The Mascarene teal (Anas theodori), also known as the Mauritius duck and Sauzier's teal, is an extinct species of duck that lived on the Mascarene Islands of Mauritius and Réunion. A small duck was mentioned in 17th century accounts from these islands which is thought to be this species, and it may also be depicted in one illustration. In 1893, various bones from the Mare aux Songes swamp of Mauritius were used to name a new species in the duck genus Anas, A. theodori, after Théodore Sauzier [de] who directed the excavations. Additional remains from Réunion were considered to belong to the same species in 1999, and it was determined to be related to the Sunda teal of the Indian Ocean.

The Mascarene teal was a small duck; in comparison with the extant ducks of Madagascar, it was larger than Bernier's teal, but smaller than Meller's duck. While overall most similar to the Sunda teal, the two differ in details of the sternum and lower leg bones. One contemporary account described the bird as "grey". Based on the proportions of its limb-bones, it had normal flight capabilities and was able to fly between Mauritius and Réunion, explaining how it occurred on both islands. It possibly nested in tree holes similar to related teals. The species was listed among the favourite prey of hunters; while abundant in 1681, it declined quickly thereafter, becoming extinct on Mauritius around 1700 and on Réunion a decade later. It was probably driven to extinction due to overhunting and predation by introduced animals, particularly cats.

A few inadequate accounts mentioned a small duck on the Mascarene Islands of Mauritius and Réunion in the 17th century. In 1889, the Mauritius government requested exploration of the Mare aux Songes swamp for "historical souvenirs", where vast amounts of dodo remains had earlier been found. The new excavations, under the direction of the French naturalist Théodore Sauzier [de], were successful, and apart from dodo bones, subfossil remains of other extinct animals, previously known as well as new species, were found. These bones were sent to the Cambridge Museum, where they were examined and described by the British ornithologist Edward Newton and the German ornithologist Hans Gadow in 1893. Based on the front part of a sternum (breast bone), a pair of coracoids (part of the shoulder), eight humeri (upper arm bones), and a pair of tarsometatarsi (lower leg bones) in bad condition, they determined the existence of a duck differing from those of Madagascar while being similar to Meller's duck (Anas melleri) of that island, but smaller. They named it as a new species in the duck genus Anas, A. theodori, in honour of Sauzier.

In 1987, the British ornithologist Graham S. Cowles reported a hitherto undescribed skull of a duck from the collection of Louis Étienne Thirioux, an amateur naturalist from Mauritius. As it differed from any other extant ducks from the island, it was suggested to be the first known skull of the species described by Newton and Gadow. In the same publication, the British ecologist Anthony S. Cheke suggested that a 1674 mention of ducks on Réunion by the French traveller Sieur Dubois could refer to this species, though the garganey (Spatula querquedula) also regularly visits the island in small numbers. He also pointed out that two reports mention sarcelles (teals) and canards (wild ducks) in addition to geese on Réunion, which may indicate there were two species of duck. The British ornithologist Hywel Glyn Young stated in 1996 that the exact relations of the species was uncertain, though it could be a southern teal (term for a small, wild duck) or a mallard.

In 1999, the French palaeontologist Cécile Mourer-Chauviré and colleagues reported the front part of a sternum, an ulna (a lower arm bone), and a tibiotarsus (a lower leg bone) from the Marais de l’Ermitage swamp on Réunion. By comparing these with bones of the Mascarene teal held in the National Museum of Natural History in France, which include a coracoid, four humeri, an incomplete juvenile carpometacarpus (part of the hand), a tibiotarsus, and two juvenile tarsometatarsi, they assigned the Réunion bones to the Mascarene teal, since the tibiotarsi were identical (the assigned sternum was too poorly preserved for comparison). By comparing the known bones of the Mascarene teal with those of other ducks, Mourer-Chauviré and colleagues found it to be most similar to the Sunda teal (Anas gibberifrons) from the East and North East of the Indian Ocean, while larger and smaller than the extant ducks of Madagascar, Bernier's teal (Anas bernieri) and Meller's duck, respectively.

Mourer-Chauviré and colleagues noted that Réunion lacks the kinds of distinctive flightless birds found on Mauritius such as the dodo, and that since Réunion is three million years old, this would be long enough for birds to lose their flight ability to the extent that it can be detected in their skeletons. They suggested that though members of the same birds groups initially colonised both Mauritius and Réunion, many of them evolving flightlessness, these species disappeared from Réunion due to volcanic eruptions of Piton des Neiges between 300,000 and 180,000 years ago. Thereafter the island would have been recolonised by flighted species like Mascarene teals from Mauritius, with none of them having time to become flightless since. They also reported two anatid (the family of ducks, geese, and swans) carpometacarpi from Réunion that differed from those of Anas, but were similar to those of Aythya.

In 2008, Cheke and the British palaeontologist Julian P. Hume noted that while the Mascarene teal was similar to both the Sunda teal and Bernier's teal, the latter is brown, while one description indicates the Mascarene species was grey like the Sunda species; these species belong to a group called "grey teals", indicating dispersal from the east. Several of these types were previously thought to be races of the Sunda teal, but are now considered full species. Cheke and Hume raised the possibility that the Sunda teal bones Mourer-Chauviré and colleagues compared with the Mascarene teal actually belonged to the Australian grey teal (A. gracilis). These writers suggested the ambiguous account of two kinds of ducks on Réunion could be explained by the possible Aythya remains, which they considered similar enough to the extant Madagascar pochard (Aythya innotata) that it could be that species.

The Mascarene teal was a small duck, and in comparison with the extant ducks of Madagascar, it was larger than Bernier's teal but smaller than Meller's duck. Based on the known bones, the sternum was 27.7 mm (1 in) wide and 21.7 mm (1 in) deep, the coracoid 41.1–42.5 mm (2–2 in) long, the humerus 69.6–73.1 mm (3–3 in) long, the ulna 63 mm (2 in) long, the tibiotarsus 71 mm (3 in) long, and the tarsometatarsus 42 mm (2 in) long.