In a way, it's much more difficult to identify the smallest dinosaurs (and other prehistoric creatures) than the biggest ones--after all, a tiny, foot-long reptile might easily have been the juvenile of a much larger species, but there's no mistaking a 100-ton behemoth. Here's a list of the 10 smallest prehistoric animals, according to our current state of knowledge. (Just how small were these ancient creatures? Compare them to the 10 Biggest Dinosaurs and 10 Giant Mammals that Succeeded the Dinosaurs.)
1. Smallest Raptor - Microraptor
With its feathers and four, count 'em, four primitive wings (one set each on its forearms and hind legs), the early Cretaceous Microraptor might easily have been mistaken for a bizarrely mutated pigeon. This was, however, a genuine raptor, albeit one that only measured about two feet from head to tail and weighed a few pounds soaking wet. Befitting its tiny size, paleontologists believe that Microraptor subsisted on a diet of insects.
2. Smallest Tyrannosaur - Raptorex
The king of all dinosaurs, Tyrannosaurus Rex, measured about 40 feet from head to tail and weighed 7 or 8 tons--but its fellow tyrannosaur Raptorex, which lived about 60 million years earlier, tipped the scales at 150 pounds, max, an object lesson in how plus-sized creatures evolve from wee ancestors. (By the way, some people insist that Nanotyrannus, the "tiny tyrant," was the smallest tyrannosaur, but the weight of current opinion is that this controversial genus was actually a juvenile T. Rex.)
3. Smallest Sauropod - Europasaurus
When most people think of sauropods, they picture huge, house-sized herbivores like Diplodocus and Argentinosaurus, some of which exceeded 100 tons in weight and stretched 50 yards from head to tail. Europasaurus, though, wasn't much bigger than a modern ox, only about 10 feet long and weighing less than a ton. The probable explanation is that the late Jurassic Europasaurus lived on a small island cut off from the European mainland, where the predatory dinosaurs weren't much bigger than their prey.
4. Smallest Marine Reptile - Lariosaurus
Pound for pound, the sea-going Lariosaurus wasn't nearly as tiny as the smallest dinosaurs, but--at about 2 feet long and 20 pounds--it's one of the smallest marine reptiles yet identified. Perhaps not surprisingly, the difficult-to-classify Lariosaurus went extinct at the end of the Triassic period, when it was supplanted by larger, more agile plesiosaurs and pliosaurs (which included truly enormous genera like Liopleurodon).
5. Smallest Ceratopsian - Microceratops
Microceratops (also known as Microceratus), the smallest of all the ceratopsian ("horn-faced") dinosaurs, was only about a foot and a half high and weighed all of four or five pounds. Unlike its much larger cousins--such as Triceratops and Pentaceratops--Microceratops scooted around on two feet, which probably helped it to avoid getting stomped on by bigger dinosaurs. Its closest relative was the slightly bigger Psittacosaurus, which is famous for being one of the few dinosaurs to have been eaten by Mesozoic mammals, rather than the other way around!
6. Smallest Pterosaur - Nemicolopterus
A couple of years ago, it seemed that the remains of new pterosaurs were being dug up in China every week. In February of 2008, paleontologists discovered the fossil of Nemicolopterus, the smallest pterosaur yet identified, which had a wingspan of only 10 inches and weighed a few ounces. Oddly enough, this pigeon-sized reptile may have occupied the same branch of pterosaur evolution that gave rise to the enormous Quetzalcoatlus 50 million years later.
7. Smallest Ankylosaur - Minmi
You couldn't ask for a better name for a tiny dinosaur than Minmi--even if this early Cretaceous ankylosaur was named after Australia's Minmi Crossing, and not "Mini-Me" from the Austin Powers movies. About 10 feet long from head to tail, Minmi weighed anywhere from 500 to 1,000 pounds, which may not seem small until you compare this genus to much later ankylosaurs like Ankylosaurus or Euoplocephalus. Judging by the size of its brain cavity, Minmi also seems to have been spectacularly dumb, another thing it shared in common with Austin Powers.
8. Smallest Prehistoric Elephant - Phosphatherium
Phosphatherium popped up in the fossil record only about 5 million years after the dinosaurs went extinct, and if you didn't know where it fit on the mammalian evolutionary tree, you wouldn't know whether this three-foot-long, 100-pound herbivore was fated to become a pig, a hippo, or a prehistoric elephant. It turns out that the last guess is the right one: modern elephants, as well as Ice Age Mastodons and Mammoths, can trace their family tree back to this squat, ungainly creature.
9. Smallest Prehistoric Whale - Pakicetus
10. Smallest Living Dinosaur - The Hummingbird
From the perspective of evolutionary biologists, dinosaurs never went extinct: they simply evolved into prehistoric birds (or, at least, the small, feathered, theropod dinosaurs did while their plant-eating sauropod, ornithopod and ceratopsian cousins went kaput). Based on this reasoning, the smallest dinosaur that ever lived is the modern hummingbird, various species of which weigh as little as one-tenth of an ounce!