Data from: Anatomy, systematics, paleoenvironment, growth, and age of the sauropod dinosaur Sonorasaurus thompsoni from the Cretaceous of Arizona, USA

BACKGROUND—Sauropod dinosaurs are rare in the Cretaceous North American fossil record in general and are absent from that record for most of the Late Cretaceous. Sonorasaurus thompsoni from the Turney Ranch Formation of the Bisbee Group of Arizona, USA, potentially represents one of the youngest sauropods before their ca. 30 million-year-long hiatus from the record. The anatomy of Sonorasaurus has only been briefly described, its taxonomic validity has been questioned, several hypotheses have been proposed regarding its phylogenetic relationships, and its life history, age, and reported paleoenvironment are ambiguous. METHODOLOGY/PRINCIPLE FINDINGS—Herein we assess the systematics, paleoenvironment, life history, and age of Sonorasaurus based on firsthand observation, bone histology, and fieldwork in the holotypic quarry and environs. The validity of S. thompsoni is substantiated by autapomorphies. Cladistic analysis firmly places it within the Brachiosauridae, in contrast to some recent analyses . Bone histology suggests that the only known exemplar of Sonorasaurus grew slowly and sporadically compared to other sauropods and was approaching its adult size. In contrast with previous assessments of a coastal/estuarine paleoenvironment for the Turney Ranch Formation, our sedimentological and plant macrofossil data indicate that Sonorasaurus lived in a semiarid, low-relief evergreen woodland that received highly variable (perhaps seasonal) precipitation. We obtained detrital zircons from the holotypic quarry for U-Pb dating, which only yielded Barremian-aged and older grains, whereas other radiometric and biostratigraphic data suggest that the sediments at the quarry were deposited near the Albian– Cenomanian boundary. CONCLUSIONS/SIGNIFICANCE—Sonorasaurus is taxonomically valid, represents one of the geologically youngest brachiosaurid sauropods, and inhabited a harsh inland evergreen- dominated woodland environment that limited its growth. A review of other Bisbee Group dinosaurs suggests that its fauna, though poorly sampled, exhibits broad similarity to those from coeval North American horizons, reinforcing the apparent faunal homogeneity at the time.研究背景——蜥脚类恐龙在北美白垩纪化石记录中整体较为罕见，且在晚白垩世的绝大部分时期均未见于该记录。产自美国亚利桑那州比斯比群（Bisbee Group）图尼牧场组（Turney Ranch Formation）的索诺拉龙（Sonorasaurus thompsoni），可能是该类群从化石记录中消失前约3000万年间断期内，已知最晚期的蜥脚类恐龙之一。目前学界仅对索诺拉龙的解剖结构进行过简要描述，其分类学有效性曾受到质疑，学界已提出多项关于其系统发育关系的假说，且其生活史、个体年龄以及已报道的古环境均存在模糊不清之处。 研究方法与主要结果——本文基于对正型标本埋藏点及周边区域的野外工作、一手实地观测与骨组织学分析，对索诺拉龙的分类学、古环境、生活史与个体年龄展开系统性评估。研究通过自衍征证实了索诺拉龙（S. thompsoni）的分类学有效性。支序分类学分析明确将其归入腕龙科（Brachiosauridae），这与近期部分相关分析的结果相悖。骨组织学分析结果显示，相较于其他蜥脚类恐龙，目前仅发现的索诺拉龙标本生长速度缓慢且呈间歇性，且当时已接近成年个体体型。不同于此前对图尼牧场组海岸/河口古环境的判定，我们的沉积学与植物大化石数据表明，索诺拉龙的生存环境为半干旱、地势平缓的常绿林地，该区域的降水具有高度不稳定性（或为季节性降水）。我们从正型标本埋藏点采集了碎屑锆石（detrital zircons）用于U-Pb定年（U-Pb dating），但仅获得了巴列姆阶（Barremian）及更古老的测年数据；而其他放射测年与生物地层学数据则显示，该埋藏点的沉积物沉积于阿尔布阶（Albian）-森诺曼阶（Cenomanian）界线附近。 结论与意义——索诺拉龙的分类学有效性得到证实，其是目前已知地质年代最年轻的腕龙科蜥脚类之一，且栖息于严苛的内陆常绿优势型林地环境中，该环境限制了其生长速度。对比斯比群其他恐龙类群的回顾研究表明，尽管采样程度较低，该动物群与同期北美其他地层的动物群具有广泛相似性，这进一步印证了当时动物群的整体同质性。