Scientists may have solved one of the oldest dinosaur mysteries

Humanity has long pictured dinosaurs as slow, heavy creatures that roamed the Earth with giant feet millions of years ago.

Due to their comparison to reptiles, dinosaurs were long-assumed to be cold-blooded; however, with studies indicating it could be the opposite, experts have debated this question for a long time.

In a new study published in the journal Nature, a Yale-led team of researchers has finally shed light on this question once and for all by developing a novel method for analyzing the metabolic rates of animals -- even extinct ones! Their strategy relies on hints left in their bones by the amount of oxygen they once inhaled.

By turning up the heat on dinosaur metabolism, the researchers have discovered that the earliest dinosaurs and pterosaurs had extremely high metabolic rates and were, in fact, warm-blooded creatures.

Dinosaurs, metabolism, and oxygen

In a nutshell, metabolism is the series of chemical events that support life in organisms and is how successfully we transform the oxygen we breathe into chemical energy that nourishes our bodies.

The efficiency with which an animal turns oxygen into energy is essentially defined by its metabolism: warm-blooded animals, for example, have high metabolic rates, requiring them to take in more oxygen and eat more food to maintain their body temperature.

When animals breathe, this sets off a chain of biochemical events that leave molecular waste products in their bones, with the quantity of waste produced scaling directly with the amount of oxygen used.

This basically means that breathing records whether the animal was warm-blooded or cold-blooded. The best part is that these identifiers withstand the fossilization process.

Turning up the heat on dinosaur metabolism

According to the paper’s lead author, Jasmina Wiemann, affiliated with Yale University and the Natural History Museum of Los Angeles County, the team’s findings are “really exciting for us as paleontologists – the question of whether dinosaurs were warm- or cold-blooded is one of the oldest questions in paleontology, and now we think we have a consensus – that most dinosaurs were warm-blooded.”

The researchers utilized Raman and FTIR spectroscopy to look for these chemical markers in the femurs of 55 different animal species, including dinosaurs, flying pterosaurs, and marine plesiosaurs, as well as current birds, mammals, and reptiles, according to a press release. Then, to find out what their metabolic rates might have been, the researchers compared the molecular profiles of the latter to those of the extinct animals.

What they discovered was extraordinary: most species, including the pterosaurs, plesiosaurs, sauropods (long-necked dinosaurs like Brachiosaurus), and theropods (predatory dinosaurs like T-rex), were warm-blooded. In fact, some of them had metabolisms that were greater than mammals and closer to birds. Others, such as Stegosaurus and Triceratops, appeared to have lower metabolic rates comparable to present cold-blooded reptiles.

This finding is incredibly exciting as it offers fascinating new insights into the physiology and even lives of dinosaurs and other prehistoric creatures. Moreover, the researchers now have a new biomolecular tool for studying ancient animals, which can greatly aid paleontologists as well as zoologists. 

"Our goal is to provide a more complete picture of how animal physiology responded to past environmental and ecological change and contribute to the lessons of the past that will guide future strategies for biodiversity conservation in times of global climate change,” Wiemann said. 

Study Abstract:

Birds and mammals independently evolved the highest metabolic rates among living animals1. Their metabolism generates heat that enables active thermoregulation1, shaping the ecological niches they can occupy and their adaptability to environmental change2. The metabolic performance of birds, which exceeds that of mammals, is thought to have evolved along their stem lineage3,4,5,6,7,8,9,10. However, there is no proxy that enables the direct reconstruction of metabolic rates from fossils. Here we use in situ Raman and Fourier-transform infrared spectroscopy to quantify the in vivo accumulation of metabolic lipoxidation signals in modern and fossil amniote bones. We observe no correlation between atmospheric oxygen concentrations11 and metabolic rates. Inferred ancestral states reveal that the metabolic rates consistent with endothermy evolved independently in mammals and plesiosaurs, and are ancestral to ornithodirans, with increasing rates along the avian lineage. High metabolic rates were acquired in pterosaurs, ornithischians, sauropods and theropods well before the advent of energetically costly adaptations, such as flight in birds. Although they had higher metabolic rates ancestrally, ornithischians reduced their metabolic abilities towards ectothermy. The physiological activities of such ectotherms were dependent on environmental and behavioural thermoregulation12, in contrast to the active lifestyles of endotherms1. Giant sauropods and theropods were not gigantothermic9,10, but true endotherms. Endothermy in many Late Cretaceous taxa, in addition to crown mammals and birds, suggests that attributes other than metabolism determined their fate during the terminal Cretaceous mass extinction.