Argentina is a hot bed for exciting fossil discoveries these days, and the latest find comes in the shape of an unusual Jurassic dinosaur whose guts have revealed the secrets of prehistoric food webs.

Named Isaberrysaura mollensis, the plant-eating species measured in at 5-6 metres (16-20 feet) in length, and it trudged among river deltas some 180 million years ago in what is now the Neuquén Province. But the really exciting find was inside the belly of the beast: a chunk of fossilised gut contents. Known as a cololite, it's food that an animal wasn't quite done digesting when it died (not to be confused with a coprolite, which is food an animal was well and truly done with).

The location of the fossilised gut contents inside Isaberrysaura mollensis. Image: Salgado et al., 2017

Isaberrysaura is unusual for another reason, too: while it appears to be an early cousin of the famous duck-billed hadrosaurs that roamed in herds across the world of the Cretaceous (millions of years later), features of its skull – particularly the simple teeth and weak jaws – bring to mind dinosaurs like the plated Stegosaurus.

And those similarities with stegosaurs may have to do with diet. While the hadrosaurs had famously complex teeth, great for chewing and grinding plants into paste, the chompers of Isaberrysaura and the stegosaurs seem better suited to minimal mastication.

But deciphering the ins and outs of dino diets can be a challenge, and that's where the cololite comes in handy: it provides rare evidence of Isaberrysaura's food preferences. Inside the dinosaur's belly were mineralised seeds, the leftovers of its last meal. There were at least a couple of different types of seeds in there, but the largest ones have been identified as belonging to plants called cycads.

The cycad seeds, marked (c), along with other seeds (s), are evidence of Isaberrysaura's last meal. The dinosaur's rib is marked (r). Image: Salgado et al., 2017

This very ancient group of plants is still hanging on today, but back in the Jurassic, cycads were some of the most successful plants around. The seeds inside Isaberrysaura are mostly undamaged, lending support to the idea that this dinosaur didn't do much chewing. But there's more: these leftovers also offer clues to a tight plant-animal relationship spanning hundreds of millions of years.

Cycad seeds are surrounded by a soft, tasty coating, but the kernel at the core is very hard-shelled and loaded with a toxic substance called cycasin. A strong-jawed animal might crush the seed completely and release those toxins, but Isaberrysaura can get the tasty bits without breaking into the kernels, allowing the seeds to pass all the way through the digestive tract and be deposited elsewhere in a rich pile of fertiliser. The dinosaur gets food, and the plant gets to spread its seeds. Everybody wins!

With this in mind, it probably isn't a coincidence that the evolutionary rise of large herbivorous dinosaurs coincided with the rise of cycads – these organisms developed a mutually beneficial relationship over the millennia.

Much later, when the super-chewing hadrosaurs – those later cousins of Isaberrysaura – spread across the world in the Cretaceous Period, toxic seeds would have been a less appealing dining choice. This may have marked the beginning of the cycads' downfall, especially since fruit-bearing angiosperm plants were becoming more and more popular, providing a tasty (and safe!) alternative food source.

But this single cololite offers only a glimpse into the life of Isaberrysaura, not the whole picture. "[W]e still lack a clear idea of the diet of these dinosaurs beyond their ingestion of cycad seeds and other seed plants," write the authors of a recent study describing the new dinosaur. The other dietary habits of the species remain a mystery for now.

With its weak jaws and seed-swallowing ways, Isaberrysaura may have been part of a long-running tradition of cycad-dinosaur mutualism, although the relationship isn't totally a thing of the past. Certain birds – modern remnants of the dinosaur dynasty – have been seen going after the seeds of modern-day cycads! Some things never change.


Top header image: daniel/Flickr