An 80-million-year-old embryo has revealed some unexpected features of the most titanic animals to ever have walked the Earth.

Sauropods, such as Brontosauruses, which lumbered their way through the Jurassic and Cretaceous ages, are well known for their impressive size and long necks. But very little is known about their young.

"The preservation of embryonic dinosaurs preserved inside their eggs is extremely rare," explained palaeontologist John Nudds from the the University of Manchester in England.

That rarity makes for some interesting discoveries when these ancient embryonic remains are analysed.

"A horned faced and binocular vision are features quite different from what we expected in titanosaurian dinosaurs," said palaeobiologist Martin Kundrát from the Pavol Jozef Šafárik University in Slovakia.

The first unequivocal remains of unhatched baby sauropods were found in a sedimentary deposit called Auca Mahuevo, in the badlands of the Patagonian province of Neuquén, Argentina.

The fossilised egg behind this new discovery only recently came to researchers' attention, however, and in the US. It's thought the egg was likely removed from Patagonia illegally at some point in the past.

The remains have now been repatriated to the Museo Municipal Carmen Funes in Plaza Huincul, Argentina, but the removal means we don't know precisely where it came from in Patagonia originally.

While Auca Mahuevo would be a good guess, the geochemistry of its mineral makeup isn't a match. Still, it's a remarkable find, with unique features not seen in any adult sauropod fossil.

Using a new imaging technology called synchrotron microtomograpy, the researchers were able to produce highly detailed 3D images of the unborn sauropod dinosaur skull, complete with tiny teeth.

"Part of the skull of these embryonic sauropods was extended into an elongated snout or horn, so that they possessed a peculiarly shaped face," said Nudds.

Its eye sockets were angled forward more than seen in adult specimens, leading the team to conclude in their paper that "early juveniles of titanosaurian sauropods might benefit from a temporary ability of at least a partial binocular vision that would provide a much better visual perception".

This might have provided tiny hatchlings with the ability to better judge distances to food or prey, or possibly spot camouflaged predators. While adult sauropods may be famously giant - some possibly over 50 metres long - this embryo is only centimetres long.

Comparing its bone structures to other embryonic developmental histories suggested it had completed about 75 percent of its total egg incubation period - but how many days this would translate to is a complete mystery for these ancient beasties.

The bones of this baby's skull also had more calcium in them then seen in other dinosaur embryos, suggesting this youngster was already making use of these nutrients from its eggshell.

While the most obvious possible use of a bony horn structure would be as an egg tooth, to help them crack through the protective incubation casing of the egg, the researchers are not sure that this is what the horn was for.

Although they can't be certain of the embryo's exact position within the egg, what we know from living reptiles suggests the horn is in the wrong position to be used as an egg tooth.

Also, egg teeth are usually shed soon after hatching, but as this horn has a bone structure within the animal, it would likely persist some time after the young dinosaur emerged into the world, the researchers think.

"Because it differs in facial anatomy and size from the sauropod embryos of Auca Mahuevo, we cannot rule out that it may represent a new titanosaurian dinosaur," said Kundrat.

Their research was published in Current Biology.