Fossil primates

The early primate-like mammals; the earliest true primates; anatomical specializations; strepsirhines; haplorhines; platyrrhines; catarrhines; cercopithecoids; hominoids; few fossils, but many interpretations.

Introduction

In the last lecture I tried to give you an outline of primate taxonomy and how humans fit into the taxonomic framework. The next two lectures attempt to fit humans into the evolutionary framework of the primates. Today's lecture concentrates on the evolutionary period from just before the Palaeocene (about 70 mya) to the end of the Miocene (about 5 mya). The next lecture will follow on and look at the early hominins in the Pliocene (5 mya) and the Pleistocene (1.5 mya) stopping before the Holocene (10,000 ya)

Lecture Outline

• Palaeocene - plesiadadiformes
• Eocene - Adapids (early strepsirhines) & Omomyids (early haplorhines)
• Oligocene - Old World & New World Monkeys
• Early Miocene - Hominoids & Cercopithecoids
• Late Miocene - Hominids

What I'll be presenting is a simplified story of early primate evolution. The fossil record is really very patchy, and although the reconstructions look nice, some are based on rather scanty information. So this is very much conjecture, and will be different in different books. The arguments about which is more likely to be correct rage all the time!

Palaeocene - plesiadadiformes

The oldest fossil that anyone considers to be primate-like is a animal called Purgatorius ceratops, which consists of a single tooth found in late-Cretaceous rocks in Montana. This is dated to approximately 70 mya.

This is what it looks like based on a fossil mandible!

The oldest primate-like animal with a reasonable fossil record is Plesiadapis, which is often considered the first prosimian. However, it has recently be re-evaluated in light of new fossil finds and is maybe best considered to not to be a primate, but a form of Dermoptoran (Colugo, or flying lemur).

Here is a list of Plesiadapis features:

• Long tail
• Agile limbs
• Claws, not nails
• Rodent-like jaws and teeth
• Eyes at side of head
• Long snout
• No post-orbital bar

Only the first two features would indicate a primate affinity. There are other mammal fragments that date to the Palaeocene that just might be ancestral primate material, but until some more complete fossils are found, it is best to remain sceptical about claims of primates from this period.

Eocene - Adapids (early strepsirhines) & Omomyids (early haplorhines)

The first unequivocal primates occur about 50 mya. There are two main groups identified: Adapids which are usually considered to be ancestral to modern Strepsirhines; Omomyids which are (mostly) considered to be early Haplorhines.

Adapids are clearly primate-like with:

• Forward facing eyes (binocular vision)
• Post-orbital bar
• Large brain
• Reduced snout
• Vertical incisors

Notharctus, an Adapid where we have a particularly good skeleton, has:

• Long muzzle
• Nails, not claws
• Opposable thumb and big-toes
• Flexible limbs
• Long tail
• Supple back

and is very clearly prosimian-like.

Omomyids are clearly early Tarsiiformes. These animals have some features to associate them with later anthropoids, including:

• Short face
• Big eyes
• Narrow gap between eyes
• Large brain
• Tubular ectotympanic bone

And in some respects, they are more similar to anthropoids than extant tarsiiformes:

• 2.1.2.3 or 2.1.3.3 dental formula (1.1.3.3 in modern tarsiers)

But to add to the confusion, they have a tubular ectotympanic bone, like extant anthropoids, but early anthropoids have a ring-like ectotympanic bone.

Indeed, there is no agreement as to which of Adapids or Omomyids are more closely related to the anthropoids. Adapids also share features in common with anthropoids: fused mandibular symphysis; interlocking canines; sexual dimorphism. And Omomyids have features not found on the anthropoids: fused tibia and fibula; tritubercular molars. Neither Adapids nor Omomyids have post-orbital closure.

Oligocene - Old World & New World Monkeys

The distinction between Catarrhines and Platyrrhines occurred sometime in the Oligocene. However, there is a distinct shortage of fossils (isn't there always). Branisella is a good example of a New World Monkey. The African fossils from this period are almost all from the El Fayum valley in Egypt. The example here is Apidium.

There are a number of questions concerning New World Monkeys. South America was an island continent at this period, so where did they come from? Rafting or island hopping have been suggested. Also, primates died out in North America... Generalisations about Oligocene NWM are difficult to make because of a severe shortage of specimens. There is some indication that they may not have yet achieved full orbital closure and have more laterally directed orbits than extant species, but this is disputed.

The considerably better fossils from El Fayum allow us to be more definite about early OWMs:

• Orbital closure is almost complete
• Fused manidibular symphysis
• Absence of stapedial artery and canal
• Lacrimal bone within orbit

However, a number of ancestral features are still retained in some species:

• Smaller brains than extant OWMs
• Ectotympanic ring

Early Miocene - Hominoids & Cercopithecoids

In the Miocene, we see the rise of Hominoids. Early on, there is a split between Cercopithecoids and Hominoids with the appearance on animals such as Proconsul (again a remarkable fossil) about 18 mya.

This split is characterized by the following features (of apes):

• Y-5 Molar pattern
• Loss of tail
• Bigger brain
• Mobile shoulder and elbow (for suspension)

But Proconsul has a long monkey-like trunk as opposed to the short trunks of modern apes.

In the early Miocene, there were a great many Hominoids, but later on the Cercopithecoids seem to have largely taken over. Except for humans, Cercopithecoids are certainly much more successful today than Hominoids.

However, the nomenclature for the early hominoids is fairly uncertain. This is best expressed by describing the ancestral gibbon forms as "small bodied hominoids" and the ancestral great ape forms as "large bodied hominoids". This latter group can be further split into Asian forms leading to the orang-utans and the African forms leading to the chimps, gorillas and the hominins.

Late Miocene - Hominins

5 mya we'll stop here with the appearance of the Australopithecines...

Bibliography

Library

 AUTHOR(S)       :JONES, Steve  et al.,
 TITLE           :The Cambridge encyclopedia of human evolution / edited by
                  Steve Jones, Robert Martin and David Pilbeam / executive
                  editor Sarah Bunney
 IMPRINT         :Cambridge : Cambridge University Press, 1994.
      

Other

Conroy, G. C., 1990. Primate Evolution, Norton, London.

Bahain, J. J., Grimaud-Hervé, D., Serre F., 1993. Histoire D'Ancêtres, la grande aventure de la préhistoire, Musées/Homme, Paris.