THECTARDIS AVALONENSIS: A NEW EDIACARAN FOSSIL FROM THE MISTAKEN POINT BIOTA, NEWFOUNDLAND

Journal of Paleontology, Nov 2004 by Clapham, Matthew E, Narbonne, Guy M, Gehling, James G, Greentree, Carolyn, Anderson, Michael M

ABSTRACT-

The Neoproterozoic Ediacara biota at Mistaken Point contains the oldest diverse Ediacaran assemblages and is one of the few known deepwater localities, yet the biota is dominated by endemic forms, nearly all of which remain undeseribed. Thectardis avalonensis new genus and species, one of these endemic forms, is a cm-scale triangular fossil with a raised rim and a featureless-to-faintly-segmented central depression. More than 200 specimens occur on two bedding plane surfaces: the 565 Ma E surface and the 575 Ma Pigeon Cove surface, nearly 2,000 m lower in the succession. Morphological and taphonomic data suggest that the organism was an elongate cone that may have lived as a suspension-feeding "mat sticker" with its pointed base inserted into the microbially bound sediment. If true, Thectardis n. gen. would be the tallest-known mat sticker, reaching a maximum height of over 15 cm. Specimens display little ontogcnetic change in length:width ratio, suggesting that Thectardis grew uniformly by incremental addition of material to its distal end. Morphological differences between specimens at two well-separated stratigraphic levels may have resulted from evolutionary or ecophenotypic variation.

INTRODUCTION

DiSTINCTIVE EDIACARA-TYPE megafossils were first reported from the Mistaken Point area of southeastern Newfoundland more than 30 years ago (Anderson and Misra, 1968; Misra, 1969); subsequent work has suggested that the "Mistaken Point biota" may include as many as 20-30 taxa (Anderson and Conway Morris, 1982; Landing et al., 1988; Narbonne et al., 2001). Mistaken Point (Fig. 1) is one of the most important Ediacaran fossil localities because it contains the oldest-known diverse Ediacaran fossil assemblages (Martin et al., 2000; Bowring et al., 2003; Narbonne and Gehling, 2003) and records one of the few Neoproterozoic deepwater slope biotas (Misra, 1971 ; Narbonne et al., 2001; Wood et al., 2003). In addition, the occurrence of the fossils as untransported census populations beneath beds of volcanic ash (Seilacher, 1992; Narbonne, 1998) provides perhaps the best opportunity anywhere to study the ecology of the Ediacara biota (e.g., Seilacher, 1999; Clapham and Narbonne, 2002; Clapham et al., 2003). Although the Mistaken Point biota contains several cosmopolitan taxa (e.g., Aspidella Billings, 1872; Charniodiscus Ford, 1958; and Charnia Ford, 1958) and some forms also known from Charnwood Forest, England (e.g., Bradgatia Boynton and Ford, 1995 and Ivesia Boynton and Ford, 1995), the majority of fossils are endemic and remain undescribed, hindering evolutionary and ecological comparisons with other Ediacaran localities. With the exception of the recently named trilobate form Triforillonia costellae Gehling et al., 2000 and the new frondose species Charnia wardi Narbonne and Gehling, 2003, all endemic forms have been referred to in the literature using informal but consistently applied names (e.g., "spindles," "pectinates;" see Waggoner, 1999). This paper describes one of these forms, Theclardis avalonensis n. gen. and sp., previously referred to as "triangles" (Narbonne et al., 2001; Clapham and Narbonne, 2002; Clapham et al., 2003; Narbonne and Gehling, 2003).

SYSTEMATIC PALEONTOLOGY

Genus THECTARDIS new genus

Type species.-Thectardis avalonensis new species, by monotypy.

Diagnosis.-As per species.

Etymology.-from the Greek "thektos" (sharp-pointed) and "ardis" (arrowhead), in reference to the pointed triangular shape of the fossil.

THECTARDIS AVALONENSIS new species

Figures 2.1-2.6, 4.1-4.4, 5

Diagnosis.-A cm-scale imprint in the shape of a triangle, wilh a prominent raised margin and an interior that is featureless or exhibits faint transverse markings.

Description.-All specimens are preserved as slightly raised impressions on upper bedding surfaces (positive epirelief). The holotype (Fig. 2.1) is an elongate triangular-shaped fossil 90 mm long and 30 mm wide at the triangle base, with a prominent 5-7 mm-wide raised margin that tapers towards the apex. The featureless interior has the same texture as the surrounding bedding surface. Other specimens range in length from 26 to 165 mm and in width from 24 to 96 mm (Fig. 3) after the effects of tectonic shortening have been removed (see Wood et al., 2003 for methodology). Specimens from Mistaken Point lend to be narrower for a given length than those from Pigeon Cove (Fig. 3); most have length:width ratios of 2-3.5 (mean 2.59, n = 53), whereas the length:width ratio of Pigeon Cove forms typically ranges between 1 and 2.5 (mean 1.81, n = 136). All display a prominent raised margin (e.g., Fig. 2.1-2.6); some also have a completely raised central area (Fig. 2.2, 2.3), others have a depressed center (Fig. 2.1, 2.5), and some are intermediate, partially depressed forms (Fig. 2.6). Mistaken Point forms display straight margins, whereas the edges of many Pigeon Cove specimens tend to curve inwards apically, giving them a more bullet-shaped appearance. Most have a featureless interior, although some display one or two transverse ridges (Fig. 2.1), either continuous or discontinuous, at varying distances from the apex. One specimen (Fig. 4.3, 4.4) has a central longitudinal ridge that decreases in width from 20 mm wide at the base toward the apex, disappearing 20 mm from the apex. This specimen also bears a deformation in the margin near the apex where the fossil overlaps a holdfast disc belonging to an unidentified frondose form. Several other specimens overlap underlying recumbent organisms (Fig. 4.1, 4.2) or smaller frondose taxa (Fig. 2.3).