#skeemella
Species:S. clavula
Etymology: “Little Skeem,” after the Skeem family who found the type specimen
Age and Location: Middle Cambrian of Utah
Classification:Eukarya: Opisthokonta: Metazoa: Eumetazoa: Bilateria: incertae sedis
Ah, the Cambrian explosion. We’ll be spending a lot of time here. Like Pikaia,which I wrote about yesterday,Skeemellais a debated bilaterian that may be close to the ancestry of chordates. It’s a possible member of the enigmatic Vetulicolia, a clade of exoskeleton-bearing animals that consist of a boxy anterior region and a segmented tail, which might be deuterostomes, and specifically seem to be stem-group tunicates. Vetulicolians, including Skeemella, were blind and probably poor swimmers that kept close to the seafloor. Skeemella, however, has a strikingly longer “tail” than any other vetulicolian, and it apparently lacks gill slits, which are otherwise prominent in many vetulicolians. Furthermore, it has a telson and a clearly segmented anterior body. By and large Skeemellaseems more arthropod-like than any other vetulicolian, though it’s totally unclear what kind of arthropod it could be. Vetulicolians, Skeemellain particular, also resemble the microscopic mud dragons (Kinorhyncha), a group somewhat related to arthropods, within the clade Ecdysozoa. At 14 cm long, Skeemellawas large for a Cambrian animal.
IsSkeemellaa vetulicolian? It’s hard to tell, given that only one specimen is known. It certainly resembles ecdysozoans more than other vetulicolians do, which raises the possibility that it’s an ecdysozoan that converged on vetulicolians. Alternatively, it could be a vetulicolian that converged on ecdysozoans, or it could be convergent on both, or it could be proof that vetulicolians were specialized, deuterostome-like ecdysozoans.
Sources:
Aldridge RJ., Hou X-G., Siveter DJ., Siveter DJ., Gabbott SE. 2007. the Systematics and Phylogenetic Relationships of Vetulicolians. Palaeontology 50:131–168.
Briggs DEG., Lieberman BS., Halgedahl SL., Jarrard RD. 2005. A new metazoan from the middle Cambrian of Utah and the nature of the Vetulicolia. Palaeontology 48:681–686.
García-Bellido DC., Lee MSY., Edgecombe GD., Jago JB., Gehling JG., Paterson JR. 2014. A new vetulicolian from Australia and its bearing on the chordate affinities of an enigmatic Cambrian group. BMC evolutionary biology 14:214.
Lieberman BS. 2008. The Cambrian radiation of bilaterians: Evolutionary origins and palaeontological emergence; earth history change and biotic factors. Palaeogeography, Palaeoclimatology, Palaeoecology 258:180–188.
Shu D-G., Conway Morris S., Zhang Z-F., Han J. 2010. The earliest history of the deuterostomes: the importance of the Chengjiang Fossil-Lagerstatte. Proceedings of the Royal Society B: Biological Sciences 277:165–174.
