Strepsirrhini or Strepsirhini (; ) is a suborder of primates that includes the lemuriform primates, which consist of the lemurs of Madagascar, galagos ("bushbabies") and pottos from Africa, and the lorises from India and Southeast Asia. Collectively they are referred to as strepsirrhines.
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. Adapiforms are sometimes referred to as being "lemur-like", although the diversity of both lemurs and adapiforms does not support this comparison.
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. Their eyes contain a reflective layer to improve their night vision, and their eye sockets include a ring of bone around the eye, but they lack a wall of thin bone behind it.
Strepsirrhine primates produce their own vitamin C, whereas haplorhine primates must obtain it from their diets. 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.
Many of today's living strepsirrhines are endangered due to habitat destruction, hunting for bushmeat, and live capture for the exotic pet trade. Both living and extinct strepsirrhines are behaviorally diverse, although all are primarily arboreal (tree-dwelling). Most living lemuriforms are nocturnal, while most adapiforms were diurnal. Both living and extinct groups primarily fed on fruit, leaves, and insects.
Etymology The taxonomic name Strepsirrhini derives from the Greek strepsis "a turning round" and rhis "nose, snout, (in pl.) nostrils" (GEN rhinos),, . which refers to the appearance of the sinuous (comma-shaped) nostrils on the rhinarium or wet nose. The name was first used by French naturalist Étienne Geoffroy Saint-Hilaire in 1812 as a subordinal rank comparable to Platyrrhini (New World monkeys) and Catarrhini (Old World monkeys). In his description, he mentioned "Les narines terminales et sinueuses" ("Nostrils terminal and winding").
When British zoologist Reginald Innes Pocock revived Strepsirrhini and defined Haplorhini in 1918, he omitted the secondGreek rho ⟨ρ⟩ r is written with a spiritus asper when word-initial, i.e. ⟨ῥ⟩ rh, and often gets doubled to ⟨ῤῥ⟩ rrh (also spelled ⟨ρρ⟩) when such a word constitutes the second part of a compound word. See Ancient Greek phonology#Liquids, #Doubled consonants and #Consonant spelling. "r" from both ("Strepsirhini" and "Haplorhini" instead of "Strepsirrhini" and "Haplorrhini"), although he did not remove the second "r" from Platyrrhini or Catarrhini, both of which were also named by É. Geoffroy in 1812. Following Pocock, many researchers continued to spell Strepsirrhini with a single "r" until primatologists Paulina Jenkins and Prue Napier pointed out the error in 1987.
Evolutionary history
Strepsirrhines include the extinct adapiforms and the lemuriform primates, which include lemurs and lorisoids (lorises, pottos, and galagos). Strepsirrhines diverged from the haplorhine primates near the beginning of the primate radiation between 55 and 90 mya. Older divergence dates are based on genetic analysis estimates, while younger dates are based on the scarce fossil record. Lemuriform primates may have evolved from either cercamoniines or sivaladapids, both of which were adapiforms that may have originated in Asia. They were once thought to have evolved from adapids, a more specialized and younger branch of adapiform primarily from Europe.
Lemurs rafted from Africa to Madagascar between 47 and 54 mya, whereas the lorises split from the African galagos around 40 mya and later colonized Asia. The lemuriforms, and particularly the lemurs of Madagascar, are often portrayed inappropriately as "living fossils" or as examples of "basal", or "inferior" primates. These views have historically hindered the understanding of mammalian evolution and the evolution of strepsirrhine traits, such as their reliance on smell (olfaction), characteristics of their skeletal anatomy, and their brain size, which is relatively small. 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.
Unclear origin The divergence between strepsirrhines, simians, and tarsiers likely followed almost immediately after primates first evolved. Although few fossils of living primate groups – lemuriforms, tarsiers, and simians – are known from the Early to Middle Eocene, evidence from genetics and recent fossil finds both suggest they may have been present during the early adaptive radiation.
The origin of the earliest primates that the simians and tarsiers both evolved from is a mystery. Both their place of origin and the group from which they emerged are uncertain. Although the fossil record demonstrating their initial radiation across the Northern Hemisphere is very detailed, the fossil record from the tropics (where primates most likely first developed) is very sparse, particularly around the time that primates and other major clades of eutherian mammals first appeared.
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. Using this molecular clock, divergence dates for the major primate lineages have suggested that primates evolved more than 80–90 mya, nearly 40 million years before the first examples appear in the fossil record.
The early primates include both nocturnal and diurnal small-bodied species, and all were arboreal, with hands and feet specially adapted for maneuvering on small branches. Plesiadapiforms from the early Paleocene are sometimes considered "archaic primates", because their teeth resembled those of early primates and because they possessed adaptations to living in trees, such as a divergent big toe (hallux). Although plesiadapiforms were closely related to primates, they may represent a paraphyletic group from which primates may or may not have directly evolved, and some genera may have been more closely related to colugos, which are thought to be more closely related to primates.
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. These first primates included Cantius, Donrussellia, Altanius, and Teilhardina on the northern continents, as well as the more questionable (and fragmentary) fossil Altiatlasius from Paleocene Africa. These earliest fossil primates are often divided into two groups, adapiforms and omomyiforms. Both appeared suddenly in the fossil record without transitional forms to indicate ancestry, and both groups were rich in diversity and were widespread throughout the Eocene.
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. They disappeared from most of the Northern Hemisphere as the climate cooled: The last of the adapiforms died out at the end of the Miocene (~7 mya).
Adapiform evolution
Adapiform primates are extinct strepsirrhines that shared many anatomical similarities with lemurs. They are sometimes referred to as lemur-like primates, although the diversity of both lemurs and adapiforms do not support this analogy.
Like the living strepsirrhines, adapiforms were extremely diverse, with at least 30 genera and 80 species known from the fossil record as of the early 2000s. They diversified across Laurasia during the Eocene, some reaching North America via a land bridge.They were among the most common mammals found in the fossil beds from that time. A few rare species have also been found in northern Africa. The most basal of the adapiforms include the genera Cantius from North America and Europe and Donrussellia from Europe. The latter bears the most ancestral traits, so it is often considered a sister group or stem group of the other adapiforms.
Adapiforms are often divided into three major groups: Adapids were most commonly found in Europe, although the oldest specimens (Adapoides from middle Eocene China) indicate that they most likely evolved in Asia and immigrated. They died out in Europe during the Grande Coupure, part of a significant extinction event at the end of the Eocene. Notharctids, which most closely resembled some of Madagascar's lemurs, come from Europe and North America. The European branch is often referred to as cercamoniines. The North American branch thrived during the Eocene, but did not survive into the Oligocene. Like the adapids, the European branch were also extinct by the end of the Eocene. Sivaladapids of southern and eastern Asia are best known from the Miocene, and the only adapiforms to survive past the Eocene/Oligocene boundary (~34 mya). Their relationship to the other adapiforms remains unclear. They had vanished before the end of the Miocene (~7 mya).
The relationship between adapiform and lemuriform primates has not been clearly demonstrated, so the position of adapiforms as a paraphyletic stem group is questionable. 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. New calibration methods may reconcile the discrepancies between the molecular clock and the fossil record, favoring more recent divergence dates. The fossil record suggests that the strepsirrhine adapiforms and the haplorhine omomyiforms had been evolving independently before the early Eocene, although their most basal members share enough dental similarities to suggest that they diverged during the Paleocene (66–55 mya).
Lemuriform evolution
Lemuriform origins are unclear and debated. American paleontologist Philip Gingerich proposed that lemuriform primates evolved from one of several genera of European adapids based on similarities between the front lower teeth of adapids and the toothcomb of extant lemuriforms; however, this view is not strongly supported due to a lack of clear transitional fossils. Instead, lemuriforms may be descended from a very early branch of Asian cercamoniines or sivaladapids that migrated to northern Africa.
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. These newer finds demonstrate that lemuriform primates were present during the middle Eocene in Afro-Arabia and that the lemuriform lineage and all other strepsirrhine taxa had diverged before then. Djebelemur from Tunisia dates to the late early or early middle Eocene (52 to 46 mya) and has been considered a cercamoniine, but also may have been a stem lemuriform. Azibiids from Algeria date to roughly the same time and may be a sister group of the djebelemurids. Together with Plesiopithecus from the late Eocene Egypt, the three may qualify as the stem lemuriforms from Africa.