Jurassic Period

During the beginning of the Jurassic, Pangea supercontinent was rifting apart, North America moved northwestward.  The shallow seas that covered the interior of North America retreated to the western edge of the continent. The highlands that dominated eastern North America eroded and large dune deposits formed along the coast, which formed a narrow arm to the east of the modern Canadian Rocky Mountains. A subduction zone on the western edge of the continent caused frequent volcanic eruptions, forming the igneous rocks in the core of the ancestral Cordilleran. As continental separation continued, the rift basins along the rift zone in Texas buried by thick deposits of Middle Jurassic Louann marine salt in the newly formed East Texas and Gulf Coast basins (young Gulf of Mexico).  The polar ice caps completely melted during the Jurassic Period, and a general warm and moist climate extended across most of the planet.

Dinosaurs diversified during the Jurassic to fill most of the major ecological niches for land animals.  Jurassic dinosaurs reached tremendous sizes. 

Jurassic Period Scene

From Smithsonian Collection

The warm and moist worldwide climate during the Jurassic Period, produced an abundant population of conifers, ferns, seed-ferns, lycopods, horsetails, and cycads.  This time was dominated by Gymnosperms.  Gymnosperms are a group of vascular plants whose seeds are not enclosed by a ripened ovary (fruit).  The seeds of many gymnosperms (literally, "naked seed") are borne in cones and are not visible.

Except for a limited number of surface exposures near Sierra Blanca, along Interstate 10 in far west Texas, the Jurassic is virtually absent at the surface in Texas.  Middle subsurface Jurassic deposits (Norphlet, Smackover, Buckner, Haynesville, Cotton valley and Shuler formations to name a few), formed broad carbonate shelves a deltaic sandstones and shales at the edge of the widening Gulf of Mexico. Upper Jurassic on the basin margins consisted of shoreline, coastal plain and lagoon and fuuvial valley fill deposits.  These are presently covered by younger Cretaceous and Tertiary deposits.  No dinosaur or reptile fossils have been found in the Texas Jurassic, owing to the lack of good surface outcrops.  However, the environment suggestive of the Upper Jurassic sediments would have been attractive for dinosaur and reptile habitats.

Supersaurus, a saurischian herbivore found in Wyoming, possibly reached 33 to 34 meters (108 to 112 ft) in length, and a weight of  35 to 40 tons (31,751 to 36,288 kg).  The brachiosaurus was similar to the supersaurus, weighting about 55 tons (49,896 kg).  

	Supersaurus Sketeton	Supersaurus

Stegosaurs, an ornithischian, became abundant during the Jurassic and had rows of special bones, called osteoderms, which develop into plates and spines along the back and tail.   It was a herbivore that had a long, narrow head and a horn-covered beak.  There were 11 different kinds of stegosaurs.  It stood between 7 to 14 feet (2.1 to 4.2m)tall on all four legs and weighted between 5 to 10 tons ( 4,536 to 9,072 kg).  

	Allosaurus Skelton Left - Stegosaurs Skeleton Right	Stegosaurs

 

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