Phyllocarid Branchiocaris pretiosa from Utah

Description: This phyllocarid arthropod is known as Branchiocaris pretiosa, and has a long legacy since the description of the genus from a Burgess Shale specimen by Resser in 1929. The affinity of Branchiocaris is disputed. Some rank it as being within a stem group of euarthropods, and others as a primitive branchiopod crustacean within the arthropod crown group, the latter being the most accepted classification.

Branchiocaris had a large carapace that protected the anterior body parts. Its carapace structure hinged along the dorsal edge in the same manner as seen in bivalves. Usually only the carapace is found fossilized, and only very rarely are the soft parts preserved in as in this case in lagerstatte fossil sites such as the Wheeler Formation. That said, such soft-tissue preservation is rare in the formation.

Besides the Burgess Shale of British Columbia, where it is also rare (less than .01% of the biota), another member of the genus (Branchiocaris yunnanensishas) is described from the Chengjiang Biota.

Resser (1929) named the genus Protocaris and classified it as a phyllocarid crustacean, but other workers later considered it more closely related to trilobites. Four decades later Briggs (1976) erected the new genus Branchiocaris, prefering a stem arthropod status. Bergström (1997) later placed Branchiocaris was a calmanostracan branchiopod, while other's phylogenetic analyses placed it close to Marrella (Briggs, Fortey and Wills, 1992). Budd (2008) argued Branchiocaris to be a stem euarthropod, belonging to a clade also containing Odaraia, Fuxianhuia, Perspicaris and Canadaspis. Thus, Branchiocaris continues its enigmatic legacy.

The subclass Phyllocarida contains three orders, one extant (Leptostraca, classified as a Malacostracan), and two extinct (Hymenostraca and Archaeostraca).

Also See: Utah Cambrian Explosion Fossils

References:
Briggs, DE. 1976. The arthropod Branchiocaris n. gen. Middle Cambrian, Burgess Shale, British Columbia. Geological Survey of Canada Bulletin, 264: 1-29.
Briggs, DE. Fortey, MA., and Wills, MA. 1992. Morphological disparity in the Cambrian. Science, 256: 1670-3.
Chen, J., Waloszek, D. & Maas, A. 2004 A new ‘great-appendage’ arthropod from the Lower Cambrian of China and homology of the chelicerate chelicerae and raptorial antero-ventral appendages. Lethaia 37, 3–20.
Briggs, D.E., Lieberman, B., Hendricks, JR., Halgedahl, SL., & Jarrard, RD. 2008. Middle Cambrian arthropods from Utah. Journal of Paleontology, 82(2): 238-254.
Budd, GE. 2002. A palaeontological solution to the arthropod head problem. Nature, 417: 271-275
Budd, GE. 2008. Head structures in upper stem-group euarthropods. Palaeontology, 51: 561-573.