Texas Cretaceous Dinosaurs
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A second major episode of continental rifting of Pangea began in the Early Cretaceous. North America was moving northwest, closer to its present position. South America and Africa separated slowly from south to north creating the South Atlantic Ocean, while India and Madagascar rifted away from the western margins of Australia and Antarctica to form the Indian Ocean. At the same time, rifting between North America and Europe began, and Iberia rotated counter-clockwise away from France.
Throughout the Cretaceous, sea level was an average of 328 ft (100 m) higher than today due to continental rifting and sea-floor spreading. Shallow seaways spread over many of the continents, including North America, South America, Africa and Eurasia. The climate was warm during the Cretaceous, although there was a cooling of the climate from the Jurassic Period. The warm climate was partly due to the climatic effects of the shallow seas and because the continental positions allowed warm waters to circulate around the globe.
During Early and Middle Cretaceous throughout central and southern North America, a large shallow inland sea was present that covered much of Texas as far west as the Trans-Pecos region and north almost to the state line. Low mountains were present along modern Appalachian Mountains, the ancestral Rocky Mountains were uplifted and lowlands dominated from the modern Great Lake States into eastern Canada. The subduction zone along the western continental margin was still active, if not more, initiating intrusive and extrusive igneous rocks to the ancestral Sierra Nevada.
During most of the Late Cretaceous, much of Texas lay beneath marine waters that were deeper than the Early Cretaceous seas. The Austin chalk, which crops out from Eagle Pass on the Rio Grande northeastward to the Texarkana area, is a prominent example of the deeper water deposits of the Late Cretaceous until delta and strandline sandstones of the Woodbine Formation were deposited in near shore areas. Late Cretaceous volcanoes were widespread in a northeast-southwest band south and east of the buried Ouachita Mountains through Texas. Pilot Knob in Travis County, is the eroded remnant of one of these. At the end of the Cretaceous (Mesozoic Era) and the beginning of the Cenozoic Era, major uplift and mountain building in the western United States during the Laramide Orogeny also affected the southwestern and western border regions of Texas.
Cretaceous Period Scene
From Smithsonian Collection
The gymnosperms which had dominated the flora in the Triassic Period began to decline and be replaced by a new group of plants called the Angiosperms (flowering plants) in the Early Cretaceous. This was due to the change in the climate and the shifting of continents, as the flora did not have the genetic potential to adjust to the changes. Additionally, the seas rose and covered vast tracts of land where the already stressed forests of conifers, cycads and ferns were growing The Cretaceous Period was the first era which included modern flora, and it was in the Rift Valley between Africa and South America where the revolution began.
The Cretaceous Period may be best known for its ending 65 million years ago. Marking the boundary between the Mesozoic and Cenozoic Eras, the end of the Cretaceous (K/T Boundary or K/T Extinction Event) is defined by one of the most famous mass extinctions in the history of life on Earth. The extinction was rather uneven, some groups of organisms became extinct, some suffered heavy losses and some appear to have been only minimally affected. It has been estimated that perhaps 60-70 percent of all marine species and nearly 15 percent of all terrestrial genera, including many mammals, went extinct at the end of the Cretaceous. All land animals larger than about 50 lbs (20 kg) died out, including the dinosaurs. Only their descendents, the birds, survived. Many species of echinoderms, brachiopods and molluscs, including ammonites and the reef-building bivalves die out. The event opened the way for mammals and birds to become the dominant land vertebrates.
In 1980 a physicist Luis Alvarez and his son Walter Alvarez, a geologist, proposed that an asteroid 4-9 miles (6-15 km) in diameter hit the Earth about 65 million years ago, creating the Chicxulub crater at the tip of Mexico's Yucatán Peninsula. The impact penetrated the Earth's crust, sending dust, debris and steam into the all levels of the atmosphere. It would have caused huge forest fires, tsunamis, earthquakes, severe storms, and eliminated all sunlight. Some have proposed that acidic rain occurred, and perhaps even volcanic activity causing chemical changes in the Earth's atmosphere, increasing concentrations of sulfuric acid, nitric acid, and fluoride compounds.
The dust and debris in the atmosphere would have reduced sun light reaching the Earth causing temperatures to lower and ending the tropical climate that was prevalent in the Cretaceous and its tropical plants. If the plant eaters died there would be nothing for the meat eaters to eat and they would eventually die out also. Plankton also seemed to decrease in number during the Tertiary period which could explain the death of the marine dinosaurs that fed on them.
Volcanic eruptions alone or in connection with an asteroid impact, would have changed the atmospheric composition and thus changing the environment for plant life.
Disease has been proposed as the killer of the dinosaurs. As the Cretaceous period went on, more and more land bridges started to appear on the earth. The oceans started to dry up and the dinosaurs species were able to intermix and bring their diseases with them, via the land bridges.
Environments were favorable during the Cretaceous for dinosaurs and associated reptiles in Texas. Today their fossils are found in the Central Hill Country Area, Prairies and Lakes Area and Big Bend Country Area of Texas. Over the last twenty years, a significant volume of work has been performed and published by paleontologists that have added to our understanding of vertebrate paleontology in Texas.
Central Hill Country and Prairies and Lakes Areas
In the Central Hill Country and Prairies and Lakes areas of Texas six different taxonomy families of dinosaurs (three Saurischia – lizard-hipped and three Ornithischia – bird-hipped) have been identified with one family of Nodosauridae having three different genera and one family of Dromaeosauridae having two different genera. Non-dinosaur reptiles include two families of Crocodylomorpha, three families of Sauropterygia (in the order Plesiosauria) including one family with two genera, one Ichthyopterygia family, five genera of Lepidosauria (in the order Squamata) in one family and three families of Pterodactyloidea (in the order Pterosauria). These are listed in the following table. For further discussion and images of Dinosaurs and associated Reptiles from the Central Hill Country and Prairies and Lakes Areas follow this Cretaceous Period - Central Texas and Prairies and Lakes link.
Dinosaurs and Reptiles
Central Hill Country and Prairies and Lakes Areas
Dinosaurs and Associated Reptiles
Dinosaurs and Associated Reptiles Found in the Big Bend Country Area