Shrinking the Gap

10cm carnosaur tooth from Doellings Bowl. 

 The Cedar Mountain formation is America's oldest Cretaceous unit. Interrupted at the top of the Morrison, the dinosaur record resumes with the Cedar Mountain. Yet the intervening interval has long been considered great. The lowest member of the Cedar Mountain, the Yellow Cat, was thought to be of Barremian age. Therefore, a yawning gap seemed to separate the late Jurassic record from that of the early Cretaceous. Morrison time ended c 147 Ma but the Cedar Mountain deposition didn't start, it appeared, until about 130 Ma. The putative hiatus spanned four stages--much of the Tithonian and all of the Berriasian, Valanginian and Hauterivian. 

 Newer dating, however, points to a much older age for the Yellow Cat, hence a considerably smaller gap between the Morrison and Cedar Mountain. The Yellow Cat is now considered a Valanginian unit not a Barremian one. Instead of being deposited 130-125 Ma, it is now dated at 139-134.6 Ma. It is even older at its base--c 140-142 Ma. Remarkably, the age of the Yellow Cat has been lowered by two full stages or even three, as an age of over 140 Ma indicates deposition began in the Berriasian, the first stage of the Cretaceous. Instead of lasting around 17 million years or 3-4 stages, the hiatus now appears to have lasted just 5-7 million years or parts of two stages. Essentially the gap has shrunk to the point where it is no longer significant. Researchers now have an almost continous record of the Jurassic-Cretaceous transition in America. 

What was the transition like? A sizeable (10cm)"carnosaur" tooth (probably allosauroid) from the basal, Doellings Bowl site may provide valuable insight. This specimen suggests some continuity with the Jurasssic in the earliest Cretaceous c 140 Ma. The familiar Morrison menagerie--sauropods and allosaurs--might've lasted into Berriasian time. The number and diversity of polacanthids, including two new taxa however, suggests turnover among thyreophorans had already occurred. Stegosaurs may have gone out with the Jurassic. Even the lower Yellow Cat suggests faunal change spurred by encroaching seas. Loss of familiar prey ultimately affected the carnivores. Utahraptor took over the apex predator niche. Diminished land and prey resources had temporarily eclipsed giant theropods. Likewise, even before Gastonia, polacanthids flourished in a paleoenvironment devoid of plated beasts. Not surprisingly, a highstand severe enough to undermine the usual ("carnosaurian") top predators extirpated the plated dinosaurs.