A Ceratopsian Dinosaur from the Lower Cretaceous of Western North America, and the Biogeography of Neoceratopsia

The fossil record for neoceratopsian (horned) dinosaurs in the Lower Cretaceous of North America primarily comprises isolated teeth and postcrania of limited taxonomic resolution, hampering previous efforts to reconstruct the early evolution of this group in North America. An associated cranium and lower jaw from the Cloverly Formation (?middle–late Albian, between 104 and 109 million years old) of southern Montana is designated as the holotype for Aquilops americanus gen. et sp. nov. Aquilops americanus is distinguished by several autapomorphies, including a strongly hooked rostral bone with a midline boss and an elongate and sharply pointed antorbital fossa. The skull in the only known specimen is comparatively small, measuring 84 mm between the tips of the rostral and jugal. The taxon is interpreted as a basal neoceratopsian closely related to Early Cretaceous Asian taxa, such as Liaoceratops and Auroraceratops. Biogeographically, A. americanus probably originated via a dispersal from Asia into North America; the exact route of this dispersal is ambiguous, although a Beringian rather than European route seems more likely in light of the absence of ceratopsians in the Early Cretaceous of Europe. Other amniote clades show similar biogeographic patterns, supporting an intercontinental migratory event between Asia and North America during the late Early Cretaceous. The temporal and geographic distribution of Upper Cretaceous neoceratopsians (leptoceratopsids and ceratopsoids) suggests at least intermittent connections between North America and Asia through the early Late Cretaceous, likely followed by an interval of isolation and finally reconnection during the latest Cretaceous.

北美下白垩统（Lower Cretaceous）新角龙类（neoceratopsian）恐龙的化石记录，主要由分类学分辨率有限的孤立牙齿与颅后骨骼构成，此前曾阻碍学界重建该类群在北美的早期演化历程。产自蒙大拿州南部克洛弗利组（Cloverly Formation，？阿尔布阶中晚期，距今1.04亿至1.09亿年）的关联保存头骨与下颌骨，被指定为美洲鹰角龙（Aquilops americanus）新属新种（gen. et sp. nov.）的正模标本。该物种具备多项独有衍征，包括带有中脊突起的强钩状吻骨，以及形态细长且尖端尖锐的眶前窝（antorbital fossa）。目前已知的唯一一件标本的头骨相对小巧，吻骨与颧骨（jugal）尖端之间的长度为84毫米。该分类单元被认为是基干新角龙类，与辽角龙（Liaoceratops）、黎明角龙（Auroraceratops）等早白垩世亚洲类群亲缘关系密切。从生物地理学角度来看，美洲鹰角龙大概率起源于从亚洲至北美的扩散事件；此次扩散的具体路径尚不明确，但鉴于欧洲早白垩世地层中未发现角龙类化石，白令陆桥路径较欧洲路径更为合理。其他羊膜类支系也呈现出类似的生物地理分布模式，这支持了早白垩世晚期存在亚洲与北美之间的跨洲际迁徙事件。上白垩统新角龙类（纤角龙科leptoceratopsids与角龙超科ceratopsoids）的时空分布表明，北美与亚洲在晚白垩世早期至少存在间断性的交流联系，随后大概率经历了一段隔离时期，最终在晚白垩世末期重新建立连接。