Cambrian stem-group annelids and a metameric origin of the annelid head.

Luke Parry,Jakob Vinther,Gregory D Edgecombe

doi:10.1098/rsbl.2015.0763

Luke Parry, Jakob Vinther + Show 1 more

Open Access

https://doi.org/10.1098/rsbl.2015.0763

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Journal: Biology Letters	Publication Date: Oct 1, 2015
Citations: 46	License type: cc-by

Affiliation: University of Bristol, Natural History Museum

Abstract

The oldest fossil annelids come from the Early Cambrian Sirius Passet and Guanshan biotas and Middle Cambrian Burgess Shale. While these are among the best preserved polychaete fossils, their relationship to living taxa is contentious, having been interpreted either as members of extant clades or as a grade outside the crown group. New morphological observations from five Cambrian species include the oldest polychaete with head appendages, a new specimen of Pygocirrus from Sirius Passet, and an undescribed form from the Burgess Shale. We propose that the palps of Canadia are on an anterior segment bearing neuropodia and that the head of Phragmochaeta is formed of a segment bearing biramous parapodia and chaetae. The unusual anatomy of these taxa suggests that the head is not differentiated into a prostomium and peristomium, that palps are derived from a modified parapodium and that the annelid head was originally a parapodium-bearing segment. Canadia, Phragmochaeta and the Marble Canyon annelid share the presence of protective notochaetae, interpreted as a primitive character state subsequently lost in Pygocirrus and Burgessochaeta, in which the head is clearly differentiated from the trunk.

Highlights

The annelid fossil record reveals morphological disparity in extinct groups of polychaetes, especially in the Palaeozoic
While Canadia was previously interpreted as a member of the Phyllodocida [8], the absence of jaws, antennae and parapodial cirri argues for a placement outside this group and, crucially, the absence of pygidial cirri suggests that both Canadia and Burgessochaeta are stem-group annelids [6]
We present evidence that the palps of Canadia are attached to a parapodium-bearing segment anterior to the mouth and discuss the implications of these findings in the context of annelid head evolution

Summary

Introduction

The annelid fossil record reveals morphological disparity in extinct groups of polychaetes, especially in the Palaeozoic. Some taxa bear a single pair of anterior appendages, including Burgessochaeta, Canadia and Peronochaeta from the Burgess Shale [5]. We describe new anatomical observations of palps from Cambrian taxa, clarifying their attachment and distribution This includes new material from Sirius Passet, notably a specimen of Pygocirrus with head appendages. A Burgess Shale biota from Marble Canyon in the Canadian Rockies includes a new species initially compared with Burgessochaeta [14] This taxon presents a combination of anteriorly directed palps situated on a structure differentiated from the trunk and a dorsal covering of protective notochaetae (figure 2e; electronic supplementary material, figure S2a,b). The anterior region of Phragmochaeta terminates as a single segment with biramous parapodia, but lacks anterior structures identifiable as the peristomium, prostomium or paired palps [15] (figure 2c,b).

Discussion

Conclusion