Afrasia Djijidae

''Afrasia djijidae'' is a fossil primate that lived in Myanmar approximately 37 million years ago, during the late middle Eocene. The only species in the genus ''Afrasia'', it was a small primate, estimated to weigh around . Despite the significant geographic distance between them, ''Afrasia'' is thought to be closely related to ''Afrotarsius'', an enigmatic fossil found in Libya and Egypt that dates to 38–39 million years ago. If this relationship is correct, it suggests that early simians (a related group or clade consisting of monkeys, apes, and humans) dispersed from Asia to Africa during the middle Eocene and would add further support to the hypothesis that the first simians evolved in Asia, not Africa. Neither ''Afrasia'' nor ''Afrotarsius'', which together form the family Afrotarsiidae, is considered ancestral to living simians, but they are part of a side branch or stem group known as eosimiiforms. Because they did not give rise to the stem simians that are known from the same deposits in Africa, early Asian simians are thought to have dispersed from Asia to Africa more than once prior to the late middle Eocene. Such dispersals from Asia to Africa also were seen around the same time in other mammalian groups, including hystricognathous rodents and anthracotheres.

''Afrasia'' is known from four isolated molar teeth found in the Pondaung Formation of Myanmar. These teeth are similar to those of ''Afrotarsius'' and Eosimiidae, and differ only in details of the chewing surface. For example, the back part of the third lower molar is relatively well-developed. In the Pondaung Formation, ''Afrasia'' was part of a diverse primate community that also includes the eosimiid '' Bahinia'' and members of the families Amphipithecidae and Sivaladapidae.

Etymology

The name ''Afrasia'' derives from the continental names "Africa" and "Asia", and refers to the occurrence of eosimiiform primates on both continents. The species, ''A. djijidae'', was named in memory of a young girl from Mogaung village in central Myanmar.

Evolutionary history

''Afrasia djijidae'' was first described in 2012 on the basis of isolated teeth from the 37-million-year-old Pondaung Formation, which is close to the village of Nyaungpinle in Myanmar. Prior to the discovery of ''A. djijidae'', early Asian simians were typically classified in two families, Eosimiidae and Amphipithecidae. While eosimiids are generally considered the most basal simian clade (a stem group of simians), the phylogenetic placement of amphipithecids is more disputed. Amphipithecids are usually considered to share affinities with either eosimiids or crown simians (those simians that are part of the smallest clade that contains the living simians); the latter view is favored.

Eosimiids were first described from findings in China in 1994 and are still best known there (two genera are now known, ''Eosimias'' and '' Phenacopithecus''), but have also been recorded in Pakistan (''Phileosimias'') and Myanmar ('' Bahinia''). All species had a small body size and a mix of primitive (ancestral) and derived traits. The largest eosimiid, ''Bahinia'', is from the Pondaung Formation, the same stratum as ''Afrasia'', and the morphology of its molars bridges the gap between the more primitive molars of ''Eosimias'' and the more derived molars of the later Eocene African simians. ''Afrasia'', on the other hand, is more comparable in size to the eosimiids from China.

The upper molars of ''Afrasia'' are nearly identical in morphology and size to those of ''Afrotarsius'', an animal known from 38- to 39-million-year-old deposits in Libya (species ''Afrotarsius libycus'') and about 30-million-year-old deposits in Egypt (''A. chatrathi''). ''Afrotarsius'' was originally described as a tarsier, but later suggested to be related to primitive simians. Because of their close similarities, ''Afrasia'' and ''Afrotarsius'' are together placed in the family Afrotarsiidae. A phylogenetic analysis placed ''Afrasia'' as a sister group to ''Afrotarsius'', forming the family Afrotarsiidae. Afrotarsiidae was found to be most closely related to Eosimiidae, and unrelated to tarsiers. The clade formed by Afrotarsiidae and Eosimiidae was designated as the infraorder Eosimiiformes by Chaimanee ''et al.'' in 2012. Eosimiiformes are the sister group of crown simians. However, the classification of ''Afrotarsius'' as a stem simian is not accepted by all researchers, and the phylogenetic claims and their implications rest on only the four molars of ''Afrasia'' initially discovered.

Biogeography

Several hypotheses compete to explain the geographic origins of simians. Of the two most popular views, one hypothesis states that simians originated in Asia, while the other favors Africa as their place of origin. Both their ancient origins and the sparse fossil record in Africa and Asia complicate the debate. Convergent evolution also makes it difficult for researchers to determine the relationships between fossils. Also, the earliest fossils, which do not match up with molecular clock estimates for simian origins, have been found in both Afro-Arabia and Asia. At the time, both landmasses would have been separated by the Tethys Sea.

The oldest undisputed simians from Africa also date to the late middle Eocene. ''Afrotarsius'' was found in the Dur At-Talah escarpment in Libya, which dates to 38–39 million years ago and also contains the parapithecid ''Biretia'' and the oligopithecid '' Talahpithecus''. Although diverse, these primates were all very small in size, much like eosimiids. Simian classification for older fossils from Africa are more contested. ''Altiatlasius'', the oldest suspected fossil primate known from the late Paleocene of Morocco, is too fragmentary to ascertain its true affinities. ''Algeripithecus'' was popularly considered an early middle Eocene simian; however, less fragmentary remains uncovered between 2003 and 2009 suggest it was a type of azibiid, a group of strepsirrhine primates likely related to lemuriform primates (lemurs and lorisoids).

According to the team who discovered ''Afrasia'', headed by Jean-Jacques Jaeger, the similarities in tooth morphology, body size, and age suggest a phylogenetic relationship with ''Afrotarsius'' despite the geographic separation. They suggested that afrotarsiid simians from Asia dispersed across the Tethys Sea to Africa around the middle Eocene, giving rise to ''Afrotarsius''. A reverse migration—as suggested by the hypothesis of the African origin for simians—is not favored due to the close phylogenetic relationship between the older Asian eosimiids and ''Afrasia''. However, to explain the origins of Proteopithecidae, Parapithecidae, and crown simians from the African fossil record, a higher simian clade also needed to disperse from Asia to Africa before the late middle Eocene since neither ''Afrasia'' nor ''Afrotarsius'' could have given rise to this group. Such dispersals from Asia to Africa were not uncommon, since hystricognathous rodents, anthracotheres, and possibly other mammalian fauna from Asia arrived around the same time.

Anatomy

''Afrasia'' is known from four isolated teeth: a first upper molar (M1), with a length of 2.13 mm and width of 3.06 mm; a second upper molar (M2), the holotype, of 2.27 mm by 3.35 mm; a second lower molar (m2) of 2.09 mm by 1.41 mm; and a third lower molar (m3) of 2.05 mm by 1.27 mm. Like other eosimiiforms, it was a small primate that weighed around , which is estimated based on the measurements of its m2 and M1, respectively. It is generally similar to other eosimiiforms in general and ''Afrotarsius'' in particular; some distinguishing features are the well-developed small cusps (conules) on the upper molars, the variable development of the lingual cingulum (a shelf on the inner, or lingual, side of the tooth) of the upper molars, and the unreduced talonid (posterior portion) on the m3. Its teeth are smaller than those of the eosimiids ''Phenacopithecus'', ''Bahinia'', and ''Phileosimias''.

The M2 is similar to that of eosimiids and almost identical to that of ''Afrotarsius libycus''. It shows well-developed small accessory cusps (the paraconule and metaconule) that are connected to equally well-developed crests. The conules are stronger than in ''Eosimias'' and ''Bahinia'', but weaker than in ''Phileosimias''. The stylar shelf, the outer area of the tooth, is broad, particularly at the metacone cusp. However, the shelf is better developed near the paracone

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