Hesperosaurus

Chapter • Meet the dinosaur

Overview

Hesperosaurus mjosi was a mid-sized stegosaur that walked on all fours, with powerful hind limbs, short forelimbs and a back that arched high over the hips. Two rows of bony plates ran along its spine, but unlike the tall, pointed plates of Stegosaurus, those of Hesperosaurus were lower, rounder and much longer from front to back. The skull was shorter and broader than in its relatives, ending in a horn-covered beak, with small leaf-shaped teeth set further back in the jaws. The tail tip carried a thagomizer of four long spikes, a weapon that could be swung sideways at an attacker. Like all stegosaurs, Hesperosaurus moved in a horizontal posture with the tail carried clear of the ground – never dragging. The animal was slow and heavy, but its tail muscles were strong and the spikes were placed exactly where they did the most damage. The plates have been debated for decades: they were riddled with vascular canals and were covered in life by a keratin sheath, as the Wyoming specimen nicknamed "Victoria" shows through preserved skin impressions. Most researchers today read the plates mainly as display and species-recognition structures, with any thermoregulatory effect a likely side benefit rather than the main function. The diet consisted of low-growing plants: ferns, horsetails, cycads and conifer shoots. The head was held low and the jaw joint sat below the tooth row, so the bite worked like a simple pair of shears. The teeth were small and could not chew efficiently, which suggests Hesperosaurus swallowed large volumes of relatively soft plant matter and broke it down in a capacious gut. The genus was described in 2001 by Kenneth Carpenter, Clifford Miles and Karen Cloward from a skeleton found in the Morrison Formation of Johnson County, northern Wyoming. The species name mjosi honours Ronald G. Mjos, who helped prepare the specimen, while Hesperosaurus means roughly "western lizard". Several workers, including Susannah Maidment and colleagues, have argued that the animal should be called Stegosaurus mjosi, but a number of later analyses keep Hesperosaurus as a separate genus. A bone-rich site in the Little Snowy Mountains of Montana has yielded several individuals and underpins the hypothesis of sex differences in plate shape.

Chapter • Where it lived

Habitat

Hesperosaurus lived about 157–152 million years ago, in the Late Jurassic (Kimmeridgian), across the broad lowland plains now preserved as the Morrison Formation of western North America. The setting was a semi-arid landscape with strongly seasonal rains, meandering rivers, wide floodplains and open stands of conifers, ginkgoes and tree ferns. Fossils come from Johnson County in northern Wyoming, the Howe-Stephens Quarry near Shell in Wyoming, and the JRDI quarry in the Little Snowy Mountains of Montana. The same beds preserve sauropods such as Diplodocus and Camarasaurus, and predators such as Allosaurus.

Chapter • Wild behavior

Behavior & Ecology

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Chapter • Scientific place

Taxonomy