Australia: The Land Where Time Began
Brachiopods of the Soom Shale, South Africa
Brachiopods are among the most common microfossils found in the Soom Shale, though very few microfossils are present in the deposits. Those found include orbiculoid and lingulate inarticulates (that have phosphatic shells), and some ribbed articulates (that have calcified shells). An oxygenated substrate is required by infaunal Lingula, found only in shallow water. The sea floor at the time the deposits were being laid down is indicated to have been at least occasionally oxygenated as the authors1 say any evidence of it drifting in from elsewhere is lacking, suggesting it is autochthonous. In the shale at the Keurbos locality more than 100 orbiculoid brachiopods have been found scattered in the shale, the numbers found associated with orthocone cephalopods are vastly higher. The orthocone shells have been found with orbiculoids attached as epizoans, and as they occur attached to all parts of the orthocone shells, no part of the shell being preferred, it has been assumed that the brachiopods colonised the living orthocones swimming in the nekton (animals that swim or move freely in the ocean). The authors1 suggest that it would be expected that the larger brachiopods attached near the apex of the cone, as this was the first formed shell, the latest brachiopods to attach would be near the aperture, the end of the shell that continues growing.
A particularly large orthocone shell and its epizoa that has been studied, that was well preserved has been studied (Gabbott, 1999). The size difference between the brachiopods on the aperture and the apex was not great, but there were fewer near the apex, and in the matrix around the orthocone the loose brachiopods were generally larger than those still attached to the shell. It has been suggested that when the dead animal dropped to the sea floor and began to decay. The combination of the buoyancy gases in the shell and the gases produced by petrification would buoy up the apex of the shell, the aperture becoming buried in the sediment, covering the brachiopods near the aperture and dying. The larger brachiopods near the apex might also die and drop off the section of the shell that was not buried, as the water near the seabed could be anoxic, or at least poorly oxygenated.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading