Spinosauridae

Spinosauridae is a family of theropod dinosaurs mostly from the Early Cretaceous period. Members of this family were large, with elongated, crocodile-like skulls, sporting conical teeth with either no serrations or very small serrations. The front dentary teeth fanned out, giving the animal a characteristic look. The name of this family alludes to the typically conspicuous

A scale chart showing potentially the only named Spinosaurids

sail-like structure protruding from the back of species in the type genus, Spinosaurus. The purpose of the sail is disputed, whereas popular explanations are that it may have served as a thermoregulator, a threat display, or as a sexual display during courtship, some palaeontologists rather interpret the neural spine elongation in Spinosaurus as a support of a muscular/fatty hump. Some other dinosaurs, especially Ouranosaurus, also had similar sail-like structures. It is not known if these sails were similar to those of pelycosaurs such as Dimetrodon or Edaphosaurus.

First spinosaurids appeared during the Late Jurassic and became abundant in the Early Cretaceous. So far, the Late Jurassic record of spinosaurids consists only of referred teeth. They seem to disappear in the Cenomanian though teeth from the Turonian of Argentina have been attributed to spinosaurid dinosaurs.

Paleobiology

Studies of spinosaurids (specifically of Irritator) have shown that they had a very different skull shape and construction compared to other large predatory dinosaurs like Allosaurus and Tyrannosaurus Rex. In most predatory dinosaurs, the jaws were broad either in width, height, or both, while spinosaurid jaws were thin and narrow. This lead paleontologists like Sues, Frey and Martill to conclude that spinosaurids, unlike other theropods, were not specialized in attacking large, struggling prey.

Sues and colleagues studied the construction of the spinosaurid skull, and concluded that their mode of feeding was to use extremely quick, powerful strikes to seize small prey items with the jaws, employing the powerful neck muscles in rapid up and down motion. Due to the narrow snout, powerful side to side motion of the skull in prey capture is unlikely.

Diet

Spinosaurids have in the past often been considered mainly fish-eaters (piscivores), based on comparisons of their jaws with the jaws of modern Crocodilians. Rayfield and colleagues, in 2007, were the first to conduct actual biomechanical studies on a spinosaurid skull (using the European spinosaurid Baryonyx). They found that the structure and bite force of baryonychine jaws was almost identical to modern Gharials, supporting the idea that at least baryonychines were mainly fish-eaters, though the jaws spinosaurines appeared to have been more generalized.

Direct fossil evidence shows that spinosaurids fed on fish as well as a variety of other small to medium-sized animals, including small dinosaurs. Baryonyx was found with fish scales and the digested bones of a young Iguanodon in its stomach cavity, and there is one documented example of a spinosaurid having eaten a Pterosaur. It is likely that spinosaurids were Generalists, specializing in small prey of any kind, fish included.

Habitat

A 2010 publication by Romain Amiot and colleagues found that oxygen isotope ratios of spinosaurid bones indicates semiaquatic lifestyles. Isotope ratios from teeth from the spinosaurids Baryonyx, Irritator, Siamosaurus, and Spinosaurus were compared with isotopic compositions from contemporaneous theropods, turtles, and crocodilians. The study found that, among theropods, spinosaurid isotope ratios were closer to those of turtles and crocodilians. Siamosaurus specimens tended to have the largest difference from the ratios of other theropods, and Spinosaurus tended to be have the least difference. The authors concluded that spinosaurids, like modern crocodilians and hippopotamuses, spent much of their daily lives in water. The authors also suggested that semiaquatic habits and piscivory in spinosaurids can explain how spinosaurids coexisted with other large theropods: by feeding on different prey items and living in different habitats, the different types of theropods would have been out of direct competition.

Taxonomy

The family Spinosauridae was named by Ernst Stromer in 1915 to include the single genus Spinosaurus. The family was expanded as more close relatives of Spinosaurus were uncovered. The first cladistic definition of Spinosauridae was provided by Paul Sereno in 1998 (as "All spinosaurids closer to Spinosaurus than to Torvosaurus).

Spinosauridae contains two subfamilies--Spinosaurinae and Baryonychinae. The subfamily Spinosaurinae was named by Sereno in 1998, and defined by Holtz et al. (2004) as all taxa closer to Spinosaurus aegyptiacus than to Baryonyx walkeri. The subfamily Baryonychinae was named by Charig & Milner in 1986. They erected both the subfamily and the family Baryonychidae for the newly discovered Baryonyx, before it was referred to the Spinosauridae. Their subfamily was defined by Holtz et al. in 2004, as the complementary clade of all taxa closer to Baryonyx walkeri than to Spinosaurus aegyptiacus.

Classification

• Superfamily Spinosauroidea
  • Family Spinosauridae
    • • Asiamericana asiatica?
      • Siamosaurus suteethorni
    • Subfamily Baryonychinae
      • Baryonyx walkeri
      • Cristatusaurus lapparenti
      • Suchomimus tenerensis'
      • Suchosaurus cultridens
    • Subfamily Spinosaurinae
      • Irritator challengeri
      • Spinosaurus aegyptiacus'
• Indeterminate Spinosauridae
  • "Sinopliosaurus" fusuiensis

Complications

The number of genera is rather disputed. Spinosauridae may contain anywhere from 4 to 10 sepperate species.

The number of Baryonychines is particulary varied. It has been suggested that Cristatusaurus and Suchomimus are the same animal. In which case the name Cristatusaurus would have priority. However, it is thought Suchomimus may also be Baryonyx, meaning both Cristatusaurus and Suchomimus would be renamed Baryonyx. What complicates things further is that Baryonyx has a clade named after it, Baryonychinea. But there is another Baryonychine with name priority over all the other Baryonychines; Suchosaurus. This should mean that the only Baryonychine is Suchosaurus. But as Baryonychinea is named after Baryonyx, it is disputed weather Suchosaurus or Baryonyx would have priority should they be the same animal. Which even that is not concrete as of yet.

The other subfamily, Spinosaurinea, has a less varied number of Theropods. Either 2 or 3 are currently recognised. Potentially, it contains Irritator, Angaturama and Spinosaurus. However, it is highly likely Angaturama is a specimen of Irritator.

Mickey Mortimer has suggested "Sinopliosaurus" is a specimen of Siamosaurus. Also the describer of Asiamericana has cautioned that the teeth he described may belong to a Saurodont fish.

If all of the above syonymising occurs, then the only valid species would be: Spinosaurus, Irritator, Siamosaurus and Suchosaurus/Baryonyx. All of these have evidence of a sail like structure on their back, making 'Spinosauridae' a fitting name.