July 22 , 2016

by Vasika Udurawane

The aptly-named titanosaurs were the biggest creatures to walk the Earth yet they had to grow up from rather tiny beginnings. From group nesting in Argentina to fast-growing babies that succumbed to Madagascan droughts, these fossils have left their mark and are helping scientists to uncover the secrets of these little giants.

It was recently announced that a new titanosaur fossil had come out of the Late Cretaceous rocks of Madagascar. Paleontologist Kristina Curry Rogers of Macalester College in St. Paul, Minnesota found the remains of a baby Rapetosaurus krausei quite by accident in a museum drawer. The bones were uncovered between 1998 and 2003, and had remained unclassified until Rogers’ rediscovery. She and her colleagues at Stony Brook University began to examine the animal’s bones. Their studies revealed something shocking; not only was the dinosaur very young when it died, but it was also an incredible grower.

The Rapetosaurus juvenile was between 39 and 77 days old when it died in a seasonal drought, and already it weighed 40 kilograms and stood 35 centimeters at the hip. This is as large as a golden retriever, quite a big baby to say the least. Compared to the adults though, this was incredibly small. Rogers’ studies also revealed that it had been ten times lighter when it first hatched, thus concluding that Rapetosaurus had a phenomenal growth rate. The bones in its legs also matched those of the large adults. According to Rogers and team, it meant that the baby titanosaur’s body shape changed very little when it grew up.

This was surprising news, given that the babies of other plant-eating dinosaurs, and dinosaurs in general, looked so different from their parents. Duckbills and horned dinosaurs especially had traditionally “cute” baby features like short heads and massive eyes and were unable to fend for themselves once they hatched. Many went through a so-called growth series, with different body shapes appearing across different ages as the animal aged. So far we have growth stages in many carnivorous and herbivorous dinosaurs while actual juvenile titanosaurs this small were unknown.

Rapetosaurus’ shape at hatching indicates that unlike duckbills, the hatchlings of titanosaurs needed very little parental care. They were precocial babies like those of a modern ostrich, able to run around, play and feed themselves straight after hatching. Named for a mischievous mythical giant in local lore, Rapetosaurus is one of the most completely known titanosaurs. It is known from very well preserved remains, including the skull, which is a rare find for any titanosaurs. It was named and described by Rogers herself in 2001, who estimates the adult at 15 meters long, with its head roughly 8 meters in the air. It was easily the biggest creature in its environment, the seasonal and drought-prone island of Madagascar. For now, it is our best look at how these creatures grew up.

Titanosaurs are the last and among the biggest of the sauropods, the famous long-necked and large-bodied herbivorous dinosaurs. They populated every continent and lived until the end of the dinosaur age, around 66 million years ago. The titanosaurs were very robust and often armored with bony plates and spikes. They were also the biggest animals to ever live on land. The largest of these already gargantuan creatures weighed an excess of 70 tons. Yet all this size came from very small eggs, mostly no larger than a soccer ball. Any larger than this, and the hard shells would have been too thick for the babies to break out of, or even breathe through.

Thanks to several hundred fossilized titanosaur eggs, paleontologists are fairly confident about how titanosaurs bred. The most famous titanosaur nesting site is the Argentinian nesting ground of Auca Mahuevo. This site was uncovered in the Mid to Late Cretaceous Neuquen Group, a long-running series of rock beds that cover a large area of the country’s Patagonia region. Auca Mahuevo was first discovered by paleontologist Luis Chiappe and colleagues in 1998.

The eggs were found in four rock layers, and were spherical structures laid in clutches between 15 and 40. It was thought that the eggs had been laid next to a stream or freshwater channel. In reality though, the eggs had been swamped by episodes of flood, thus leaking in through pores in the shells and suffocating the babies inside.

Other sites in Argentina include one recently discovered in Sanagasta. Here, paleontologists have found a species of titanosaur using a very different nesting strategy from those at Auca Mahuevo, which are essentially group nesters laying in scooped-out nests. It had been depositing its eggs a geothermal area, similar to the places in which geysers are found. Another species in Spain had been actively building nests or nest-like structures out of surrounding vegetation, just like megapodes today. Megapode birds are a family of Australasian gamebirds, known not just for laying in burrows but also for building massive nests of rotting vegetation over their eggs. The vegetation slowly turned to compost through the actions of microorganisms, thus generating heat that the eggshells can absorb.

Thus it is clear that while they did not invest much care in their offspring’s development, titanosaur mothers were certainly doing something right. These selective nesting behaviors certainly varied among species, and many more might yet be discovered. Despite these protective measures, female titanosaurs still had other issues to contend with: Nest predation. In 2010, an astonishing new find from India’s Lameta Formation revealed one such predator. It was not a predatory dinosaur but a primitive snake known as Sanajeh indicus. The snake resembled a modern-day boa constrictor and at 3.5 meters long, was similar in size and lacked venom. Its gape however, was somewhat less than that of present boas and pythons and it could not unhitch its jaws to swallow massive prey.

Sanajeh’s bones were found in the remains of a titanosaur nest containing three crushed eggs, with the fossil of a 50-centimeter hatchling preserved alongside it. It is uncertain as to what genus the titanosaur represents. What is clear though is that the snake had died while in the act of preying on the eggs and possibly the babies of these huge dinosaurs. And there was not just one nest with snake fossils in it. Instead, there are three titanosaur nests, each associated with Sanajeh remains in this one locality. This suggests that not only were these snakes preying on baby titanosaurs, they may also have been specialized raiders that lived in the nesting sites and nests themselves.

References

Hechenleitner et al. (2015) What do giant titanosaur dinosaurs and modern Australasian megapodes have in common?

Curry Rogers et al. (2016) Precocity in a tiny titanosaur from the Cretaceous of Madagascar

Wilson et al. (2010) Predation upon Hatchling Dinosaurs by a New Snake from the Late Cretaceous of India

http://www.eartharchives.org/articles/from-egg-to-enormous-dinosaur-the-making-of-a-titanosaur/