Oxalaia (in reference to the African deity Oxalá) is a controversial genus of spinosaurid dinosaur that lived in what is now the Northeast Region of Brazil during the Cenomanian stage of the Late Cretaceous period, sometime between 100.5 and 93.9 million years ago.

[1][2] Outcropping at the northern coast of the formation, the Laje do Coringa locality is made up mostly of sandstones and mudstones, along with conglomerate rock layers containing fossil plant and vertebrate fragments.

[5] Palaeontologist Elaine Machado, of the National Museum of Rio de Janeiro, was surprised to find such a well-preserved fossil at the site and stated in a press release that "this is how most scientific discoveries happen, it was by accident".

[6] The finding was a rare occurrence due to the erosive nature of the tides at the deposit, which are responsible for the fragmented state of most fossils in the bone bed; remains not found on site are often removed from the formation by wave action.

[5] Generally, the majority of fossil remains found at the Alcântara Formation consist of teeth and isolated skeletal elements, of which the Laje do Coringa site has yielded hundreds.

[10][11] The holotype specimen of Oxalaia quilombensis, designated MN 6117-V, was found in situ (at its original place of deposition) with part of the left side embedded in the rock matrix; it consists of the fused premaxillae (frontmost snout bones) from a large individual.

[18] The species description of Oxalaia was written by Brazilian palaeontologists Alexander Kellner, Elaine Machado, Sergio Azevedo, Deise Henriques, and Luciana Carvalho.

[22] The rostrum of Oxalaia features broad, deep foramina (holes) that are possibly nutrient canals for blood vessels and nerves; it is also rounder in side view than that of Spinosaurus, whose upper jaw ends in a more acute downward angle as shown by specimens MSNM V4047 and MNHN SAM 124.

The maxillae show a pair of elongated and thin processes extending forwards along the midline of the roof of the mouth; they are encased between the premaxillae and border an elaborate, triangle-shaped pit at their front end.

Small fragments inside some of the remaining alveoli show that unlike its Early Cretaceous relatives Suchomimus and Cristatusaurus, Oxalaia lacked serrations on its teeth.

Kellner and colleagues differentiated Oxalaia from it and other spinosaurids by its autapomorphic (distinguishing) craniodental features, like its sculptured palatal part of the premaxillae, and the possession of two replacement teeth in each position.

The habit of naming theropods from isolated teeth or tooth fragments has resulted in many invalid and synonymous genera; it has also occurred with spinosaurids and is compounded by the common lack of overlapping skeletal remains—a precondition of validly distinguishing taxa.

In 2021, Lacerda, Grillo and Romano noted that the anteromedial processes of the holotype maxillae (MN 6117-V) contact medially, a condition not observed in MSNM V4047 which has been referred to as a specimen of Spinosaurus, and thus adding a new possible diagnostic feature of Oxalaia.

[29] In 2023, Isasmendi and colleagues considered Oxalaia as a valid taxon based on the examination of its referred maxilla (MN 6119-V) which suggests that the position of its external naris would have been more anteriorly located, a condition similar to that of Irritator and baryonychines, differing from Spinosaurus aegyptiacus.

[1][32] Dinosaur fossil remains suggest the presence of diplodocoids like Itapeuasaurus cajapioensis, basal titanosaurs, a giant Carcharodontosaurid sp., a noasaurid closely related to Masiakasaurus, and a dromaeosaurid.

According to Medeiros and colleagues, the Laje do Coringa assemblage may also be linked to the contemporaneous Bahariya Formation in Egypt, which holds a distinct combination of key taxa constituting Spinosaurus aegyptiacus, Carcharodontosaurus saharicus, and Onchopristis numidus.

This extreme similarity between the Cretaceous biota of Brazil with that of Africa is a result of their connection as parts of the supercontinent Gondwana (which comprised most landmasses of the modern southern hemisphere).

Such behavior is observed in cases such as juvenile Iguanodontid bones found in the stomach cavity of a Baryonyx fossil and an Irritator tooth embedded in pterosaur remains.

[10][36] The conical, transversely oval-shaped teeth of Oxalaia and its nasal openings, that were retracted further back on the skull than in most theropods (likely to avoid water entering its nostrils while fishing) are characteristic of spinosaurids.

[5][10][22] The expanded, interlocking front jaws and piercing teeth of spinosaurs worked as an efficient fish trap, a trait also exhibited by the Indian gharial—the most piscivorous extant crocodilian.