In the 1993 Steven Spielberg-helmed epic blockbuster Jurassic Park, on beholding a living dinosaur (Brachiosaurus) for the first time on the fictitious Isla Nublar, protagonist Dr Alan Grant emphatically says, “We could tear up the book on cold-bloodedness. It doesn’t apply. They’re totally wrong. This is a ‘warm-blooded’ creature.” After decades of study and speculation, the more accepted theory today is that dinosaurs, once considered cold-blooded reptilian lizard ancestors, were indeed warm-blooded creatures, much like their bird descendants and mammals of today.
A new study, led by researchers at University College London and University of Vigo, Spain, proposes that the ability to regulate body temperature, a trait all mammals and birds possess today, may have evolved among some dinosaurs in the early stage of the Jurassic period, about 180 million years ago. This questions the earlier plausible belief that stemmed in the early 20th century, that dinosaurs were slow-moving, ‘cold-blooded’ animals like modern-day reptiles, relying on heat from the Sun to regulate their temperature. Newer discoveries, however, indicate some dinosaur types were likely capable of generating their own body heat, but when this adaptation occurred is a mystery.
The new study, published in journal Current Biology, observed the spread of dinosaurs across different climates on Earth throughout the Mesozoic Era (the dinosaur age lasting 230-66 mya), drawing on 1,000 fossils, climate models and the geography of that period, and the dinosaurs’ evolutionary trees. The research team found that two of the three main groupings of dinosaurs, theropods (eg. Tyrannosaurus rex and Velociraptor) and ornithischians, including relatives of the herbivorous Stegosaurus and Triceratops, moved to colder climates during the Early Jurassic, suggesting that they may have developed endothermy (the mechanism of heat generation without shivering that increases body temperature and resting metabolic rate) at this time. In contrast, sauropods, the other main grouping, which includes the Brontosaurus and the Diplodocus, kept to warmer areas of the planet.
Previous research has found traits linked to warm-bloodedness among ornithischians and theropods, with some known to have had feathers or proto-feathers, insulating internal heat. “Our analyses show that different climate preferences emerged among the main dinosaur groups around the time of the Jenkyns Event 183 million years ago, when intense volcanic activity led to global warming and extinction of plant groups,” first author Dr Alfio Alessandro Chiarenza of UCL Earth Sciences says, adding, “At this time, many new dinosaur groups emerged. The adoption of endothermy, perhaps a result of this environmental crisis, may have enabled theropods and ornithischians to thrive in colder environments, allowing them to be highly active and sustain activity over longer periods, to develop and grow faster and produce more offspring.”
Extinction event
The Jenkyns Event, also called the Toarcian Extinction Event, the Pliensbachian-Toarcian Extinction Event, the Early Toarcian Mass Extinction, or the Early Toarcian Palaeoenvironmental Crisis, was an extinction event that occurred during the early part of the Toarcian Age, approximately 183 mya, during the Early Jurassic. It occurred after lava and volcanic gasses erupted from long fissures in the Earth’s surface, covering large areas of the planet.
Meanwhile, co-author Dr Sara Varela of the Universidade de Vigo suggests, “Theropods also include birds and our study suggests that their unique temperature regulation may have had its origin in this Early Jurassic epoch. Sauropods, on the other hand, which stayed in warmer climates, grew to gigantic sizes at around this time – another possible adaptation due to environmental pressure. Their smaller surface area to volume ratio would have meant these larger creatures would lose heat at a reduced rate, allowing them to stay active for longer.”
In the paper, the researchers have also investigated if sauropods might have stayed at lower latitudes to eat richer foliage unavailable in colder polar regions. Instead, they found sauropods seemed to thrive in arid, savannah-like environments, supporting the idea that their restriction to warmer climates was more related to higher temperature and then to a more cold-blooded physiology. During that time, polar regions were warmer, with abundant vegetation.
Co-author Dr Juan L Cantalapiedra of the Museo Nacional de Ciencias Naturales, Spain, says, “This research suggests a close connection between climate and how dinosaurs evolved. It sheds new light on how birds might have inherited a unique biological trait from dinosaur ancestors and the different ways dinosaurs adapted to complex and long-term environmental changes.”
Even as the Earth and its climate continued to transform rapidly, dinosaurs dominated the planet’s fauna for over 165 million years, surviving, adapting, evolving, and succeeding to change.
What’s the difference?
Cold-blooded
Cold-blooded animals cannot generate their own body temperature, relying on the environment to regulate it
They obtain their energy from the surrounding environment
Their rate of metabolism is dictated by the change in environment
They cannot sustain/survive in extreme temperatures
Types: Insects, molluscs, arthropods, fish, amphibians, and reptiles
Warm-blooded
Warm-blooded animals can generate their own body temperature, not needing to depend on the environment to regulate it
They obtain their energy through proteins and vitamins from the food they consume
Their rate of metabolism does not rely upon any change in environment
They are able to maintain a steady body temperature of 37-40°C
Types: Mammals and birds
Human Body Temperature
The internal body temperature in human beings is regulated by a part of the brain called the hypothalamus. The hypothalamus checks our current temperature and compares it with the normal temperature of about 37°C. If our temperature is too low, the hypothalamus makes sure that the body generates and maintains heat. Any readings above the normal temperature threshold is considered as fever.
Inertial Homeothermy
According to some studies, large dinosaurs may have also been able to maintain their temperatures by inertial homeothermy, also known as ‘bulk homeothermy’ or ‘mass homeothermy’. Inertial homeothermy refers to the maintenance of a relatively constant body temperature that occurs simply because of large size. This concept is often applied to large dinosaurs. Moreover, biophysical modelling and actual measurements show that large crocodiles can behaviourally achieve body temperatures above 30°C. In other words, the thermal capacity of such large animals was so high that it would take two days or more for their temperatures to change significantly, and this would have smoothed out variations caused by daily temperature cycles. This effect has been observed in large turtles and crocodilians, but Plateosaurus, which weighed about 700 kg, may have been the smallest dinosaur in which it would have been effective.
