Articulated halkieriids from the Lower Cambrian of North Greenland and their role in early protostome evolution

Abstract

Articulated halkieriids of Halkieria evangelista sp. nov. are described from the Sirius Passet fauna in the Lower Cambrian Buen Formation of Peary Land, North Greenland. Three zones of sclerites are recognizable: obliquely inclined rows of dorsal palmates, quincuncially inserted lateral cultrates and imbricated bundles of ventro-lateral siculates. In addition there is a prominent shell at both ends, each with radial ornamentation. Both sclerites and shells were probably calcareous, but increase in body size led to insertion of additional sclerites but marginal accretion of the shells. The ventral sole was soft and, in life, presumably muscular. Recognizable features of internal anatomy include a gut trace and possible musculature, inferred from imprints on the interior of the anterior shell. Halkieriids are closely related to the Middle Cambrian Wixaxia , best known from the Burgess Shale: this clade appears to have played an important role in early protostome evolution. From an animal fairly closely related to Wixaxia arose the polychaete annelids; the bundles of siculate sclerites prefigure the neurochaetae whereas the dorsal notochaetae derive from the palmates. Wixaxia appears to have a relic shell and a similar structure in the sternaspid polychaetes may be an evolutionary remnant. The primitive state in extant polychaetes is best expressed in groups such as chrysopetalids, aphroditaceans and amphinomids. The homology between polychaete chaetae and the mantle setae of brachiopods is one line of evidence to suggest that the latter phylum arose from a juvenile halkieriid in which the posterior shell was first in juxtaposition to the anterior and rotated beneath it to provide the bivalved condition of an ancestral brachiopod. H. evangelista sp. nov. has shells which resemble those of a brachiopod; in particular the posterior one. From predecessors of the halkieriids known as siphogonuchitids it is possible that both chitons (polyplacophorans) and conchiferan molluscs arose. The hypothesis of halkieriids and their relatives having a key role in annelid—brachiopod—mollusc evolution is in accord with some earlier proposals and recent evidence from molecular biology. It casts doubt, however, on a number of favoured concepts including the primitive annelid being oligochaetoid and a burrower, the brachiopods being deuterostomes and the coelom being an archaic feature of metazoans. Rather, the annelid coelom arose as a functional consequence of the transition from a creeping halkieriid to a polychaete with stepping parapodial locomotion.

Metamorphosis in Craniiformea revisited: Novocrania anomala shows delayed development of the ventral valve

“…The finding, that both valves of the juveniles are secreted by mantle epithelia that originates at the dorsal side of the posterior lobe led to extensive conclusions about a folding event during brachiopod evolution (Cohen et al, 2003;Freeman, 2000;Malakhov and Kuzmina, 2006;Nielsen, 1991). This brachiopod fold hypothesis was supported by the existence of fossils that are worm like with shells on the dorsal side of the anterior and posterior end of the body, such as the halkieriid Halkieria evangelista (Conway Morris and Peel, 1995). The hypothesis that Halkieria evangelista belongs within the brachiopod stem, has since been questioned by several authors and other Lower Cambrian fossils (e.g.…”

A molecular palaeobiological hypothesis for the origin of aplacophoran molluscs and their derivation from chiton-like ancestors

“…Older sclerite-bearing taxa, the sachitids (Early -MidCambrian) [54], have been interpreted as aculiferan relatives [24,55 -57], including Halkieria [56,58], Maikhanella [55], Orthrozanclus [59] and Wiwaxia [60,61], all taxa known as partial to completely articulated specimens with a variable number of shell plates and sclerites arranged in morphological zones similar to aculiferans [57]. The sclerites are constructed of longitudinal fibres of presumed aragonite, a structure similar to that of chiton sclerites, and are hollow with a branching canal system that has been compared with the aesthete canals in the shell plates of modern chitons and the sclerites in the extinct multiplacophorans [57].…”

Darwin's dilemma: the realities of the Cambrian ‘explosion’

“…Halkieriids have also attracted attention on account of the hypothesis that the prominent shells at either end of the body are precursors of the dorsal and ventral valves of the brachiopods (Conway Morris & Peel 1995). It has long been known that a number of otherwise enigmatic tommotiids, a diverse group of phosphatic shelly fossils from the Lower Cambrian, have shell structures that are strikingly brachiopod-like (e.g.…”

“…So far as stem-groups are concerned, potentially key groups include the halkieriids and wiwaxiids. The former, as the articulated Halkieria evangelista (Conway Morris & Peel 1995), was argued to be instructive with respect to several lophotrochozoan phyla, including the annelids, brachiopods and molluscs. Vinther & Nielsen (2005), however, preferred to shoe-horn the halkieriids back into the molluscs, but in setting this essentialist agenda failed to grasp the evolutionary importance of stem groups and the nature of transitional organ systems.…”