For more than a century, dinosaurs were imagined to be cold-blooded, reptilian beasts. Then, in the 1960s, John Ostrom argued that dinosaurs were warm-blooded, and had evolved into birds. Actually, that view had been espoused a century earlier by scientific gadfly Thomas Huxley, but never caught on. Ostrom’s discovery and study of a Deinonychus* fossil ushered in a paradigm shift: the belief that dinosaurs were warm-blooded, active, social, and bird-like. Yes, social. It turns out, they do move in herds:
*Deinonychus was the model for the velociraptors in Jurassic Park; actual velociraptors were closer to chicken-sized. Sometimes I think I’m the only one who finds it more terrifying to imagine these things as huge flightless eagles rather than lizards.)
Paleontologist Jack Horner “made his bones” when he discovered a clutch of hornbill dinosaur eggs and nesting sites in Montana. He was one of the first to study dinosaur rearing and juvenile dinosaur behavior, which further changed old notions about dinosaurs. Previously, they were assumed to be cold-blooded and cold-hearted, absentee parents that did not care for their young, a view that Horner helped overturn. Dinosaur Lives describes some of Horner’s digs and beliefs about the the behavior and physiology of dinosaurs.
• • •
Let’s go back to the warm-blooded vs. cold-blooded distinction: though useful shorthands, they don’t fully describe animal physiology. In unearthing duck-billed dinosaurs, they discovered that infant duckbill bones are primarily cartilage, which suggests that dinosaurs are altricial and require parental care after birth (like humans). Altricial species also tend to grow quickly, as that minimizes the amount of time they spend helpless after birth, and skeletal evidence suggests duckbills were fast growers. And because rapid growth usually demands warm-bloodedness and a fast metabolism to convert food energy into body weight, it leads to the straightforward conclusion that duckbills were warm-blooded endotherms.
There’s a but here: if that rapid metabolism held throughout their life, the duckbills would grow to astronomical proportions. If metabolism was fast for infants and juveniles, it almost assuredly slowed down for adults.* And when you look at the bone structure of adult duckbills, it’s similar to modern-day crocodiles, which are cold-blooded. This leads to Horner’s argument that duckbills were likely warm-blooded as infants, with a fast metabolism to support rapid growth, but cold-blooded as adults—and so we can’t easily classify them in a simple warm vs. cold dichotomy.
*Extremely large animals can be gigantothermic, which happens when less of their mass is exposed to the elements than smaller animals, so that it takes a long time for their internal body temperature to change.
That dinosaur metabolism changed across their lifespan is part of a larger point Horner makes about taxonomy and the study of extinct animals. Modern Linnean taxonomy is a description of the world as it exists. And while we believe the categories that fall out of this taxonomic structure reveal how and when different species diverged, evolutionary ancestors, especially very old ones like dinosaurs, need not fit within these categories. Dinosaurs may have exhibited traits of both warm- and cold-bloodedness in ways that extant species don’t, and haven’t for tens of millions of years.
Horner is now trying to genetically engineer a dinosaur, a plan less Jurassic Park than it sounds. His approach, in essence, is to forcibly de-evolve a chicken into its long-ago evolutionary forefather, which is all weirdly fascinating. Suddenly, the plight of the robot turkeys against the master chickens in the year 9595 makes sense.