Another cool Cambrian critter

It is not easy working on Cambrian fossils. The petrified treasures are found in only a few places in the world, and even though many exhibit exquisite preservation they come from a time when life on earth would have looked very unfamiliar. One such creature, Anomalocaris, was a three foot long invertebrate that swam by undulating a series of lobes on either side of its body. In front of its mouth were two spiked tendrils that may have helped situate prey items to be processed by its conveyor belt of crushing plates that was its mouth. There is nothing quite like it alive today.

Indeed, Anomalocaris was so unusual that it was misidentified multiple times. Parts of it were taken as representing jellyfish, crustaceans, or other strange Cambrian creatures. Now we known differently, but similar problems still affect other Cambrian fossils. In the book Wonderful Life the late paleontologist Stephen Jay Gould wrote that "Three genera (Hurdia, Tuzoia, and Carnarvonia) are bivalved arthropod carapaces with no soft parts preserved; they cannot be properly allocated to any arthropod subgroup, and remain unclassified today." Twenty years later Tuzoia and Carnarvonia remain mysterious, but in the journal Science a team a researchers has just revealed a new identity for Hurdia.

Admittedly I am a little late to the game on this one (see what others have had to say at Ediacaran, The Dragon's Tales, and Deep Sea News), but it is hard to resist talking about such a strange creature. It has a story as complex as, and entwined with, that of Anomalocaris.

As Allison Daley and colleagues note, Hurdia has had a complex history. Parts of it were identified as belonging to jellyfish and various hard-shelled invertebrates, but during the major revisions that occurred during the 1980's these parts were assigned to two particular genera: Anomalocaris and Laggania. This placed the pieces of Hurdia in the right ballpark, but it was not until the 1990's that Desmond Collins saw that Hurdia deserved its own genus designation. A reinvestigation of the material relating to all three genera has supported this conclusion and given us an image of a creature even stranger than Anomalocaris.

Generally speaking Hurdia looked a lot like Anomalocaris. Being close relatives they shared a common body plan with anterior tendrils, stalked eyes, and a trash-compactor-of-doom type mouth. A major feature that made Hurdia distinct, though, was a large head shield that stuck out in front of its eyes. These shields were a little over three inches long, making up for about half the body length of the entire animal.

Hurdia has also provided increased insight into the place of creatures like Anomalocaris among invertebrates. Together Hurdia, Anomalocaris, and Laggania belong to a group called Radiodonta, and this group is placed close to the Euarthropoda, or "true" arthropods like trilobites. This makes the members of the Radiodonta very significant to questions of arthropod origins, and the recent discovery of another Anomalocaris-relative called Schinderhannes has provided researchers with even more material to investigate. These creatures were strange enough to start with, but with each new discovery they seem get a little bit stranger.

Daley, A., Budd, G., Caron, J., Edgecombe, G., & Collins, D. (2009). The Burgess Shale Anomalocaridid Hurdia and Its Significance for Early Euarthropod Evolution Science, 323 (5921), 1597-1600 DOI: 10.1126/science.1169514