cavity; EC, epiotic crest; EF, endolymphatic fossa; ELF, endolymphatic foramen; FM, foramen magnum; GB, glossopharygeal base; HF, hyomandibular facet; LC, lateral commissure; OEC, otic external canal; OC, otic capsule; OG, orbital groove; OW, orbital wall; UPOP, upper postorbital process.

FIG2

Fig. 2. Chondrocranium of Squatina sp. (IRScNB P.9485), Sint Niklaas Phosphorite Bed (Rupelian, Upper Oligocene), SVK clay pit (Oost-Vlaanderen, Belgium) in anterior (A) and occipital (B) views. Scale bar equals 20 mm. Abbreviations: IX, glossopharygeal nerve foramen; X, vagus nerve foramen; BP, basal plate; CC, cranial cavity; CR, cranial roof; EC, epiotic crest; FM, foramen magnum; GB, glossopharygeal base; LC, lateral commissure; OCC, occipital condyle; OG, orbital groove; OHC, occipital hemicentrum; OP, occipital process; OW, orbital wall; POW, postorbital wall; UPOP, upper postorbital process.

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.

Otic region (i.e., labyrinth or auditory region)

The otic region, which is separated from the orbital and occipital region by the posterior margin of the lateral commissure and vagus nerve foramen, respectively, is almost complete and well preserved, representing most of the present specimen. In lateral view, the otic region is elongated and divided over its entire length, by a well-marked, horizontal ridge of the otic external canal (OEC; ‘β‘ of Iselstöger, 1937), which borders the dorsal margin of a large and relatively uniform hyomandibular facet (HF) (see Fig. 1C). In dorsal view, the otic region is bordered over its entire length laterally by a parallel pair of longitudinally oriented epiotic crests (EC). The right epiotic crest (‘sphenopterotic ridge’ of Shirai, 1992b) is less well preserved than the left one, but does show a clear outline of the epiotic crest base in dorsal view, which is anteriorly bounded by the base of the upper postorbital process, and widest in its median portion (see Fig. 1A). Also in dorsal view, the posterior end of both epiotic crests are bounded by a continuous, well-marked and rectilinear transverse crest (‘ridged, posterior tectum’ of de Carvalho et al., 2008: 480) which is more robust in its mid-portion (= otic-occipital junction). Just anterior to the transverse crest, and in between the otic capsules, is the endolymphatic fossa, situated within a shallow depression. The endolymphatic fossa (EF; ‘parietal fossa’ of Maisey, 1983) is wider than long, with a rounded posterior margin. However, most of the endolymphatic fossa is filled with matrix, covering the lymphatic foramina, except for the left endolymphatic foramen (ELF), which is relatively large, with its centre situated at the anterior and distalmost margins of the endolymphatic fossa. In ventral view, the basal plate, except for the otic capsules (OC), is well preserved, its surface consisting of tessellated, prismatic cartilage. Although the basal plate is not preserved ventrally to the otic capsules, much of their margins is still intact, rectangular to oval in appearance, and protruding as if they were bulging. As a result, the otic capsules and medial portion of the basal plate are separated by a pair of longitudinal depressions running from the anterior margin of the OC bulge towards the occipital region. In cross section, the basal plate is slightly convex between both grooves. In its mid-portion, a well-marked medial, longitudinal groove is present (Fig. 1B).

Occipital region

The occipital region, well preserved in the SVK specimen, is more or less dorso-ventrally flattened and widest near the glossopharyngeal bases (see Fig. 2B). It is separated from the otic region by the vagus nerve foramen (following Gegenbaur, 1872; Hoffmann, 1912), making the occipital region extremely short in lateral view. In this view, the occipital region is oriented obliquely, inclining ventrally in posterior direction (Fig. 1C-D). As a result, the occipital region can also be observed in dorsal view, albeit under an angle. In this view, the dorsal margin of the occipital region is more or less concave between the posterior ends of the epiotic crests in general outline, but rectilinear in its mid-portion (which equals the width of the foramen magnum). Here, the dorsal margin of the occipital region is completely merged with the mid-portion of the transverse crest, making it very robust in its median part (Fig. 1A). In occipital view, the dorsal margin of the occipital region is slightly concave between the epiotic crests, and slightly overhanging posteriorly in its mid-portion (= otic-occipital junction), forming a pair of short, near-horizontal grooves dorsal to the large foramen magnum. The foramen magnum (FM) is partially filled and covered with matrix, especially near its ventral margin, but appears to have had an oval, vertically oriented shape (Fig. 2B). Matrix also covers the regions lateral to the foramen magnum, thus obscuring possible posterior vein foramina. A pair of oval occipital processes (OP) are present near the dorsal margin of the foramen magnum, slightly ventral to the horizontal grooves formed by the overhanging otic-occipital junction; their dorsal margins are more pronounced than the ventral ones. The foramen magnum is flanked by at least one pair of openings for the vagus nerve (X) which are situated posterior to the foramen magnum in dorsal view (see Fig. 2B). Glossopharyngeal bases (GB) and glossopharyngeal nerve foramina (IX) are present, but are only partially preserved. As a result of longitudinal grooves in the basicranium (see description of otic region), the ventral margin of the occipital region is UUU shaped, and protrudes posteriorly. It consists of tesselated, prismatic cartilage, forming a complete, robust, yet not swollen ridge, with the possible exception of its portion ventral to the foramen magnum which is slightly damaged, leaving a broken, faintly oval, wedge-shaped structure (for an interpretation, see Results and Discussion).