Athena Review Vol. 5, no. 1
Records of Life: Fossils as Original Sources
33. Early Primates 1
Proprimates and Primates
Primates, the ancestral mammalian order including monkeys, apes, and humans, first appear at the boundary of the Cretaceous and Paleocene periods, around the time of the major end-of-Cretacrous extinction events. They were small, insectivore- or shrew-like animals who, during the Paleocene and Eocene periods, appear to have migrated extensively between North America and Asia. The first monkey-like primates do not appear before the Oligocene period.
Gingerich (1984) organized the eviden
ce for early primates
into a sequential tree diagram (fig.1). This begins
with Proprimates in the Palecene period, including the
Plesiadapiformes and Purgatorius. It continues into the Eocene, a time of major primate evolution,when primates are split into two major groups, adapoids and tarsoids (also called omomyds). These names were originally based on their resemblaces to modern prosimians, the former (adapoids) resembling lemurs and the
latter (omomyids) resembling tarsiers. As many have pointed out, current evidence has outgrown this original
distinction, and many of the early primates have traits of both groups
(i.e., Rooneyia from the Late Eocene).Fig.1: Primate evolution in the Paleocene and Eocene periods (after Gingerich 1984).
The question of which of the two groups ultimately led to anthropoids (monkeys, apes, and humans) is currently far from resolved, and debate on the topic remains universal as new evidence continually appears. The evidence will be followed chronologically in this report.
Hell Creek Formation
.
The Hell Creek Formation is a Late Cretaceous formation in Montana and the Dakotas, dating from the Maastrichtian stage (70-65 mya). Fossil taxa included multituberculates (insectivores), turtles, birds, and crocodilians, as well as dinosaurs ranging from Tyrannosaurus to Triceratops. The Hell Creek Formation extends right to the K/T boundary at 66-65 mya, marking the end-of-Cretaceous extinction events, and for this reason has been much studied (Clemens 2004). One of the fossil mammals found in this timeframe is Purgatorius, a candidate for the earliest known primate.
Purgatorius
Purgatorius (fig.2) was a small, chipmunk-sized mammal found in eastern Montana in deposits dating from 66-63 mya. The first fossils of Purgatorius unio were reported in 1965 at Purgatory Hill in
the Early Paleocene Tullock Formation, as part of the Puercan faunal assemblage, and at Harbicht Hill in the late
Cretaceous and lower Paleocene Hell Creek Formation (Van Valen and Sloan 1965). Other early
Paleocene examples have been found in the Ravenscrag Formation and the
Bug Creek fauna (Clemens 1974).Purgatorius, known only from teeth, jaw fragments, and a few ankle fragments, was originally considered by many to be the earliest known primate, with some fossils dating from the very end of the Cretaceous period (Simpson ).
Fig.2: Jaw and reconstructed skull of Purgatorius unio.
Purgatorius is interpreted to have been a diurnal insectivore who lived in burrows.The precise identification of this small mammal, however, has been much debated, as has its chronology. Purgatorius is more recently thought to date only from the early Paleocene. Some consider it may be a proto-primate, and a precursor to the Plesiadapiformes, based on similarities in dentition (Gingerich ; Gunnell 2002). Others consider that it may not a primate at all, but a multituberculate, an insectivore named for its unique tooth forms, which was a widespread early placental mammal (Clemens 2004), Until more evidence of Purgatorius is found, these questions will remain unresolved.
Plesiadapiformes
Plesiadapis
Plesiadapis cookei (figs.3,4) was a late Paleocene Proprimate found in Wyoming, part of a large group of Plesiadipiforms found in North America and Europe. It was an arboreal mammal with long, slender limbs, and long, grasping hands and feet, suitable for climbing and leaping among branches.
The teeth, including large incisors, are typical of insectivores. The brain was relatively small and primitive, with large olfactory lobes indicating an enhanced sense of smell (Gingerich and Gunnell 2005)
Fig.3: Skeleton of Plesiadapis cookei (AMNH; photo Athena Review).
The skull, 9 cm, in length, is very similar to that of P. tricuspidens, found in Europe. Uniquely, however, one P. cookei skull (Cat. UM-87990 at the University of Michigan), found in 1986 at Locality SC-117 at Clark's Fork Basin in Wyoming. included a partial brain endocast (fig.4).
The brain as a whole shows a small size, with relatively large olfactory bulbs, a long and smooth cerebrum, and extensive exposure of the midbrain with distinct impressions of caudal colliculi. Gingerich and Gunnell conclude from this brain evidence that Plesiadapis was more primitive than either a primate or a dermoptera (arboreal gliding mammals from southeast Asia).
Fig.4: Skull and brain endocast of Plesiadapis cookei (after Gingerich and Gunnell 2005).
Adapidae
Adapids were named for lemur-like traits first seen in the Late Eocene primate Adapis, as contrasted with tarsier-like omomyids. A group of adapid primates in the Notharctidae family (named for Notharctus) are abundant in early and middle Eocene faunal assemblages from western North America. Early Eocene forms in the Wasatchian fauna are dominated by Cantius, replaced in the middle Eocene, Bridgerian fauna by Notharctus and Smilodectes, both found most commonly in southwestern Wyoming (Gingerich and Simons 1977).
Cantius
Cantius (fig.5) was a small adapiform primate that lived in the early Eocene in N
orth America. Belonging to the family Nothactidae and a predecessor
of Notharctus, Cantius was more advanced than the plesiadapiformes of
the Paleocene period. The species Cantius frugivorus was named by Cope (1875), and more recently, other species including Cantius abditus have been named (Rose et al. 1999). Cantius is most abundantly found in the Willwood Formation of the Bighorn Basin, Wyoming, where it sometimes represents about 10% of the mammalian fauna (Gingerich and Simons 1977). While thousands of specimens of Cantius have been found, most comprise teeth and jaw fragments, with few skulls known until a nearly complete cranium and lower jaw of C. abditus was discovered in 1993 (Rose et al. 1999).
Fig.5: Skull of Cantius (partly reconstructed).
Cantius was small, with a body mass of around 2.8 kilograms. The limb bones suggest it moved by arboreal quadrupedalism and leaping. It had a dental formula of 2:1:4:3 on both the upper and lower jaw. The incisors are small and vertical, the canines are prominent, and the mandibular symphysis is unfused.
The anterior teeth of Cantius in some ways resemble those of primitive anthropoids such as the Late Eocene Fayum primate Catopithecus (Simons 1995). Canine size, and incisor proportions and morphology, but not orientation, closely resemble conditions in some primitive anthropoids (e.g., Catopithecus, but not Eosimias from Asia who is linked with omomyids). Such similarities also occur in some omomyids, however, leaving unclear any possible significance for determining adapiform-anthropoid relationships (Rose et al. 1999, Based on the dental morphology of Cantius frugivorus, it is thought to have had a frugivorous diet (reflected in its original species name), and was probably active during the day rather than nocturnal..
Notharctus
Notharctus tenebrosus (figs.6-8) was an early, adapiform, tree-dwelling primate from the Eocene period, dating from 54-38 mya. It is typically found in middle Eocene, Bridgerian faunal assemblages from southwestern Wyoming.
Fig.6: Skeleton of Notharctus tenebrosus (AMNH; photo Athena Review).
First described by Leidy (1870), Notharctus tenebrosus, measuring about 40 cm in length, is considered one of the earliest known primates in the lineage of humans. More primitive than living lemurs, Notharctus still had claws, not nails.
The forelimbs were long and slender. With these it was able to spring from bran
ch to branch across great gaps. The radius bowed away from the ulna
enabling greater pronation and supination. The fingers were very long,
wh
ile the thumb was more robust and rotated medially, opposing the
others. Notharctus held on to branches by grasping them with its hands,
not by embedding its claws.Fig.7: (right) Skull of Notharctus, showing postorbital ring and cranial crest.
The hind limbs were long and gracile. The foot had a better thumb than did the hand. Assuming it behaved like a living lemur, Notharctus practiced vertical clinging and leaping, an innovative method of locomotion.
Fig.8: (left) Skull and dentition of Notharctus.
The skull of Notharctus had a complete postorbital ring and a cranial crest (fig.7), which served as a jaw muscle attachment. Both eyes were directed forwards for binocular vision. The eyes were smaller than those of omomyids such as Tetonius and were higher on the skull. The jawbones fused at the chin, creating a single solid mandible (fig.8). The incisors and canines were smaller than those of some other early primates such as Vulpavus.
Smilodectes
Smilodectes (fig.9) was an adapid primate from the Early and Middle Eocene in Wyoming. It is related to Notharctus, and is classed in the Nothactidae family of adapids. The species S. gracilis was named by Marsh in 1871. Since then, two additional species have been named, S. mcgrewi and S. gingerichi.
Smilodectes has a post-orbital bar, common to both adapidae and omomyid primates. It has a small cranium, with the foramen magnum placed at the back of the skull, on the occipatal bone. It has a relatively short snout, with rounded frontal bone as compared to other nothactines.
Fig.9: Skeleton of Smilodectes (after Fleagle 1988).
S. gracilis had a dental formula of 2:1:4:3 on both the upper and lower jaw. It has comparatively reduced olfactory bulbs and a more expanded visual cortex, which suggests that it was a diurnal species. S. gracilis had a cranial capacity of 9.5 cc.
S. gracilis had an average body mass of around 2.1 kilograms. Based upon its postcranial skeleton, it was a vertical clinger and leaper with and grasping thumbs and toes.
References:
Fleagle, J.G. 1988, 1999. Primate Adaptation and Evolution. Academic Press: San Diego.
Clemens, W. 1974. Purgatorius, an Early Paromomyid Primate. Science. 184 (4139): 903–05.
Clemens, W. 2004. Purgatorius (Plesiadapiformes, Primates?, Mammalia), A Paleocene Immigrant into Northeastern Montana: Stratigraphic Occurrences and Incisor Proportions. Bulletin of Carnegie Museum of Natural History. 36: 3–13.
Gingerich, P.D. 1984 Primate Evolution.
Gingerich, P.D. and E.L. Simons EL. 1977. Systematics, phylogeny, and evolution of Early Eocene Adapidae (Mammalia, Primates) in North America. Contrib Mus Paleontol Univ Mich 24:245–279.
Gingerich, P.D. and G. Gunnell 2005. Brain of Plesiadapis cookei (Mammalia, Proprimates): Surface Morphology and Encephalization Compared to those of Primates and Dermoptera. University of Michigan, Contrib. from Museum of Paleontology, vol. 31, no. 8, pp.185-195.
Gregory W.K. 1920. On the structure and relations of Notharctus, an American Eocene primate. Memoirs of the American Museum of Natural History.
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Rose, K.D., R. MacPhee, and J.P. Alexander 1999, Skull of Early Eocene Cantius abditus (Primates: Adapiformes) and Its Phylogenetic Implications, With a Reevaluation of ‘‘Hesperolemur’’ actius. Amer. Jour. Phys. Anthr. 109: 523-539.
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Glossary