Pachycephalosaurus Had a 25 cm Dome Skull That May Have Been Used for Head-Butting

Despite being the largest land animals ever, sauropod dinosaurs laid surprisingly small eggs relative to their body size. The largest known dinosaur eggs (Macroelongatoolithus) were about 45–60 cm long, yet the adults that laid them might weigh 40–70 tonnes. This is because eggshell thickness is limited by the need for oxygen exchange — a larger egg would suffocate the embryo. Hatchlings were therefore tiny compared to adults, growing at astonishing rates throughout their lives.

The Cretaceous-Paleogene (K-Pg) extinction event 66 million years ago did not only kill non-avian dinosaurs — it eliminated approximately 75% of all species on Earth, including many marine reptiles (mosasaurs, plesiosaurs), flying pterosaurs, ammonites, and countless plant and invertebrate species. Survivors included small mammals, birds, crocodilians, turtles, snakes, and frogs — groups that could shelter underground, burrow, or tolerate the sudden darkness and cold.

Despite being featured alongside dinosaurs in popular media, pterosaurs were not dinosaurs. They were archosaurs — the same broad group that includes dinosaurs and crocodilians — but evolved flight independently on their own lineage. The first pterosaur appeared around 228 million years ago. The largest, Quetzalcoatlus, had a wingspan of up to 10–11 metres and stood as tall as a giraffe, making it the largest known flying animal in Earth's history.

Micropachycephalosaurus ('tiny thick-headed lizard') from Late Cretaceous China holds the record for the longest dinosaur name at 23 letters. Ironically, it was one of the smallest known dinosaurs, estimated at under 1 metre long. It belonged to the pachycephalosaur family and was first described in 1978 by Chinese palaeontologist Dong Zhiming. Its name is a paradox — meaning small, yet bearing one of the most unwieldy labels in all of palaeontology.