Fossil tracks on a rock surface in South Africa offer amazing insights into life 250 million years ago: the imprints preserved in the rock show how large predatory amphibians from the late Permian moved. Accordingly, the so-called rhinesuchid temnospondyls swam in a similar way to modern-day crocodiles by drawing up their legs and using their tails for propulsion. The traces could be evaluated using modern 3D scans, which enabled a detailed analysis of the imprints, which were more than one meter long.
In the late Permian, around 250 million years ago, large, predatory amphibians, the so-called temnospondyls, were among the top predators in the freshwater habitats of the continents. An important site is the Karoo Basin in South Africa. There the temnospondyles were represented by the Rhinesuchidae family. Skeleton finds suggest that some representatives could be several meters tall and that their physique resembled that of modern-day crocodiles or large salamanders. Until now, however, little was known about their movement patterns.
New methods required
"Most assumptions about the behavior of the Rhinesuchidae are based on interpretations of their skeletal form," reports a team led by David Groenewald from the University of the Witwatersrand in Johannesburg in South Africa. But such estimates based on the bone structure leave many questions unanswered. Groenewald and his team have now clarified some of them using rare trace fossils. These are petrified imprints in the subsoil that prehistoric predators left more than 250 million years ago at what was then the bottom of a vast lake.
The tracks were discovered several years ago in an area of the Karoo Basin known as the Dave Green Paleo-Area after its discoverer. "Although the unique morphology of these tracks and the importance of the site have been recognized, neither the site nor the tracks have been fully described. This is mainly because the traditional casting methods have not been successful due to the size of the molds, which is more than one meter and the shallow depth of less than five millimetres," explains the team.
Winding swim tracks
Therefore, instead of traditional casts, Groenewald and his team used modern 3D scans and aerial photographs, with which they were able to record the imprints in the rock in high resolution and then analyze them. "The most conspicuous tracks are seven large body imprints and associated swimming tracks, which we assign based on our data to a medium-sized rhinesuchid temnospondylus with a body length of around 1.90 meters," reports the team.
According to the researchers, the spatial arrangement of the tracks indicates that they originate from one or two individuals that swam from one resting place to the other. The tracks also shed light on how they did this: "The tortuous shape of some tracks suggests that the maker of the tracks swam with a continuous tail propulsion, moving the tail from side to side," the authors say . Since there were no footprints next to the swimming tracks, Groenewald and his team assume that the Rhinesuchidae pressed their legs close to their bodies when swimming - similar to today's crocodiles. "These tracks provide evidence of an active lifestyle involving swimming and bottom-walking behavior in rhinesuchid amphibians, possibly foraging or hunting," explains the team.
Other smaller trace fossils at the site come from other quadrupeds as well as fish and invertebrates. 250 million years ago, along with the Rhinesuchidae, they populated the shallow sandy lagoons and mudflat-like landscapes that then dominated the area of what is now the Dave Green Palaeoplane. "The remarkable tracks and tracks preserved at the Dave Green paleosurface are a window into the Karoo Basin shoreline some 255 million years ago and provide direct evidence of how animals of the time moved and interacted with their environment ' the researchers said. "The results of the study help fill in gaps in our knowledge of these ancient animals."
Source: David Groenewald (University of the Witwatersrand, Johannesburg, South Africa) et al., PLoS ONE, doi: 10.1371/journal.pone.0282354