Devonian Tetrapod Trackways
Devonian tetrapod trackways (footprints) have been reported from Australia, Brazil, Greenland, Scotland and Ireland. These reports have generated considerable controversy because theyve often been presented as evidence of terrestriality from strata that predate those from which the earliest and apparently aquatic tetrapod body fossils have been recovered.
Footprints and other trace fossils present particular problems with respect to dating, identification and interpretation. The typical absence of index fossils usually means that dating the trace fossils is dependent on their stratigraphic relationships to other, more precisely datable strata. Consequently, the range in potential ages for any given trace fossil is often considerable. The identification of fossil footprints can be difficult, particularly in the case of early tetrapods, since knowledge of their foot anatomy is limited to three taxa (Acanthostega, Ichthyostega and Tulerpeton). In contrast, the analyses of other trackway are often based on animals for which we have a greater anatomical and biomechanical understanding (e.g., theropod dinosaurs). Finally, its often very difficult to determine whether trace fossils were made under water (subaqueously), on land (subaerially) or at the indistinct boundary between the two.
Difficulties aside, there are four credible records of Devonian tetrapod trackways. Two of these (Genoa River and Glen Isla) come from Australia, while the other two (Tarbat Ness and Valentia Island) comes from the British Islands.
Genoa River (New South Wales, Australia)
Three trackways have been reported from the Genoa River Beds of New South Wales, Australia. Two of these, Trackways I & II, were evidently created by tetrapods, but the creator of the third is much less clear. James Warren and Norman Wakefield originally concluded that these trackways were probably early Late Devonian (Frasnian) in age, but Gavin Young later reported that they were clearly younger (late Late Devonian or Famennian in age).
Trackway I, which is 1.1 m long, has the clearest and best-preserved impressions of any of the Devonian trackways. Some of the tracks even have indications of digits. Warren and Wakefield reported that the pes had a total of five digits, but Jenny Clack concluded that they had at least five and possibly more digits. This trackway shows a pattern of alternating pairs of what are probably manus and pes impressions that one would expect for tetrapods. Unlike in the other trackways, however, there is a partial overlap of the pes impression onto that of the manus. (This kind of overlap is not unusual for tetrapod tracks.) There are also no tail- nor belly-drag marks. Interestingly, the digits were oriented laterally and away from the body. In contrast, those of more derived tetrapods (i.e., Carboniferous to modern taxa) are generally oriented forwards which helps maximize backward thrust.
Trackway II, which is adjacent to and on the same bedding plane as Trackway I, differs considerably from its neighbor. The tracks consist of alternating manus and pes impressions and there is a sinuous mark that probably resulted from a tail or belly drag. The impressions are indistinct and distorted along the line of the track. Moreover, the stride-length is longer relative to the pace-width than it is in Trackway I. Differences in the stride-lengths and distorted pes impressions could have been caused either by faster moving animal or the backward displacement of softer substrates.
James Warren and Norman Wakefield concluded that both of these trackways were made subaerially. However, well-preserved tracks can form subaqueously. Moreover, the absence of tail or body drag marks in Trackway I suggest that the tail and possibly trunk were supported in the water. On the other hand, the distorted tracks and drag marks in Trackway II suggest that the tetrapods body was not completely supported by water.
The best-preserved pes impressions in Trackway I were 3.5 cm wide and the tetrapod that made these tracks is estimated to be 55 cm long. No estimate for the size of the animal was published.
Glen Isla (Victoria, Australia)
The Glen Isla trackway is perhaps the most controversial, in large part because of its reported age. The trackways were found on a block formerly used as a paving stone. As such, dating the rock is problematic. Even the age of the presumed source quarry had proved difficult to determine, with ages ranging from the Late Silurian to the Early Carboniferous; the most recent estimates range from late Middle Devonian (Givetian) to the earliest Late Devonian (basal Frasnian). Essentially, the date will remain unresolved until there is more information about the specimens origin.
The trackway is about 1.5 m long and consist of two parallel lines of indistinct prints. The two lines contain aligned rather than alternating prints of variable shapes and sizes; manus and pes prints cannot be differentiated. In addition, the pace-width is relatively narrow compared to the stride-length. There is also no indication of tail nor body drag. In other words, this trackway lacks many of the characteristics one would expect from one made by a tetrapod. However, given our limited knowledge of Devonian tetrapods, its not entirely sure what we should expect.
The paving stone in which the footprints are preserved was a ripple-marked sandstone, with the prints occurring as natural casts (i.e., mounds rather than impressions). In other words, it probably formed as a layer of sand that covered the original sediments and infilled the footprints. Jenny Clack reported that the original specimen was not available for study and that sediment size could not be deduced from a fiberglass replica. It can't be determined whether this trackway was made subaqueously or subaerially. It also appears that any future analysis of this trackway may prove frustratingly limited.
Tarbat Ness (Scotland)
This trackway consist of one short sequence of two parallel rows of impressions and three other impressions which are presumably part of another trackway. The main trackway consists of impressions of varying shapes and sizes that could plausibly be interpreted as alternating manus and pes tracks. There is some evidence, however, that these tracks were not made by a tetrapod. They vaguely resemble large arthropod tracks reported from the same locality. The size of each imprint is about 10 cm.
Unlike the other trackways, there is some evidence suggesting that this one was formed subaerially. The surface is marked by probable rain marks while the bedding plane is overlain by a layer of fine mud that may have been aeolin (wind-blown). Unfortunately, dating is poorly resolved. The age of this trackway probably ranges between late Middle Devonian (Givetian) and Lower Carboniferous (Tournaisian).
Valentia Island (Ireland)
The Valentia trackways, which were discovered by Iwan Stössel in 1994, are the most extensive of the Devonian trackways. They consist of at least five separate, but adjacent trackways, the longest of which is 15 m. The trackways generally show alternating but non-overlapping pairs of prints suggesting both manus and pes impressions. Some of the trackways also exhibited what appears to be slightly undulating belly- and tail-drag marks. No details from the footprints are preserved. Judging from the size and spacing of the tracks Stössel concluded that the animal was about 1 m in length.
Stössel originally interpreted the age of the trackways to be somewhere between middle Middle Devonian (Eifelian) and late Late Devonian (Famennian), but he more recently concluded that they were formed sometime in the late Middle Devonian (Givetian) or early Late Devonian (Frasnian). The sediments from which the tracks were found formed a ripple-marked bedding plane that were probably laid down during flooding. The tracks were probably formed either on newly exposed deposits or submerged under shallow water.
- The Irish Scientist's web page on the Valentia trackways:
- Clack, J.A. 2002. Gaining Ground: The Origin and Early Evolution of Tetrapods. Bloomington: Indiana Univ. Press.
- National Geographic. May 1999. "From Fins to Feet."
- Scientific Papers:
- Clack, J.A. 1997. "Devonian tetrapod trackways and trackmakers: a review of the fossils and footprints". Paleogeography, Paleoclimatology, Paleoecology 130: 227-250.
- Stössel, I. 2000. "Early tetrapods: trackways as controversial contribution." Viert. Natur. Gesell. Zurich 145(1): 31-40.
- Warren, A. A., R. Jupp, and R. Boulton. 1986. "Earliest tetrapod trackway." Alcheringa 10: 183-196.
- Warren, J.W. and N. A. Wakefeld, 1972. "Trackways of tetrapods from the Upper Devonian of Victoria, Australia." Nature 238: 469-470.