WORLD SCIENCE

"Long before it's in the papers"
December 20, 2005

Posted July 28, 2005
Courtesy University of Toronto
and World Science staff

Newly studied embryos of a long-necked, plant-eating dinosaur are the earliest known for any terrestrial vertebrate, or backboned animal, researchers say.

Reconstruction showing the curled up embryo of the dinosaur Massospondylus inside the egg. The egg is 6 centimeters long. (Artwork by Kevin Dupuis/courtesy of University of Toronto at Mississauga)

Embryonic skeleton of the Massospondylus preserved inside the egg. Note that the head and strongly bent neck are lying outside of the egg. It is unknown whether this is due to unsuccessful hatching or if the head and neck were pushed out after death. Estimated length of embryo is 15 centimeters. (Courtesy Robert Reisz/University of Toronto at Mississauga)

The complete embryo compared to the skull of the largest known adult. The bones of the skull are outlined in black. (Courtesy Robert Reisz/University of Toronto at Mississauga)

Reconstruction of Massospondylus carinatus as an adult. Length is 5 meters. (Artwork by Gabriel Lio/courtesy University of Toronto at Mississauga)

The scientists claim the discovery yields new clues to how primitive dinosaurs evolved into the largest animals ever to walk on Earth.

It also provides a rare glimpse into the life of the Massospondylus, an early dinosaur that grew to five metres (more than five yards) and was fairly common in South Africa, they said.

The 190 million year-old embryos are from the beginning of the Jurassic Period, the middle of an epoch known as the age of dinosaurs. 

While the delicate bones of most dinosaur embryos have disintegrated over time, these embryos have well-preserved skeletons, the researchers said. One is nicely curled up in an egg. 

The findings, by scientists from the University of Toronto at Mississauga and colleagues, are published in the July 29 issue of the research journal Science.

“The work on the embryo, its identification, and the fact we can see the detailed anatomy of the earliest known dinosaur embryo is extremely exciting,” said the university’s Robert Reisz. 

“Most dinosaur embryos are from the Cretaceous period,” a later era lasting from 146 to 65 millions years ago, he added. 

Reisz said the finding is interesting because it enables scientists to put the embryos into a growth series and work out for the first time how these animals grew from a tiny, 15-centimetre (6-inch) embryo into a five-metre adult. 

“This has never been done for a dinosaur. Only Massospondylus is represented by embryos as well as by numerous articulated [jointed] skeletons of juveniles and adults. The results have major implications for our understanding of how these animals grew and evolved.” 

The hatchling was born four-legged, with a short tail, horizontally held neck, long forelimbs and huge head, the researchers said. 

As the beast matured, the neck grew faster than the rest of the body but the forelimb and head grew more slowly. The end result was a two-legged animal that looked very different from the four-legged embryo.

Reisz said the embryos provide insight into the origins of the later four-legged giant sauropods, a group that includes the passenger-jet sized Seismosaurus. 

Sauropods, a group of dinosaurs that include the largest land animals of all time, were small-headed, long-necked plant-eaters.  

The Massospondylus embryo looks like a tiny sauropod with massive limbs and a four-legged gait, he said. 

For that reason, Reisz and colleagues propose that the later sauropods’ gait probably evolved through a phenomenon called paedomorphosis, the retention of embryonic and juvenile features in the adult. 

Another point of interest is the absence of teeth, Reisz said. 

“These embryos, which were clearly ready to hatch, had overall awkward body proportions and no mechanism for feeding themselves, which suggest they required parental care,” said Reisz. “If this interpretation is correct, we have here the oldest known indication of parental care in the fossil record.” 

While the embryos were discovered in 1978 in South Africa, researchers only now managed to clear surrounding rock and eggshell from the embryo.

Experts called the study a major step forward. “This discovery is exciting in providing a major piece of the puzzle of how sauropodomorphs grew and reproduced,” said James Clark of George Washington University in Washington, D.C.

In particular it helps to show that this animal was four-legged when it first hatched and then gradually grew to be two-legged, he added.

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