No Stegosaur Comeback

Some researchers believe a stegosaur will be found at Doellings Bowl, an early Cretaceous site in Utah. Forming the base of the Yellow Cat, the lowest member of the Cedar Mountain formation, Doellings Bowl has yielded iguanodonts, dromaeosaurs, polacanthid ankylosaurs, sauropods, therizinosaurs and an allosauroid. Currently, no stegosaurs are known from the site. Their presence is thought probable, however, because Doellings Bowl formed only about a stage after the stegosaur heyday of the late Jurassic. In addition, stegosaurs persisted into the early Cretaceous on other continents. Inasmuch as Morrison sauropods appear to have vanished by the Cretaceous, replaced by European forms, Stegosaurus could have suffered the same fate. Perhaps its niche was taken by European taxa such as Dacentrurus or Miragaia. The latter, in fact, was already present in the late Jurassic (Morrison formation) of America.

Survival of Stegosauria into Yellow Cat times is not likely, however. The Jurassic-Cretaceous transition witnessed a wetter climate, which proved inimical to plated dinosaurs. With the exception of Hesperosaurus mjosi from the base of the Morrison, local stegosaurs were adapted to a dry climate. This appears to have been true of Stegosaurus. Occuring higher in the Morrison (or later) than Hesperosaurus, Stegosaurus would've been vulnerable to wetter conditions. It wouldn't be surprising if the iconic stegosaur, like coeval sauropods (also dry-adapted) vanished at the end of the Jurassic.

Based on the record of the Yellow Cat, no stegosaur, local or European, persisted into the Berriasian stage. Doellings Bowl taxa are essentially the same as higher (Valanginian) taxa. Thriving under humid conditions, therizinosaurs, iguandonts and polacanthids predominated. Inasmuch as no stegosaur was ever found in the Valanginian aged deposits, they seem equally unlikely at Doellings Bowl, where similar taxa suggest a similar environment. And this environment excluded plated dinosaurs.

Even in Cretaceous environments highly reminiscent of the Jurassic, stegosaurs apparently never returned to regions where they had vanished. Africa was one example.

The Cenomanian Baharije environment had big sauropods, carcharodontosaurs and spinosaurs. The much older (late Jurassic) Tendaguru had a similar fauna suggestive of similar habitats. The Baharije looks like the Tendaguru revived, but with an important difference. Stegosaurs had vanished. Not only had they disappeared, for some reason they couldn't reappear despite prima facie evidence for a favorable environment.

The same appears true in the middle Cretaceous of America. At the time there was faunal exchange between Asia and America. Caenagnathids, hadrosauroids, ceratopsians, tyrannosauroids etc radiated across beringia and entered both continents. Surprisingly, though, stegosaurs--still present in Asia down to at least Aptian--did not recolonize America. There is no sign of a stegosaur in the Antlers formation or the middle Cedar Mountain (Ruby ranch) member. Both have yielded big allosauroids and sauropod giants, yet even in habitats which seem to mimic the Morrison to a considerable degree, plated dinosaurs remained defunct. Although allosauroid and sauropod remains in the Doellings Bowl suggest persistence of some favorable habitat, hence the possibility of a stegosaur, the record elsewhere argues this is illusory.

Why were stegosaur habitats ruined permanently, in one region after another? Wetter conditions probably undermined the key vegetation of stegosaurs--cycadopohytes and bennitatales. Stomach contents of Isaberrysaura confirm that stegosaurs required such plants, yet they became increasingly rare in the Cretaceous. Angiosperms proliferated at their expense. As narrow-snouted, selective feeders, stegosaurs could not easily adapt to other plants, or if they could they faced too much competition from ankylosaurs and iguanodonts which could more efficiently process it.