Good tidings and well-wishes!
NOTE: Much of the information contained in this article has been effectively rendered out-dated by a very recent discovery. Click here to find out why.
As promised, TTT is now proud to present a series of articles concerning its author’s favorite Burgess Shale oddities. Anyone familiar with the dynamic formation has doubtlessly run across the following conclusion described by reknowned Cambrian paleontologist Simon Conway Morris: namely that “Current research is showing that a number of species from the Burgess Shale cannot reasonably be accomodated in any extant phylum”. In 1973, Conway Morris composed a series of papers describing a bizarre collection of five Cambrian oddities ecased within the local rock prior to his doctoral thesis. These exceptional oddities are as follows: Nectocaris, Odontogriphus, Dinomischus, Amiskwia, and Hallucigenia. Though this week-long event won’t lend its coverage to each of these admittedly very deserving candidates, I’ve nevertheless elected to kick things off with the former member of this motley congregation: Nectocaris pteryx, a sinuous chimaera of a beast which for all the world resembles a cross between an arthropod and a chordate.
Any attempt to disambiguate the affiliations of this peculiar critter are gravely unassisted by the fact that Nectocaris is, to date, known from but a single specimen. Although I’ve most regrettably been unable to upload an image of the fossil in question, I’d advise anyone interested in viewing it to consult this link. However, as is often the case with Burgess Shale organisms, what the fossil record of Nectocaris lacks in abundance, it makes up for in anatomical clarity.
From this solitary example, the scientific community has been able to erect a reasonably-complete reconstruction of how this fascinating animal looked. As alluded in the introduction, the most striking feature of Nectocaris is the coupling of its vertebrate-like tail and body with its very arthropod-esque head and “neck”. As depicted in the above pair of reconstructions, the animal’s head sports a pair of short, forward-projecting appendages. While these structures may conjure thoughts of crawdads and other crustaceans, their lack of joints renders them decidedly more primitive. The posterior end of the head is covered by an ovular shield which, according to some experts, may have been bivalved. Similarly debatable is whether or not Nectocaris‘ eyes rested upon stalks, although most paleo-artists appear to have arrived upon the affirmative conclusion in this regard.
All this being said, Nectocaris lacks the single most commonly cited defining characteristic of the arthropoda phylum: jointed appendages. In place of these, evolution has elected to grant Nectocaris a laterally-compressed body composed of approximately forty segments. This lengthy form is dorsally and ventrally draped by a pair of fins which, as noted by Conway Morris himself (along with Stephen Jay Gould), strongly resemble those belonging to the Actinopterygii (or “ray-finned”) class of fish.
According to Gould’s excellent book entitled “Wonderful Life: The Burgess Shale And The Nature Of History” (which, not suprisingly, will be referenced quite routinely during the course of this event), the following features of these fins insinuate some sort of affiliation (or, possibly, a case of convergent evolution) with modern chordates:
-While arthropod limbs are almost exclusively connected internally, a dark, slight, filmy layer of some dark structure appears to externally unite the parallel series of rays into a single “fin”.
-Arthropod appendages sprout from a lateral base with near invariability. This is contrasted by Nectocaris’ dorsal and ventral fin set.
-Arthropod bodies are constrained by a general rule which prohibits the attatchment of multiple appendages to each individual segment. Nectocaris‘ fins contain an average of three stiffening rays anchored in each bodily division.
Whatever the phylogenetic and taxonomic affiliations of Nectocaris may in fact be, the eccentric creature reminds us quite vividly that although paleontological science has greatly expanded our view of life’s historical saga, a great deal remains to be learned.
May the fossil record continue to enchant us all!
UPDATE: Be sure to check out the ‘comments’ section for a very intriguing hypothesis concerning Nectocaris‘ evolutionary affiliations.