| Kentrosaurus | |
|---|---|
 | |
| Mounted skeleton,Museum für Naturkunde, Berlin | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Thyreophora |
| Clade: | †Stegosauria |
| Family: | †Stegosauridae |
| Genus: | †Kentrosaurus Hennig,1915 |
| Type species | |
| †Kentrosaurus aethiopicus Hennig, 1915 | |
| Synonyms | |
Kentrosaurus (/ˌkɛntroʊˈsɔːrəs/KEN-troh-SOR-əs;lit. 'prickle lizard') is agenus ofstegosauriddinosaur from theLate Jurassic inLindi Region ofTanzania. Thetype species isK. aethiopicus, named and described byGermanpalaeontologistEdwin Hennig in 1915. Often thought to be a "primitive" member of theStegosauria, several recentcladistic analyses find it as more derived than many other stegosaurs, and a close relative ofStegosaurus from theNorth AmericanMorrison Formation within theStegosauridae.
Fossils ofK. aethiopicus have been found only in theTendaguru Formation, dated to the lateKimmeridgian and earlyTithonian ages, about 152million years ago. Hundreds of bones were unearthed by German expeditions toGerman 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 carnivoroustheropodsElaphrosaurus andVeterupristisaurus, giant herbivoroussauropodsGiraffatitan andTornieria, and thedryosauridDysalotosaurus.
Kentrosaurus generally measured around 4–4.5 metres (13–15 ft) in length as an adult, and weighed about 700–1,600 kilograms (1,500–3,500 lb). 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. Thethigh bones come in twodifferent types, suggesting that one sex was larger and more stout than the other.
The first fossils ofKentrosaurus were discovered by theGerman Tendaguru Expedition in 1909, recognised as belonging to astegosaur by expedition leaderWerner Janensch on 24 July 1910, and described byGermanpalaeontologistEdwin Hennig in 1915.[1] The nameKentrosaurus was coined by Hennig and comes from theGreekkentron/κέντρον, meaning "sharp point" or "prickle", andsauros/σαῦρος meaning "lizard",[2] Hennig added thespecific nameaethiopicus to denote the provenance from Africa.[1] Soon after its description, a controversy arose over the stegosaur's name, which is very similar to theceratopsianCentrosaurus. Under the rules of biological nomenclature, forbiddinghomonymy, two animals may not be given the same name. Hennig renamed his stegosaurKentrurosaurus, "pointed-tail lizard", in 1916,[3] while Hungarian paleontologistFranz Nopcsa renamed the genusDoryphorosaurus, "lance-bearing lizard", the same year.[4][5] If a renaming had been necessary, Hennig's would have had priority.[6] However, because the spelling is different, bothDoryphorosaurus andKentrurosaurus are unneeded replacement names;Kentrosaurus remains the valid name for the genus withKentrurosaurus andDoryphorosaurus being itsjunior objective synonyms.[7]
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 theMuseum für Naturkunde by Janensch.[8] The mount was dismantled during the museum renovation in 2006/2007, and re-mounted in an improved pose by Research Casting International.[9] Some other material, including a braincase and spine, was thought to have been misplaced or destroyed duringWorld War II.[10] However, all the supposedly lost cranial material was later found in a drawer of a basement cupboard.[11]
From 1909 onwards,Kentrosaurus remains were uncovered in four quarries in theMittlere Saurierschichten (Middle Saurian Beds) and one quarry in theObere Saurierschichten (Upper Saurian Beds).[12] During four field seasons, the German Expedition found over 1200 bones ofKentrosaurus, belonging to about fifty individuals,[13] many of which were destroyed during the Second World War.[14] 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 theUniversity of Tübingen houses a composite mount, roughly 50% of it being original bones.[12]
In the original description, Hennig did not designate aholotype specimen. However, in a detailed monography on the osteology, systematic position and palaeobiology ofKentrosaurus in 1925, Hennig picked the most complete partial skeleton, today inventorised asMB.R.4800.1 through MB.R.4800.37, as a lectotype (seesyntype).[15][16] This material includes a nearly complete series of tail vertebrae, several vertebrae of the back, asacrum with five sacral vertebrae and bothilia, bothfemora and anulna, and is included in the mounted skeleton at the Museum für Naturkunde inBerlin,Germany. The type locality is Kindope,Tanzania, north of Tendaguru hill.[16]
Unaware that Hennig had already defined a lectotype,Peter Galton[17] 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, whenHermann Heck concluded the last German excavations, areparalectotypes, and that MB.R.4800 is the correct lectotype.[18]
Kentrosaurus was a small stegosaur. It had the typicaldinosaurian bodybauplan, characterised by a small head, a long neck, short forelimbs and long hindlimbs, and a long, horizontal and muscular tail. Typicalstegosaurid 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.[15]
Kentrosaurus aethiopicus was a relatively small stegosaur, reaching 4–4.5 m (13–15 ft) in length and 700–1,600 kg (1,500–3,500 lb) in body mass.[8][A][20][21] Some specimens suggest that relatively larger individuals could have existed.[1][13] These specimens are comparable to someStegosaurus specimens in terms of the olecranon process in development.[22]
The long tail ofKentrosaurus results in a position of thecenter of mass that is unusually far back for aquadrupedal animal. It rests just in front of the hip, a position usually seen inbipedal dinosaurs. However, thefemora are straight inKentrosaurus, as opposed to typical bipeds, indicating a straight and vertical limb position. Thus, the hindlimbs, though powered by massive thigh muscles attached to a longilium, did not support the animal alone, and the very robust forelimbs took up 10 to 15% of the bodyweight.[9] The center of mass was not heavily modified by theosteoderms (bony structures in skin) inKentrosaurus orStegosaurus, which allowed the animals to stay mobile despite their armament. The hindlimbs’ thigh muscles were very powerful, allowingKentrosaurus to reach a tripod stance on its hindlegs and tail.[23]
Eight specimens from the skull, mandible, and teeth have been collected and described from the Tendaguru Formation, most of them being isolated elements.[24] Twoquadrates (bones from the jaw joint) were referred toKentrosaurus, but they instead belong to a juvenilebrachiosaurid.[25]
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The long and narrowskull was small in proportion to the body. It had a smallantorbital fenestra, the hole between the nose and eye common to mostarchosaurs, including modern birds, though lost in extant crocodylians. The skull's low position suggests thatKentrosaurus 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 orrhamphotheca. The presence of a beak extended along much of the jaws may have precluded the presence of cheeks in stegosaurs.[26] Due to its phylogenetic position, it is unlikely thatKentrosaurus had an extensive beak likeStegosaurus and it instead probably had a beak restricted to the jaw tips.[27][28] Other researchers have interpreted these ridges as modified versions of similar structures in other ornithischians which might have supported fleshy cheeks, rather than beaks.[7]
There are two nearly completebraincases known fromKentrosaurus though they exhibit some taphonomic distortion.[24] Thefrontals andparietals 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 theorbit (eye socket) and the medial side creates the anterior part of theendocranial cavity (braincase).Basioccipitals (where the skull articulated with the cervical vertebrae) form the posterior floor of the brain and theoccipital condyle, which is large and spherical inKentrosaurus. The rest of the braincase is formed by thepresphenoid composing the anterior end. The overall braincase morphology is very similar to those ofTuojiangosaurus,Huayangosaurus, andStegosaurus. However, the occipital condyle is a closer distance to thebasisphenoid tubera (bone at the front of the braincase) inKentrosaurus andHuayangosaurus than inTuojiangosaurus and some specimens ofStegosaurus. Due to dinosaurs having more molding in their braincases,endocasts ofKentrosaurus can be reconstructed using the preserved fossils. The brain is relatively short, deep, and small, with a strongcerebral andpontine 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 (lacking bone) region between the top of thesupraoccipital (bone at the top-back of the braincase) and overlying parietal that was likely covered incartilage. 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.[24]
In themandible (lower jaw), only an incomplete right dentary is known fromKentrosaurus.[29] The deep dentary is almost identical in shape to that ofStegosaurus, 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 preservedalveoli 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 smallforamina (small openings in bone) that is similar to grooves on the dentary of theCretaceous neornithischianHypsilophodon, with one foramina per tooth position. Stegosaurian teeth were small, triangular, and flat; wear facets show that they did grind their food.[30] A single complete cheek tooth is preserved, with a large crown and long root. The crown notably has fewer marginaldenticles and a prominentcingulum compared toStegosaurus,Tuojiangosaurus, andHuayangosaurus.[24]
The neck was composed of 13cervical (neck) vertebrae, the first being theatlas which was strongly fused to the occipital region of the skull and followed by theaxis. The other 11 cervicals had hourglass-shapedcentra (the base of a vertebra) and rounded ventral keels. Thediapophyses are large and strongly angled posteriorily and parallel to each other. The spinous processes got larger towards the posterior end, while thepostzygapophyses became smaller and less horizontal, giving the anterior part of the neck lots of mobility laterally. The dorsal column consists of 13dorsal (back) vertebrae which are tall and have short centra. They have aneural arch more than twice as high as the centrum, the vertebral body, and almost completely occupied by the extremely spaciousneural canal, a trait unique toKentrosaurus. The diapophyses too were laterally elongated, creating a Y-shape in anterior view. Thesacrum (part of pelvis with vertebrae) consists of 6 fused centra, the first being a loose sacrodorsal, while the rest of the centra'stransverse processes (extensions of bone) are fused to the dorsal parts of thesacral ribs into a solid sacral plate. The ribs also fuse to theilium (the upper part of the pelvis) creating a fully ankylosed and solid sacrum. The ilium is notable in that the preacetabular process, front blade, of theilium widens laterally, to the front outer side, and does not taper unlike in all other stegosaurs. Another characteristic is that the length of the ilium equals, or is greater than, that of the thigh bone.[12] Thecaudal (tail) vertebrae are 29 in number, though 27-29 are coossified for attachment to the thagomizers (tail spikes). The caudal vertebrae are unique, as they have a combination of transverse processes up to the 28th vertebra and rod-shaped processes on the posterior caudals. These posterior caudal processes have narrow bases that do not tough the plate formed by the fusion of the processes of the sacral vertebrae.Kentrosaurus can be distinguished from other members of theStegosauria by a number of processes 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. Thechevrons, bones pointing to below from the bottom side of the tail vertebrae, have the shape of an inverted T.[12]
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Thescapula (shoulder blade) is sub-rectangular, with a robust blade. Though it is not always perfectly preserved, theacromion ridge is slightly smaller than inStegosaurus. The blade is relatively straight, although it curves towards the back. There is a small bump on the back of the blade, that would have served as the base of thetriceps muscle. Thecoracoid is sub-circular.[31] The fore limbs were much shorter than the stocky hind limbs, which resulted in an unusual posture. Thehumerus (upper arm bone), like other stegosaurs, has greatly expanded proximal and distal ends that were attachment points between the coracoid andulna-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. Thefemur (thigh bone) is the longest element in the body, with the largest known femur measuring 665 mm from the proximal to distal end. Thetibia (shin bone) was wide and robust, while the fibula was skinny and thin without a greatly expanded distal end. Thepes (foot) terminated in 3 toes, all of which had hoof-likeunguals (claws).[15][32][24]
Typically for a stegosaur,Kentrosaurus had extensiveosteoderm (bony structures in the skin) covering, including small plates (probably located on the neck and anterior trunk), and spikes of various shapes. The spikes ofKentrosaurus are very elongated, with one specimen having a bone core length of 731 millimetres.[20] The plates have a thickened section in the middle, as if they were modified spines.[33] The spikes and plates were likely covered byhorn. Aside from a few exceptions they were not found in close association with other skeletal remains. Thus, the exact position of mostosteoderms 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.[13]
Hennig[13] and Janensch,[8] 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.[33] 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 AmericanStegosaurus, 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 bonemorphology 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 stegosaursGigantspinosaurus andHuayangosaurus.[33]
Like the spikes and shields ofankylosaurs, thebony plates and spines of stegosaurians evolved from the low-keeled osteoderms characteristic of basal thyreophorans.[34]Galton (2019) interpreted plates of an armored dinosaur from theLower Jurassic (Sinemurian-Pliensbachian) LowerKota Formation ofIndia as fossils of a member ofAnkylosauria; the author argued that this finding indicates a probable early Early Jurassic origin for both Ankylosauria and itssister group Stegosauria.[35]
The vast majority of stegosaurian dinosaurs thus far recovered belong to theStegosauridae, which lived in the later part of the Jurassic and early Cretaceous, and which were defined byPaul Sereno as all stegosaurians more closely related toStegosaurus than toHuayangosaurus.[36] This group is widespread, with members across the Northern Hemisphere,Africa and possiblySouth America.[37] The South American remains come fromChubut,Argentina and consist only of a partial humerus, but the anatomy of the humerus is very similar to that ofKentrosaurus and both date to the Late Jurassic. In a phylogenetic analysis, the Chubut stegosaurid was recovered in polytomy withKentrosaurus as basal stegosaurids, further suggesting that they are closely related.[37]
In Hennig's 1915 description,Kentrosaurus was assigned to the familyStegosauridae due to the preservation of dermal armor and features like posterodorsally angled neural spines on the caudal vertebrae.[1] This is confirmed by moderncladistic analyses, although in 1915 Stegosauridae was a far more inclusive concept that included some taxa now classified as ankylosaurs. A consecutive narrowing down of this concept causedKentrosaurus, until the 1980s to be seen as a typical "primitive" stegosaurian,[38] to be placed in a more derived, higher, position in the stegosaur evolutionary tree. However, recent analyses have consistently foundKentrosaurus to be in Stegosauridae, though typically as one of the most basal genera in the family.[39][37][40]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.[17][41]
Thetype and sole accepted species ofKentrosaurus isKentrosaurus aethiopicus, named by Hennig in 1915. Fragmentary fossil material from Wyoming, namedStegosaurus longispinus byCharles Gilmore in 1914,[42] was in 1993 classified as a North American species ofKentrosaurus, asK. longispinus.[43] However, this action was not accepted by the paleontological community, andS. longispinus has been assigned to its own genus,Alcovasaurus, differing fromKentrosaurus in having more elongated tail spikes and the structure of the pelvis and vertebrae.[44][45] Cladogram of the phylogenetic analysis of Stegosauridae conducted by Maidmentet al (2019), which recovers a distinctAlcovasaurus:[46]
Like allornithischians,Kentrosaurus was aherbivore. 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.[47]Kentrosaurus was capable of eating at heights of up to 1.7 m (5 ft 7 in) when on all fours. It may also have been possible for it to rear up on its hindlegs to reach vegetation higher in trees.[9]
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 byRobert Thomas Bakker. In this pose,Kentrosaurus could have fed at heights of 3.3 m (11 ft).[9]
Differences in the proportions, not the size, of the femurs (thighbones) ledHolly Barden andSusannah Maidment to realize thatKentrosaurus probably showedsexual 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.[48]
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 bothKentrosaurus andDacentrurus 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.[48]
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 alordosis posture as the male maneuvers his penis into hercloaca. The shoulder spikes would have made the female unable to lie on her side during mating as is proposed forStegosaurus.[49]
In 2013, a study byRagna Redelstorff e.a. concluded that the bonehistology ofKentrosaurus indicated that it had a higher growth rate than reported forStegosaurus andScutellosaurus, in view of the relatively rapid deposition of highly vascularised fibrolamellar bone. AsStegosaurus was larger thanKentrosaurus, this contradicts the general rule that larger dinosaurs grew quicker than smaller ones.[50]
Because the tail had at least forty caudal vertebrae,[13] it was highly mobile.[9] It could possibly swing at an arc of 180 degrees, covering the entire half circle behind it.[20][9] 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.[20] Earlier interpretations of the defensive behaviour ofKentrosaurus included the suggestion that the animal might have charged to the rear, to run through attackers with its spines, in the way of modernporcupines.[38]
ThoughKentrosaurus 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.[9]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 ofKentrosaurus may have required group hunting. Compared to the more robust spikes ofStegosaurus, the thinner spikes ofKentrosaurus were at greater risk of bending.[20]
Kentrosaurus lived in what is now Tanzania in theLate JurassicTendaguru Formation. The mainKentrosaurus 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.[51] Since 2012, the boundary between the Kimmeridgian and Tithonian is dated at 152.1 million year ago.[52]
The Tendaguruecosystem 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 thefair weather wave base and behindsiliciclastic andooid barriers. It appeared to have had little change in salinity levels and experienced tides and storms. The coastal environments consisted ofbrackish 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 byconifers. Overall, the Late Jurassic Tendaguru climate was subtropical to tropical with seasonal rains and pronounced dry periods. During theEarly Cretaceous, the Tendaguru became more humid.[53] The Tendaguru Beds are similar to theMorrison Formation of North America except in its marine interbeds.[54]
Kentrosaurus would have coexisted with fellow ornithischians likeDysalotosaurus lettowvorbecki; thesauropodsGiraffatitan brancai,Dicraeosaurus hansemanni andD. sattleri,Janenschia africana,Tendaguriatanzaniensis andTornieria africanus;theropods "Allosaurus"tendagurensis, "Ceratosaurus"roechlingi, "Torvosaurus"ingens,Elaphrosaurus bambergi,Veterupristisaurus milneri andOstafrikasaurus crassiserratus; and thepterosaurTendaguripterus recki.[55][56][57][58] Other organisms that inhabited the Tendaguru includedcorals,echinoderms,cephalopods,bivalves,gastropods,decapods, sharks,neopterygian fish, crocodilians and small mammals likeBrancatherulum tendagurensis.[59]
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