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:
- 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
- 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.
- 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
- Selden, Paul & Nudds, John, 2004, Evolution of Fossil
Ecosystems, Manson Publishing.
Eurypterida "sea scorpions"