Middle Cambrian ctenophores from the Stephen Formation, British Columbia, Canada

Abstract

The Ctenophora are a marine phylum of gelatinous swimmers and crawlers, with a minimal fossilization potential. To date only two acceptable fossil specimens are known, both from the Devonian Hunsruck Slate. Here we re-describe the single specimen of Fasciculus vesanus from the Phyllopod bed of the Burgess Shale (Middle Cambrian). The globose body bore two sets of comb-rows: one elongate and estimated to total ca. 16, the other shorter and totalling ca. 64, so giving a total of ca. 80. Internally there were at least two and possibly four prominent organs, each consisting of an elongate series of lobes. Xanioascus canadensis gen. nov., sp. nov. is recorded from a horizon low in the Stephen Formation, from the Glossopleura zone. It is more similar to extant ctenophores than F. vesanus, but the comb-rows total ca. 24. These converge on the aboral pole, where there is evidence for the polar fields. The comb-rows stop short of the oral area, which is poorly defined. Internally prominent ovoid bodies are sometimes present, but their significance is uncertain. Ctenorhabdotus capulus gen. nov., sp. nov. occurs in the Bathyuriscus-Elrathina zone of the Stephen Formation. It is best known from the Raymond Quarry of the Burgess Shale, but also occurs in the underlying strata, including the Phyllopod bed. The comb-rows appear to have totalled 24, and converged towards the aboral pole, adjacent to which they amalgamate as groups of three rows each. In the fossils the comb-rows then join eight strands, possibly representing the meridional canals, that meet as a ring. In addition, in each group of three comb-rows the central row appears to have been conspicuously shorter than those flanking it. The aboral pole also bore a prominent capsule, presumably housing the statocyst. The oral area, which lacks the comb-rows, bore a voluminous mouth, apparently surrounded by a folded margin with possible musculature. These Cambrian ctenophores differ from the Devonian and Recent taxa in a number of ways. They have a larger number of comb-rows and apparently an absence of tentacles. In addition, structures found in the Cambrian ctenophores, such as the lobed organs of F. vesanus and the ovoid bodies of X. canadensis lack obvious counterparts in living species. The wider affinities of the ctenophores remain mysterious, but they probably evolved very early in the metazoan radiations, perhaps from an animal with an anterio-posterior axis and a ciliated surface.