by Greg Mayer
Update: An alert reader, has objected to the theory presented below, or at least the specific evidence used; he has proffered what he contends is “much more pertinent evidence”, which I append below.
Jerry posted a couple of days ago on a specimen of an early tetrapod, Ossinodus, which seems to have had a partially healed injury to the radius of its right forearm. The authors who described the injured specimen interpreted the injury as a fracture that could only have occurred on land, arguing that Ossinodus therefore is the oldest tetrapod that can confidently be said to be terrestrial. (The first tetrapods, from the upper Devonian, are considerably older than Ossinodus, which is from the following Mississippian subperiod of the Carboniferous; but these earlier tetrapods, which had caudal fins and functional gills, may not have been terrestrial.) Ossinodus is thus potentially an important point in the transition from fishes to amphibians.
Another major transition in the history of vertebrate life was that from reptiles to mammals, which we have discussed here before at WEIT. As important as the morphological changes which can be seen in the fossils, are the changes in ecology and behavior, which, along with environmental changes, lead to changes in the extent to which one group or another dominates the ecosystems of its time. Although mammals originated in the mid-Mesozoic era, it was not until the Cenozoic (colloquially known as the “Age of Mammals”) that the mammals became the dominant terrestrial vertebrates. Most ideas on the rise of mammals to ecological dominance focus on the fate of dinosaurs and other large reptiles at the end of the Cretaceous, when the disappearance of the latter may have been caused or accelerated by the impact of an extraterrestrial body. I was recently forwarded another theory, visually expressed, about how it was that mammals replaced reptiles as the dominant land animals on Earth.
And now, the more pertinent evidence:
I will allow as the mammal in the new evidence does seem to have a more dominant position over the reptile.
h/t: C. Mayer, J.B. Losos