Great Discovery!

The Hebei stegosaur

Plated dinosaurs survived past their Jurassic heyday, into the Cretaceous. The latter members of this group, however, are poorly known. Whereas Jurassic stegosaurs are represented by an abundance of material from the Morrison, Shaximiao, Tendaguru, Villar del Arzobispo etc, Cretaceous remains tend to be rare and fragmentary. Only a few bones and teeth have been found in the La Amarga, Khuktekskayasvita, Kirkwood and other units. Fortunately a great discovery promises to enhance our knowledge. As if to compensate for the paucity of EK material previously known, a new specimen is almost complete, in natural articulation and with preserved skin.

Found in Fengning Man autonomous county, Jiecaigou village, Hebei province, China, the specimen is from the lower Jehol biota, c 130 Ma (Dabeigou formation, Valanginian-Hauterivian boundary). It is 5m long and essentially complete although lacking most of the dorsal osteoderms characteristic of stegosaurs. There may be a taphonomic bias against preservation of plates in Asia, accounting for the truncated ones of Wuerhosaurus and their near-absence in this specimen.

The Hebei stegosaur probably died in a drought. As Kenneth Carpenter once wrote, "so many animals die (in a drought) the predators can't keep up." That explains the pristine, unscavenged condition of some specimens. But when "compensatory" flooding occurred, the dorsal osteoderms (originally embedded in the skin but loosened by decomposition) were swept away and not buried with the rest of the skeleton.

A length of only five meters may indicate a immature individual. Stegosaurus and other taxa were considerably larger. Yet the Hebei stegosaur has a prodigious thagomizer. Still immature but too large to hide, it needed adult-sized weaponry. Clearly, caudal spikes were still effective as they lasted into the Cretaceous. What theropod attempted to prey on it?

The Dabeigou also yielded a theropod. Identified as a ceratosaur, it suggests persistence of a relict Jurassic fauna to Hauterivian time. It's noteworthy, though, that Kelmayisaurus petrolicus was originally identified as a ceratosaur, until its carcharodontosaurid affinities became clear. The Hebei stegosaur may be too early to be Wuerhosaurus, known from a Psittacosaurus bearing unit of probable Aptian age. But carcharodontosaurs ranged far and wide throughout much of the EK, so the Dabeigo beast could've faced a relative of K. petrolicus.

Whereas previouly known EK stegosaurs told us little, the Hebei stegosaur will tell us much. Ample fossilized skin, similiar to that of snakes and lizards, is "incredibly distinct and clear." We are very fortunate to have a late surviving stegosaur in this condition, and can now learn a lot about it.

The affinities of the new stegosaur have yet to be assessed. With the possible exception of the skull, it looks conservative, not much different from Jurassic taxa. Temporally the stegosaur is intermediate between Stegosaurus and Mongolostegus. Phylogenetically, it might be closer to the former. It would be interesting to compare its jaws and dentition with those of contemporary taxa such as Paranthodon, and the Siberian stegosaur. Does the Dabeigou stegosaur also show signs of adaptation to tough vegetation? Soon, studies will clarify its phylogenetic position and shed light on the paleoecology and evolution of Cretaceous stegosaurs.

Another view of the specimen showing the thagomizer. Dorsal osteoderms might've changed since the Jurassic but not means of active defense.