You might think Smilodon fatalis--commonly, and mistakenly, known as the "saber-toothed tiger"--was a vicious, relentless, unstoppable predator. However, a new study comparing the sizes of male and female Smilodon specimens has reached the opposite conclusion: this prehistoric cat, which prowled the North American plains from 1.5 million to 10,000 years ago, may not have been as aggressive as it's been portrayed in countless Hollywood movies.

The reasoning comes down to this: in populations of modern big cats, like lions and tigers, males are significantly bigger than females, the evolutionary result of having to fight one another for the right to mate. All this sparring and tussling breeds aggression, increased size and greater speed, resulting in male-dominated packs that make short shrift of Savannah wildlife.

The lack of sexual dimorphism in Smilodon, on the other hand, hints at a much more egalitarian pack structure, with a comparable lack of meanness. However, it's important not to take this argument too far: even if saber-toothed cats were much tamer than modern lions and tigers, they were just as dangerous to their prey (which probably included early humans).

Tyrannosaurs didn't come into their own until the late Cretaceous period, when monsters like T. Rex, Albertosaurus and Tarbosaurus terrorized North America and Asia. Now, a new study of the sole fossil specimen of Proceratosaurus--a mysterious theropod dating from 170 million years ago--has shown it to be an extremely primitive member of the breed, a find that dials back the first tyrannosaurs to the mid-late Jurassic period. Interestingly, the tiny (75 pounds fully grown) Proceratosaurus was unearthed in England, where no later tyrannosaurs have been found.

Confusingly enough, similar T. Rex ancestors have been announced to the public with great fanfare--first the tiny, feathered Guanlong (dating back about 160 million years) from China, and more recently the pint-sized, central Asian Raptorex, a relative newcomer at a mere 125 million years old. What this means is that dinosaur evolution in general--and tyrannosaur evolution in particular--is a far more convoluted affair, far more dependent on the vagaries of fossil finds, than is presented in most popular accounts!

One of the problems with investigating pterosaurs is that these flying reptiles were so numerous, and so similar looking, that they can be hard to distinguish from one another on the genus (much less the species) level. A case in point is Germanodactylus, which for years was thought to be a species of Pterodactylus, until a more rigorous analysis showed that it deserved its own genus.

As pterosaurs go, Germanodactylus tended toward plain vanilla, except for its prominent (and probably prominently colored) head crest--which was composed of solid bone on the bottom and soft tissue on the top.

Read more about pterosaurs like Germanodactylus: Pterosaurs - The Flying Reptiles

Illustration: Wikimedia Commons

Every week, it seems, there's more evidence that dinosaurs of the same species regularly harassed, attacked, and even cannibalized one another. The latest is the discovery of tyrannosaur-sized bite marks on the skull of Tyrannosaurus Jane, a juvenile T. Rex that was probably about 12 years old when it died. The 1,500-pound Jane (who may or may not have been a female) almost certainly scuffled with a fellow T. Rex juvenile, based on the size and position of the numerous, non-fatal bite marks.

Why would Jane have tussled with a fellow tyrannosaur? Well, kids will be kids, and it's possible that two T. Rex siblings were simply engaging in a little horseplay. But it's also possible that the battle was more serious, and may have involved another genus of juvenile tyrannosaur attempting to horn in on T. Rex territory. (There's no evidence, by the way, that the puncture wounds were caused by trichomonosis, a parasitic infection implicated in the death of another T. Rex specimen, Tyrannosaurus Sue.)