Repost: Hurdia – Another cool Cambrian critter

Author’s Note: A post currently in preparation reminded me of Hurdia, a bizarre Cambrian creature that was initially divvied up into parts attributed various invertebrate groups and has only recently been united into a single creature allied with Anomalocaris. Check back later today for a tale about the debated affinities of a possibly related creature, proposed as the ur-squid last year.

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 looked very unfamiliar. One creature extracted from the famous Burgess Shale, Anomalocaris, was a three foot long invertebrate that swam by undulating a series of lobes on either side of its body. At the front of its head were two spiked tendrils that may have helped situate prey items to be processed by the camera-shutter arrangement 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 know differently, but similar problems still affect other Cambrian fossils, including one mystery animal named Hurdia. In his 1989 book Wonderful Life, the 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 enigmatic, but, as reported in the journal Science, a team a researchers has just revealed a new identity for Hurdia. Its complex story is entwined with that of Anomalocaris.

(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.)

As Allison Daley and co-authors note, Hurdia remained mysterious for so long because researchers were never looking at the whole animal. Parts of it were identified as belonging to jellyfish and various hard-shelled invertebrates, but during the major revisions of the Cambrian fauna 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. They shared a common body plan with anterior tendrils, stalked eyes, and a trash-compactor-of-doom 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. Why this feature evolved is unknown.

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 supported this close association. These creatures were strange enough to start with, but with each new discovery they seem get a little bit stranger.

References:

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