Wednesday, January 24, 2018

[Paleontology • 2018] Kootenayscolex barbarensis • A New Burgess Shale Polychaete and the Origin of the Annelid Head Revisited

Kootenayscolex barbarensis
Nanglu & Caron, 2018 

•An abundant Cambrian polychaete preserves exceptional morphological details
•The new species possesses a median antenna and large palps on the prostomium
•Neuropodial chaetae are present on the mouth-bearing peristomium
•A chaetigerous origin for the peristomial portion of the annelid head is proposed

Annelida is one of the most speciose (∼17,000 species) and ecologically successful phyla. Key to this success is their flexible body plan with metameric trunk segments and bipartite heads consisting of a prostomium bearing sensory structures and a peristomium containing the mouth. The flexibility of this body plan has traditionally proven problematic for reconstructing the evolutionary relationships within the Annelida. Although recent phylogenies have focused on resolving the interrelationships of the crown group, many questions remain regarding the early evolution of the annelid body plan itself, including the origin of the head. Here we describe an abundant and exceptionally well-preserved polychaete with traces of putative neural and vascular tissues for the first time in a fossilized annelid. Up to three centimeters in length, Kootenayscolex barbarensis gen. et sp. nov. is described based on more than 500 specimens from Marble Canyon and several specimens from the original Burgess Shale site (both in British Columbia, Canada). K. barbarensis possesses biramous parapodia along the trunk, bearing similar elongate and thin notochaetae and neurochaetae. A pair of large palps and one median antenna project from the anteriormost dorsal margin of the prostomium. The mouth-bearing peristomium bears neuropodial chaetae, a condition that is also inferred in Canadia and Burgessochaeta from the Burgess Shale, suggesting a chaetigorous origin for the peristomial portion of the head and a secondary loss of peristomial parapodia and chaetae in modern polychaetes.

Keywords: Annelida, polychaete, Burgess Shale, Cambrian Explosion, body plan, prostomium, peristomium, annelid head evolution, Marble Canyon

Life reconstruction of Kootenayscolex barbarensis.

Illustration: Danielle Dufault/Royal Ontario Museum  

Kootenayscolex barbarensis is part of a group of animals called annelids (or the 'ringed worms'). It had a pair of long sensory structures called palps on its head, with a small medial antenna between them (right). Its body was covered in fleshy appendages called parapodia which bear bristles called chaetae. These structures are used for movement.

 photo: Jean-Bernard Caron/Royal Ontario Museum

Karma Nanglu and Jean-Bernard Caron. 2018. A New Burgess Shale Polychaete and the Origin of the Annelid Head Revisited. Current Biology. 28(2); p319–326.e1.  DOI: 10.1016/j.cub.2017.12.019

New 508-million-year-old bristle worm species from British Columbia's Burgess Shale wiggles into evolutionary history via @physorg_com
Half Billion-Year-Old Fossil Clue to How Worms Evolved via @NatGeo