Spinosaurs

Spinosaurids occupy one of the strangest ecological niches of any large dinosaur: semi-aquatic piscivores. Their skulls are unlike those of any other theropod — long and narrow with a rounded tip, pressure-sensitive pits on the snout for detecting fish movement, and conical teeth that interlock when the jaws close. This anatomy is almost identical to that of modern crocodilians and gharials, and almost certainly evolved for the same purpose: snatching large fish from rivers.

The group is divided into two major subfamilies. The baryonychines — Baryonyx and Suchomimus — had proportionally narrower snouts and large hooked thumb claws, possibly used as fish gaffs. The spinosaurines — Spinosaurus, Irritator — developed taller sail structures and, as revealed by a landmark 2020 paper, tails flattened into paddle-like swimming structures. Spinosaurus in particular has been transformed by recent discoveries from a terrestrial biped into something more like a giant, spine-backed crocodile — capable of sustained aquatic locomotion.

Spinosaurs flourished in the Cretaceous waterways of what are now North Africa and South America — regions that in the Early Cretaceous were vast river systems teeming with enormous fish like Coelacanths and giant lungfish. The group disappeared by ~93 Ma, possibly as sea level rise transformed their riverine habitats into shallow inland seas.