Thursday, 21 March 2013

Dinosaurs: What did their size really mean?

Dinosaur skeletons at Melbourne Museum :: Photo by: S. Barker

Is big always better? Is big normal? What can body size tell us about an animal?

These are some of the questions explored in a recent paper about the distribution of body size in dinosaurs. It also looked at whether the huge sizes some reached were common in nature. 

Why should you care whether dinosaurs were bigger more often than other animals? 
There are 3 key points:

  1. It can demonstrate how big animals fit into their ecosystem, 
  2. what might drive animals to get really big and 
  3. what being big means at time of global change.

The researchers trawled through vast amounts of information to collect as many records of body size as possible. As well as dinosaurs, they collected body size information of:

  • Birds
  • Reptiles
  • Amphibians
  • Fish
  • Terrestrial mammals
  • Extinct Cenozoic (65Ma-now) terrestrial mammals
  • Extinct pterosaurs

The first 5 were all extant (currently living) species. The two groups of extinct animals were included to make sure there was no bias in the fossil record, just in case we only dig up big bones!

As well as comparing all the various animals, they compared the 3 main types of dinosaurs: 

  • Ornithiscian: For example plant eating duckbills, stegosaurs & Triceratops; 
  • Sauropodomorphs: The long necked plant eating dinosaurs, for example brachiosaurs, apatosaurs, diplodicus; and 
  • Therapoda: Meat eaters, which ultimately evolved into birds, for example T. rex and velociraptors.

Frequency distribution of species body 
size for eight different animal groups: 
(a) extinct dinosaurs; (b) extant birds; (c) extant reptiles; 
(d) extant amphibians; (e) extant fish; (f) extant mammals; 
(g) extinct pterosaurs; and (h) Cenozoic mammals.  O'Gorman & Hone
The scientists also compared dinosaurs found in 2 prolific dinosaur fossil formations, the Morrison formation and Dinosaur Park, and across time periods of the Age of Dinosaurs, the Mesozoic. These comparisons were to make absolutely sure that there were no biases in the results.

All the non-dinosaurian animals showed a higher, or normal, frequency of smaller body size, and so did the Theropods. The herbivorous Ornithischians and Sauropodomorphs however, showed a uniquely high frequency of large size.

The other interesting result was that the dinosaurs got larger more often towards the end of each time period. The Late Triassic, Late Jurassic and Late Cretaceous all see a higher frequency of these massive animals. From this we can speculate that:

  1. Times of stability can allow animals to evolve into ginormous size.
  2. Being big is not so good when things start to change in your environment. 

So why were the big plant eating dinosaurs so darn big? It appears to be a combination of factors. The first being escape from predators: once you get really big, not much can hurt you. The other factor has to do with processing food. These dinosaurs were eating very tough, nutrient poor, plant material that took a lot of breaking down. As they got bigger, their gastric tract also increased and allowed for more efficient processing of food.

This paper shows that when it comes to size, the dinosaurs were truly unrivaled. No other group of animals has such a high frequency of gigantic size. The large size of the herbivorous dinosaurs was a double edged sword. While it helped them avoid predation and made them fantastic plant processing machines, it left them vulnerable to changes in their environment so that they suffered greatly at each major extinction event they faced. 

It also raises the question: was it the preference for smaller size, that lead to the survival of Therapods in the form of birds?

What do you think? Could the smaller size of Therapods be part of the reason they continue to live successfully today?