Also belonging to the suborder are the extinct adapiform primates which thrived during the Eocene in Europe, North America, and Asia, but disappeared from most of the Northern Hemisphere as the climate cooled.

Strepsirrhines are defined by their "wet" (moist) rhinarium (the tip of the snout) – hence the colloquial but inaccurate term "wet-nosed" – similar to the rhinaria of canines and felines.

They also have a smaller brain than comparably sized simians, large olfactory lobes for smell, a vomeronasal organ to detect pheromones, and a bicornuate uterus with an epitheliochorial placenta.

Lemuriform primates are characterized by a toothcomb, a specialized set of teeth in the front, lower part of the mouth mostly used for combing fur during grooming.

The taxonomic name Strepsirrhini derives from the Greek στρέψις strepsis "a turning round" and ῥίς rhis "nose, snout, (in pl.)

In the case of lemurs, natural selection has driven this isolated population of primates to diversify significantly and fill a rich variety of ecological niches, despite their smaller and less complex brains compared to simians.

Lacking detailed tropical fossils, geneticists and primatologists have used genetic analyses to determine the relatedness between primate lineages and the amount of time since they diverged.

[25] The early primates include both nocturnal and diurnal small-bodied species,[26] and all were arboreal, with hands and feet specially adapted for maneuvering on small branches.

[29] The first true primates (euprimates) do not appear in the fossil record until the early Eocene (~55 mya), at which point they radiated across the Northern Hemisphere during a brief period of rapid global warming known as the Paleocene–Eocene Thermal Maximum.

[23] These first primates included Cantius, Donrussellia, Altanius, and Teilhardina on the northern continents,[30] as well as the more questionable (and fragmentary) fossil Altiatlasius from Paleocene Africa.

The last branch to develop were the adapiforms, a diverse and widespread group that thrived during the Eocene (56 to 34 million years ago [mya]) in Europe, North America, and Asia.

Both molecular clock data and new fossil finds suggest that the lemuriform divergence from the other primates and the subsequent lemur-lorisoid split both predate the appearance of adapiforms in the early Eocene.

[22][45] Until discoveries of three 40 million-year-old fossil lorisoids (Karanisia, Saharagalago, and Wadilemur) in the El Fayum deposits of Egypt between 1997 and 2005, the oldest known lemuriforms had come from the early Miocene (~20 mya) of Kenya and Uganda.

Confused taxonomic terminology and oversimplified anatomical comparisons have created misconceptions about primate and strepsirrhine phylogeny, illustrated by the media attention surrounding the single "Ida" fossil in 2009.

The use of the tarsier-galago classification continued for many years until 1898, when Dutch zoologist Ambrosius Hubrecht demonstrated two different types of placentation (formation of a placenta) in the two groups.

[11][53] It was not until 1953, when British anatomist William Charles Osman Hill wrote an entire volume on strepsirrhine anatomy, that Pocock's taxonomic suggestion became noticed and more widely used.

[69] The idea reemerged briefly in 2009 during the media attention surrounding Darwinius masillae (dubbed "Ida"), a cercamoniine from Germany that was touted as a "missing link between humans and earlier primates" (simians and adapiforms).

[32] The first fossil primate described was the adapiform Adapis parisiensis by French naturalist Georges Cuvier in 1821,[49] who compared it to a hyrax ("le Daman"), then considered a member of a now obsolete group called pachyderms.

One of these two European forms was identified as cercamoniines, which were allied with the notharctids found mostly in North America, while the other group falls into the traditional adapid classification.

[99] These differences give strepsirrhines the ability to make more complex rotations of the ankle and indicate that their feet are habitually inverted, or turned inward, an adaptation for grasping vertical supports.

[102] Some adapiforms were sexually dimorphic, with males bearing a larger sagittal crest (a ridge of bone on the top of the skull to which jaw muscles attach) and canine teeth.

The rhinarium, upper lip, and gums are tightly connected by a fold of mucous membrane called the philtrum, which runs from the tip of the nose to the mouth.

[106][109] The strepsirrhine rhinarium can collect relatively non-volatile, fluid-based chemicals (traditionally categorized as pheromones) and transmit them to the vomeronasal organ (VNO),[110] which is located below and in front of the nasal cavity, above the mouth.

[111] Fluids traveling from the rhinarium to the mouth and then up the nasopalatine ducts to the VNO are detected, and information is relayed to the accessory olfactory bulb, which is relatively large in strepsirrhines.

[124] In females, the clitoris is sometimes enlarged and pendulous, resembling the male penis, which can make sex identification difficult for human observers.

[127] Approximately three-quarters of all extant strepsirrhine species are nocturnal, sleeping in nests made from dead leaves or tree hollows during the day.

[143] The European adapids Adapis, Palaeolemur, and Leptadapis shared adaptations for slow climbing like the lorises, although they may have been quadrupedal runners like small New World monkeys.

[27] The threats facing strepsirrhine primates fall into three main categories: habitat destruction, hunting (for bushmeat or traditional medicine), and live capture for export or local exotic pet trade.

In 2012, the International Union for Conservation of Nature (IUCN) announced that lemurs were the "most endangered mammals", due largely to elevated illegal logging and hunting following a political crisis in 2009.

[149] In Southeast Asia, slow lorises are threatened by the exotic pet trade and traditional medicine, in addition to habitat destruction.