Burgess Shale Extravaganza: Canadia

21 05 2010

Good tidings and well-wishes!

I’ve saved the concluding entry in this week-long diagnosis of Burgess Shale weirdos for what I consider to be one of the most extravagant and yet under-appreciated denizens of this most paleontologically vital of fossiliferous congregations: the 3-centimeter annelid Canadia spinosa.

The bizarre, vaguely feather-shaped extensions of the creature’s anatomy as shown in the previous photograph are in fact an innumerable series of short, rigid bristles scientifically known as “setae”. More specifically, they can be classified as “notosetae”: these are setae which are rooted in an animal’s dorsal lobes. These rather “showy” contraptions cover essentially the entire dorsal surface of Canadia. In famed Burgess paleontologist Simon Conway Morris’ book entitled “The Crucible Of Creation: The Burgess Shale And The Rise Of Animals” (which was largely written from the perspective of a time-traveling scientific research team, hence the heavy editing I’ve provided), he writes:

“Each bundle [of notosetae overlaps] the one behind it, so as to give a tile-like covering to the upper surface of the body. There can be little doubt that this entire arrangement [was] primarily protective. The neuropodia [or “ventral branches”], in contrast, are much more lobe-like and strongly muscular. Each neuropodia bears a prominent podium of [setae], and it is on these structures that [the creature walked]…[theoretically] by a series of locomotory waves passing along the neuropodia. By precise coordination each neuropodium is first placed on the seabed and then pushed back so that the [notosetae] act as levers to push the animal forwards. Finally, at the end of the stroke the neuropodium lifts the [setae] clear of the sediment and swings them forward in preparation for the next shove against the sea floor.”

In addition to hypothetically providing a degree of protection, the notosetae may have enabled Canadia to swim were they rhythmically beaten while cruising above the sea floor.

Canadia‘s gut was straight and was capable of anteriorly extending from the main body to form a proboscis of sorts. Along with the frequent presence of sediment in the beast’s gut, this digestive setup has given rise to the hypothesis that Canadia was a detrivore. The function of the anterior pair of tentacles is unknown, though they likely assisted the animal’s sensory capabilities.

I’ll close this article with an intriguing idea championed by Conway Morris’ aforementioned volume: that Canadia may in fact be that most elusive of Cambrian fauna: a relative of WiwaxiaWere this true, it would greatly disambiguate the spiny little eccentric’s incomparably vague phylogeny. Consider the following passage:

“When the [setae] is placed under the microscope, it is seen to have a microstructure very similar to that observed in Wiwaxia. Evidently there is some evolutionary connection between Canadia and Wiwaxia.”

Furthermore, Conway Morris notes that the ventrally-situated gills of Canadia strongly resemble those of modern molluscs, which persuasively insinuates a kinship between the genus and the mollusca phylum as well. Hopefully, additional discoveries will yield a plethora of new information concerning this functional morphology, evolution, and taxonomic affiliations of most engaging organism.

May the fossil record continue to enchant us all!

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