Scientists assess how large dinosaurs could really get

The researchers assert that the huge sizes attained by many dinosaurs make them a source of endless fascination, raising the question as to how these animals evolved to be so big. There are perennial claims and counter-claims about which dinosaur species was the largest of its group or even the largest ever.

Most dinosaur species are known from only one or a handful of specimens, so it's extraordinarily unlikely that their size ranges will include the largest individuals that ever existed. The question remains: how big were the largest individuals, and are we likely to find them?

To address this question, Mallon and Hone used computer modelling to assess a population of T. rex. They factored in variables such as population size, growth rate, lifespan, the incompleteness of the fossil record, and more.

T. rex was chosen for the model because it is a familiar dinosaur for which many of these details are already well estimated. Body-size variance at adulthood, which is still poorly known in T. rex, was modelled with and without sex differences, and is based on examples of living alligators, chosen for their large size and close kinship with the dinosaurs.

The palaeontologists found that the largest known T. rex fossils probably fall in the 99th percentile, representing the top 1% of body size, but to find an animal in the top 99.99% (a one-in-ten-thousand individual) scientists would need to excavate fossils at the current rate for another 1,000 years.

The computer models suggest that the largest individual that could have existed (one in 2.5 billion animals) may have been 70% more massive than the current largest-known T. rex specimens (an estimated 15 tonnes vs 8.8 tonnes) and 25% longer (15 metres vs 12 metres).

The values are estimates based on the model, but patterns of discovery of giants of modern species tell us there must have been larger dinosaurs out there that have not yet been found. "Some isolated bones and pieces certainly hint at still larger individuals than for which we currently have skeletons," says Hone.

This study adds to the debates about the largest fossil animals. Many of the largest dinosaurs in various groups are known from a single good specimen, so it's impossible to know if that one animal was a big or small example of the species. An apparently large species might be based on a single giant individual, and a small species based on a particularly tiny individual -- neither of which reflect the average size of their respective species.

The chances that palaeontologists will find the largest ever individuals for a given species are incredibly small. So, despite the giant skeletons that can be seen in museums around the world, the very largest individuals of these species were likely even larger than those on display.

Dr. Jordan Mallon, from the Canadian Museum of Nature, said: "Our study suggests that, for big fossil animals like T. rex, we really have no idea from the fossil record about the absolute sizes they might have reached. It's fun to think about a 15 tonne T. rex, but the implications are also interesting from a biomechanical or ecological perspective."