A NEW LATE SILURIAN (PRIDOLIAN) NARAOIID (EUARTHROPODA: NEKTASPIDA) FROM THE BERTIE FORMATION OF SOUTHERN ONTARIO, CANADA-DELAYED FALLOUT FROM THE CAMBRIAN EXPLOSION

Journal of Paleontology, Nov 2004 by Caron, Jean-Bernard, Rudkin, David M, Milliken, Stuart

ABSTRACT-

The discovery of a new naraoiid nektaspid in the Upper Silurian (Pridolian) of southeastern Ontario significantly extends the range of this unusual group. Nektaspids are nonmineralized arthropods typical of Early and Middle Cambrian soft-bottom communities, but were thought to have become extinct in the Late Ordovician. The unique holotype specimen of Naraoia bertiensis n. sp. comes from a Konservat-Lagerst�tte deposit renowned for its eurypterid fauna (the Williamsville Member of the Bertie Formation). Naraoia bertiensis lacks thoracic segments and is morphologically similar to Naraoia compacta from the Middle Cambrian Burgess Shale, save for the presence of a long ventral cephalic doublure and a subtly pointed posterior shield. To examine the phylogenetic relationships of the new naraoiid, we coded characters of the holotype specimen and of nine previously described nektaspids. The results confirm a sister taxon relationship between Naraoia compacta and Naraoia bertiensis and the monophyly of nektaspid forms lacking thoracic segments (family Naraoiidae). This latter group may have arisen from an ancestral segment-bearing form through heterochronic loss of thoracic segments early in the Cambrian. The disjunct occurrence of a naraoiid nektaspid in the Late Silurian resembles the reappearance of other "Lazarus taxa" that were thought to have been eliminated during mass extinction events. The naraoiid lineage survived the Late Ordovician biotic crisis, but in this case the "Lazarus effect" seems likely to be taphonomic in origin.

INTRODUCTION

THE FAMILIES Liwudae and Naraoiidae, which together include fewer than 10 species of noncalcified, Cambro-Ordovician trilobitomorph arthropods (Fig. 1.1), have recently been assigned to a resurrected order Nektaspida (Budd, 1999). Nektaspids, along with helmetiids and tegopeltids, comprise a small group of early Palaeozoic euarthropods lacking calcified exoskeletons that figure prominently in discussions on the origin and composition of the class Trilobita (Ramsk�ld and Edgecombe, 1991). In particular, the Naraoiidae, as previously conceived, has been variously regarded as sister group to the calcified Trilobita (Briggs et al., 1992), as representing a higher level taxon ("order uncertain") within the Trilobita (Fortey, 1997), or as a more restricted trilobitomorph group excluded from Trilobita a helmetiid-tegopeltid clade. Because the exoskeleton of adult nektaspids consists of two main parts, a cephalic shield and a posterior shield of similar length separated by variable numbers (0-6) of thoracic segments, some authors have argued that one way of potentially resolving nektaspid phylogeny would be to explore heterochronic variations in thoracic segment release during ontogeny (Fortey and Theron, 1994). Based upon the presence or absence of an articulated thorax in the nektaspids, two opposing phylogenetic scenarios have been proposed (Fortey and Theron, 1994): the absence of thoracic segments could be a derived condition within the nektaspids and other arthropods (Briggs and Fortey, 1992) (Fig. 1.2), or, the absence of thoracic segments could be primitive (Fig. 1.3). In the first scenario, the stratigraphical order of appearance of nektaspid genera is not fully congruent with the phylogenetic hypothesis; the articulated segment-bearing forms are both Cambrian and Ordovician in age, but the non-segment-bearing forms are limited to the Cambrian. The second scenario, in which the absence of thoracic segments is primitive, implies a secondary expression of thoracic segments in a descendant of a form that lost the articulated thorax by a process of terminal progenesis or neoteny (rather than hypermorphosis, even though both peramorphic and paedomorphic processes are possible, see Budd, 1999). This second scenario is less parsimonious and the stratigraphical argument is weakened by the presence of thorax-bearing forms in the Early Cambrian (Fortey and Theron, 1994).

This paper has three main goals: to describe the first known post-Ordovician nektaspid, based on a single specimen recovered from the Upper Silurian (Pridolian) Bertie Formation (Williamsville Member) of southern Ontario; to analy/.e its phylogenetic relationships; and to make some inferences concerning the mode of segment release in the nektaspids.

GEOLOGICAL AND PALEOENVIRONMBNTAL SETTING

The Bertie Formation in southern Ontario consists of about 17 m of dark brown-to-buff, finely laminated, argillaceous dolostones with abundant bituminous partings (Fig. 2) (Johnson et al., 1992; Brett et al., 1998). The sequence represents deposition in a range of environments from brackish estuarine, through shallow evaporitic and sabkha settings, to near-normal marine lagoonal and subtidal conditions. The development of these marginal environments is associated with restricted circulation across a broad carbonate platform (Algonquin Arch) on the northern flank of the Appalachian Basin during the Late Silurian (Brett et al., 1998). The Williamsville Member is a thin (1.25- 1.5m) unit of massive, buff grey, planar-laminated dolostone ("waterlime") that contains elements of the renowned Bertie eurypterid fauna (Clarke and Ruedemann, 1912; Braddy, 2001). In addition to at least four species of eurypterids, the unit also yields phyllocarid crustaceans (Copeland and Bolton, 1985), xiphosurans, nautiloid cephalopods, ostracodes, chlorophyte algae, and cooksoniid rhyniophytes. The Williamsville Member was likely deposited in an estuarine setting, where brackish conditions resulted from flushing of freshwater streams into shallow hypersaline lagoons. The often superb preservation of the unbiomineralized biota in this Konservat-Lagerst�tte can be attributed to rapid burial in exceptionally fine carbonate muds, and exclusion of benthic scavengers and bioturbators by virtue of a combination of periodically elevated salinities and bottom anoxia (Kluessendorf and Mikulic, 1991; Brett et al, 1998).