TINY TITANOSAURS: MICRO-COMPUTED TOMOGRAPHY AND PRIMARY GROWTH RATES IN HATCHLING SAUROPODS FROM MADAGASCAR

Sauropods exhibit the greatest ontogenetic variation in body size for any vertebrate, extant or extinct. Eggs and embryos have been recovered but the smallest post-hatching juveniles are only a little less than half of known adult size, and details of the earliest stages of sauropod ontogeny particularly poorly understood. Here we report on two partial skeletons of hatchling Rapetosaurus krausei, a titanosaur from the Upper Cretaceous Maevarano Formation of Madagascar that provide new data on primary early stage growth rates in sauropods.

The skeletons come from two localities and exhibit no duplication of elements for either specimen. We compared greatest length ratios for appendicular elements to those of a complete sub-adult Rapetosaurus and confirmed that there are only two individuals present, that there is no significant allometry in Rapetosaurus postcranial ontogeny, and that each individual is less than 15% adult size. The smaller specimen includes sacral and caudal vertebrae, pubis, femur (maximum length [ml] = 19.3 cm), tibia (ml = 12.7 cm), fibulae, metatarsal I, humeri, metacarpal III, and a phalanx. The larger specimen includes caudal vertebrae, tibia (ml = 17.9 cm), and metacarpals I and IV.

We employed micro-computed tomography to garner bone histological data on earliest stage growth rates in these juveniles. The micro-computed tomography was carried out using an X5000 high-resolution dual-head 225kV microfocus X-ray CT system (North Star Imaging Inc., USA) located in the Department of Earth Sciences, University of Minnesota. We achieved an effective pixel pitch of 36 – 48 microns for the larger samples and 14 – 28 microns for sub-volumes. We collected 2-D radiographs and reconstructed these data to produce a 3-D volume for visual analysis and slices of the 3-D volume for quantitative analysis.

Primary bone growth in Rapetosaurus is highly vascularized woven and fibrolamellar bone with mid-diaphyseal remodeling. A single line of arrested growth is recorded in each specimen. These results are surprising given the small size of the individuals, and support the hypothesis that intensive remodeling observed in the bones of older juvenile Rapetosaurus may be dictated, at least in part, by resource limitations during periods of drought/ecological stress recorded in the Maevarano Formation.