The Tertiary Fossil Record Outside Madagascar

The earliest fossil primate that undisputably bears a toothcomb is Karanisia clarki, described from a small sample of isolated teeth and jaw fragments by Seiffert et al. (2003). These specimens, which include a canine crown indicating the presence of a toothcomb, come from late middle Eocene (probably ca. 40 myr old) sediments of the Birket Qarun Formation, in the Egyptian Fayum. Karanisia is interpreted by its describers as dentally not only lorisiform but lorisid, possibly representing a sister genus to the living West African genus Arctocebus. Two teeth from the same stratigraphic were assigned to the galagid genus Saharagalago mis-rensis. Seiffert and his coauthors believe that these Fayum fossils establish the divergence of the two living non-Malagasy strepsirhine families by the mid-to-late Eocene.

Marivaux et al. (2001) allocated several isolated teeth from the Bugti Hills of Pakistan to the new species Bugtilemur mathesoni, in which a lower canine is said to confirm the presence of a toothcomb. Marivaux et al. assigned these very tiny early Oligocene (ca. 30 myr old) fossils to the Malagasy family Cheirogaleidae on the basis of cheektooth morphology, suggesting transfer via a putative sweepstakes/filter route involving a Chagos/Laccadive paleoridge system. Within Cheirogaleidae, Marivaux and colleagues most closely compared the molars of Bugtilemur to those of Cheirogaleus, and morphologically the resemblance is indeed remarkable. However, the living Malagasy genus is much larger in body size than the fossil one is, and it has a much longer and slenderer toothcomb than Bugtilemur apparently had.

The molar morphology of Bugtilemur is particularly interesting given that it has been argued that the cheirogaleid lemurs may in fact be more closely related to the Afro-Asian strepsirhines than to the other Malagasy lemurs (e.g., Szalay and Katz, 1973; Schwartz and Tattersall, 1985). In 1970 Charles-Dominique and Martin drew attention to the suite of behavioral similarities uniting the cheirogaleids and lorisoids. Charles-Dominique and Martin were content to regard these similarities as ancestral retentions; but Szalay and Katz (1973) proposed that many characters shared between lorisoids and cheirogaleids are in fact derived, and that in consequence the cheirogaleids and lorisoids are more closely related to each other than the cheirogaleids are to the other lemurs. Cartmill (1975) later added other features to the list of apparent cheirogaleid-lorisid synapomorphies. And a few years later Schwartz and Tattersall (1985) pointed to evidence from molar morphology that also supports this association. In addition, these authors found morphological justification for regarding the entire balance of the Malagasy primate fauna, including the aye-aye, as a monophyletic unit.

The paraphyly that these observations suggested had radical implications for lemur biogeography and origins. For if the cheirogaleids are in fact lorisoids, then there are only two biogeographical possibilities. The first of these is that the cheirogaleids are descended from an African (or conceivably Indian) ancestor that invaded Madagascar separately from the ancestor of the remaining lemur fauna (i.e., that there were two strepsirhine colonizations of Madagascar, the later one subsequent to the apparently Eocene or earlier divergence of the lorisiform and lemuriform groups in Africa). The second possibility is that, from a single African or Asian common ancestor, the lemurs diversified on Madagascar to the family level we now recognize and that today's Afro-Asian lorisoids are descended from a cheirogaleid ancestor that recolonized Africa (or conceivably India) from Madagascar, probably before the late Eocene. The pattern of ocean currents makes a Madagascar-Africa crossing considerably more probable than the reverse trip; but if the modern Afro-Asian families had already emerged by the late Eocene, as Seiffert et al. (2003) suggest, then the cheirogaleid back-crossing must have been an early one indeed. At the same time, however, a very early back-crossing of this kind might also be consonant with the presence of a cheirogaleid-like Bugtilemur in the early Oligocene of Pakistan, although Marivaux et al. (2001) prefer a dispersal event between Madagascar and Greater India.

A few years ago, before the finds in the Fayum and the Bugti Hills, any discussion of lemur origins would have begun with a look at the Eocene adapiform primates of Eurasia. This group formed part of the great Eocene radiation of early euprimates "primates of modern aspect," and produced a profusion of genera and species among which number some of the best-documented fossil primate species known. In the very vague sense of an evolutionary "grade" the adapiforms (possibly themselves paraphyletic in laxer definitions) seem generally to have resembled today's strepsirhine primates; and indeed, some recent primate classifications have included Adapiformes as an extinct infraorder of the suborder Strepsirhini (e.g., Delson et al., 2000). Within Adapiformes, signs of lemuriform ancestry have been particularly sought within the family Adapidae (first and still best known from Europe), despite the fact that all known adapiforms, including the adapids, primitively lack the principal strepsirhine synapomorphy, the toothcomb. Adapids also typically possess four premolars in each quadrant of the jaw as opposed to the three or two of lemurs, and show a fused mandibular symphysis. Intriguingly, though, a presumed adapid hindbody skeleton from Germany's middle Eocene Messel site does quite clearly show a grooming claw on the second digit of the foot (von Koenigswald, 1979); and adapids are well documented to have very lemurlike auditory bullae.

Impressed by these complex basicranial resemblances, Gregory (1920) argued that close molar similarities between the European Eocene adapid Adapis and the living Malagasy Lepilemur were of ancient derivation and provided a sort of evolutionary link between AAdapis and the other modern strepsirhines. Gingerich (1975) later suggested that the closest molar resemblances were to be found between Adapis and Hapalemur, suggesting that Adapis had given rise to the other lemurs via a form that at least dentally resembled Hapalemur. Schwartz and Tattersall (1979) proposed the alternative notion that the dental morphologies of Hapalemur, Lepilemur, and jAdapis were derived, thus indicating that the fossil taxon somehow nests within the strepsirhine clade, rather than lying at its origin. They later noted (Schwartz and Tattersall, 1985) that the indriid lemurs, in particular, shared a whole suite of dental and mandibular characters with adapids, whereas some of the dental characters of the cheirogaleid+lorisid+galagid group recalled those of some Eocene non-adapid adapiforms such as Anchomomys and Periconodon. However, Beard et al. (1988) observed that a wrist structure in which the os centrale overlaps the capitate to contact the hamate is unique to lemurs, to the exclusion of Adapis.

The paleontological argument over both the origin of the lemurs and the existence of potential lemur relatives in the Eocene is bedevilled by slender Paleocene and Eocene fossil records in the possible source areas of Africa and India. Altiatlasius from the early Eocene of Morocco is pretty indeterminate, while Djebelemur from the early Eocene of Tunisia is plausibly adapid but shows no particular affinity with any lemur. In the late Eocene of the Egyptian Fayum, the sketchily known genera Aframonius (Simons et al., 1995; Simons and Miller, 1997) and Wadilemur (Simons, 1997), plus an apparent representative of the European genus Anchomomys (Simons, 1997), show that adapiforms (though not adapids) may have survived there not only after the strepsirhines had originated, but also after the two modern lorisoid families had differentiated.

In Arabia, the poorly known Omanodon and Shizarodon from Oman indicate not much more than that adapiforms were present there in the early Oligocene. In the Indian region the two sivaladapid adapiforms Indraloris and Sivaladapis survived into the late Miocene, as recently as 8 myr ago (Gingerich and Sahni, 1984), and the possibly adapid genus Panobius has been described on the basis of a couple of teeth from early-to-middle Eocene deposits in Pakistan (Russell and Gingerich, 1987). However, Sivaladapis conspicuously lacks a toothcomb and otherwise bears no close resemblances to any lemur. Several other Asian fossil forms are also classified in the family Sivaladapidae, including the Eocene Chinese forms Guanxilemur, Rencunius, and Hoanghonius. The Eocene Chinese genus Adapoides may in contrast be a true adapid, as may Wailekia from Thailand. All in all, however, these various forms do little more at present than suggest that both adapids and other adapiforms were abundant in forests throughout the Old World tropics for much of both the Paleogene and the Neogene, at least back to about 55 myr ago. They do not shed any direct light on the origin of the Malagasy lemurs, and indeed the presence of Aframonius and the others in the Fayum, more or less contemporaneously with Karanisia and Saharagalago, implies that if the direct strepsirhine ancestor was an early member of the African adapid radiation it was very ancient, possibly even predating the early Eocene. This is in line with molecular phylogenies that suggest a much earlier initial diversification of the lemuriforms than of the lorisoids (see below).

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