Hooking some stem‐group “worms”: fossil lophotrochozoans in the Burgess Shale

Abstract

The fossil record plays a key role in reconstructing deep evolutionary relationships through its documentation of the early diverging stem groups leading to extant phyla. In the middle Cambrian Burgess Shale, two famously problematic worms, Odontogriphus and Wiwaxia, have recently been reinterpreted as stem-group molluscs based on their shared expression of a putative radula and putative ctenidia in Odontogriphus. More detailed analysis of these fossil structures, however, reveals pronounced anatomical and histological discrepancies with molluscan analogues, such that they are more reliably interpreted as primitive features of the superphylum Lophotrochozoa. In the absence of any obviously derived characters, Odontogriphus could be placed in the stem group of the Lophotrochozoa or on the stem of any of its constituent phyla, whereas the dorsal covering of chaetae in Wiwaxia identifies it as a stem-group polychaete. Despite their close relationship, these two jawed, segmented worms could conceivably represent the early stages of two separate phyla.

The earliest fossil record of the animals and its significance

“…Persistent claims are made that members of the Ediacaran biota should be considered to be bilaterians, especially the clearly complex Kimberella from the White Sea area (Fedonkin & Waggoner 1997), a claim that has been revitalized by the discovery of the molluscan affinities of the rather similar Odontogriphus from the Burgess Shale (Butterfield 2006;Caron et al 2006).…”

Monoplacophorans and the Origin and Relationships of Mollusks

“…Some paleontologists and zoologists have argued that these animals are mollusks and therefore are yet another representative of the placophoran grade of organization. Others argue that while they are closely related to mollusks and other lophotrochozoan groups such as annelid worms or perhaps brachiopods, they are not members of the Mollusca (see Vinther and Nielsen 2005;Caron et al 2006;Butterfield 2006 for a sampling of both pro and con arguments).…”

The late Ordovician Soom Shale Lagerstätte: an extraordinary post-glacial fossil and sedimentary record

“…Radulae are a key synapomorphy of the Mollusca and provide important phylogenetic and ecological information, but they are rarely preserved. For example, from the entire Lower Palaeozoic succession, radulae occur in the early Cambrian Mahto Formation, Alberta, Canada (Butterfield 2008) and are interpreted in both Wiwaxia and Odontogriphus from the Burgess Shale (Caron et al 2006; but see Butterfield 2006, for an alternative view). In the Silurian there are two occurrences (Mehl 1984;Sutton et al 2006).…”