Science, Tech, Math › Animals & Nature The Dinosaurs and Prehistoric Animals of Oregon Share Flipboard Email Print Animals & Nature Dinosaurs Basics Paleontologists Carnivores Dinosaurs & Birds Herbivores Marine Reptiles Prehistoric Mammals Amphibians Birds Habitat Profiles Mammals Reptiles Wildlife Conservation Insects Marine Life Forestry Evolution View More By Bob Strauss Science Writer B.S., Cornell University Bob Strauss is a science writer and the author of several books, including "The Big Book of What, How and Why" and "A Field Guide to the Dinosaurs of North America." our editorial process Bob Strauss Updated May 09, 2019 Let's dispense with the bad news first: because Oregon was underwater for most of the Mesozoic Era, from 250 to 65 million years ago, no dinosaurs have ever been discovered in this state (with the exception of a single, disputed fossil, which seems to have belonged to a hadrosaur that washed up from a neighboring locality!) The good news is that the Beaver State was well-stocked with prehistoric whales and marine reptiles, not to mention various megafauna mammals, as you can read about below. 01 of 05 Various Marine Reptiles James Kuether There's little doubt that the shallow ocean covering Oregon during the Mesozoic Era harbored its fair share of marine reptiles, including ichthyosaurs ("fish lizards"), plesiosaurs, and mosasaurs, which dominated the Mesozoic undersea food chain. The problem is that very few of these undersea predators took the trouble to actually fossilize, with the result that the discovery of a single plesiosaur tooth, in 2004, generated big headlines in the Beaver State. To date, paleontologists have yet to identify the exact genus of marine reptile to which this tooth belonged. 02 of 05 Aetiocetus Nobu Tamura The most complete prehistoric animal ever to be discovered in Oregon, Aetiocetus was a 25-million-year-old whale ancestor that possessed both fully developed teeth and baleen plates, meaning it fed mostly on fish but also supplemented its diet with healthy servings of near-microscopic plankton and other invertebrates. (Modern whales subsist on either one food source or the other, but not both.) One well-known species of Aetiocetus, A. cotylalveus, hails from Oregon's Yaquina Formation; other species have been discovered along the east and west edges of the Pacific Rim, including Japan. 03 of 05 Thalattosuchia Dmitry Bogdanov A marine crocodile of the Jurassic period, Thalattosuchia only makes it onto this list with a big asterisk attached: it's believed that the fossil specimen discovered in Oregon actually died in Asia tens of millions of years ago, and then drifted slowly to its final resting place via the intervening eons of plate tectonics. Thalattosuchia is informally known as a marine crocodile, though it was not directly ancestral to modern crocs and gators; however, it was closely related to one of the fiercest marine reptiles of the Mesozoic Era, Dakosaurus. 04 of 05 Arctotherium Wikimedia Commons Here's another big asterisk for you: paleontologists have yet to discover a single fossil of Arctotherium, otherwise known as the South American Giant Short-Faced Bear, in the state of Oregon. However, a series of fossilized footprints discovered in Lake County, in the south-central part of the state, bear an uncanny resemblance to footprints from other regions known to have been left by Arctotherium. The only logical conclusion: either Arctotherium itself or a close relative, lived in the Beaver State during the Pleistocene epoch. 05 of 05 Microtheriomys Wikimedia Commons No list of prehistoric animals of the Beaver State would be complete without, well, a prehistoric beaver. In May 2015, researchers at the John Day Fossil Beds announced the discovery of Microtheriomys, a 30-million-year-old, squirrel-sized ancestor of the modern beaver genus, Castor. Unlike modern beavers, Microtheriomys didn't have teeth sturdy enough to gnaw down trees and build dams; rather, this tiny, inoffensive mammal probably subsisted on soft leaves and kept its distance from the larger megafauna mammals of its coastal habitat.