Dinosaur tracks from the Frontier Formation, Montana: preservation, distribution and palaeoecological significance for the middle Cretaceous terrestrial ecosystems of North America

The Coniacian-Santonian stages in western North America are characterized by a sparse fossil record. We present here the first account of dinosaur tracks from nine sites in the Frontier Formation (Coniacian-Santonian) of southwestern Montana. Tracks are largely preserved in distal alluvial facies as sandstone casts, with a single example of shallow epirelief impression interpreted as undertrack. Sandstone casts show significant relief, kinematic features (i.e., scale marks), and variable morphologies, arguing for a strong substrate control on their preservation. Putative producers are assigned to ornithopod, ankylosaurian, and theropod dinosaurs in decreasing order of abundance. This record reflects a composition similar to those of other middle Cretaceous formations in North America. Moreover, ankylosaurian tracks of the Frontier Formation represent the first known from Coniacian strata in North America. The incorporation of body and trace fossils yielded by the Frontier Formation indicates the presence of a fauna with North American endemic elements. The establishment of Coniacian-Santonian dinosaurian palaeocommunities akin to those observed in Campanian and Maastrichtian formations may reflect the trend towards provincialism attested for the end of the Mesozoic in North America.

北美西部的科尼亚克阶-桑托阶（Coniacian-Santonian）地层以化石记录稀缺为典型特征。本文首次报道了蒙大拿州西南部边境组（Frontier Formation，科尼亚克阶-桑托阶）9个遗址产出的恐龙足迹化石。这些足迹大多以砂岩铸型的形式保存在远端冲积相地层中，仅见一例浅表层上覆浮雕印痕，被解释为下覆足迹（undertrack）。砂岩铸型具有显著的起伏形态、运动学特征（即鳞片状痕迹）以及多样的外形，表明基底环境对其保存具有极强的控制作用。推测的造迹者按丰度从高到低依次归为鸟脚类、甲龙类和兽脚类恐龙。该化石组合的组成与北美其他中白垩世地层的同类记录相似。此外，边境组产出的甲龙类足迹是北美地区已知首个来自科尼亚克阶地层的甲龙类足迹记录。结合边境组产出的实体化石与遗迹化石可知，该地区存在具有北美地方性特征的动物群。科尼亚克阶-桑托阶恐龙古群落的构成与坎潘阶（Campanian）和马斯特里赫特阶（Maastrichtian）地层中所见的古群落相似，这或许反映了北美中生代末期所印证的生物地理分区趋势。