Cambrian suspension-feeding tubicolous hemichordates

Karma Nanglu,Christopher B. Cameron,Simon Conway Morris,Jean-Bernard Caron

doi:10.1186/s12915-016-0271-4

Karma Nanglu, Christopher B. Cameron + Show 2 more

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https://doi.org/10.1186/s12915-016-0271-4

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Abstract

BackgroundThe combination of a meager fossil record of vermiform enteropneusts and their disparity with the tubicolous pterobranchs renders early hemichordate evolution conjectural. The middle Cambrian Oesia disjuncta from the Burgess Shale has been compared to annelids, tunicates and chaetognaths, but on the basis of abundant new material is now identified as a primitive hemichordate.ResultsNotable features include a facultative tubicolous habit, a posterior grasping structure and an extensive pharynx. These characters, along with the spirally arranged openings in the associated organic tube (previously assigned to the green alga Margaretia), confirm Oesia as a tiered suspension feeder.ConclusionsIncreasing predation pressure was probably one of the main causes of a transition to the infauna. In crown group enteropneusts this was accompanied by a loss of the tube and reduction in gill bars, with a corresponding shift to deposit feeding. The posterior grasping structure may represent an ancestral precursor to the pterobranch stolon, so facilitating their colonial lifestyle. The focus on suspension feeding as a primary mode of life amongst the basal hemichordates adds further evidence to the hypothesis that suspension feeding is the ancestral state for the major clade Deuterostomia.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-016-0271-4) contains supplementary material, which is available to authorized users.

Highlights

The combination of a meager fossil record of vermiform enteropneusts and their disparity with the tubicolous pterobranchs renders early hemichordate evolution conjectural
The posterior end of the trunk is bulbous (Fig. 2b, g; Additional files 1D–E, 2H, 4, 5D, E, G) and sometimes terminates in a bilobed structure (Fig. 2a, f; Additional files 1B, C, 2A–D, F, H) that is usually wider than long
Establishing Oesia as an enteropneust that inhabited the tube previously identified as the alga Margaretia has significant implications for the Cambrian paleogeography and paleoecology of this group

Summary

Introduction

The combination of a meager fossil record of vermiform enteropneusts and their disparity with the tubicolous pterobranchs renders early hemichordate evolution conjectural. The two classes (tubicolous Pterobranchia and vermiform Enteropneusta) are monophyletic [1,2,3], but are morphologically disparate (see [4, 5] for an alternate viewpoint of Pterobranchia as sister to the family Harrimaniidae within a paraphyletic Enteropneusta). They give only generalized clues as to both the anatomy and mode of life of the last common ancestor as well as its connections to the sister phylum Echinodermata (collectively Ambulacraria). One exception is a tubicolous taxon (Spartobranchus tenuis) from the middle Cambrian Burgess Shale [10] This enteropneust is closely comparable to extant harrimaniids, its organic tube finds

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