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Research

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Publication details for Dr Timothy Topper

Topper, Timothy P., Strotz, Luke C., Holmer, Lars E., Zhang, Zhifei, Tait, Noel N. & Caron, Jean-Bernard (2015). Competition and mimicry: the curious case of chaetae in brachiopods from the middle Cambrian Burgess Shale. BMC Evolutionary Biology 15(1): 42.

Author(s) from Durham

Abstract

Background: One of the first phyla to acquire biomineralized skeletal elements in the Cambrian, brachiopods
represent a vital component in unraveling the early evolution and relationships of the Lophotrochozoa. Critical to
improving our understanding of lophotrochozoans is the origin, evolution and function of unbiomineralized
morphological features, in particular features such as chaetae that are shared between brachiopods and other
lophotrochozoans but are poorly understood and rarely preserved. Micromitra burgessensis and Paterina zenobia
from the middle Cambrian Burgess Shale are among the most remarkable examples of fossilized chaetae-bearing
brachiopods. The form, functional morphology, evolutionary and ecological significance of their chaetae are studied
herein.
Results: Like in Recent forms, the moveable but semi-rigid chaetae fringe both the dorsal and ventral mantle margins,
but in terms of length, the chaetae of Burgess Shale taxa can exceed twice the maximum length of the shell from
which it projects. This is unique amongst Recent and fossil brachiopod taxa and given their size, prominence and
energy investment to the organism certainly had an important functional significance. Micromitra burgessensis
individuals are preserved on hard skeletal elements, including conspecific shells, Tubulella and frequently on the
spicules of the sponge Pirania muricata, providing direct evidence of an ecological association between two
species. Morphological analysis and comparisons with fossil and extant brachiopod chaetae point to a number
of potential functions, including sensory, defence, feeding, defouling, mimicry and spatial competition.
Conclusions: Our study indicates that it is feasible to link chaetae length to the lack of suitable substrate in the
Burgess Shale environment and the increased intraspecific competition associated with this. Our results however,
also lend support to the elongated chaetae as an example of Batesian mimicry, of the unpalatable sponge Pirania
muricata. We also cannot discount brachiopod chaetae acting as a sensory grille, extending the tactile sensitivity
of the mantle into the environment, as an early warning system to approaching predators.