Fossil Primates 1
Dr. Bill Sellars
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). Tomorrow's 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 prosimians) & Omomyids
(early anthropoids)
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
Oldest primate
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 - Adapiformes (early prosimians) & Tarsiiformes (early anthropoids)
The first unequivocal primates occur about 50 mya. There are two main groups identified: Adapiformes which are usually considered to be ancestral to modern Strepsirhines; Tarsiiformes which are (mostly) considered to be early Haplorhines.Adapids are the main group of early Adapiformes. They 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 the best examples of early Tarsiiformes. For example Rooneyia or Necrolemur. These early Tarsiiformes 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.
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 die 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 mandibular 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 animal is classified as a Dryopithecin - one of the three sub-families of the Hominids.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.
NOTE: as an aside, many authorities class the Hominids as starting with the Australopithecines. However, current thinking puts this as just a anthropocentric bias - the group containing Homo and Australopithecus has been down-graded from a family to a subfamily, Hominins. So, the family Hominids includes the three sub-families: Dryopithecins (all now extinct); Pogins (great apes); Hominins (human-like animals). Lesser apes are considered to have branched off the Hominid line earlier on to produce the family of Hylobatids.
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...Summary
Draw up summary tree.This page is maintained by Steve Paxton