There is something morbidly fascinating about mass extinctions. Life on Earth has always reasserted itself, sometimes from the dimmest of embers.
Old forms are sometimes reinvented, when a new lifeform colonizes an ecological niche previously dominated by another being. For instance, consider the remarkable morphological similarity between the crocodile and the Permian semiaquatic ambush predator Prionosuchus. The two creatures aren't closely related at all (Prionosuchus is most closely related to modern frogs and salamanders), but the particular body plan of crocodilians and prionosuchians happens to be an excellent design for a creature with that particular lifestyle. In fact, if the 200-million-year reign of crocodiles was to end, I'll wager that eventually another lifeform will reinvent a roughly similar body plan.
Reestablishment of biodiversity after a mass extinction event can be a long process. During the Triassic-Jurassic extinction event, life on Earth hadn't fully recovered after the Permian-Triassic event 50 million years earlier; large predators were only just reappearing at that point. Sometimes, the emptying of ecological niches after a mass extinction has opened for entirely new forms of life to appear, and the post-recovery biosphere becomes utterly unrecognizable. Grass wasn't widespread before the Cretaceous-Tertiary event (the "Dinosaur Killer"), and only became ubiquitous after that event. Consequently, the large herbivores of the time ate foliage, and thus had morphological adaptations more like modern giraffes than modern cattle.
If one was to bump into a proto-archosaurid during the heyday of Prionosuchus, it would be impossible to guess that its descendants would eventually lay claim to the monster amphibian's place in the ecosystem—or, for that matter, that others of its descendants would become tyrannosaurs, diplodocus, ostrich, mallard and swallow. Then, they were unimpressive little predatory reptiles that sprinted on their hind legs and risked getting eaten by giant amphibians if they were unfortunate. The emptying of ecological niches by later mass extinctions provided the opportunity for their descendants to become all crocodiles, all dinosaurs and all birds. Likewise, nobody would have guessed that the rather sorry little hairy vermin scurrying among dinosaur droppings would one day become elephants, tigers, bats, blue whales, agriculturists, emperors and web developers.
And I wonder: What will life look like in 50 million years, when the Anthropocene mass extinction is over?