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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General outline

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.