Reptiles provide new clues about the diets of extinct animals

Scientists have long pondered how evolution resulted in the development of both herbivores and carnivores. To understand this, it is important to clarify what type of food did extinct vertebrates eat. But how can we know what the diets of these creatures were? 

While in living animals we can directly observe what they feed on, in the case of extinct species researchers must rely on morphological or chemical information supplied by fossils. 

Now, a research team led by professor Thomas Tütken from Johannes Gutenberg University Mainz (JGU) has compiled a reference framework of isotope compositions for modern reptiles. This dataset stands to improve our ability to identify the diets of ancient reptiles, including dinosaurs.

“This reptile reference framework is a convenient toolbox that will enable us to better reconstruct the diet of extinct animals, such as dinosaurs,” Tütken explained. “A set of comparable data of this kind has not previously been available.” 

Diverse diets of modern reptiles 

Today’s reptiles showcase a remarkable range of diets, from strictly plant-based to exclusively carnivorous. Many also exhibit specialized feeding preferences, such as the marine iguana in the Galapagos, which eats only algae, or chameleons, which subsist solely on insects. 

Gila monsters prefer eggs, while top predators like the saltwater crocodile and the Komodo dragon feed entirely on meat. “This diversity makes it difficult to reconstruct the nutritional habits of extinct animals,” Tütken said.

The oldest known ancestors of modern reptiles lived more than 300 million years ago; for instance, the 315-million-year-old Hylonomus resembled a lizard. These early reptiles often bore skulls, jaws, and teeth indicative of insectivory. 

The point in geological history when they transitioned from insect-eating to purely carnivorous or herbivorous diets remains unclear.

Building a geochemical reference

In creating their new framework, Tütken’s team examined 28 modern reptile species by measuring calcium and strontium isotopes in their bones and teeth, which offer chemical clues about what these animals eat. 

“To obtain a wide spectrum for comparison, we selected reptiles with pronounced herbivorous or carnivorous behavior, and we also included those with specialized dietary preferences,” Tütken noted. The list included alligators, monitor lizards, chameleons, and more.

Focusing on calcium isotopes, the researchers highlighted the ratio of calcium-44 to calcium-42. They observed a clear trend in which these isotope concentrations decreased at every step in the food chain. 

“Overall, the values are highest in insectivorous reptiles, clearly differentiating these from animals with other feeding behaviors,” said lead author Michael Weber, a paleontologist at JGU. 

Detecting unique feeding patterns 

Herbivores showed lower isotope ratios, while carnivores presented the lowest levels. This approach even allowed them to detect unique feeding patterns, such as the algae diet of marine iguanas and the egg-based diet of other reptiles. The team also examined stable strontium-88 to strontium-86 ratios. 

“We set up the first extensive reference system of stable strontium isotopes as a dietary proxy. The results for strontium are largely consistent with those for calcium isotopes in respect of the various nutritional forms,” explained study co-author and paleontologist Katrin Weber. 

However, strontium is more vulnerable to alteration over time, making calcium isotopes the more reliable measure for extinct species.

Comparing reptiles to mammals

In analyzing these isotopes, the researchers discovered that reptiles show greater calcium isotope variability than mammals, even under similar dietary regimes. 

This phenomenon is believed to stem from physiological differences, and it illustrates why mammal-based data cannot directly determine the diets of prehistoric reptiles such as dinosaurs.

Teeth wear patterns and animal diets

Beyond chemical traces, the scientists also investigated the mechanical wear patterns on modern reptile teeth, collecting data in collaboration with Dr. Daniela Winkler at Kiel University. 

Scratches on tooth surfaces reveal whether an animal consumed tough materials such as shells or bones, or softer foods. 

When combined with isotope data, these wear patterns can distinguish faunivores that ingested abrasive substances from those relying on soft food sources.

Discovering the diets of extinct species

“In order to understand and interpret the data for chemical and mechanical patterns of nutritional behavior from fossil finds, we first had to determine the corresponding information for related modern species with their known dietary preferences,” said Tütken.

“As a result, we now have a reference frame for comparison that enables the precise diet assessment and thus a more accurate reconstruction of the feeding habits of extinct species.”

The study forms part of a larger initiative led by Tütken, who in 2016 received an ERC Consolidator Grant to examine the feeding habits of early land vertebrates. 

By combining geochemical and mechanical evidence, Tütken’s team has created an essential resource for scientists to unravel the mystery of what ancient reptiles – and possibly dinosaurs – might have eaten, guiding us toward a deeper understanding of evolutionary history.

The study is published in the journal Proceedings of the Royal Society B.

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