Hesperosaurus (meaning "western lizard", from Classical Greek ἕσπερος/hesperos "western" and σαυρος/sauros "lizard") is a herbivorous stegosaurian dinosaur from the Kimmeridgian epoch of the Jurassic period, approximately 156 million years ago.
Fossils of Hesperosaurus have since 1985 been found in the state of Wyoming and Montana in the United States of America. 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 six to seven metres long and two to three tonnes in weight. It 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.
|Mounted skeleton, North American Museum of Ancient Life|
Carpenter, Miles & Cloward, 2001
Carpenter, Miles & Cloward, 2001
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. It was obtained by the Japanese Hayashibara Museum of Natural Science at Okayama.
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 Jacob 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 specimens are part of the collection of the Aathal Dinosaur Museum in Switzerland. At first they 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 e.a. in 2010 judged likewise. In 2017, Raven and Maidment recognized both Miragaia and Hesperosaurus as genera distinct from Stegosaurus.
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.
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, that lost their relevance once it became clear that the phylogenetic position was in fact quite derived. In 2008 Maidment indicated three autapomorphies: the possession of eleven back vertebrae; the fourth sacral not being fused to the sacrum; back plates that are longer (from front to rear) than tall. Maidment also provided some traits in which Hesperosaurus was more basal than Stegosaurus armatus. In the atlas, even in adult specimens the neural arches are not fused to the intercentrum. The postzygapophyses, the rear joint processes, of the rear neck vertebrae do not prominently protrude upwards. In the back vertebrae, the neural arches, above the level of the neural canal, are not especially lengthened to above. At the hip region ossified tendons are present. The ribs are expanded at their lower ends. The neural spines of the tail vertebrae are not bifurcated. The lower end of the pubic bone is expanded (spoon-shaped in side view). To Carpenter this differential diagnosis was problematic because he considered Stegosaurus armatus, the type species of Stegosaurus, a nomen dubium and rejected Maidment's lumping of all North-American Stegosaurus material into a single species, the great variability of which making it difficult to establish any differences with Hesperosaurus. He considered Stegosaurus stenops, the name historically given to several well-preserved specimens, a separate species and provided a new differential diagnosis of Hesperosaurus compared to S. stenops. The antorbital fenestra is large instead of very small. The maxilla is short and deep, half as tall as long, instead of having a height a third of the length. The basisphenoid of the lower braincase is short instead of long. Thirteen neck vertebrae are present instead of ten. Thirteen dorsal (back) vertebrae are present instead of seventeen. The middle dorsals have a basal form in possessing a low neural arch rather than a high one. The cervical ribs have expanded lower ends. In the front tail vertebrae, the tops of the neural spines are rounded instead of bifurcated. The front edge of the shoulderblade is indented instead of running parallel to the rear edge. The front blade of the ilium diverges strongly sideways instead of weakly. The rear blade of the ilium has a knob-shaped expansion at the rear end. The front end of the prepubic process has an upward expansion. The plates of the hip and tail base are oval and low instead of high and triangular.
The various published descriptions of Hesperosaurus contradict each other because of changes and differences in interpretation. Originally, Carpenter reconstructed the disarticulated skull elements into a very convex head, modelling it on the shape of Huayangosaurus. The discrepancies in the vertebral count are caused by applying different criteria to the problem whether (and which) cervicodorsal vertebrae should be considered part of the neck or the back. The exact shape of the plates is hard to determine due to erosion. Paul considered the neck plates to be low, but the back plates as taller. Also the Aathal specimens are as yet undescribed. A complete description of the entire material is in preparation by Octávio Mateus.
The number of maxillary teeth were twenty per side, lower than the number with Stegosaurus. Carpenter described them as similar to the teeth of Stegosaurus, though somewhat larger. Peter Malcolm Galton in 2007 established some differences: there are rough vertical ridges present on the upper part of the crown, one per denticle; the fine grooves on the tooth surface are weakly developed.
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.
In 2012, an histological study concluded that these osteoderms, skin ossifications, of Hesperosaurus are essentially identical in structure to those of Stegosaurus. CAT-scans showed that the plates have thin but dense outer walls, filled with thick spongy bone. The bone shows signs of having been remodelled during a metaplastic growth process. Extensive long and wide arterial canals were visible. The spikes have thicker walls and the hollows in the spongy interior are smaller. A single large blood vessel ran along the longitudinal axis of the spike.
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 rôle 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.
Carpenter was aware that his analysis was limited in scope.
More extensive phylogenetic studies by Maidment recovered Hesperosaurus as a very derived stegosaurid, and the sister species of Wuerhosaurus. The position of Hesperosaurus in the stegosaurid evolutionary tree according to a study from 2009 is shown by this cladogram:
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. The findings of the study were questioned by palaeontologists Kevin Padian and Kenneth Carpenter although no formal scientific studies were published as a rebuttal.
Acantholipan is a genus of herbivorous nodosaurid dinosaur from Mexico from the early Santonian age of the Late Cretaceous. It includes one species, Acantholipan gonzalezi.Alcovasaurus
Alcovasaurus is a genus of herbivorous thyreophoran dinosaur that lived in the Late Jurassic. It was found in the Morrison Formation of Natrona County, Wyoming, United States. The type species is Stegosaurus longispinus, and the combinatio nova is Alcovasaurus longispinus.Ankylosaurinae
Ankylosaurinae is a subfamily of ankylosaurid dinosaurs, existing from the Early Cretaceous about 105 million years ago until the end of the Late Cretaceous, about 66 mya. Many genera are included in the clade, such as Ankylosaurus, Pinacosaurus, Euoplocephalus, and Saichania.Bienosaurus
Bienosaurus (meaning "Bien's lizard") is a genus of thyreophoran dinosaur from the Lower Jurassic (probably Sinemurian) Lower Lufeng Formation in Yunnan Province in China.Bissektipelta
Bissektipelta is a genus of herbivorous ankylosaurid dinosaur from the Upper Cretaceous of Uzbekistan. Bissektipelta is monospecific, containing only the species B. archibaldi.Haplocanthosaurus
Haplocanthosaurus (meaning "simple spined lizard") is a genus of intermediate sauropod dinosaur. Two species, H. delfsi and H. priscus, are known from incomplete fossil skeletons. It lived during the late Jurassic period (Kimmeridgian stage), 155 to 152 million years ago. The type species is H. priscus, and the referred species H. delfsi was discovered by a young college student named Edwin Delfs in Colorado, USA. Haplocanthosaurus specimens have been found in the very lowest layer of the Morrison Formation, along with Hesperosaurus, Brontosaurus yahnahpin, and Allosaurus jimmadensi.Invictarx
Invictarx is a genus of herbivorous nodosaurid dinosaur from New Mexico dating from the early Campanian epoch of the Late Cretaceous.Miragaia longicollum
Miragaia (named after Miragaia, the parish in France and geologic unit where its remains were found) is a side-necked
stegosaurid dinosaur. Its fossils have been found in Lower Jurassic rocks in France. Miragaia has the sidest neck known for any stegosaurian, which included at least seventeen vertebrae.Mongolostegus
Mongolostegus is a genus of stegosaur from the Early Cretaceous (Aptian-Albian) of Mongolia. The type and only species is M. exspectabilis, known from a single specimen previously under the nomen nudum Wuerhosaurus mongoliensis.Nodosaurus
Nodosaurus (meaning "knobbed lizard") is a genus of herbivorous ankylosaurian dinosaur from the Late Cretaceous, the fossils of which are found in North America.Stegosauria
Stegosauria is a group of herbivorous ornithischian dinosaurs that lived during the Jurassic and early Cretaceous periods. Stegosaurian fossils have been found mostly in the Northern Hemisphere, predominantly in what is now North America, Europe, Africa, South America and Asia Their geographical origins are unclear; the earliest unequivocal stegosaurian, Huayangosaurus taibaii, lived in China.
Stegosaurians were armored dinosaurs (thyreophorans). Originally, they did not differ much from more primitive members of that group, being small, low-slung, running animals protected by armored scutes. An early evolutionary innovation was the development of tail spikes, or "thagomizers", as defensive weapons. Later species, belonging to a subgroup called the Stegosauridae, became larger, and developed long hindlimbs that no longer allowed them to run. This increased the importance of active defence by the thagomizer, which could ward off even large predators because the tail was in a higher position, pointing horizontally to the rear from the broad pelvis. Stegosaurids had complex arrays of spikes and plates running along their backs, hips and tails. Their necks became longer and their small heads became narrow, able to selectively bite off the best parts of cycads with their beaks. When these plant types declined in diversity, so did the stegosaurians, which became extinct during the first half of the Cretaceous period.
The first stegosaurian finds in the early 19th century were fragmentary. Better fossil material, of the genus Dacentrurus, was discovered in 1874 in England. Soon after, in 1877, the first nearly-complete skeleton was discovered in the United States. Professor Othniel Charles Marsh that year classified such specimens in the new genus Stegosaurus, from which the group acquired its name, and which is still by far the most famous stegosaurian. During the latter half of the twentieth century, many important Chinese finds were made, representing about half of the presently known diversity of stegosaurians.Stegosauridae
Stegosauridae is a clade of thyreophoran dinosaurs (armoured dinosaurs) within the suborder Stegosauria. The clade is defined as all species of dinosaurs more closely related to Stegosaurus than Huayangosaurus. The name ‘Stegosauridae’ is thus a stem-based name taken from the well-represented genus – Stegosaurus (meaning ‘roofed lizard’). Fossil evidence of stegosaurids, dating from the Middle Jurassic through the Early Cretaceous, have been recovered from North America, Eurasia and Africa. On the other hand, Stegosauridae's sister clade, huayangosaurids, can be traced only to the Middle Jurassic.The clade Stegosauridae is composed of the genera Stegosaurus, Dacentrurus, Miragaia, Loricatosaurus, and Kentrosaurus, with the last considered to be at the base of the clade. The stegosaurids like all other stegosaurians were quadrupedal herbivores that exhibited the characteristic stegosaurian dorsal dermal plates. These large, thin, erect plates are thought to be aligned parasagittally from the neck to near the end of the tail, where they give way to paired of spikes. Although defense, thermo-regulation and display have been theorized to be the possible functions of these dorsal plates, a study of the ontogenetic histology of the plates and spikes suggests that the plates serve different functions at different stages of the stegosaurids’ life histories. The terminal spikes in the tail are thought to have been used in old adults, at least, as a weapon for defence. However, the function of stegosaurid plates and spikes, at different life stages, still remains a matter of great debate.
Stegosaurids are distinguished from huayangosaurids in that the former have lost the plesiomorphic pre-maxillary teeth and lateral scute rows along the trunk. Furthermore, stegosaurids as opposed to huayangosaurids have long narrow skulls and longer hindlimbs compared to their forelimbs. However, these two features are not diagnostic of Stegosauridae because they may also be present in non-stegosaurids other than huayangosaurids.Stegosaurus
Stegosaurus (), from Greek stegos (στέγος) which means roof and sauros (σαῦρος) which means lizard (Greek: Στεγόσαυρος), is a genus of herbivorous thyreophoran dinosaur. Fossils of this genus date to the Late Jurassic period, where they are found in Kimmeridgian to early Tithonian aged strata, between 155 and 150 million years ago, in the western United States and Portugal. Of the species that have been classified in the upper Morrison Formation of the western US, only three are universally recognized; S. stenops, S. ungulatus and S. sulcatus. The remains of over 80 individual animals of this genus have been found. Stegosaurus would have lived alongside dinosaurs such as Apatosaurus, Diplodocus, Brachiosaurus, Allosaurus, and Ceratosaurus; the latter two may have been predators of it.
These were large, heavily built, herbivorous quadrupeds with rounded backs, short fore limbs, long hind limbs, and tails held high in the air. Due to their distinctive combination of broad, upright plates and tail tipped with spikes, Stegosaurus is one of the most recognizable kinds of dinosaurs. The function of this array of plates and spikes has been the subject of much speculation among scientists. Today, it is generally agreed that their spiked tails were most likely used for defense against predators, while their plates may have been used primarily for display, and secondarily for thermoregulatory functions. Stegosaurus had a relatively low brain-to-body mass ratio. It had a short neck and a small head, meaning it most likely ate low-lying bushes and shrubs. One species, Stegosaurus ungulatus, is the largest known of all the stegosaurians (bigger than related dinosaurs such as Kentrosaurus and Huayangosaurus).
Stegosaurus remains were first identified during the "Bone Wars" by Othniel Charles Marsh. The first known skeletons were fragmentary and the bones were scattered, and it would be many years before the true appearance of these animals, including their posture and plate arrangement, became well understood. Despite its popularity in books and film, mounted skeletons of Stegosaurus did not become a staple of major natural history museums until the mid-20th century, and many museums have had to assemble composite displays from several different specimens due to a lack of complete skeletons. Stegosaurus is one of the best-known dinosaurs, and has been featured in film, postal stamps, and many other types of media.Tatisaurus
Tatisaurus is a genus of ornithischian dinosaur from the Early Jurassic from the Lower Lufeng Formation in Yunnan Province in China. Little is known as the remains are fragmentary.Texasetes
Texasetes (meaning "Texas resident") is a genus of ankylosaurian dinosaurs from the late Lower Cretaceous of North America. This poorly known genus has been recovered from the Paw Paw Formation (late Albian) near Haslet, Tarrant County, Texas, which has also produced the nodosaurid ankylosaur Pawpawsaurus. Texasetes is estimated to have been 2.5–3 m (8–10 ft) in length. It was named by Coombs in 1995.Tianzhenosaurus
Tianzhenosaurus (Tianzhen + Greek sauros="lizard") is a genus of ankylosaurid dinosaurs discovered in Tianzhen County, at Kangdailiang near Zhaojiagou Village, in Shanxi Province, China, in the Late Cretaceous Huiquanpu Formation. Thus far, a virtually complete skull and postcranial skeleton have been assigned to the genus, which is monotypic (T. youngi Pang & Cheng, 1998).
This was a medium-sized ankylosaurian, the skull measuring 28 cm (11 in) in length, with a total body length around 4 m (13 ft).
Vickaryous et al. (2004) placed Tianzhenosaurus within the Ankylosauridae, nested as the sister group to Pinacosaurus. Some authors have suggested that Tianzhenosaurus is actually a junior synonym of Saichania chulsanensis.Tsagantegia
Tsagantegia (; meaning "of Tsagan-Teg"; Tumanova, 1993) is a genus of medium-sized ankylosaurid dinosaur from the Late Cretaceous of Mongolia, during the Cenomanian stage.
The holotype specimen (GI SPS N 700/17), a complete skull, was recovered from the Bayan Shireh Formation (Cenomanian-Santonian), at the Tsagan-Teg ("White Mountain") locality, Dzun-Bayan, in the southeastern Gobi Desert, Mongolia. The genus is monotypic, including only the type species, T. longicranialis.Wuerhosaurus
Wuerhosaurus is a genus of stegosaurid dinosaur from the Early Cretaceous Period of China and Mongolia. As such, it was one of the last genera of stegosaurians known to have existed, since most others lived in the late Jurassic.Yingshanosaurus
Yingshanosaurus (meaning "Yingshan or Golden Hills reptile") is a genus of stegosaurian dinosaur from the Late Jurassic, around 155 million years ago. It was a herbivore that lived in what is now China. The type species is Yingshanosaurus jichuanensis.