Dinosaurs Get a New Family Tree

The dinosaurs left behind a treasure trove of fossils. Over 65 million years later, humans began gazing on these fossils with excitement and curiosity: what were these fearsome giants like? Early paleontologists, noting the apparent anatomical similarities between dinosaurs and modern reptiles, naturally assumed that dinosaurs were just like reptiles: cold-blooded, slow-moving beasts. This assumption went unchallenged for 150 years, and generations of children imagined an ancient Earth populated by lumbering, Godzilla-like creatures. But new research suggests these early behemoths were neither cold-blooded nor warm-blooded, but rather somewhere in between.

Knowing the true body temperature of dinosaurs would make it easier to understand how they lived and evolved. In a recent paper in Science, paleontologists John Grady and Felisa Smith from the University of New Mexico in Albuquerque suggest that dinosaurs belonged to a hitherto unknown metabolic category. Their findings may help explain how dinosaurs outcompeted other animals to rule the Earth for 135 million years.

Just Add Heat

Recently, several paleontologists challenged the 150-year-old assumption that dinosaurs were cold-blooded, and pointed to evidence that these ancient animals might actually have been warm-blooded. Strong support for this argument came from the theory that warm-blooded animals tend to grow quickly compared to cold-blooded animals. Because the dinosaurs ruled the Earth for so long, scientists reasoned they must have been comparable in growth speed to the endotherms that evolved during the age of the dinosaurs.
Called endotherms, from the Greek for "internal heat," warm-blooded animals enjoy rapid growth rates. These animals, including mammals and birds, generally have fast metabolisms, which allow them to quickly break down food and to produce large amounts of energy, much of it in the form of body heat. This allows endotherms to maintain a constant body temperature. Endotherms are active in places with a wide range of temperatures, but require a constant supply of food to maintain their body temperatures. Because they eat constantly, most endotherms grow steadily and quickly.

In contrast, the body temperature of cold-blooded ectotherms, whose name comes from the Greek for "external heat," is subject to the environment. These animals, among them crocodiles and turtles, must bask in the sun to warm up. Because of their slow metabolisms, ectotherms generate little body heat and energy, and are generally slow-growing. While ectotherms do not need to eat much or often, their activity is limited to the warmer hours of the day, after they have sunned themselves enough to raise their body temperatures. Based on these considerations, some scientists reasoned that dinosaurs would not have been able to compete with fast-growing endotherms if they themselves had been cold-blooded and slow-growing like many reptiles.


Dinosaur bone structure also suggests that these animals differ from modern ectothermic reptiles. Cold-blooded animals grow haltingly, experiencing more rapid growth during the warmer months and slower growth during the colder ones. As a result of this uneven growth, cross-sections of ectotherm bones contain rings similar to tree rings. Endotherms, which grow constantly, lack bone rings. Their bones instead take on a spongy appearance due to infiltrating blood vessels, which bring the nutrients necessary to sustain constant growth. While dinosaur bones have the growth rings associated with ectotherm bones, they also have the spongy appearance of endotherm bones.

The Metabolic Middle

Grady and Smith, along with a small team, decided that the best way to determine whether dinosaurs were endotherms or ectotherms was to compare the growth rates of dinosaurs to those of as many warm- and cold-blooded species as possible. The scientists examined pre-existing data from over 350 living species that were known to be either endothermic or ectothermic, and concluded that, as was to be expected, the endothermic animals all had faster growth rates than the ectothermic animals. Now the scientists simply had to determine the growth rates of various dinosaurs and compare them to those of the 350 living species in their study.


Luckily, the researchers could collect all the data they needed from the fossil record. Dinosaur bones contain many clues regarding growth rate: the animals' ages can be inferred from bone growth rings, similar to establishing the age of a tree, and their body sizes can be extrapolated from bone sizes. Thus, the researchers calculated the growth rates of 21 species of dinosaurs based on their inferred ages and sizes. Perhaps not so surprisingly, Grady and Smith came up with dinosaur growth rates that fell between those for endo- and ectotherms; dinosaurs, they found, grew faster than cold-blooded animals like reptiles, but slower than warm-blooded animals like mammals and birds. Because growth rate is a strong predictor of ecto- or endothermy, the scientists created a new category of animals, which they called mesotherms, from the Greek for "middle heat"; the new category was populated almost — but not quite — entirely by dinosaurs.

Mesothermy Is Not Extinct

Besides dinosaurs, the new mesotherm category includes a small but interesting assortment of living species. Among them are tuna and some species of sharks and turtles; these creatures are able to raise their body temperature from within, but, unlike endotherms, do not keep it constant. The only mammalian example of a mesotherm turned out to be the spiny anteater or echidna; it maintains a constant internal temperature, but, unlike normal endotherms, can tolerate dramatic changes in that temperature. Besides having similar metabolisms, these latter-day mesotherms also resemble dinosaurs by appearing to have growth rates falling between those of endotherms and ectotherms.


Grady, Smith and their colleagues also concluded that feathered dinosaurs like Archaeopteryx and Troodon had growth rates similar to those of other, non-feathered dinosaurs. This is surprising because it suggests that the evolution of feathers and flight, traits found in both modern birds and certain dinosaurs, can be separated from the evolution of other birdlike traits such as extremely fast growth rates, which are characteristic of modern birds but not of dinosaurs.

Is a Middling Metabolism the Way to Go?

Dinosaurs ruled the Earth for over 135 million years. It now appears likely that this domination was due in part to dinosaur mesothermy. Perhaps dinosaurs, neither warm-blooded nor cold-blooded, enjoyed an advantageous middle road: a metabolism fast enough to provide some body heat and comfort over a wide range of temperatures, but slow enough that the dinosaurs did not have to constantly search for food. Although we may never know for sure what these giant creatures were like, clues from their bones suggest that they were not simply lumbering giant reptiles. Instead, it appears they were something much more unique.

Discussion Questions

How do you think mesothermy evolved separately in species as different as dinosaurs, tuna and anteaters? Some birds have extremely fast growth rates, even compared to other endotherms, while primates have relatively slow growth rates. If growth rates fall along a spectrum, how can they be limited to three categories: endothermic, ectothermic and mesothermic?