Despite a diversity a mammal-like reptiles in the Permian, mammals evolved in a world in which dinosaurs and their reptilian relatives dominated a variety of ecological niches.  The Tertiary Period begins with the Paleocene, immediately following the final extinction of most of these reptiles.  From a small number of generalist stem mammals, mammals evolve into a diversity of forms to fill these vacant niches.  The earliest mammals had more teeth than humans; the human tooth formula is 2,1,2,3/2,1,2,3 (2 incisors, 1 canine, 2 premolars, and 3 molars in both the upper and lower jaws. The dental formula of an opossum is compared to that of a human in the images below.

opossum teeth human teeth
     One group of mammals, the primates, adapted to life in the trees.  Grasping hands are especially useful in slow movements on slender branches (Cartmill, 1974).  They developed large, forward-facing eyes and improved vision.  Higher primates evolved better color vision which was obviously important in distinguishing between leaves and fruits in the treetops, as the following images demonstrate.
berries berries

     Purgatorius is believed to be close to the base of the primate family tree. A single tooth from the Late Cretaceous may also belong to Purgatorius in addition to specimens known from the Paleocene.  It had a tooth formula of 3,1,4,3/3,1,4,3.


The primates diverged from their closest relatives, tree shrews and flying lemurs, either just before or just after the end-Cretaceous extinction. The spread of angiosperms probably contributed to the dispersal of these small, arboreal mammals which could feed on fruit. Plesiadapiforms (such as Ignacius clarkforkensis and Dryomomys szalayi) are considered to be true primates at the base of the clade. Primate features like a grasping foot and petrosal bulla evolved in plesiadapiforms. Plesiadaiforms compose the first 10 million years of primate history and were among the most successful mammals in the Paleocene and Eocene with more than 120 species classified into 11 or 12 families. They are known from all northern continents (and perhaps Africa as well). A variety of lifestyles and adaptations existed in the group. While the smallest weighed 30-40 g, the largest weighed 500 g (Bloch, 2007).Later Paleocene forms included   Phenacolemur, Plesiadapis, and Ignacius. The Plesiadapiforms may not be a monophyletic group (Martin, 1993; Martin 1990). Earlier analyses linked plesiadapiforms to flying lemurs because of their elongated phalanges of the middle fingers and toes (Beard, 1990; Kay, 1990; Hamrick, 1999).


     Palaechthon of the Middle Paleocene had a tooth formula of 2,1,3,3/2,1,3,3—the same as in modern prosimians and New World Monkeys.  Although there was no complete postorbital bar to enclose the eye socket from behind (as in modern primates), the posterior portion of the orbit was constricted (Carroll, 1988).

plesiadapis ignacius
palechthon skull comparison


    The Eocene primates branched early into two groups, the omomyids and the adapids (together called the prosimians, in the primate suborder Strepsirhini).  In the Early Eocene, these two groups were present in Europe and North America and omomyids were also found in Asia.  Both groups possess a postorbital bar, a more grasping hand, and the position of the internal carotid artery is more similar to that of modern primates. Members of each group (such as Cantius and Teilhardina) have the dental formula 2,1,4,3/2,1,4,3 which included the primitive retention of 4 premolars; this is more primitive than any known plesiadapiform dentition (Kay, 1997; Gingerich, 1984; Luckett, 1975; Martin, 1990) .  The adapids (or lemuroids) may consist of two separate groups, one of which is composed of North American and some European forms (Martin, 1993).

During the Paleocene–Eocene Thermal Maximum about 56 million years ago, true primates spread throughout Asia, Europe, and North America. The basal omomyid Teilhardina spread throughout all three of these continents, apparently reaching North America via Greenland rather than by way of a Beringia land bridge (Smith, 2006).

smilodectes smilodectes

     The adapids, the more primitive of the two groups, produced a number of lineages in the Eocene.  Many of these lineages later became extinct and this group dwindled in the Miocene. Very primitive adapids are in the lineage leading to early omomyids and the anthropoid primates (New World monkeys, Old World monkeys, apes, and hominids).

      . Adapids are represented today by lemurs and lorises, which first appeared in the Miocene.  A large number of adapid species are known, including a number which may be related to the anthropoids such as Hoanughonius, Anchomomys, and Algeripithus (some feel that the fragments of Pondaungia are anthropoid).  Pondaungia is classified with the adapiforms, although it possesses a number of traits in common with other groups as well.Others species have been placed in lineages some of which eventually lead to lemurs and lorises.  These species include Adapoides (several species including A. parisiensis of France and A. troglodytes of China), Adapis, Leptadapis, Cryptadapis, Microadapis, Mahgarita stevensi, Europolemur koenigswaldi, Wadilemur elegans, Anchomomys milleri, Smilodectes, and Notharctus osborni. (Beard, 1994; Simons, 1997; Kay, 1997; Gingerich, 1984; Luckett, 1975; Martin, 1990; Ciochon, 2001). Modern strepsirrhine prosimians—the lemurs, galagos, and lorises which possess modified lower incisors which form a tooth comb—can trace their ancestry back to the Late Eocene strepsirrhine Waldilemur and the Late Eocene galago Saharagalago of 35 to 37 million years ago (Seiffert, 2005).

lemur skull lemur skull

     Cantius provides the oldest evidence of an opposable,  grasping toe; 52 million years ago from Wyoming.  Its limbs are similar to modern primates adapted for leaping.  Its retention of 4 premolars is a primitive trait present in only one other primate. It may be ancestral to more modern primates.  The feet of Cantius and other notharctines indicate that these groups were not closely related to modern prosimians.  The largest species of Cantius was several times larger than the smallest species (Gebo, 1991; Gingerich, 1986).

     Eosimius sinensis is a small species identified from Middle Eocene fragments in China.  While it retains some characteristics of primitive primates, it has some characteristics of both adapids and higher primates as well.  Some feel that it should be identified with the base of the adapid group shortly after the omomyids separated, others feel it should be considered a higher primate (Beard, 1994; Kay, 1997).

    Lemurs evolved into a diversity of forms including the dwarf lemurs which are the size of rodents, to the giant fossil lemur Megaladapis.  Archaeoindris  weighed an estimated 200 kg and its femurs suggest that it lived on the ground rather than in the trees (Jungers, 1997)

megaladapis lemurs
notharctus archaeolemur

       Strepsirrhine primates seem to have radiated to Asia and Madagascar from their native Africa (Roos, 2004).  A number of features such as the placenta, tooth comb, occlusal pattern, and foot anatomy unite the strepsirhine prosimians as a group derived from common ancestors.  Anatomical evidence suggests that lorises evolved from cheirogaline lemurs evolved from basal lemurines. The aye-aye lacks a tooth comb (Szalay, 1973; Yoder, 1994; Rasmussen, 1998).

    Modern prosimians can vary significantly in their skeletal features.  Variations in strepsirhines include a large lacrimal which prevents the frontal and maxillary bones from contacting each other, additional infraorbital foramina, the location of the infraorbital foramen, contact between the palatine and lacrimal in the orbit, contact between the zygomatic and the lacriaml, ethmoid exposure in the orbit, position of the lacrimal foramen inside or outside the orbit, absence of a foramen rotundum, possession of an interparietal bone, a lacrimal sutural bone, lack of a malar foramen, lack of a postorbital bar (Cynocephalus), thickness of the postorbital bar, lack of a post glenoid process, position of the foramen ovale,  division of the jugular foramen, separation of the hypoglossal canal and the posterior foramen of the jugular complex, the number of palatine foramina, the presence of gnawing upper teeth, the presence of a tooth comb as opposed to comb shaped teeth, the presence/loss of one or two pairs of upper incisors, the presence/loss of one or two pairs of premolars, the loss of a lower canine, the presence of a diastema between the canine and premolars, diurnal/nocturnal, size (under .6 kg to more than 6 kg), the presence of the AIIS, the distinctness of the femur neck, the presence of a third femur trochanter, the presence of an epicondylar foramen, an open olecranon fossa, reduction of the second finger to a stump, the presence of modified grooming claws, a foramen in the pedicle of thoracic vertebrae, and the reduction of the tail, sometimes to the point of being vestigial (Yoder, 1994).  Modern lemurs vary considerably in a number of characteristics, such as their skull form.

Although living indrid lemurs are talented at leaping, their recently extinct relatives were among the primates best adapted to a hanging lifestyle. These fossil relatives are known as “sloth lemurs” because an early fossil was originally identified as a sloth because of its modified skeletal features, such as the extreme curvature of its finger bones (Jungers, 1997).

sloth lemur


There are more than 50 modern species of lemur inhabiting the island of Madagascar and at least 15 species of recently extinct lemur once lived there as well. Genetic analysis of modern and extinct forms indicate that they have all descended from a common ancestor. Two genera, Achaeolemur and Hadropithecus evolved adaptations for a terrestrial lifestyle. The giant lemur Megaladapis (adjacent image) was closely related to modern idriids (Karanth, 2005).

Humans first reached Madagascar about 2300 years ago. Elephant birds and many of the extinct lemurs still inhabited Madagascar a thousand years ago. Prior to the arrival of humans in Madagascar, there were 17 additional species of lemur on the island. All of these extinct species were larger than the largest modern lemur (the indri) and some were the size of gorillas (Burney, 2004).

skull series

giant lemur

loris skull
The Middle Eocene Anchomomys is a small adapoid with affinities to modern lemurs and lorises.  Plesiopithecus teras was a Late Eocene primate with some similarities to lorises, although it also possessed primitive features such as four premolars (Rasmussen, 1998).  The image to the left represents the skull of a modern loris.
Small lemurs and small New World monkeys (described in the next section) possess a smooth cerebrum while larger species possess convolutions of the cerebral surface.
brains brains


  The omomyids diversified to produce a variety of species throughout the world (at least 20 genera from North America, 4 from Europe, and one from Asia) but virtually all the lineages later became extinct. Apparently this extinction was due to climatic changes and to competition from the monkeys and apes which evolved later.  At present, omomyids seem to be more closely related to anthropoids than adapids but many of the specimens on which this is based are fragmentary and new finds may challenge this. Omomyids were diverse during the Eocene, dwindled during the Oligocene, and virtually all became extinct just after the end of that period.  Today the tarsier is the sole living representative of this group  (Gingerich, 1984; Luckett, 1975; Martin, 1990).

     Teutonius from North America and Necrolemur from Europe are among the earlier species. 

teutonius teutonius
Tarsius eocaemus is known from the Eocene in China; later species exist in Africa (Afrotarsius),  China (Macrotarsius macrorhysis), and North America (a number of Macrotarsius species) Kay, 1997; Beard, 1994). Tarsiers and anthropoid primates also share an additional chamber in the middle ear and the absence of the ancestral stapedial artery. By the mid-Eocene, fossils of the genus Tarsius had a developed enlarged orbits and greater bony enclosure of the orbit (Rossie, 2006).
macrotarsius rooneyia necrolemur
tarsier skull tarsier skull