THERIZINOSAURUS

Upon discovery, scientists were quick to tag Therizinosaurus a huge turtle-like reptile, because some broad ribs suggested a body "very reminiscent of the giant sea turtles Protostega and Archelon", while its ginormous hands and claws were thought to have been the business end of powerful swimming organs, ideally suited to harvesting seaweed. The animal was named in 1954 from fossils recovered from the Gobi Desert, but the remains were so incomplete and so unlike anything previously known that its true identity remained a mystery for decades. Only later discoveries of related dinosaurs revealed that Therizinosaurus was not a turtle at all, but one of the most unusual members of the theropod dinosaur lineage.

Living in Mongolia during the final few million years of the Late Cretaceous, Therizinosaurus grew to around 9-10 metres in length and stood roughly 3 metres high at the hips. Excluding the giant ornithomimosaur Deinocheirus, it was likely the tallest known theropod, its long neck and unusually upright posture giving it an imposing height that rivalled many much larger dinosaurs. For all its size, however, Therizinosaurus challenges almost everything people expect a theropod dinosaur to be. It belonged to the same great lineage that produced formidable predators such as Tyrannosaurus and Allosaurus, yet it appears to have been a dedicated hunter of... vegetation. As its ancestors moved away from carnivory, the body was transformed. The torso became deep and barrel-like, the pubis rotated backwards, and space was created for an enormous fermentation-based digestive system, producing the distinctive pot-bellied silhouette shared by other therizinosaurs.

Its most striking feature was a pair of immensely powerful forelimbs tipped with enormous claws, among the largest ever evolved by a land animal: the bony cores alone exceeded half a metre in length, and in life the complete claws may have approached a metre along their outer curve. For many years these claws were imagined as fearsome weapons or specialised tools for gathering food, but biomechanical studies have suggested they were surprisingly weak and poorly suited to either role. Instead, some researchers have proposed that the exaggerated claws functioned primarily as display structures, advertising the health, maturity, or fitness of their owners. In that respect, they may have been somewhat analogous to the oversized claw of a fiddler crab, an impressive appendage whose size and movement can intimidate rivals and attract mates.