What Is a Spinosaur? The Predator That Went Back to the Water

Spinosaurs have long narrow skulls with a kink near the front, conical unserrated teeth suited to gripping rather than slicing, and a rosette of enlarged teeth at the snout tip, which is an arrangement closely paralleled in crocodilians and in fish-eating reptiles generally. Their nostrils sit further back on the skull than in other theropods, which would allow breathing while the front of the snout was submerged. Pits at the end of the snout have been interpreted as housing pressure-sensitive receptors comparable to those crocodilians use to detect movement in water. The forelimbs are unusually large and powerfully built with strongly curved claws. Several species carry an elaborate sail supported by extended vertebral spines, whose function is debated between display, thermoregulation and hydrodynamic use. Direct dietary evidence exists, with fish scales found in the body cavity of one specimen and a spinosaur tooth embedded in a flying reptile bone, which together indicate a mixed diet rather than a single specialisation.

The extent of aquatic adaptation is one of the more active disputes in the field and both positions marshal specific evidence:

• •For swimming: dense limb bones lacking the hollow interiors typical of theropods, which reduce buoyancy and are characteristic of animals that submerge
• •For swimming: a tail described in 2020 with tall flexible spines forming a broad paddle-like surface, with a physical model producing substantially more thrust in water than comparable theropod tails
• •For swimming: isotopic composition of the teeth resembling that of aquatic animals rather than terrestrial ones
• •Against: hind limbs proportionally short and apparently unsuited to propulsion, with body proportions that modelling suggests would be unstable in water
• •Against: the argument that a wading or shoreline lifestyle explains the same features without requiring pursuit swimming
• •The current position is genuinely unsettled, with papers arguing both readings appearing within months of each other and with the disagreement concerning how the same anatomy should be modelled

The group's history is complicated by the destruction of its founding material. The original Spinosaurus specimen was described from Egypt in 1915 and held in Munich, where it was destroyed in a bombing raid in 1944, leaving only the published drawings and photographs. That loss mattered enormously, since the reconstruction of the animal thereafter depended on interpretation of those illustrations and on fragmentary material found later, and estimates of its size, proportions and posture varied widely. New material described from 2014 onward substantially revised the reconstruction, notably proposing much shorter hind limbs than previously assumed, and that revision was itself contested on the grounds that the material might combine more than one individual. The sequence illustrates how much a single well-preserved specimen matters and how difficult a field becomes when its reference material is gone, which is also an argument for the practice of distributing casts and now digital scans across institutions.

The structure along the back is formed by enormously elongated spines projecting from the vertebrae, reaching over a metre and a half in the largest specimens, and its function has attracted the same range of explanations as other conspicuous dinosaur structures. Thermoregulation was the traditional answer, with the sail acting as a radiator, and it faces the objection that related animals in similar climates managed without one. Display is supported by the variation between species and by the disproportionate investment, which is the standard argument for signalling structures. A hydrodynamic role has been proposed, with the sail acting as a stabiliser or as a display visible above water while the body is submerged, which connects it to the aquatic question. Fat storage in a hump rather than a thin sail has also been argued, based on the thickness of the spines at their bases. The distribution of the spines, tallest over the hips and reducing towards both ends, is consistent with several of these and decisive for none.

A long narrow kinked skull with conical gripping teeth, retracted nostrils and pressure-sensing pits at the snout all point at catching fish, and stomach contents confirm a mixed diet. Dense limb bones and a paddle-like tail support real swimming, while short hind limbs and stability modelling support wading instead, and the dispute is unresolved. The founding specimen was destroyed by bombing in 1944.