A Triassic fossil dated to almost 230 million years ago has a very similar skull to dinosaurs that appeared 100 million years later. Even more surprisingly, numerous other species from the same fossil deposit also show features that subsequently vanished before arising again. The resemblance appears to be a result of convergent evolution, where unrelated species produce similar solutions to the same challenges, rather than a result of direct descent.
The Late Triassic Otis Chalk assemblage was laid down 220-228 million years ago in what is now Texas. These deposits include the oldest – and best preserved – specimens of archosauromorphs, pre-dinosaur reptiles, many of which bore considerable resemblances to dinosaurs of the Jurassic and Cretaceous eras.
“We were surprised to find something like Triopticus primus at all!” Dr Michelle Stocker of Virginia Tech said in a statement. “There was nothing else like that known from the Triassic period, and we wouldn’t have guessed that we would find something that looked like that outside of the Cretaceous period. Our team was also surprised that the internal structure of the skull of Triopticus was similar to pachycephalosaur dinosaurs – it wasn’t just an external similarity.”
These internal similarities involve bony partitions, revealed in CT scans. For such a complex trait to evolve not once, but twice, it must have provided some evolutionary advantage. However, Stocker says it is not obvious what this was, particularly given that no known close relative adopted the same structure.
The fact that Triopticus primus coincided, in both space and time, with so many other species that prefigured later developments may indicate that the Triassic was a “time of experimentation,” Stocker said. “Reptiles were diversifying after the end-Permian mass extinction, and this may have been an opportunity for evolution to operate quickly and with few constraints.” Examples include elongated skulls like modern crocodiles and teeth shaped like leaves.
Such periods have been seen before, most famously in the Cambrian fossils trapped in the Burgess Shale where a host of curious body designs appeared. However, where the Burgess fossils reveal curiosities whose like was never seen again, the Triassic experiments proved successful in later eras. Triopticus primus and its fellows may have achieved world domination using this architecture had they not ran into the Triassic-Jurassic extinction event 201 million years ago, an event almost as devastating as the one that caused the extinction of the dinosaurs.
The resemblence in form of some unrelated Triassic and post-Triassic reptiles is remarkable. Stocker et al, Current Biology