Some 400 million years ago, animals emerged from the ocean for the first time and began to thrive on land. But after eons of evolutionary experimentation, some lineages peaced out of the terrestrial world and embraced a full-time life at sea again. While whales and dolphins are among the best known marine creatures to follow this path, it turns out some ancient relatives of modern crocodiles, called thalattosuchians, took the same aquatic turn while dinosaurs roamed the planet.
According to a study published Monday in the Proceedings of the National Academy of Sciences, these predatory crocodilians are the only archosaurs — the group that also includes birds and dinosaurs — known to have shifted from land to a lifestyle spent always in the ocean.
Thalattosuchian fossils are scattered across the world, and the researchers believe their transition from land to sea began about 182 million years ago. While they likely breathed through their noses rather than blowholes, these Jurassic animals eventually evolved to have smooth skin and body shapes reminiscent of whales and dolphins today.
To reach this finding, Julia Schwab, a doctoral student in paleontology at the University of Edinburgh in Scotland who led the study, and her colleagues focused on crocodilian sensory systems. They examined the rigid structure of the inner ear called the bony labyrinth. The inner ear moderates balance and equilibrium, making it “one of the most important sensory organs,” Ms. Schwab said. “You can tell a lot, looking at the shape of the inner ear, about the environment in which an animal lived.”
The team used computed tomography to analyze the bony labyrinths of 18 extinct reptiles and 14 modern relatives. The results revealed that thalattosuchians evolved thicker ear canals and a more compact labyrinth as they strayed from land, perhaps to adapt to pressure differences in the water.
“Some of them lived in coastal lagoonal areas,” where the water was relatively shallow, Ms. Schwab said.
“But there’s evidence that some of them were possibly deep divers,” she added. “They were perfectly adapted for life in the open ocean.”
The bony labyrinths of cetaceans, the family of marine mammals that includes whales and dolphins, experienced similar shifts when those animals entered the ocean some 50 million years ago. But while cetaceans adapted to ocean life rapidly, thalattosuchians experienced a protracted semiaquatic phase that lasted tens of millions of years.
“The most interesting discovery of this study is that in these marine crocs, the evolution of inner ear features related to a marine lifestyle occurred well after most of the skeleton had adapted to that lifestyle,” said Andrea Cau, a vertebrate paleontologist who has researched thalattosuchians extensively but was not involved in the study.
“For a long part of their history, even the fully marine thalattosuchians kept an inner ear built like their less-aquatic relatives, and this is intriguing.”
It’s not exactly clear why the Jurassic crocs were slower to adapt than cetaceans, but one possible explanation is that cetaceans had a reproductive edge. “Modern crocodiles lay eggs, and they need to go on land to do that,” Ms. Schwab explained. “If you’re an animal perfectly adapted for life in the ocean, and have flippers, then it might be difficult to still go on land to lay those eggs.”
Perhaps thalattosuchians had to evolve live birth before they could completely untether themselves from land. As potential backup for this hypothesis, Ms. Schwab said there is preliminary evidence that thalattosuchian hips widened over time.
The team plans to continue researching other skull features, such as the brain and sinus cavities, to learn more about the sensory changes that realm-switching animals experience over time.
“It’s important to look at the sensory systems because those systems are the key to understanding the past, and past animals,” Ms. Schwab said, “and to also understand how animals are going to evolve in the future.”