Hesperosaurus (from Ancient Greek ἕσπερος (hésperos), meaning "western", and σαῦρος (saûros), meaning "lizard") is a herbivorous stegosaurian dinosaur from the Kimmeridgian age of the Jurassic period, approximately 156 million years ago.
Fossils of Hesperosaurus have been found in the state of Wyoming and Montana in the United States of America since 1985. The type species Hesperosaurus mjosi was named in 2001. It is from an older part of the Morrison Formation, and so a little older than other Morrison stegosaurs. Several relatively complete skeletons of Hesperosaurus are known. One specimen preserves the first known impression of the horn sheath of a stegosaurian back plate.
Hesperosaurus was a member of the Stegosauridae, quadrupedal plant-eaters protected by vertical bony plates and spikes. It was closely related to Stegosaurus and was similar to it in having two rows of, possibly alternating, plates on its back and four spikes on its tail end. The plates on its back were perhaps not as tall, but were longer. It possibly had a deeper skull than Stegosaurus.
Discovery and species
thumb|left|Mounted cast skeleton, [[North American Museum of Ancient Life]]
In 1985, fossil hunter Patrick McSherry, at the ranch of S.B. Smith in Johnson County, Wyoming, found the remains of a stegosaur. As he had difficulty securing the specimen due to the hard rock matrix, he sought help from Ronald G. Mjos and Jeff Parker of Western Paleontological Laboratories, Inc. They, in turn, cooperated with paleontologist Dee Hall of Brigham Young University. At first, it was assumed it represented an exemplar of Stegosaurus. However, Clifford Miles, while preparing the remains, recognised that they belonged to a species new to science.
The type species Hesperosaurus mjosi was named and described in 2001 by Kenneth Carpenter, Clifford Miles, and Karen Cloward. The generic name is derived from the Greek ἕσπερος, hesperos, "western", in reference to its location in the western United States. The specific name honours Mjos who, apart from his involvement in the process of collecting and preparing the holotype, also had a cast of it made, exhibited with the inventory number DMNH 29431 in the Denver Museum of Natural History.
The holotype, HMNH 001 (later HMNS 14), was found in the Windy Hill Member, stratigraphic zone 1 of the lower Morrison Formation, dating from the early Kimmeridgian, about 156 million years old. In 2001, it represented the oldest known American stegosaur. It consists of a nearly complete skull and much of the skeleton. It includes the disarticulated elements of the skull, the rear lower jaws, a hyoid, thirteen neck vertebrae, thirteen back vertebrae, three sacrals, forty-four tail vertebrae, neck ribs, dorsal ribs, chevrons, a left shoulderblade, a complete pelvis, ossified tendons and ten neck and back plates. The skeleton was partly articulated and, in view of healed fractures, belongs to an old individual.
alt=Stegosaurierschadel|right|thumb|Crushed skull (30 cm long) of the Hesperosaurus "Lilly"
From 1995 onward at the Howe-Stephens Quarry in Big Horn County, Wyoming, named after the historic location of the Howe Ranch, once explored by Barnum Brown, and the new owner Press Stephens, Swiss palaeontologist Hans-Jakob Siber excavated stegosaur specimens. The first was SMA 3074-FV01 (also SMA M04), a partial skeleton dubbed "Moritz" after Max und Moritz as an earlier Galeamopus sauropod skeleton from the site had been nicknamed "Max". In 1996/97, specimen SMA 0018 (also mistakenly referred to as SMA V03) was uncovered, dubbed "Victoria" after the feeling of victory the exploring team felt when they discovered Allosaurus "Big Al Two" after the original "Big Al" had been confiscated as federal property. It represents a rather complete skeleton with skull, also preserving skin and horn sheath impressions. A third specimen was found in 2002: SMA L02, dubbed "Lilly" after the sisters Nicola and Rabea Lillich assisting the excavations as volunteers. The "Lilly" and "Moritz" specimens are part of the collection of the Aathal Dinosaur Museum in Switzerland; "Victoria" was donated by the Sauriermuseum Aathal, along with "Big Al" and other specimens, to the Natural History Museum of the University of Zurich. At first the three specimens were considered Stegosaurus exemplars. In 2009, initially only "Moritz" and "Lilly" were reclassified as cf. Hesperosaurus mjosi. In 2010, "Victoria" was referred to Hesperosaurus mjosi by Nicolai Christiansen and Emanuel Tschopp.
Carpenter had originally concluded that Hesperosaurus was a rather basal stegosaur. However, Susannah Maidment and colleagues in 2008 published a more extensive phylogenetic study in which it was recovered as a derived form, closely related to Stegosaurus and Wuerhosaurus. They proposed that Hesperosaurus should be considered a species of Stegosaurus, with Hesperosaurus mjosi becoming Stegosaurus mjosi; at the same time Wuerhosaurus was renamed into a Stegosaurus homheni. Carpenter, considering the problem more of a philosophical than a scientific nature, in 2010 rejected the synonymy of Hesperosaurus with Stegosaurus stating that in his opinion Hesperosaurus was sufficiently different from Stegosaurus to be named a separate genus. Christiansen in 2010 judged likewise.
In 2015, additional specimens were reported: a concentration of at least five individuals discovered at the JRDI 5ES Quarry near Grass Range, Montana, and two individuals found in the Meilyn Quarry at Como Bluff. In 2018, new specimen of H. mjosi was described from Montana.
Description
thumb|left|Size comparison
Hesperosaurus was a large stegosaur, reaching in length and in body mass. Some large individuals may have reached in body mass.
In 2001 Carpenter provided a diagnosis. Due to his conclusion that Hesperosaurus were rather basal, in it many comparisons were made with the basalmost known stegosaurian Huayangosaurus,
Osteoderms and skin impressions
left|thumb|"[[Thagomizer"]]
Carpenter in 2001 identified ten plates as part of the holotype. He described them as long and low. Asymmetrical bases would indicate that they ran in two rows. The end of the tail bore a "thagomizer" of two pairs of spikes, the front pair being thicker, the rear pair thinner and more horizontally directed to behind.
thumb|Traces of skin (black) on a plate of Hesperosaurus
In 2010, a study was published on the soft parts visible with the "Victoria" specimen. It preserves both true impressions of the skin into the surrounding sediment, and natural casts, where the spaces left behind by the rotting of the soft body parts have been filled in with sediment. Additionally on some areas a black layer is present, possibly consisting of organic remains or bacterial mats. A part of the lower trunk flank shows rows of small hexagonal, non-overlapping, convex scales, two to seven millimetres in diameter. Higher on the flank two rosette structures are visible with larger central scales, one being twenty by ten millimetres in size, the other ten by eight millimetres. Apart from the scales, an impression of the lower side of a back plate has been found, covering about two hundred square centimetres. This shows no scales but a smooth surface with low parallel vertical ridges. As it is a true impression, with the life animal grooves would have been present. These grooves would have been about half a millimetre deep and stood about two millimetres apart. The impression probably represented the horn sheath of the plate, as would be confirmed by vertical traces of veins. It is the first direct proof of such sheaths with any stegosaurian. The study considered the presence of a sheath to be a strong indication that the plate had primarily a defensive function, as a horn layer would have strengthened the plate as a whole and provided it with sharp cutting edges. Also the display function would have been reinforced, because the sheath would have increased the visible surface and such horn structures are often brightly coloured. Thermoregulation, on the other hand — another often assumed role of the plates — would have been hampered by an extra insulating layer and the smoothness of the surface, but cannot be entirely ruled out as extant cattle and ducks use horns and beaks to dump excess heat despite the horn covering.
In a 2019 re-evaluation of dacentrurine stegosaurids, Costa and Mateus suggested that, based on their revised diagnosis for the clade Dacentrurinae, Hesperosaurus appears to heave been closely related to Dacentrurus after all, though they refrained from formally reassigning it to that group pending the completion of an expanded phylogenetic analysis.
Paleobiology
In 2015, a study by Evan Thomas Saitta based on the finds in the JRDI 5ES Quarry concluded that Hesperosaurus showed sexual dimorphism. Plates found in the quarry came in two types: a taller one, and a low broad one. Though the back plates of the various individuals were not articulated, Saitta managed to order them into cervical, dorsal and caudal series for each type. This seemed to show that some individuals had tall plates exclusively while others bore broad plates only, which was confirmed by earlier specimens also possessing plates of one kind. Saitta suggested that the tall plates typified the females, while the males were equipped with low plates.
See also
- Timeline of stegosaur research