Skeletal morphology of Kritosaurus navajovius (Dinosauria: Hadrosauridae) from the Late Cretaceous of the North American south-west, with an evaluation of the phylogenetic systematics and biogeography of Kritosaurini

The osteology of the hadrosaurid dinosaur Kritosaurus navajovius (late Campanian of southern North America) is documented in detail, and the taxonomy and phylogenetic relationships of the genus are revised. Kritosaurus is rediagnosed based on the extensive length of the dorsolateral margin of the maxilla and a unique combination of characters that includes a jugal with orbital constriction deeper than infratemporal one, infratemporal fenestra greater than orbit and with dorsal margin greatly elevated above dorsal orbital margin in adults, frontal participating in orbital margin, and paired caudal parasagittal processes of nasals resting over frontals. The taxonomy of numerous hadrosaurid specimens previously referred to Kritosaurus is reassessed; the vast majority of these cannot be positively referred to Kritosaurus. One exception is a specimen collected from the Cerro del Pueblo Formation that extends the geographical range of K. navajovius further south in Laramidia, to present-day northern Mexico. Anasazisaurus is regarded a junior synonym of Kritosaurus; their holotypes are indistinguishable from each other when considering the overlapping elements. However, many characters support distinction of Naashoibitosaurus ostromi as a valid taxon. Kritosaurus, consisting of the sister species K. navajovius and K. horneri, is deeply nested within Saurolophinae as a member of Kritosaurini. The latter clade includes also Naashoibitosaurus, Gryposaurus, and the South American Secernosaurus. Kritosaurini is characterized by a rostral nasal dorsal process not reaching the rostral margin of the narial foramen, frontal with triangular rostrolateral projection ending in a narrow apex (convergent in Brachylophosaurini), and a subrectangular dorsal region of infratemporal fenestra, among other characters. Kritosaurin hadrosaurids are hypothesized to have originated in southern Laramidia no later than the early Campanian. Subsequently, members of the clade reached northern Laramidia and South America via dispersal no later than the early and late Campanian, respectively.本研究详细记述了鸭嘴龙科（Hadrosaurid）恐龙纳瓦霍克瑞托斯龙（Kritosaurus navajovius，分布于北美南部晚坎潘期（late Campanian））的骨骼解剖学特征，并对该属的分类学与系统发育关系进行了修订。研究基于上颌骨（maxilla）背外侧缘的显著长度，以及一系列独特的性状组合对克瑞托斯龙属展开重新诊断：包括颧骨（jugal）的眶部收缩（orbital constriction）深度大于颞下孔收缩深度；成年个体的颞下孔（infratemporal fenestra）大于眼窝，且其背缘显著高于眼窝背缘；额骨（frontal）参与构成眶缘；成对的鼻骨（nasals）尾侧矢状突架于额骨之上。研究对此前被归入克瑞托斯龙属的大量鸭嘴龙类标本的分类学地位进行了重新评估，其中绝大多数无法确切归入克瑞托斯龙属。唯一的例外是一件采自塞罗德尔普埃布罗组（Cerro del Pueblo Formation）的标本，该标本将纳瓦霍克瑞托斯龙在拉拉米迪亚（Laramidia）的分布范围向南拓展至现今的墨西哥北部。阿纳萨齐龙（Anasazisaurus）被视为克瑞托斯龙的次同物异名（junior synonym）；对比二者的重叠骨骼元素后，其正模标本（holotypes）无法相互区分。但诸多性状表明，奥斯特罗姆氏纳什奥比托龙（Naashoibitosaurus ostromi）应为有效分类单元。克瑞托斯龙包含姐妹种（sister species）纳瓦霍克瑞托斯龙与霍氏克瑞托斯龙（K. horneri），作为克瑞托斯龙族（Kritosaurini）的成员，其演化位置深度嵌入赖氏龙亚科（Saurolophinae）内部。该演化支（clade）同时包含纳什奥比托龙、格里珀龙（Gryposaurus）以及南美洲的塞克尔龙（Secernosaurus）。克瑞托斯龙族的鉴别特征包括：鼻骨背侧吻突（rostral nasal dorsal process）未延伸至鼻孔孔（narial foramen）的吻侧缘；额骨具三角形吻侧突起（triangular rostrolateral projection），末端呈窄尖（该特征在短冠龙族（Brachylophosaurini）中为趋同演化）；颞下孔的背侧区域呈亚矩形，以及其他诸多性状。研究推测，克瑞托斯龙族鸭嘴龙类最早于早坎潘期（early Campanian）之前起源于拉拉米迪亚南部；随后该演化支的类群分别不晚于早坎潘期与晚坎潘期，通过扩散抵达拉拉米迪亚北部与南美洲。