title: Competition and mimicry: the curious case of chaetae in brachiopods from the middle Cambrian Burgess Shale.
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.
authors.affiliation: Department of Earth Sciences, Palaeobiology, Uppsala University, Villavägen 16, SE - 752 36, Uppsala, Sweden. Timothy.Topper@nrm.se.|Department of Palaeobiology, Swedish Museum of Natural History, P.O. Box 50007, , SE-104 05, Stockholm, Sweden. Timothy.Topper@nrm.se.
authors.forename: Timothy P
authors.initials: TP
authors.lastname: Topper
authors.affiliation: Department of Biological Sciences, Macquarie University, Sydney, NSW, 2109, Australia. luke.strotz@mq.edu.au.
authors.forename: Luke C
authors.initials: LC
authors.lastname: Strotz
authors.affiliation: Department of Earth Sciences, Palaeobiology, Uppsala University, Villavägen 16, SE - 752 36, Uppsala, Sweden. lars.holmer@pal.uu.se.
authors.forename: Lars E
authors.initials: LE
authors.lastname: Holmer
authors.affiliation: Early Life Institute and Department of Geology, State Key Laboratory for Continental Dynamics, Northwest University, Xian, 710069, China. elizf@nwu.edu.cn.
authors.forename: Zhifei
authors.initials: Z
authors.lastname: Zhang
authors.affiliation: Department of Biological Sciences, Macquarie University, Sydney, NSW, 2109, Australia. noelrobyn@bigpond.com.
authors.forename: Noel N
authors.initials: NN
authors.lastname: Tait
authors.affiliation: Department of Natural History (Palaeobiology Section), Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario, M5S2C6, Canada. jcaron@rom.on.ca.|Department of Ecology and Evolutionary Biology, University of Toronto, 25 Willcocks Street, Toronto, Ontario, M5S3B2, Canada. jcaron@rom.on.ca.|Department of Earth Sciences, University of Toronto, 22 Russell Street, Toronto, Ontario, M5S3B1, Canada. jcaron@rom.on.ca.
authors.forename: Jean-Bernard
authors.initials: JB
authors.lastname: Caron
mesh_terms: D000818:Animals; D005075:Biological Evolution; D004463:Ecology; D005580:Fossils; D007448:Invertebrates / Q000033:anatomy & histology* / Q000145:classification / Q000235:genetics; D011161:Porifera / Q000033:anatomy & histology*