Late Cretaceous dinosaur assemblages of North America—characterized by gigantic tyrannosaurid predators, and large-bodied herbivorous ceratopsids and hadrosaurids—were highly successful from around 80 million years ago (Ma) until the end of the ‘Age of Dinosaurs’ 66 Ma. However, the origin of these iconic faunas remains poorly understood because of a large, global sampling gap in the mid-Cretaceous, associated with an extreme sea-level rise. We describe the most complete skeleton of a predatory dinosaur from this gap, which belongs to a new tyrannosauroid theropod from the Middle Turonian (~92 Ma) of southern Laramidia (western North America). This taxon, Suskityrannus hazelae gen. et sp. nov., is a small-bodied species phylogenetically intermediate between the oldest, smallest tyrannosauroids and the gigantic, last-surviving tyrannosaurids. The species already possesses many key features of the tyrannosaurid bauplan, including the phylogenetically earliest record of an arctometatarsalian foot in tyrannosauroids, indicating that the group developed enhanced cursorial abilities at a small body size. Suskityrannus is part of a transitional Moreno Hill (that is, Zuni) dinosaur assemblage that includes dinosaur groups that became rare or were completely absent in North America around the final 15 Myr of the North American Cretaceous before the end-Cretaceous mass extinction, as well as small-bodied forebears of the large-bodied clades that dominated at this time.

北美的晚白垩纪恐龙组合以巨大的霸王龙食肉动物，大型草食的拟齿兽类和鸭嘴龙为特征，从大约8000万年前（马）到“恐龙时代”末期66 Ma，都取得了巨大的成功。然而，由于白垩纪中期全球取样范围巨大，同时海平面急剧上升，这些标志性动物的起源仍知之甚少。我们从这个缝隙中描述了掠食性恐龙的最完整骨架，它属于来自南拉美迪亚州（北美洲西部）中突尼斯（〜92 Ma）的一种新的暴龙类兽脚亚目兽脚类。这个分类群，Suskityrannus hazelaegen。等 nov。，是一种小种，在系统发育上介于最老的，最小的暴龙和最大的，最后存活的暴龙之间。该物种已经具有霸王龙bauplan的许多关键特征，包括在霸王龙中弧形eta足的系统发育最早记录，这表明该物种在较小的体型上发展了增强的光标能力。Suskityrannus是过渡莫雷诺山（Zuni）恐龙组合的一部分，其中包括在白垩纪末大灭绝之前在北美白垩纪最后15 Myr附近在北美变得罕见或完全不存在的恐龙群。就像这时占主导地位的大身体进化支的小身体祖先。