The area that is now the state of Texas underwent cycles of plate movement, volcanic activity and flooding over its 600 million years of existence. The type of life that could survive in the area depended on the geologic conditions of the time. For instance, during the Cretaceous period, which ended with the extinction of the dinosaurs, most of Texas was underwater. Only a swath of West Texas from the Panhandle to the Big Bend region remained dry. However, because of the size of the state, a variety of fossils have been recovered.
Because much of Texas was once an inland sea, fossils of marine life can be found in many parts of the state. Fossil hunters commonly find snails, whelks, scallops and various seashells, as well as colonies of coral. Fossils may be embedded in limestone or buried in soil. The marine fossils date primarily to the Cretaceous period, or approximately 100 million years in the past. In West Texas, paleontologists found sharks, a reef and marine invertebrates in the Guadalupe Mountains. Two small plesiosaurs were unearthed in central Texas near Austin.
Wood becomes petrified when it is buried before it can decay. Beneath the soil, the decay happens very slowly, allowing minerals in the ground water to replace the cells of the plant. Fossilized palm wood is so common in the state that it became Texas' official state stone. Fossilized wood of various types can be found throughout the state.
Dinosaur fossils have been rare in Texas. However, in 2003, a site was found in Arlington, a large city between Dallas and Fort Worth. The area was a coastal plain during the Cretaceous period, and paleontologists date the fossils to about 95 million years ago. Finds include the most nearly intact fossil of a duckbilled dinosaur called Protohadros that has been found so far. An as-yet-unnamed theropod from the same family as Tyrannosaurus rex was also discovered. Due to the area's proximity to the coast, turtles, crocodiles and fish have been unearthed as well.
During the Cenozoic era, which began about 65 million years ago, the growth of the Rocky Mountains led to sedimentary run-off covering parts of Texas. The fossils of mammals are found in this sediment from the High Plains to the Gulf Coast. Mastodons, mammoths and bison are among the mammal fossils found in the deposits. In south Texas, scientists have found fossils of sloths, saber-toothed tigers and American zebras and lions.