Primates

The most common group of archaic primates in the Paleocene were the plesiadapiforms. They had large incisors that were suitable for prying open nuts. The most common skeleton of plesiadapiforms in the Late Paleocene was Plesiadapis, which had clawed feet. Carpolestes simoni (Figure 12‑48) was a mid-Paleocene plesiadapiformes that may have had a close relationship with true primates. They had long fingers with opposable thumbs that enabled them to grab branches. The thumbs had fingernails, like humans, but the rest of the fingers had claws.

Jonathon Bloch discovered a fossil plesiadapiforms named Ignacius clarkforkensis in the Bighorn Basin, outside of Yellowstone National Park, dated 56 Ma.[1] It was the size of a mouse, ate fruit, and lived in trees. Ignacius clarkforkensis is classified as a paraomomyid. Omomyids were early primates. The Ignacius genus (Figure 12‑49) lived from 62 – 34 Ma.

The Late Paleocene plesiadapiformes such as Ignacius have already branched off from the primate evolutionary trunk. They have derived other characteristics; thus, paleontologists state that they are too derived to be ancestral to the true primates. In general, the plesiadapiformes lacked certain traits of primates such as nails on the fingers and a complete postorbital bar on the skull. On the other hand, dental and other similarities with early primates indicate their close relationship.[2] The fact that they have some characteristics of primates but not other characteristics indicates that they were transitional.[3]

Some evidence indicates that the euprimates (true primates) originated in the region of India. The first true primates appear in the fossil record at the time of the Paleocene Eocene Thermal Maximum (55 Ma). They include Cantius, Teilhardina, and Archicebus (Figure 12-50) from China. Although North America and Asia were at this time separated by the Atlantic Ocean, the continents were periodically connected at the Bering Straits in Alaska; thus, there was transfer of animal species between them.

There were two families of early primates, The Strepsirrhini (wet-nosed primates). looked like modern lemurs (Adapoids). Notharctus (Figure 12-51) is a famous Strepsirrhini fossil from the Eocene. The first fossil was discovered in the 19^th century, and there are many fossils in Europe and North America. It had long fingers and opposable thumbs, and a relatively long snout.

The early primates in the Eocene were “lower primates” while the monkeys and apes in the Oligocene and Miocene were “higher primates,” Some fossils, such as Eosimias (Figure 12-54) in the Middle Eocene, had a mix of features from the lower and higher primates: [5] Eosimias lived in China.

The climate cooled at the end of the Eocene and primates vanished from North America and most other parts of the world, but they remained in the Mediterranean region. The higher primates (anthropoids) include monkeys, apes, and humans. The earliest undoubted anthropoids are in the Fayyum deposits in Egypt between 37 and 30 Ma, Apidium and Aegyptopithecus (Figure 12-55). Aegyptopithecus was an early Oligocene monkey (33 Ma) and is placed near the base of all catarrhines (Old World primates).

Transitional apes (no tail, etc...) appeared in the Oligocene of Kenya. Rukwapithecus (25 Ma) was an early ape that was probably a proconsulid. The proconsulids had a mix of monkey and ape features Thirteen proconsulid genera (Figure 12-58) have been found in the early in the Miocene (22-18 Ma) in East Africa. One early ape feature in proconsulid was the lack of a tail. There is dispute over whether they are within or outside the Hominoidae (apes, Figure 12-59) because they are transitional between monkeys and apes.

Gibbons (Figure 12-60) have characteristics of both apes and monkeys. They are amazing animals and can swing from branch to branch through the forest at speeds up to 34 mph. One major difference between gibbons and monkeys is that they, like the great apes, do not have tails. As with the gibbons, the ape fossils between 20 and 10 Ma have many characteristics of apes but still retain some of the characteristics of monkeys. Molecular clock analysis indicates that the Hominoidea split between gibbons and the Hominidea family, 17 Ma. The Hominidae, the great ape family, includes the orangutans, gorillas, chimpanzees, and humans.

Dryopithecus (12.5 – 11.1 Ma) was an intermediate ape from Europe that was transitional between orangutans and African apes (Homoninae). There are many dryopithecine jaw (Figure 12-63), teeth, and skeleton fossils, which enabled scientists to paint an accurate picture of the dryopithecine physiology and to even determine some of their habits. Even so, the classification of Dryopithecus has been controversial. For example, it walked on all fours like an African ape, but it walked on its palms while chimps and gorillas are knuckle walkers. Because the climate cooled during this period, the dryopithecines left Europe and continued in Africa, where it was warmer (tropical), which is the reason that the Homoninae evolved in Africa.

Chimpanzees (Figure 12-64) are the most intelligent primates other than humans. Until recently, most scientists thought that last common ancestor of the Homoninae (humans and chimps) was a dryopithecine that lived approximately 7 million years ago. However, recent DNA research has shown far more genetic difference between humans and chimps than previous estimates and has pushed the estimate of the time of the last common ancestor of humans and chimps further back in time.