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
Discussion

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Comparison with fossil and modern squatinoids

Squatinoids are represented by two monotypical families (Squatinidae, Pseudorhinidae) that comprise Squatina Duméril, 1806 and Pseudorhina Jaekel, 1898, respectively. The family Pseudorhinidae has been erected only recently by Klug and Kriwet (2013) to accommodate the latter genus and its constituent species, P. acanthoderma (Fraas, 1854), P. alifera (von Münster, 1842) and P. frequens (Underwood, 2002). Stratigraphically, these species range from the upper Oxfordian to the lower Tithonian (Upper Jurassic; see Cappetta, 2012). In contrast, all post-Jurassic squatinoids, inclusive of modern angel sharks, have been assigned to Squatina (Squatinidae) (see e.g., Guinot et al., 2012). Klug and Kriwet (2013) estimated the date of the origin of (identifiable characters of) Squatiniformes to lie between 181.74 and 156.2 Ma, and that of Squatinidae to be between 157.59 and 114 Ma, representing ‘soft’ maximum and ‘hard’ minimum age constraints, respectively. The early age of radiation of extant angel shark species was dated at 33.17 ± 9.85 Ma by Stelbrink et al. (2010: Table 6), which overlaps with the Rupelian age of the SVK specimen. In this respect, this particular specimen is of interest in our understanding of modern squatinid diversity as well.

Pseudorhina (i.e., Pseudorhinidae) differs from Squatina (i.e., Squatinidae) by a large number of characters, including a longer anterior fontanelle with a less rounded posterior margin, less concave lateral capsular walls of the otic region and more laterally oriented postorbital processes. Of these three cranial features, only the upper postorbital processes are retained in the SVK chondrocranium. Although not complete in our specimen, the bases of both upper postorbital processes are oriented more anteriorly, which is a morphological feature typical of Squatina (see de Carvalho et al., 2008; Klug and Kriwet, 2013), and consistent with what could be expected for a squatinoid of Oligocene (i.e., post-Jurassic) age. Other characters diagnostic of Pseudorhina comprise proportionally larger vertebral hemicentra, comparably shorter and more tightly connected basiventral processes and a broad labial protuberance (‘apron’ of Cappetta, 1987) of teeth not supported by roots (Claeson, 2008; de Carvalho et al., 2008; Klug and Kriwet, 2013). The relative size of vertebral hemicentra is difficult to assess in the isolated vertebrae from the SVK clay pit, but the anterior vertebrae found at the level that yielded the chondrocranium display relatively robust and long basiventrals (see Fig. 5A-B), which make assignment to Squatina most likely. In addition, squatinoid teeth from the Sint Niklaas Phosphorite Bed are diagnostic of Squatina, in having a slender apron that is supported by the root.

Although chondrocrania of modern angel sharks are generally conservative, de Carvalho et al. (2012) listed several differential characters in three species of Squatina from the southwest Atlantic (i.e., S. argentina (Marini, 1930), S. guggenheim and S. occulta). These include rostral projections, anterior fontanelle, supraorbital crest, upper postorbital process, lower postorbital process, otic capsules, suborbital crest and pterotic process. These diagnostic features prove to be even more revealing when comparing species of Squatina from different geographic origins, which correlates to different clades based on molecular evidence (Stelbrink et al., 2010) (see Figs 6-7-8). In spite of the fact that the SVK chondrocranium is closely similar to those of modern angel sharks, two morphological features might actually fall outside the range of interspecific variation seen in modern Squatina. In the latter, the ventral margin of the occipital region is rectilinear and the occipital hemicentrum is flanked by a pair of swollen occipital condyles that are oriented horizontally, with only their lateral margins slightly bent dorsally (see Fig. 8A-E). In contrast, the ventral margin of the occipital region of the SVK specimen is UUU-shaped and not swollen (see Fig. 8F). Although a broken, slightly oval, wedge-shaped structure is visible ventral to the foramen magnum, there is no clear indication of an occipital hemicentrum in this specimen. However, in modern angel sharks this structure easily detaches in fetal specimens (Claeson and Hilger, 2011) and is only weakly calcified in adults (de Carvalho et al., 2008) which might explain its absence in the SVK specimen. Secondly, the anterior margin of the upper postorbital process is distally expanded in all modern angel sharks (de Carvalho et al., 2012; the present study), forming a completely or nearly closed orbital groove in those species that present a well-developed supraorbital flange (see Figs 67; compare de Carvalho et al., 2012). Although the upper postorbital processes are not completely preserved in the SVK specimen, remains of this structure seem rounded in both dorso-ventral and lateral views and thus are not expanded. As a result, the orbital groove was most likely not fully closed. Such horizontal expansion is absent in Jurassic Pseudorhina, and has not yet been observed in Cretaceous species of Squatina either (see Guinot et al., 2012; NHMUK P.12213). Consequently, the distal expansion of the anterior margin of the upper postorbital proces might well represent a derived character for Recent angel sharks only, or at least post-Oligocene ones.

FIG2

Fig. 6. Interspecific variations in Recent Squatina spp. (A-E) in dorsal view, compared to the fossil specimen (F; IRScNB P.9485). Recent chondrocrania are presented as 3D-reconstructions using CT scanning of S. squatina (A; ERB 1029), S. africana (B; ERB 0971), S. australis (C; ERB 0514), S. dumeril (D; ERB 1026) and S. guggenheim (E; ERB 0881). Species represent the European/North African/Asian (A), South African (B), Australian (C) and North/South American (D-E) clades as defined by Stelbrink et al. (2010). Scale bars equal 20 mm. Abbreviations: AF, anterior fontanelle; CR, cranial roof; EC, epiotic crest; EF, endolymphatic fossa; ELF, endolymphatic foramen; FM, foramen magnum; GB, glossopharygeal base; LPOP, lower postorbital process; OG, orbital groove; OHC, occipital hemicentrum; PEP, preorbital process; RP, rostral projection; SF, supraorbital flange; UPOP, upper postorbital process.

FIG2

Fig. 7. Interspecific variations in Recent Squatina spp. (A-E) in ventral view, compared to the fossil specimen (F; IRScNB P.9485). Recent chondrocrania are presented as 3D-reconstructions using CT scanning of S. squatina (A; ERB 1029), S. africana (B; ERB 0971), S. australis (C; ERB 0514), S. dumeril (D; ERB 1026) and S. guggenheim (E; ERB 0881). Species represent the European/North African/Asian (A), South African (B), Australian (C) and North/South American (D-E) clades as defined by Stelbrink et al. (2010). Scale bars equal 20 mm. Abbreviations: IX, glossopharygeal nerve foramen; BP, basal plate; GB, glossopharygeal base; ICF, internal carotid artery foramen; LPOP, lower postorbital process; OC, otic capsule; OG, orbital groove; OHC, occipital hemicentrum; PEP, preorbital process; PP, pterotic process; RP, rostral projection; SBC, suborbital crest; UPOP, upper postorbital process.

FIG2

Fig. 8. Interspecific variations in Recent Squatina spp. (A-E) in occipital view, compared to the fossil specimen (F; IRScNB P.9485). Recent chondrocrania are presented as 3D-reconstructions using CT scanning of S. squatina (A; ERB 1029), S. africana (B; ERB 0971), S. australis (C; ERB 0514), S. dumeril (D; ERB 1026) and S. guggenheim (E; ERB 0881). These represent the European/North African/Asian (A), South African (B), Australian (C) and North/South American (D-E) clades as defined by Stelbrink et al. (2010). Scale bars equal 20 mm. Abbreviations: IX, glossopharygeal nerve foramen; X, vagus nerve foramen; EC, epiotic crest; FM, foramen magnum; LPOP, lower postorbital process; OCC, occipital condyle; OHC, occipital hemicentrum; PEP, preorbital process; UPOP, upper postorbital process; VC, posterior vein foramen; VHJ, ventral margin of hyoid junction.

As a result, the UUU shape of the ventral margin of the occipital region, combined with rounded margins of the upper post orbital process (vs distally expanded), might justify the erection of a new genus to accommodate the SVK chondrocranium. However, in view of the fact that the fossilisation process might have had an impact on the shape of the ventral margin of the occipital region and of the occipital condyles in particular (e.g., shrunk, abraded), and the upper postorbital processes are not completely preserved, we prefer to await the discovery of additional material and/or pieces of evidence (e.g., on brain nerve foramina). Unfortunately, the inner structures of the phosphatic chondrocranium have proved impossible to penetrate by the medical CT scanner available to us.