Written by Graham | Created: Saturday 6th July 2019 @ 0034hrs | Revised: Thursday 1st October 2020 @ 1205hrs
66-23.03 million years ago.
The shuffling of the non-avian dinosaurs off this mortal coil led to situations vacant in a wide range of ecological niches. The early Paleocene saw mammals, birds (including the successful and long-lived flightless turkey from hell Gastornis) and crocodilians, among others, stepping up to the plate in a competition to secure the best of these. Mammals - which were by no means exempt from the effects of the extinction (metatherians in particular being hard hit) - began small, being at this stage a disparate bunch of rodent-like animals, though, once this opportunity presented itself, they began to diversify in a series of weird and wonderful ways.
The types of placental mammals which are attested in known Paleocene geological levels remain mysterious, with their exact relationships to the orders known from the Eocene to today, not to mention their affinities to one another, still the topic of a good deal of debate among paleontologists. What is known is that, among their number, remains of animals closely related to the ancestors of a number of later orders, including the Primates, are to be found.
Many of the remainder have been placed in the convenient "wastebasket taxon" Condylarthra: these early ungulates (hoofed mammals) are a motley bunch, including the Arctocyonidae (which includes the omnivore Arctocyon, which reached sizes up to that of a small bear during the Paleocene, as well as the arboreal Chiracus), herbivores such as Phenacodus and others such as the Periptychidae. Perhaps related to the Arctocyonidae is the order Mesonychia, encompassing a host of hoofed carnivores dating from the Paleocene into the Oligocene, the largest of which during the Paleocene was the early form Ankalagon saurognathus. The Creodonta, which largely replaced the Mesonychids as top proto-dogs, were also evolving during this period, while the ancestors of the Carnivora which would take their place are also known: these are small, probably tree-dwelling animals grouped in the family Miacidae. Early herbivores include the Pantodonta, Tillodonta and Taeniodonta, with a pantodont, Titanoides, reaching lengths of up to 10 feet during the Late Paleocene.
Also present in the Paleocene was another order, the Plesiadapiformes. These animals, probably descended from the K-Pg extinction survivor Purgatorius, were closely related to the ancestors of today's Primates. Members of this order include Micromys, Carpolestes and Plesiadapis, as well as a possible glider in Phenacolemur.
By the early Eocene, a number of other Primate groups appear in the fossil record, including the Adapidae (relatives of the lemurs) and Omomyidae (with affinities with the tarsiers). The earliest monkey-like primates to appear are members of the family Eosimiidae, the earliest of which, Phileosimias, dates to around 45 mya. One possible member, Afrasia djijidae, appears in both Asia and Africa about 37 mya.
The boundary between the Paleocene and Eocene is marked by an increase in global temperature, culminating in the Paleocene–Eocene Thermal Maximum, around 55.5 mya.
While ocean life suffered (with 35-50% of benthic foraminifera disappearing), this gave an incredible opportunity on land, enabling plants and animals to spread out in the direction of the poles. Temperatures decreased steadily throughout the Eocene, however, leading to the eventual glaciation of Antarctica about 34 mya - a cold spell which still persists to this day.
The early Eocene also provides evidence of the starting points for two well-understood evolutionary lineages: whales and horses. The former group emerged from a primitive family of Artiodactlys (even-toed ungulates, such as cattle, sheep, pigs and goats, as well as camels and hippos) called the Raoellidae, which includes the genus Indohyus (c.50-48 mya). Indohyus was a contemporary of Pakicetus inachus, which may have already begun to adapt to a semi-aquatic lifestyle. A few million years later, Ambulocetus natans was already fully aquatic and adapted to both fresh and salt water, though the animal retains large toes. Remingtonocetus (c.45-43 mya) marks the next phase in Cetacean evolution, followed by the Protoceticae, the best-known of which is probably Rodhocetus.
By about 40 mya, the Archaeocetes had produced two sub-families well-adapted to the seas: the Basilosaurinae and Dorudontinae. These were in turn closely affiliated with the Odontoceti (toothed whales) and Mysticeti (baleen whales) which still roam the seas today, as they have since the late Eocene.
Meanwhile, the primitive horse-like Eohippus gave rise to Orohippus, which evolved into Epihippus by about 47 mya. After the evolution of grasses and the subsequent conversion of much of the forest into prairies and grasslands as the Eocene gave way to the Oligocene, Mesohippus appeared (c.37 mya), followed in short order by Miohippus, the ancestor of the later equines which appeared in the Miocene.
The Eocene and Oligocene saw a great many fearsome and often bizarre creatures come and go, a number of which are related, however distantly, to animals still alive today. Foremost among this category are a number of large, probably omnivorous, species belonging to the Artiodactyls. The most iconic of these is without doubt Andrewsarchus mongoliensis, which is known from the Middle Eocene of Inner Mongolia. The skull of this animal is vast in comparison with mammals exploiting similar niches at other times, possibly indicating that Andrewsarchus was the largest mammalian predator of all time.
Andrewsarchus' precise affinities are unclear, though it is likely that the creature was related to the ancestors of the Cetaceans, as well as another group of large omnivores which first appear about 10 million years after Andrewsarchus' time. These are the entelodonts, perhaps better known by their byname of "hell (or terminator) pigs," a moniker created to reflect their morphological similarities to modern pigs and their relatives. The entelodonts survived into the Miocene and reached their largest size during the period around the Oligocene-Miocene boundary in the form of Daeodon shoshonensis. The skull of Daeodon indeed suggests that the animal was comparable in size to Andrewsarchus. Contemporary with Andrewsarchus was Mesonyx, which, along with the slightly earlier Pachyaena, were among the largest of the later Mesonychids.
By this time, however, these hoofed predators had to contend with the Creodonta, which were growing in size and increasingly competing with them. The creodonts belong to two families, the Oxyaenidae and Hyaenodonta. The latter are named for Hyaenodon, the most substantial of which was H. gigas, reaching lengths of three metres. The Oxyaenidae includes the likes of Patriofelis and the huge Sarkastodon mongoliensis, which reached a similar length to H. gigas, and lived about 35 mya.
Early carnivorans of the Eocene and Oligocene include the Nimravidae, members of the suborder Feliforma, which lived between around 40 to 7.25 mya. These are named for the Oligocene member Nimravus, and are sometimes given the nickname of "false sabre-toothed cats," due to their providing early evidence for this famous feline innovation. The Barbourofelidae, which arose during the Miocene, continued this trend, which eventually led to the fearsome Smilodon gracilis, a member of the Machairodontinae subfamily of the true cats or Felidae, a family first attested around the time of the Eocene-Oligocene boundary, in the form of Proailurus.
The Caniforma are represented by early members of the clades Amphicyonidae ("bear-dogs") and, to add to the confusion, the Hemicyonidae or Hemicyoninae ("dog-bears"), the latter of which are closely affiliated to modern bears in the family Ursidae, as well as the Hesperocyoninae, alongside their offshoots the Borophaginae ("bone-crushing dogs") and the early true Canidae, such as Cynodictis. Towards the end of the Oligocene, another couple of members of this order, Puijila darwini and the genus Enaliarctos, represent early steps in the evolution of modern pinnipeds, the clade encompassing seals, sea-lions and walruses.
Meanwhile, a series of herbivore groups were also gaining in size and number. Early on, one particular group, the Dinocerata - whose best-known member is the Eocene Uintatherium of North America - were reaching large sizes. The earliest members of this group were Prodinoceras in Asia and the North American Probathyopsis, during the Late Paleocene, while later members include Eobasileus cornutus, which boasted an elaborate set of horns.
The affinities of the Dinocerata have long been unclear, though they now appear to have been relatives of Carodnia, a genus of early Eocene ungulates from South America. This continent, which was isolated prior to the creation of the Isthmus of Panama and subsequent Great American Interchange during the Pliocene, was home to a rather unique fauna, which included the metatherian Sparassodonta (a group which included another sabre-tooth, Thylacosmilus) and giant Xenarthra (relatives of modern sloths, armadillos and anteaters), was also home of a number of placental ungulate groups grouped together as Meridiungulata. These include: the Xenungulata (the order which includes Carodnia, which spans a period from about 58 to 48 mya); the probably-semi-aquatic Pyrotheria ("fire-beasts"), which died out during the Oligocene; the Astrapotheria; Litopterna; and Notoungulata.
Africa too had a fauna all of its own, the Afrotheria. This superorder, which is represented today by elephants, sirenians (the dugong and manatee), tenrecs, hyraxes and others, includes the Eocene-Oligocene Embrithopoda, which also boasted large horns, as well as the Desmostylia, an extinct group of sea dwellers which appeared in the Oligocene and survived until around 7.2 mya. The carnivorous Ptolemaiida also prowled the African landscape during the Oligocene.
23.03-2.588 million years ago.
As noted above, the transition from the Eocene to the Oligocene was marked by a significant radiation of the grasses, with an attendant changing of landscape from forest to plain. Concomitant with this was another evolutionary turnover, the Eocene–Oligocene extinction event or Grande Coupure ("great break"), which saw the extinction of many groups and their replacement by others. Paleontologist J.J. Hooker and his colleagues sum up the situation as follows: -
The Neogene also saw a radiation among the Proboscidea, the group which contains the modern species of elephants, as well as the iconic mastodons and mammoths of the surprisingly-recent past. Early Elephantiformes include the Eocene and Oligocene Phiomia and Palaeomastodon, and it is this group which would fill out considerably during the Miocene and beyond. However, some members of groups which radiated earlier (the Plesielephantiformes) lived on, including the Deinotheriidae, whose latest representative, from the species Deinotherium bozasi, survived into the Pleistocene.
The Mammutidae developed from Oligocene animals such as Eozygodon and Losodokodon into the likes of Zygolophodon tapiroides with its vast tusks, and Mammut americanum, the American mastodon. Later offshoots from the surviving lineage include the Gomphotheriidae and the truly bizarre Amebelodontidae, as well as the relatives of the huge Stegodon. The remaining subfamily are the Elephantinae, which includes the genera Loxodonta (African elephants) and Elephas (Indian/Asian elephants), as well as the genus Mammuthus (the mammoths, not to be confused with the Mammutidae or mastodons). The best-known representatives of this genus are the Columbian (M. columbi) and - another icon - the wooly mammoth (M. primigenius), the latter of which survived on Wrangel Island in Arctic Russia until as recently as perhaps 300 BC.
Australia too was home to a variety of marsupial and other life then as now. These include the enormous Diprotodon and the "marsupial lion" Thylacoleo carnifex, alongside the titanic lizard Megalania (or Varanus) priscus - all of which survived to witness the first appearance of our own species on the continent - as well as a number of other, earlier creatures, such as large, carnivorous relatives of today's rat-kangaroos (Ekaltadeta and Propleopus), and the lineage of the recently-extinct Tasmanian tiger, Thylacinus cynocephalus. Less well-known are the tapir-like Palorchetidae.
By the Oligocene, the ancestors of the Catarrhini (Old World monkeys and apes) and Platyrrhini (New World monkeys) of today had separated, each following its own path. In the case of the former, this led to Saadanius hijazensis, a close affiliate of the last common ancestor of both the Old World monkeys and our own lineage, the Hominoidea, which lived about 28 mya.
By the beginning of the Miocene, a number of genera belonging to the superfamily Hominoidea had arisen, including Ekembo, the Proconsulidae (such as Proconsul and Ugandapithecus), Dendropithecidae (including Dendropithecus, Micropithecus and Simiolus) and the Afropithecidae (such as Morotopithecus and Afropithecus). The last mutual ancestor shared by the Hominidae (great apes) and Hylobatidae (gibbons) lived around 20 mya during roughly this same period.
By about 15 mya, the African group (ancestral to ourselves, chimpanzees and gorillas) had diverged from the Asian ancestors of the orang utans, with the latter, the Ponginae well-represented in the fossil record by genera such as the 12 mya Sivapithecus and the famous Gigantopithecus - whose largest member, G. blacki, reached a height of up to ten feet and survived up until only about 100,000 years ago.
The ancestors of the Homininae may well have spent a good deal of time in Europe, where a number of Miocene fossils are attested, starting with Pierolapithecus catalaunicus, which dwelt in Catalonia some 13 mya. An early tribe of great apes, the Dryopithecini, includes a number of genera dating from about 10 mya, including Nakalipithecus from Africa and the Eurasian Dryopithecus and possibly Oreopithecus. Also present in Europe between 8.7 and 7.4 mya was the genus Ouranopithecus, the precise affiliations of which are as yet unclear, while Samburupithecus lived in Africa about 9.5 mya.
The Gorillini make their first appearance around 8 mya, in the form of Chororapithecus abyssinicus from the Afar Depression of Ethiopia, while the last common ancestor shared by ourselves and the genus Pan - which includes the common chimpanzee and bonobo - lived perhaps some 7.5 mya. The species Graecopithecus freybergi, which inhabited Greece around 7.2 mya, might fit the bill, or else it could be the oldest human ancestor after the separation. However, at this point, Africa takes centre stage in the story of human evolution.