A New Burgess Shale Polychaete and the Origin of the Annelid Head Revisited

Abstract

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 [1-3], many questions remain regarding the early evolution of the annelid body plan itself, including the origin of the head [4]. 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 [5] 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.

Sclerite-bearing annelids from the lower Cambrian of South China

“…This analysis results in a well-resolved phylogeny that is broadly congruent with the most recently published phylogeny 15 . A parallel Bayesian phylogenetic analysis recovers a broadly similar topology (Supplementary Information Fig.…”

“…Recent phylogenetic analyses of the annelids 6–10 have been augmented by those that include a variety of fossil taxa 12,14,15 . These new phylogenies 14,15 represent important advances, but indicate comparatively little morphological character overlap between extant species and Cambrian fossil taxa, which generally preserve far fewer soft-tissue or micro-scale characters. Because of the recognition of sclerites in Ipoliknus our principal aim here is to place these new fossils in the context of early annelid evolution.…”

“…Accordingly our main analysis (Fig. 4a) encompasses Adelochaeta and Ipoliknus as well as those fossil taxa previously considered 14,15 .…”

“…In this latter context most notable are the potential links to the brachiopods and molluscs. Thus, despite the growing roster of stem-group annelids 11–15 that have been recorded from a number of Cambrian Fossil-Lagerstätten 16–23 , the connections of these disparate taxa to what have generally been identified as the most primitive of extant families 6–10 are not obvious 11 . Moreover, thumb-nail sketches of the purported appearance of the ancestral annelid 6,7,24 find few obvious counterparts amongst Cambrian polychaetes 12 , although here allowances need to be made for the potentially low fossilization potential of critical features 25 .…”

Biogenic Iron Preserves Structures during Fossilization: A Hypothesis

“…[ 1–3 ] For instance, the Burgess Shale deposit in Canada has yielded a considerable number of spectacular soft bodied fossils of Cambrian age (508 million years old) whose bizarre anatomy, preserved in high fidelity, has been crucial for revealing the earliest evolution of animals. [ 4–10 ] Similarly, exceptionally preserved soft parts in fossils from the slightly younger Fezouata Shale (Ordovician, Morocco) were decisive in ending long‐standing debates on the systematic affinities of various enigmatic taxa (e.g., machaeridians, stylophorans). [ 11–13 ] The Chengjiang Biota (Cambrian, China) has also yielded a considerable number of soft arthropod taxa with complex nervous systems.…”