Written by Tim Vernimmen
The crocodilian pakasuchus on the illustration is extinct, and scientists have analyzed fossilized teeth and showed that it is a herbivorous animal.
Illustration: Jorge Gonzalez
In children's books and cartoons, crocodiles and their kind often have terrifying fangs, each with sharp teeth ready to rip apart the flesh of their prey. In fact, genetic variation occurs a lot, and so do crocodile teeth, says Keegan Melstrom, a paleontologist at the University of Utah.
"That's nothing compared to the amazing diversity of teeth of extinct crocodile-like reptiles or crocodyliformes," he said. "Some of the extinct crocodile teeth are very strange."
Recently, scientists analyzed 146 teeth of 16 crocodile relatives that have become extinct and came to a shocking conclusion: ancient crocodile relatives were vegetarian at least 3 times.
"This shows that vegetarianism was once a successful dietary strategy," said Melstrom, whose team recently published the study in the journal Contemporary Biology. "I think as more fossil teeth are unearthed, we may find more herbivorous crocodile populations."
Focused chewing
In their analysis of fossil teeth, Melstrom and colleague Randall Irmis used a method specifically designed to compare differences in teeth, a method that paleontologists often used when studying ancient mammals before.
"The point is, we need to calculate how many individual facades each tooth has," melstrom says. "If they tilt in different directions, we think they're separate."
According to studies of extant mammals and reptiles, scientists know that carnivores tend to have simple teeth with few separate facades. For example, the carnivorous Komodo monitor lizard, whose teeth look as thin and sharp as a steak knife, straight and simple, without any grooves and bumps. Such teeth are great for catching prey and cutting them into small pieces so that they can be swallowed without chewing. Some animals have teeth covered with holes and gaps, which increases their surface area and makes the space for teeth to come into contact with food larger, so that they can grind various plant rhizomes that are more wear-resistant.
A pair of 3D printed models show the complex tooth morphology of two crocodile relatives. In terms of tooth morphology, these long-lost ancient crocodiles are herbivores.
Photographed by Mark Johnston, NHMU
"Such teeth are generally herbivores because the leaves, branches and stems of plants usually need to be chewed constantly before they are digested," Melstrom said.
Extant crocodiles are mostly carnivores, and their tooth structure is very simple, Melstrom explains, but in some extinct crocodile species, their teeth have as many as 20 different facades. This suggests that they may require very focused chewing when eating.
"In this group of fossils we studied, the teeth of the mini crocodile (simosuchus) are one of the most structurally complex. The mini crocodile is small, with a mouth and nose that resembles a pig's, as if it had been struck with a shovel," Melstrom said. Their teeth are very similar to those of the sea iguanas of the Galapagos Islands, which feed on algae in the rocks. "Mini crocodiles are not aquatic animals, but are likely to live at the water's edge, so one would speculate that their diet may be similar to that of the marine iguana," Melstrom said.
Surprisingly, Melstrom's research has now given clear evidence that there is more than one vegetarian population. Different and more complex tooth structures have occurred in at least three separate populations, suggesting that the shift from carnivorous to plant-based diets occurred multiple times throughout evolution.
Ohio State University paleontologist Patrick O'Connor was not involved in the study, but he was very interested in the team's approach.
"With the discovery of new fossils, this method of research can be applied and developed, so that several different ideas about why herbivorousness evolves repeatedly in crocodiles can be verified," he said. Diego Pol, who currently works at the Museum of Paleontology in Argentina, agrees, but he also warns that the method of extrapolating dietary structures based on tooth complexity is not flawless, and that scientists should look for other evidence to support their conclusions.
Select Survivors
Herbivorous crocodiles thrived, but the unprecedented catastrophe 65 million years ago destroyed about three-quarters of the planet's species, and herbivorous crocodiles were not spared. Although the existing crocodiles were among the few tetrapods that survived at the time, no vegetarian groups have evolved since then, perhaps because mammals occupy their niche.
"Becoming a herbivore always involves some type of specialization," says Hungarian paleontologist Attila Ősi. When their feeding plants disappear, herbivorousness may become a weakness. It is also worth pondering that not only herbivorous crocodiles, but all land crocodiles are also extinct. Most of the more than 20 surviving crocodile species live in lakes or rivers and occasionally live by the sea, feeding mainly on meat and fish.
However, today's crocodiles are not strictly carnivores. Many species occasionally eat fruit. After giving alligators to plant-based diets for months, the researchers found that nothing was abnormal about their bodies. Obviously, crocodiles are much more flexible than we think, and today's crocodiles are more adaptable than those so-called "living fossils.".
Mikael fortelius of the University of Helsinki in Finland was not involved in the study, but he has applied this method to mammals, and he believes that we should analyze the problem on a case-by-case basis, not generalize, let alone label the animals casually.
"Many extinct crocodiles were not carnivores, most ancient hyenas were not brutal 'bone crushers,' and most ancient rhinos had no horns," he said. "After several generations of evolution, many of today's animals may have very different relationships with their ancestors."
(Translator: Strange Flowers Blossom)
Source: National Geographic Chinese Network (Official v)