Listracanthus pectenatus, Mutter & Neuman, 2006

Mutter, Raoul J. & Neuman, Andrew G., 2006, An enigmatic chondrichthyan with Paleozoic affinities from the Lower Triassic of western Canada, Acta Palaeontologica Polonica 51 (2), pp. 271-282 : 274-277

Listracanthus pectenatus sp. nov.

Derivation of the name: Latin pecten, a comb, referring to the comb−like arrangement of processes at the posterior border in the large denticles.

Holotype: UALVP 47002 ( Fig. 2 ); assemblage of small and large dermal denticles.

Locality and horizon: Wapiti Lake Provincial Park , near “Fossil Fish Lake” (54.51 ° N / 120.71 ° W) center of C cirque (see Neuman and Mutter 2005 for detailed information on locality). Exact layer is unknown (holotype is from talus), but specimens come most likely from between 34 and 42 meters above the Paleozoic–Mesozoic contact (lower Smithian, see discussion above) .

Diagnosis.— Listracanthus pectenatus sp. nov. differs from all other species of Listracanthus in having the posterior processes of type I−denticles showing comb−like and very regular, parallel arrangement. These processes project perpendicular to the denticle’s long axis and are not confluent with the ridges in the shaft. The anterior border is ornamented by about one dozen short and curved fulcra.

Description.—The denticles recognized as belonging to Listracanthus pectenatus sp. nov. range in size from 40 to 70mm in length (denticle type I; width about 1/4 to 1/3 of the length) or from 1 to 4mm (denticle type II).

The arrangement of type I−denticles on several specimens suggest that actual parts of the body of the fish may be preserved (e.g., holotype specimen Fig. 2 ; also specimens TMP 89.131 .1, UALVP 47004 , 47005 , 47015 ). The smaller dermal denticles (type II) have various conical shapes, some are recurved at their tips, are straight−conical or are almost completely flat ( Fig. 3). The bases are not conspicuously separated from the crowns (there is no neck), and the crowns were probably circular in cross−section (not laterally compressed), bearing on average eight to ten ridges running basad from the

MUTTER AND NEUMAN—ENIGMATIC CHONDRICHTHYAN FROM LOWER TRIASSIC OF CANADA 275

1 mm 1 mm crownlshaft base 1 mm

apex ( Fig. 3). Type II−denticles possess a short, compact crown with usually converging ridges. The bases of the small denticles are rarely clearly visible, were probably weakly developed and had no visible foramina ( Fig. 3B 3).

The larger denticles (type I) consist of a long, laterally flattened shaft (~crown) that tapers towards the apex and has longitudinal ridges. The shafts show distinctive anterior and posterior borders. The ridges in this type vary in number, comprising as a maximum 45 near the base of the shaft and 15 near the apex ( UALVP 46575 and 46577). The upper half of the convex anterior border is hemmed by about one dozen, one millimeter to one and a half millimeters long, single, short and curved fulcra, that are regularly and increasingly wider spaced towards the base and are absent in some denticles in the basal third of the denticle’s entire length (as can be seen in Fig. 4 ). The concave posterior border sends out numerous straight and striated processes that give the posterior border of the denticle a comb−like appearance. These small processes run perpendicular to the denticle’s long axis, are oriented more apicad near the base and are not “confluent” with the longitudinal ridges on the lateral wall of the shaft (see Fig. 4 ). There are smaller, triangular shaped processes intercalated at the posterior border just lateral to the comb−like straight processes .

The base is slightly broader than the maximum width of the crown of the denticle and is usually poorly preserved or was originally weakly developed. As seen from thin sections (see discussion below), the shaft of type I−denticle is probably secondarily ossified near the core centre and is histologically indistinguishable from the base. In ornamentation, the base is clearly delimited from the crown by lacking a superficial striation and by possessing wide cavities. The ventral border of the base is normally convex but may occasionally be straight or even concave ( Figs. 4 , 5 ). The apex consists of fulcral ridges arranged as a “tuft” ( Fig. 4B ) that exhibits individual variation in structure and complexity.

The denticles weather to bluish−grey or white. Most specimens consist of one type of denticle or the respective slabs either contain the great majority of large or small denticles. However, there are several larger slabs, that show an accumulation of largest and smallest denticles (e.g., UALVP 46568 , 47004 , 47005 , and 47015). Two specimens show that denticles may become “split” in half during growth and that the largest denticles were probably also fairly regularly arranged and connected to the bases ( Fig. 5 ) .

No head or fin region or articulated remains of jaws or a dentition have been detected in any specimen. The internal skeleton was undoubtedly largely uncalcified. However, a?tooth−like fragment or skeletal element is preserved with large and small denticles in specimen UALVP 38562 ( Fig. 6 ). The questionable skeletal element is only partly preserved, oval in cross−section and apparently hollow. It is not clear whether this remain at all represents a skeletal part of Listracanthus pectenatus sp. nov.

Figures

Fig. 2. Small and large dermal denticles in holotype specimen UALVP 47002, a possible body part of Listracanthus pectenatus sp. nov., Lower Triassic, Wapiti Lake, British Columbia. A. A large number of denticles is irregularly arranged but an unusually high number is clustered to the left. On the right, denticle types I and II are widely spaced, clearly unrelated to each other. B. Three large denticles (type I) as preserved (close−up).C. Small denticles (type II) as preserved, showing variable crown shape (close−up).

Fig. 4. Listracanthus pectenatus sp. nov., Lower Triassic, Wapiti Lake, British Columbia. Specimen UALVP 46551 showing morphological details of a large denticle (type I). A. A complete large denticle. Large denticles may vary in length, width and curvature. B. The apex of a large denticle with the fulcral ridges arranged like a “tuft”. C. The posterior border as preserved in the same specimen. Note that each straight process possesses a striated shaft (1) and a triangular base (2) lateral to the process (see also thin sections in Fig. 7). D. Detail of postero−basal corner of the posterior border.

Fig. 5. Type I−denticles of Listracanthus pectenatus sp. nov., Lower Triassic, Wapiti Lake, British Columbia, showing in situ arrangement (A) and aberrant morphology (B). A. As preserved in specimen TMP 95.114.52. B. Aberrant denticle as preserved in specimen TMP 2001.18.01. B1. General view. B2. Close−up of the mid−portion.

Fig. 6. Associated fragment (A) and sketch (B) of a?tooth−like fragment or skeletal fragment preserved on a slab with both types of denticles (not shown) of Listracanthus pectenatus sp. nov., Lower Triassic, Wapiti Lake, British Columbia, in specimen UALVP 38562.