page_title: Ostafrikasaurus
section_title: Evolution
hierarchical_section_title: Ostafrikasaurus / Classification / Evolution
caption_attribution_description: English: Spinosaurid tooth from the Middle Jurassic Tegama Group, Agadez, Niger. MUPE HB-87 in distal (D1), lingual (D2), mesial (D3), and labial (D4) views; close-up view of the labial side (D5); notethe deeply veined enamel surface texture and the shape and size of the distal denticles.
context_page_description: Ostafrikasaurus is a genus of theropod dinosaur from the Late Jurassic period of what is now Tanzania. It is known only from fossil teeth discovered sometime between 1909 and 1912, during an expedition to the Tendaguru Formation by the Natural History Museum of Berlin. Eight teeth were originally attributed to the dubious dinosaur genus Labrosaurus, and later to Ceratosaurus, both known from the North American Morrison Formation. Subsequent studies attributed two of these teeth to a spinosaurid dinosaur, and in 2012, Ostafrikasaurus crassiserratus was named by French palaeontologist Eric Buffetaut, with one tooth as the holotype, and the other referred to the same species. The generic name comes from the German word for German East Africa, the former name of the colony in which the fossils were found, while the specific name comes from the Latin words for "thick" and "serrated", in reference to the form of the animal's teeth.
  Ostafrikasaurus has been tentatively estimated at 8.4 metres long and weighing 1.15 tonnes. The holotype tooth is 46 millimetres long, has a curved front edge, and is oval-shaped in cross section.
context_section_description: Fowler in 2007 put forward the possibility of spinosaurids having evolved from ceratosaurian ancestors, given that baryonychine teeth have ridges on their crowns reminiscent to those seen on the premaxillary and dentary teeth of Ceratosaurus. In 2008, Buffetaut rejected this proposal, citing that the D-shaped cross section of said Ceratosaurus teeth is not present in those of baryonychines.
  
  The main difference between MB R 1084 and all other known spinosaurid teeth, as Buffetaut noted, was in the large size of the denticles borne by the carinae. This led him to hypothesize in 2008 that spinosaurid dental evolution was largely characterized by the shrinking and eventual loss of serrations. In 2012, Buffetaut posited that this pattern would begin with large serrations in primitive Jurassic taxa like Ostafrikasaurus (from the Tithonian age), which resembled those of typical, similarly-sized theropods. These would then evolve into the fine, reduced, and more numerous serrations of Early Cretaceous baryonychines, like Baryonyx from the Barremian of Europe, and Suchomimus from the Aptian to Albian of West Africa. Baryonyx, for example, had seven denticles per mm (0.04 in) in comparison to Ostafrikasaurus's two to four. Finally, the denticles would disappear entirely in spinosaurines such as Spinosaurus from the Albian to Turonian of North Africa. Buffetaut points out that the heavily diminished serrations in taxa like Siamosaurus, from the Barremian of Thailand, seem to represent an intermediate form. He also noted similarities between Ostafrikasaurus's dentition and a set of Baryonyx-like teeth from the pre-Aptian Cabao Formation of Libya, with potentially important biogeographical implications. These teeth are similar in their general shape, oval-cross section, and wrinkled crown surface, but differ in having smaller serrations, and flutes instead of ridges.
  In conjunction with their disappearing serrations, the cross sections of spinosaur teeth were initially blade-like and lenticular in earlier taxa like Ostafrikasaurus and Baryonyx, and became more circular in derived forms like Spinosaurus. The ornamentation of spinosaur tooth enamel, however, developed more irregularly than their serrations and cross section shape. Most non-avian theropods generally had smooth teeth with, at most, minimal wrinkles. Ostafrikasaurus teeth in contrast bore strong lengthwise ridges, a feature also seen in Baryonyx and its close relatives, though their prominence varies between taxa. Spinosaurus teeth, like those of typical theropods, were usually smooth, and Asian forms like Siamosaurus exhibited an increase in the amount of dental ridges. Furthermore, the tooth crown was wrinkled in Ostafrikasaurus, Baryonyx, and Suchomimus, and Asian spinosaurids, but smooth in Spinosaurus, with only some specimens of the latter showing fine wrinkles.
  In 2016, Spanish palaeontologist Aleandro Serrano-Martínez and colleagues described a possible spinosaurid tooth, catalogued as MUPE HB-87, from the Irhazer Shale of Niger. Found in association with a skeleton of the sauropod dinosaur Spinophorosaurus, the tooth likely dates to the Bathonian stage of the Middle Jurassic, 14 million years prior to Ostafrikasaurus. If this identification is correct, MUPE HB-87 represents the oldest known evidence of spinosaurids in the fossil record. Based on this specimen, the authors proposed a new evolutionary model for the transition between "normal" theropod teeth, to those of spinosaurids. The tooth is oval in cross section, is recurved, and bears small serrations, as well as strong fluting that does not reach the tip of the crown.
caption: Possible spinosaurid tooth from Middle Jurassic Niger in multiple views (D1, D2, D3, D4), and a closeup of the serrations (D5)
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