剑龙
维基百科，自由的百科全书
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 位在波蘭的劍龍模型
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絕滅 (化石)
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劍龍是一種已滅絕的草食性四足動物。在分類上歸於裝甲亞目之下，並且是劍龍下目（Stegosauria）中的一個屬。屬名為Stegosaurus，源自希臘文中的στέγος-（stegos-；「屋頂」之意）與σαῦρος（-sauros；「蜥蜴」之意）[1]。
劍龍生活在侏儸紀晚期，大約是1億5千5百萬年前到1億4千5百萬年前左右，也就是啟莫里階（Kimmeridgean）至提通階（Tithonian）期間。這段期間正是一些巨型蜥腳類（sauropoda），如梁龍、圓頂龍與迷惑龍（雷龍）支配生態系的時代。在美國與加拿大西部的莫里遜組（Morrison Formation）地層中，大約已經挖掘出80具劍龍屬的遺骸，且其中至少可辨識出3個物種。過去人們認為劍龍只分佈於現今的北美洲地區，不過到了2006年，葡萄牙境內也發現了新的劍龍屬標本，顯示現今的歐洲也有劍龍化石的存在[2]。
劍龍的身體龐大且沉重，是所有劍龍下目之中最大，大概相當於一輛巴士。牠們的背部曲線呈弓狀彎曲，後肢比前肢更長。頭部靠近地面，而尾部則伸向空中。關於劍龍身上的尖刺與板狀物的用途，有許多不同的推論。尖刺很可能是用來防禦，而板狀物除了防禦之外，或許還能用來誇耀與調節體溫。劍龍以及其他屬於劍龍下目的恐龍，具有許多共同的特色，例如釘狀龍屬（Kentrosaurus）與華陽龍屬（Huayangosaurus），同樣擁有板狀結構與尖刺。
劍龍也是最知名的恐龍之一，就像霸王龍、三角龍以及迷惑龍一樣。
目录 |
[编辑] 描述
劍龍的頭尾長大約是9公尺，高度則大約4公尺。對人類來說，劍龍是相當龐大的動物。但是在牠們所生存的年代中，還有許多更為巨大的蜥腳類（sauropoda）恐龍。另外沿著弓起的背部脊線，有兩道形狀類似風箏的板狀物平行排列；在尾部靠近末端的區域，則有兩對尖刺向水平方向突起。這些裝甲，可以用來防禦一些屬於獸腳類（theropoda）的掠食者，例如異特龍（Allosaurus）與角鼻龍（Ceratosaurus）。
劍龍有4支腳，牠們的後腳有3個腳趾，而前腳則有5個。四肢皆由位於腳趾後方的腳掌支撐[3]。劍龍的後腳比前腳更長也更強壯，使姿態變得前低後高。牠們的尾部明顯高於地面許多，而頭部則相對地較為貼近地面，可能離地不超過1公尺[4]。
狹長的顱骨在整個劍龍身體中只佔一小部分。與大多數的恐龍不同，在劍龍的眼睛與鼻子之間，並沒有一個稱為眶前孔（antorbital fenestra）的洞口。這種特徵出現在大多初龍類（Archosauria；包含鳥類、恐龍、翼手龍與鱷魚等動物的分類群）動物中，其中現存的鱷魚已經失去了這個特徵[3]。位置較低的頭部，可能可以用來觀察低矮的植物，並且以這些植物為食。劍龍的門齒消失，取而代之的是喙狀結構，這也顯示了牠們的食性。 劍龍的牙齒較小，並且呈三角形，因為缺乏研磨面，所以這些牙齒用來進行研磨的作用不大。此外，牙齒在下頜的排列方式，顯示出劍龍擁有突出的臉頰（腮幫子）[5]。
劍龍的腦容量不比狗的腦容量更大，因此與整個身體相比之下便顯得相當地渺小。劍龍的命名者奧斯尼爾·查爾斯·馬許（Othniel Charles Marsh），曾經在1880年代獲得一具保存完善的顱骨，顯示出劍龍的腦容量非常小，可能是所有恐龍中最小的一個。事實上，當動物的體重大於4.5公噸時，其腦部重量將不會超過80公克。這個現象使過去的人們認為恐龍是相當愚笨的動物，不過這種想法現在已經被廣泛地否定。
大多數關於劍龍的資訊是來自成熟的個體，直到較晚近的年代，才發現了一些幼年時期的劍龍。美國的懷俄明州在1994年發現了一具半成熟個體的標本，頭尾長4.6公尺，高度為2公尺；活著時的體重則估計大約是2.3公噸。這具標本目前收藏在懷俄明大學地質學博物館（University of Wyoming Geological Museum）[6]。
[编辑] 分類
Stegosaurus was the first-named genus of the family Stegosauridae. It is the type genus, that gives its name to the family. Stegosauridae is one of two families within the infraorder Stegosauria, with the other being Huayangosauridae. Stegosauria lies within the Thyreophora, or armoured dinosaurs, a suborder which also includes the more diverse ankylosaurs. The stegosaurs were a clade of animals similar in appearance, posture and shape, that mainly differed in their array of spikes and plates. Among the closest relatives to Stegosaurus are Wuerhosaurus from China and Kentrosaurus from east Africa.
[编辑] 起源
The origin of Stegosaurus is uncertain, as few remains of basal stegosaurs and their ancestors are known. Recently, stegosaurids have been shown to be present in the lower Morrison Formation, existing several million years before the occurrence of Stegosaurus itself, with the discovery of the related Hesperosaurus from the early Kimmeridgian.[7] The earliest stegosaurid (the genus Lexovisaurus) is known from the Oxford Clay Formation of England and France, giving it an age of early to middle Callovian.
The earlier, and more basal genus Huayangosaurus from the Middle Jurassic of China (some 165 million years ago) predates Stegosaurus by 20 million years and is the only genus in the family Huayangosauridae. Earlier still is Scelidosaurus, from Early Jurassic England, which lived approximately 190 million years ago. Interestingly, it possessed features of both stegosaurs and ankylosaurs. Emausaurus from Germany was another small quadruped, while Scutellosaurus from Arizona in the USA was an even earlier genus and was facultatively bipedal. These small, lightly-armoured dinosaurs were closely related to the direct ancestor of both stegosaurs and ankylosaurs. A trackway of a possible early armoured dinosaur, from around 195 million years ago, has been found in France.[8]
[编辑] 物種與發現
Stegosaurus, one of the many dinosaurs first collected and described in the Bone Wars, was originally named by Othniel Charles Marsh in 1877,[9] from remains recovered north of Morrison, Colorado. These first bones became the holotype of Stegosaurus armatus. The basis for its scientific name, 'roof(ed) lizard' has been thought to have been Marsh's initial belief that the plates lay flat over the animal's back, overlapping like the shingles (tiles) on a roof. A wealth of Stegosaurus material was recovered over the next few years and Marsh published several papers on the genus. Initially, several species were described. However, many of these have since been considered to be invalid or synonymous with existing species,[10] leaving two well-known and one poorly-known species.
[编辑] 公認存在的物種
Stegosaurus armatus, meaning "armoured roof lizard", was the first species to be found and is known from two partial skeletons, two partial skulls and at least thirty fragmentary individuals.[9] This species had four horizontal tail spikes and relatively small plates. At 9 metres (30 ft), it was the longest species within the genus Stegosaurus.
Stegosaurus stenops, meaning "narrow-faced roof lizard", was named by Marsh in 1887,[3] with the holotype having been collected by Marshal Felch at Garden Park, north of Cañon City, Colorado, in 1886. This is the best-known species of Stegosaurus, mainly because its remains include at least one complete articulated skeleton. It had large, broad plates and four tail spikes. Stegosaurus stenops is known from at least 50 partial skeletons of both adults and juveniles, one complete skull and four partial skulls. It was shorter than S. armatus, at 7 metres (23 ft).
Stegosaurus longispinus, meaning "long-spined roof lizard", was named by Charles W. Gilmore[11] and known from one partial skeleton, from the Morrison Formation in Wyoming. Stegosaurus longispinus was notable for its set of four unusually long tail spines. Some consider it a species of Kentrosaurus. Like S. stenops, it grew to 7 metres (23 ft) in length.
[编辑] 仍受懷疑的物種
Stegosaurus ungulatus, meaning "hoofed roof lizard", was named by Marsh in 1879, from remains recovered at Como Bluff, Wyoming.[12] It is known from a few vertebrae and armour plates. It might be a juvenile form of S. armatus,[13] although the original material of S. armatus is yet to be fully described. The specimen discovered in Portugal and dating from the upper Kimmeridgian-lower Tithonian stage has been ascribed to this species.[2]
Stegosaurus sulcatus, meaning "furrowed roof lizard" was described by Marsh in 1887 based on a partial skeleton.[3] It is considered a synonym of S. armatus.[13] Stegosaurus duplex, meaning "two plexus roof lizard" (in allusion to the greatly enlarged neural canal of the sacrum which Marsh characterized as a "posterior brain case"), is probably the same as S. armatus.[13] Although named by Marsh in 1887 (including the holotype specimen), the disarticulated bones were actually collected in 1879 by Edward Ashley at Como Bluff, Wyoming.
Stegosaurus seeleyanus, originally named Hypsirophus, is probably the same as S. armatus. Stegosaurus (Diracodon) laticeps was described by Marsh in 1881, from some jawbone fragments.[14] Just as some consider S. stenops a species of Diracodon, others consider Diracodon itself to be a species of Stegosaurus. Bakker had resurrected D. laticeps in 1986,[15] although others note that the material is non-diagnostic and likely synonymous with S. stenops.[10]
Stegosaurus affinis, described by Marsh in 1881, is only known from a pubis and is considered nomen dubium.[13] It is possibly synonymous with S. armatus.[11]
[编辑] 同時受到懷疑與公認的物種
"Stegosaurus" madagascariensis from Madagascar is known solely from teeth and was described by Piveteau in 1926. The teeth were variously attributed to a stegosaur, the theropod Majungatholus,[16] a hadrosaur or even a crocodylian. Other remains, originally attributed to Stegosaurus, are now considered to belong to different genera. This is the case for "Stegosaurus" marshi, which was described by Lucas in 1901. It was renamed Hoplitosaurus in 1902. "Stegosaurus" priscus, described by Nopcsa in 1911, is a synonym of Lexovisaurus.[13]
[编辑] 古生物學
Stegosaurus was the largest stegosaur, reaching up to 12 metres (39 ft) in length and possibly weighing up to 5,000 kilograms (5.5 short tons). However, 7 to 9 metres was a more usual length. Soon after its discovery, Marsh considered Stegosaurus to have been bipedal, due to its short forelimbs.[17] He had changed his mind however, by 1891, after considering the heavy build of the animal.[3] Although Stegosaurus is undoubtedly now considered to have been quadrupedal, there has been some discussion over whether it could have reared up on its hind legs, using its tail to form a tripod with its hind limbs and browsing for higher foliage.[13] This has been proposed by Bakker[18][15] and opposed by Carpenter.[4]
Stegosaurus did have very short forelimbs, in relation to its hind legs. Furthermore, within the hindlimbs, the lower section (comprising the tibia and fibula) was short compared with the femur. This suggests that it couldn't walk very fast, as the stride of the back legs at speed would have overtaken the front legs, giving a maximum speed of 6-7 kilometres per hour (4-5 mi/hr).[5]
[编辑] 「第二顆腦」
Soon after describing Stegosaurus, Marsh noted a large canal in the hip region of the spinal cord, which could have accommodated a structure up to 20 times larger than the brain. This has led to the famous idea that dinosaurs like Stegosaurus had a 'second brain' in the tail, which may have been responsible for controlling reflexes in the rear portion of the body.It has also been suggested that this "brain" might have given the stegosaurus a temporary boost when it was under threat from predators.[5] More recently, it has been argued that this space (also found in sauropods) may have been the location of a glycogen body, a structure in living birds whose function is not definitely known but which is postulated to facilitate the supply of glycogen to the animal's nervous system.[19]
[编辑] 板狀結構
The most recognizable features of Stegosaurus are its dermal plates, which consisted of 17 separate flat plates. These were highly modified osteoderms (bony-cored scales), similar to those seen in crocodiles and many lizards today. They were not directly attached to the animal's skeleton, instead arising from the skin. In the past, some palaeontologists, notably Robert Bakker, have speculated the plates may have been mobile to some degree, although others disagree.[20] The largest plates were found over the animal's hips and measured 60 centimetres (2 ft) wide and 60 centimetres tall. The arrangement of the plates has long been a subject of debate but most palaeontologists now agree that they formed a pair of alternating rows, one running down each side of the midline of the animal's back.
The function of the plates has been much debated. Initially thought of as some form of armour,[17] they appear to have been too fragile and ill-placed for defensive purposes, leaving the animal's sides unprotected.[21] More recently, researchers have proposed that they may have helped to control the body temperature of the animal,[20] in a similar way to the sails of the large carnivorous Spinosaurus or of the pelycosaur Dimetrodon (and the ears of modern elephants and jackrabbits). The plates had blood vessels running through grooves and air flowing around the plates would have cooled the blood.[22] This theory has been seriously questioned,[23] since the closest relative to the common plate-wielding species, Stegosaurus stenops, had low surface area spikes instead of plates, implying that cooling was not important enough to require specialised structural formations such as plates.
Their large size suggests that the plates may have served to increase the apparent height of the animal, in order either to intimidate enemies[11] or to impress other members of the same species, in some form of sexual display,[21] although both male and female specimens seemed to have had them. A study published in 2005 supports the idea of their use in identification. Researchers believe this may be the function of other unique anatomical features, found in various dinosaur species.[24] Stegosaurus stenops also had disk-shaped plates on its hips.
One of the major subjects of books and articles about Stegosaurus is the plate arrangement.[25] The argument has been a major one in the history of dinosaur reconstruction. Four possible plate arrangements have been mooted over the years:
- The plates lay flat along the back, like armour. This was Marsh's initial interpretation, which led to the name 'Roof Lizard'. As further and complete plates were found, their form showed that they stood on edge, rather than lying flat.
- By 1891, Marsh published a more familiar view of Stegosaurus,[3] with a single row of plates. This was dropped fairly early on (apparently because it was poorly understood how the plates were embedded in the skin and it was thought that they would overlap too much in this arrangement). It was revived, in somewhat modified form, in the 1980s, by an artist (Stephen Czerkas),[26] based on the arrangement of iguana dorsal spines.
- The plates paired in a double row along the back. This is probably the most common arrangement in pictures, especially earlier ones (until the 'Dinosaur Renaissance' in the '70s). (The Stegosaurus in the 1933 film, King Kong has this arrangement.) However, no two plates of identical size and shape have ever been found within the same animal.
- Two rows of alternating plates. By the early 1960s, this had become (and remains) the prevalent idea, mainly because the one Stegosaurus fossil with the plates still articulated indicates this arrangement. An objection to it is that this phenomenon is unknown among other reptiles and it is difficult to understand how such a disparity could evolve.
[编辑] 尾部的尖刺
There has been debate about whether the tail spikes were used for display only, as posited by Gilmore in 1914[11] or used as a weapon. Robert Bakker noted the tail was likely to have been much more flexible than that of other dinosaurs, as it lacked ossified tendons, thus lending credence to the idea of the tail as a weapon. He also observed that Stegosaurus could have manoeuvred its rear easily, by keeping its large hindlimbs stationary and pushing off with its very powerfully muscled but short forelimbs, allowing it to swivel deftly to deal with attack.[15] More recently, a study of tail spikes by McWhinney et al.,[27] which showed a high incidence of trauma-related damage, confirms the spikes were indeed used in combat.
Stegosaurus stenops had four dermal spikes, each about 60-90 centimetres (2-3 ft) long. Discoveries of articulated stegosaur armour show that, at least in some species, these spikes protruded horizontally from the tail, not vertically as is often depicted. Initially, Marsh described S. armatus as having eight spikes in its tail, unlike S. stenops. However, recent research re-examined this and concluded this species also had four.[10]
[编辑] 覓食
Stegosaurus and related genera were herbivores. However, they adopted a feeding strategy different from that of the other herbivorous ornithischian dinosaurs. The other ornithischians possessed teeth capable of grinding plant material and a jaw structure capable of movements in planes other than simply orthal (i.e they could chew plants). This contrasts with Stegosaurus (and all stegosaurians), which had small teeth lacking the wear facets associated with tooth-tooth occulsion and a jaw capable of only orthal movements.[13]
The stegosaurians must have been successful, as they became speciose and geographically widely distributed, in the late Jurassic.[13] Palaeontologists believe it would have eaten plants such as mosses, ferns, horsetails, cycads and conifers or fruits[28] and swallowed gastroliths to aid food processing (due to the lack of chewing ability), in the same manner used by modern birds and crocodiles.[29] Low-level browsing on grasses, seen in modern mammalian herbivores, would not have been possible for Stegosaurus, as grasses did not evolve until late into the Cretaceous Period, long after Stegosaurus had become extinct.
One hypothesised feeding behaviour strategy considers them to be low-level browsers, eating low-growing fruit of various non-flowering plants, as well as foliage. This scenario has Stegosaurus foraging at most one metre above the ground.[30] On the other hand, if Stegosaurus could have raised itself on two legs, as suggested by Bakker, then it could have browsed on vegetation and fruits quite high up, with adults being able to forage up to 6 metres (20 ft) above the ground.[5]
[编辑] 大眾文化
As one of the most recognizable dinosaurs, Stegosaurus has been widely depicted on film, in cartoons, comics, as children's toys, and has even been declared the State Dinosaur of Colorado in 1982.[31]
