Holocephali Bonaparte, 1831

Ginter, Michał & Piechota, Agnieszka, 2004, The first Devonian holocephalian tooth from Poland, Acta Palaeontologica Polonica 49 (3), pp. 409-415 : 410-412

Superorder Holocephali Bonaparte, 1831 Order Cochliodontiformes Obruchev, 1953 Family Psephodontidae Zangerl, 1981 Genus Psephodus Morris and Roberts, 1862

(ex Agassiz ms. 1859)

Psephodus cf. magnus (Agassiz, 1838)

Figs. 2A , 3 .

Material.—Single, partly broken and abraded tooth, GIUS−4− 2314 Kow−1, from the middle Famennian, Late Pa. trachytera conodont Zone, of the Kowala Quarry, south−western Holy Cross Mountains, Poland.

Description.—The specimen is very dark brown, almost black, and lacks probably about a half of a lateral ramus. It broke off apparently quite soon after falling on the sea floor, because the edges of the breaking surface are rounded by abrasion ( Fig. 3A 3 ). Before losing a piece, the tooth seems to have been virtually symmetrical mesio−distally, but the broken ramus might also have been a little shorter. In oral view the tooth is gently arched ( Fig. 3A 2 ), with the labial side concave and the lingual side convex. The end of the preserved ramus is rounded. The tooth has a wavy outline in lingual and labial views. The central part of the tooth is elevated, in the form of a broad swelling ( Fig. 3A 1 ), and the corresponding part of the basal surface is concave ( Fig. 3A 3 ).

Three faces of the base, labial, lingual and basal, can be distinguished. The slightly concave shape of the labial face ( Fig. 3A 3 ) suggests that it overlapped the lingual side of the base of a preceding (more labial) tooth in a family. There are some pores on the labial face, but they are hardly visible due to the sediment filling them. Traces of large nutritive canals can be observed on the lingual face of the base, mainly in a form of grooves ( Fig. 3A 1 ). The basal surface is smooth and devoid of any traces of foramina ( Fig. 3A 3 ). The crown is composed of tubular dentine, with openings of tubules present all over the crown surface ( Fig 3A 1 , A 2 ).

Remarks.—Problems with the systematics of Palaeozoic Holocephali, commonly called “bradyodonts” and known almost only from isolated teeth and tooth−plates (but see Lund and Grogan 1997), were presented in detail by Stahl (1999) in the Handbook of Paleoichthyology. Of this, only a few facts need be repeated here. From a few articulated specimens and fragments of dentitions we know that bradyodonts display a very high degree of heterodonty. Their dentition can be composed of tooth−families, consisting of individual teeth ( Fig. 3B, C 1 , C 3 ) and of tooth−plates ( Fig. 3C 2 ), probably primarily developed by fusion of teeth in a particular family and, at least in some taxa, of lateral fusion between two adjacent tooth−families. Broadly speaking, with the exception of Chondrenchelyiformes and Chimaeriformes, Devonian –Carboniferous bradyodonts are subdivided, at the ordinal level, according to the numerical ratio of “free” tooth−families to the tooth−families with tooth−plates, the total number of tooth−families, and the shape of tooth−plates. Representatives of only two orders, Helodontiformes and Cochliodontiformes are known to have individual teeth. Of the latter group, numerous such teeth were recorded only from Psephodontidae and from two genera ( Lophodus and Venustodus ) treated by Stahl (1999) as incertae sedis. In the other cochliodontiform dentitions there are virtually only tooth−plates.

Lophodus and Venustodus are very characteristic and can be excluded from the comparison with our tooth from Kowala. The dentition of Helodus , apparently the only genus of the Helodontiformes , consists almost entirely of unfused teeth except a few tooth−plates (four in Helodus simplex , according to the restoration by Moy−Thomas 1936: text−fig. 4) of a Pleuroplax − type, i.e., with the crowns of fused teeth clearly differentiated ( Stahl 1999: fig. 47). Typical individual helodont teeth are subsymmetrical, only gently elongated mesio−distally, and they have crowns with a strongly elevated median part, usually rounded but often developed into a tip, which is slightly directed labially. The crown is composed of tubular dentine and the base of trabecular dentine with numerous canal openings and grooves. From the first description of Helodus by Agassiz (1833 –44) many more or less similar forms were ascribed to this genus. It is generally impossible to confirm or reject most of these identifications, because H. simplex is the only species represented by articulated specimens. Moreover, thanks to the discovery of the specimen from the Lower Carboniferous of Scotland, referred to as Psephodus magnus by Traquair (1885), in which a large part of the dental apparatus is preserved in a fairly undisturbed condition, it became clear that helodont−like teeth can occur in a dentition largely different from that of H. simplex . In P. magnus ( Fig. 3C ; Traquair 1885: figs. 1, 2; Stahl 1999: fig. 58A) only a few anterior tooth−families are composed of small helodont−like teeth with elevated median parts ( Fig. 3C 1 ). Then, at least one large, flat tooth−plate on each jaw ramus occurs in a more distant position ( Traquair 1885: figs. 1b, 2b; see also such plate, displaced and overturned, in Fig. 3C 2 ), probably forming a tooth−family with a few labially situated, elongated and flattened individual teeth ( Traquair 1885: fig. 2d; Stahl 1999: fig. 58A). Teeth of the subsequent three or four tooth−families are smaller, but similar in form to the latter ( Fig. 3C 3 ; Traquair 1885: fig. 2e). Unlike the Pleuroplax− like tooth–plates of H. simplex , those of P. magnus show no sign of fused teeth.

The bradyodont tooth from Kowala resembles the helodont−like teeth of P. magnus most closely. Slightly stronger mesio−distal elongation of a tooth and a less distinct elevation of the median part makes it different from typical teeth of H. simplex . Although many teeth, attributed formerly to Helodus (see e.g., Stahl 1999: fig. 57D, J), are very similar to the tooth under description, they do not necessarily belong to that genus. Moreover, a tooth from the Carboniferous Limestone of Armagh, Northern Ireland, housed in the Natural History Museum, labelled P. magnus ( Fig. 2B ), and accepted as such by Stahl (1999: fig. 58H), seems the closest to the tooth from Kowala of all the specimens examined by us, both as far as the crown and the base features are concerned. Therefore, we decided tentatively to assign our tooth as P. cf. magnus , leaving it in open nomenclature due to the lack of diagnostic tooth−plates and a large stratigraphical distance.

The deep water, oxygen depleted facies in which the tooth was found is rather unusual for Lower Carboniferous cochliodonts, typically occurring in bright limestones, rich in benthic fauna, such as the Mountain Limestone of Armagh ( Agassiz 1833 –44) . We therefore presume that the tooth might have been deposited as a gastric residue of a larger predator which had preyed in some neritic area and later travelled through the surface waters of the Kowala basin. Such an interpretation was provided by Williams (1990) for the rare occurrence of orodont crushing teeth in the Cleveland Shale of Ohio.

Figures

Fig. 2. A. Position of the tooth of Psephodus cf. magnus (GIUS−4−2314 Kow−1) from Kowala, as found on the rock. Note shells of a bivalve Guerichia in the upper right. B. Tooth of Psephodus magnus (NHM 28746) from the Viséan, Carboniferous Limestone of Armagh, Northern Ireland, in occlusal (B1) and lingual (B2) views; same specimen as figured by Stahl (1999: fig. 58H). Scale bars 10 mm.

Fig. 3. A. Tooth of Psephodus cf. magnus (GIUS−4−2314 Kow−1) from Kowala, in lingual (A1), oral (A2), and labial/basal (A3) views. Scale bar 5 mm. B. Arrangement and direction of replacement of helodont−like teeth in a tooth−family.Arrow indicates labial side. From Janvier (1996: fig. 4.36.G1). C. Diversity of tooth and tooth−plate morphotypes in Psephodus magnus (NSM 1950.38.51) from the Carboniferous Limestone, East Kilbride, Lanarkshire, Scotland; same specimen as figured by Traquair (1885: figs. 1, 2). 1, a group of helodont−like anterior teeth; 2, displaced and overturned tooth−plate; 3, flat lateral teeth. Scale bar 10 mm.

Fig. 1. A. Position of the Holy Cross Mountains massive (HCM) against the outline of Poland and the location of the Kowala section in the western Holy Cross Mountains (after Racki 1993). B. Stratigraphic column of the northern part of the Kowala Quarry, with the position of the discovered tooth (cross). C. Location of the studied section in the Kowala Quarry, indicated by an arrow.