Palaeobates, : PIMUZ T, 1179

Mutter, Raoul J., Blanger, Keith De & Neuman, Andrew G., 2007, Elasmobranchs from the Lower Triassic Sulphur Mountain Formation near Wapiti Lake (BC, Canada), Zoological Journal of the Linnean Society 149 (3), pp. 309-337 : 334-335

COMPARISON WITH PALAEOBATES

Several specimens from the Vega-Phroso Siltstone Member of Wapiti Lake consist of fragmentary sections of the torsi and/or dorsal fins with neither teeth nor cranial morphology preserved. One of these very incomplete specimens, CMN 9980, consists mainly of patches of denticles and is preserved in outline only. Lacking most fins and all teeth, it was referred to Hybodus by Gardiner (1966) and to cf. Palaeobates by Schaeffer & Mangus (1976).

Comparison between Wapitiodus gen. nov. and Palaeobates sp. , the only previously mentioned hybodont taxon from the Sulphur Mountain Formation, stands on weak grounds as the original material and description are insufficient. Palaeobates is now known by various skeletal elements and differs from the material described here in many features in hybodont fin spine structure and in tooth structure (discussed below). Polyacrodus , Palaeobates and Wapitiodus gen. nov. share one plesiomorphic feature, the thick singlecrystallite enameloid.

Schaeffer & Mangus (1976) described four (or multiple)-pronged acuminate projections in the dermal denticles that are possibly conspecific with ‘genus A’ (Superorder incertae sedis) described here. In the presence of conspicuous diverging prongs, the denticles resemble the shape of the crown in certain denticles of Sphenacanthus serrulatus ( Dick, 1998) . The welldeveloped multiforaminate pedicles, however, clearly distinguished ‘genus A’ from S. serrulatus .

A similar type of platform with several long diverging ridges and a well-developed pedicle has, to our knowledge, only been described under the nominal name Parvidiabolus longisulcus ( Johns et al. 1997) from the Middle Triassic Liard Formation in western Canada (Ladinian). This material consists of scales only and does not settle the systematic position of these specimens. Although the histological ultrastructure observed in these scales is superficially similar to the ultrastructure of hybodont single-scale histology (see e.g. Rieppel, 1981: fig 13E), we are hesitant to assign any systematic value to this feature. This type of denticle is clearly distinctive from all other denticles found in patches of shark squamations from the Vega- Phroso Siltstone Member.

The denticles described in TMP 97.74.10 ( W. aplopagus gen. et sp. nov.) exhibit extraordinary variation, even within a single dorsal fin, and there are several kinds of denticles on the entire body, but none is comparable to the denticle-type referred to as ‘ Palaeobates’ (compare Figs 9 and 19C ). In particular, Schaeffer & Mangus (1976) considered denticle morphology to be similar to Palaeobates polaris ( Stensiö, 1921) . Stensiö (1921), however, describes the denticles of P. polaris as ‘poorly preserved’ and states that a number of ridges extend backwards as ‘long slender processes’ (?ridges on mesial platform), but also states that the number of these ridges ‘... cannot be stated with certainty’. Nevertheless, as described in the holotype of W. aplopagus gen. et sp. nov., one given specimen may possess denticles of considerable variability, and even on the same fin it is possible to find denticles that have from none to numerous ridges in variably shaped platforms. The range of individual, intraspecific and interspecific variation in the denticles is rather unsatisfactorily known in Lower Triassic sharks, and it seems impossible at present to distinguish Early Mesozoic sharks on the basis of dermal denticle morphology alone (see also Mutter & Rieber, 2005). The assignment of any specimen from the Sulphur Mountain Formation to Palaeobates sp. is cast further into doubt in the light of the total evidence from this locality. There are several isolated teeth and a number of teeth preserved in the dentitions of the two most complete specimens of Wapitiodus gen. nov., and none of these meet the criteria set out by Stensiö (1921) to describe Palaeobates from Spitzbergen, i.e. ‘crown long and narrow, without lateral cones (cusps), but sometimes with principal cone’. A ‘longitudinal crista’ (i.e. ‘longitudinal ridge’ sensu Reif, 1973: fig. 2) is often present but may also be absent. These features are very vague and could (erroneously) be taken to be present in the teeth of Wapitiodus gen. nov. In fact, the strongly asymmetric teeth with broad and flat crowns in Palaeobates show an ornament consisting of faintly elevated and fine striae, sometimes either anastomosing or forming a network ( Rieppel, 1981: fig. 9). Like the sectioned polyacrodontid tooth (TMP 88.98.51; Fig. 18 ), the crowns of teeth of Palaeobates are also covered by a layer of enameloid, but this layer is thicker in Wapitiodus gen. nov., and almost the entire tooth consists of osteodentine.

COMPARISON WITH LOWER TRIASSIC MATERIAL FROM THE?DIENERIAN OF SPITZBERGEN

The lack of diagnostic skeletal elements associated with teeth in the sample from Wapiti Lake recalls the suspicion of Stensiö (1921: 42) that the ‘generically indeterminable fin-spines’ from Spitzbergen, recovered as isolated fragments, may possibly be referred to Polyacrodus (or Palaeobates ). The fin spines briefly described by Stensiö (1921: 40–42) resemble the fin spines from Wapiti Lake in various features, but most of these features can only be observed in very few and fragmentary specimens: stellate tubercles (all three of Stensiö’s specimens and all specimens with fin spines described here), broadly triangular shaped cross-section and convex posterior border (Stensiö’s specimen P.44 and UALVP 46528), the ‘enamel keel’ (Stensiö’s specimen P.35 and specimens TMP 97.74.10 and UALVP 46529) and the ultrastructure of the fin spines as far as is discernible ( Stensiö, 1921: 41).

In 1932, Stensiö described a few more teeth on which he erected P. claveringensis and four additional fragmentary fin spines, two of which he called ‘ Nemacanthus -like’.

Stensiö (1932) also reported dermal denticles from the head region of Polyacrodus claveringensis that actually resemble our Figures 9 (B–D) and 12. As mentioned above, there are no anteriormost teeth preserved and the lateral/postero-lateral teeth retain a small central cusp. Two of the Stensiö (1932) fin spines (nrs 2 and 3) show a tubercular ornament and an enamel keel (cf. Fig. 8E ) that are similar to Wapitiodus aplopagus gen. et sp. nov. Because of the imperfect state of preservation of these remains, however, these finds cannot be further compared.

Figures

Figure 8. Wapitiodus homalorhizo sp. nov.: structure of fin spine in cross-sections (A–D) through the apical half of specimen UALVP 46528 (thin sections T1–3) and the external view of the apex of the fin spine in specimen UALVP 46529 (E, silicon peel dusted with NH4Cl). A, overview: note the large but secondarily obliterated, partly remodeled cavity and the large tubercles clearly delimited from the core of the fin spine by lines of arrested growth. B, note the couple of displaced and stacked tubercles in the left postero-lateral corner (as indicated in A) at a slightly lower level of the spine (thin section T3). C, note the internal structure in the posterolateral wall of the fin spine that is characteristic of ctenacanthoids. D, attempts at restoration of the cross-section through the apical third. E, note the well-spaced stellate tubercles (arrowhead 2), the absence of recurved denticles in the posterolateral wall (arrowhead 1, but also see Fig. 10 and the description of specimen UALVP 46528) and the partly preserved anterior rim (arrowhead 3).

Figure 9. Wapitiodus aplopagus gen. et sp. nov.: morphologic variation of dermal denticles as found in various body regions in holotype TMP 97.74.10. A, dermal denticles from between the pelvic and anal fins. B, dermal denticles from the tip of the lower jaw. C, dermal denticles from the dorsal trunk area. D, dermal denticles from the tip of the anterior dorsal fin. E, dermal denticles from the mid-area of the posterior dorsal fin. F, dermal denticles from the anterior rim of the fin above the posterior dorsal fin spine. The denticles are not necessarily either in situ or orientated in the same way, see the text for the description. The shark was drawn by Beat Scheffold, PIMUZ.

Figure 18. Morpho-histology of teeth (cf. Fig. 2). A, Wapitiodus aplopagus gen. et sp. nov.: an isolated tooth in labial view (holotype specimen TMP 97.74.10). B, Polyacrodontidae gen. et sp. indet.: vertical section through an isolated tooth, specimen TMP 88.98.51. Note that the root and most of the crown is osteodont. C–E, Polyacrodontidae gen. et sp. indet.: various scanning electron microscopy (SEM) magnifications of the SLE in specimen TMP 88.98.51 (white asterisks mark the same position in B–E). Abbreviations: ORT, orthodentine; OST, osteodentine; SLE, single layer enameloid.

Figure 19. Ctenacanthoidea incertae sedis ‘genus A’: dermal denticles and unidentified elements as preserved in specimen UALVP 46572. A and B, bar-like?skeletal elements and impressions of elements aligned among numerous small denticles (arrows). C, denticle, and a sketch of the denticle, with diverging ridges in the crown. Possibly, this is the denticle-type referred to as ‘Palaeobates’ by Schaeffer & Mangus (1976). D, thin section (UALVP 46572-T1) through one of the denticles, showing the osteodont pedicle and the mesial platform with orthodont prongs covered entirely with enameloid.