Canadia spinosa and the early evolution of the annelid nervous system

Abstract

Annelid worms are a disparate, primitively segmented clade of bilaterians that first appear during the early Cambrian Period. Reconstructing their early evolution is complicated by the extreme morphological diversity in early diverging lineages, rapid diversification, and sparse fossil record. Canadia spinosa, a Burgess Shale fossil polychaete, is redescribed as having palps with feeding grooves, a dorsal median antenna and biramous parapodia associated with the head and flanking a ventral mouth. Carbonaceously preserved features are identified as a terminal brain, circumoral connectives, a midventral ganglionated nerve cord and prominent parapodial nerves. Phylogenetic analysis recovers neuroanatomically simple extant taxa as the sister group of other annelids, but the phylogenetic position of Canadia suggests that the annelid ancestor was reasonably complex neuroanatomically and that reduction of the nervous system occurred several times independently in the subsequent 500 million years of annelid evolution.

Proclivity of nervous system preservation in Cambrian Burgess Shale-type deposits

“…Our results are directly relevant to euarthropod fossils, but these conclusions are also broadly applicable to other organisms from Burgess Shale-type deposits with neurological features such as the priapulid Ottoia prolifica [26] and the annelid Canadia spinosa [38]. Contrary to recent claims that the preservation of neurological tissues may require alternative taphonomic models [34,36], early and middle Cambrian Konservat-Lagerstätten from South China [21][22][23][25][26][27], North Greenland [37] and North America ( [24,29,30,38], this study) consistently show CNS expressed as carbonaceous films (figure 5), in accordance with the proposed mechanism of Burgess Shale-type preservation of labile tissues [39,73]. Instead, we may seek to further refine our knowledge of how the conditions that produce Burgess Shale-type fossils contribute to the stabilization of non-biomineralizing tissues in Cambrian deposits in order to better understand the limits of exceptional preservation [74][75][76].…”

A Cambrian crown annelid reconciles phylogenomics and the fossil record

“…Both molecular 1,5,17 and morphological 18 phylogenies have converged on scenarios where annelids evolved from polychaete-like ancestors. However, reconciling fossil and phylogenomic evidence has been challenging 2,6,12,15 . Molecular phylogenies recover a grade of infaunal, sessile and tube-dwelling taxa as deep branches 1,5 , including Magelonidae, Oweniidae, Chaetopteriformia and Sipuncula.…”

“…These groups differ from Cambrian polychaetes in terms of gross morphology and inferred mode of life 6 . A tube-dwelling annelid ancestor has been proposed 19 (although see 20 ), which is contradicted by interpretations of the fossil record 15 , and morphological hypotheses regarding the origin of annelid body plan features, e.g. segmentation and parapodia 18,20 .…”

“…If correctly interpreted, the oldest fossil crown annelids therefore belong to taxa that have lost most annelid synapomorphies 6 , including segmentation 22 . Magelonidae and Oweniidae (Palaeoannelida 1 ) are recovered as the sister group of all other annelids and so have featured prominently in recent discussions of the annelid ancestor 15,17,19 . These families are unusual among polychaetes, as they lack nuchal organs, possess monociliated epidermal cells and simple nervous systems 1, 17 .…”

Biogenic Iron Preserves Structures during Fossilization: A Hypothesis

“…It opens new avenues of research by highlighting the importance of tissue chemistry during the fossilization process especially in the case of nervous tissues that are preserved in carbonaceous compressions without any pyrite. [ 59–61 ]…”