Why have crocodiles changed so little over the last 200 million years? Crocodiles today look very similar to those that lived during the Jurassic period alongside the dinosaurs, and they have also not diversified much since that time.
In a new study from the University of Bristol, scientists have determined that a “stop-start” pattern of evolution, governed by environmental change, may explain why crocodiles have changed so little over millions of years.
There are only 25 species of crocodiles alive today. Other animals, like birds and lizards, have branched off into thousands of species in the same time frame.
The researchers pointed out that there were also types of ancient crocodiles that vanished, including giants as big as dinosaurs, plant-eaters, fast runners, and serpentine forms that lived in the sea.
In their study, the scientists explain how crocodiles follow a pattern of evolution known as “punctuated equilibrium.” This means that they evolve slowly, but this rate picks up speed occasionally due to environmental changes.
In particular, the researchers found that the evolution of crocodiles is accelerated in a warmer climate, and their body size increases.
“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,” said study lead author Dr. Max Stockdale.
“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 findings show that a slow rate of evolution is responsible for the limited diversity of crocodiles. It seems that the crocodiles arrived at a body plan that was very efficient and versatile enough that they did not need to change it in order to survive.
Such versatility could explained how crocodiles survived the meteor impact and subsequent environmental changes that killed the dinosaurs.
Crocodiles cannot control their body temperature and require warmth from the environment. The climate during the Jurassic period was warmer than it is today, which likely supported the diverse types of crocs that no longer exist.
“It is fascinating to see how intricate a relationship exists between the Earth and the living things we share it with,” said Dr. Stockdale. “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.”
Next, the Bristol team will investigate why some types of prehistoric crocodile died out, while others persisted.
The study is published in the journal Communications Biology.