Campodus agassizianus De Koninck, 1844

Ginter, Michał, 2018, The dentition of a eugeneodontiform shark from the Lower Pennsylvanian of Derbyshire, UK, Acta Palaeontologica Polonica 63 (4), pp. 725-735 : 727-732

Campodus agassizianus De Koninck, 1844

Figs. 3–8 .

1844 Campodus agassizianus gen. et sp. nov.; De Koninck 1844: 617–

618, pl. 55: 1. 1864 Orodus elegans sp. nov.; Romanovsky 1864: 157–158, pl. 3: 1. 1884 Campodus agassizianus De Koninck, 1844 ; Lohest 1884: 295–

305, pl. 3: 1–3, pl. 4: 1, 4–6. 2004 Orodus sp. ; Wilkinson et al. 2004: pl. 2. 2006 Orodus sp. ; Wilby et al. 2006: 311, fig. 2.

Emended diagnosis.—A eugeneodontiform with a crushing, heterodont dentition in which the largest and most characteristic teeth display the following combination of features. The teeth are elongated mesio-distally. The crown, built entirely of tubular dentine, is symmetrical labio-lingually and slightly asymmetrical mesio-distally. The median part is elevated and the median cusp is broad, pyramidal, with a rounded tip. There are three similar cusps on each side of the median cusp. All the cusps are connected with a median crest and bear strong transverse ridges. The median crest and the transverse ridges are ornamented with secondary ridges, transverse or oblique to the major forms. All the ridges are serrated. The base is of euselachian type. The smallest teeth, probably from the anterior and posterior parts of the jaw, are bar-like, with no elevation in the crown. Numerous minute transverse ridges with serrated edges are situated symmetrically on labial and lingual sides.

This diagnosis is based almost exclusively on the most characteristic teeth, probably situated in the middle of the jaw ramus, because only such teeth are really diagnostic. Teeth of this type are present in three of the reference materials: Romanovsky (1864), Lohest (1884) ( Figs. 4 , 7A–C ), and the collection from Derbyshire described herein ( Figs. 3 , 5 , 6 ), but unfortunately do not occur in the holotype ( Figs. 7D , 8 ), of which only the small, bar-like teeth are known (see the discussion below).

Description.—The following text is based mostly on the characteristics of the fragments of the anterior part of a shark dispersed in a limestone nodule (GSM 105458– 105522) from Lower Hays Farm borrow pit (now flooded) near Carsington, Derbyshire, England, UK (Kinderscoutian, Bashkirian, Pennsylvanian), and the description provided by Lohest (1884) of his specimen from the Chokierian of Belgium. The whereabouts of the latter specimen are unknown; a cast of it is allegedly present at the University of Liège (Valentin Fischer, personal communication 2018), but the description and drawings are so good that they are sufficient for comparison. The word “mamelon” adopted here from Lohest’s text means in this case a broad cusp.

General features: The teeth of Campodus agassizianus are elongated mesio-distally, straight or gently curved. The height of the crown is approximately equal to that of the root; the middle region, in general, is more elevated than the extremities. The teeth are formed of five to twelve conical mamelons, slightly elongated transversally to the length of a tooth and connected with each other by a concave area. In the middle of a tooth the mamelons are larger and the spaces between them are wider than in the lateral areas.

Along the tooth crown there runs a high median crest. The crest is ornamented by low, transverse or oblique ridges which may be delicately serrated. From time to time such transverse ridges are larger and in that case they form median crests of the mamelons. These crests are also ornamented by small, sharp ridges, similar to those ornamenting the main, mesio-distal crest. Therefore, there are four orders of ornamentation: the median crest of a tooth (mesio-distal) with its own low, serrated ridges; the median crests of mamelons (labio-lingual); the ridges ornamenting the mamelons’ crests; the serrations on the latter ridges.

The base (root) is of a classic euselachian type sensu Ginter et al. (2010). There are numerous openings of basal vascular canals in the orolingual and aboral-labial areas and a flat surface devoid of foramina in the aboral-lingual part ( Fig. 6D , arrows). Due to the overlapping of bases, when the teeth are in place in a tooth-family, these flat surfaces form together a continuous aboral surface with no canal openings. The tooth families observed in the Belgian material consist of a revolver of at least five teeth.

The dentition is characterised by a strong monognathic heterodonty. The largest teeth are generally symmetrical with the median part highly elevated ( Figs. 4 , 5 , 6A, E ). The smaller the tooth, the lower the elevation of the crown. There also occur very small, bar-like teeth with no median elevation at all in which the mamelons are replaced by numerous transverse (labio-lingual), serrated ridges ( Figs. 7C, D , 8 ).

In the concretion from Derbyshire there occur extremely long teeth ( Figs. 3 , 6C ), absent from the Belgian material. They apparently have no more than three mamelons in the median part and the rest is reduced to a long median crest with labial and lingual transverse ridges. Due to the fragmentary character of the material it is difficult to say whether such teeth are symmetrical or not. Usually only one lateral part of a tooth is completely preserved and this can measure up to 20 mm in its mesio-distal dimension.

The symphyseal tooth whorl, typical of the eugeneodontiforms, has not been found in any of the studied materials.

Microstructure: The crowns of the teeth are composed entirely of tubular dentine (orthotrabeculine sensu Zangerl et al. 1993), a tissue typical of all the Euchondrocephali, and which was highly resistant to compressive stress. In Mesozoic and modern holocephalans it forms the tritors on the tooth-plates, in petalodonts it covers the biting parts of the teeth with a relatively thin layer, but in orodonts, Palaeozoic holocephalans and eugeneodontiforms it is the major component of the crown. Here, in Campodus , the tubules, surrounded by hypermineralised orthodentine, run from the base-crown interface upwards, almost to the surface of the crown ( Figs. 5 , 6E ; Lohest 1884: pl. 4: 4–6; compare Ginter et al. 2010: fig. 10 D and Stahl 1999: fig. 20). However, their upper ends are closed by a thin layer of compact dentine, so the crown surface is smooth and shiny ( Figs. 5 , 6B, F ). Lohest (1884: 297–298), in the section dedicated to the tooth microstructure of his Belgian specimen, noted that the surface is somewhat rough, shagreen-like, due to the nodes formed by the ends of the tubules. However, although I can confirm this observation on a few surfaces of the teeth from Derbyshire, I suppose that this is the result of initial abrasion (compare Stahl 1999: fig. 21.2). Stronger abrasion leads to uncovering of the tubules and a perforated surface ( Fig. 6D ).

The base is built of trabecular dentine, with a spongey network of canals. Because the preservation of the bases is, in most cases, much worse than that of the crowns, I could not trace any special system of basal canals. Lohest (1884) did not analyse the microstructure of the base.

Remarks.—The diversity of teeth in the specimens from Derbyshire and Belgium corresponds to the heterodonty observed on a jaw of “ Agassizodus variabilis ” from Osage, Kansas (St. John and Worthen 1875: pl. 8: 1; for the validity of this name, see the Discussion below). In that splendid specimen, the bar-like, minute teeth occur in the anterior and posterior parts of the jaw and the largest teeth with the most elevated median part in the middle region of the jaw Fig. 9 ). The teeth of intermediate size and shape occupy the intermediate positions. The only teeth from Derbyshire which have no equivalents in the specimen from Osage are those which are extremely long, with reduced mamelons Fig. 6C ). They are also absent from the Lohest’s Belgian specimen. However, they are very similar to an isolated tooth also referred to by St. John and Worthen (1875: pl. 8: 2) as “ A. variabilis ”, but from the Mills County, Iowa. There was no firm ground for the latter authors’ decision to place the bunch of loose teeth from Iowa (St. John and Worthen 1875: pl. 8: 2–21) in the same species as the jaw from Kansas, but their intuition that they represent upper and lower jaws, respectively, might have been correct.

The potential similarity of the dentition model between Agassizodus variabilis ” and Campodus agassizianus does not mean that the individual teeth of these two species are identical. Indeed, this is not the case: the teeth of Campodus are virtually symmetrical across the mesio-distal midline (see particularly Figs. 6A, E , 7A ) while those of Agassizodus ” have strong transverse ridges on the labial side of the crown and the lingual side is almost flat. In the words of St. John and Worthen (1875: 315) “[The teeth of Agassizodus are] distinguished by the prevailing prominence of the buttressed condition of the anterior [= labial] coronal borders, and the relative uniformity or evenness of the posterior [= lingual] face.” This difference between the labial and lingual side is typical of all later Pennsylvanian to Triassic eugeneodontiforms, whereas the Orodus -like labio-lingual symmetry of the teeth of Campodus situates this genus at the basal position of the group (see the Discussion below).

The single tooth of Campodus illustrated by Romanovsky 1864: pl. 3: 1) as Orodus elegans probably represents the middle part of a jaw ramus. It is mesio-distally symmetrical, relatively short, preserving the elevated middle part and only three mamelons on each side. Unfortunately the Romanovsky collection has been lost (Alexander Ivanov and Oleg Lebedev, personal communication 2018) and only the illustration is available.

Stratigraphic and geographic range.—Serpukhovian–Bashkirian boundary beds, Carboniferous. Serpukhovian, Mississippian, Moscow Syneclise, Russia ( Romanovsky 1864); Chokierian, Bashkirian, Pennsylvanian, Liège, Belgium ( De Koninck 1844; Lohest 1884); Kinderscoutian, Bashkirian, Pennsylvanian, Derbyshire, England, UK (this paper).

Figures

Fig. 3. A part of the dark limestone nodule (GSM 105524) from the Kinderscoutian of Derbyshire, UK, showing the diversity of fossils.

Fig. 4. Morphological terminology of the teeth of Campodus agassizianus De Koninck, 1844. The drawing based on Lohest’s (1884) Belgian material re-illustrated in Ginter et al. (2010).

Fig. 5. The largest and best preserved tooth (GSM 105459) of the eugeneodontiform shark Campodus agassizianus De Koninck, 1844 from the Kinderscoutian of Derbyshire, UK. Note the tubular dentine in the broken median cusp (black arrow) and a probable bar-like tooth at the bottom of the photograph (white arrow).

Fig. 6. Teeth of the eugeneodontiform shark Campodus agassizianus De Koninck, 1844, in the fragments of nodule from the Kinderscoutian of Derbyshire, UK. A. A half of horizontal section of a large tooth (GSM 105471a). B. Lingual view of a half of a medium-sized tooth with well preserved surface (GSM 105471b). C. Close-up of long teeth (GSM 105524). D. Two cusps (mamelons) of a large tooth with abraded surface and uncovered tubular dentine canals; at the bottom part (arrows): aboral views of bases of two teeth (GSM 105471c). E. A half of a section of a large tooth (specimen not located in the collection during the second investigation). F. The upper part of a crown with a relatively high median cusp (GSM 105471d). Scale bars 5 mm apart from E where it is unknown.

Fig. 7. Teeth of Campodus agassizianus De Koninck, 1844, from the Chokierian of Belgium, near Liège. A–C. Specimens whereabouts unknown, from Lohest (1884). D. Holotype (left part NHMUK P.28754, see Fig. 8 herein; right part whereabouts uncertain), from De Koninck (1844). Not to scale.

Fig. 8. Left part of the holotype, NHMUK P.28754 (right part, see Fig. 7D, whereabouts uncertain, probably in France) of the eugeneodontiform shark Campodus agassizianus De Koninck, 1844, from the Chokierian of Belgium, near Liège. A. General view. B. Close-up of a group of teeth.

Fig. 9. Fragment of the mandibular dentition of “Agassizodus variabilis” from the Coal Measures (Pennsylvanian) of Osage County, Kansas, USA; from St. John and Worthen (1875: pl. 8: 1).