Carbonaceous preservation of Cambrian hexactinellid sponge spicules

Harvey, T. H. P. (2010) Carbonaceous preservation of Cambrian hexactinellid sponge spicules. Biology Letters. DOI 10.1098/rsbl.2010.0377

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Abstract

Early fossil sponges offer a direct window onto the evolutionary emergence of animals, but insights are limited by the paucity of characters preserved in the conventional fossil record. Here, a new preservational mode for sponge spicules is reported from the lower Cambrian Forteau Formation (Newfoundland, Canada), prompting a re-examination of proposed homologies and sponge inter-relationships. The spicules occur as wholly carbonaceous films, and are interpreted as the remains of robust organic spicule sheaths. Comparable sheaths are restricted among living taxa to calcarean sponges, although the symmetries of the fossil spicules are characteristic of hexactinellid sponges. A similar extinct character combination has been documented in the Burgess Shale fossil Eiffelia. Interpreting the shared characters as homologous implies complex patterns of spicule evolution, but an alternative interpretation as convergent autapomorphies is more parsimonious. In light of the mutually exclusive distributions of these same characters among the crown groups, this result suggests that sponges exhibited an early episode of disparity expansion followed by comparatively constrained evolution, a pattern shared with many other metazoans but obscured by the conventional fossil record of sponges.

Item Type:	Article
Uncontrolled Keywords:	2010AREP, IA60; phd
Subjects:	04 - Palaeobiology
Divisions:	04 - Palaeobiology
Journal or Publication Title:	Biology Letters
Identification Number:	10.1098/rsbl.2010.0377
Depositing User:	Sarah Humbert
Date Deposited:	23 Jun 2010 07:49
Last Modified:	23 Jul 2013 09:57
URI:	http://eprints.esc.cam.ac.uk/id/eprint/1412

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