Oligocene squatinid taxonomy
The fossil record of Squatina comprises at least 35 nominal species, most of them published in the 1800s (Cappetta, 2006). Some are based on (partial) skeletons, but most descriptions rely on isolated teeth or vertebrae only. For isolated material, differential characters are rarely provided and interspecific variation has not been studied. In other cases, new species have been erected merely on the basis of stratigraphic provenance of the material. The naming of new so-called chronospecies, without proper differential diagnosis, was very common at the time, especially for taxa whose (dental) morphology had been quite conservative over time, such as Squatina. Therefore, taxonomic revisions of fossil angel sharks (e.g. Guinot et al., 2012) are called for, particularly post-Mesozoic species. In some cases, assignment of isolated teeth and vertebrae to species is fraught with difficulties, and might even prove impossible (Cappetta, 2012; Herman et al., 2013; the present study). Authors often opt to leave angel shark material in open nomenclature (Squatina sp.), in particular when this originates from strata of Cenozoic age (see Klug and Kriwet, 2013).
However, Oligocene angel shark teeth have often been identified as S. angeloides Van Beneden, 1873 (e.g., Nolf, 1988; Reinecke et al., 2001; Cicimurri and Knight, 2009; Génault, 2012). The same holds true for teeth of Squatina from the SVK clay pit (see van den Bosch, 1981). The original species description by Van Beneden (1873: 384) was based on a collection of unassociated vertebrae from an undocumented locality in Belgium (current whereabouts unknown), not illustrated and described extremely briefly as follows, ‘Squatina angeloïdes. Van Ben. Nous avons des vertèbres en assez grand nombre pour reconstituer une colonne vertébrale plus ou moins complète. Ces vertèbres sont surtout reconnaissable au corps, qui est plus large que haut.’ (We have a large number of vertebrae to reconstruct a vertebral column that is more or less complete. These vertebrae are easy to identify, trunk vertebrae in particular, that are wider than tall). Van Beneden’s description is quite remarkable as new chondrichthyan taxa are rarely based on skeletal elements because such are seldom preserved, especially in strata of Cenozoic age (Mollen, 2010; Mollen and Jagt, 2012). Moreover, isolated skeletal material and, in particular vertebrae, often are undiagnostic, making it unsuitable for describing new taxa. In Van Beneden’s case, we agree that the vertebrae (which are wider than tall) are diagnostic of the genus Squatina, but not of its constituent species (see e.g., Hasse, 1876, 1877, 1882; Ridewood, 1921). Van Beneden, probably aware of this shortcoming, did not provide differential characters to discriminate among species of Squatina. In view of this, we consider S. angeloides to have been based merely on stratigraphic data. Three other angel shark species have been described from Oligocene deposits, all postdating S. angeloides, namely S. beyrichi Noetling, 1885, S. rupeliensis Daimeries, 1889 and S. crecelii Weiler, 1922. The last-named species was based on teeth, whereas the other two were erected on the basis of isolated vertebrae.
Storms (1894) was the first to describe and illustrate teeth (and vertebrae) of Squatina from the Belgian Rupelian (R2c, equalling the Boom Clay Formation, Upper Oligocene), and assigned them to S. angeloides, assuming the vertebrae of Squatina described by Van Beneden had originated from the same level. Although the description provides detailed differential diagnoses with fossil as well as modern angel shark species (at the time, this was very progressive), evidence that teeth and vertebrae stem from the same species of Squatina is lacking. The geographic distribution of modern species of Squatina overlap extensively (Stelbrink et al., 2010), and the material from the Belgian Oligocene might well represent more than a single species.
Unlike Storms (1894), we do not consider the stratigraphic origin of S. angeloides to be beyond doubt. In fact, Van Beneden did not record any stratigraphic data in his description of S. angeloides, nor can the stratigraphic context be deduced from the text beyond doubt. Although Van Beneden’s chapter in Patria Belgica was more or less in reversed stratochronological order, beginning with the Pliocene and ending with the Cretaceous, and most of these sections included references to faunal lists from the same stratum that had been published previously, no clear subdivisions were made in the text, nor was a species list for the Oligocene provided. Moreover, in the text, several species were stated to occur at more than one level, making it unclear which one Van Beneden was discussing at the time. This holds true especially for S. angeloides.
Although rare, most of the elasmobranch skeletal material known from the Belgian Cenozoic originates from the Boom Clay Formation (see Leriche, 1910), supporting Storms’s (1894) assumption. However, in Leriche’s series of papers on fossil fishes from the Belgian Cenozoic, vertebrae of Squatina were recorded from all time intervals; Paleocene (Leriche, 1902: 17; Pl. 1, Figs 21-22), Eocene (Leriche, 1905: 96), Oligocene (Leriche, 1910: 251-252) and Mio-Pliocene (Leriche, 1926: 382, Fig. 163-163a; 383, Fig. 164-164b). A fossil angel shark vertebra donated by Van Beneden to Hasse (1877: 349; 1882: 135), was consistently mislabelled as ‘Pliocaen (Terrain rupel(l)ien)’. Daimeries (1889) noted this conflicting label in Hasse (1882), and favoured the ‘Terrain rupelien’, yet failed to provide arguments for his choice. Subsequently, Daimeries (1889) described this particular specimen as S. rupeliensis, a species erroneously reported by Cappetta (2006: 201) as originating from the Eocene, apparently unaware of Van Beneden’s (1873) description.
In conclusion, the original description of S. angeloides is extremely poor and based on vertebrae of uncertain stratigraphic provenance that are not diagnostic at the species level. The material was not illustrated and its current whereabouts are unknown. In view of the fact that criteria stipulated by the code (ICZN, art. 12.1) are barely met, we consider S. angeloides to be a nomen nudum, and thus unavailable. The same applies for S. rupeliensis the description of which suffers the same shortcomings. However, Daimeries (1889) referred to material mentioned by Hasse (1882). Although a name can be made available by indication (ICZN, art. 12.1), requirements of art. 12.2. are not met. In contrast, Noetling (1885) did provide a proper description of S. beyrichi, inclusive of illustrations and stratigraphic origin of the vertebrae. Although this name is available according to the code, we consider it to be a nomen dubium because angel shark vertebrae are not diagnostic at the species level; in doing so we agree with Cappetta (2006), who rejected the name.