Early Dinosaur Embryos
Embryonic skeleton of the prosauropod dinosaur Massospondylus as preserved inside the egg. Note that the head and strongly bent neck are lying outside of the egg. It is not possible to determine if this is the result of an unsuccessful hatching or if the head and neck were pushed out postmortally. Image courtesy of Robert Reisz/University of Toronto at Mississauga.
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Researchers have identified several dinosaur embryos in eggs from South Africa that date to about 190 million years ago, within the Jurassic period, making them much older than many other known dinosaur embryo fossils, which are generally from the Cretaceous period.
The embryos' features indicate that they are Massospondylus dinosaurs, according to Robert Reisz and colleagues in the 29 July 2005 issue of the journal Science.
As adults, these early "prosauropods" were about 5 meters long, with long necks and short heads, and they were capable of bipedal motion and posture. The embryos, however, have relatively large forelimbs and heads, horizontally held necks and short tails, implying that they hatched as quadrupeds.
This may suggest that later sauropods, which are four-legged, evolved by preservation of this early developmental state. The embryos' body proportions, including their large heads, suggest that it would have been difficult for the hatchlings to move around efficiently.
The authors also found very little evidence of teeth. The hatchlings may therefore have required parental care for some time.
Kathy Wren
29 July 2005
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