The cranial morphology and systematics of Triceratops, with a preliminary analysis of Ceratopsian phylogeny
Triceratops is a Late Cretaceous (Maastrichtian) ceratopsian from the western interior of North America. This study redescribes the cranial morphology and paleoneurology of Triceratops, analyses the species level systematics of the genus, and provides a preliminary phylogenetic analysis of the Ceratopsia. The re-examination of the skull of Triceratops reveals that it is highly variable, particularly in the morphology of the horns and frill. Skull elements are often well fused and sutural relationships blurred. The nasal horn is augmented by a separate ossification, the frontals and postorbitals form a deep sinus over the braincase that is confluent with the conrnual sinuses and open onto the surface of the skull through a small frontal fontanelle, the laterosphenoid encloses the rostral brain cavity, the frill is saddle-shaped, and the parietals are greatly thickened and lack fenestrae. Sixteen species of Triceratops have been proposed in the paleontological literature since 1889. The reality of these species was investigated through a combination of cladistic (character) and morphometric (shape) analysis. Cladistic analysis was performed using characters discovered during the redescription of the cranium and the phylogenetic analysis of the Ceratopsia. Morphometric analyses based on Principal Components Analysis were performed on 14 Triceratops skulls using sets of straight-line distances measured on each skull as variables. Nearly all specimens belong to a single species, Triceratops horridus. AMNH 5116 falls outside this group and is here designated a new species, Triceratops sternbergii. USNM 2412, originally described as a separate genus, is removed once more to that genus as Diceratops hatcheri. To place Triceratops within the Ceratopsia, a preliminary cladistic analysis of the phylogeny of this major clade of dinosaurs was performed. Nineteen genera and 129 characters were analysed via PAUP, using Psittacosaurus and the Pachycephalosauridae as outgroups. This analysis shows that the Protoceratopsidae are a paraphyletic group, the Centrosaurinae form an unresolved trichotomy, and the Chasmosaurinae form a clade. Triceratops is a highly derived member of the Chasmosaurinae.
Forster, Catherine Ann, "The cranial morphology and systematics of Triceratops, with a preliminary analysis of Ceratopsian phylogeny" (1990). Dissertations available from ProQuest. AAI9101154.