Rare embryo discovery gives hints on how dinosaurs reproduced

Sometime in the Late Jurassic era, a dinosaur nest was hit by a fatal tragedy and its eggs never hatched. Whatever killed the baby dinos - perhaps a hungry predator or a flood - was a stroke of luck for the team of paleontologists who, about 150 million years later, stumbled on the crushed eggs and embryo remains in the Lourinhã geological formation, in Portugal.  

“Most of the time what happens is that you find eggs without embryos, to find them together is really a matter of chance,” says Ricardo Araújo, lead author in the recent Scientific Reports study describing the Lourinhã fossils.

 

Lourinhã geological formation (Credit: wikipedia commons)

Findings of fossilised eggs with embryos are extremely rare - no more than a handful have ever been found. But without embryos it is virtually impossible to link an egg to a specific dinosaur species. So although many dinosaur eggs have been discovered all around the world, sadly we still know little about how dinosaurs reproduced and looked after their babies.

Araújo and colleagues at the Museum of Lourinhã didn’t immediately realise the importance of their fossil discovery; it wasn’t until they prepared the specimens in the lab that they saw tiny teeth and bones amongst the broken eggshells. “That’s when the good news really happened,” Araújo says.

The dinosaur baby teeth had nothing cute about them - they were long, pointy and sharp. Together with other bone features, the shape of the fossilised teeth gave away their identity: they belonged to Torvosaurus, the top meat-eating predator of the Late Jurassic era.

Sketch of the anterior part of the embryonic maxilla, showing the sharp teeth.
(Credit: Museu da Lourinhã)

Torvosaurus is an older, or 'primitive', dinosaur of the theropod family, which includes large carnivorous predators like the famous Tyrannosaurus and also modern birds. Even though Torvosaurus lived (and was extinct) well before Tyrannosaurus was around, it looked a lot like its cousin- it had huge jaws and walked on two legs, but it had longer and stronger arms. Torvosaurus was also nearly as big as Tyrannosaurus, measuring up to 11 meters and weighing about two tons.

Partial fossils of adult Torvosaurus have been found in North America and in the Lourinhã formation, but no embryos had ever been discovered. Up until now that is. The new specimens are the oldest theropod embryos found to date.

"Before we had examples of eggs and embryos of very advanced theropod dinosaurs, but we didn’t know anything at all of what was happening at the base of the family tree," says Araújo “[…] this finding is one of the oldest in the world, and it’s certainly the oldest for theropod dinosaurs,” he adds.

So what do these new fossils tell us about how primitive theropods lived?

By using a bunch of high tech methods, like high-power electron microscopy, the Lourinhã researchers looked in extreme detail at the microstructure of the eggshell pieces. They found that Torvosaurus’ eggs had a single structural layer, in contrast to advanced theropods that have more complex eggshells with two or even three layers (including modern birds, which are technically living dinosaurs).

It was known that primitive dinosaurs from other families had single-layered eggs, but theropods were the missing piece in the puzzle. “Now we have the evidence that eggshells of primitive dinosaurs only have one structural layer,” Araújo says. It appears that eggshell complexity increased throughout dinosaur egg evolution. But this isn’t all.

Torvosaurus is a basal or 'primitive' member of the theropod dinosaur family (Credit: Vladimir Bondar
& GEAL - CIID - Museu da Lourinhã)

The Torvosaurus’ eggshells have another interesting (and strange) feature- they have huge pores, or holes. Bird, reptile and dinosaur eggshells have pores so gases can be exchanged between the inside and the outside of the egg, so the embryos can breath. As a rule of thumb, eggs with larger pores are laid in a moist substrate, while eggs with smaller pores are incubated in nests exposed to air.

The Torvosaurus eggshells have large pores that “interconnect in a network towards the top of the eggshells,” explains Araújo “this is really different from what was found to date”. The eggshell pore size suggests that Torvosaurus buried their eggs, just like most modern reptiles. 

Another indication that Torvosaurus’ eggs were buried is the fact that the fossilised eggshells and embryos are exceptionally well preserved. "The eggshells are nearly exactly the same as they were 150 million years ago," says Araújo. Being underground would have protected the fossils from bacteria and atmospheric erosion.

“Dinosaur embryos are very rare and also challenging to identify,” says David Varricchio, a paleontologist from Montana State University “This study provides an important addition to our understanding on dinosaur reproduction.”

The Lourinhã formation is very rich in Late Jurassic fossils; it has many dinosaur nests and footprints, and countless invertebrate fossils. Araújo notes “There have been more discoveries and […] they will give further insights into dinosaurs and other types of vertebrates from 150 million years ago in Portugal. Finding other eggshells and embryo associations from other groups of dinosaurs would be really helpful to complete the picture.”

Reference:

Araújo R., Castanhinha R., Martins R.M.S., Mateus O., Hendrickx C., Beckmann F., Schell N. & Alves L.C. (2013). Filling the gaps of dinosaur eggshell phylogeny: Late Jurassic Theropod clutch with embryos from Portugal, Scientific Reports, 3 DOI: 10.1038/srep01924

This article was publish in Lab Times on the 13-08-2013. You can read it here.