Just how heavy were extinct species? And why should we care?

When we think of the weight of an animal, that contributes to our understanding of its biology, including how that animal grows throughout its existence and its place in the ecosystem.

Consider, for example, an Asian elephant. Babies can be up to 200 pounds; adults can reach weights of 2-5 tons. They have the potential to live upwards of 60 years in the wild. Surviving takes a considerable amount of local vegetation and water, some of which requires traveling long distances to obtain it. That’s a lot of impact on the ecosystem.

A pygmy shrew, on the other hand, is a mere 1 and a half – 2 inches long. It may eat more than its body weight daily, but that body weight is less than an ounce. Its life span is approximately a year. When a pygmy shrew travels and eats, its environmental impact is remarkably different than that of an elephant.

Big or small, every species has an important place in the environment.

And while it’s not difficult to compare the gigantic fossil of a titanosaur to that of the much smaller ankylosaur and immediately grasp that their weights were vastly different, estimating just how much extinct animals weighed when they lived is profoundly challenging.

Co-authors Asier Larramendi, Gregory Paul and Shu-yu Hsu rise to that challenge. Their recent paper, published online this past December in The Anatomical Record, offers detailed insight into the density, or specific gravity (SG), of the living and the extinct.

It’s not the first time scientists have tried to estimate the density of extinct species. This new research reviews previous research and finds that earlier estimates may have been too conservative. “The results show that the commonly applied densities [of] these extinct critters are up to 20% too low in case of some dinosaurs, and more in the case of pterosaurs,” they write in their press release.

Part of their method involved determining how buoyant a dinosaur or pterosaur might be in freshwater. This involved including possible bone and tissue weights and–what they say other researchers haven’t taken into account–how much a species might have to dispense the air in its lungs to maintain buoyancy. In other words, density is related to how much it takes for a species to sink or float in water.

Surprisingly, the authors explain that the densities of living animals is not always known.

“In fact,” Asier wrote in response to my question about this, “we have records of many extant animals’ densities. The problem is that these had been obtained on corpses where their respiratory system is usually empty (a data, which in any case, has been very useful for our study). So these densities do not correspond to that of animals in life, and estimating the density of a living animals is tremendously difficult since they would have to be trained or sedated in some way. However, thanks to the fact that there are many published data of different animals respiratory capacities, we were able to develop a novel methodology to reach as close as possible to the real densities of current animals, and from there and using comparative anatomy, also to that of extinct ones.”

An illustration showing dinosaurs of different densities in freshwater. The lightest, Diplodocus (top left) should have shown over 10% of its body above surface level when swimming, while at the other extreme the heavily armored Gargoyleosaurus (top right) was so dense that it probably bottom walked like today’s hippos.  (Image credit: Sante Mazzei).

One exciting aspect of their work involves long-necked sauropods. The authors scanned the entire skeleton of a Diplodocus using photogrammetry and then used 3D software to help flesh out the body. No easy task, as they write in their press release that ventilation within sauropods included lungs and “a bird-like system of air sacs and diverticula, plus some pneumatic bones that could include some air inside them.”

Asier, Research Director of Eofauna Scientific Research, has a unique interest in specific gravity. His previous research focuses on body mass and size in various animals, including proboscideans.

He attributes his fascination with density to local fishing trips he did with his friends when they were children. In order to fish, they needed to have sinkers–weights of various sizes that would drop bait and keep it under water. The variety of weights in the fishing store, from very small to incredibly large, captured his imagination.

“Since then,” he explained, “I began to be fixated on the density of everything, including people.”

Which, he wrote, included learning that he himself had a different type of density than those around him.

“Unlike all my friends, I literally used to sink into the pool without having to blow air out of my lungs out.”

And he began testing this.

“The test consists of asking people to [get into] a fetal position (wrapping the arms around the knees) in a relaxed position and/or exhaling all the air from their lungs (Vital volume + Expiratory Reserve Volume). What I’ve observed is that, in freshwater and in a relaxed position, the vast majority (probably over 90%) floats, but very very barely. In order to sink, they have to exhale vital volume (that only consists on about the 0.7% of the body volume) or just scarcely more, indicating a neutral specific gravity very close to 0.99–1.0 for most of human beings, as it is true for most of land mammals too.”

He learned a lot through online footage, writing, “I could later check that most animals including reptiles, share a very similar density: close to 1.0. However, when many years ago, I started reading papers relating to body masses and other stuff, I realized that the generally-applied densities of vertebrates-–extant and extinct–were generally too low and unrealistic because they did not match with what was observable. This made me start talking about the subject in my first papers as well as in the Dinosaur Fact and Figures series of books.”

“But the real push to finally carry out the present study took place when Gregory Paul and me were working on another massive methodological comparative study. For that research work, we needed to estimate the most credible weights possible for a very large number of extinct creatures. As we were using volumetric methods to get the final masses, we needed to apply the correct densities, so we finally embarked on the the study that has recently been published. Fortunately, this acquired knowledge help us to continue with the other project we had underway.”

We’ve come a long way in reconstructing the extinct and understanding ancient ecosystems, but we have a long way yet to go. This research is a strong foot forward on that path.

Many thanks to Asier for his generous time and help in answering my questions about this very complicated, comprehensive research!

References:

  1. Koirala, Raj Kumar; Raubenheimer, David; Aryal, Achyut; Pathak, Mitra Lal; Ji, Weihong. Feeding preferences of the Asian elephant (Elephas maximus) in NepalBMC Ecol 16, 54 (2016). https://doi.org/10.1186/s12898-016-0105-9
  2. Larramendi, Asier. New analysis finds that dinosaurs and pterosaurs were denser, and therefore heavier than we thought; Eofauna press release, 2021.
  3. Larramendi, Asier; Paul, Gregory; Hsu, Shu-yu. A review and reappraisal of the specific gravities of present and past multicellular organisms, with an emphasis on tetrapods. The Anatomical Record. https://doi.org/10.1002/ar.24574