Marrella splendens is an extinct Cambrian stem-group arthropod that flourished during the middle Cambrian period (~ 500 mya).

Discovered by Charles Doolittle Walcott in 1909 in the famous Burgess Shale Formation of British Columbia, Marrella splendens is one of the most emblematic Paleozoic organisms - closely associated with the concept of the "Cambrian Explosion" and consequently our understandings of evolutionary biology.

Marrella was the first fossil discovered in the Burgess Shale by Charles Walcott and it is also the most numerous soft bodied organism - over 12,000 specimens - found within the shale. It has a distinctive cephalic shield with two pairs of spines that sweep backwards for almost the entire length of the body (possibly as a form of protection), 24-26 biramous limbs, a pair of long antennae and large swimming appendages (Garcia-Bellido, Collins, 2006).

Initially classified as a kind of trilobite Marrella was reinterpreted as possibly belonging to its own distinctive clade (Marrellomorpha) but it is now thought to belong within the stem-group arthropoda. Its difficult classification partially led to the debates on the nature of phyla emergence within evolutionary history. Why did the various phylums arise so early and so simultaneously at the beginning of the Paleozoic era? Were there many other phylums that arose during the same period only to disappear? The debate became a focus for evolutionary biology during the 1970's, famously highlighted by dialogues between Stephen Jay Gould and Simon ConwayMorris on the twin hypotheses of convergent and contingent evolutionary processes.

Merrella splendens model by Paleozoo Evolutionary Models by paleoartist Bruce Currie

Marrella has a series of micro-corrugations along the dorsal surface of the cephalic shield and these have been interpreted a capable of reflecting iridescent hues (A. R. Parker, 1998). Marrella is thought to have evolved complex eyes - as displaying colour and swimming about suggests an ability to see - however only putative eyes have been uncovered to date. 

A number of hypotheses have been put forward to explain the sudden jump in complexity that occurred during the Cambrian period - ranging from a rise in oxygen levels that facilitated larger and more dynamic organisms to the advent of predation or an accumulation of oceanic calcium carbonate - enabling shell-like armour and mobility which generated a type of arms race. The ability to see through complex eyes is another candidate for the triggering of the rapid evolution associated with the Cambrian Explosion. The consensus is that it was a combination of these and other possiblefactors that were responsible for the sudden evolutionary outburst.

  With thanks to Assoc. Prof. Diego Garcia-Bellido  for feedback on anatomical  detail.