In many species, the upwards-projecting neural spines of the vertebrae (backbones) were significantly elongated and formed a sail on the animal's back (hence the family's etymology), which supported either a layer of skin or a fatty hump.

This is further indicated by various anatomical adaptations, such as retracted eyes and nostrils; and the deepening of the tail in some taxa, which has been suggested to have aided in underwater propulsion akin to that of modern crocodilians.

Consisting of vertebrae, skull fragments, and teeth, these remains became the holotype specimen of the new genus and species Spinosaurus aegyptiacus in 1915, when they were described by German paleontologist Ernst Stromer.

[30] The tips of the premaxillae (frontmost snout bones) were expanded in a spoon shape, forming what has been called a "terminal rosette" of enlarged teeth.

[33] Lengthwise atop their skulls ran a thin and shallow sagittal crest that was usually tallest near or above the eyes, either becoming shorter or disappearing entirely towards the front of the head.

[37] Angaturama (a possible synonym of Irritator) had an unusually tall crest on its premaxillae that nearly overhung the tip of the snout with a small forward protrusion.

Their teeth ranged from slightly recurved, such as those of Baryonyx and Suchomimus, to straight, such as those of Spinosaurus and Siamosaurus, and the crown was often ornamented with longitudinal grooves or ridges.

In genera like Baryonyx and Suchomimus, the phalanges (finger bones) were of conventional length for large theropods, and bore hook-shaped, strongly curved hand claws.

In life, these spines would have been covered in skin or fat tissue and formed a sail down the animal's back, a condition that has also been observed in some carcharodontosaurid and ornithopod dinosaurs.

Examinations in 2017 by Marcos Sales and Cesar Schultz indicate that the South American spinosaurids Angaturama and Irritator were intermediate between Baronychinae and Spinosaurinae based on their craniodental features and cladistic analysis.

[1][53] They shared features such as long, narrow, crocodile-like skulls; sub-circular teeth, with fine to no serrations; the terminal rosette of the snout; and a secondary palate that made them more resistant to torsion.

Since Suchomimus was more closely related to Baryonyx (from Europe) than to Spinosaurus—although that genus also lived in Africa—the distribution of spinosaurids cannot be explained as vicariance resulting from continental rifting.

Therefore, teeth with small or no serrations, such as in spinosaurids, were not good for cutting or ripping into flesh but instead helped to ensure a strong grip on a struggling prey animal.

[65] Spinosaurid jaws were likened by Romain Vullo and colleagues to those of the pike conger eel, in what they hypothesized was convergent evolution for aquatic feeding.

[65] Based on the size and positions of their nostrils, Marcos Sales and Cesar Schultz in 2017 suggested that Spinosaurus possessed a greater reliance on its sense of smell and had a more piscivorous lifestyle than Irritator and baryonychines.

[34] In the Sao Khua Formation of Thailand, isolated tooth crowns from Siamosaurus have been found in association with sauropod remains, indicating possible predation or scavenging.

The researchers proposed that the diet of spinosaurines from this environment may have included—in addition to pterosaurs—terrestrial and aquatic crocodyliforms, juveniles of their own species, turtles, and small to medium-sized dinosaurs.

This study likewise suggests their jaws and teeth were well-suited to quick strikes and deep, puncturing bites, but not for slicing flesh or crushing bones.

[17] In 1986, Charig and Milner suggested that the robust forelimbs and giant thumb claws would have been Baryonyx's primary method of capturing, killing, and tearing apart large prey; whereas its long snout would have been used mostly for fishing.

[36] A 2005 study by Canadian paleontologist the François Therrien and colleagues agreed that spinosaur forelimbs were probably used for hunting larger prey items, given that their snouts could not resist the bending stress.

[76] In a 2017 review of the family, David Hone and Holtz considered possible functions in digging for water sources or hard to reach prey, as well as burrowing into soil to construct nests.

[77] Though there has been little discussion on the head crests of spinosaurs, Hone and Holtz in 2017 considered that their most likely use was for displaying to potential mates or as a means of threatening rivals and other predators.

[77] Many theories have been proposed over the years for the use of spinosaurid dorsal sails, such as thermoregulation;[78] to aid in swimming;[79] to store energy or insulate the animal; or for display purposes, such as intimidating rivals and predators, or attracting mates.

[82] In a 2013 blog post, Darren Naish considered the latter function unlikely, favouring the hypothesis of sexual selection for Ichthyovenator's sail because it appears to have evolved on its own, without very close relatives.

[88] In 2018, an analysis was conducted on the partial tibia of an indeterminate spinosaurine from the early Albian, the bone was from a sub-adult between 7 and 13 m (22 and 42 ft) in length still growing moderately fast before its death.

This specimen (LPP-PV-0042) was found in the Araripe Basin of Brazil and taken to the University of San Carlos for a CT Scan, where it revealed osteosclerosis (high bone density).

[89] However, research conducted in 2023 cited the immediate assumption of Spinosaurids being avid divers due to correlations in bone compactness as being subject to errors, such as flawed statistical methods and measurements, as well as sampling bias.

[91] A 2018 study of buoyancy (through simulation with 3D models) by the Canadian palaeontologist Donald M. Henderson found that distantly related theropods floated as well as the tested spinosaurs, and instead supported they would have stayed by the shorelines or shallow water rather than being semi-aquatic.

[96] Partial skeletons and numerous fossil teeth indicate spinosaurids were widespread in Asia; three taxa—all spinosaurines—have been named: Siamosaurus suteethorni from Thailand, "Sinopliosaurus" fusuiensis from China, and Ichthyovenator laosensis from Laos.

[101] At la Cantalera-1, a site in the early Barremian Blesa Formation in Treul, Spain, two types of spinosaurid teeth were found, and they were assigned, tentatively, as indeterminate spinosaurine and baryonychine taxa.