Eccentrotheca is a genus of extinct tommotiid known from Early Cambrian deposits. Tommotiids are small, organophosphatic skeletal fossils that represent stem-group lophophorates, related to modern brachiopods and phoronids.
Taxonomy and Classification
Eccentrotheca was first described by Landing, Nowlan, and Fletcher in 1980 from lower Cambrian rocks of Nova Scotia, Canada. It belongs to the order Tommotiida within the superphylum Lophotrochozoa. The genus includes at least two recognized species: the type species Eccentrotheca kanesia and Eccentrotheca helenia (described by Skovsted et al. in 2011). The Paleobiology Database also classifies Eccentrotheca within the phylum Brachiopoda, reflecting its inferred phylogenetic position as a stem-group brachiopod or close relative.
Morphology and Scleritome Structure
Eccentrotheca possessed a scleritome (external skeleton) composed of numerous individual phosphatic sclerites (small plates) that fused together during growth. The sclerites formed rings that were stacked vertically to produce a tapering, widening-upwards conical or tubular structure with a circular to subcircular cross-section. Individual sclerites exhibit two broad morphotypes: (1) low, cap-shaped sclerites of variable dorsal outline, and (2) high, laterally compressed, roughly triangular sclerites. A complete gradational series exists between these types.
The first partially articulated scleritome of a tommotiid was described for Eccentrotheca in 2008 from the Lower Cambrian of South Australia. This discovery fundamentally changed the understanding of tommotiid body plans. Previously, tommotiid sclerites were assumed to form a dorsal armor of imbricating phosphatic plates in slug-like bilaterians, analogous to the calcareous sclerites of halkieriids. The articulated specimens revealed that Eccentrotheca instead formed a rigid, protective tubular structure.
Paleobiology and Ecology
The narrow (apical) end of the scleritome terminated in an open aperture with concentric growth lines, which likely housed organic structures for attachment to a hard substrate. The pointed end was probably fastened to a hard sea floor or other firm surface via a basal disc. The open (adapical) end has been interpreted as a filter-feeding aperture. Rather than being a mobile, benthic organism, Eccentrotheca most likely represented a sessile, vermiform (worm-shaped) filter feeder.
In the Arrowie Basin of South Australia, Eccentrotheca helenia is found in association with archaeocyath-microbial-spongiomorph-dominated bioherms (reef-like structures) and most likely inhabited pendant or cryptic habitats within these bioherms, attached to overhangs or cavity ceilings.
Phylogenetic Significance
Eccentrotheca is considered to occupy a position close to the base of the lophophorate tree. Individual sclerites have been homologized with the valves of brachiopods, and a relationship with the phoronids is also likely at a stem-group level. The discovery of the tubular scleritome construction provided important evidence linking tommotiids to lophophorates (brachiopods and phoronids), a clade that also possesses phosphatic shell chemistry and a sessile life habit. The bivalved brachiopod body plan is thought to have evolved from an Eccentrotheca-like ancestor through specialization of sclerite function and shortening of the tube.
Stratigraphic Range and Distribution
Eccentrotheca has a temporal range from the Terreneuvian through Cambrian Series 2 (approximately 525–512 million years ago). Fossils have been recovered from Canada (Nova Scotia, Newfoundland and Labrador), the United States (Massachusetts), Australia (South Australia, Northern Territory), and the United Kingdom (England). The genus is particularly well documented from the Avalonia Terrane and from the Arrowie and Stansbury basins of Australia.
References
- Landing, E., Nowlan, G. S., & Fletcher, T. P. (1980). A microfauna associated with Early Cambrian trilobites of the Callavia Zone, northern Antigonish Highlands, Nova Scotia. Canadian Journal of Earth Sciences, 17(3), 400–418.
- Skovsted, C. B., Brock, G. A., Paterson, J. R., Holmer, L. E., & Budd, G. E. (2008). The scleritome of Eccentrotheca from the Lower Cambrian of South Australia: Lophophorate affinities and implications for tommotiid phylogeny. Geology, 36(2), 171–174.
- Skovsted, C. B., Brock, G. A., Topper, T. P., Paterson, J. R., & Holmer, L. E. (2011). Scleritome construction, biofacies, biostratigraphy and systematics of the tommotiid Eccentrotheca helenia sp. nov. from the Early Cambrian of South Australia. Palaeontology, 54(2), 253–286.
