Kentrosaurus

Kentrosaurus ( ) is a genus of stegosaurian dinosaur from the Late Jurassic 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.
"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 species.
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 '/, meaning "sharp point" or "prickle", and '/ meaning "lizard", Hennig added the specific name "aethiopicus" to denote the provenance from Africa.
From 1909 onwards, "Kentrosaurus" remains were uncovered in four quarries in the "Mittlere Saurierschichten" (Middle Saurian Beds) and one quarry in the "Obere Saurierschichten" (Upper Saurian Beds). 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 (roughly 350 remaining specimens), while the museum of the Institute for Geosciences of the University of Tübingen houses a composite mount, roughly 50% of it being original bones.
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.
The type and sole accepted species of "Kentrosaurus" is "Kentrosaurus aethiopicus", named by Hennig in 1915. Fragmentary fossil material from Wyoming, named "Stegosaurus longispinus" by Charles Gilmore in 1914, was in 1993 classified as a North American species of "Kentrosaurus", as "K. longispinus". 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.
Type specimens and type locality.
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 (see syntype). 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, near the 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.
Naming controversy.
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 saurian", in 1916, while Hungarian paleontologist Franz Nopcsa renamed the genus "Doryphorosaurus", "lance-bearing saurian", 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.
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. Only a single complete tooth was known when Hennig published his monography in 1925. Later, a part of a dentary, the tooth-bearing bone of the front lower jaw, was found, which bears a just emerging tooth, and some tooth fragments were recovered from matrix sticking to other bones. 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.
Size and posture.
"Kentrosaurus aethiopicus" was smaller than "Stegosaurus ungulatus", "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 . Slightly more than half of this length is made up by the tail. Larger single elements were found, so that the animal could probably attain a total length of . In 2010, Gregory S. Paul estimated the weight of a "Kentrosaurus" at . An estimate for the 4.5 m long composite mount in the Museum für Naturkunde Berlin by Mallison, on the basis of a virtual 3D skeleton and 3D model, varied between 1073 L and 1267 L, and a body mass between , depending on the amount of musculature reconstructed for the tail.
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.
Distinguishing features.
"Kentrosaurus" can be distinguished from other members of the Stegosauria by a number of extensions of the vertebrae, which in the tail do not run sub-parallel, as in most dinosaurs. In the front 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 obliquely forward. Also typical is that the dorsal (back) vertebrae have a neural arch more than twice as high as the centrum, the vertebral body, and almost completely occupied by the extremely spacious neural canal. The preacetabular process, front blade, of the ilium widens laterally, to the front outer side, and does not taper.
Furthermore, there is a unique combination of traits not in themselves unique. The transverse processes, the side extensions, of the tail are present up to the 28th vertebra of the series. The transverse processes of the front tail vertebrae are rod-shaped with narrow bases and do not touch the plate formed by the fusion of the processes of the sacral vertebrae. The chevrons, bones pointing to below from the bottom side of the tail vertebrae, have the shape of an inverted T. The length of the ilium equals, or is greater than, that of the thigh bone.
Armour.
Typically for a stegosaur, "Kentrosaurus" had extensive osteoderm (bony structures in the skin) covering, including small plates (probably located on the neck and anterior trunk), and spikes of various shapes. The spikes of "Kentrosaurus" are very elongated, with one specimen having a bone core length of 731 millimetres. The plates have a thickened section in the middle, as if they were modified spines. The spikes and plates 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.
Hennig and Janensch, while grouping the dermal armour elements into four distinct types, recognised an apparently continuous change of shape among them, shorter and flatter plates at the front gradually merging into longer and more pointed spikes towards the rear, suggesting an uninterrupted distribution along the entire body, in fifteen pairs. Because each type of osteoderm was found in mirrored left and right versions, it seems probable 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, and having a very broad base. Because of bone morphology classic reconstructions placed it on the hips, at the iliac blade, while many recent reconstructions place it on the shoulder, because a similarly shaped spike is known to have existed on the shoulder in the Chinese stegosaurs "Gigantspinosaurus" and "Huayangosaurus".
Phylogeny.
"Kentrosaurus" was by Hennig assigned to the Stegosauridae in 1915. This is confirmed by modern cladistic analyses, although in 1915 Stegosauridae was a far more inclusive concept. A consecutive narrowing down of this concept caused "Kentrosaurus", until the 1980s to be seen as a typical "primitive" stegosaurian, to be placed in a more derived, higher, position in the stegosaur evolutionary tree. Derived traits include a sacral yoke, a long prepubic process, a long thighbone and two rows of plates or spikes. A study by Octávio Mateus e.a. in 2009 recovered "Kentrosaurus" in a basal position in the Stegosauridae as shown by this cladogram:
Earlier analyses had shown "Kentrosaurus" closer in the tree to "Stegosaurus". Basal traits include a prominent paraquadratic foramen at the quadrate in the skull; maxillary teeth with only seven denticles at the margin; and a shoulder spine.
Paleobiology.
Feeding.
Like all ornithischians, "Kentrosaurus" was a herbivore. The fodder was barely chewed and swallowed in large chunks. One theory on stegosaurid diet holds that they were low-level browsers, eating foliage and low-growing fruit from various non-flowering plants. "Kentrosaurus" was capable of eating at heights of up to when on all fours. It may also have been possible for it to rear up on its hindlegs to reach vegetation higher in trees. With its centre of mass close to the hind-limbs, the animal could potentially support itself as it stood up. The hips were likely capable of allowing a vertical trunk rotation of about 60 degrees and the tail probably would either have been fully lifted, not blocking this movement or have enough curvature to rest on the ground; thus it could have provided additional support, though precisely because of this flexibility it is not certain whether much support was actually provided: it was not stiff enough to function as a "third leg" as had been suggested by Robert Thomas Bakker. In this pose, "Kentrosaurus" could have fed at heights of .
Defense.
Because the tail had at least forty caudal vertebrae, it was highly mobile. It could possibly swing at an arc of 180 degrees, covering the entire half circle behind it. Swing speeds at the tail end may have been as high as 50 km/h. Continuous rapid swings would have allowed the spikes to slash open the skin of its attacker or to stab the soft tissues and break the ribs or facial bones. More directed blows would have resulted in the sides of the spikes fracturing even sturdy longbones of the legs by blunt trauma. These attacks would have crippled small and medium-sized theropods and may even have done some damage to large ones. Earlier interpretations of the defensive behaviour of "Kentrosaurus" included the suggestion that the animal might have charged to the rear, to run through attackers with its spines, in the way of modern porcupines.
Though "Kentrosaurus" likely stood with forelimbs erect like in other dinosaurs, it is hypothesised that the animal adopted a sprawling posture when defending itself. Its neck was flexible enough to allow it to keep sight of predators, as it could reach the sides of its body with its snout and look over the back. In addition, 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 the attacker. "Kentrosaurus" was nevertheless not invulnerable. A quick predator could have made it to the tail base (where the impact speed would be much lower) when the tail passed and the neck and upper-part of the body would have been unprotected by the tail swings. A successful predation of "Kentrosaurus" may have required group hunting. Compared to the more robust spikes of "Stegosaurus", the thinner spikes of "Kentrosaurus" were at greater risk of bending.
Growth.
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.
Sexual dimorphism.
Differences in the proportions, not the size, of the femurs (thighbones) led Holly Barden and Susannah Maidment to realize that "Kentrosaurus" probably showed sexual dimorphism. This dimorphism of the femurs consisted in them being either more or less robust than the other. The occurrence ratio of the robust morph to the gracile one was 2:1, and it is likely that the higher percentage of animals were females. Because of this ratio, it was considered reasonable to assume that in their society, "Kentrosaurus" males mated with more than one female, a behaviour also found in other vertebrates.
The problem posed by the ratio is that the multiple specimens studied, died in the same place, but probably not in a sudden mass-death and so do not represent a single herd or contemporary population. The results may have been distorted by a greater chance for robust animals of getting fossilised or discovered. In an earlier study by Galton in 1982, it was suggested that individual difference in the sacral rib count of both "Kentrosaurus" and "Dacentrurus" might be an indication of dimorphism: females would have had an extra pair of sacral ribs, having also the first sacral vertebra connected to the ilium, in addition to the subsequent four sacrals.
Reproduction.
As the plates and spikes would have been obstacles during copulation, it is possible that pairs mated back-to-back with the female staying still in a lordosis posture as the male manoeuvres this penis into her cloaca. The shoulder spikes would have made the female unable to lay on her side during mating as is proposed for "Stegosaurus".
Paleoecology.
"Kentrosaurus" lived in what is now Tanzania in the Late Jurassic Tendaguru Formation. The main "Kentrosaurus" quarries were located in the Middle Saurian Beds dating from the upper Kimmeridgian. Some remains were found in the Upper Saurian Beds dating from the Tithonian. Since 2012, the boundary between the Kimmeridgian and Tithonian is dated at 152.1 million year ago.
The Tendaguru ecosystem primarily consisted of three types of environment: shallow, lagoon-like marine environments, tidal flats and low coastal environments; and vegetated inland environments. The marine environment existed above the fair weather wave base and behind siliciclastic and ooid barriers. It appeared to have had little change in salinity levels and experienced tides and storms. The coastal environments consisted of brackish coastal lakes, ponds and pools. These environments had little vegetation and were probably visited by herbivorous dinosaurs mostly during droughts. The well vegetated inlands were dominated by conifers. Overall, the Late Jurassic Tendaguru climate was subtropical to tropical with seasonal rains and pronounced dry periods. During the Early Cretaceous, the Tendaguru became more humid. The Tendaguru Beds are similar to the Morrison Formation of North America except in its marine interbeds.
"Kentrosaurus" would have coexisted with fellow ornithischians like "Dysalotosaurus lettowvorbecki"; the sauropods "Giraffatitan brancai", "Dicraeosaurus hansemanni" and "D. sattleri", "Janenschia africana", "Tendaguria" "tanzaniensis" and "Tornieria africanus"; theropods "Allosaurus" "tendagurensis", "Ceratosaurus" "roechlingi", "Ceratosaurus" "ingens," "Elaphrosaurus bambergi", "Veterupristisaurus milneri" and "Ostafrikasaurus crassiserratus"; and the pterosaur "Tendaguripterus recki". Other organisms that inhabited the Tendaguru included corals, echinoderms, cephalopods, bivalves, gastropods, decapods, sharks, neopterygian fish, crocodilians and small mammals like "Brancatherulum tendagurensis".