Hybodontoidea, Owen, 1846

Roelofs, Brett, Barham, Milo, Mory, Arthur J. & Trinajstic, Kate, 2016, Late Devonian and Early Carboniferous chondrichthyans from the Fairfield Group, Canning Basin, Western Australia, Palaeontologia Electronica (Barking, Essex: 1987) 262, pp. 1-28 : 19-20

Hybodontoidea gen. et sp. indet.

Figure 7.6-7

Material. One tooth from sample TS-1, Laurel Formation, Laurel Downs, Tournaisian.

Description. A well-preserved tooth with a large vertical bulbous crown that overhangs the lingual and distal faces of the base. The lower half of the crown, on the lingual face, is raised and bears a row of low profiled ovoid cusplets running mesio-distally ( Figure 7.6 ). A pair of large pits is present on the distal sides of the distinct labial peg ( Figure 7.7 ). The crown base interface on the labial side is highly arched with a thickened margin. The lingual margin of the base is semicircular and approximately one and a half times as deep as the crown height. The lingual face is slightly convex and perforated by three horizontally aligned rows of pores ( Figure 7.6 ). Small pores are present in and around the concave labial side of the base.

Remarks. This tooth is unusual with the elongate base directed under the crown. The labial peg and fused crown is similar to some Hybodonts such as Lissodus . However, the base lacks the large vascular foramina typical of that genus. In overall morphology, the tooth bears a resemblance with a hybodont tooth figured in Ginter and Sun (2007, figure 6A 1-3) with the large labial peg, pyramidal crown and deep base orientated directly under the crown.

Euselachii gen. et sp. indet. 1

Figure 7.8 -11

Material. Six teeth from 198404, two teeth from 198480, nine teeth from TS-1, Laurel Formation, Laurel Downs, Tournaisian.

Description. Tooth crowns with two morphologies but both with a similar euselachian basal form. The crown and base are separated by a small but distinct lingual groove on the lingual face of the tooth. The base extends lingually and ranges from symmetrical ( Figure 7.8 ) to asymmetrical in outline ( Figure 7.9 ). A row of large canals extend from baso-lingual margin to the crown base. On the labial face of the tooth, the margin between the crown and base is often marked by a significant concavity ( Figure 7.11 ). A large pore network covers the concave underside of the crown whereas the underside of the baso-lingual extension is smooth.

Crown Morphotype 1 is symmetrical in form ( Figure 7.8 ) with a high, prominent medial point and steeply tapering margins. The crown is smooth with only faint, rounded cusp type projections along the occlusal surface of some teeth ( Figure 7.8 ). The crown margins are rounded and do not overhang the base.

Crown Morphotype 2 is asymmetrical with a large bulbous central cusp overhanging the lingual margin ( Figure 7.10 ), and two to three smaller lateral cusps. Tooth crowns with three cusps, possess a slightly flattened and distally tapering lateral cusp on one side ( Figure 7.10 -11). The opposite lateral cusp forms a small dome, approximately half the size of the central cusp, and in some specimens overhangs the labial face of the crown. In one specimen ( WAM 15.6 .20, Figure 7.9 ), the tooth comprises four cusps; a central cusp with a single flattened lateral cusp on one side and two smaller rounded cusps, decreasing in size distally, on the other. The cusps all possess a slight distally directed orientation with the largest cusp overhanging both the lingual and labial margins .

Remarks. Both morphotypes are here considered to represent a single taxon based on the high degree of heterodonty amongst other sharks with clutching-crushing dentitions (e.g., Heterodontus ). Some of the teeth ( Figure 7.10 -11) superficially resemble the teeth attributed to Holocephali gen. et sp. indet. with their low profile bulbous cusps and lingually extending base perforated by a series of elongate canals. The crowns of the Holocephali gen. et sp. indet. teeth, however, differ from Euselachii gen. et sp. indet. 1 due to the small pores covering the crown and a central cusp that overhangs the labial face of the crown-base interface rather than overhanging the lingual margin.

Euselachii gen. et sp. indet. 2

Figure 7.12 -13

Material. Seven teeth from sample 198404, nine partial teeth from sample TS-1, Laurel Formation, Laurel Downs, Tournaisian.

Description. Asymmetrical teeth that are highly flattened along the occlusal surface. The crown is almost rectangular in outline with a low convex occlusal surface ( Figure 7.12 -13). The surface of the tooth is covered by small rounded areas of discolouration, which are sometimes associated with small pits that do not appear to project into the dentine layer. There are two rows of cusplets: large irregularly shaped cusplets along the labial face ( Figure 7.13 ) and smaller rounded cusplets along the lingual face ( Figure 7.12 ). The occlusal facing base extends lingually and is perforated by a series of elongate canals. The baso-labial side of the crown is thin and lacks a clear crown-base interface.

Remarks. These teeth possess a range of characters including the wide canal openings similar to Euselachii gen. et sp. indet. 1; as well as different cusplet types on the labial and lingual faces of the crown that appear similar to Protacrodontidae gen. et. sp. The mesio-distal elongation of the teeth in conjunction with lingual and labial cusplets is diagnostic of lateral teeth attributed to the genus Cassisodus . In addition, the crown positions on the teeth of Cassisodus margaritae and Euselachii gen. et sp. indet. 2 are more labial, creating a slightly labio-lingual elongation. However, the crown of Cassisodus is far more pronounced with accessory labial and lingual cusplets of equal size. Due to the rarity of these teeth and the asymmetrical form, it is possible these teeth are part of a more complex dentition of another species.

Figures

FIGURE 7. Shark teeth from the Upper Devonian Gumhole Formation, Oscar Hill and Lower Carboniferous Laurel Formation, Laurel Downs, Lennard Shelf, Canning Basin, Western Australia. 1-2, Protacrodontidae gen. et sp. indet., WAM 15.6.29, sample OH-4, in lingual (1) and labial (2) views; 3-5, Lissodus sp., WAM 15.6.42, sample TS-1, in lingual (3), occlusal (4) and labial (5) views; 6-7, Hybodontoidea gen. et sp. indet., WAM 15.6.41, sample TS-1, in lingual (6) and labial (7) views; 8-11, Euselachii gen. et sp. indet.1, WAM 15.6.21, sample 198404, in lingual view (8), WAM 15.6.20, sample 198404, in occlusal view (9), WAM 15.6.27, sample 198404, in occluso-lingual view (10), and WAM 15.6.39, sample TS-1 in labial view (11); 12-13, Euselachii gen. et sp. indet. 2, WAM 15.6.40, sample TS-1, in occluso-lingual view (12) and WAM 15.6.22, sample TS-1, in labial view (13); 14-17, Holocephali gen. et sp. indet. 1, WAM 15.6.4, sample 198404, in occlusal view (14), and WAM 15.6.3, sample 198404, in lingual (15), labial (16) and occlusal (17) views. Scale bar length: 1-2, 0.5 mm; 3-5, 0.4 mm; 6-7, 0.75 mm; 8-10, 0.3 mm; 11, 1 mm; 12-13, 0.6 mm; 14-17, 5 mm.