The Snowy Plains Formation Trackways, Berrepit, Australia: a seismically important epilogue of early tetrapod and amniote evolution
The three sets of tracks in the study have clear footprints with indentations from claws, a feature of reptiles but not amphibians. The hooked claws on the trackways indicate they are a reptile-like animal, says Long.
The sandstone block is part of a larger structure that had been dated to the early Carboniferous based on evidence from the time period. Fossilized tracks of aquatic invertebrates and fish found in the same layer were also dated to this time period.
The claw tracks were found in a sandstone block on the bank of the Broken River at Barjarg in the state of Victoria, by two co-authors of the paper who are not professional scientists. Berrepit is a name given to this part of the river by the Indigenous Taungurung people.
The Snowy Plains Formation trackways have a disproportionate impact on our understanding of early tetrapod evolution because of their combination of diagnostic amniote characteristics and early, securely constrained date. They demonstrate, once more, the extraordinary importance of happenstance and serendipity in the study of severely under-sampled parts of the fossil record. Against this background, two things stand out: first, that the interpretation of such a fossil record is critically dependent on phylogenetic inferences and cannot be ‘read’ as a literal account of the history of a group; and second, the fundamental, continuing importance of palaeontological fieldwork as a source of new knowledge.
A substantial time interval between the tetrapod and amniote crown-group nodes is a universal feature of recent molecular phylogenies in which both nodes are defined (Supplementary Information Part 1); the median age difference between these nodes in the calibrated phylogenies curated by TimeTree is 33 million years. These age differences are inferred from branch lengths that in turn reflect base substitutions recorded in the genomes of extant animals, and are thus not susceptible to the distorting effects of gaps in the fossil record. Even though the exact ages vary between phylogenies, depending both on the fossil calibrations and phylogenetic algorithms used, the substantial age separation between the amniote and tetrapod crown-group nodes is consistent and must be real; these cladogenetic events were separated by tens of millions of years.
Eggs were encased in anamniotic membrane that stopped them drying out, which is what led to the evolution of amniotes. For about 319 million years before this study, the earliest known amniote fossils had been found in Nova Scotia, Canada. The early Carboniferous period is believed to have had amniotes around 355 million years ago.
By contrast, the Snowy Plains Formation trackways do cast substantial new light on the effect of the end-Devonian mass extinction event on tetrapod evolution. Romer’s gap, a hiatus of 20 million years, was visible in the fossil record until recently. The pre-gap and post-gap tetrapods appeared substantially different in character, with the post-gap forms showing much higher diversity and disparity, as well as being more advanced and including crown-group tetrapods in their ranks10,37. The extinction event gave rise to the idea that there was a resetting of the evolution of tetrapods, allowing the emergence of more modern groups. It was linked to terrestrialization after a supposed aquatic phase of evolution in the period of theDevonian10,37. This somewhat simplistic conception of Romer’s gap and its relationship to tetrapod evolutionary history can now be replaced by a more nuanced interpretation. The presence of sauropsid tracks in the early Tour is believed to indicate that the tetrapod crown-group radiation was well under way, as well as other lineages such as seymouriamorphs and diadectomorphs. If this is correct, the mass extinction did not have a role in the emergence of these derived lineages, although it is still possible that the amniote crown group arose in its immediate aftermath. The impact of the extinction on diversity is more difficult to assess, and may have been substantial. None of the known Devonian tetrapods seem to be a crownward segment of the stem. This suggests a selective extinction with appreciable effects on ecosystem structure.
