How did sauropods grow?

Standing from about 15 to 20 m tall, with body lengths in excess of 30 m, and weighing roughly the same as 8 to 10 African bull elephants, the long-necked dinosaurs (the sauropods), were the largest animals to live on our planet. How did these animals attain their gigantic stature? How long did they grow? Answers to these questions are presented in an Argentinian–South African collaborative study involving University of Cape Town’s Professor Anusuya Chinsamy-Turan, which was recently published in PLOS One.

The sauropods, and their ancestors the “prosauropods”, belong to a group of dinosaurs called the Sauropodomorpha. About 190 million years ago South Africa and Argentina were common stomping grounds for these early dinosaurs, and the remains of such animals are often found as fossils in these countries. Over the past decade or so, several more sauropodomorph dinosaurs have been discovered, which has helped shed light on how the transition from basal to more derived forms occurred.

Several of these newly described dinosaurs appear to have a mosaic of basal and derived features, which clearly makes them “transitional” dinosaurs. A good example of such an intermediate dinosaur is the South African dinosaur Antetonitrus, which was described in 2009.

For almost two decades, scientists thought that the “prosauropods” took several years to reach adult body sizes, as indicated by growth rings in their bones. The more derived sauropods appear to have had faster growth rates, which allowed them to grow rapidly without periods of slowing down, and only during the late stages of their lives did they develop growth rings in their bones.

An extraordinary conclusion

After studying the microscopic structure of the bones of 13 different sauropod dinosaurs from South Africa and Argentina, as well as that of 12 others described previously, the researchers reached an extraordinary conclusion. Although some sauropods grew up rapidly without pauses in their growth, there were others (like Lessemosaurus pictured above, which grew to about 10 m in length) that were also giants, but they took the energetically less expensive route of growing cyclically, ie with alternating slow and fast rates of growth, thus, taking several years to reach their grand size.

On the other hand, these scientists found that some of the basal forms, like Mussaurus (about 5 to 6 m in length), contrary to the previous hypothesis, grew rapidly like the more derived sauropods, and did not have alternating periods of slowed and fast growth expected for basal forms. Thus, the study shows that the long-held view that basal and derived Sauropodomorpha had different growth patterns is no longer valid.

As Argentinian palaeontologist Dr Ignacio Cerda says, “More research can often lead to a reassessment of previous hypotheses . . . this is the nature of science.”

It now appears that among the Sauropodomorpha, some of them (especially the most derived ones) grew rapidly to adulthood, while others (irrespective of whether they were basal or derived sauropods) took the energetically less expensive, but more protracted route to reaching gigantic proportions.

Professor Chinsamy-Turan said, “Our study of more recently discovered dinosaurs permitted fresh perspectives into the lives of these giant dinosaurs.”

Argentinian palaeontologist Diego Pol – one of the key people involved in the discovery two years ago of the largest ever titanosaur – says, “Finding new dinosaurs is thrilling, but it is just the first step in understanding the biology of these colossal animals.”