Australia: The Land Where Time Began

A biography of the Australian continent 

Tetrapods – First Feet in the Famennian

Vertebrates which had limbs with digits had appeared in the fossil record by the Famennian, the latter part of the Late Devonian. Fossil material from several localities has been found that is remarkably well preserved and provides details of their anatomy and lifestyles.

Tetrapods in East Greenland in the Famennian

Most detailed information about tetrapods from the Devonian has been from the fossils found in East Greenland, a location that has been studied by geologists for many decades. One of the reasons for the great interest in the geology of the area is that the terrain of interest is in the polar semidesert, where the vegetation is sparse, which makes for easy access to the geology once the ice and snow have melted around the coast in summer.

Part of central East Greenland is comprised of rocks that are from the Middle and Late Devonian, and the area around Kejser Franz Joseph Fjord, which is 40 km north of the Arctic Circle, has been yielding vertebrate fossils of Devonian age since early in the 20th century. The main fossil-producing locations are centred on Celsius Bjerg on Ymer Ø and the mountains Sederholm, Smithwoodward, Stensiö and Wiman Bjerg that comprise the peninsula of Gauss Halvø. All the locations in East Greenland from which tetrapods have been recovered, as well as the specimens recovered from them, have been documented (Blom et al., 2005, 2007).

Fossil fish and tetrapods, at the time the earliest known, were recovered between the 1920s and the 1950s. There are 2 tetrapod genera that were recovered from this site that have since become particularly well known from large collections. The first genus of tetrapod from the Devonian to be widely known and written about was Ichthyostega, which has sometimes been referred to as a 4-legged fish. Many specimens of Ichthyostega recovered during the early years of exploration of East Greenland have been in a monograph on this animal (Jarvik, 1996). Acanthostega gunnari was named and described in 1952, and since then has become much better known from collections made in 1987 (Bendix-Almgreen et al., 1990; Clack, 1988). New ichthyostegid material was collected by Clack1 in 1998 that allowed the discovery of more about the circumstances of deposition of the tetrapods (Clack1 & Neininger, 2000) in an expedition that was supported by the National Geographic Society.

According to Clack1 Greenland would have been in the tropics, and the climate of the region appears to have been monsoonal, the animals living in an extensive river basin, meandering streams flowing from mountains to the south to add water to the system. There appears to have been a large freshwater lake, or possibly an inland sea, the margins of which were determined by a series of faults that carved a broad valley with parallel sides into the mountains of the Old Red Sandstone continent (Olsen & Larsen, 1993; Olsen, 1993; Larsen et al., 2008).

Tetrapods have been recovered from 2 formations, the lower one being called the Aina Dal Formation, and the upper one the Britta Dal Formation, along a peninsula that has since been named the Gauss Halvø. These formations represent 2 periods of seasonal deposition in the river basin, and they are separated by another formation, the Wimans Bjerg, from which few vertebrate fossils have been recovered, though a rich fauna of bottom living invertebrates has been indicated by the presence of trace fossils and burrows (Clack & Neininger, 2000). The Celsius Bjerg Group is comprised of these 3 formations. It is believed a drier period is represented by the Wimans Bjerg Formation, when the lake that was once deeper in Aina Dal times had been replaced of remnants consisting of a shallow lake or playa. The lake and rivers were refilled in the succeeding Britta Dal times when the climate returned to a wetter phase. These formations cannot be distinguished as easily in the other parts of the Kejser Franz Joseph area, which according to Clack1 suggests the geological conditions were slightly different (Olsen & Larsen, 1993; Olsen, 1993).

The presence of large plants in the region is indicated by the finding of plant fossils, though they have been poorly preserved, that Clack1 suggests probably grew along riverbanks, though not enough detail has been preserved to precisely identify them, one them is an impression of a single branched specimen; that has been likened to Rhacophyton, a fernlike plant (Astin et al., 2010). Most regions of the sediments have been oxidised, and most of the plant organic matter has been destroyed, including their spores. The precise dating of these rocks from the Devonian has been hampered by the lack of spores in them, as plant spores have proven to be useful for the dating of the sediments they are found in.

Spores have been recovered from above the Britta Dal Formation and beneath the Aina Dal Formation that allowed these formations to be dated to the late Famennian (Marshall et al., 1999). According to Clack1 some of the tetrapods from the Devonian that have been most securely dated are Acanthostega and Ichthyostega, as they are bracketed between 2 marker bands that define a range of time that is quite narrow. They can now be compared with similar spore data associated with other tetrapods from the Devonian as a result of these spore dates having been worked out. The dating of Devonian tetrapod localities has been reviewed (Blieck et al., 2007, 2010), based on zonation of European miospores, which has shown that Acanthostega and Ichthyostega are not the earliest to appear in the fossil record, they are the only ones that have definable stratigraphic ranges.

Many articulated skeletons of Acanthostega were found in a small rock lens, and above and below there were bones that were disarticulated. The small outcrop that contained most of the Acanthostega material is about 800 m up Stensiö Bjerg. Acanthostega is suggested by the first studies of the sediments it was found in to have lived in river channels that were flowing actively (Bendix-Almgreen et al., 1990). The deposit has been interpreted as sediment accumulation on a riverbank at that inside of a bend, a point bar, where there were many individuals heaped up, possibly after death during a flash flood, the animal bones and carcasses being dumped on the bar, that had been formed by fine silty sand, as well as coarser sediments being carried by the fast-flowing floodwater. The good state of preservation of some of the skeletons suggests they hadn’t been transported far before being deposited and fossilised.

It has been suggested more recently by a new interpretation that the environment resembled that of Cooper Creek in Australia (Astin et al, 2010), a multichannel ephemeral river system that is subjected to rare though intense flooding. Following the flood event the water gradually retreats leaving a lake which acts as a waterhole, which attracts local fauna. It is suggested (Astin et al, 2010) that animals in the Acanthostega horizon were transported or migrated downstream towards the remaining waterhole, either during or after a major flood event. The animals could not, or did not, return upstream as the water receded, either during or after the flood event. They also suggested that 2 major flood events that occurred in the same channel are represented by the Acanthostega sites, that are separated in time. The Acanthostega could therefore have occupied the basin for a considerable period. They suggest that the carcasses could have been exposed to the air for some time after being deposited on the point bar. There are mud cracks, which were subsequently filled with finer sediment, running through the rocks, and disrupting the skeletons in some places.

Fish scales were also recovered from the site containing the Acanthostega specimens, which have been found to be from fishes such as Holoptychius, a porolepiform, and a lungfish, though the fish remains were all isolated, disarticulated elements. Nearly all the fish remains represented animals that were much larger than Acanthostega, and are suggested to have possibly been living in a lake with deeper water or being carried to the Acanthostega site from a river or lake upstream by floodwaters.

Ichthyostega has been found in sediments varying from coarse red to finer-grained blackish-red sandstones, which frequently contain fish scales and placoderm plates. Ichthyostega is the tetrapod that is the most frequently found in the Aina Dal Formation, usually being found in massive blocks of talus slope. Though scales from fishes such as Holoptychius and lungfish are found alongside ichthyostegids, placoderms are as often as not found in different sediment lenses from those containing tetrapods. Placoderms and Holoptychius are often either found in lenses or bedding planes that contain only those animals. Most Ichthyostegid specimens appear to have been preserved in flood deposited as they are disarticulated. The few specimens that are partially articulated have been suggested to possibly have been mummified prior to being transported in floodwater. The ichthyostegid specimens appear to have been transported from quite a large distance from their original habitat (Clack & Neininger, 2000).

At the base of the Britta Dal Formation is one of the most productive horizons for Ichthyostega, which is a thick layer of sandstone and at its base there abundant plant remains (Blom et al., 2005). Several Ichthyostega skeletons have been found piled up chaotically on top of one another, as well as isolated plates from placoderms and scales of Holoptychius, and these sediments have also been interpreted as being the result of a flood event (Astin et al., 2010).

According to Clack1 2 different animals are represented by Ichthyostega and Acanthostega that are not particularly closely related to each other, being adapted in different ways. Clack1 says there is a tendency in some literature to portray both as being ichthyostegalians which she says is mistaken and quite misleading). They have not been recovered from exactly the same deposits, though they were contemporaries. They show that tetrapods were already of diverse body forms and modes of life by the Famennian.

Sources & Further reading

1.      Clack, JA, (2012). "Gaining Ground: The origin and evolution of tetrapods", Indiana University Press

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated  15/10/2014
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading