Dinosaur Embryos

Two 190-million-year-old dinosaur embryos, out of a group of seven eggs, have been identified as the world's oldest dinosaur embryos found to date. Discovered in South Africa, they are also the oldest known embryos for any terrestrial vertebrate from anywhere in the world.

Artist's reconstruction of the embryo in its egg

The embryos are of an Early Jurassic prosauropod dinosaur according to Dr Mike Raath, from the Wits Bernard Price Institute for Palaeontological Research (BPI). Raath is one of five authors who describe the embryos and their evolutionary significance in the leading international journal Science published on July 29, 2005. "The embryos belong to the early sauropodomorph dinosaur Massospondylus carinatus. These skeletons are quite common in South Africa and range in size from small juveniles to full adults, up to about 5m in length. This identification is a major coup, because embryos are often difficult to identify to species," says Raath.

The late Professor James Kitching from the BPI originally discovered the cluster of eggs containing the embryos nearly thirty years ago in the Golden Gate Highlands National Park in the northeastern Free State, South Africa.They have lain on the shelves of the fossil store in Wits University since then, awaiting someone with the necessary training and skill to prepare the fossil eggs for detailed study because of the extremely delicate nature of the tiny embryonic bones and their intricately curled up position in the eggs. "In January 2000, Professor Robert Reisz from the University of Toronto at Mississauga in Canada was on a research visit to South Africa and borrowed the fossil eggs to take back to Canada, where Diane Scott of his lab carried out the detailed and difficult preparation under high magnification using a special microscope and achieved spectacular results," explains Raath.

Prof James Kitching who discovered the eggs in 1977

The embryos provide significant insights into the growth and development of this early dinosaur. Raath explains that this discovery allowed the team to reconstruct in detail the growth trajectory of Massospondylus, from pre-hatchling to full adult - a first for any dinosaur. Prof Reisz, the project leader, points out that adults and juveniles of other types of dinosaur are known, but they are usually either recovered from bone beds, where the skeletons are broken up, disarticulated and scattered, or the rare articulated skeletons are not sufficient to reflect a growth series.

The growth trajectory of Massospondylus shows that this dinosaur started out as an awkward-looking little creature that was an obligate quadruped, had a relatively short tail, a horizontally held neck, long forelimbs and a huge head. As the animal grew, the neck grew faster than the rest of the body, but the forelimb and head grew much more slowly than the rest of the body, so the body proportions changed dramatically as the animal grew. This means that Massospondylus changed from a tiny, awkward-looking quadruped into a weird-looking large animal which had a very long neck (still held horizontally) a thick, massive tail, a very small head, short forelimbs, and long hindlimbs.

Skull of adult Massospondylus above and embryo in its egg below.

The result is an adult animal that looked very different from the embryo and was probably at least partly bipedal. In other cases where embryos and adults are known, as in the hadrosaurs or duck-billed dinosaurs, such dramatic changes in body proportions are not shown.

The embryos also provide clues about the origin of the quadrupedal gait of the giant sauropods (the 'brontosaurs') of later times, which are descendants of the prosauropods. The embryo of Massospondylus looks like a tiny sauropod with massive limbs and a quadrupedal gait, which the authors believe shows that the quadrupedal gait of sauropods probably evolved through a phenomenon called paedomorphosis - the retention of embryonic and juvenile features in the adult. "Some people think that humans too are products of paedomorphosis," says Raath.

The absence of well developed teeth in the two preserved embryos, which were clearly on the point of hatching, and the overall awkward body proportions suggest that the hatchlings required parental care of some kind for some time after emerging from the egg. The authors say that if this interpretation is correct, it constitutes the oldest known indication of parental care in the fossil record.
27 July 2005.

2. What is so special about these embryos? Can you describe how they look?
These are the oldest known dinosaur embryos. Two embryos were exposed in the group of seven eggs. One of them is almost complete and nicely curled up inside its egg. It appears to be trapped in the act of hatching. The embryos are also the oldest known for any terrestrial vertebrate and thus the oldest embryos in an amniote egg known from anywhere in the world.

3. How old are these embryos? What period are they from?
They are about 190 million years old, from the Early Jurassic Period. Most of the other known dinosaur embryos are from the Cretaceous period and are least 100 million years younger (80-65 million years ago). There are nicely articulated embryos from the Late Cretaceous, identified as belonging to duck-bill dinosaurs or theropods.

4. To which species do the embryos belong?
The embryos belong to the early sauropodomorph dinosaur Massospondylus. Massospondylus skeletons are quite common in South Africa and range in size from small juveniles to full adults up to about 5 metres in length. This identification is a major coup, because embryos are difficult to identify to species.

5. How will the identification of these embryos contribute to knowledge about these early dinosaurs?
This identification allowed the team to reconstruct in detail the growth trajectory in Massospondylus, from pre-hatchling to full adult - a first for a dinosaur. This is because the adults and juveniles of other dinosaur embryos are known either from bone beds, where the skeletons are disarticulated and scattered or the rare articulated skeletons are not sufficient to reflect a growth series.

6. What does this tell us about the life history of these dinosaurs?
The growth trajectory of Massospondylus shows us that this dinosaur started out as an awkward-looking little creature that was an obligate quadruped, had a relatively short tail, a horizontally held neck, long forelimbs and a huge head. As the animal grew, the neck grew faster than the rest of the body, but the forelimb and head grew much more slowly than the rest of the body, so the body proportions changed dramatically as the animal grew. This dinosaur changed from a tiny, awkward-looking quadruped into a weird-looking large animal which had a very long neck (still held horizontally) a thick, massive tail, a very small head, short forelimbs and long hindlimbs. The result is an animal that looks very different from the embryo and is probably at least partly bipedal. Except for human beings, most animals either stay bipedal as they grow or are able to run bipedally as juveniles but become quadrupeds as adults like some lizards, crocodiles and hadrosaurian dinosaurs. Other dinosaurs like hadrosaurs - the duck-billed dinosaurs do not show such dramatic changes in body proportions.

7. Does this identification shed any new light on the giant dinosaurs that walked the earth in later geological times?
The embryos of this prosauropod have provided clues about the origin of the quadrupedal gait of the giant sauropods (the 'brontosaurs') of later times, which are descendants of the prosauropods. As the embryo of Massospondylus looks like a tiny sauropod with massive limbs and a quadrupedal gait, we assume that the quadrupedal gait of sauropods probably evolved through a phenomenon called paedomorphosis, the retention of embryonic and juvenile features in the adult. Some people think that humans, too, are products of paedomorphosis.

8. Is there any sign of dinosaur parental care in these embryos?
The absence of well developed teeth in the two preserved embryos, which were clearly on the point of hatching, and the overall awkward body proportions suggest that the hatchlings required parental care of some kind for some time after emerging from the egg. If this interpretation is correct, we have here the oldest known indication of parental care in the fossil record.

9. Who made the discovery?
The cluster of seven eggs with the two exposed embryos was originally discovered nearly 30 years ago by the late legendary Professor James Kitching from the Bernard Price Institute at Wits University. They come from the Golden Gate Highlands National Park in the northeastern Free State, South Africa.

10. Why were the eggs not touched for thirty years?
At the time of their discovery, nobody in South Africa had the necessary training or skill to prepare the tiny, delicate embryonic bones, in their intricately curled up position, from the fossil eggs for detailed study. So there they have lain on the shelves at Wits University until January 2000. Professor Robert Reisz from the University of Toronto at Mississauga in Canada was then on a research visit to South Africa and borrowed the fossil eggs. He took them to Canada where Diane Scott of his lab carried out the detailed and difficult preparation under the high magnification of a special microscope and achieved spectacular results.

11. Who are the members of the research project?
There are five members of the team who are co-authors of a paper published in the leading scientific journal Science on July 29, 2005. The team was led by Professor Robert Reisz of the University of Toronto at Mississauga in Canada, and included research assistant Diane Scott and PhD student David Evans from his laboratory, Dr. Hans-Dieter Sues, a researcher at the Smithsonian Institution, and Dr. Mike Raath from the Bernard Price Institute for Palaeontological Research, University of the Witwatersrand, Johannesburg.(See biographies).

12. Why is this project so exciting?
This project opens an exciting window into the early history and evolution of dinosaurs. Prosauropods were the first dinosaurs to diversify extensively and they quickly became the most widely spread group, so their biology is particularly interesting. They represent in many ways the dawn of the age of dinosaurs, the group that was to become so dominant on earth for so long (more than 100 million years) in later geological times.