The Triassic was one of the strangest times in the history of the planet. Rebounding from the worst mass extinction of all time, life flourished into startling new varieties, including the first dinosaurs, weird marine reptiles, and croc-line critters that came in forms like “armadillodiles” and huge, jagged-toothed carnivores, to name just a few Triassic stars. But the strange nature of the Triassic extends beyond the odd anatomy of the creatures that evolved during the period.
While paleontologists are still piecing together the details of Triassic life for the early parts of the period, researchers know that the landscapes of the Late Triassic were dominated by carnivores. At many classic Late Triassic localities – such as those in Petrified Forest National Park and Ghost Ranch, New Mexico – flesh-eaters outnumber herbivores in both abundance and species diversity. This doesn’t fit with the classic ecosystem pyramid we learn in grade school, with a greater number of herbivores providing fodder for a small number of carnivores. No, something strange was going on during the Late Triassic, and a pair of damaged leg bones may hint at why the Late Triassic held an embarrassment of predators.
The two bones, studied by paleontologists Stephanie Drumheller, Michelle Stocker, and Sterling Nesbitt, were found in different localities at different times. But they have three important features in common. They both belong to large Late Triassic carnivores called paracrocodylomorphs, both are upper leg bones called femora, and both are pocked by bitemarks from different carnivores.
There isn’t any animal quite like a paracrocodylomorph alive today. These were archaic cousins of today’s alligators on crocodiles, but vastly different in shape, with legs held beneath their bodies and boxy skulls full of blade-like teeth. Their ranks included the largest terrestrial predators of their time, but, as the bite marks attest, paracrocs could still end up as nourishment for their carnivorous neighbors.
The smaller of the two bones was found in the 218 million year old rock around Fort Wingate, New Mexico. Exactly what species the bone represents isn’t clear, but another animal chomped onto the paracroc’s upper leg around the time of the animal’s death.
The larger femur tells a more complex tale. Found somewhere in the vicinity of Ghost Ranch, New Mexico, this bone came from an approximately 27-foot-long paracroc that was savagely bitten on the upper leg. Healed bite wounds and an embedded tooth, itself surrounded by repaired tissue, show that the victim survived the assault, but, in time, the reptile still became a meal. A second set of bite marks on the same bone show no indication of healing – the sign of a successful predator or a snacking scavenger.
What was attacking and eating these apex predators? The broken tooth tip identifies the initial attacker of the larger paracroc as a phytosaur – one of the long-snouted, crocodile-like ambush predators that inhabited the rivers and lakes of the Late Triassic. In particular, the attacker was likely smaller than the victim. Drumheller, Stocker, and Nesbit estimate the biting phytosaur was about 20-feet-long. And while whatever bit the smaller paracroc isn’t as clear, as several Triassic carnivores could have left such marks, phytosaurs are among the possible culprits.
Petrified punctures can only take us so far in reconstructing these scenes. The unhealed bite marks could have been made by carnivores that killed the paracrocs, or by other individuals who came to feed on the carrion and wrench the meaty legs off the carcasses. And even in the case of the healed wounds, it’s impossible to say whether the phytosaur was trying to prey on the paracroc, the paracroc botched an attempt to feed on the phytosaur, or the two got into a fight for some other reason.
All the same, the two battered bones definitively show that Triassic bodies did not go to waste. More than that, the researchers behind the study note, the bones show a connection between the terrestrial and aquatic realms. It wasn’t as if the paracrocs ruled the land and the phytosaurs kept to themselves in the water. Fresh meat was exchanged across the boundary of the terrestrial and aquatic realms, with phytosaurs perhaps being both predators and part of the carcass clean-up crew. In short, the Late Triassic was a carnivore-eat-carnivore world.
Drumheller, S., Stocker, M., Nesbitt, S. 2014. Direct evidence of trophic interactions among apex predators in the Late Triassic of western North America. Naturwissenschaften. doi: 10.1007/s00114-014-1238-3