|Bigfoot: giant tetrapod footprints|
Korolki Ravine and the Permo-Triassic boundaryDuring the 2004 expedition, we kept ever alert for fossil bones, but did not find much other than isolated jaw bones and teeth in channel lag deposits. Then we found some giant footprints in the Korolki Ravine, at a locality called Boyevaya Gora, 14 km NNW of Sol-Iletsk, on the Asiatic side of the Ural River, and close to the border between Russia and Kazakhstan. The ravine cuts through a fine section of the Permian to Triassic transition (PTB).
In the Korolki Ravine, the Kulchomovskaya Svita (latest Permian) lies below the ledge, and the
Kopanskaya Svita (basalmost Triassic) above. Members of the 2004 expedition (left to right),
Mikhail Surkov, Michael Benton and Valentin Tverdokhlebov inspect the sandstone lying right at the
Left. Sedimentary succession exposed at the Korolki Ravine locality, in a tributary on the northern flank of the Ilek River. Base of section at 51 29805 N, 54 90949 E. Grain sizes indicated from left to right are clay (c), silt (s), fine (f), medium (m), coarse (c) sand, gravel (g). Sedimentary log by Richard Twitchett and Andy Newell.
Careful logging by the Russian and British geologists showed that the footprints occurred in the Vyatskian zone, some 50 m below the PTB. The footprints were emplaced in a reddish-brown mudstone deposited from suspension beneath shallow ponded water in a floodplain environment. The footprints were subsequently cast by the base of the overlying fine-grained sandstone, which was deposited from a sheet flood event.
In more detail, approximately 124 m of mudstones, siltstones, sandstones and conglomerates are exposed in this section (Figure left). The local PTB is located 94 m above the base of the measured section, at an abrupt facies change from red mudstones with well developed caliche to trough cross-bedded pebbly sandstones and conglomerates. These latter beds have yielded Early Triassic Tupilakosaurus remains, a left angular of Wetlugasaurus samarensis (?), and limb and dermal bones of indeterminate Temnospondyli. The mudstones a few metres beneath the boundary contain the Vyatkian ostracods Volganella magna, V. ex. gr. laevigata, Wjatkellina fragilina, Gerdalia sp., Suchonellina inornata var. macra, S. inornata var. magna, S. parallela, S. parallela var. typica, S. futschiki and Suchonella typica. The youngest Permian vertebrate remains are found 22 m beneath the PTB, and comprise tetrapods (Scutosaurus sp., Karpinskiosauridae gen. indet., Chroniosuchus paradoxus, Theriodontia fam. indet.) and fishes (Isadia aristoviensis, Toyemia blumentalis, Mutovinia stella, Saurichthys sp., Gnathoriza sp.).
The sedimentary succession at the track locality is dominated by red mudstones and sandstones. The mudstones are generally massive and often contain root traces and weakly developed palaeosols with calcrete nodules. The associated sandstones are generally less than 0.5 m thick, have sharp erosive bases, and are cross-bedded or ripple cross-laminated with bioturbated tops. The red colouration of the mudstone and the presence of rootlets and palaeosol horizons with calcrete probably indicate a continental setting with a semi-arid to subhumid climate. The mudstones were probably deposited from suspension in shallow ephemeral lakes (Tverdokhlebov et al. 2005) and on floodplains adjacent to broad, shallow river channels, represented by thin, erosive-based sandstones. Cross-bedding indicates that the flow direction was towards the west. Recession of the flood water allowed plant colonization of the muds and longer subaerial exposure led to the development of palaeosol horizons with calcrete nodules. Overall, the range of facies is similar to that described by Newell et al. (1999) from Vyatkian deposits 75 km to the north-east, and they suggested that the overall depositional system was a fluvial 'terminal fan' characterized by a network of shallow channels ending in a mud-dominated flood basin.
'Bottoms up': classic view of geologists at work, in the Korolki Ravine; Richard Twitchett (foregound), Andy Newell (right), and Cindy Looy (background).
The discoveryOne day during our five days at the site, Richard Twitchett spotted a discarded block in the ravine that appeared to have three rather pointed impressions radiating from the centre (Photograph below left). The block was perhaps 30 cm across. Then, just behind it, in the face of a step in the bottom of the ravine, he saw a curved depression at the base of a sandstone bed (Photograph below right). This could have been a small channel or a load cast, but we decided to investigate further. We took the loose block back to camp, and showed it to our Russian colleagues.
At first, our Russian colleagues were unimpressed, but agreed to walk down to the site. Richard was convinced the two blocks were fossilized footprints of some large animal. Valentin Tverdokhleobov instructed Sasha and Edvard, two students with the expedition, to work at the sandstone bed, and they turned over slab after slab, which we fitted together on the step above (Photographs below). The impressions were so huge, from 50 cm to 1 m across, and so mixed up that it was hard to make out what we had found. After ten minutes of hard work, everyone present agreed that we had, indeed, found some massive footprints of a five-toed animal, and that there were several intersecting tracks criss-crossing the area we had exposed. Valentin immediately christened the beast who had made the tracks 'bigfoot'.
Seventeen prints in all were observed. We could have gone on turning slabs for some time, but there would have been no point. Our Russian colleagues loaded up the best examples to add to the museum collection at Saratov State University.
The footprint horizon is located 45 m above the base of the measured section (Section, above). The footprints occur in the upper part of a 0.34-m-thick reddish-brown mudstone bed that has a blocky texture and lacks root traces (Photograph above). A grey, fine-grained sandstone immediately overlies this mudstone, and natural sandstone casts of the footprints are preserved in epirelief on the base of this bed. The tabular sandstone body is some 0.4 m thick, although the thickness varies, with a sharp base and a basal lag of mudstone rip-up clasts and isolated calcrete pebbles 10-20 mm in diameter. The sandstone contains current ripple cross-laminae, with a uniform flow direction to the west-south-west (260o).
The landscape may also have sustained some perennial water bodies, indicated by thin (up to 1.5 m) and wide (up to 100 m) lenses of dark grey mudstone, siltstone and sandstone with abundant plant remains in nearby sites. The mudstone bed containing the impressed footprints overlies a pinkish-grey, cemented palaeosol containing abundant root traces, and is interpreted as the uppermost unit of a heterolithic channel fill. The well-preserved footprint casts with detailed hoof scratches would have required a firm, but not waterlogged, substrate and were probably emplaced on an emergent, but not desiccated, surface. Given the consistent palaeocurrent directions above and below the footprint level, it is likely that the animals were moving in a shallow, ENE/ WSW-orientated depression that had recently been flooded.
Who made the footprints?Fossil tracks are remarkably rare in the Late Permian of both Russia and South Africa. In Russia, as far as we know, we had reported the first finds in 1997 (Tverdokhlebov et al. 1997), tracks of a small reptile that Valentin Tverdokhlebov had found some years before near our campsite on the Sakmara at a site called Kulchomovo. A second find of larger tracks of a pareiasaur from the Sukhona river in the northern part of Russia was reported by Gubin et al. (2003). So this was apparently only the third find ever. But it was important to determine the track-maker.
We first compared the tracks with other specimens that had been described from other parts of the world. It seemed clear that our tracks were not a new form, but were more or less identical with Brontopus giganteus described by Heyler & Lessertisseur (1963) from the latest Permian of France. The name was chosen by the French authors to indicate that the maker was ponderous and huge - it means literally 'gigantic thunder foot'.
The prints indicate that the animal was plantigrade or semi-plantigrade (placed the soles of its feet flat on the ground. The hand print has five short stumpy fingers of similar length, while the foot also has five toes, but decreasing in length from the second toe to the fifth. The fingers and toes are broad and end in pointed claw marks. Behind the fingers and toes is an impression of the sole of the foot. The hand prints are 230 mm long and 360 mm wide, the foot prints 175 mm long and 380 mm wide; the stride length is about 1.2 m. Some of the tracks cross over each other, showing that bigfoot had trampled back and forwards (Figure below), and some show clear scratch marks where the claws had slipped down through the mud.
The fact that hands and feet each have five digits shows that the maker was almost certainly a reptile, and not an amphibian (nearly all amphibians had, and have, four fingers on the hand). Among Late Permian reptiles there are three possible candidates: pareiasaurs, dinocephalians or dicynodonts. Heyler & Lessertisseur (1963) were clear that Brontopus was made by a pareiasaur, whereas other authors assigned the track to a 'pelycosaur'. The latter proposal seems unlikely because the 'pelycosaurs' were basal synapsids ('mammal-like reptiles') known from the Late Carboniferous and Early and Middle Permian. They had been extinct for at least 10 myr before the Vyatskian. Pareiasaurs are also ruled out because of the inferred posture of bigfoot. Pareiasaurs had sprawling fore and hind limbs, with their elbows and knees sticking out sideways, whereas the Brontopus tracks indicate an only partially sprawling stance. That leaves dinocephalians and dicynodonts as possible track-makers. Both groups included some members large enough to have made the Brontopus tracks.
The largest Russian herbivorous dinocephalians that could have produced Brontopus-sized footprints are Ulemosaurus and Deuterosaurus, but their skeletal remains are too old, being known only from the upper part of the Urzhumian (Tverdokhlebov et al. 2005). World-wide, dinocephalians had largely disappeared by the end of the Tatarian and mostly by the mid to late Tatarian (Kemp 2005). Dinocephalians are thus rejected from consideration as potential track-makers for the Russian material on stratigraphic grounds.
That leaves dicynodonts. Some of these synapsids with 0.5 m skulls (Rhachiocephalus, Aulacephalodon) are known from the uppermost beds of the Upper Permian in South Africa (Rubidge 1995). Although such truly giant dicynodonts have not been reported from Russia, the genus Vivaxosaurus (Kalandadze & Kurkin 2000) has nearly the same skull size (0.4 m) and is known from the Vyatskian of Russia.
Right: The right hand of the dicynodont Lystrosaurus drawn over the manus (hand) print of Brontopus, to scale, showing that this is probably the print-maker.
As an experiment, we scaled a dicynodont foot skeleton to 'bigfoot' size and found that this fitted more or less perfectly within a Brontopus footprint (Figure to right). Further, dicynodonts had a kind of dual mode of locomotion in which the hind limbs were held in an erect or semi-erect position, and the forelimbs sprawled somewhat, expressed memorably by Kemp (1980), who described the locomotion as akin to a man pushing a wheelbarrow: the hind limbs striding in erect, parasagittal posture and the sprawling forelimbs scrabbling along in front.
So, even though giant skeletons of dicynodonts are not known from the terminal Permian in Russia, the tracks of 'bigfoot' appear to confirm their presence. The track-maker was then a massive herbivore, about the size of a hippopotamus, that scraped up plant material with a pair of massive tusks in its upper jaws and sliced the vegetable matter with horn-covered jaw margins. The footprints are so huge that they cannot be regarded as particularly beautiful fossils, but they have added considerably to knowledge of life just before the PT mass extinction (Surkov et al. 2007).