Weighing a Dinosaur

Welcome back. If you’ve lost count, there have been five Jurassic Park or Jurassic World movies and a sixth is scheduled for 2021. I missed most, though I did see the first, which is still ranked as the best by those who rank films.

Even if your dinosaur-movie watching is like mine, you may have wondered how much dinosaurs weighed, the big ones, like Tyrannosaurus rex (T. rex) and bigger. A recent study filled in the blanks, at least the blank of how they, paleontologists, come up with the estimates. As you’d expect, it’s kind of cool.

Dinosaurs reached an amazing range in size; horizontal lines are 1 meter apart (Royal Ontario Museum, image by Vitor Silva, from eurekalert.org/multimedia/pub/241791.php).

What’s with Jurassic?
The Jurassic movie franchise is based on the late Michel Crichton’s 1990 best-selling novel, Jurassic Park. Not knowing what you learned or remember from geology, I thought it might be useful to comment on why Crichton used “Jurassic” before I get into dinosaur weight.

The geologic time scale plots the order of major events in the history of the Earth based on evidence from rocks, fossils, astronomy and more. The largest scale intervals are eons, which span hundreds of millions of years; eons are divided into eras, eras into periods and periods into epochs. Where these intervals begin and end is based on reading the rock record; new methods have continually refined the dating accuracy. (The International Commission on Stratigraphy sets the standards.)

Geological Society of America’s current geologic time scale, version 5.0, August 2018; for oldest to present, read right to left, bottom to top, and go to link to read smaller print (www.geosociety.org/GSA/Education_Careers/Geologic_Time_Scale/GSA/timescale/home.aspx).

Dinosaurs are associated with the Mesozoic Era, which followed and ended with a major mass extinction. The era lasted some 180 million years, from about 250 to 65 million years ago, and includes three periods, the Triassic, Jurassic and Cretaceous.

Dinosaurs evolved in the late-Triassic period. The Triassic ended with a mass extinction, yet many dinosaur species survived, evolving even more as the most dominant species on Earth in the Jurassic period. They remained the dominant species during the Cretaceous period. (Yes, the book and movies could have been named Cretaceous Park and World.) 

Dinosaurs appeared and went extinct during the Mesozoic Era (graphic from www.enchantedlearning.com/subjects/dinosaurs/mesozoic/).

Dinosaur Body Mass Estimation
A pair of collaborating researchers affiliated with Australia’s University of New England and Canada’s University of Toronto and the Royal Ontario Museum reviewed dinosaur body mass estimation techniques carried out over more than a century.

Regarding the importance of the topic, they note that body size, in particular body mass, influences all aspects of the animal’s life, including diet, ability to maintain body temperature, reproduction and locomotion. Adding order to the topic, they found the different body mass estimation techniques could be categorized into two fundamental approaches: volumetric density and extant scaling.

Volumetric Density involves calculating the body’s weight based on a three-dimensional reconstruction of what the animal might have looked like. This method has received most attention with non-avian dinosaurs, progressing from physical scale models to virtual techniques that utilize data scanned from entire skeletons.

Extant Scaling measures the size of limb bones in living animals--usually the circumference of the femur (thigh bone) or humerus (upper arm bone)--then scales up the body weight based on measurements from the same bones in the dinosaur skeleton.

Although there’s been considerable debate over which approach produces the best results, the researchers found that estimates by the two approaches are largely consistent--73% of volumetric density reconstructions are within the expected 95% prediction intervals of the extant scaling relationship.

Comparing approaches: volumetric density reconstructions projected onto extant scaling (from onlinelibrary.wiley.com/doi/abs/10.1111/brv.12638).

Wrap Up
Extant scaling from living animals is accurate but often of low precision. Volumetric density is precise but of unknown accuracy; reconstructions depend on our changing thoughts of what extinct animals looked like.

The decision of which approach to use is largely question dependent. In general, biomechanical and physiological studies benefit from the full body reconstruction provided through a volumetric density approach, whereas large scale evolutionary and ecological studies require the extensive data sets afforded by an extant scaling approach.

The researchers recommend that future work seeking to estimate the sizes of Mesozoic dinosaurs and other extinct animals better integrate the two approaches.

Oh, an adult T. rex would have weighed about 7 metric tons (7.7 US tons). Of course, dinosaurs like humans vary, so figure 5 to 10 metric tons (5.5 to 11 US tons).

Thanks for stopping by.

P.S.
Jurassic Park movies--Jurassic Park (1993), The Lost World: Jurassic Park (1997), Jurassic Park III (2001), Jurassic World (2015), Jurassic World: Fallen Kingdom (2018), Jurassic World: Dominion (2021):
en.wikipedia.org/wiki/Jurassic_Park
parade.com/679441/samuelmurrian/we-ranked-all-five-jurassic-park-movies-including-jurassic-world-fallen-kingdom/
www.businessinsider.com/every-jurassic-park-movie-ranked-worst-to-best-2018-6
Geologic time scale and Mesozoic Era:
geology.com/time.htm
scienceviews.com/dinosaurs/mesozoic.html
en.wikipedia.org/wiki/Mesozoic
Study of dinosaur body mass estimation in Biological Reviews journal: onlinelibrary.wiley.com/doi/abs/10.1111/brv.12638
Article on study on EurekAlert! website: eurekalert.org/pub_releases/2020-09/rom-htw090120.php