"We were able to actually plot various elements of skeleton onto a chart and figure out how particular parts of skeletons grew relative to other parts," Reisz said.
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That growth pattern turned out to be highly unusual. The hatchling had a huge head and forelimbs as long as its hind legs. As the animal grew, its neck stretched dramatically, while its head got increasingly smaller relative to its body. Its hind legs grew more than twice as long as its forelimbs.
"This kind of change hasn't been shown in any other dinosaur to my knowledge," said James Clark, a dinosaur expert at George Washington University in Washington, D.C.
An adult Massospondylus had a head that was only 8 inches (20 centimeters) long. Its upper limbs were only half the size of its thighbones. It grew to be about 16 feet (5 meters) long, with a beanstalk-like neck and an 8-foot (2.4-meter) tail.
The earliest sauropods may have also developed with quadrupedal proportions, like their Massospondylus cousins. But these early sauropods retained their four-footed stance into adulthood.
The growth pattern of the Massospondylus could therefore provide clues about how the giant sauropods evolved.
"These animals are essentially predecessors to those large sauropods," Reisz said.
The scientists also found that both skulls in the embryos completely lacked teeth.
"Put together with the large head and generally awkward proportions of the body, this raises the possibility that these animals were not able to move around efficiently after they were hatched and may have required parental care in the form of [food] regurgitation from the mother," Reisz said.
If this interpretation is correct, it would be the oldest known indication of parental care in the dinosaur fossil record.
"This is clearly speculative," Reisz said. "We would like to go back and examine this in greater detail. There is a lot more to be done on this project."
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