Big baby hypothesis supported by fossil fragments

Whether you use 99 backup hard drives or store everything on the cloud, if you are a scientist today, data loss is one of your worst nightmares. However, for vertebrates, there is one hard drive that continuously collects data and never needs backing up. Beginning before birth, our skeletal system records the details of how we develop and mature throughout our lifetime. Long after our bones have fossilized, we can rest assured the data will still be there. Dr. Robin O’Keefe and his team of paleontologists are using data stored in the long bones of a group of ancient marine reptiles, the polycotylid plesiosaurs, to understand the life history of these animals, including the major puzzle of how they grew such big babies.

Plesiosaurs lived throughout the Mesozoic Era, and many had a body plan that resembles the Loch Ness monster, with a long neck and large paddle shaped limbs. These large aquatic predators probably used their limbs for underwater flight, similar to sea turtles and penguins. In 2011 O’Keefe and his colleague, Dr. Luis Chiappe, suggested that at least some plesiosaurs gave birth to live young based on a fossilized mother and fetus Polycotylus latipinnis. The most surprising discovery was the relative size of the baby, estimated to be about 40% of mom’s length before birth! To further investigate growth patterns throughout ontogeny, Dr. O’Keefe and his team examined the histology and microanatomy of tiny fragments from the upper limb bones of additional polycotylid fossils. The team used bone slivers from fetal, neonate, and adult specimens to examine how the skeleton grew throughout development.

The results from this study support the hypothesis that the plesiosaurs did indeed give birth to large young that grew quickly after birth. In their neonate specimen, the team found a birth line, a clear growth mark in the bone that distinguishes rapid growth before and after birth. However, this rapid growth came at a cost to young plesiosaurs. A tradeoff between fast bone growth and bone growth that would be better for locomotion was found in the bone density and differences in blood vessel traces left in the bones. The blood vessel patterns of fetal and neonate specimens suggested rapid growth but low bone density, whereas adults had features suited for resisting forces experienced while swimming. Reading the data from these fossil fragments revealed that these big babies probably were not the strongest swimmers.

By Kelly Diamond – I am an integrative biologist interested in how animals move and interact within their constantly changing environments. Currently I am a PhD candidate at Clemson University studying how the environment impacts predator-prey interactions in Hawaiian stream fishes. To find out more, check out my website  or follow me on twitter @DiamondKMG

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