The Late Jurassic sauropod dinosaur ‘Morosaurus’ agilis Marsh, 1889 reexamined and reinterpreted as a dicraeosaurid

The holotypic materials of the sauropod ‘Morosaurus’ agilis—a partial skull, proatlases, and first three cervical vertebrae—have been a taxonomic and phylogenetic mystery since their initial description by O. C. Marsh in 1889 and redescription by C. W. Gilmore in 1907. Although most species of Morosaurus were subsumed into Camarasaurus in 1919, ‘M.’ agilis was left in the defunct genus without a proper taxonomic assignment. Similarities have been noted between ‘M.’ agilis and other Morrison Formation sauropod taxa, including Camarasaurus, Haplocanthosaurus, Diplodocus, and Brachiosauridae, but it had yet to be included in a phylogenetic analysis. Here we present new data following additional preparation and study that suggest ‘M.’ agilis is a basally diverging member of Dicraeosauridae, along with the recently described sauropod Kaatedocus. Based upon its recovery as a distinct taxon, we propose that ‘M.’ agilis receive the new generic name Smitanosaurus, yielding the new combination Smitanosaurus agilis. http://zoobank.org/urn:lsid:zoobank.org:pub:6977D649-EE6B-42EE-9E1C-C043A38AC2AC蜥脚类（sauropod）恐龙敏捷莫里龙（‘Morosaurus’ agilis）的正模标本材料——包含部分头骨、寰椎前弓（proatlases）及前3节颈椎——自1889年由O. C. 马什（O. C. Marsh）首次描述、1907年由C. W. 吉尔莫（C. W. Gilmore）重新描述以来，始终是分类学与系统发育领域的未解之谜。尽管绝大多数莫里龙属（Morosaurus）物种已于1919年被归入圆顶龙属（Camarasaurus），但‘M.’ agilis仍被留在该废弃属中，未获得恰当的分类归属。此前已有研究指出‘M.’ agilis与其他莫里逊组（Morrison Formation）蜥脚类类群存在形态相似性，涵盖圆顶龙属（Camarasaurus）、轻龙属（Haplocanthosaurus）、梁龙属（Diplodocus）以及腕龙科（Brachiosauridae），但该物种始终未被纳入系统发育分析研究。本文通过额外的标本制备与研究获取了新数据，结果表明‘M.’ agilis是叉龙科（Dicraeosauridae）的基部分化支系成员，与近年新描述的蜥脚类卡塔杜库斯龙（Kaatedocus）同属该演化类群。基于该物种作为独立分类单元的系统发育恢复结果，我们提议为‘M.’ agilis建立新属斯米塔龙属（Smitanosaurus），新组合为斯米塔龙敏捷种（Smitanosaurus agilis）。相关注册信息可查阅：http://zoobank.org/urn:lsid:zoobank.org:pub:6977D649-EE6B-42EE-9E1C-C043A38AC2AC