古生物学：初期のドーム頭を持つ恐竜

The most complete and oldest-known dome-headed dinosaur, known as a pachycephalosaur, is described in Nature this week. The fossil, from the Early Cretaceous of Mongolia (dating to around 115–108 million years ago) pushes back evidence for the origin of the frontoparietal dome by at least 14 million years. 

Pachycephalosaurs had dome-shaped heads owing to a thickened skull, thought to either provide a means for self-defence or have a role in sociosexual behaviour (to attract a mate or fend off competition). An absence of early examples in the fossil record have impeded efforts to determine the origins and early evolution of this type of dinosaur. 

This gap in the record is filled by a new specimen described by Lindsay Zanno and colleagues. Zavacephale rinpoche is a small, juvenile pachycephalosaur from the Early Cretaceous of Mongolia and is the most skeletally complete and geologically oldest dome-head in the world, the authors report. They estimate that it was around 1 m long and weighed 5.85 kg. The dinosaur has a well-developed dome, suggesting that it was sexually mature. The findings reveal early stages in the evolution of the dome and shed light on the development of this structure, the authors conclude.