Coelacanths are famous for being “living fossils”, completely disappearing from the fossil record at the end of the Cretaceous but then being rediscovered alive just 80 years ago. But although they’re often thought to have physically changed very little over the last 300 million years or so, more recent discoveries are starting to show that coelacanth body forms and lifestyles were actually more varied in the distant past.
Meet the wonderfully-named Rebellatrix divaricerca, from the Early Triassic of British Columbia, Canada (~251-247 mya). Measuring around 1.3m long (4′3″), its body shape and large symmetrical forked tail suggest it was adapted for fast swimming. Unlike its slow-moving deep-water modern relatives this coelacanth was a speedy oceanic active predator, convergently similar to tuna or some sharks.
Since it lived in the immediate wake of the end-Permian “Great Dying” mass extinction, Rebellatrix may have rapidly evolved from more standard-looking coelacanths to take advantage of a suddenly vacant ecological niche – or it might be part of a more extensive unusual lineage whose other members simply haven’t been discovered yet.
Originally known only from a few fragments, this 2.5-3m long (8′2″-9′10″) animal was first considered to be an indeterminate early archosaur, then a non-archosaurian doswelliid. But new fossil material and a recent analysis have instead placed it as a member of the erpetosuchids, an early group of pseudosuchians (the branch of the archosaurs that includes modern crocodilians).
Erpetosuchids were some of the earliest well-armored archosaurs, with several rows of bony osteoderms along their neck, back, and tail, and scattered oval osteoderms covering their limbs. Their fairly gracile build and slender limbs suggest they were active terrestrial carnivores – but it’s hard to say exactly what they were preying on due to their somewhat odd skulls.
They had only a few teeth at the very front of their upper jaws, with the rest being toothless, but meanwhile the lower jaw was fully-toothed. Their skulls had narrow snouts at the front but became much wider further back, suggesting the presence of powerful jaw muscles, and they had slightly upward-facing eye sockets.
Smaller erpetosuchids are speculated to have been specialized for insect-eating, catching their small prey with their front teeth and then crushing it with the semi-toothless part of their jaws further back. But something the size of Tarjadia probably couldn’t have survived on a purely insectivorous diet, and it must have been doing something else with its weird jaws.
Nicrosaurus kapffi from the Late Triassic of Germany, about 221-205 million years ago. Although rather crocodile-like in appearance, this 4-6m long (13′-19′8″) animal was actually part of an extinct group called phytosaurs – long-snouted heavily-armored reptiles with their nostrils high up on their heads near their eyes.
Phytosaurs’ exact evolutionary relationships are still disputed, with opinions currently going back and forth between them being archosauriformes or an early branch of the croc lineage within the true archosaurs. But either way they weren’t directly ancestral to modern crocodilians, and instead developed a very similar body plan via convergent evolution.
While some phytosaurs had very slender gharial-like snouts and probably fed mostly on fish, others like Nicrosaurus had much more robust jaws and seem to have secondarily adapted to a terrestrial predator lifestyle. They had longer limbs and a more upright posture than their semi-aquatic relatives, and enlarged fangs at the hooked tips of their jaws that may have been used to deliver a powerful stabbing blow to their prey.
Its short toothless snout suggests it was a suction feeder, using water pressure differences to pull small soft-bodied prey straight into its mouth like a syringe. Along with a heavily built body similar to those of hupehsuchians, and a very long tail that made up over half of its 1.6m length (5′3″), it was probably a fairly slow swimmer living in shallow coastal waters.
It was a close relative of Cartorhynchus, and may have been similarly capable of hauling itself onto land like a modern pinniped.