Crocodiles are large reptiles with scaly complexions and menacing marble-like eyes. If we were to go back in time 200 million years, we would find that the crocodile's appearance would be unchanged. Today, there are only 25 different crocodile species, a stark difference in diversity compared to birds, which appeared on the fossil record millions of years after crocodiles did but have since evolved into 10,000 other species.
In a study published this month in Communications Biology, researchers of the University of Bristol found crocodiles have kept their distinct features over millions of years and lack diversity because of punctuated equilibrium, or long periods where species are stable. During these eras, the need to evolve only arises when the animal’s environment forces them to adapt to new conditions, writes Max T. Stockdale, an expert in vertebrae macroevolution and paleontology and who was involved in the study, in the Conversation.
Fossil remains of crocodiles from as early as the Jurassic period show identifying characteristics in modern crocodiles. They both have lengthy snouts, husky tails, and rough, scaly hides. Stockdale and his team used an evolutionary mathematical model to analyze and compare modern crocodiles' body sizes with fossils of prehistoric crocodiles, he explains in the Conversation. The data collected revealed that most prehistoric crocodiles were evolving at a slower rate over time, with a few outliers evolving faster.
It turns out, slow and steady wins the race. The crocodiles that evolved slower were more likely to survive normal Earth-like conditions if there are no pressures to change, reports Caroline Delbert for Popular Mechanics. The other crocodiles that evolved faster went extinct.
Surviving crocodiles did not change throughout millions of years because they arrived at an equilibrium where they were efficient and versatile enough that they did not need to evolve to exist, reports the Conversation. Punctuated equilibrium is seen in species that evolve based on external factors like climate change or mass extinctions rather than evolving through sexual selection or predator versus prey situations. The crocodile may have reached this equilibrium through their abilities to survive without eating for long periods, and their sensitivity to temperature changes.
The researchers hope to use these findings to piece together Earth's evolutionary history and continue to look for other environmental factors that could have been a factor in the crocodile's consistent appearance.