Australia: The Land Where Time Began

A biography of the Australian continent 

Early Chordates - diversity

According to Benton there are 4 main categories that are possibly early chordates: possible urochordates, possible cephalochordates, Vetulicolians, and carpoids. The conodonts, for some time found only as tooth elements in many fossil deposits, were originally considered to be dubious chordates, but are now known to be jawless fish, so are undoubted vertebrates, as are some of the taxa from the Chengjiang Fossil Site in China, such as Haikouichthys and Myllokunmingia.

Urochordates do not have a strong fossil record, often being found as impressions of sac-like bodies and trace fossils in the form of marks on the sediment surface that are believed to result from animal activity, some being ascribed to tunicates. At the time of writing, the best known fossils of these organisms come from the Chengjiang Fossil deposits of China, Shankouclava showing a large perforated branchial basket, branchial slits, and an endostyle that is elongated (Chen et al., 2003). It is believed that this specimen may be a lava that had settled on the bottom of the sea as it has what appears to be a degenerating tail, suggesting it may be a larval stage.

For the cephalochordates the fossil record is not much better. Again it is the Chengjiang fossil deposits that have produced what appears superficially to be like an amphioxus-like cephalochordate Cathaymyrus, and the yunnanozoons, that have been identified as cephalochordates, though they have been assigned by others to the Ventulicolia. The organisms, that are non-vertebrate chordates, have no hard parts so are usually not often preserved.

Sources & Further reading

1. Benton, Michael J., 2005, Vertebrate Palaeontology, 3 rd ed., Blackwell Publishing.