Argentine Sauropods Were Big

But even more interesting, other animals grew really big, too.

This summer, researchers report a new fossil find in Argentina, yet another gigantic animal (more than 15 meters nose to tail, weighing something like 7 tons), this one from the late Triassic [1].  The fossil looks a lot like sauropods, but dates earlier than the well know giant dinosaurs of the Jurassic, such as Diplodocus.

This find is interesting for several reasons.  It shows that dinosaurs grew very large even earlier than suspected, indeed, before the die off at the end of the Triassic.  And then, other species grew very large again, independently, during the Jurassic.  So we seem to have two independent cases of giganticism.

This second case forces reexamination of theories of the development of giganticism, what leads to what, and what is necessary and what is accidental in the evolution of really large land animals.

“The evolution from small bipedal to giant quadrupedal sau- ropodomorphs involved numerous anatomical changes,” ([1], p. 1)

The new species is as large as later cousins, but  has anatomical differences from them.  The researchers report that the neck is not elongated and the front limbs were not columnar, traits found in later sauropods.  These differences show that “that these fea- tures were not strictly necessary for the acquisition of gigantic body size.” ([1], p. 4)

The fossil remains suggest that the animal had a adaptation of avian like lungs and air sacs in the skeleton, unlike later suaropods.  In birds, this contributes to efficient respiration and also makes the bones lighter.  In this case, lighter bones would make larger bones easier to support.

These cavities may also play a role in thermoregulation.  Large animals need ot keep cool, so pneumatic cavities might have made larger size feasible.

In addition to the respiratory system, examination of the growth rings in the bones reveals evidence of “cyclical and remarkably high growth rates.”   One way to get big is to grow fast, which seems to be what these animals did.

Clearly, there is more than one way for a species to evolve extreme size. Future work will have to try to sift out more understanding the necessary and accidental features of these two cases, and what environmental and other factors were involved.  (What did they eat?  What ate them?  and so on)

  1. Cecilia Apaldetti, Ricardo N. Martínez, Ignacio A. Cerda, Diego Pol, and Oscar Alcober, An early trend towards gigantism in Triassic sauropodomorph dinosaurs. Nature Ecology & Evolution, 2018/07/09 2018.
  2. Helen Briggs, Fossil of ‘first giant’ dinosaur discovered in Argentina, in BBC News – Science & Environment. 2018.

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