Friday, March 20, 2009

500 Million Year Old Predator Reconstructed

The Burgess Shale Anomalocaridid Hurdia and Its Significance for Early Euarthropod Evolution. 2009. A. C. Daley, et al. Science 323: 1597 – 1600.

Hurdia victoria was originally described in 1912 as a crustacean-like animal from the famous 505 million year old Burgess Shale, a UNESCO World Heritage Site in British Columbia, Canada. Researchers have found more pieces of this critter and reconstructed it as a formidable predator.

When the first fragments of Hurdia were described nearly one hundred years ago, they were assumed to be part of a crustacean-like animal. It was not then realised that other parts of the animal were also in collections, but had been described independently as jellyfish, sea cucumbers and other arthropods. New material collected in the 1990s and the discovery of the best-preserved specimen in the collections at the Smithsonian Museum shows Hurdia to be related to Anomalocaris. Like Anomalocaris, Hurdia had a segmented body with a head bearing a pair of spinous claws and a circular jaw structure with many teeth. But it differs from Anomalocaris by the possession of a huge three-part carapace that projects out from the front of the animal's head.

"The use of the large carapace extending from the front of its head is a mystery. In many animals, a shell or carapace is used to protect the soft-parts of the body, as you would see in a crab or lobster, but this structure in Hurdia is empty and does not cover or protect the rest of the body. We can only guess at what its function might have been."

Hurdia and Anomalocaris are both early offshoots of the evolutionary lineage that led to the arthropods, the large modern group that contains the insects, crustaceans, spiders, millipedes and centipedes. They reveal details of the origins of important features that define the modern arthropods such as their head structures and limbs. As well as its bizarre frontal carapace, Hurdia reveals exquisite details of the gills associated with the body, some of the best preserved in the fossil record.