There have been no major increases in size among land-dwelling mammals since that time. In fact, every single group of mammals appears to have reached their respective maximum body sizes by about 40 million years ago. In a world denuded of dinosaurs, mammals underwent a major evolutionary radiation, including increases in size among multiple lineages. In a paper published last year in Science, researchers Jessica Theodor, Mark Uhen, and collaborators tracked changes in mammal body size after the end-Cretaceous mass extinction. The operation of these unwritten rules of anatomical architecture can be seen in the evolution of the largest land mammals. There are constraints that limit how big organisms can get and what shape they can take as they get bigger. The elephant-sized, bloodthirsty bunnies from Night of the Lepus would require changes in proportion to cope with the different stresses and strains that come with increased body size. Unless you’re deathly afraid of huge millipedes, anyway.) The same goes for vertebrates. ( There were giant arthropods over 300 million years ago, but even they were not of horror-movie size. They would not be able to obtain enough oxygen to survive, and, without major modifications to their bodies, would collapse under their own weight. the Spider? Mercifully for us, arthropods of city-destroying size could never exist.

Remember all those classic b-movies featuring giant insects, like THEM!, The Deadly Mantis, and Earth vs. The ecological dominance of the dinosaurs kept mammals small, but, once the dinosaurs disappeared, what prevented them from attaining enormous size? Why have there never been elephants as big as Apatosaurus or rhinos equal in stature to Brachiosaurus? The three-foot long Repenomamus – a 130 million year old mammal that ate baby dinosaurs – was as big as they came in those days. During the long tenure of the dinosaurs, mammals did not get very large at all. The giant rhino arrived on the scene about 28 million years after the last of the non-avian dinosaurs became extinct, but its ancestors and evolutionary cousins lived alongside dinosaurs for 135 million years. Thankfully for the pride of Paraceratherium, a wide gulf in time separated it from the Mesozoic titans. Compared to these giants, Paraceratherium seems rather puny. Estimates based on the long-lost bones of the sauropod Amphicoelias reconstruct this dinosaur as being over 131 feet long and weighing in at over 122 tons. The largest dinosaurs may have been even bigger still. Futalognkosaurus, a sauropod dinosaur that was apparently elsewhere when the good names were being given out, stretched over 100 feet long from its tiny head to the tip of its tail, and it probably weighed in excess of 75 tons. This immense rhino – part of an entirely-extinct group of hornless rhinos called hyracodonts – lived in Asia between 37 and 23 million years ago, but even this giant would have been dwarfed by the dragons of earlier epochs. What you have just made is a dead ringer for Paraceratherium, the largest land mammal of all time.

Dinosaur like a rhino