Growth
The holotype of Scipionyx is a rare example of a non-avian theropod hatchling; the most important other very young specimens are the chicks of Byronosaurus that however are much less complete. The young age is reflected by the proportions and the low degree of ossification and fusion of several skeletal elements. The most obvious youthful trait is the relatively large and short head.
Dal Sasso & Maganuco have tried to determine the absolute age of the hatchling. The fact that the fontanelle had not closed yet, poses an upper age limit of about five weeks. An even lower limit is indicated by the lack of any tooth replacement, which with Archosauria begins after a few weeks at the latest. The most exact age is given by the size of the yolk sac, which indicates a probable age of three days, with an upper limit of a week. Despite its very young age, the hatchling was able to walk, as is shown by the complete ossification of the ilium. However, this does not imply Scipionyx was precocial as even with altricial birds this pelvis bone fully ossifies within a few days after hatching.
Read more about this topic: Scipionyx, Paleobiology, Physiology
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