Poor incisivosaurus. Although we don't know for sure what its skin and feathers looked like, we do know that the early Cretaceous theropod had prominent incisors at the front of its mount, looking very much as though it could use a good orthodontist. Actually, those incisors are really interesting. Although it is a primitive species oviraptor, some of which were definitely carnivorous, the wear patterns on its teeth are more consistent with a herbivorous diet.
What is even up with atopodentatus! Its mouth is T-shaped! Its mouth is literally in its forehead! It looks like a "Pacific Rim" kaiju! This Middle Triassic marine reptile (OK, not technically a dinosaur) is actually named for its disturbing mouth. Its teeth are fused to its jaw, rather than growing from sockets, and its jaw splits into three parts, none of which had strong enough musculature for chomping. This suggests that the animal, which could live on both sea and land, fed more like whales, by filter feeding on the seafloor.
There are sharks. And then there are prehistoric sharks that have a wheel of teeth in their jaws like a circular saw. While the image above is a little outdated -- the current theory is that the tooth-wheel was set a little farther back into the shark's mouth -- the basic concept is the same. Heliocoprion, alive from the Late Carboniferous through to the Early Triassic, baffled palaeontologists with its whorl of teeth, left behind to fossilise as the shark's cartilaginous frame was not. It is supposed that it contains all of the shark's teeth from birth to death; as the shark ages, the whorl advances new teeth, pushing the older teeth into the centre of the whorl.
If you thought the T.rex's tiny arms were silly-looking, you just hadn't met linhenykus yet. The Late Cretaceous' alvarezsaur has forelimbs that are so unformed as to be useless. To be fair, it didn't need them, relying on its hindlimbs for locomotion and speed, and its powerful chest and neck muscles -- relative to its small size of just a few feet -- to hunt prey, possibly termites. Its useless little arms had just one digit, or phalange, to speak of -- although recent evidence suggests that it has a reduced second digit.
Epidexipteryx is not someone you want to be friends with, no matter which way you slice it. In the drawing on the right, it looks like a rabid bird-monkey that would eat your eyes as soon as look at you. On the right, it looks like the Orco or the Snarf of the dinosaur kingdom. Known from only a single specimen, epidexipteryx, living in the Middle or Late Jurassic, looks like a hodge-podge of an animal, and it is -- the beak and feathers of a bird, the forelimbs and phalanges of a climbing animal, and teeth more in common with oviraptors. It is also the earliest known example of ornamental feathering: the specimen included four long tail feathers which seemed to have served no purpose, indicating that feathers may have initially evolved for display.
There's probably a really good reason gliding reptiles have their membranes around their forelimbs -- the pectoral girdle -- but sharovipteryx just had to be different, with its gliding membrane weirdly located around its hindlimbs. Actually, it's the only animal known to have had its gliding membrane located this way. Known from just a single fossil, the Middle to Late Triassic protorosaur has vexed and perplexed paleontologists as they try to locate its evolutionary position, with much disagreement over whether it could possibly be an early relative of the pterosaurs. Most seem to think it is not, due to the methodology of the researcher who proposed the relationship.
If deinotherium had survived instead of the elephant, we'd probably be sitting here saying that elephants look really weird, but they didn't: they lived from the Middle Miocene and died out in the Early Pleistocene, with their crazy, backward-curving tusks. Exactly what they used those tusks for, we don't know; they're not positioned in such a way to make them particularly useful for, say, carrying things, clearing obstacles or fighting. Researchers guess that they may have used the tusks for pulling branches down to reach leaves, digging for tubers or scraping away soft tree bark, but we'll probably never know for sure.
The therizinosaurus looks very much as though it was made of parts of other dinosaurs, all stuck together: a teeny tiny head (presumably; a skull has never been found, but has been extrapolated from similar dinosaurs); a giant pot belly; and absolutely enormous claws, the longest claws ever found on Earth. The Late Cretaceous theropod didn't, however, use them for hunting prey: although diet is usually gleaned from the teeth, and therefore its diet is unknown, its enormous belly, as well as specimens from other therizinosaur species, indicate that it was probably herbivorous, using its claws to hook branches and defend itself from predators.
Pegomastax looks like what would happen if a Skeksis got bitten by a porcupine vampire. The Lower Jurassic heterodontosaur had a parrot-like beak containing enlarged canines at the front of the lower jaw, and its body may have been covered in a layer of quills. What these teeth and quills are for is not entirely known; the 60-centimetre (2ft) animal was herbivorous, so it wouldn't have needed its sharp teeth for prey, and the quills weren't as strong as a porcupine's, so how effective they were for defense is unknown. Hypotheses for the teeth include defense, in-fighting or digging for roots, while the quills might have been used for display.
On first glance, masiakasurus looks like a pretty standard carnivorous theropod… but hoo boy, those teeth! Although it looks like it could use some braces, though, the Late Cretaceous predator's teeth were actually really useful. The dinos were smallish, growing no larger than around two metres (6.6ft) from nose to tail-tip, and could have used those teeth as a scavenger, cutting and tearing meat from a carcass. They could also have been useful for snaring fish, or catching small animals with the top teeth, which could have been used for grasping, and shredding them with the bottom.
The dinosaur's species name could be fortunate or unfortunate, depending on your musical taste: masiakasaurus knopfleri was named for musician Mark Knopfler, whose work the dig crew enjoyed.
OK, so it's unlikely that the pterodaustro was pink, but even without the colouring, this Cretaceous pterosaur (not technically a dinosaur) still looks pretty dang weird. That lower jaw, with its thousand bristly teeth, look more like baleen than a beak. That's because, like flamingos -- and, indeed, whales -- the pterodaustro was probably a filter feeder, living on crustaceans, algae and plankton. This similarity to how flamingos eat led to the hypothesis that the pterodaustro might be pink -- the flamingo gets its colouration from the shrimp it eats. However, only modern birds have this ability to retain pigment, so the pterodaustro is at least safe from that particular fashion faux pas.
Marine animals nowadays have a flat dorsal fin, which they generally use for stabilisation, but not the Early Carboniferous stethacanthus. The early shark's dorsal fin was shaped more like an anvil, covered in an array of spikes -- enlarged versions of the rough placoid scales with which sharks' skin is covered, matching a patch of similar spikes on its forehead. Exactly how the shark used the fin is unknown; perhaps it was merely for stabilisation. Perhaps it had a purpose in mating. Perhaps the sharks used it to attach itself to other animals, or to threaten off larger predators -- stethacanthus only grew to about 70cm (27in) long. Or maybe the males used them to fight each other.