Re-assessment of the Late Jurassic eusauropod <i>Mamenchisaurus sinocanadorum</i> Russell and Zheng, 1993, and the evolution of exceptionally long necks in mamenchisaurids
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The sauropod genus <i>Mamenchisaurus</i>, from the Late Jurassic–Early Cretaceous of East Asia, has a convoluted taxonomic history. Although included in the first cladistic analysis of sauropods, only recently has the monophyly of <i>Mamenchisaurus</i>, and the anatomical diversity of the many penecontemporaneous East Asian eusauropods, been evaluated critically. Here, we re-describe the holotype and only specimen of <i>M. sinocanadorum</i>. Although the original diagnosis is no longer adequate, we identify several autapomorphies that support the validity of this species, including an elongate external mandibular fenestra and distinctive pneumatic structures on the cervical centra. We incorporate new data into a phylogenetic character matrix that also includes <i>Bellusaurus</i> and <i>Daanosaurus</i>, both of which are known only from juvenile material and are often hypothesized to be neosauropods (or close relatives thereof). We recover all species of <i>Mamenchisaurus</i> as part of a radiation of predominantly Middle–Late Jurassic East Asian eusauropods, but the genus is non-monophyletic, underscoring the need for further systematic revision of mamenchisaurid taxonomy. Analyses that score ontogenetically variable characters ambiguously recover <i>Bellusaurus</i> and <i>Daanosaurus</i> as juvenile mamenchisaurids, a hypothesis supported by several features that are unique to mamenchisaurids or exhibit little homoplasy, including anteriorly bifurcate cervical ribs. Finally, computed-tomography reveals extensive vertebral pneumaticity in <i>M. sinocanadorum</i> that is comparable to that of the largest sauropods, and updated scaling analyses imply a neck over 14 m long, rivalling estimates for other exceptionally long-necked sauropods. Previous work has suggested that the elongated cervical ribs of particularly long-necked sauropods such as <i>M. sinocanadorum</i> stabilized the neck by limiting its mobility. Given that extent of pneumaticity responds dynamically to a bone’s habitual loading, we propose that long cervical ribs – and other structural modifications that limited flexibility – promoted the evolution of increasingly long necks by producing a more predictable biomechanical milieu amenable to increased pneumatization.
产自东亚晚侏罗世—早白垩世的马门溪龙属（Mamenchisaurus）有着错综复杂的分类学历史。尽管该属最初被纳入蜥脚类分支系统学分析的范畴，但直至近期，学界才对马门溪龙的单系性，以及诸多同期东亚真蜥脚类的解剖学多样性展开了严谨评估。本文重新描述了中华马门溪龙（M. sinocanadorum）的正模且唯一一件标本。尽管原始鉴别特征已不再适用，但我们识别出多项独有衍征以支撑该物种的有效性，包括延长的外下颌孔以及颈椎体上独特的气腔构造。我们将新数据整合至一套系统发育性状矩阵中，该矩阵同时涵盖仅知幼年化石材料、且常被推测为新蜥脚类（或其近缘类群）的贝拉龙（Bellusaurus）与大安龙（Daanosaurus）。系统发育分析结果显示，所有马门溪龙物种均隶属于以中晚侏罗世东亚真蜥脚类为主的辐射演化支系，但该属并非单系群，这表明马门溪龙科的分类学亟需进一步的系统性修订。对个体发育可变性状进行模糊计分的分析，将贝拉龙与大安龙恢复为幼年马门溪龙类群，该假说得到多项马门溪龙科独有或同塑性极低的特征支持，包括颈椎肋前部分叉。计算机断层扫描（computed-tomography）结果显示，中华马门溪龙具有可与大型蜥脚类媲美的广泛椎骨气腔化特征；更新后的体型缩放分析表明，其颈部长度超过14米，可与其他超长颈蜥脚类的估算颈部长度相媲美。此前研究提出，诸如中华马门溪龙这类超长颈蜥脚类的延长颈椎肋，通过限制颈部活动度起到了稳定颈部的作用。鉴于气腔化程度会随骨骼的习惯性负荷发生动态响应，我们推测，延长的颈椎肋以及其他限制颈部灵活性的结构改造，通过营造更可预测的生物力学环境以适配气腔化程度提升，进而推动了颈部不断向更长的方向演化。
