There are some animals on earth that can be traced back to the Mesozoic Era when dinosaurs walked the earth; these animals include monotremes such as the duckbill platypus, reptiles such as the crocodilians and sea creatures such as the sea urchin. For the most part, the animals that are alive today have made evolutionary changes from the forms they had when dinosaurs were alive. For example, though the event that caused dinosaurs to die out also killed many other reptile species, several surviving reptile species have direct ancestors from that era, including the crocodilians, which are smaller than their Mesozoic ancestors.
Sharks are another example of creatures that survived whatever event wiped out the dinosaurs. In fact, sharks are one of the most enduring types of animals on the planet, with direct ancestors that can be traced back further than even the first dinosaur. Other marine animals, including a fish known as the coelacanth, are the last living examples of animal families that were much more populous during the Mesozoic era. In fact, the coelacanth, which is the last known surviving sarcopterygian, was thought to be completely extinct until scientists found evidence of its continued existence in the early 1900s.