Theropods are any of numerous carnivorous dinosaurs that lived from the Triassic period to the Cretaceous period. Most had powerful jaws, three fingers, and almost all ate meat. The theropods came in many sizes and shapes, and they were among the most successful carnivores to ever walk the earth. Most had relatively short arms, though some, like the spinosaurs, had long powerful arms that were used for a variety of purposes. Most carnivores actually had relatively small heads, with the exceptions of tyrannosaurids, spinosaurids, and carcharodontosaurids. Theropods were also the ancestors of all birds today, and many of the smaller theropods like the dromaoesaurs share over a hundred features with birds.