Burgess Shale Biota: The Lace Crab from the Middle Cambrian



Marrella Splendens is beautifully preserved in black shale. Marrella had two large, curved lateral cephalic spines, a pair of posteriorly-directed cephalic spines and two pairs of antennae extending outward from its unusual head shield. Each body segment, of which there are many (17-26), bear a pair of jointed limbs and associated gills for respiration. Also notice the dark stain emanating from the tail-portion of the specimen, the result of fluids seeping from the body cavity during burial. This is a valued fossil in my collection.  

CHARLES D. WALCOTT
The celebrated geologist and paleontologist Charles Doolittle Walcott from Utica, New York, discovered the fossils of the Burgess Shale biota (or life forms) on the side of a mountaintop in the Canadian Rockies in British Columbia in 1909. It was a discovery of phenomenal importance! It remains one of the world's best-known and best-studied fossil deposits!

Charles D. Walcott proudly standing in the Phyllopod Bed of the Burgess Shale Quarry
From the Smithsonian Institution Archives

Working throughout his life, Walcott amassed over 65,000 specimens that lived in the Middle Cambrian Period some 545 million years ago. This is an example of one of those fossils, Marrella Splendens, embedded in black shale, but informally called the “lace crab”, the site’s most common fossil. Over 15,000 specimens have been collected from Walcott's quarry.

Paleogeographic reconstruction of the Early Cambrian position of the landmasses and oceans. The approximate marine locality of the Burgess Shale is circled.
Modified from Ron Blakey (http://www2.nau.edu/rcb7/globaltext2.html).

THE BURGESS SHALE BIOTA
The Burgess Shale biota is regarded as evidence of the Cambrian Explosion of life, when complex (more than single-celled), bilaterian animals (with a symmetrical body plan having a front and back, top and bottom) proliferated (multiplied) and rapidly diversified (branched out and specialized). The Burgess Shale site is one of the world’s most celebrated fossil fields. It is considered to be a Lagerstatte, a sedimentary rock deposit exhibiting extraordinary fossil richness, completeness and state of preservation. It is proclaimed a UNESCO World Heritage site of global significance and worthy of protection from overgathering.

A FORTUITOUS TAPHONOMY
The decomposition of the Burgess Shale biota was prevented by an underwater, muddy avalanche that transported and rapidly buried them in deeper, oxygen-free waters. This circumstance preserved the finest details of their delicate, soft-bodied components such as antennae, legs and gills that would otherwise be destroyed. Also, rapid burial in shale has allowed microscopic dissection of the fossils in such a way as to recreate their three-dimensional, anatomical structure. This has provided paleontologists with the means to infer aspects of their behavior and lifestyle, and provide us with a comprehensive view of a typical pellagic and benthic paleocommunity during the Cambrian Explosion.

This is the science of taphonomy, a fundamental branch of paleontology. Its goal is the understanding of the processes behind the preservation of fossils. It involves the "ecological, biogeochemical and sedimentary processes that occur in the environment before and after burial of organisms" thus "is necessarily a science that integrates biology, chemistry and geology" (Exceptional Fossil Preservation by Bottjer et al). 

MARRELLA SPLENDENS
Of the thousands of specimens that Walcott sent from the quarry back to Washington, over 170 species are currently recognized today. Many aspects of Walcott's classification system has met with contemporary controversy, but that doesn't detract from the importance of his discovery. Marrella Splendens, based on its segmented body-structure and jointed-legs, was initially thought by Walcott to be a member of the trilobite family and was falsely assigned to the Trilobitoidea, a group that he established.

Overall, it can be stated that the morphology of Marrella is primitive. It was the kind of arthropod that could have given rise to any of the three great aquatic arthropod groups: crustaceans (shrimp, crab and lobster), chelicerates (scorpions and spiders) or trilobites. But, more recent re-analysis has assigned it as a stem-group (primitive precursor) arthropod (lobsters, crabs, insects, etc.). Marrella is now listed within the class Marrellomorpha. 

BURGESS SHALE-TYPE LOCALITIES
The Burgess Shale is of singular importance, but it is not unique. Since the discovery of the Burgess Shale biota in the Canadian Rockies, similar Early and Middle Cambrian fossil assemblages with exceptional soft-tissue preservation have been found both locally and worldwide (Chengjiang Fauna of China and Greenland for example). Those occurrences are called Burgess Shale-type faunas and localities. It demonstrates that these faunas were widespread and that a number of these organisms possessed a long history.

Here's an interpretive YouTube "Burgess Shale Animation." Marrella navigates by at 56 seconds. Check it out! 

http://www.youtube.com/watch?v=LbhGWDjOkP0&feature=related