Кентрозавр: Танзаниядағы жоғалып кеткен динозаврлар тұқымы

Extinct genus of dinosaurs from late Jurassic in Lindi Region, Tanzania

Kentrosaurus (,/k//ɛ//n//t//r//oʊ/'/s//ɔːr//ə//s/ KENtrohSORəs; ) 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.

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. Some specimens suggest that relatively larger individuals could have existed. 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.

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 (bones from the jaw joint) 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 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 alvelovi on the dorsomedial side and it they are slightly convex in lateral and dorsal views. On the surface adjacent to the alveoli, there is a shallow groove bearing small foramina (small openings in bone) 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. The fore limbs were much shorter than the stocky hind limbs, which resulted in an unusual posture. The humerus (upper arm bone), like other stegosaurs, has greatly expanded proximal and distal ends that were attachment points between the coracoid and ulna radius (forearm bones) respectively. The radius was larger than the ulna and had a wedge shaped proximal end. The manus (hand) was small and had five toes with 2 toes bearing only a single phalange. The hindlimbs were much larger and too are similar to those of other stegosaurs. The femur (thigh bone) is the longest element in the body, with the largest known femur measuring 665 mm from the proximal to distal end. The tibia (shin bone) was wide and robust, while the fibula was skinny and thin without a greatly expanded distal end. The pes (foot) terminated in 3 toes, all of which had hoof like unguals (claws). Galton (2019) interpreted plates of an armored dinosaur from the Lower Jurassic (Sinemurian Pliensbachian) Lower Kota Formation of India as fossils of a member of Ankylosauria; the author argued that this finding indicates a probable early Early Jurassic origin for both Ankylosauria and its sister group Stegosauria. The vast majority of stegosaurian dinosaurs thus far recovered belong to the Stegosauridae, which lived in the later part of the Jurassic and early Cretaceous, and which were defined by Paul Sereno as all stegosaurians more closely related to Stegosaurus than to Huayangosaurus. This group is widespread, with members across the Northern Hemisphere, Africa and possibly South America. The South American remains come from Chubut, Argentina and consist only of a partial humerus, but the anatomy of the humerus is very similar to that of Kentrosaurus and both date to the Late Jurassic. In a phylogenetic analysis, the Chubut stegosaurid was recovered in polytomy with Kentrosaurus as basal stegosaurids, further suggesting that they are closely related. Kentrosaurus has many traits not seen in other stegosaurids but seen in basal stegosaurians, such as the presence of a parascapular spine and maxillary teeth with only seven denticles at the margin. However, this action was not accepted by the paleontological community, and S. longispinus has been assigned to its own genus, Alcovasaurus, differing from Kentrosaurus in having more elongated tail spikes and the structure of the pelvis and vertebrae. Cladogram of the phylogenetic analysis of Stegosauridae conducted by Maidment et al (2019), which recovers a distinct Alcovasaurus:

Like all ornithischians, Kentrosaurus was a herbivore. The fodder was barely chewed and swallowed in large chunks. One hypothesis on stegosaurid diet holds that they were low level browsers, eating foliage and low growing fruit from various non flowering plants. In 2013, a study by Ragna Redelstorff e. a. concluded that the bone histology of Kentrosaurus indicated that it had a higher growth rate than reported for Stegosaurus and Scutellosaurus, in view of the relatively rapid deposition of highly vascularised fibrolamellar bone. As Stegosaurus was larger than Kentrosaurus, this contradicts the general rule that larger dinosaurs grew quicker than smaller ones.