Supplementary photographs from: The Canadian fossil record supports anagenesis in Triceratops (Ornithischia, Ceratopsia)

Evolution of the ceratopsid dinosaur Triceratops has been hypothesized to
have proceeded anagenetically, with the geologically older Triceratops
horridus having directly given rise to the younger Triceratops prorsus.
This evolutionary scenario is based primarily on the upper Maastricthian
fossil record of the Hell Creek Formation in Montana. The Canadian fossil
record affords an opportunity to test this hypothesis beyond the
geographic bounds where it was originally developed. The Canadian strata
(lower Scollard Formation of Alberta and Frenchman Formation of
Saskatchewan) are roughly contemporaneous with the upper third of the Hell
Creek Formation where T. prorsus is found. On the anagenesis hypothesis,
the Canadian strata should, therefore, uniquely contain that species. Our
exhaustive survey of the Canadian Triceratops material, supported by
cladistic and ordination analyses, confirms the presence of a single
species, T. prorsus, in Canada, yielding robust support for the anagenesis
hypothesis conceived for the genus.

角龙科恐龙三角龙（Triceratops）的演化曾被假设以线系演化（anagenesis）的方式进行，即地质年代更古老的恐怖三角龙（Triceratops horridus）直接演化为更为年轻的前突三角龙（Triceratops prorsus）。该演化假说主要基于美国蒙大拿州地狱溪组（Hell Creek Formation）马斯特里赫特阶（Maastrichtian）上部的化石记录。加拿大的化石记录则为在该假说最初提出的地理范围之外检验该假说提供了契机。加拿大的地层（阿尔伯塔省斯科拉德组（Scollard Formation）下部与萨斯喀彻温省法国溪组（Frenchman Formation））大致与产出前突三角龙的地狱溪组上部三分之一层位同期。基于线系演化假说，加拿大地层理应唯一产出该物种。我们对加拿大三角龙标本的全面调查，结合分支系统学分析与排序分析的支撑结果，证实加拿大境内仅存在前突三角龙这一个物种，从而为三角龙属的线系演化假说提供了强有力的支撑。