What is a dino?

If you happen to know a dinosaur-loving kid aged from 3 to 93, you may have been told that the pigeons and ducks in the park are dinosaurs, or an innocent “my favourite dinosaur is the Pterodactyl” may have been replied with “Pteranodon is not a dinosaur”. As the implication is that not all long-gone reptiles qualify for the label, AND that they may still be around, you may be wondering “what is actually a dinosaur?”

“Dinosaur” (roughly meaning “terrific reptile”) was coined as a word in the 1840s, to refer to a trio of rhino-sized animals discovered in the previous decades: the carnivore Megalosaurus, the herbivore Iguanodon, and the armoured Hylaeosaurus. The remains were scarce, and thus the restorations imprecise, yet they were found to share anatomical details that pointed to them being active land animals with an upright stance, more like mammals than to large lizards, despite clearly being reptilian.

However, ideological shifts in the early 20th century disregarded most of those assumptions: dinosaurs were deemed evolutive failures unrelated to any living animal, perhaps not even to each other. The word itself got tainted into pretty much “big dead lizard”. However, in the 1970s, the situation started to change: The discovery of Deinonychus (forever known in pop culture by the name of its relative Velociraptor), a nimble and birdlike predator built for agility, lead a new generation of palaeontologists to challenge then-conventional knowledge. Dinosaurs steadily became understood as birdlike, warm-blooded active animals, a view that made it into pop culture with a highly popular 1993 film. Subsequent discoveries have only reinforced those notions.

So then, what is a dinosaur? The advent of classification based on most recent common ancestry (cladistics) in the 1980s gave us a definition: a dinosaur is a descendant of the most recent common ancestor shared by Iguanodon and Megalosaurus (and Diplodocus, just to avoid potential issues). And well, this is an evolutive definition, which tells us what belongs and doesn’t belong in the group (birds do, as they are descendants of that common ancestor, while stuff like pterosaurs doesn’t, as they aren’t). But what physical characters do we have to look for in a dinosaur? Large size? It appears to have evolved independently many times within dinosaurs. Fuzziness and a birdlike physiology? Those appear to be older; ancestral to dinosaurs but also present in close non-dinosaurian relatives. The key, among tiny yet highly informative details of skeletal anatomy, appears to be related to the old character of “upright posture”: the shape of the hip and ankle joints is unique to dinosaurs. It tells them apart from their closest non-dinosaurian relatives and gives them the upright posture that caught the eyes of scientists 2 centuries ago.

So, the common children’s book definition of “land-based reptile which lived in the Mesozoic era” falls flat: there are plenty of non-dinosaurs which fit that definition, while a humble duck, which lives in the present and is a semiaquatic flying animal, is a proper dinosaur.