Homalocephale

Chapter • Meet the dinosaur

Overview

Homalocephale was a small, two-legged plant-eater belonging to the pachycephalosaurs, the group popularly known as the thick-headed dinosaurs. Unlike its famous relative Pachycephalosaurus, Homalocephale had a flat, broad and slightly bumpy skull roof rather than a high dome, which is exactly what its name refers to: "level head". The body was compact, with short arms, powerful hind legs and a strikingly wide pelvis. The animal reached roughly 1.5–2 metres in length and probably weighed between 20 and 45 kilograms, about the bulk of a large dog. Its neck curved in an S-shape and the head was carried forward, while the long tail was held horizontally as a counterweight — no pachycephalosaur dragged its tail along the ground. The most distinctive feature of Homalocephale is its tail. The species name calathocercos means "basket tail", referring to a lattice of ossified tendons and thin rib-like rods (myorhabdoi) woven along the tail vertebrae. This basket-like scaffolding made the tail stiffer and more resistant to twisting, but its precise function is still debated. Suggestions include balance during fast running, support during body-to-body contact between individuals, and protection of the underside of the tail. The broad pelvis has been interpreted both as space for a large fermenting gut and — on much weaker grounds — as evidence of live birth. That last idea is not supported by modern research. The teeth were small, leaf-shaped and coarsely serrated along the front edge, well suited to cropping soft leaves, shoots, fruits and seeds. Many researchers argue that pachycephalosaurs had a mixed diet and that smaller species may have supplemented plants with insects and other small animals, though no gut contents are known for Homalocephale itself to confirm this. The animal had large eye sockets and relatively broad olfactory regions in the brain, suggesting good vision and a decent sense of smell — useful traits for a small animal sharing its world with predators such as Tarbosaurus. Homalocephale was described in 1974 by the Polish palaeontologists Teresa Maryańska and Halszka Osmólska, based on material collected during the Polish–Mongolian expeditions to the Gobi Desert. The holotype comes from the Nemegt Formation of southern Mongolia and consists of a partial skull roof together with much of the backbone, pelvis, hind limbs and the characteristic tail. Since the 2000s several workers, notably Robert Sullivan, have argued that the flat skull roof is simply a juvenile trait and that Homalocephale may in fact be a young individual of Prenocephale prenes, found in the same rocks. Other researchers point to differences in the pelvis and tail and regard the genus as valid. The question remains open.

Chapter • Where it lived

Habitat

Homalocephale lived about 72–68 million years ago, during the Maastrichtian at the end of the Cretaceous, in what is now southern Mongolia. Its fossils come from the Nemegt Formation at the Nemegt locality in the Gobi Desert. Unlike the older, more desert-like beds of the same region, the Nemegt Formation preserves a landscape of large meandering rivers, floodplains, temporary lakes and forested riverbanks — a semi-humid inland setting with pronounced dry seasons. Here Homalocephale shared its habitat with hadrosaurs such as Saurolophus, the sauropod Nemegtosaurus, oviraptorosaurs, the bizarre Therizinosaurus and the apex predator Tarbosaurus.

Chapter • Wild behavior

Behavior & Ecology

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Chapter • Scientific place

Taxonomy