Paleontology and geology
The Precambrian: Precambrian rocks can be seen today in the area commonly referred to as the Llano Uplift of central Texas and in far western Texas. Fossils are rare to non-existent in these metamorphic and igneous rocks.
The Paleozoic: Paleozoic rocks are well represented in Texas. The presence of Cambrian sandstones indicates that sediments were brought into the sea from land areas to the northwest of the state. The limy sediments deposited farther out to sea preserved the remains of trilobites, brachiopods, bivalves, sponges, and other invertebrates. This deposition of limy sediments continued through the Silurian and Devonian; however, fossils are rare in the Silurian rocks and most Texas Devonian rocks are subsurface and can only be seen in drill cores. Rocks of the Early Carboniferous are not exposed at the surface in Texas, while rocks of the Late Carboniferous are well exposed in north-central Texas, and fossils are quite common in these outcrops.
During the Permian, north-central Texas was an area of changing environments, where shallow marine waters gave way to lowland coastal areas. Extensive Permian salt and gypsum deposits indicate that evaporation rates were high, and the red beds in this region contain an abundant vertebrate fossil record. An ancient reef system preserved in the Guadalupe Mountains of far western Texas is evidence that the sea still covered that part of the state. These rocks are rich in marine invertebrate fossils as well as rare vertebrate fossils, primarily sharks.
The Mesozoic: Mesozoic rocks are common in several parts of Texas. Triassic sands and shales were deposited as part of a large river system. These rocks contain a well-documented vertebrate fossil record that includes numerous reptiles and amphibians. Rocks of the Jurassic Period are virtually absent at the surface in Texas, but can be seen in a few small exposures in far western Texas and in subsurface drill cores. The Cretaceous was a time of fluctuating sea levels in the state. The shoreline shifted back and forth many times, leaving behind a record of both marine and non-marine environments. These rocks contain a rich fossil record including dinosaurs, other reptiles, amphibians, birds, fish, and mammals.
The Cenozoic: During the Tertiary, extensive volcanic activity poured lavas across the landscape in far western Texas; however, much of the modern High Plains and Texas Coastal Plains are covered by rocks formed from sediment washed down from the rising Rocky Mountains to the northwest. These sedimentary rocks contain a rich vertebrate fossil record, including a large assortment of fossil mammals. There is a wide band of Quaternary deposits along the modern Gulf Coast and in several places in west Texas. Fossils found in these deposits include bones of bison, mammoths, and mastodons.