Australia: The Land Where Time Began

A biography of the Australian continent 

Eurypterids

Eurypterids of the Soom Shale, South Africa

To the authors¹ find the Eurypterid Onychopterella that has been described (Braddy et al., 1995) to be the most interesting of the other arthropods found in the Soom Shale, the other being myodocopid  ostracods found in association with orthocone cephalopods. The eurypterids reached their peak in the Silurian, though they ranged from the Ordovician to the Permian. The earliest known of the eurypterids were exclusively marine, apparently invading brackish and freshwaters by the Silurian, some becoming amphibious. They are believed to have grown to about 2 m (6 ft) in length, evidence having been found of an eurypterid in the form of trackways indicating an animal of about this size near Laingsburg, about 150 km  southeast of the Cedarberg Range, where there are trackways the authors¹ suggests may be of the largest ever arthropod that date to the Permian. For about 200 million years they were top predators. In the Soom Shale the larger of the 2 known eurypterids was about 150 mm (6 inches) long.

The authors¹ say the eurypterids of the Soom Shale are important for a number of reasons:

1. Few eurypterids are known from the Southern Hemisphere Onychopterella being previously known only from North America in deposits dating to the Silurian. The eurypterids of the Soom Shale extend the range of the genus back to the Ordovician, and the family Erieopteridae to Gondwana.
2. The eurypterid had a peculiar structure that has been interpreted (Braddy et al.,1995) as a spiral valve of the gut, that was situated between the giant coxae of the last pair of appendages, the swimming legs. This feature is usually associated with scavenging animals that feed on the detritus in the mud, though it is also known in Cyrtoctenus, another eurypterid from South Africa. The  authors¹ suggest that Onychopterella could not have been a benthic sediment feeder as life would have been precluded from the sediments and lower levels of the ocean because of the anoxia, and the spiral valve present in fish and Cyrtoctenus is situated further back in the alimentary tract, resulting in the problem with its presence in Onychopterella. 
3. The gill lamellae of Onychopterella have been preserved. Though known for a long time to have had gills for respiration, the only material that had previously been found preserved was a spongy material that had a surface area that would have been too small to support an animal as large as a eurypterid. This palaeophysiological problem has been discussed previously (Selden, 1985). He concluded that the spongy material was actually an accessory respiratory organ that could be used on land to breathe air, similar to structures present in amphibious crabs and woodlice. He predicted that true gill lamellae would be thin and preserve only under exceptional conditions of fossilisation. The exceptional conditions were present in the deposition of the Soom Shale (see Braddy et al., 1999).

Sources & Further reading

1. Selden, Paul & Nudds, John, 2004, Evolution of Fossil Ecosystems, Manson Publishing.

Links

1. Eurypterida "sea scorpions"
2. Eurypterid images