Australia: The Land Where Time Began
Gogo Reef Formation
The Upper Devonian reef formation is in the Geikie Gorge National Park, 100 km southwest of Fitzroy Crossing, in the Kimberley region of north-western Western Australia, dates to 380 million years ago. There are about 45 species of fish preserved at this site. It is the only Devonian site in the world where complete fish have been preserved in perfect, uncrushed, condition, at least in some specimens.
The limestone sediments of this formation were deposited in the Late Devonian. At the time Australia was still part of Gondwana, and the Gogo Reef was a 1400 km long barrier reef around at least part of the supercontinent. The corals responsible for this reef were of a different type from those responsible for the present barrier reef. They were predominantly solitary corals from a group that is now extinct - the tabular corals and rugose corals. The inhabitants of the reef system consisted of many types of fish, including lungfish. There were also Ammonoids, and eurypterids (sea scorpions).
The Gogo Reef Formation fossils are found within calcareous nodules. On the outer periphery of the reef and the area inside the reef were places where the sediments were slightly anaerobic (dysaerobic). These sediments are shales within the limestone formation of the site. The remains of the animals formed heterogeneities in the limestone, water moved into the heterogeneities and calcareous concretions formed around the bone. These nodules are much harder than the surrounding material of the limestone reef. The fossils have been preserved in very fine detail and in 3 dimensions. Acid etching of the placoderm head shields has revealed the fine structures of the head, such as nerve pathways and the olfactory system.
The concretions formed as carbonate precipitated, possibly as the result of organic decomposition, forming before many delicate structures like gill arches were damaged. Some contain whole fish while others contain only parts, isolated bones. The concretions were concentrated in the Gogo region by high erosion rates, partly by deflation, and partly by transport by water during the flash floods that occur in the wet season, being deposited in the surrounding creek beds.
They are composed mainly of hard calcium carbonate surrounded by dark, clay-rich marine sediments. The carbonate matrix has 2 big benefits for the preservation of fine structure. It protects the fossils from damage by crushing or erosion because of its hardness compared to that of the surrounding material, and it is easily dissolved by weak acids such as acetic acid, that don't damage any contained bone.
All the vertebrate fossils in the Gogo Formation are of fish, predominantly placoderms. Among the placoderms many had a duraphagous feeding style, e.g., Bullerichthys, Bruntonichthys and Kendrickichthys, as did some of the ptyctodontids, probably living on the reef feeding in cracks and crevices in the structure of the reef.
The most abundant and diverse fish in the formation are Coccosteomorph arthrodires. Some of the arthrodires are rare, such as Harrytoombsia. Most fish in the deposit are less than 1 m long but some, such as the carnivorous Eastmanosteus grew to more than 2 m. Among the most common fish found in the Gogo deposit are the arthrodires that are closely related to Harrytoombsia, Kimberleyichthys and Goujetoestus. These are all plourdosteids, a group of predatory arthrodires, between 30 and 50 cm long, that are believed to have been ambush hunters, probably hiding in crevices in the reef until prey came within reach of a lunge.
There were also mid-water to surface feeders. Among these were camuropiscid arthrodires, streamlined fish adapted for surface feeding
Other arthrodires in the Gogo Formation included Rolfosteus, Fallacosteus and Tubonasus, that were all streamlined fish about 30 cm long, with long, tubular snouts with crushing teeth. From the shape of their bodies and the form of their dentition they are believed to have been surface feeders. They probably ate shrimps or similar crustaceans.
These placoderms had large heads with big eyes and crushing tooth plates and long bodies, spiky claspers, and whip-like tails. In the Gogo Formation the 2 main ptyctodonts were Austropytodus about 20 cm long, and Campbellodus about 30-40 cm long. On the back of Campbellodus was a long spine composed of 3 of the body plates. Both are believed to have fed on seafloor invertebrates with hard shells.
The antiarch placoderm Bothriolepis are also present. This mainly lived in freshwater habitats, but apparently was able to survive in sea water, suggesting that it could migrate from one freshwater system to another via the sea. Bothriolepis was common at a number places around the world but rare on the Gogo reef. Those that have been found were near the reef front and it is suspected they may have been moving from the mouth of one river to another where they moved into the freshwater parts of the rivers.
A number of specimens of Onycgodus have been found in the Gogo Formation. It is believed these fish may have reached as much as about 4 m long, and they were probably been an ambush predators.
Gogonasus (snout from Gogo), a lobe-finned fish. The first fossil, a snout (ethmosphenoid), was found in 1985, and in 1986 a more complete fossil was found. Based on the original snout it had been suggested that the large fangs of the coronoid bones inserted into the choana of the palate, but this later find showed that it didn't, the fangs fitting into special grooves. Several have been found since then, the latest being an almost complete skeleton. This specimen is at Museum Victoria.
Both main types of lungfish during the Devonian were represented in the Gogo Formation, the denticle-shedders, that had small plates covered with denticles, small tooth-like structures. These were continuously shed and replaced throughout the life of the fish, and the other group had tooth plates suitable for crushing food, that were not shed. There were also some lungfish that were intermediate between these 2 groups in the Gogo deposits.
Dipnoans, lungfish, are present in the Gogo Formation is a variety of shapes, so occupied a variety of niches. A Short-snouted form with tooth-plates was Chirodipterus, was also common in North America, China and Europe. Griphognathus whitei the duck-billed lungfish, common in the Gogo deposit, had a long snout and Denticulate dentition. Holodipterus, a large dipnoan, appeared to have crushed hard-shelled invertebrates. Griphognathus was also common in places such as North American, Chinese, and European sites in the Late Devonian.
Evidence of a possible marine origin and cause for the development of air-breathing
It has been found that the atmospheric oxygen dropped to about 12 % (now about 20 %) at around the time the air-gulping fish evolved, about 375 million years ago. In a paper published in Biology Letters it is proposed that the development of air breathing may have been the result of the oxygen drop forcing fish to adapt to low oxygen levels in the water. It also apparently occurred in a marine fish, Rhinodipterus, from the Gogo Formation in Western Australia. It has previously been believed that the adaptation to air gulping occurred in freshwater environments.
Minia and Moythomasia, primitive actinopterygian palaeoniscoids, are believed to have been mid water carnivores. One fossil of Moythomasia was found with a conodont inside it.
Among the fish in this formation is the placoderm fish Mcnamaraspis kaprois, it was about 25 cm long, and had a bony head shield and a shark-like body. Placoderms were the first jawed fish, this one had annular cartilage rings in its snout, making it the only placoderm found to have such a structure. This structure allowed water to flow over its sensory organs so it probably had an acute sense of smell. Combined with the large teeth, it seem clear this fish was a predator.
There was also an arthrodire placoderm with hard bony teeth. It apparently fed on shrimp-like crustaceans. A long-snouted placoderm, Fallocosteus turnerae. The Gogo lungfish, Griphognathus whitei, Chirodipterus australis.
A new species of eurypterid is Rhenopterus waterstoni. This species differs from similar forms in having tuberculation of the foremost tergite. and the posterior margins of its carapace and opisthosomal segments are crenulated. It is the only eurypterid known from the Gogo Formation, and the most complete eurypterid from Australia, and the youngest member of the Rhenopterus genus in the fossil record. Structures between the prosomal-opisthosomal juncture, tubes 30-40 micrometres in diameter and believed to be sarcomeral sheaths of muscle tissue.
An almost complete specimen of a new ptyctodontid placoderm, Campbellodus decipiens. This specimen is so well preserved that it enabled the description of the skull roof, visceral skeleton, pelvic girdle, dermal scale cover and parts of the vertebral cover.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|