在兽脚类中,异特龙的颅骨、牙齿与身体的比例适中。古生物学家格雷戈里·S·保罗(英语:Gregory S. Paul)根颅一个长为845 mm(33.3英寸)的颅骨估计该个体的体长为7.9米(26英尺)。[19]每个前颌骨(构成鼻尖的骨骼)有5颗牙齿,横截面呈D形,每个上颌骨(主要用于承载牙齿的上颌骨骼)有14到17颗牙齿;牙齿数量并不完全符合骨骼尺寸。每块齿骨(承载牙齿的下颌骨骼)有14到17颗牙齿,平均数为16颗。牙齿不断变短变窄,向颅后弯曲,皆有锯齿状边缘,极易脱落且不断替换,成为常见的化石。[8]颅骨轻而坚固,长有几十颗锋利的锯齿状牙齿,平均长度为1米(3.3英尺),但也可能达到1.5米(4.9英尺)。
异特龙命名自标本YPM 1930――部分零碎骨骼,包括部分三节椎骨、一个肋骨碎片、一颗牙齿、一块趾骨以及最有助于后期争论的右肱骨干(上臂)。奥塞内尔·查利斯·马什(Othniel Charles Marsh)于1877年将这些遗骸正式命名为脆弱异特龙(Allosaurus fragilis)。属名取自希腊语αλλος/allos(“奇怪的”或“不同的”)和σαυρος/sauros(“蜥蜴”)。[35]它之所以被命名为“不同的蜥蜴”是因为脊椎形状不同于当时已知的任何恐龙。[36][37]种名在拉丁语中意为“脆弱的”,指脊椎轻盈的特点。这些骨骼采集于卡农城北部花园公园(英语:Garden Park, Colorado)的莫里逊组。[36]马什和爱德华·德林克·科普(Edward Drinker Cope)在科学上相互竞争,并继续根据类似的零碎材料命名其它几个属,这些材料会出现在异特龙分类的后期争论中,包括马什命名的肌肉龙[38]和贪食龙[39]以及科普命名的依潘龙。[40]
在竞争中,科普和马什并不总是对其发现(更常见的是其下属的发现)进行深入研究。例如,本杰明·富兰克林·穆奇(英语:Benjamin Franklin Mudge)在科罗拉多州发现异特龙正模标本后,马什选择在怀俄明州集中工作;1883年花园公园恢复工作时,M·P·费尔奇(M. P. Felch)发现一具近乎完整的异特龙标本及部分骨骼。[21]1879年,H·F·哈贝尔(H. F. Hubbell)在怀俄明州的科摩崖(英语:Como Bluff)发现一件标本,但显然没有提到其完整性,科普也从未将其从岩石中取出。1903年(科普去世几年以后)取出后,人们发现它是当时已知最完整的兽脚类标本之一。1908年,该标本被编目为AMNH 5753。[41]它是一座著名的骨架,稳固站立于部分迷惑龙骨骼上,仿佛在清理尸体,查尔斯·耐特的插图正反映了该场景。尽管其以兽脚类的第一座独立骨架而闻名,且有大量插图和照片发表,但从未被正式叙述。[42]
1991年发现一具绰号“大艾尔”("Big Al",即MOR(英语:Museum of the Rockies) 693)的具有关节的部分异特龙骨骼。该骨骼长约8米(约26英尺),完整度可达95%,由落基山博物馆(英语:Museum of the Rockies)和怀俄明大学地质博物馆组织、科比·西贝尔(Kirby Siber)领导的一个瑞士团队在怀俄明州舍尔(英语:Shell, Wyoming)附近发现。[60]丘尔和洛文于2020年将其确认为新种詹氏异特龙(Allosaurus jimmadseni)。同一团队于1996年发现第二件标本――“大艾尔2号”("Big Al II")。该标本是迄今为止保存最完好的异特龙骨骼,完整度可达99%,也被归类于詹氏异特龙。[1]
脆弱异特龙、詹氏异特龙、大异特龙和卢氏异特龙的化石皆出自美国上侏罗统启莫里阶至提通阶的莫里逊组,分布于科罗拉多州、蒙大拿州、新墨西哥州、俄克拉荷马州、南达科他州、犹他州和怀俄明州。脆弱异特龙被视为最常见的物种,所知于至少60具残骸。[24]在20世纪80年代末和90年代初的一段时间里,人们普遍认为脆弱异特龙鼻部较短、残暴异特龙鼻部较长,[19][71]然而,后期对克利夫兰劳埃德恐龙采石场、科摩崖和干岩台采石场标本进行的分析表明,莫里逊组材料的差异可归因于个体变异。[14][16]对克利夫兰劳埃德遗址的颅骨的研究发现异龙个体间存在很大差异,对以前基于泪骨角形状等特征的物种区分及拟议的基于颧骨形状的詹氏异特龙划分提出质疑。[17]欧洲异特龙发现于洛里尼扬组(英语:Lourinhã Formation)启莫里阶的新港段(Porto Novo Member),[65]可能是脆弱异特龙的异名。[67]
至少已在一件异特龙标本(克利夫兰劳埃德采石场发现的股骨)中发现了髓质骨组织(Medullary bone tissue,即妊娠雌鸟长骨骨髓中短期存在的骨内膜矿化物质)。[120]如今,这种骨组织仅在妊娠雌鸟体内形成,可为卵壳提供钙元素。异特龙体内髓质组织的存在被用于确定性别以及该个体是否达到生殖年龄,然而,其它研究对部分恐龙髓质组织鉴定结果提出质疑,包括上文提到的异特龙标本。来自现存鸟类的数据表明,该标本中的“髓质骨组织化石”很可能是骨骼病变的结果。[121]然而,一具暴龙标本中髓质组织及其性别的确认可能确定这只异特龙是否为雌性。[122]
埃里克·斯尼弗利(Eric Snifly)及其同僚于2013年发表的一项生物力学研究显示,与其它兽脚类(如暴龙)相比,异特龙颅骨的头浅最长肌(Longissimus capitis superficialis)颈部肌肉附着点异常低矮,使动物能够快速有力地垂直移动颅骨。作者发现,巴克和雷菲尔德提出的“垂直打击”策略与这种能力一致。他们还发现,异特龙可能运用与隼(如红隼)相似的“垂直运动”方式进食尸体:用颅骨和脚爪固定猎物,然后向后向上拉动以咬下肌肉。这与暴龙的进食方式不同,暴龙可能会通过向两侧晃动颅骨来撕裂肌肉,类似鳄鱼。[131]此外,异龙能够相对快速地移动其头部和颈部,并具有相当大的控制力,但代价是消耗力量。[132]
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