Boyd Clint A, Pagnac Darrin C
North Dakota Geological Survey , Bismarck, North Dakota , USA.
Department of Geology and Geological Engineering, South Dakota School of Mines and Technology , Rapid City, South Dakota , USA.
PeerJ. 2015 Sep 22;3:e1263. doi: 10.7717/peerj.1263. eCollection 2015.
Knowledge regarding the early evolution within the dinosaurian clade Ankylopollexia drastically increased over the past two decades, in part because of an increase in described taxa from the Early Cretaceous of North America. These advances motivated the recent completion of extensive preparation and conservation work on the holotype and only known specimen of Dakotadon lakotaensis, a basal ankylopollexian from the Lakota Formation of South Dakota. That specimen (SDSM 8656) preserves a partial skull, lower jaws, a single dorsal vertebra, and two caudal vertebrae. That new preparation work exposed several bones not included in the original description and revealed that other bones were previously misidentified. The presence of extensive deformation in areas of the skull is also noted that influenced inaccuracies in prior descriptions and reconstructions of this taxon. In addition to providing an extensive re-description of D. lakotaensis, this study reviews previously proposed diagnoses for this taxon, identifies two autapomorphies, and provides an extensive differential diagnosis. Dakotadon lakotaensis is distinct from the only other ankylopollexian taxon known from the Lakota Formation, Osmakasaurus depressus, in the presence of two prominent, anteroposteriorly oriented ridges on the ventral surfaces of the caudal vertebrae, the only overlapping material preserved between these taxa. The systematic relationships of D. lakotaensis are evaluated using both the parsimony and posterior probability optimality criteria, with both sets of analyses recovering D. lakotaensis as a non-hadrosauriform ankylopollexian that is more closely related to taxa from the Early Cretaceous (e.g., Iguanacolossus, Hippodraco, and Theiophytalia) than to more basally situated taxa from the Jurassic (e.g., Camptosaurus, Uteodon). This taxonomic work is supplemented by field work that relocated the type locality, confirming its provenance from unit L2 (lower Fuson Member equivalent) of the Lakota Formation. Those data, combined with recently revised ages for the members of the Lakota Formation based on charophyte and ostracod biostratigraphy, constrain the age of this taxon to the late Valanginian to early Barremian.
在过去二十年里，关于恐龙类群装甲龙下目早期演化的知识大幅增加，部分原因是北美早白垩世已描述分类单元的数量有所增加。这些进展推动了近期对达科他齿龙（Dakotadon lakotaensis）的正模标本及唯一已知标本进行广泛的清理和保护工作，达科他齿龙是一种来自南达科他州拉科塔组的基础装甲龙下目恐龙。该标本（SDSM 8656）保存了部分头骨、下颌、一枚背椎和两枚尾椎。新的清理工作使一些在最初描述中未包含的骨骼得以显露，并发现其他一些骨骼之前被误认。还注意到头骨部分区域存在广泛变形，这影响了对该分类单元先前描述和重建的准确性。除了对达科他齿龙进行广泛的重新描述外，本研究回顾了先前对该分类单元提出的诊断，确定了两个自近裔性状，并提供了广泛的鉴别诊断。达科他齿龙与拉科塔组已知的另一个装甲龙下目分类单元——凹陷奥氏龙（Osmakasaurus depressus）不同，在尾椎腹面有两条明显的前后向脊，这是这两个分类单元之间唯一保存的重叠材料。利用简约法和后验概率最优标准对达科他齿龙的系统关系进行了评估，两组分析均将达科他齿龙恢复为非鸭嘴龙形的装甲龙下目恐龙，它与早白垩世的分类单元（如伊瓜纳巨蜥、马驹龙和神植龙）的关系比与侏罗纪更基础的分类单元（如弯龙、犹他齿龙）更为密切。这项分类学工作得到了野外工作的补充，野外工作重新确定了模式产地，确认其来自拉科塔组的L2单元（相当于下富森段）。这些数据，结合最近基于轮藻和介形虫生物地层学对拉科塔组各成员修订后的年龄，将该分类单元的年龄限制在晚瓦兰吉尼阶至早巴列姆阶。
