Kentrosaurus
Kentrosaurus is a genus of stegosaurid dinosaur from the Late Jurassic in Lindi Region of Tanzania. The type species is K. aethiopicus, named and described by German palaeontologist Edwin Hennig in 1915. Often thought to be a "primitive" member of the Stegosauria, several recent cladistic analyses find it as more derived than many other stegosaurs, and a close relative of Stegosaurus from the North American Morrison Formation within the Stegosauridae.
Fossils of K. aethiopicus have been found only in the Tendaguru Formation, dated to the late Kimmeridgian and early Tithonian ages, about 152 million years ago. Hundreds of bones were unearthed by German expeditions to German East Africa between 1909 and 1912. Although no complete skeletons are known, the remains provided a nearly complete picture of the build of the animal. In the Tendaguru Formation, it coexisted with a variety of dinosaurs such as the carnivorous theropods Elaphrosaurus and Veterupristisaurus, giant herbivorous sauropods Giraffatitan and Tornieria, and the dryosaurid Dysalotosaurus.
Kentrosaurus generally measured around in length as an adult, and weighed about. It walked on all fours with straight hindlimbs. It had a small, elongated head with a beak used to bite off plant material that would be digested in a large gut. It had a, probably double, row of small plates running down its neck and back. These plates gradually merged into spikes on the hip and tail. The longest spikes were on the tail end and were used to actively defend the animal. There also was a long spike on each shoulder. The thigh bones come in two different types, suggesting that one sex was larger and more stout than the other.
Discovery and naming
The first fossils of Kentrosaurus were discovered by the German Tendaguru Expedition in 1909, recognised as belonging to a stegosaur by expedition leader Werner Janensch on 24 July 1910, and described by German palaeontologist Edwin Hennig in 1915. The name Kentrosaurus was coined by Hennig and comes from the Greek kentron/κέντρον, meaning "sharp point" or "prickle", and sauros/σαῦρος meaning "lizard", Hennig added the specific name aethiopicus to denote the provenance from Africa. Soon after its description, a controversy arose over the stegosaur's name, which is very similar to the ceratopsian Centrosaurus. Under the rules of biological nomenclature, forbidding homonymy, two animals may not be given the same name. Hennig renamed his stegosaur Kentrurosaurus, "pointed-tail lizard", in 1916, while Hungarian paleontologist Franz Nopcsa renamed the genus Doryphorosaurus, "lance-bearing lizard", the same year. If a renaming had been necessary, Hennig's would have had priority. However, because the spelling is different, both Doryphorosaurus and Kentrurosaurus are unneeded replacement names; Kentrosaurus remains the valid name for the genus with Kentrurosaurus and Doryphorosaurus being its junior objective synonyms.File:Kentrosaurus brain and ganglium.JPG|thumb|Endocasts of the sacral ganglion and brain at Museum of Paleontology of Tübingen.Although no complete individuals were found, some material was discovered in association, including a nearly complete tail, hip, several dorsal vertebrae and some limb elements of one individual. These form the core of a mount in the Museum für Naturkunde by Janensch. The mount was dismantled during the museum renovation in 2006/2007, and re-mounted in an improved pose by Research Casting International. Some other material, including a braincase and spine, was thought to have been misplaced or destroyed during World War II. However, all the supposedly lost cranial material was later found in a drawer of a basement cupboard.
From 1909 onwards, Kentrosaurus remains were uncovered in four quarries in the Mittlere Saurierschichten and one quarry in the Obere Saurierschichten. During four field seasons, the German Expedition found over 1200 bones of Kentrosaurus, belonging to about fifty individuals, many of which were destroyed during the Second World War. Today, almost all remaining material is housed in the Museum für Naturkunde Berlin, while the museum of the Institute for Geosciences of the University of Tübingen houses a composite mount, roughly 50% of it being original bones.
In the original description, Hennig did not designate a holotype specimen. However, in a detailed monography on the osteology, systematic position and palaeobiology of Kentrosaurus in 1925, Hennig picked the most complete partial skeleton, today inventorised as MB.R.4800.1 through MB.R.4800.37, as a lectotype. This material includes a nearly complete series of tail vertebrae, several vertebrae of the back, a sacrum with five sacral vertebrae and both ilia, both femora and an ulna, and is included in the mounted skeleton at the Museum für Naturkunde in Berlin, Germany. The type locality is Kindope, Tanzania, north of Tendaguru hill.
Unaware that Hennig had already defined a lectotype, Peter Galton selected two dorsal vertebrae, specimens MB.R.1930 and MB.R.1931, from the material figured in Hennig's 1915 description, as 'holotypes'. This definition of a holotype is not valid, because Hennig's selection has priority. In 2011, Heinrich Mallison clarified that all the material known to Hennig in 1915, i.e. all the bones discovered before 1912, when Hermann Heck concluded the last German excavations, are paralectotypes, and that MB.R.4800 is the correct lectotype.
Description
Kentrosaurus was a small stegosaur. It had the typical dinosaurian body bauplan, characterised by a small head, a long neck, short forelimbs and long hindlimbs, and a long, horizontal and muscular tail. Typical stegosaurid traits included the elongation and flatness of the head, the powerful build of the forelimbs, erect and pillar-like hindlimbs and an array of plates and spikes running along both sides of the top mid-line of the animal.Size and posture
Kentrosaurus aethiopicus was a relatively small stegosaur, reaching in length and in body mass. Some specimens suggest that relatively larger individuals could have existed. These specimens are comparable to some Stegosaurus specimens in terms of the olecranon process in development.The long tail of Kentrosaurus results in a position of the center of mass that is unusually far back for a quadrupedal animal. It rests just in front of the hip, a position usually seen in bipedal dinosaurs. However, the femora are straight in Kentrosaurus, as opposed to typical bipeds, indicating a straight and vertical limb position. Thus, the hindlimbs, though powered by massive thigh muscles attached to a long ilium, did not support the animal alone, and the very robust forelimbs took up 10 to 15% of the bodyweight. The center of mass was not heavily modified by the osteoderms in Kentrosaurus or Stegosaurus, which allowed the animals to stay mobile despite their armament. The hindlimbs' thigh muscles were very powerful, allowing Kentrosaurus to reach a tripod stance on its hindlegs and tail.
Skull and dentition
Eight specimens from the skull, mandible, and teeth have been collected and described from the Tendaguru Formation, most of them being isolated elements. Two quadrates were referred to Kentrosaurus, but they instead belong to a juvenile brachiosaurid.The long and narrow skull was small in proportion to the body. It had a small antorbital fenestra, the hole between the nose and eye common to most archosaurs, including modern birds, though lost in extant crocodylians. The skull's low position suggests that Kentrosaurus may have been a browser of low-growing vegetation. This interpretation is supported by the absence of premaxillary teeth and their likely replacement by a horny beak or rhamphotheca. The presence of a beak extended along much of the jaws may have precluded the presence of cheeks in stegosaurs. Due to its phylogenetic position, it is unlikely that Kentrosaurus had an extensive beak like Stegosaurus and it instead probably had a beak restricted to the jaw tips. Other researchers have interpreted these ridges as modified versions of similar structures in other ornithischians which might have supported fleshy cheeks, rather than beaks.
There are two nearly complete braincases known from Kentrosaurus though they exhibit some taphonomic distortion. The frontals and parietals are flat and broad, with the latter bearing two transversely concave ventral sides with a ridge running down the middle that divides them. The lateral surface of the frontals form part of the orbit and the medial side creates the anterior part of the endocranial cavity. Basioccipitals form the posterior floor of the brain and the occipital condyle, which is large and spherical in Kentrosaurus. The rest of the braincase is formed by the presphenoid composing the anterior end. The overall braincase morphology is very similar to those of Tuojiangosaurus, Huayangosaurus, and Stegosaurus. However, the occipital condyle is a closer distance to the basisphenoid tubera in Kentrosaurus and Huayangosaurus than in Tuojiangosaurus and some specimens of Stegosaurus. Due to dinosaurs having more molding in their braincases, endocasts of Kentrosaurus can be reconstructed using the preserved fossils. The brain is relatively short, deep, and small, with a strong cerebral and pontine flexures and a steeply inclined posterodorsal edge when compared to those of other ornithischians. There is a small dorsal projection in the endocast where an unossified region between the top of the supraoccipital and overlying parietal that was likely covered in cartilage. This characteristic is seen in other ornithischians. Because of the prominent flexures, many of the aspects of the brain can only be interpreted by the present structures.
In the mandible, only an incomplete right dentary is known from Kentrosaurus. The deep dentary is almost identical in shape to that of Stegosaurus, albeit much smaller. Similarly, the tooth is a typical stegosaurian tooth, small with a widened base and vertical grooves creating five ridges. The dentary has 13 preserved alveoli on the dorsomedial side and they are slightly convex in lateral and dorsal views. On the surface adjacent to the alveoli, there is a shallow groove bearing small foramina that is similar to grooves on the dentary of the Cretaceous neornithischian Hypsilophodon, with one foramina per tooth position. Stegosaurian teeth were small, triangular, and flat; wear facets show that they did grind their food. A single complete cheek tooth is preserved, with a large crown and long root. The crown notably has fewer marginal denticles and a prominent cingulum compared to Stegosaurus, Tuojiangosaurus, and Huayangosaurus.