Some individuals hit upon excellent conditions for piling on the pounds, while others fell on lean times.
Dinosaur researcher Paul Barrett, of the Natural History Museum in London, agrees that the study appears to provide "nice evidence of some kind of temporary growth spurt rather than continued, similar rate of growth throughout the animal's lifetime."
This "might correlate with greater instances of food or maybe a warmer summer," Barrett said. "That would make sense in interpreting [Plateosaurus] in a crocodile- or lizardlike model."
Growth spurts and big size differences within a species are characteristic of ectothermic animalsthose with a metabolism dependent on external conditions. Lizards, for instance, are active when it's hot but become sluggish in cold weather.
Meanwhile mammals and birds, which have growth rates largely independent of outside factors, are endothermicthey heat and cool their bodies internally.
Which of these two camps dinosaurs belonged to has long been a bone of contention among experts.
Sander says Plateosaurus was probably somewhere between the two, with the animal perhaps representing a first stage in the evolution of endothermic dinosaurs.
Barrett of the Natural History Museum says early large dinosaurs such as Plateosaurus were not endothermic, but likely were warm-blooded.
"They're probably warm-blooded because they're big and they produce a lot of heat through digestion and through muscular action," he said.
"They have a relatively small surface area to their volume ratio, and any excess heat they generated just by accident would mean they have a higher body temperature than the outside environment."
Based on their body structure, some later, smaller dinosaurs also appear to have been endothermic.
"It's a size argument," Barrett said. "Because they are small and active, it looks like they should be warm-blooded to fuel their active lifestyle."
"Some of these little dinosaurs are now known to have had an insulating coveringfeathers or fuzz of some kind," he added. "This suggests they were generating heat internally and were trying to retain it."
These small theropod dinosaurs are on the evolutionary line that leads eventually to birds, Barrett says.
"Certainly, by the time you get to birds, [the animals] are generating heat internally," he added.
"Birds are only modified dinosaurs, and at some point in their ancestry you need to switch [endothermy] on. Deciding when that happened is quite difficult, as we can't just go and measure the temperature of an extinct dinosaur."
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