The extinction of all dinosaurs (except birds) at the end of the Cretaceous Age was made famous 45 years ago when a crater was found in the Gulf of Mexico–confirming a comet impact as the cause of the extinctions 66 million years ago. A lesser-known local extinction of dinosaurs occurred 145 million years ago at the end of the Jurassic Age. From geological and paleontological evidence scientists determined dinosaurs were diverse and abundant in western North America from 152 million years ago to 145 million years BP, but these species became extinct after a sudden cataclysmic event. Scientists suspect a mega volcano eruption rubbed out all the dinosaurs in this region then. During the late Jurassic sauropods such as diplodocus (the largest known land animal in earth’s history), allosaurs, stegosaurs, and small ornithomimids (bird-like dinosaurs but not ancestral to birds) dominated the faunal composition. After the extinctions Cretaceous Age species gradually colonized the region and ecologically replaced the Jurassic Age species, although sauropods continued to live in other regions of the earth. Iguanodons, ankylosaurs, and smaller species of ornithomimids were the new inhabitants of the region. The early Cretaceous dinosaur composition was less diverse in this region than the late Jurassic.
The Morrison formation consists of sedimentary rocks that have the best evidence of Jurassic Age dinosaurs in North America.
Scientists found an interesting faunal turnover between the late Jurassic and the early Cretaceous in the Morrison Formation located mostly in Utah. Poster from James Kirk’s twitter feed.
Plant fossils from the Morris Formation include conifers, cycads, and ancestors of ginko.
The northern part of the Morris Formation yielded fossil remains of a different species of allosaur than the southern part. Scientists don’t know why 2 different species evolved. They don’t know of any physical barrier between the 2 areas.
Sauropods like this diplodocus and stegosaurs were the main species of large plant-eaters in western North America before the Jurassic Morrison extinctions.
Stegosaurs didn’t make it to the Cretaceous.
Evidence of this faunal turnover comes from the Morrison Formation–mountains of sedimentary rock found in western North America, mostly Utah. During the late Jurassic this area was a floodplain bordered by a newly forming mountain range. Cycads and conifers grew on the plain, and rivers flowed from the mountains into a vast marshy lake. Some of these trees have been preserved in petrified forests located in Dinosaur National Monument. The northern and southern part of this region hosted 2 different species allosaurs and 2 different species of carnosaurs. Scientists haven’t figured out why the northern and southern parts of this region had different species of carnivorous dinosaurs. They know of no physical barrier that would have contributed to this speciation. The region shifted frequently between wet and dry climate cycles, but scientists don’t think this was a factor in the sudden extinctions of Jurassic Age dinosaurs here. Dinosaurs were thriving and diverse up until their sudden end here.
Reference:
Kirkland, J.; E. Sampson, M. Wizecurch, and D. Deblieux
“Paleosols in the Lower Yellow Cat Member of the Cedar Mountain Formation in Eastern Utah Indicate the Earliest Cretaceous (Borrasian) in the Colorado Plateau was Exceptionally Wet”
75th Annual Meeting of the Rocky Mountain Geological Survey 2025
Maidmont, S.
“Diversity Through Time and Space in the Upper Jurassic Morrison Formation, western USA”
Journal of Vertebrate Zoology 43 (5) 2023
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