Oxalaia

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Oxalaia quilombensis: Brazil’s giant spinosaur

Oxalaia quilombensis is a large meat-eating dinosaur from Brazil that lived during the Cenomanian age of the Late Cretaceous, about 100.5 to 93.9 million years ago. It belonged to a group of dinosaurs called spinosaurids, known for long skulls and teeth adapted for catching fish. Oxalaia’s fossils come from Cajual Island in the Alcântara Formation, in the northeastern state of Maranhão. The rocks there were deposited in a warm, forested landscape near water, with some dry periods.

Discovery and fossils
Fossils of Oxalaia were found in 1999 and described in 2011 by paleontologist Alexander Kellner and colleagues. The holotype consists of part of the snout: fused premaxilla bones from a large individual (MN 6117-V). An additional fragment from the upper jaw (MN 6119-V) was also attributed to Oxalaia. These two skull pieces are the main basis for recognizing Oxalaia as a separate dinosaur. The site on Cajual Island has yielded many teeth and fragments, but complete skeletons are rare.

Size and appearance
Based on the skull bones and comparisons with related dinosaurs, Oxalaia’s skull was about 1.35 meters long. Estimates for the whole animal put it at about 12 to 14 meters (40 to 46 feet) in length, and it likely weighed around 5 to 7 tonnes. That would make Oxalaia one of the largest carnivorous dinosaurs found in Brazil.

Unique features and diet
Oxalaia stood out from other spinosaurids because of several distinctive skull and tooth traits. It had two replacement teeth in each tooth socket, which is unusual for theropods. The palate near the roof of the mouth was heavily sculptured, and the upper jaw (premaxilla and maxilla) showed a set of unusual structures that helped form a strong, broad snout. Its teeth were conical and mostly had no serrations, a trait common to fish-eaters. The openings for nerves and blood vessels in the snout were unusually large, and the snout itself was relatively broad and rounded from the side, a shape different from some relatives like Spinosaurus. Taken together, these features indicate a partially fish-eating (piscivorous) lifestyle, with the forelimbs and tail likely helping in the water.

Classification and relationships
Oxalaia is placed in the spinosaurid family, specifically the Spinosaurinae, which also includes Spinosaurus. Its exact relationship to Spinosaurus and other Brazilian spinosaurids has been debated:

- Some analyses suggest Oxalaia is a close relative or even a sister form to Spinosaurus, and that Brazil’s Oxalaia and Angaturama were among the closest Brazilian kin to Spinosaurus.
- Other studies have produced different results, with Oxalaia placed in broader groups with Irritator or other spinosaurines, or even considered a possible synonym of Spinosaurus in some analyses.
- By the early 2020s, most researchers viewed Oxalaia as a valid taxon, though some still debate whether it might be the same species as Spinosaurus aegyptiacus. New comparisons of the skull pieces and more fossil material continue to refine its position.

Paleoenvironment and biogeography
The Alcântara Formation records a humid, tropical environment with forests, near-shore habitats, and an arid-to-semi-arid landscape. A diverse ecosystem lived there, including other dinosaurs, crocodilians, fish, pterosaurs, and plants. The fauna shows striking similarities to contemporaneous Africa, which reflects the ancient connection of the continents as part of Gondwana. Over time, sea-level changes and tectonic shifts separated South America from Africa, leading to regional differences in dinosaur life.

What happened to Oxalaia fossils
In 2018, a fire at the National Museum of Rio de Janeiro damaged many fossils, including Oxalaia material. There has been recovery of some damaged remains, and researchers continue studying the available material to better understand this dinosaur. The name Oxalaia honors the African deity Oxalá, and quilombensis refers to the quilombo settlements like Cajual Island where its fossils were found.

Why Oxalaia matters
Oxalaia helps show how diverse spinosaurids were in South America and how closely connected ancient dinosaur faunas were across Gondwana. Even with only a couple of skull bones, scientists can learn a lot about its biology, likely lifestyle near water, and how it fits into the story of dinosaur evolution on the ancient supercontinent.