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. 2011 Dec 23;7(6):929-32.
doi: 10.1098/rsbl.2011.0592. Epub 2011 Jul 6.

An Early Cambrian stem polychaete with pygidial cirri

Jakob Vinther  1 , Danny Eibye-Jacobsen, David A T Harper
Affiliations

An Early Cambrian stem polychaete with pygidial cirri

Jakob Vinther et al. Biol Lett. .

Abstract

The oldest annelid fossils are polychaetes from the Cambrian Period. They are representatives of the annelid stem group and thus vital in any discussion of how we polarize the evolution of the crown group. Here, we describe a fossil polychaete from the Early Cambrian Sirius Passet fauna, Pygocirrus butyricampum gen. et sp. nov., with structures identified as pygidial cirri, which are recorded for the first time from Cambrian annelids. The body is slender and has biramous parapodia with chaetae organized in laterally oriented bundles. The presence of pygidial cirri is one of the characters that hitherto has defined the annelid crown group, which diversified during the Cambrian-Ordovician transition. The newly described fossil shows that this character had already developed within the total group by the Early Cambrian.

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Figures

Figure 1.
Figure 1.
Pygocirrus butyricampum sp. et gen. nov. Holotype MGUH 29288. From Sirius Passet, North Greenland, Early Cambrian. (a) Part and (b) corresponding interpretive camera lucida drawing; numbering denotes individual parapodia. (c) Counterpart and (d) corresponding interpretive camera lucida drawing. (e) Parapodium, detail of area indicated in (b). (f) Two chaetal bundles emerging from a parapodium, detail of anterior area indicated in (d). (g) The pygidial cirri on the counterpart, detail of posterior area indicated in (d). Scale bars, (a–d) 5 mm; (e–g) 1 mm.
Figure 2.
Figure 2.
Hypothesized position of Pygocirrus butyricampum on the annelid stem lineage denoting the appearances of important morphological characters. The position of P. butyricampum is hypothesized based on a cladistic analysis (electronic supplementary material, S2) as a stem form subtending the crown group.
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