Why crocodiles have changed so little since the age of the dinosaurs?

New research by scientists at the University of Bristol explains how a ‘stop-start’ pattern of evolution, governed by environmental change, could explain why crocodiles have changed so little since the age of the dinosaurs.

The distributions and ecology of living crocodilians are controlled by environmental factors such as temperature. Crocodilians have a rich history, including amphibious, marine, and terrestrial forms spanning the past 247 Myr.

Crocodiles today look fundamentally the same as ones from the Jurassic time frame nearly 200 million years ago. Prehistory also saw types of crocodiles we don’t see today, including giants as big as dinosaurs, plant-eaters, fast runners, and serpentine forms that lived in the sea.

A new study explains how crocodiles follow a pattern of evolution known as ‘punctuated equilibrium.’ The study describes how a ‘stop-start’ pattern of evolution, governed by environmental change, could explain why crocodiles have changed so little since the dinosaurs’ age.

Crocodiles
The crocodiles have had a much greater diversity of forms in the past. Examples include fast runners, digging and burrowing forms, herbivores, and ocean-going species Image: University of Bristol

Their evolution rate is usually slow, but occasionally they evolve quickly when the climate has changed. Specifically, this new study recommends that their evolution accelerates when the climate is warmer and increases its body size.

Lead author Dr. Max Stockdale from the University of Bristol‘s School of Geographical Sciences, said: “Our analysis used a machine-learning algorithm to estimate rates of evolution. Evolutionary rate is the amount of change that has taken place over a given amount of time, which we can work out by comparing measurements from fossils and taking into account how old they are.”

“For our study, we measured body size, which is important because it interacts with how fast animals grow, how much food they need, how big their populations are, and how likely they are to become extinct.”

The limited diversity of crocodiles and their apparent lack of evolution result from a slow evolutionary rate. In other words, crocodiles arrived with an already efficient and versatile body plan that doesn’t require changes. The versatility might also be why crocodiles survived the meteor impact at the end of the Cretaceous period, in which the dinosaurs perished.

Crocodiles generally thrive better in warm conditions because they cannot control their body temperature and require warmth from the environment. The climate during the age of dinosaurs was more generous than today, which may explain why there were many more crocodile varieties than we see now.

Dr. Stockdale added“It is fascinating to see how intricate a relationship exists between the earth and the living things we share it with. The crocodiles landed upon a lifestyle that was versatile enough to adapt to the enormous environmental changes that have taken place since the dinosaurs were around.”

Journal Reference:
  1. ‘Environmental Drivers of Body Size Evolution in Crocodile-Line Archosaurs’ by Dr. Maximilian T. Stockdale and Professor Michael J. Benton in Nature Communications Biology. DOI: 10.1038/s42003-020-01561-5

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