The Marrella Mystery
The 520 million year old Burgess Shale in British Columbia (Canada), is one of the world’s most important fossil sites as it contains the remains of soft-bodied organisms that thrived in a shallow sea in the Cambrian Period. It also documents the marine ecosystem that existed in the mid Cambrian, including the first evidence of hard-shelled organisms.
Although, the location is now thought not to be unique, other sites in Canada that seem to have been subjected to very special geological conditions that permitted the fine fossilisation of a marine community have been found, the Burgess Shale remains exceptionally important.
To read more about similar locations to the Burgess Shale: Putting the Burgess Shales in the Shade.
One of the most abundant of all the fossils found at the site is Marrella splendens with something like over 15,000 known specimens. However, this Canadian site is the only location known in the world where fossils of this strange creature have been found.
So despite the large number of fossils of Marrella found in British Columbia, these fossils have not been discovered elsewhere in the world. Does this mean that Marrella sp. were localised to the Canadian province? Or does this indicate that Marrella sp. had such a low fossilisation potential that only under exceptional conditions could fossils of this animal be formed?
There is another mystery surrounding this creature, it does not resemble any extant animal (animals living today). Scientists debate how best to classify this bizarre creature. Informally regarded as a “lace crab” by the American palaeontologist Charles Walcott, this strange creature may not be closely related to lace crabs. Marrella had two distinct pairs of large, backward facing appendages, these grew out of a head-shield but rather than being hard like the calcium carbonate exoskeletons of arthropods, the head-shield seems to have been soft. The body was composed of approximately twenty segments, each segment had pairs of jointed limbs and gill branches. The antennae (two pairs) faced forward of the body, one pair was short and spiky with the second outer pair, much longer and thinner.
This thumb-sized animal may have been a member of the Order Arthropoda, but whether it was an ancient ancestor of crabs, trilobites or spiders remains open to intense scientific debate.