Marrella splendens is an extinct stem-group arthropod that lived during the Cambrian period (~505 mya). It is one of the most emblematic of Paleozoic lifeforms - 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 of Canada by Charles Walcott. It is also the most numerous organism within the famous shale - over 12,000 specimens. 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). It also has 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.
The difficulty in classifying Marrella contributed to heated debates on the nature of emergent phyla within evolutionary history. Why did the various phyla (body-plans) arise simultaneously at the beginning of the Paleozoic era? Were there many other phylum that arose during the same period only to disappear? These questions became a focus for evolutionary science 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.
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 the accumulation of oceanic calcium carbonate enabling the build of hard skeletons and armour -engendering a type of arms race.
The evolution of sight and mobility are also candidates for triggers of the Cambrian Explosion.
It may have been a combination of all these and other factors that were responsible for the sudden evolutionary outburst associated with this period in Earth's history.
With thanks to Assoc. Prof. Diego Garcia-Bellido for feedback on anatomical detail.