Starting from a reasonable size of about 3.5 kg at birth, a specimen of the dinosaur Rapetosaurus krausei shot up to about 40 kg in just about 39-77 days, when it succumbed to starvation. Kristina Curry Rogers from Macalester College, U.S., and collaborators, who were studying the fossil remains of a young one of this species, deduced that the juveniles closely resembled the adult in shape. This meant that the young ones were quite independent at an early age unlike other species where young ones enjoyed parental care till they were able to fend for themselves. The results were published recently in the journal Science.
Identifying the fossils The fossils of these titanosaurs, so called for their huge size, were originally collected during the researchers’ field seasons in Madagascar, but were not “discovered” until they had been prepared in the lab, and placed within drawers along with unidentified fossils of smaller animals from the same geological formation. “I was working in those drawers in search of bones for another research project about growth in those reptiles, and began finding so many tiny titanosaur bones,” writes Dr Curry Rogers in an email to this correspondent. In fact, in 2001, along with Catherine Forster, it was Dr Curry Rogers who gave this dinosaur its name.
R. krausei is a dinosaur that used to inhabit the Madagascar islands about 70 million years ago. A fully grown Rapetosaurus could measure about 15 metres in length, which is about two metres longer than a Volvo bus. The generic name Rapetosaurus itself stands for “giant lizard,” suggestive of the fact that it was among the largest dinosaurs that walked the earth. While paleontologists knew the shape and size of this dinosaur from observations of skeletal remains, until recently, the early life of these so-called titanosaurs has remained a mystery.
“We have lots of adult, sub-adult, and juvenile Rapetosaurus bones, and these tiny specimens literally look like little miniatures of the bigger bones in our sample. We could tell right away that the skeleton was from Rapetosaurus, on the basis of the shapes of bones and the positions of muscle scars...it was just from a baby, only 11 per cent the size of the biggest known specimen,” she says.
Using bone histology and x-ray computer tomography, the researchers were able to understand the dinosaur’s growth pattern. From the compactness of the bones, the researchers deduced that the bones likely retained their shape as the young one grew into the adult. Thus the young ones resembled the adults to a very close degree, unlike in the case of other dinosaurs like theropods and ornithischians. The authors further propose that the young ones must have been independent at a very early age.
Lines of evidence
Dr Curry Rogers explains, “We used three lines of evidence to support this hypothesis [that the young ones were precociously independent]: (1) bone proportions stay the same throughout life, as in modern precocial animals; (2) under the microscope, bones show evidence of remodeling, which indicates that bones are being stressed during locomotion, and need to repair themselves already — this is not seen in other baby dinosaurs (like Maiasaura, a duck-billed dinosaur; or Troodon, a theropod dinosaur); and (3) we looked carefully at the preserved calcified cartilages at the ends of long bones — these provide a clue about how bones growth in length, and their thickness in baby Rapetosaurus was very similar to the observed thicknesses in modern precocial birds.” These led the authors to propose that R. krausei must have been independent of parental care even at very early stages in their development.
