Thursday 6 November 2014

A Pre-Tooth-Combed Primate from the Early Eocene of Tunisia.

The Strepsirhini (Lemurs, Lorises and Galagos) are often considered to be ‘Lower Primates’, surviving representatives of a primitive stock from which the ‘Higher’ Anthropoid Primates (Monkeys and Apes) arose. However they are just as specialized in their way as the Anthropoids, and have a set of adaptations not present in the other group, most notably a specialized ‘tooth-comb’ (specialized set of front teeth used in grooming). Estimates of the age of the group vary, with molecular dating methods suggesting that the most recent common ancestor of all living Strepsirhini lived some time in the Late Cretaceous, while the earliest known fossils of the group date from the Early Eocene, suggesting that either the group has a long ‘fossil gap’ (period when the animals were present but left no fossils) or the ‘molecular clock’ dating method is inaccurate for Strepsihines.

In a paper published in the journal PLoS One on 4 December 2013, a team of French and Tunisian scientists led by Laurent Marivaux of the Laboratoirede Paléontologie at Université Montpellier redescribe Djebelemur martinezi, an Early Eocene Primate that has variously been considered an Adapiforme (an extinct group of Lemur-like Primates lacking a tooth-comb, which includes the ‘Lemurs’ of Europe and North America), or an early Anthropoid.

Djebelemur martinezi was originally described from a left dentary (jawbone) with two preserved molars, from the Natural Park of Djebel Chambi in Tunisia, estimated to be about 50 million years old. Since this time a number of other specimens attributable to the species have become available, including a facial fragment with a partial maxila (upper jaw) and numerous isolated teeth.

Lower jaw of Djebelemur martinezi from the Djebel Chambi locality.(A) Fragments of right mandible, whichconsists of three isolated pieces found together and reassembled here: the anterior part of the dentary bears the p3 and m1, and alveoli for p4, p2and c, while the posterior part preserves m3 and a portion of the ascending ramus; m2 was found isolated but in the same small calcareous blocktreated by acid processing; (B) photograph of the proximal part of the right mandible (for p4 and p3, note that their mesialalveolus is slightly offset buccallywith respect to their distal alveolus; the single alveolus for p2 is mesiodistally compressed and oblique, while the alveolus for the canine, althoughonly partially preserved, appears slightly larger, suboval, and more lingually positioned with respect to the main axis of the toothrow). (C–G) noncompositelower toothrow with the mandible, canine (reversed), p2 (reversed), and p4, in occlusal (C), lingual(D), frontal (F), and distal (G) views; (H) left p2 in (from left to right) occlusal, buccal, lingual, distal, and mesial views (not reversed); (I) left canine in (from left to right) occlusal, buccal, lingual, distal, and mesial views (not reversed). The 3D representations have been obtained by X-ray mCT surface reconstruction. The crown and roots of teeth of the mandible have beenvirtually delimited by manual segmentation. Marivaux et al. (2014).

Marivaux et al. conclude from this material that Djebelemur martinezi had a primitive tooth-comb apparatus, and that it should therefore be considered an early member of the Strepsihine lineage, though an analysis of its phylogeny suggests that it branched off before the most recent common ancestor of all modern members of the group. This in turn suggests that the Strepsihines are younger than previously thought, probably originating in the early Eocene. If this is the case, then the Strepsihines are considerably younger as a group than the Adapiformes of Europe, and the may have arisen from a small Adapiforme population that reached Africa from Europe, rather than both groups arising from an more ancient African ancestor – which has implications for the origins of Primates as a whole.

Facial fragment of Djebelemur martinezi from the Djebel Chambi locality.(A–E) Left maxilla preserving P3-M3 andalveoli for P2 and C1, in frontal (A), coronal section (cs) through M1 (B), palatal (C), dorsal (D), and lateral (E) views; (F) left P4 in occlusalview. The 3D representations have been obtained by X-ray mCT surface reconstruction. On the maxilla, the crown androots of teeth have been virtually delimited by manual segmentation. Abbreviations: IOF, infraorbital foramen; of, orbital floor; hp, hard palate; pzm,processuszygomaticus maxillae; ra, root apex; jms, jugo-maxillary suture. Marivaux et al. (2014).

Marivaux et al. also tried to reconstruct the diet of Djebelemur martinezi, based upon analysis of tooth wear and the potential shearing force that could be applied by its jaws. Based upon this they conclude that it was probably an insectivore. The shearing force it could apply would be far in excess of that expected for a frugivore (fruit eater) of similar size, falling comfortably within the range for leaf-eaters and a little on the low side for an insect eater; however leaves require an extended digestive tract to process, and at an estimated 80g Djebelemur martinezi is highly unlikely to have been able to do this. The dental wear seen is also more consistent with insectivory than any other diet.

See also…

Bushbabies (Galagidae) are African nocturnal Primates related to Lorises (Lorisidae). Like the Lorises they are solitary nocturnal foragers living in the tree canopy, but while Lorises are slow, deliberate animals, the Bushbabies often move rapidly through the forest, and are capable of leaping from tree-to-tree like Monkeys...

The Omomyids were a group of Palaeocene-Oligocene Primates similar, and probably related to, the modern Tarsiers. They had large, forward facing, eyes and grasping hands and feet with nails instead of claws. Their teeth sugest that they had a diet of fruit and insects, and their skeltel morphology that they lived in trees and that most species were nocturnal. They appeared in the fossill...

Mouse lemurs are the smallest primates. They are nocturnal animals that resemble Tarsiers or Bush Babies, but are true lemurs, all 27 known species belonging to a single genus Microcebus. Like all modern lemurs they are found only in Madagascar (fossil lemurs are known from Africa, Asia and Europe).

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