Australia: The Land Where Time Began
Emu Bay Fossils
Emu Bay, Kangaroo Island, South Australia
Lower Cambrian Shale
The Emu Bay Lower Cambrian shale, dated to about 520 million years old, between the slightly older Chengjiang deposits of southern China and the slightly younger Burgess Shale, has been known for some time for well preserved specimens of the trilobites Redlichia takooensis & Hsuepsis bilobata. It differs from other Burgess Shale-type biotas in that it is believed to result from relatively shallow water deposition, as opposed to the deeper water deposition of the other known biotas of this type. The most notable difference between the Emu Bay assemblage and the other examples of this biota is the extensive labile soft tissue. All 3 fossil localities in which this type of fauna have been found are from that time, called the Cambrian Explosion, a time when metazoan animals with hard parts that could fossilise suddenly appeared in the fossil record in several localities. Once the Ediacaran fauna were recognised as being at an earlier stage of evolution it became obvious that the Ediacaran type fauna was a forerunner of the Burgess Shale Fauna.
Emu Bay remains are notable as the oldest known phosphatised muscle tissue and the first so far reported from the Cambrian. This mineralisation is rarely found in other Burgess-Shale depomineralisation sites.
It is also the only known Australian site of a Burgess-Shale-type biota. The Emu Bay Fauna differs from the other known Burgess Shale-type faunas, the older upper Atdabanian Chengjiang Fauna of China, and the younger Middle Cambrian Burgess Shale Fauna.
The type section of the Emu Bay Shale outcrops is on the east side of Emu Bay. The assemblage is comprised of a number of organisms, among them Hsuaspis, Redlichia, hyolithids, brachiopods, and the scleritome-bearing Chancelloria.
The Big Gully deposit contains soft-bodied fossils in addition to the trilobites, including the giant predator Anomalocaris, as well as Isoxys, Tuzoia, Palaeoscolex (presumed to be a worm), the problematic Myoscolex, and a number of rarer forms. A few specimens of Redlichia have been found to have antennae. At this site a number of fossils have been found that are so well preserved that they are called a Konservat-Lagerstatten (German for "resting place").
The Emu Bay deposit was previously believed to be of late Early Cambrian age, but this date has been re-assessed as a result of the realisation that it is probably equivalent to the Early Cambrian Tsanglangpuian in the China. Contemporary South Australian faunas have been correlated with the Botomian of Siberia.
A new species of Anomalocaris, A. briggsi, is endemic in the Emu Bay deposit. A feature of Anomalocarids was the paired frontal appendages lined with spines and a powerful mouth. The frontal appendages consisted of 14 segments tapering towards the ends, with 2 broad blades like daggers on the first segment, and segments 2-13 have paired spines on the inner surface. The appendages of A. briggsi may not have been robust enough for hunting but the stronger appendages on the Big Gully species would have no trouble being used for hunting. It is believed A. briggsi, that had appendages that had the largest segments in the middle, may have used them to move prey back to blades on the 1st segment.
At about 60 cm long it is one of the largest known animals from the Cambrian. The mantis shrimp of the present, as found on the Great Barrier Reef, Queensland, has been compared to Anomalocaris, having a very similar body appearance, though its head is very different. It is a member of the order Stromatopoda, not really a shrimp.
Both Anomalocaris and the mantis shrimp feature in David Attenborough's documentary First Life.
Direct body fossil evidence of predation
The most direct evidence of predation, probably by Anomalocaris, on trilobites is seen in a specimen of Naraoia From Big Gully, that had a chunk missing from one side that matched the mouth of Anomalocaris, and on the opposite edge of the body, what appears to have been an indentation where the animal seems to have been pinched in, that is thought to be the result of being held by the frontal appendages.
A coprolite, fossilised dung, about 43 mm long and 28 mm wide, large for the time, has been found in the Big Gully deposit that contains trilobite fragments of what is thought to be Redlichia that is estimated to have been about 4 cm long. The only known trilobite hunter large enough to make it is thought to be Anomalocaris.
Fossils of these animals from Big Gully are the oldest known examples of fossilised muscle in the world. It had a segmented cigar-shaped body, that was normally strait or slightly curved, with 3 eyes at the front, beneath which is a short trunk-like structure. A tergite, a mineralised shell, covers each segment, as is seen in prawns, and a flap-like pair of appendages on the ventral side. These appendages are missing from many of the fossils, probably lost after death. Some of these appendages had fine, parallel grooves on their surfaces, leading to the suggestion they may have been gills, or possibly used in swimming. Some specimens clearly show narrow muscle bands running along the dorsal and ventral sides, and another, broader set of muscles along the sides from the dorsal to ventral parts of the body. Some specimens of Myoscolex have pairs of curved rod-like structures on the ventral surface of each segment, the function of which is unknown. It has been suggested they may have been used for moving about, but they are different from such structures in polychaete worms. Another suggestion is that they may have been defensive.
They were originally thought to have been related to the polychaete worms, but further work has indicated they may have been early arthropods, maybe distantly related to modern spiders. They had some features similar to those seen in Opabinia found in the Burgess Shale, both possessing a short trunk-like structure under the eyes and at least 1 pair of compound eyes, both have many segments, each of which is protected by a separate shell. Both have longitudinal and dorso-ventral muscle blocks and both have the curved rod-like structures of unknown function. This similarity between 2 widely separated animals suggests they, and probably many other animals, may have been widely distributed across the ocean floor.
Trilobites are the most common fossils from the Cambrian, largely because most of the fossils are the cast-off shells from Trilobites as they grew. The fossilisation of these cast off shells results in the fossilisation of all stages of growth of the animals. 3 trilobites species have been found in the Emu Bay Shale, one of which is very unusual, the other 2 are true trilobites. The animals from Big Gully show a large number of unusual types. The 2 species that are true trilobites, Redlichi takooensis and Estangia bilobata, are the most common fossils at the Big Gully site, comprising almost 2/3 of the total number of fossils from the site.
E. bilobata has also been found in western New South Wales and the Flinders Ranges, as well as in China. Fossils of every stage of the life of Estangia have been found. This has shown that another trilobite, Xystridura, displays many features in its adult stage that Estangia has in its juvenile stage. This is an example of paedomorphy (neoteny), in which a species develops from another species by retaining features of the juvenile stage of the parent species. An obvious feature involved was the retention of long spines in adult Xystridura that Estangia had at juvenile stages.
At 25 cm, Redlichia is the largest of the fossils in the Emu Bay Shale. Some of the fossils of this species from Emu Bay are of animals but most are of the moulted shells, and the antennae and legs are often preserved, unlike at other sites. A large spine projects from the 11th body segment of this species. The Big Gully species also has a spine on the 6th segment.
A specimen of the trilobite Naraoia from the Early Cambrian Big Gully, Kangaroo Island, South Australia, is the earliest direct body fossil evidence of predation on nonmineralised individuals. The giant Cambrian Anomalocaris is believed to have been a raptorial predator of trilobites. Doubt had been raised on its ability to prey on highly mineralised trilobites (hardened cuticles like crustaceans). Analysis on arthropod cuticles and the injuries inflicted on the specimen of Naraoia from Big Gully suggest it was probably preyed upon by Anomalocaris. It seems at least some Anomalocaris used their large frontal appendages to flex trilobites prior to eating. Similar wounds were found on mineralised trilobites elsewhere, indicating that the flexing of the trilobite allowed the Anomalocarids to prey on the mineralised trilobites (with hardened cuticles like crustaceans). This is now seen as evidence that predation pressure led to the development of hardened cuticles. The discovery that there was variation in the frontal appendages of members of the Genus Anomalocaris indicates that by the late Cambrian the genus niche partitioning was already well under way.
Naraoia was first found in the Burgess Shale in Canada and since then it has been found in other sites in North America and China from the Early and Middle Cambrian, indicating it had a worldwide distribution.
University of Adelaide, Department of Geology and Geophysics, Adelaide, South Aust., Australia
It is thought this animal from Big Gully may be related to the arthropod group arachnomorphs (spider-like forms). Previously found in China, a single specimen has been found in the Big Gully deposit that is not well preserved. Unlike the eyes of other species, where the eyes are directed to the sides, this species has eyes directed up and to the sides. On its head there appears to be widely spaced antennae and 3 pairs of appendages. The 7 body segments, those between the head (cephalon) and the tail (pygidium), each has a thin shell that develops into backward pointing spines at the margins, there is no hardened shell.
Crustaceans with shells
Isoxys and Tuzoiaare are 2 of these that are about 10 cm long and look like clams, but they are crustaceans like shrimps with a head shield that encloses the whole body of the animal and is hinged along the back. There is disagreement about where they belong, some thinking they should be assigned to the phyllocarids (leaf-shrimps) and others think they belong with a closely related crustacean group. Both genera are common in the Big Gully deposit.
The Isoxys from Australia is much larger than those found in the Burgess Shale or elsewhere. It has been proposed that this large size may be a response to low oxygen levels or from reduced competition in low oxygen environments. The first known appearance of eyes in this genus is found in some specimens from Big Gully. Some have large, globular eyes on the ends of stalks projecting from the head. The shell of the 2 genera differs in the presence of surface ornamentation on that of Tozoia where it takes the form of a reticulated pattern and spines around the edges, and a ridge on the surface of the shell. The shell surface of Isoxys is smooth.
This worm-like animal with a small head made up of 4 outwardly-directed 'nipples'. They are believed to have been worms with soft, segmented bodies covered with a cuticle with rows of plates separated by minute polygonal platelets that are not fused together, the skin between these plates and platelets allowing a range of movement. They probably moulted their cuticle as they grew, so most of the specimens from Kangaroo Island were most likely the shells rather than actual animal. It has also been found in Cambrian sites in the Georgina Basin in Queensland. Some species from Kangaroo Island are P. piscatonium, P. antiquus.
Of the specimens from Big Gully, more than 40, some were coiled tightly, a response often seen in modern worms in response to environmental stress, such as low oxygen or high salinity.
The affinities of Palaeoscolex are still uncertain. Some of the possibilities suggested are that it is a true worm or annelid or a worm-like group such as priapulids or aschelminthes.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|