Unusual Goings-On in the Triassic

A typical Triassic ecosystem. The large predator in the back is the crocodile ancestor Postosuchus, who massively outweighs the dinosaur Coelophysis, seen cowering under a log along with two plant-eating dicynodont protomammals.
ABelov2014 (CC BY-SA 3.0)

When we think of the age of dinosaurs, we usually think of three periods - Triassic, Jurassic and Cretaceous, spanning from about 250 million years ago to 66 million years ago when a giant asteroid put an end to their reign over Earth's terrestrial ecosystems.

However, those more versed in palaeontology will know that this is an oversimplification. This is because during the Triassic, dinosaurs were definitely not the top dogs of the ancient ecosystem. At the time, the only large dinosaurs were plant-eating sauropodomorphs, who competed for vegetation with various other types of herbivorous reptile like the beaked rhynchosaurs and crocodile-like aetosaurs, along with the tusked dicynodonts, which were distant relatives of mammals. The predatory dinosaurs were in an even more unsecure position than the herbivores, as there was no ecosystem on Earth where they reigned supreme. The largest meat-eaters around were the rauisuchians, a collection of active, powerful carnivorous reptiles who ultimately gave rise to today's crocodiles. The largest rauisuchians, such as Saurosuchus and Fasolasuchus, could get eight metres long and weighed over a tonne, making them the largest non-dinosaur predators ever to walk the Earth. They kept predatory dinosaurs small and unimpressive until they were wiped out by climate change at the end of the Triassic, freeing up space for their distant dinosaurian cousins to take the seat of apex predator.

The Newtonsaurus specimen.
Evans et al. (2025) (CC BY 4.0)

But very recently, a few new fossils have emerged that may challenge the rauisuchians' exclusive claim to the Triassic's apex carnivore role. Meet Newtonsaurus cambrensis, a fossil from the latest Triassic of Wales discovered all the way back in 1899 and languished in a museum collection for over a century before coming into the light.

Objectively speaking, Newtonsaurus is a pretty garbage specimen. The fossil is a wraparound impression of a single jawbone with some teeth in it which resembles that of the large Jurassic theropod Dilophosaurus. Now Dilophosaurus was about seven metres long and half a tonne in weight, making it something even the largest rauisuchians couldn't take lightly. If Newtonsaurus was indeed of similar size, then it would tell us that at least some predatory dinosaurs did manage to carve out a role at the top of some Triassic ecosystems in spite of rauisuchian competition.

Unfortunately, the authors of the new paper weren't able to say anything too defintive about Newtonsaurus. The fossil is just too fragmentary, and though it superficially looks a lot like a dinosaur jaw we can't say for sure since it doesn't preserve any of the key anatomical traits which unite early dinosaurs. While the authors think it's unlikely, it is still possible that Newtonsaurus was in fact a rauisuchian with a dinosaur-like head.

Fortunately, we may not have to rely on Newtonsaurus to inform our view of predatory dinosaur size in the Triassic. Two other Triassic dinosaurs, Liliensternus and Gojirasaurus, are known from five to six-metre specimens which seem to have been juvenile. Extremely fragmentary remains and occasional footprints suggest that both could have gotten upwards of 7-9 metres long, which would make them on par with Dilophosaurus (though they were more lightly built). The largest specimens of the primitive predatory dinosaur Herrerasaurus got up to six metres, and these are not merely fragments but entire skulls and necks. Herrerasaurus lived in a rich ecosystem filled with early dinosaurs and other types of reptile, but even at its size it would still have had to live in fear of the resident giant rauisuchian Saurosuchus.

Regardless of these rare cases, it is clear that dinosaurs were not a smashing success from the get-go. But after playing understudy for thirty million years, their luck finally took a turn as the major players who kept them down bit the dust, too specialised to cope with the changing conditions at the end of the Triassic. So keep your eyes open. You never know when it'll be your big breakthrough.