Lemuridotherium madagascariense Standing, 1910 Archaeoindris fontoynontii is an extinct giant lemur and the largest primate known to have evolved on Madagascar, comparable in size to a male gorilla.

Archaeoindris was first described by Herbert F. Standing in 1909 on the basis of subfossil fragmentary jaws, although Charles Lamberton later discovered a complete skull.

Size estimates based on the limited remains have varied widely, ranging as high as 244 kilograms (538 pounds), but the most thorough statistical investigation using regression analyses predicts a mass of 160 kg (350 lb).

Misattributions and limited remains have resulted in varying opinions about the way Archaeoindris moved in its environment, ranging from tree-dwelling to ground-dwelling.

The diet of Archaeoindris was mostly leaves, and its habitat—prior to human arrival—was a mix of woodlands, bushlands, and savanna, rich in lemur diversity.

Today, the region is dominated by grasslands and lemur diversity is very low in the nearest protected area, Ambohitantely Special Reserve.

[3] The species name, fontoynontii (sometimes spelled fontoynonti), was selected in honor of Antoine Maurice Fontoynont, the president of the Académie Malgache (Malagasy Academy) at the time.

Sixteen years after Standing's discovery, Charles Lamberton discovered the first and only complete cranium (skull) and associated mandible for Archaeoindris, both of which were well preserved.

[15] Historically, some remains from other subfossil lemurs have been mistakenly attributed to Archaeoindris, resulting in incorrect interpretations of its anatomy and behavior.

[12] In 1934, Lamberton missed earlier attribution errors[16] and incorrectly labeled a tibia and two fibulae (lower leg bones) from a species of koala lemur (Megaladapis grandidieri) as belonging to Archaeoindris.

[12][18] In 1910, twenty-four years before Lamberton's monograph on Archaeoindris, Standing identified a massive right femur from Ampasambazimba as a new species, Lemuridotherium madagascariense.

Although Standing recognized the strong similarities between Lemuridotherium and Archaeoindris, he placed them in separate genera due to what he perceived as a great size difference.

[18] Vuillaume-Randriamanantena also established associations between the postcrania and crania of Archaeoindris,[16] summarized what is known about the postcranial skeleton, and documented the strong similarity with the genus Palaeopropithecus.

The ratio of its orbit area to the size of its optic canal indicates that Archaeoindris had low retinal summation, meaning its eyes were as sensitive to light as those of living diurnal lemurs.

[35] Its fused mandibular symphyses and the likelihood of accelerated dental development suggest that it began processing fibrous foods in the mouth with thorough chewing.

[37] Both Standing and Lamberton assumed Archaeoindris to be a slow-moving tree-dweller like Megaladapis, primarily due to the misattributed bones.

Lamberton also speculated that it would have resembled a ground sloth—a view later supported by Jungers in 1980 after several misattributions had been corrected and having considered its gorilla-like size.

It is described as a deliberate, scansorial (climbing) browser, and it is unknown whether it was like Palaeopropithecus in performing hang-feeding since hand and foot bones are missing.

[40] Prior to human arrival, the area around Ampasambazimba was not completely forested, but more of an open habitat, consisting of a mix of woodlands, bushlands, and savanna.

[41] Animal remains at this subfossil site have yielded about 20 species of lemur living in sympatry (sharing the same geographic area).

In comparison, the nearby Ambohitantely Special Reserve today contains only four species, roughly 20% of the area's original lemur diversity.

[43] From these dates, it is likely that Archaeoindris was still alive on the high plateau in 350 BCE when the first humans reached the west coast of Madagascar,[44] despite being rare by that time.