Cardabiodon venator, Siverson & Lindgren, 2005

Siverson, Mikael & Lindgren, Johan, 2005, Late Cretaceous sharks Cretoxyrhina and Cardabiodon from Montana, USA, Acta Palaeontologica Polonica 50 (2), pp. 301-314 : 308-311

Cardabiodon venator sp. nov.

Figs. 3, 4 .

Pseudoisurus tomosus ; Siverson 1992: 530.

Cretolamna woodwardi ; Williamson et al. 1993: 457, figs. 4, 5.

Pseudoisurus ? sp. [partim]; Siverson 1996: 834, pl. 4: 8–10 [not figs. 5–7, which depict a tooth very similar to the third lower anterior tooth in the reconstructed dentition of Cardabiodon ricki (see Siverson 1999)].

Etymology: Venator (Latin for hunter), referring to its presumed position near or at the very top of the food chain.

Holotype: WAM 04.10 .64, interpreted as a left second lower anterior tooth.

Paratypes: WAM 04.10.65–04.10.71 .

Additional material: 29 teeth (total number, including types, is 37), WAM 04.10.72–04.10.100.

Type horizon: A 1.7 m (5.6 feet) thick interval of the Fairport Member with dark−grey shale and thin limestone lenses, weathering to orange brown. The beds are 1.83–3.54 m (6.0– 11.6 feet) above the base of the Carlile Shale (see Johnson and Smith 1964: 33).

Type locality: Badlands near Mosby, in Petroleum and Garfield Counties, Montana. Precise locality data is on file at the Department of Earth and Planetary Sciences, Western Australian Museum.

Diagnosis.—Dignathic heterodonty relatively weaker in lateroposterior files than in C. ricki , with both upper and lower teeth exhibiting a distally curved cusp. Cusplets usually present on lateroposterior teeth of large size but often poorly developed, especially on the mesial side of the cusp. In contrast, both mesial and distal cusplets are large in C. ricki . Lateroposterior teeth of small juveniles frequently lack a mesial cusplet, and some also lack a distal cusplet. Root and crown flushed in the same plane labially in most large lateroposterior teeth, whereas the base of the crown overhangs the root in most teeth from small juvenile individuals, both in anterior and lateroposterior files. The root is thick and bulky, and differs significantly in this regard from the more slender root of C. ricki . Its lingual protuberance lacks a median groove, except in some teeth from very young individuals, which exhibit a very shallow groove. Most teeth display a string of densely spaced, circular foramina along the labial crown−root boundary, whereas labial foramina on the root are fewer and more randomly distributed in C. ricki .

Description.—All type specimens are described below. WAM 04.10.64 (holotype; Fig. 3A 1 –A 3); a perfectly preserved, second, left lower anterior tooth (position based on the type specimen of Cardabiodon ricki, WAM 94.4.45). It is 31.6 mm high (32.6 mm in maximum slant height). The robust cusp is erect and has a conspicuous constriction near its base. The cutting edges have a sigmoid profile view. Labially, the cusp is markedly convex in its apical third, but rather flat in the middle part. The lingual side of the cusp is strongly convex. The mesial and distal cusplets are both rudimentary and barely noticeable. The lingual neck, separating the crown from the root is very broad. It measures 3.6 mm in height medially, which corresponds to 11 per cent of the height of the tooth. The basal edge of the root is tightly curved. A string of small, labial foramina is present on the root, just below the strongly curved basal edge of the crown.

WAM 04.10.65 (paratype; Fig. 3B 1 , B 2); a lower? right lateroposterior tooth, probably from the middle part of the lateroposterior hollow (possibly lp7–lp9; see Siverson 1999: fig. 5), measuring 22 mm in height (26.1 mm in maximum slant height). The cusp is curved towards the commisure. Labially, the cusp is markedly convex. The distal cusplet is blunt, lacking a well defined apex. The outer edges of the mesial cusplet are much longer than the inner edge. A small indentation separates both the mesial and the distal cusplets from the cusp. The neck is 2.5 mm high medially. The crown and the root are flushed in the same plane labially. The mesial lobe of the root is missing.

WAM 04.10.66 (paratype; Fig. 3C 1 , C 2); a well preserved, 20.5 mm high (26.9 mm in maximum slant height) upper right lateroposterior tooth. The neck is 2.1 mm high medially. In combination, the relatively large size of the tooth, the strong distal curvature of the cusp and the symmetrical root indicate that the tooth belonged to a file from the middle part of the lateroposterior hollow (see Siverson 1999: fig. 5). Although the labial angle between the root−lobes and the cusp (almost 180 °) is more in line with that of lower lateroposterior teeth, this tooth is assigned to the upper jaw on the basis of the strong distal curvature and flat labial face of the cusp, which is rather broad in its apical half (again indicating an upper jaw position). Like in WAM 04.10.65, there is no labial overhang of the root by the crown. The string of labial foramina on the root, just below the base of the crown, that characterizes this species, is particularly well developed on this tooth.

WAM 04.10.67 (paratype; Fig. 3D 1 –D 3); an incomplete (distal root−lobe and cusplet broken off), 23.5 mm high (28.3 mm in maximum slant height) upper left lateroposterior tooth. The neck is 2.6 mm high medially. The angle between the labial face of the only preserved root−lobe and the labial face of the cusp is considerably less than 180 °, strongly indicating that this is an upper jaw tooth. An upper jaw position is also indicated by the comparatively flat labial face of the cusp.

WAM 04.10.68 (paratype; Fig. 3E 1 , E 2); an incomplete (lacking both lobes of the root) lower left lateroposterior tooth from a large individual, measuring, as preserved, 24 mm in height (31.5 mm in maximum slant height). The tooth is referred to the lower lateroposterior hollow (as opposed to the upper lateroposterior hollow) on the basis of its broad−based cusp (see Siverson 1999: fig. 5), marked convexity of the labial side of the cusp, and the moderate distal curvature of the cusp. The relatively short cusp in combination with the large size of the tooth indicates that it originated from the middle part of the lateroposterior hollow. The mesial cusplet is much reduced in size and exhibits an outer edge much longer than the inner one. There is no indentation at the junction between the inner cutting edge of the mesial cusplet and the mesial cutting edge of the cusp. The cutting edges of the distal cusplet are more equal in length and there is an indentation present at the junction between the inner cutting edge of the distal cusplet and the distal cutting edge of the cusp. The lingual neck, separating the crown from the root, is 3.0 mm high medially. There is a conspicuous string of densely spaced, labial foramina on the root, just below the base of the crown. Several foramina open medially on the basal face of the root, near the summit of the lingual protuberance.

WAM 04.10.69 (paratype; Fig. 3F 1 , F 2); a well preserved, 5.8 mm high (8.8 mm in maximum slant height) lower? left lateroposterior tooth from a very small juvenile. The cusp is strongly distally curved and worn at its apex. A single, damaged cusplet is present on the distal blade of the crown. The tooth is tentatively assigned to the lower jaw on the basis of its moderately developed lingual protuberance of the root.

WAM 04.10.70 (paratype; Fig. 3G 1 , G 2); a perfectly preserved 8.1 mm high (10.9 mm in maximum slant height) upper? left lateroposterior tooth from a small juvenile. The cusp is strongly distally curved. Like in most juvenile lateroposterior teeth of this species, there is no mesial cusplet. The inner edge of the distal cusplet is far shorter than the outer edge. The root is bulky, measuring 3.7 mm in labiolingual thickness, and symmetrical.

WAM 04.10.71 (paratype; Fig. 3H 1 , H 2); a damaged (mesial root−lobe broken off), 5.7 mm high (5.9 mm in maximum slant height) upper left lateroposterior tooth from a juvenile. The low cusp and the short preserved distal lobe of the root indicate that it belonged to one of the more posteriorly situated lateroposterior files. The distal cusplet is relatively large and the breakage surface at the mesial border of the tooth indicates that a mesial cusplet was originally present on this tooth.

Comparisons.—The Fairport Cardabiodon differs from the middle Cenomanian type species C. ricki by the following features: (1) in C. ricki , there is a marked dignathic heterodonty in the anterior half of the lateroposterior files. The lower teeth have a wide and slightly distally bent cusp, whereas the cusp on the corresponding upper teeth is narrower and markedly distally curved. The difference in distal curvature of the cusp appears to be less prominent in C. venator sp. nov., making it more difficult to separate isolated upper and lower ones in this species; (2) the cusp is considerably taller in C. ricki than it is on teeth of the same width and approximately the same position of C. venator sp. nov.; (3) the cusp on the second lower anterior tooth of C. venator sp. nov. have a distinct constriction near its base, a feature absent in the type specimen of C. ricki ; (4) lateral cusplets are reduced to a larger extent in C. venator sp. nov. than they are in C. ricki ; (5) the root is significantly thicker labiolingually in C. venator sp. nov. than it is in C. ricki ; (6) most teeth of C. venator sp. nov. have a conspicuous string of densely spaced, circular foramina just below the base of the crown, whereas the labial foramina are more scattered in C. ricki .

There may be differences in the ontogeny but juvenile teeth of Cardabiodon are only known from the early middle Turonian (this study).

Dental differences between Cardabiodon and the dentally similar genera Archaeolamna Siverson, 1992 and Dwardius Siverson, 1999 , were outlined by Siverson (1999: 61). In addition to these differences, the Parotodus −like morphology of juvenile C. venator teeth (which have much reduced cusplets) sets them apart from juvenile teeth of Archaeolamna . In the latter genus, juvenile teeth have relatively larger lateral cusplets than have teeth from large, presumably adult individuals (see Siverson 1997: fig. 4A, H).

Discussion.—The presence of Cardabiodon venator sp. nov. in the Fairport Member of the Carlile Shale, extends the published stratigraphic range of Cardabiodon to span the middle Cenomanian–lower middle Turonian, an interval spanning approximately four million years. Siverson (1996: 834, pl. 4: 5–10) described two teeth as Pseudoisurus ? sp. from the mid−Cretaceous deposits at Thirindine Point in the lower Murchison River area, Western Australia. They were both surface collected, and their origin was thought to be either the uppermost metre of the Alinga Formation or the basal 0.1 m of an overlying pinkish claystone referred to the Beedagong Claystone by Siverson (1996). The colour and state of preservation of one of the teeth ( Siverson 1996: pl. 4: 8–10) indicate, however, that it is derived from the nodule bed at the base of the Toolonga Calcilutite (see Siverson 1996: text−fig. 3). The upper part of the pinkish claystone at Thirindine Point has yielded Turonian planktic foraminifera (David Haig, UWA, personal communication 1995), whereas the base of the overlying Toolonga Calcilutite is of Santonian ( Belford 1958) or possibly late Coniacian age ( Shafik 1990). The tooth from the claystone−Toolonga contact is very similar to those of the Mosby Cardabiodon and indicates an intercontinental distribution of C. venator sp. nov.

The second tooth referred to Pseudoisurus ? sp. by Siverson (1996) is possibly a Cardabiodon ricki (third lower anterior tooth) and was probably derived from the uppermost part of the Alinga Formation (which is somewhere within the late Albian to mid−Cenomanian in age; see Siverson 1999: 50–52).

It is our hope that this investigation will stimulate further research on this genus, as it is quite common in upper Cenomanian–middle Turonian offshore facies of the Western Interior Seaway and northeastern Texas. Teeth of Cardabiodon are consistently mislabelled, either as Cretoxyrhina , Cretodus or “ Cretolamna ” woodwardi (= Dwardius woodwardi ; see Siverson 1999), in US collections that we have examined but this work is hopefully highlighting the rather marked dental differences between Cardabiodon and Cretoxyrhina .

Figures

Fig. 4. Comparison of Cardabiodon venator sp. nov. from the Fairport Member, Carlile Shale, Mosby, Montana, USA (A) and Cardabiodon ricki Siverson, 1999 from the uppermost Gearle Siltstone, Giralia Anticline, Western Australia (B).

Fig. 1. Map showing the location of studied area in Montana (A) and location of Mosby (B). All selachian remains were collected on private land. Precise locality data is on file at the Department of Earth and Planetary Sciences, Western Australian Museum, Perth.