Foraminacephale

''Foraminacephale'' (meaning "foramina head") is a genus of pachycephalosaurid dinosaur from Late Cretaceous (Campanian stage) deposits of Canada.

Description

''Foraminacephale'', as a pachycephalosaurid, was a small, bipedal herbivore with a thickened dome on its skull. In 2016, Gregory S. Paul estimated its length at , its weight at . In ''Foraminacephale'', the top surface of the dome is punctuated by many small pits, the eponymous foramina; the dome itself consists of a large, central lobe with a sloped frontal half, and two smaller lateral lobes at the front. The squamosal bone forms a tall bar of completely smooth bone underneath the dome, save for six bony nodes that line the bottom edge of the dome and an additional "corner" node just below. These features differentiate ''Foraminacephale'' from all other pachycephalosaurids.

Unlike ''Stegoceras'', ''Hanssuesia'', and '' Colepiocephale'', the parietal bone of ''Foraminacephale'' (which constitutes the back part of the dome) projects backwards and downwards over the base of the skull. Two traits also differentiate ''Foraminacephale'' from ''Sphaerotholus'' and ''Prenocephale'': there are prominent grooves between the central lobe and the lateral lobes, and the opening of the temporal bone is slit-like.

Ontogeny

As with ''Stegoceras'', the wide age range present in specimens of ''Foraminacephale'' allows for analysis of the ontogeny, or growth, of the dome. Measurements of 27 different points on 21 ''Foraminacephale'' skulls showed that the dome became proportionally taller with age, but did not become significantly wider. Histology of the specimens showed that the domes became less porous with age - the smallest specimen's skull was 1.67% empty space, while the largest specimen's skull was 0.25% empty space. Generally, the frontal part of the dome was more porous than the parietal part.

The growth series of the dome allows ''Foraminacephale'' to be differentiated further from ''Stegoceras''. In young ''Stegoceras'', the dome is flat; however, in even the smallest ''Foraminacephale'' specimens, the parietal is already slightly domed. The supratemporal fenestrae on the top of the skulls of ''Foraminacephale'' are slit-like, as opposed to ''Stegoceras'' where they are round, and they seal shut very early in ontogeny as opposed to remaining open more-or-less throughout the animal's lifespan. The amount of empty space in the skull roof was also smaller relative to ''Stegoceras'', and more similar to ''Acrotholus''. Furthermore, in ''Foraminacephale'', the squamosal and postorbital bones became part of the dome much quicker, the dome thickened at a slower rate, and the sides of the dome were less angled than in ''Stegoceras''.

Discovery and naming

The holotype of ''Foraminacephale'' is CMN 1423, a nearly complete frontoparietal dome collected in Dinosaur Provincial Park, Alberta in 1902 and reported on during the same year. The layers of rock where the specimen were found belong to the Dinosaur Park Formation of the Belly River Group. Various other fragments of the skull are also known from the Dinosaur Park Formation, representing juvenile, subadult, and adult individuals.

Although it has been reported that many specimens are known from the contemporary Oldman Formation, only one (catalogued as TMP 2015.044.0041) can definitely be assigned to ''Foraminacephale''. Another specimen from the Horseshoe Canyon Formation formerly assigned to ''Foraminacephale'', CMN 11316, was re-identified as a small, juvenile indeterminate pachycephalosaurine.

CMN 1423 was first assigned to a new species of ''Stegoceras'' by Lawrence Lambe in 1918; there was considerable debate over whether ''"S." brevis'' represented a truly distinct animal or simply a morph, perhaps the female, of ''S. validum''.
Later, it was assigned to ''Prenocephale'' by Robert M. Sullivan in 2000, and then it was assigned to ''Sphaerotholus'' by Longrich, Sankey & Tanke in 2010. In 2011, Ryan Schott suggested a new generic name, ''Foraminacephale'', in his Master of Science thesis, resulting in a new combination ''Foraminacephale brevis''. It remained an invalid ''nomen ex dissertatione'' until Schott and David Evans formally renamed ''"S." brevis'' to ''Foraminacephale brevis'' in 2016. The generic name combines Latin ''foramina'' ("foramina") with ''cephale'', Latinised Greek for "head", referring to the many pits that covered the top of its dome.

Classification

A phylogenetic analysis in 2016 found that ''Foraminacephale'' was a member of the Pachycephalosaurinae, in a more derived position than ''Stegoceras'', but in a more basal position than ''Prenocephale''. The consensus of the phylogenetic trees recovered is shown below.

The topology of this phylogenetic tree is not very stable, likely due to the incompleteness of most pachycephalosaur specimens.

Paleoecology

The Belly River Group is particularly rich in pachycephalosaur remains; 70% of all known pachycephalosaur fossils come from this region. Because of a lack of understanding of pachycephalosaur ontogeny, various authors have assigned this material to anywhere from a single species, ''Stegoceras validum'', to four different genera. This has significant implications for the Late Cretaceous biodiversity of herbivorous dinosaurs. The 2016 description of ''Foraminacephale'' recognized four distinct species of pachycephalosaur from the Belly River Group: ''Stegoceras'', ''Foraminacephale'', ''Hanssuesia'', and '' Colepiocephale'', although the specimens that have been referred to ''Hanssuesia'' likely represent more than one of the existing species.

See also

*2016 in paleontology

References

{{Taxonbar, from=Q5863739

Late Cretaceous dinosaurs of North America
Pachycephalosaurs
Fossil taxa described in 2016
Paleontology in Alberta
Campanian genus first appearances
Campanian genus extinctions
Ornithischian genera