2 August 2005
Two 190-million-year-old dinosaur embryos from a group of seven eggs have been identified as the oldest dinosaur embryos yet found. Discovered in South Africa, they are also the oldest known embryos for any terrestrial vertebrate – and the oldest evidence that dinosaurs were caring parents.
The embryos are of an Early Jurassic prosauropod dinosaur, according to Dr Mike Raath of the Wits Bernard Price Institute for Palaeontological Research (BPI). Raath is one of five authors who describe the embryos in the 29 July 2005 isssue of leading international journal Science.
“The embryos belong to the early sauropodomorph dinosaur Massospondylus carinatus,” he says. “These skeletons are quite common in South Africa and range in size from small juveniles to full adults, up to about five metres in length. This identification is a major coup, because embryos are often difficult to identify to species.”
The late Professor James Kitching of the BPI discovered the cluster of eggs and their the embryos at the Golden Gate Highlands National Park in the northeastern Free State in 1977.
These are the oldest known dinosaur embryos. Two were exposed in the group of seven eggs. One of them is almost complete, and appears to be trapped in the act of hatching.
The embryos are the oldest known for any terrestrial vertebrate, and so the oldest embryos in an amniote egg known from anywhere in the world.
Since their discovery the eggs sat on a shelf in Wits University’s fossil store, awaiting someone with the necessary skill to prepare the fossil eggs for detailed study. The tiny embryonic bones are extremely delicate, and intricately curled up in the eggs.
“In January 2000, Professor Robert Reisz of 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,” says Raath. “There Diane Scott of his lab carried out the detailed and difficult preparation under high magnification using a special microscope and achieved spectacular results.”
Robert Reisz (left) with James Kitching, who discovered the fossilised dinosaur embryos in 1977 (Photo: Wits University)
The growing dinosaur
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.
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 quadruped with 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 quadruped into a weird-looking large animal with a long neck (still held horizontally), a thick, massive tail, a very small head, short forelimbs, and long hind limbs.
The result is an adult animal very different from the embryo, and 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 walking on all fours. This means the quadrupedal gait of sauropods may have evolved through 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.
If this interpretation is correct, it constitutes the oldest known indication of parental care in the fossil record.
The embryos are about 190-million years old, from the Early Jurassic Period. Most other known dinosaur embryos are at least 100-million years younger, from the Cretaceous period of 80- to 65-million years ago.
The five co-authers of the Science paper are Professor Robert Reisz, Diane Scott, David Evans, Dr Hans-Dieter Sues, and Dr Mike Raath.