They were the largest land creatures the Earth has ever known. But what survived millions of years of fossilization in one specific area of the Ponte Alta region of Brazil was not their massive bones, rather, it was their rare and relatively tiny eggs. And many of them! The first titanosaur nesting site in the country was recently announced in a paper published in Scientific Reports.
Sauropods, a group of long-necked herbivores, were a diverse type of dinosaur that lived from the Jurassic era through the Cretaceous, a period spanning from 201 million years to 66 million years ago. Titanosaurs were a clade of sauropod—a group with a common ancestor—that was the last of this lineage to exist on this planet in the Late Cretaceous. While their name justifiably implies an enormous size, not all of them were huge.
South America is well-known for its titanosaur fossils, particularly in Argentina, home to some of the world's most spectacular titanosaur nesting sites and embryonic remains. Titanosaur eggshells and egg fragments are known in Uruguay, Peru, and Brazil, but a fossilized egg here and there doesn't provide evidence of a nesting site. Several egg clutches, numerous eggs and egg fragments in more than one layer of sediment, does.
The discovery marks the northernmost titanosaur nesting site in South America. While we knew the dinosaurs ranged farther north, the lack of known nesting sites there suggested they might have migrated south for egg-laying. The discovery indicates that this wasn't necessarily the case.
Lost in the limestone
These fossils were found by one of the authors of the paper, João Ismael da Silva, a paleontology technician who works at the Federal University of Triângulo Mineiro in Brazil.
"In the 1990s," he said in a press release, "I became aware of the occurrence of dinosaur eggs in Ponte Alta. In conversation with friends of mine who worked in limestone mining, I was able to recover some isolated eggs and, finally, an association of ten spherical eggs."
Limestone mining was key to the find, which came from the former Lafarge Quarry, which was in operation for 26 years—meaning that substantial ground layers now lie open. But the mine also undoubtedly destroyed many fossils that might have contributed to our understanding of lost ecosystems. The quarry remnants mean this area might have been of extraordinary paleontological value.
In addition to the eggs, the site yielded fossil evidence of crocodyliforms, bits of the bipedal carnivores known as theropods, fragments of titanosaurs, fish, and gastropods.
And this, wrote Dr. Thiago Marinho in an email, "shows how important [it] is to have a paleontologist on large-scale excavations of sedimentary rocks." (Also from the Federal University of Triângulo Mineiro, Marinho is a paleontologist and co-author on the paper.) "The presence of these beautifully preserved eggs," he continued, "demonstrate[s] that this was an exceptional paleontological site that could have provided many other important materials if this simple action of having a paleontologist in loco was taken."
The most well-preserved egg clutch contains 10 eggs huddled together, eight eggs facing the surface and two lying beneath the others. The specimens, mostly collected by Da Silva, were found in at least two layers of sediment, indicating that these long-necked behemoths returned year after year to this location to reproduce.
Vertically challenged
Given the other fossils they've found in the mine and the relative lack of titanosaur bones, one might wonder how these scientists determined which species laid these eggs. The answer lies in their particular shape. The Ponte Alta eggs are slightly smaller and thinner than in other locations but otherwise match the distinctly spherical eggs found in other global localities. Almost 20 eggs have been recovered, each about 12-18 cm in diameter. To put that into context, that's roughly the diameter of an ostrich egg. An ostrich, however, is about 1.8 m (or 5 to 6 feet) tall. The type of titanosaurs known in Brazil could have reached heights of anywhere from 3.5 to 18 m (or 12 to 59 feet). In other words, the eggs these sauropods lay were tiny, relative to their overall size.
We have so many examples of creatures today that lay eggs, but what they all have in common is something pretty specific: They are all low to the ground. Crocodiles, lizards, turtles, and even today's dinosaur—birds—at their very largest are still far from comparable to the juggernaut titanosaurs. Today's massive creatures—elephants, hippos, and whales—all give birth to live young. Which begs the question: How could something that enormous lay something as small and fragile as an egg?
Ph.D. candidate Kamila Bandeira, a vertebrate paleontologist focusing primarily on sauropod dinosaurs, noted that this is "still a very complex question and there is no consensus. The large size of most of the known sauropod species, in addition to the fact that the eggs are quite small (mostly with 15 to 20 centimeters in diameter), make it difficult for us to understand what this process would be like."
Weird & wonderful #sauropod Amargasaurus mounted as though laying eggs. Argentinian Museum of Natural Sciences, Buenos Aires. #FossilFriday pic.twitter.com/JAFd6P2MQ3
— John Pickrell 🌈 (@john_pickrell) September 22, 2017
She described the "relatively flexible" eggshell of today's birds and crocodilians that hardens once it hits the air, pondering whether dinosaurs may have employed a similar strategy. Squatting closer to the ground, she added, would have helped "to reduce the impact of the eggs falling to the ground."
Others, she wrote, have proposed "some sort of conductive channel, a kind of flexible organic 'tube' that could extend outside the body of females, as sea turtles present today." But no such evidence for that tube yet exists.
Local style
However the eggs got there, millions of years of fossilization have offered us insight into the incubation conditions. Many Ponte Alta eggs show cracks and signs of sedimentary pressure, indicating that titanosaurs burrowed their nests. A number of the eggs are also broken, with pieces of eggshells found within, suggesting either hatched or scavenged babies.
And, the authors note, some of the pores on these eggshells differ slightly from those in other locations. Co-lead author and paleontologist Dr. Lucas E. Fiorelli (CRILAR – CONICET, La Rioja, Argentina) explained in an email that "[a]mniote eggs have pores in the shell to facilitate the diffusion of gasses produced by the embryo's respiration (oxygen enters and carbon dioxide leaves, among other dissolved gases)." The distinctive pores of these eggs may indicate they were specifically adapted to the environment of Ponte Alta.
"There was something in Ponte Alta chosen by those titanosaurs that was optimal for them to nest there," he wrote, wondering whether it had to do with the type of soil, its possible aridity or humidity, for example. "We don't know that yet. It is part of future research in the area."
"It is an anatomical survival game," he added. "You have to think that they were generalists, like sea turtles, they laid many eggs and survival was low: few reached adulthood."
Fiorelli pointed out that he and his co-authors consider the eggs part of the sauropod, rather than a trace fossil or an ichnofossil, the term given to fossils that record traces made during an animal's life. Examples include fossil feces (coprolites), footprints, nests, and even fossil vomit (regurgitalites).
"Thus," he said, "we avoid the parataxonomy of eggs. Eggs are part of the organism, genetically regulated/determined and possess phylogenetic information, differing from footprints or coprolites that do not have genetic material of the organism and are indirect evidence of its presence."
He added, "We think that this point will bring further and heated discussions among paleontologists."
Egg contents to come?
Dr. Agustín Martinelli is a paleontologist from the Bernardino Rivadavia Natural Science Museum associated with the CPPLIP, Uberaba, Brazil, and co-lead author on this paper. He was emphatic about their discovery, writing, "Finding the first nesting place for Brazil was amazing, although we did not find embryos yet… We will continue with the work at the quarry and try to find more eggs and make tomographies in order to try to find bones inside. At some point, fossil embryos will be found!!"
Fiorelli explained that the eggs are too big for synchrotron imaging, but they hope to CT scan more of them.
"The paper is certainly an important finding to help in the knowledge about titanosaurs not only in Brazil, but in South America as a whole, especially for being the northernmost nesting occurrence for this continent," Ph.D. candidate Bandeira said in an email, explaining that before this discovery, it was thought that South American titanosaurs migrated over long distances to reach the areas in Argentina for reproduction. "The new research also corroborates something researchers have already reported for other titanosaurs around the world: that these animals were colonial (or at least, in grouping) nesting animals, much like other sauropods."
Jeanne Timmons (@mostlymammoths) is a freelance writer with a strong passion for paleontology. Based in New Hampshire, she writes about paleontology (and some archaeology) on her blog mostlymammoths.wordpress.com.
