It's hard to imagine that anything could have lived through the Cretaceous-Tertiary extinction that wiped out the dinosaurs, but many species did in fact survive the catastrophe, also known as the K-T event. Most insects that live in the world today date back to the time of the dinosaurs, as well as many small varieties of ocean life. Many of these surviving creatures continued to evolve and diversify and still flourish even today. While the dinosaurs couldn't survive the initial disaster and subsequent climate and environmental change, mammals were easily adaptable and began to spread immediately after the KT event. One of the oddest of these creatures to survive and one that still exists today, is the duck-billed platypus. Referred to as a monotreme, a mammal that lays eggs, the duck-billed platypus is thought to have existed as far back as 125 million years ago. This would put the tiny creature directly beside some of the most ferocious dinosaurs known to man, but it was not alone.
The late Paleozoic era to the end of the Mesozoic era was considered the age of the reptile due to the abundance and diversity of dinosaur life on earth. Although many snakes, frogs and turtles that we see today have ancestors that survived the extinction period, none of them are actually the exact same as that which came before them. Most reptiles of the Mesozoic era died off with the exception of crocodilians, as well as one creature called the Tuatara that is from an ancient order of reptilians called the Sphenodontians family. Of the crocodilians, twenty-three species survived the K-T event, making the crocodile the oldest remaining species related to the dinosaurs. An estimated 240 million years old, the crocodile is the closest living link that we have to dinosaurs. Although the Mesozoic era crocodilians were larger than the crocodiles we are accustomed to seeing now, their appearance and evolutionary traits have changed very little.
There was an abundance of life within the oceans that survived the asteroid impact as well, and not surprisingly the world's deadliest ocean predators, the sharks, were among those that continued to prosper after the extinction of the dinosaurs. A small predator in the Mesozoic period, the modern shark quickly became one of the supreme ocean predators after the dinosaurs died out. There were also many other familiar creatures that held on after the mass extinction, including shrimp, lobsters, crab, clams, and snails. All of these creatures existed before the K-T event and lived with dinosaurs. While many of these creatures diversified after the extinction, when their populations grew so dramatically, they still retain the same basic structure as those animals that came before them. One creature that was thought to have gone extinct with the dinosaurs was the Coelacanths, but when a specimen turned up in the 1930's, and again in the 1990's, scientists were persuaded to change their opinion about the fate of the ancient fish.
Another group of creatures who existed with the dinosaurs are all modern varieties of insects, and nearly the complete family of insects survived. One species that existed with the dinosaurs actually benefitted from its late arrival toward the end of the cretaceous period, the bee. Eusocial bees are known today as colony bees and without them many mammals would have had a much harder time adjusting to life after the age of dinosaurs. Their survival is traced back to their late appearance in the cretaceous period, where it is thought that they evolved alongside the flowering plants that were beginning to emerge.
One theory that has been gaining acceptance among scholars is that dinosaurs never actually died out, but that they evolved. What they evolved into is the controversial factor here; this is because many people believe that some dinosaurs evolved into the modern bird. There is evidence of dinosaurs evolving towards something like a bird, with the ornithischia order having developed a bird-like hip structure. It isn't these creatures, such as the stegosaur and triceratops that are thought to have evolved into birds however; it is creatures such as the Tyrannosaur Rex and the Velociraptor, the keen predators of the period. While it will take much time to determine whether or not we are actually walking with dinosaurs currently, we can look to animals such as the platypus and the crocodile as our closest connection to the creatures of the Mesozoic era.