New chemical abrasion thermal ionization mass spectrometry dates from the Brushy Basin Member, Morrison Formation, western Colorado; implications for dinosaur evolution

Two new chemical abrasion thermal ionization mass spectrometry U/Pb dates establish the age of the top of the Brushy Basin Member, Morrison Formation, at Flat Top (also known as Terrace Hill), north of the Fruita Paleontological Research Natural Area (FP), and of the lower part of the Brushy Basin Member, in the region of the Trail through Time (TT) and Mygatt-Moore Dinosaur Quarry in westernmost Colorado. Strata at both locations preserve a fossil record of large dinosaurs, including bones, dinosaur nests, and trace fossils, as well as fossils of unionid bivalves, fish, turtles, crocodilians, microvertebrates, logs, and other plant material. We report here a new weighted mean (super 206) Pb/ (super 238) U date of 149.43+ or -0.059 Ma for three zircon grains sampled from a stratigraphic level 99 m above the base of a 101-m-thick Brushy Basin Member FP section and a new weighted mean (super 206) Pb/ (super 238) U date of 150.208+ or -0.094 Ma for three zircon grains sampled from a stratigraphic level 39 m above the base of a 79-m-thick Brushy Basin Member TT section. These dates place the Brushy Basin strata in the Tithonian Stage of the Upper Jurassic Series. The 149.43 Ma date from Fruita is the youngest reliable date from near the top of the formation. The new dates also indicate a relatively short interval of deposition-less than 1 My-for the upper Brushy Basin Member. Thus, two-thirds of the dinosaur and other vertebrate fossils known from the Morrison Formation represent a biota that existed for only 1 My. However, it is possible that some taxa persisted during some of the time represented by the K-1 unconformity.