Thursday 11 October 2012

Are the Vetulicolians Deuterostomes?

The Vetulicolians are are group of organisms known from Cambrian deposits at a number of sites around the world. They Have segmented bodies superficially resembling those of Arthropods, but lack any visible limbs. Since palaeontologists and biologist theorize that the earliest Arthropods lacked limbs, having evolved from something similar to Nematode Worms, this absence of limbs in an Arthropod-like animal is not considered problematic. However the foreparts of the animals have been known to show complex and unArthropod-like lateral structures, which until now have been hard to resolve.

In a paper published in the journal BMC Biology on 2 October 2012, a team of scientists led by Qiang Ou of the Early Life Evolution Laboratory at the School of Earth Sciences at the China University of Geosciences in Beijing discuss the discovery of several new Vetulicolian fossils from the Chengjiang Lagerstätte in Yunnan Province, China (palaeontologists use the German word Lagerstätte to denote a particularly abundant or well preserved fossil bed).

These fossils show much more clearly the structure of the mysterious lateral structures of the Vetulicolians, which appear to closely resemble the pharynxes of Deuterostomes. The Deuterostomes are one of the major sub-divisions of the Animal Kingdom, and the one to which the Vertebrates belong, along with other Chordates (the Phylum which includes the Vertebrates) such as Tunicates (Sea Squirts) and Lancets (simple fish-like creatures lacking brains), Hemichordates (Acorn-worms, Pterobranches and the extinct Graptolites) and Echinoderms.

Echinoderms are thought to have been so remodeled by evolution that their original anatomy is hard to determine, though molecular studies suggest they are closely related to the Chordates and Hemichordates. Both these other groups posses Pharynxes, which is part of the entrance to both the entrance to both the respiratory and digestive systems in terrestrial Vertebrates, and the structure which supports the gills in Fish. In more primitive members of the group the Pharynx is used as a filter-feeding device, with water entering through the mouth and being pumped out through slits analogous to the gill slits in Fish. The pharynxes of Vetulicolians seem to be on the more primitive end of this spectrum, essentially muscular sacks with slits in them.

Reconstruction of the body-plan of a Vetulicolian, based upon the Chengjiang material. Ou et al. (2012).

The anatomy of a Lancet. The Earth Through Time/Wiley Online Library.

The anatomy of a Sea Squirt. Earthlife.

Images of Vetulicolians from Chengjiang. Scale bars are 1 cm. Ou et al. (2012).

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