Nanocetorhinus tuberculatus, Underwood & Schlögl, 2013

Underwood, Charlie J. & Schlögl, Jan, 2013, Deep-water chondrichthyans from the Early Miocene of the Vienna Basin (Central Paratethys, Slovakia), Acta Palaeontologica Polonica 58 (3), pp. 487-509 : 502-505

Nanocetorhinus tuberculatus sp. nov.

Fig. 9A–H .

2005 Elasmobranch dermal denticle or possible tooth, Form I, Johns et al. 2005: fig. 37.

Etymology: From the tuberculate surface ornament of the teeth.

Holotype: SNM Z 27485 .

Type locality: Cerová−Lieskové, Vienna Basin, Slovakia.

Type horizon: Lakšárska Nová Ves Formation, late Karpatian (equivalent to latest Burdigalian).

Material.—28 additional partial and complete teeth including SNM Z 27486 to SNM Z 27492.

Diagnosis.—As for genus.

Description.—The teeth comprise a single cusp above a rather irregular root, with the cusp comprising at least half of the total tooth height. They are very small, typically less than 1 mm high, and more than twice as high as wide. There is low degree of heterodonty, other than a single symphyseal tooth ( Fig. 9F ) that is extremely mesio−distally compressed compared to the other teeth. The cusp is elongate and roughly circular in cross section, being straight or having a slight curvature. The base of the cusp is marked by a pronounced and slightly constricted “collar”, the base of which expands slightly onto the upper surface of the root. Broken cusps reveal that no pulp cavity is present within the tooth crown. The labial face of the cusp is strongly ornamented with irregular tubercles, some of which are sharp−edged, which are on occasion connected to form irregular longitudinal ridges. The ornament extends from the top of the “collar” and reaches close to the cusp apex. The lingual face is smooth or ornamented by one to three weak longitudinal enameloid folds. The “collar” is constricted at the top where it joins the cusp, and rather flared at the base where it meets the root. There is a fine granulate ornament over the entire “collar”. The root is wider than the cusp, and the cusp stands proud of the root labial surface. Although rather variable in form, the root is quite bulky and has two, variably well developed, root lobes that project basally. The root lobes are round or oval in cross section, and have pointed apices. The basal face of the root is typically at least partly open to reveal a pulp cavity within the root. In some specimens this is not the case, and the central part of a convex lingual root face is penetrated by several, rather irregular, foramina. Smaller foramina are present on the lingual face of the root lobes, and a weak groove may be present between these and the main opening. The symphyseal tooth is generally similar to the other teeth, especially in side view, but is very strongly compressed, resulting in the depth of the tooth being at least three times the width.

Remarks.—The teeth of this taxon have a very distinct morphology that is rather different from that of any other known chondrichthyan, so the affinity is uncertain. The morphology is simple, comprising a single cusp and rather irregular root, and is of a general type shared with many planktivorous sharks and rays. The teeth of juvenile specimens of Cetorhinus have a complex rugose ornament and irregular cutting edge, similar to that seen in Nanocetorhinus tuberculatus (see Herman et al. 1993), although most of this ornament is lost in the teeth of adults. Teeth of Cetorhinus , however, differ from those of N. tuberculatus in having a globular, anaulacorhize root and lacking a distinct collar at the base of the cusp, in addition to being far larger. Furthermore, symphyseal teeth are absent in lamniform sharks including Cetorhinus . As a result, the similarities between the teeth of Cetorhinus and N. tuberculatus are here considered to be superficial and not indicative of taxonomic affinity. Teeth of the some planktivorous batoids resemble those of N. tuberculatus (see Herman et al. 2000) in possessing an irregular reticulate ornament, but have a very different morphology to both the root and crown. In addition, there is some resemblance between teeth of N. tuberculatus and those of some Squaliformes. Teeth of Aculeola nigra de Buen, 1959 also possess a single cusp with some ornamentation at its base and a bilobed root that commonly has a large basal opening (see Herman et al. 1989). Despite this, the crown and root of A. nigra and other similar squaliform teeth, are much more regular, and the roots have a well defined morphology with a flat basal face.

The simple and rather irregular form of the teeth of Nanocetorhinus tuberculatus is typical of the non−functional teeth of planktivorous taxa. In addition, there is no evidence for wear on the apex or cutting edges of the teeth, and the irregular ornament is inconsistent with the teeth having been used to penetrate food items. It is therefore probable that this taxon was a planktivore, or possibly microphageous suction feeder, where the teeth played little or no direct role in food

http://dx.doi.org/10.4202/app.2011.0101

manipulation. While the teeth of N. tuberculatus are very small, this is not necessarily indicative of a small body size, as teeth of planktivores do not scale with body size; Herman et al. (2000) figures teeth of a 4.8 m width Manta that are all below 1.5 mm in high. Johns et al. (2005) figure imperfect specimens of this taxon from the Late Oligocene or Early Miocene of western Canada. It is also present in Miocene of France (Sylvain Adnet, personal communication 2010).

Neoselachii incertae sedis: denticles

Denticle Type 1

Fig. 10A, B.

Material.—Frequent denticles including SNM Z 27493, SNM Z 27494.

Description.—Denticles of this type are somewhat rhomboid in form, but follow a similar basic morphology. The denticle crown is flat and thin and overlies narrow and short neck and root region; the neck and root are typically poorly preserved. The crown is biconvex and “lozenge−shaped” in surface view, with a number of sharp and continuous longitudinal ridges. In some denticles, there are small lateral points along the distal edge either side of the main central projection. The crown surface between the ridges is weakly concave. Typically three or six ridges are present, in the case of the latter these appear to be arranged in parallel pairs. At the anterior margin of the denticle crown, some of the ridges widen. Wear is not seen on the upper surface, although damage to the denticle edges may be present.

Remarks.—Denticles of this general morphology are not diagnostic to taxonomic group, and are present within many pelagic, and some benthic, shark taxa. The lack of wear due to contact with the substrate suggests that these denticles were not present on the underside of a benthic species.

Denticle Type 2

Fig. 10C–E.

Material.—Very common denticles including SNM Z 27495 to SNM Z 27497.

Description.—Denticles of this type are abundant and range in form from short and very robust to high and gracile, but intermediate forms are also present. In all cases there is a strong and continuous curvature of the crown neck and crown, with a flat surface of the crown being absent unless it has been formed by wear. The anterior part of the crown has four to seven (most commonly five) ridges that continue across the upper surface unless they are removed by wear. Where these reach the distal edge of the denticle, they form a variable pectinate edge. The stem and crown is typically completely covered by a fine reticulate ornamentation. This comprises irregular and anastomosing ridges, each with a weak anterior edge and sharp posterior edge. The stem is variably constricted, but is always robust, and expands basally into a rhombic root what is commonly of a similar width to the crown. Wear is commonly present and often severe, with the surface both polished and bearing abundant longitudinal scratches.

Remarks.—The degree of wear typically present on these denticles is indicative of abrasion by the substrate on the skin of a benthic taxon. It is unclear whether all denticles of this type relate to a single taxon, but the characteristic fine−scale ornamentation would suggest that this is likely. These denticles are very similar to those of some extant deep−water scyliorhinids, and it is therefore likely that these also relate to this group, and may be conspecific with the Galeus species recognized from dental remains.

Denticle Type 3

Fig. 10F.

Material.—Single denticle, SNM Z 27498.

Description.—The single denticle of this morphology is superficially like Type 2, but is far more robust, lacks a well defined neck and a fine surface ornament. The denticle is about as wide as long, and has a flat and smooth, roughly triangular, crown surface. Several short but strong ridges are present along the anterior edge of the crown. The neck is very short and only weakly constricted and originates from a root of similar overall dimensions to the crown.

Remarks.—The robust form of the denticle is similar to that of several groups of benthic chondrichthyans, and as a result the affinity is uncertain.

Denticle Type 4

Fig. 10G–H.

Material.—Rare denticles including SNM Z 27499, SNM Z 27500.

Description.—These uncommon denticles have a very characteristic “arrowhead” crown which is longer than wide and comprises a pointed distal apex which widens proximally into rounded “wings”. Anterior to these is a roughly parallel−sided projection which forms about half of the length of the crown. A single or pair of very robust ridges run along the projection to close to the apex. There is a constricted and short neck over a wide root that is commonly roughly “T” shaped in basal view.

Remarks.—Denticles similar to this are present on the dorsal surfaces of a range of slow swimming chondrichthyans, including species of Heterodontus , Squatina , and some rhinobatid rays. As a result, the affinities of this denticle are uncertain.

Denticle Type 5

Fig. 10I.

Material.—Single denticle, SNM Z 27501.

Description.—This denticle is damaged, but the general morphology is clear. It is irregularly oval, with a rather conical root capped by a smaller crown, with no obvious constricted neck. The crown is irregular and has a number of short conical projections extending from its upper surface

UNDERWOOD AND SCHLÖGL—MIOCENE DEEP−WATER SHARKS FROM SLOVAKIA 505

0.5 mm

and edges. Below this, the sides of the root have a series of string vertical ridges, some bifurcating towards the base.

Remarks.—The overall form of this denticle is similar to that present on the dorsal surface of a number of batoid taxa, but the detailed form of the crown does not appear to closely resemble that of any examples that have been figured or observed by us so the generic affinity is unclear.

Denticle Type 6

Fig. 10J, K.

Material.—Frequent denticles including SNM Z 27502, SNM Z 27503.

Description.—These extremely characteristic denticles are quite abundant. They are far longer than wide and are symmetrical or slightly curved. The crown is slender and leafshaped, with a pointed apex. This has a smooth surface and lacks sharp edges. The crown is strongly displaced to the posterior relative to the root. The anterior end of the crown passes downwards into the short neck that forms a robust ridge on the anterior end of the root. The root is smaller than the crown and has an elongate rhombic profile.

Remarks.—This form of denticle does not appear to relate well to any figured or observed in modern chondrichthyans, and therefore is likely to belong to an extinct genus. The overall shape of the denticle is very similar to that of many extant etmopterids, but these all appear to differ in possessing a single or multiple sharp ridges along the length of the denticle and having as more star−shaped root. It is therefore likely that these specimens belong to an extinct etmopterid genus, with their relative abundance suggesting that one of the commoner tooth taxa would be likely. This denticle morphology is therefore tentatively considered as conspecific with Paraetmopterus horvathi sp. nov.

Figures

Fig. 9. Miocene neoselachian and myliobatiform sharks of Nannocetorhinus and Gymnura teeth respectively, from Cerová section in Malé Karpaty, Slovakia. A–H. Nannocetorhinus tuberculatus gen. et sp. nov. A. Holotype, SNM Z 27485, bed 21–22, in labial (A1) and lingual (A2) views. B. SNM Z 27486, bed 8–9, in labial (B1) and lingual (B2) views. C. SNM Z 27487, bed 17–18, in labial (C1) and lingual (C2) views. D. SNM Z 27488, bed 8–9, in labial (D1) and lingual (D2) views. E. SNM Z 27489, bed 13, in labial (E1) and lingual (E2) views. F. SNM Z 27490, bed 17–18, in labial (F1) and lateral (F2) views. G. SNM Z 27491, bed 20–21, in labial (G1) and lingual (G2) views.H. SNM Z 27492, bed 20–21, in labial (H1) and lingual (H2) views, detail (H3). I. Gymnura sp., SNM Z 27484, bed 14, in occlusal (I1) and basal (I2) views.