Kentrosaurus

Kentrosaurus is a genus of stegosaurid dinosaur from the Late Jurassic of Tanzania. Its fossils have been found only in the Tendaguru Formation of Tanzania, dated to the Kimmeridgian stage, between about 155.7 ± 4 Ma and 150.8 ± 4 Ma (million years ago). Apparently, all finds belong to one species, K. aethiopicus Hennig 1915.

Kentrosaurus was described by German palaeontologist Edwin Hennig in 1915. Often thought to be a primitive member of the Stegosauria, several recent cladistic analyses find it to be derived, and a close relative to Stegosaurus from the North American Morrison Formation.

Kentrosaurus generally measured around 4.5 metres (15 ft) in length as an adult, probably had a double row of small plates and spikes running down its back, and could use its tail as a "thagomizer" for defense. The femora (thigh bones) are strongly dimorphic, suggesting that one gender (likely the females) was larger and more stout than the other.

Description
Kentrosaurus was a small stegosaur, with the typical dinosaurian body bauplan characterised by a small head, a long neck, short forelimbs and long hindlimbs, and a long and muscular tail. Typically for a stegosaur, it has extensive osteoderms (bony structures in the skin), including small plates (probably located on the neck and anterior trunk), and spikes of different shape.

Body size and posture
Kentrosaurus aethiopicus was a small stegosaur, smaller than Stegosaurus armatus, Hesperosaurus mjosi, Dacentrurus armatus and Tuojiangosaurus multispinus, and about as large as Huayangosaurus taibaii. The total length of a composite skeletal mount in the Museum für Naturkunde Berlin, Germany, from the tip of the snout to the tip of the tail is 4.5 m (15 ft). Slightly more than half of this length is made up by the tail.[1] Larger single elements were found,[2] so that the animal could probably attain a total length of 5.5 m (18 ft).

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 hind feet did not support the animal alone, and the fore feet took up 10 to 15% of the bodyweight. The posterior position of the center of mass may not have been advantageous for rapid locomotion, but meant that the animal could quickly rotate around the hips by pushing sideways with the arms, keeping the tail pointed at threats.

Autapomorphies
Kentrosaurus can be distinguished from other members of the Stegosauria by a number of osteological characters. Most notably, the neural spines in the tail are not sub-parallel, as in most dinosaurs. In the anterior third of the tail, they point backwards, the usual direction. In the middle tail, however, they are almost vertical, and further back they are hook-shaped and point forward. Also typical are, among other features, that the dorsal vertebrae have a neural arch more than twice as high as the centrum, and completely occupied by the extremely spacious neural canal. The preacetabular process of the ilium widens laterally, and does not taper.

Armor
The spikes and plates of Kentrosaurus were likely covered by horn. Aside from a few exceptions they were not found in close association with other skeletal remains. Thus, the exact position of most osteoderms is uncertain. A pair of closely spaced spikes was found articulated with a tail tip, and a number of spikes were found apparently regularly spaced in pairs along the path of an articulated tail.[2] Hennig[2] and Janensch,[1] while grouping the dermal armour elements into four distinct types, recognized an apparently continuous change of shape among them, suggesting an uninterrupted distribution along the entire body. Because each type of osteoderm was found in two handed versions, it seems probably that all types of osteoderms were distributed in two rows along the back of the animal, a marked contrast to the better-known North American Stegosaurus, which had one row of plates on the neck, trunk and tail, and two rows of spikes on the tail tip. There is one type of spike that differs from all others in being strongly, and not only slightly, asymmetrical. Because of bone morphology classic reconstructions placed it on the hips, while many recent reconstructions place it on the shoulder, because a similarly shaped spike is known to have existed on the should in the Chinese stegosaur Gigantspinosaurus.

Discovery and species
The first fossils of Kentrosaurus were discovered by the German Tendaguru Expedition in 1909, recognized as belonging to a stegosaur by expedition leader Werner Janensch on 24 July 1910, and described by German palaeontologist Edwin Hennig in 1915.[3] During four field seasons, the German Expedition found over 1200 bones of Kentrosaurus,[2] many of which were destroyed during the Second World War.[4] Today, almost all remaining material is housed in the Museum für Naturkunde Berlin (roughly 350 remaining specimens), while the museum of the Institute for Geosciences of the Eberhard-Karls-University Tübingen houses a composite mount, roughly 50% of it being original bones.[5] Although no complete individuals were found, some material was found 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.[1] 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.[6] However, all the supposedly lost cranial material was later found in a drawer of a basement cupboard.[7] The British Tendaguru Expedition also found material, but it is unclear how much, in what state of preservation, and where it is today. The type and sole species of Kentrosaurus is K. aethiopicus. Fragmentary fossil material from Wyoming, named Stegosaurus longispinus by Charles Gilmore in 1914,[8] has been suggested to belong to a North American species of Kentrosaurus. However, this notion has not found any support in the professional community.

Etymology
When Hennig named his new stegosaur, he chose to highlight the extensive dermal armour in the generic name. From the Greek kentron/κεντρον, meaning "point" or "prickle", and sauros/σαυρος meaning "lizard",[9] Hennig created Kentrosaurus (/ˌkɛntrɵˈsɔrəs/ KEN-tro-SAWR-əs), adding the species name aethiopicus to denote the provenance.

Naming controversy
Kentrosaurus was described by Edwin Hennig in 1915,[3] but soon after its description, a controversy arose over its name, which is very similar to the ceratopsian dinosaur Centrosaurus. Under the rules of biological nomenclature, two animals may not be given the same name. Hennig renamed his stegosaur Kentrurosaurus,[10] while Hungarian paleontologist Franz Nopcsa renamed the genus Doryphorosaurus.[11] If a renaming had been necessary, Hennig's would have had priority.[12] However, because both the spellings and the pronunciations are different (Centrosaurus is pronounced with a soft C), both Doryphorosaurus and Kentrurosaurus are unneeded replacement names; Kentrosaurus remains the valid name for the genus.