Australia: The Land Where Time Began
Devonian – Tetrapod Trackways
Trackways cannot always be used reliably as evidence of the habitus (physical characteristics, body design and structure) of the earliest tetrapods, as it is extremely difficult to interpret foot print trackways. There is another problem, it seems the kind of fossilisation required to preserve trackways is not compatible with the preservation of bone, and though it is uncertain why this should be, the identification of trackway markers requires indirect evidence and inference. It is also sometimes difficult to determine if the prints are original, where the animal actually walked on the surface causing the impression in the surface, or underprints at various levels, the top layer having been eroded away to leave the impressions from deeper in the sediments. In the case of underprints the size of the original print is often overestimated.
Several fossil trackways have been described as being made by tetrapods in the Devonian. The dating of such trackways is a problem with most trackways, with the date of only 1 of these trackways not requiring placement within a range that is rather broad, with the result that it is not really possible to list them in a strict time sequence. According to Clack1 the easiest way to deal with them is by the use of the dates when their descriptions were first published.
The first tetrapod trackways to be described are a pair of them on the same block from the Genoa River in Australia. Described in 1972, they were given a date “probably Frasnian” by the authors of the description (Warren & Wakefield, 1972), therefore probably making then contemporary with Obruchevichthys and Elginerpeton.
Clack1 suggests the trackway pair appear to have been made by 2 animals that were rather different, or at least animals of different sizes that moved in contrasting ways. In one trackway a series of foot impressions have the hind footprint overlapping the fore footprints in an alternating sequence on left and right sides. This is a characteristic pattern of tetrapods that are produced by the fore and hind limbs being moved alternately, as in a modern animal such as a dog trotting on a beach. Some digit impressions are seen situated to the side of the print, not in front as they would be in a modern animal. At least 5 digits are represented, though there could have been more.
There was no tail or belly drag mark on the substrate, which has been reported as the animal walking with its body supported well clear of the ground. A rough calculation that is based on the separation of the prints suggests the body size was about 220 mm not including the tail.
The second trackway is parallel with the first and this consists of a small print from one foot that is probably the forefoot, which alternated on each side with a larger print that has been assumed to be the hind foot, which made a drag mark behind it. There is a sinuous tail or body trace down the centre of the trackway, which suggests that in this case at least part of the body was not being supported. The animal that mad this trackway was apparently somewhat smaller than the maker of the first trackway.
These trackways were made by animals that were supposed to be walking on the land surface, though there is little independent evidence that has been presented in support of this assumption, with the exception that they were made by tetrapods. There are also trackways made by invertebrates present on the same bedding plane, though these have not been described. When the footprints are compared with what is known of the earliest tetrapods it is suggested that none of the known forms is likely to have been responsible for these tracks unless they were formed at least partially under water (Clack, 1997a). In spite of this the trackways from the Genoa River represent some of the best substantiated evidence of locomotion of a tetrapod with digits.
Another set of trackways from the Valentia Slate Formation on the west coast of Ireland were also certainly made by tetrapods (Stössel, 1995). Though these trackways are more extensive than those from the Genoa River the individual prints are not as clear. These trackways are comprised of several series of footprints that were made by more than a single animal, and all have been preserved on the same bedding plane. These trackways show the alternating sequence of prints, with the fore and hind prints being distinguishable, in spite of the rocks being distorted as the rocks were stretched and pulled by nearby geological events. The original shape of these prints has been restored by computer techniques.
At this site there is a long, sinuous trackway and several shorter stretches that were clearly made by different individuals. There are broad, shallow furrows between the footprints of one of the trackways that were apparently made by the body as it was pulled along the ground while not being fully supported by the limbs. Assuming that all these trackways were made by the same kind of animal, the one with the drag marks is the best evidence that if the body was not supported by much water the body dragged along the ground and the limbs projected to the sides of it. There is another trackway in which the footprints are shown with a much longer stride length; though in all the tracks the distance between the left and right prints remains the same. There are no digit marks on the footprint impressions, though they are quite deep and rather egg-shaped, and the pointed end is at the outer edge. According to Clack1 it is suggested by the suite of trackways that the animals making these tracks were to some extent supported by water, with some being more immersed than others. It appears all the trackways were made at the same time, and all of the animals were moving in the same direction, more or less, though it is not clear whether the differences between them resulted from animals moving at different speeds, or animals that were moving in different depths of water, or by animals that were of different shapes.
Clack1 suggest that clues to the dating might be given by the fact that there are no other fossils or spores that have been found in the same beds as these tracks. There are, however, some placoderms that have been found at a different point on the western coast of Ireland in a nearby formation that Clack1 suggests is probably not too far distant in time from those in which the tracks were found. The presence of the placoderms is consistent with the rocks being of Frasnian age, which would not be considered surprising for the age of the trackways. Clack1 suggests they could have been made by an animal such as Elginerpeton, though a date around the boundary of the Middle-Late Devonian was supported by analysis of magmatic zircons from sediments just below the Valentia Slate Formation (Williams et al., 1997), and an earlier date for the tetrapod origin than that indicated by the body fossil data is implied by this.
Other trackways dating from within the Devonian that have been attributed to tetrapods are all subject to interpretation problems as to whether they are of terrestrial origin as well as their dating (Clack, 1997a). Among these other trackways is a ladderlike trackway in the Grampian Mountains, Victoria, Australia, that is suggested by the latest evidence to be of Late Silurian or Early Devonian age. Clack1 suggests that ladderlike trackways, similar-sized individual prints in opposite pairs, are inherently not likely to have been made by tetrapods, some evidence that has recently been discovered suggesting they may have been made by some kind of large invertebrate. Based on comparable ladderlike trackways found in East Greenland dating from the Middle Devonian, another interpretation is that they were formed by the forelimbs of a placoderm such as Bothriolepis moving under water. It has been shown by recent evidence resulting from a study that the trackways in East Greenland are exactly the correct size to have been made by the fins of contemporary placoderms. At Tarbat Ness in Scotland there is a trackway that is more likely to have been made by a tetrapod, which has been tentatively dated to the Middle to Late Devonian, though it could possibly be as late as the Early Carboniferous (Rogers, 1990).
The origin of the Tarbat Ness and Irish trackways must be viewed in a new context. The most astonishing yet are sets of trackways and imprints are the most recently described. The tracks and trackways that have most recently been discovered in the Holy Cross Mountains in southeast Poland, rich sequences of rock from most ages from the early Palaeozoic to the Triassic are present. The trackways are in a disused quarry, Zachelmie Quarry, the horizons in which they were found have been well-dated by conodont stratigraphy (Niedzwiedzki et la., 2010). They date to 395 Ma, in the early Eifelian, the sediments representing a shallow water coastal lagoon environment. Also present are desiccation cracks and impressions of raindrops.
Included among the trackways are several series of prints that are paired or alternating, that are in the form of oval depressions, of which the latter has been interpreted as having the typical alternating manus and pes sequence of indisputable tetrapods. Varying in size, animals from about 40 to 250 cm in length are indicated. Isolated single footprints have been interpreted to have been made by an Ichthyostega-like limb. There are some large isolated prints that are about 15 cm in diameter that bear what appear to be prints of 6 or 7 digitlike extensions.
The age of these trackways is the first unexpected characteristic, about 18 My older than the oldest known tetrapod body fossil, even predating the tetrapod-like sacropterygians such as Tiktaalic and Panderichthys. Their size is the second surprising feature of these animals, though Tiktaalic had an estimated size of up to 2.75 m, these trackways that are much earlier, dating to a time when animals such as Osteolepis were the largest known sarcopterygians. The third surprise was the environment in which they were found, that represents a shallow water nearshore lagoonal environment that was subject to occasional marine incursions. Clack1 suggests this is possibly not as surprising as it would have been in the past in light of other recent body fossil finds, though it raises questions about how the animals were living.
The whole question of timing, sequence and circumstances of tetrapod origins is possibly indicating that the entire scenario that is implied by these trackways may need to be revised.
1. Clack, JA, (2012). "Gaining Ground: The origin and evolution of tetrapods", Indiana University Press
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|