Pachylemur is thought to have gone extinct between 680 and 960 CE, although subfossil remains found in a cave pit in southwestern Madagascar may indicate that it survived up until 500 years ago.

In 1899, Guillaume Grandidier named a new genus and species, Palaeochirogalus jullyi,[a] on the basis of two teeth from Antsirabe, central Madagascar, which he thought similar to dwarf lemurs (Cheirogaleus).

However, both names are based on material from the central plateau of Madagascar and Tattersall therefore presumed that they belong to the same species, which he could continue to call Lemur jullyi.

[22] First, Filhol had himself used the name Pachylemur in 1874 for a group of primitive primates, including Adapis, that he considered intermediate between pachyderms and lemurs.

[25] To conserve the name Pachylemur, Jelle Zijlstra, Colin Groves, and Alex Dunkel submitted a petition to the International Commission on Zoological Nomenclature in 2011.

P. insignis had narrower lower premolars and molars, and the buccal (outer) cusps on these teeth are located to the front of their lingual counterparts.

[3] The skull of Pachylemur is relatively broad, but the orbits (eye sockets) are smaller and oriented more towards the front than in the ruffed lemurs.

It had shorter and more robust limbs than the ruffed lemurs, and the fore- and hindlimbs were closer in length (intermembral index of approximately 97).

[4] Compared to the axial skeleton of ruffed lemurs, the vertebrae of Pachylemur had shorter vertebral bodies and the spinous process had less anticliny.

[31] Within the spiny thickets of southwest Madagascar, only P. insignis and Archaeolemur majori, a type of extinct monkey lemur, are suspected of having been large-seed dispersers, particularly for plants that use a form of photosynthesis known as C3 carbon fixation.

[37] The plants that may have depended on these giant extinct lemurs include Adansonia (baobabs), Cedrelopsis, Commiphora, Delonix, Diospyros, Grewia, Pachypodium, Salvadora, Strychnos, Tamarindus, and Uncarina.

[39] Many small trees and shrubs in the spiny thickets, such as endemic Uncarina, conserve water by producing seeds with hooks and spines rather than fleshy fruits.

[2][29] Like the ruffed lemurs, Pachylemur was also an arboreal quadruped that frequently exhibited hindlimb suspension in order to reach fruit and leaves on smaller branches.

However, Pachylemur was a slow, deliberate climber unlike the ruffed lemurs, which leap and bound through the upper canopy.

[41][42] Like both the living and extinct lemurs, Pachylemur likely conserved energy because of its diet, small brain, and slow climbing.

[45] The two species are typically found in the spiny thickets and succulent woodlands of southern/southwestern Madagascar (P. insignis) and the subhumid forests of the central highlands (P. jullyi), although other indeterminate or fragmentary remains have been discovered at Ankilitelo Cave in southwestern Madagascar,[46] as well as in the dry deciduous forests at Amparihingidro in the northwest (possibly P. insignis) and Ankarana in the northern tip of the island (possibly P. jullyi).

[47] Subfossil sites with P. insignis include Andolonomby,[48] Beloha (near Anavoha), Bemafandry, Andrahomana, Manombo-Toliara, Ambolisatra, Ambararata-Mahabo, Ampoza-Ankazoabo, Belo-sur-mer, Lamboharana, Taolambiby, Tsiandroina, and Tsirave in south and southwestern Madagascar.

[1] In general, lemur diversity has declined since the arrival of humans due to habitat loss, forest fragmentation, and bushmeat hunting.

Instead, many human-related factors, such as habitat loss, forest fragmentation, bushmeat hunting, and the introduction of invasive species, along with the gradual desiccation of certain parts of the island, caused their decline and eventual extinction over more than a millennium.

[51] Large lemurs, including Pachylemur, survived in the Central Highlands, succulent woodlands, and spiny thickets until around 950 CE.