Then… What is not a Dino?

In a previous article, we looked at what dinosaurs actually are: a well-defined evolutive group. However, there are a few animal groups (mostly large extinct reptiles) that we associate with dinosaurs, or even think of as dinosaurs, but they are not. So, if not dinosaurs, what were those membrane-winged reptiles and those fully aquatic creatures?

Pterosaurs: The winged reptiles, those fuzzy, warm-blooded animals were the closest relatives of dinosaurs without being dinosaurs proper. They evolved from dinosaur-like animals like Lagerpeton and exploded in diversity alongside their dinosaurian cousins (around 200 million years ago), being the first flight-capable vertebrates, and the most efficient ones thus far. The largest ones, like Quetzalcoatlus, reached a 10-meter wingspan, and remained flight-capable despite having a build comparable to a giraffe. They went extinct alongside non-bird dinosaurs 66 million years ago, when a meteor impacted modern Yucatán and wrecked the biosphere.

Figure 1. a) Skeletal reconstruction of Lagerpeton, a close relative of the ancestral pterosaur. By Wikimedia Commons user Maurissauro, licensed under Creative Commons Attribution-Share Alike 4.0. b) Original specimen of Pterodactylus, found in the Bavarian Solnhofen limestone circa 1780. Photo licensed under Creative Commons Attribution-Share Alike 3.0 Unported. c) Pteranodon mount in launching posture, with clear view of the unusual proportions of these animals. Photo licensed under Creative Commons Attribution-Share Alike 4.0 International. d) Artistic depiction of Quetzalcoatlus foraging behaviour, by Mark Witton (2008). Licensed under Creative Commons Attribution-Share 3.0 Unported.

Marine reptiles: Just like modern marine mammals, they are not a single group, but an assortment of different groups adapted to a marine lifestyle of which only sea turtles survive. Other than sea turtles, those animals tended to evolve warm bloodedness and live bearing. The most notable groups are the following:

–Plesiosaurs: Broad-bodied, short-tailed, with four flippers and coming in two main flavours (long necked and small headed fish eaters like Elasmosaurus, and short necked and big-headed meat eaters like Pliosaurus), These reptiles are a bit hard to pin in the family tree: turtles may be their closest living relatives, but that’s not as certain as the nature of dinosaurs or pterosaurs. They became diverse at the same time as dinosaurs and pterosaurs being also meteor victims (however, the big headed pliosaurs went extinct some 20 million years earlier).

–Ichthyosaurs: Fish reptiles that were highly adapted to an aquatic life, being very similar in shape to dolphins and sharks, with the larger ones such as Shonisaurus being whale-sized and shaped. Their position in the family tree is even murkier than that of plesiosaurs (maybe their relatives?). They diversified before dinosaurs, pterosaurs and plesiosaurs did; yet shared the oceans with plesiosaurs for most of their history. However, they didn’t make it to the meteor, going extinct at the same time as pliosaurian plesiosaurs.

–Mosasaurs: Those shark-like reptiles, whose pop culture presence has surged in the last decade despite being known for two centuries, are a curious case as they are actual lizards, close relatives of monitors, Gilas and slowworms. They rapidly evolved from monitor-like lizards after ichthyosaurs and pliosaurs went extinct, taking over their niches as shark-like predators in the final stretch of the Mesozoic era. But this highly successful evolution experiment was cut short by the meteor impact.

Figure 2. a) Museum mount of the long-necked plesiosaur Elasmosaurus. Creative Commons Attribution-Share Alike 2.0 Generic. b) Reconstruction of the short-necked plesiosaur Pliosaurus. Creative Commons Attribution-Share Alike 4.0 International. c) Pregnant specimen of the ichthyosaur Stenopterygius. Finding show that those animals had a blubber and soft tissue fin. Creative Commons Attribution-Share Alike 4.0. d) Museum mount of the giant marine lizard Mosasaurus, one of the largest macropredators of all time. Creative Commons Attribution-Share Alike 3.0 Unported.

Dimetrodon and co.: The superficially lizard-like, sail-backed Dimetrodon is not even a reptile, but an early mammal relative (synapsid) which lived 65 million years BEFORE the oldest known dinosaurs. Even if it is the most seen synapsid, there are other mammal relatives that often make it into the dinosaur toy bin: the beaked and tusked dicynodonts (such as Lystrosaurus) and the saber-toothed, somewhat badger-like gorgonopsians (such as Inostrancevia) are both common sights in dinosaur-adjacent media.

Figure 3. a) Reconstruction of Dimedon, with skin texture based on relative, by Max Belonio (2019). Creative Commons Attribution-Share Alike 4.0. b) Reconstruction of the dicynodont Placerias, by Jeff Martz (2012). Creative Commons Attribution-Share Alike 2.0 Generic. c) Museum mount of the gorgonopsian Lycaenops. Public domain image.

Mammoths and co.: Mammoths, saber-tooth cats, ground sloths and other ice age fauna are often associated with dinosaurs, but those animals are, for all intents and purposes, extinct modern animals. Woolly mammoths, for example, were a species of elephant that evolved alongside reindeer and musk oxen. They went extinct while the pyramids were being built while the rapid spread of early modern humans, placed heavy strain in its habitat.

Figure 4. Artistic depiction of northern Spain, 12000 years ago. In a tundra environment, wooly mammoths and wooly rhinos thrive alongside wild horses, reindeer, and lions. Artwork by Mauricio Anton (2008). Creative Commons Attribution 2.5 Generic.  

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