Thursday, October 28, 2010
October 28, 2010 : Nectocaris (Extinct)
Nectocaris is a genus containing one species (N. pteryx) of early cephalopod known from the Middle Cambrian Burgess Shale. Walcott, the discoverer of the shale, had photographed the one specimen he had collected in the 1910s, but never had time to investigate it further. It was not until 1976 that it was formally described, by Simon Conway Morris. The head had two stalked eyes, one pair of tentacles, and a flexible siphon underneath its body. Fleshy fins supported by internal spines ran along the sides of the flattened, kite-shaped body.
Because the genus was only known from a single specimen, Conway Morris was unable to deduce its affinity. It had some features which were reminiscent of the arthropods, but these could well have been convergently derived. Its fins were very unlike the arthropods. Working from photographs, the Italian palaeontologist Alberto Simonetta believed he could classify Nectocaris within the chordates. He focussed mainly on the tail and fin morphology, interpreting Conway Morris's 'gut' as a notochord - a distinctive chordate feature. However, his case was unconvincing, and its classification remained uncertain until 2010, when Martin Smith and Jean-Bernard Caron described 91 additional specimens, many better preserved than the type. These allowed them to reinterpret Nectocaris as a primitive cephalopod, with two tentacles instead of the 8 or 10 of modern cephalopods. The structure previous researchers had identified as an oval carapace or shield behind the eyes was shown to actually be a soft funnel, similar to the ones used for propulsion by modern cephalopods. The interpretation would push back the origin of cephalopods by at least 30 million years, much closer to the first appearance of complex animals, in the Cambrian explosion.
Nectocaris was a free-swimming, predatory or scavenging organism, possibly occupying a niche similar to the arrow worms. This lifestyle is honoured in its binomial name: Nectocaris means "swimming crab" (Latin pteryx means "fin").