Sunday, August 6, 2017

[Paleontology • 2017] Capinatator praetermissus • A Large Cambrian Chaetognath with Supernumerary Grasping Spines

Capinatator praetermissus Briggs & Caron, 2017

• Multiple specimens of a new Cambrian chaetognath preserve evidence of soft tissues
• At ∼10 cm long, it is one of the largest chaetognaths known, living or fossil
• The grasping apparatus is unique in bearing as many as 25 spines in each half
• Chaetognaths may have been benthic before becoming important planktonic predators

Chaetognaths (arrow worms) are a separate phylum (Chaetognatha) of small carnivorous animals, dominantly pelagic, and a major component of today’s plankton. The position of Chaetognatha among metazoan phyla remains equivocal—neither morphological nor molecular data provide definitive evidence. Originating early in the Cambrian period, if not earlier, chaetognaths quickly became important members of marine metazoan communities. Chaetognath grasping spines, originally reported as conodonts, occur worldwide in many Cambrian marine sediments. Fossilized chaetognath bodies, in contrast, are very rare: only two unequivocal specimens have been reported, both from the early Cambrian of China. Here we describe Capinatator praetermissus, a new genus and species, based on ∼50 specimens from several middle Cambrian Burgess Shale localities in British Columbia, many of which preserve evidence of soft tissues. Capinatator praetermissus reached body lengths of nearly 10 cm exclusive of fins, a much larger size than that of most living forms. Clusters of specimens preserving the body indicate that they were rapidly buried, providing indirect evidence that they swam near the seabed. The feeding apparatus comprises up to ∼25 spines in each half, almost double the maximum number in living chaetognaths. Early chaetognaths apparently occupied ecological niches associated with predatory euarthropods. The large body size and high number of grasping spines in C. praetermissus may indicate that miniaturization and migration to a planktonic lifestyle were secondary.

Keywords: Chaetognath; Cambrian; Burgess Shale; soft parts; paleoecology

Of the specimens studied, many preserve the feeding apparatus attached to the head, with some showing evidence of the rest of the body. Forty-eight specimens are held by the Royal Ontario Museum and one, with an isolated set of grasping appendages, is from the Smithsonian National Museum of Natural History. Specimen from the Collins Quarry, Mount Stephen (Yoho National Park, BC). (Photo by J.B. Caron/Royal Ontario Museum)

Figure 4. Life Reconstruction of Capinatator praetermissus, The location, size, and shape of the fins are speculative.
 Drawing by Marianne Collins/Royal Ontario Museum.

The generic name is derived from capioto grasp and natatorswimmer, reflecting its predatory habit. The species name praetermissusoverlooked, refers to the long gestation between discovery and description: a few specimens were found more than 30 years ago by the Royal Ontario Museum (ROM) during early exploration for new Burgess Shale sites in the Canadian Rockies.

Derek E.G. Briggs and Jean-Bernard Caron. 2017. A Large Cambrian Chaetognath with Supernumerary Grasping Spines. Current Biology. in press. DOI:  10.1016/j.cub.2017.07.003

Capinatator praetermissus: a prehistoric sea creature with spines to spare

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