Australia: The Land Where Time Began
Soom Shale - a lagerstätte
An Ordovician horizon of the Table Mountain Group from the Lower Palaeozoic sediments of the Western Cape, South Africa, the Soom Shale Member produced giant conodont animals in the 1990s that preserved musculature and feeding apparatuses. The Soom Shale is unique in that it is situated at high latitudes, 60o S, in a marine habitat that was cool, being glacially influenced. Some of the animals discovered in the Soom Shale hark back to the biotas of the Middle Cambrian, such as Soomapsis, a naraoiid-like trilobite. Among other animals found in this shale are Onychopterella, a eurypterid, presaged the great diversity of the future.
In the region of the southwestern Cape the landscape is dominated by rugged sandstone escarpments and plateaus, the Soom Shale forms a distinct bench, as a result of its being relatively soft, and this has led to it being well known for a long time. The predominantly sandstone rocks of the Table Mountain Group, which the Soom Shale is a pert of, hasn't produced many fossils, and the same appeared to apply to the Soom Shale. It wasn't until 1970, when the finding of brachiopods and trilobite fragments was published by Cocks et al. gave the first indication of the Ordovician age of this fossil deposit, the first fauna from the Lower Palaeozoic from South Africa. The Soom Shale first become prominent in 1993 when it was reported that a fossil conodont animal had been found in which eyes were present. Evidence of cartilage supporting large eyes in this conodont, Promissum pulchrum, helped show that conodonts were actually early fish. Conodonts are usually found as small phosphatic tooth rows, often in the form of a basket, that were found associated with soft tissue that proved to be early fish. A wide variety of plants and animals have now been found in the Soom Shale, including more specimens of Promissum, that increased the knowledge of the morphology of these fish, one of which was reported in 1995 that had preserved the muscle tissue. Among other animals found in the Soom Shale are Onychopterella, a 'sea scorpion', which also showed the structure of the gut and the musculature, and orthocone cephalopods, naraoiid trilobites that the author1 describes as enigmatic, related to squids and ammonites, that had straight, comical shells.
Stratigraphy and taphonomy
The thickness of the Soom Shale Member is a maximum of 10 m (30 ft). It is characteristically finely laminated, and is coloured yellow-brown to light or dark grey. The Disa Siltstone Member, about 130 m (425 ft) thick is above it. The Cedarberg Formation, named for the Cedarberg Mountains, is comprised of these 2 members. The Cedarberg Formation is in the form of a north-south ridge between Citrusdal and Clanwilliam in the Western Cape, about 150 km (90 miles) north of Cape Town. It has been determined that the Soom Shale is of Late Ordovician (Ashgill) age, based on the presence of the trilobite Mucronaspis olini (Cocks & Fortey, 1986) in an outcrop in the Hexrivier Mountains, about 100 km (60 miles) from the main locality at Clanwilliam. The Keurbos Farm locality, about 13 km (8 miles) south of Clanwilliam has produced most of the biota, with some coming from Sandfontein Farm, 25 km (15 miles) east of Citrusdal.
The Table Mountain Group is mostly a deposit of sandy sediments about 4,000 m (13,000 ft) thick. At Cape Town the distinctive Table Mountain is comprised of the Graafwater Formation and the Peninsula Formation, 2 of the lower formations, the higher beds are found further north. Clues to understanding the climate at the time the deposits were laid down comes from the Pakhuis Formation, between the Peninsula Formation and the Cedarberg Formation. The most prominent material of the Pakhuis Formation is tillite - fossilised till or boulder clay, both produced by glaciers. The tillite rests on a grooved pavement of the Peninsula Formation at many places beneath the Pakhuis Formation. The grooves are glacial striae that were gouged out as ice moved across the rocks of the Peninsula Formation. The base of the Pakhuis Formation is an erosion surface, making it an unconformity. The top of this formation grades gradually into Soom Shale that is laminated finely.
The result is that the Soom Shale appears to have been deposited in quiet water close to the ice front. The gentle settling of fine particles to eventually form the laminae of silt and mud about 1 mm (0.04 inches) thick, only rarely reaching as much as 10 mm (0.4 inches) thick, persist laterally. The authors1 suggest distal turbidities may be represented by the silt layers, the final settling out as a flurry of sediment spread across the sea floor (possibly a brackish bay), or possibly freezing and thawing that was seasonal (varves). Dropstones provide evidence of floating ice. Coarser siltstones comprise the overlying Disa Siltstone Member. These siltstones occasionally contain evidence of wave and current activity, indicating it was deposited in a shallow marine environment, though with a greater input from melting ice sheet sediment.
There is no known evidence in the Soom Shale of bioturbation (signs of animal activity in or on the sediment), that suggests that animals could not live in or on the sediment. The authors1 suggest the lack of benthic animals, though there is evidence that animals occupied the water column above, may have been because of very low temperatures, or possibly that the sediment and the lower layers of water were anoxic, or possibly toxic in some way. There are ribbon-like algae ('sea weeds') on the surfaces of many of the laminae, but they may have fallen to the sea bed after death, though it is unknown if they were actually living on the sediment.
The bodies of the animals preserved in the Soom Shale deposits are preserved as thin films of clay minerals that have replaced the original organic tissues, as well as the mineral tissues, of the dead animals' bodies. The fossils are flattened, as shale is very compressible. Some aspects of the sequence of events leading to the preservation of the fossils are unique. From the available evidence it can be concluded that the water was cool, shallow, or not too deep, and was free of currents. In addition to possibly some rippling of the shale surface, the elongate fossils, such as the enigmatic spines, Siphonacis, would be expected to show some degree of alignment if there were currents in the water. The evidence, such as virtually no benthic animals, a lack of bioturbation, and most of the animals that were on the sediment surface are almost complete, showing little or no signs of predation, scavenging or even much decay, all conditions that are the opposite of what is seen oxygenated sea beds, indicated that the sediments and lower levels of the water column were anoxic. The authors1 suggest that it is possible that the sediments were not always anoxic, becoming anoxic periodically, and the organisms only being preserved at such anoxic times.
It appears that decay is inhibited by cold water, as well as increasing the dissolution of calcium carbonate, thus helping explain the lack of calcite shells while preserving the organic tissues. Illite, a clay mineral, replaced the organic material at an early stage, silica replacing apatite, as found in conodont teeth and parts of some brachiopod shells. Some debate has occurred (Gabbott, 1998 Gabbott et al., 2001) regarding the mechanism of replacement of organic material by clay minerals, whether kaolinite was the material of the original replacement, changing later to illite, or possibly the direct replacement of the organic tissues by illite. Gabbott favours of direct replacement by illite.
The factor making the preservation in the Soom Shale unique is the lack of other Lagerstätten in which it can be demonstrated that direct replacement of organic material by clay minerals has taken place. The preservation of animals in the Burgess Shale was as films of shiny micaceous minerals, that are similar to clays, as well as some adherence of clays to trilobite cuticles. In the Burgess Shale deposits the minerals appear to have adsorbed onto the organic surfaces, forming later in the diagenetic process than occurred with the clay replacement in the Soom Shale. The authors1 suggest it is possible adsorption of clay minerals preceded replacement in the Soom Shale. According to the authors1 it is not certain at this point in time if the Soom Shale is indeed unique in this respect, the other possibility being that it represents an end-member of a geochemical spectrum.
Biota of the Soom Shale
The diversity of the biota in the Soom Shale is considerably less than that of other Ordovician communities from shallow water such as are found in North Wales and Scotland. Among the microfossils, that are particularly well preserved in the Soom Shale, found at this site are the spores of plants as well as acritarchs and chitinozoans, both of which are cysts with organic walls that are of unknown affinity. Chitinozoans and acritarchs are both microfossils that are generally flask-shaped, and of uncertain affinity. Chitinozoans are known from the Ordovician to the Early Carboniferous. Based on their biochemical composition, that is similar to sporopollenin, they are suggested by the authors1 to appear to be related to plants.
The composition of chitinozoans is a chitin-like substance, that the authors1 say suggests an affinity with animals. Though the chitinozoans are usually found as isolated individuals when the rock matrix is macerated, when they are found on rock surfaces they are often in strings, radial aggregates and clusters inside a membrane, a structure that is suggestive of a cocoon. When the area on and around the Soom Shale conodont animals and orthocone cephalopods (Gabbott et al., 1998) were searched all 4 types of occurrence were found, the 3 unenclosed types and enclosed in cocoons. The authors1 consider the most likely reason chitinozoans show a zoological affinity is that they are actually either eggs and egg-masses or cocoons of a group of animals that is in the same stratigraphic range in the strata between the Ordovician and the Carboniferous. Assuming the animal in question is restricted to the range from the Ordovician to Carboniferous and further assuming that the animal is one of those found only in the restricted fauna of the Soom Shale, the only possibilities are the conodont animals or the orthocone cephalopods. The authors1 say of the other possible animals, such as graptolites, they can be ruled out as they are either not present, or only rarely present, in the Soom Shale. In the Soom Shale the chitinozoans are associated more with the cephalopods than the conodont animals, suggesting it is slightly more likely that they are egg-masses of the orthocone cephalopods.
According to the authors1 the Soom Shale is probably best known for the exceptionally good preservation of conodont animals. The animals the conodont structures, that were from are simple to complex, spiny, phosphatic structures, were from were not known for more than 100 years after conodonts were first discovered.
Among the biota of the Soom Shale there are some that cannot be placed in any known groups. One of these, that the authors1 find the most interesting, is a soft-bodied animal with a segmented body that hasn't been fully studied and published. There are also scattered spines with organic walls that have been named Siphonacis that have been suggested to have come from an unknown spiny animal. There are also some cornulitids, cone-shaped fossils of uncertain affinity, from the Palaeozoic that were found among the epizoa attached to the orthocone shell.
Based on the available evidence it has been suggested that the environment of the site at the time of deposition was probably a shallow sea with a muddy, cold bottom that was mostly lifeless, but with many organisms that were swimming (Nekton). Among the animals eurypterids, conodonts and orthocones are presumed to have been carnivores and/or scavengers, and the cornulitids and brachiopods filter feeders. At times there were some benthos such as lingulate brachiopods, filter feeders, and myodocopid ostracods, scavengers. On the sediment surface there is also evidence of unknown large predators in the form of coprolites, crushed brachiopod shells and broken conodont elements.
The Soom Shale compared to other biotas from the Lower Palaeozoic
The Soom Shale contains virtually the only exceptionally preserved biotas known from the Ordovician, though other sites from this time show similar biotas, that are less well preserved. The Soom Shale has been dated to the Late Ordovician, the Ashgill Epoch, the Hernantian Age specifically, the last part of the Ashgill. Dating from this time evidence has been found indicating that the part of Gondwana occupied by East and West Africa was glaciated, though the effects were more wide-ranging. There was a large decline in the diversity of graptolites, members of the plankton normally found in low, subtropical latitudes. As with the end-Cretaceous mass extinction event, the end-Ordovician mass extinction event extinguished almost 24 % of families that existed at the time, and has been suggested to have been caused by the Hernantian glaciation. The animals that went extinct at this time included 75 % of all trilobites and 25 % of all brachiopods, to be replaced by the Hernantia Fauna that were better adapted to cold water. A number of localities around the world contain Hernantian Fauna. Exceptional biotas are also sparse in the Silurian, though from the Wenlock, Lower Silurian, a new Lagerstätte has been reported (Briggs et al., 1996). At the time of writing1 this fauna, that is preserved in volcanic ash, was in the early stages of description.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|