The Last Stegosaur

Yanbeilong ultimus is too poorly known to be restored. The above restoration is based on late Jurassic taxa and is probably inaccurate

The latest stegosaur to be named, Yanbeilong ultimus, is more important for what it tells us about stratigraphy and extinction than morphology and relationships of Cretaceous stegosaurs.

Like stegosaurs from the Louhandong and Kukhtekian beds,Yanbeilong is based on just pelvic and vertebral elements. Discovered in the Zuoyun formation of Shanxi province, China, it consists of a sacrum, both ilia, the left ischium, the right pubis, seven dorsal verebrae and one caudal. The holotype lacks cranial or limb bones. Unlike the Hebei material, Yanbeilong is of limited value to taxonomists.

The new taxon can, however, shed much light on the demise of Stegosauria--both the timing and cause of that event. With the possible exception of Dravidosaurus, Yanbeilong is the geologically youngest stegosaur known. Unlike Mongolostegus, Wuerhosaurus and the Louhandong specimen, which are not precisely dated ("Aptian-Albian" or in the case of Wuerhosaurus "Valanginian-Albian") Yanbeilong is unambiguously dated. The Zuoyun is of Albian age. This places Yanbeilong at the end of the early Cretaceous. No other stegosaur definitely occurs as late as the last stage of the EK.

An Albian age is interesting inasmuch as no stegosaur is known from the subsequent Cenomanian stage. The Baynshiree formation, for example, never yielded a stegosaur and nor did Iren Dabasu. It seems reasonable to infer that Stegosauria essentially went out with the EK or Albian. Realizing Yanbeilong was the denouement of Stegosauria, Chinese researchers gave it the species name "ultimus"--the last.

Why did stegosaurs vanish in the Albian, or by the end of that stage? Studies have noted evidence for drastic climatic change in Asia, accompanided by floral change, around the end of that period.

Krassilov et al. note the appearance of Sequoia in Mongolia "indicates a radical change from subtropical redbed climate of pronounced seasonal dryness towards more humid and equable conditions. A climate change like this must have inflicted a major restructuring of terrestrial ecosystems at about the Albian-Cenomanian boundary."

Golozoubov et al. report similar findings. In the Partizansky basin of far eastern Russia, "The late Albian saw a sharp change of vegetation...Diversity markedly decreased...numerous warmth-requiring species became extinct...the released niches occupied by angiosperms." In the Razdolnensky basin "..diversity sharply decreased in the Albian...Many of the cycadophytes, conifers and ferns became extinct."

Loss of cycadophytes and bennetitales, in particular, likely proved fatal to stegosaurs. Their niches taken by angiosperms, the cycadophytes never revived, and nor did the stegosaurs requiring them. Drastic climate change, specifically wetter habitats, spurred the change.

Climate/floral transition, the root of the stegosaur demise, did not occur everywhere at once. There is evidence for it at the start of the Cretaceous as well as its end. In western North Ameria, the Jurassic-Cretaceous boundary saw a wetter climate. Stegosaurs, notably the iconic Stegosaurus, thrived in the late Jurassic of America, in dry, upper Morrison habitats. But they are unknown after the wetter Jurassic-Cretaceous transition. None have been found in the lower Yellow Cat sediments. Since increased rainfall proved inimical to American stegosaurs, it isn't surprising that the same conditions were their nemesis in Asia. Dry habitats apparently persisted longest in Asia, but the late Albian finally witnessed the fatal change. Wetter conditions, resulting from a severe highstand, extirpated the last stegosaur.

References

Krassilov et al. New Fossil Plant and Insect Records Bearing on Cretaceous Climate of Western Gobi, Mongolia. American Journal of Plant Biology Vol. 2, No. 2, 2017 pp 43-48.

V. Golozoubov et al. Early Cretaceous change of Vegetation and Environment in East Asia Paleogeography, Paleoclimatolgy, Paleoecology 153, 1999 pp 139-146

Addendum

Dorsal vertebrae of Yanbeilong ultimus

The vertebrae have higher neural arches than those of other stegosaurs, and their neural canals are smaller. Yanbeilong is also distinguished by incomplete fusion of the sacrals (possible evidence for immaturity?). I'm not sure of the functional significance of small neural canals. High neural arches probably evolved to maximize the height of the stegosaur's back, making it appear more formidable (like the plates; a cat arches its back for the same resason). Not surprisingly, Yanbeilong is considered most closely related to Wuerhosaurus and Stegosaurus but differs from them.