Mammals Family Tree
Multituberculates
The multituberculates were the first plant-eating mammals to appear, during the Late Jurassic, and they survived until the Oligocene. During the Jurassic and Cretaceous they were the most common mammals, ranging from the size of a mouse to that of a badger, which was very large for mammals at the time.
Their teeth were adapted for the same type of lifestyle as modern rodents, with gnawing front teeth and tough grinding back teeth. As with many of these early mammals it is often only the teeth which survive, but some almost complete skeletons have been found in Mongolia.
The group became extinct in the Oligocene and appear not to be related to any other mammalian groups. They seem to have evolved in Asia, and then migrated to North America and Europe during the Cretaceous.
Early Mammals
The early mammals were all relatively small, shrew-like animals and so their delicate bones have not preserved well. Hard-wearing teeth is often all that we have, and so it is difficult to know how they were related to each other. They evolved around the same time as the dinosaurs, in the Late Triassic.
Whilst most of the dinosaurs were relatively large (the smallest is about the size of a chicken) and ate either the tough vegetation or meat, the mammals exploited the forests. They were mostly small, insectivorous and probably active at night, using their acute sense of smell to find their prey and mates. The two lived side by side like this for hundreds of millions of years.
Morganucodonts
The morganucodonts are an ancient group of mammals, and like all other mammals were probably descended from the cynodonts. They were small furry shrew-like animals living in the Late Triassic with some of the earliest dinosaurs. They had well developed, pointed teeth showing that they were probably insectivores, and they probably behaved much like modern shrews too. Their fossils have been found in Europe, southern Africa and eastern Asia, but being so small it is often only the teeth that survive. A few almost complete skeletons have been found, though, which give us a good picture of what they looked like.
Although they are one of the earliest groups of mammals, its not possible to say whether they went entirely extinct or whether they are in fact the ancestors of any of the other, later groups of mammals.
Early Therians
There are three groups of modern mammal placental (which carry their young inside themselves for the full pregnancy, like ourselves), marsupials (which give birth to under-developed young which are then carried in the pouch, like the kangaroo) and monotremes (the egg laying mammals like the duck-billed platypus and the echidnas). These groups seem to have diverged from a common ancestor in the Jurassic or early Cretaceous.
Placentals
Placental mammals, like ourselves, carry their young inside their body until they are almost fully formed. There are a huge number of placental mammals alive today, but recent DNA analysis has given us a great insight into how they are related to each other. It seems that they split from the marsupials in the Late Jurassic or Early Cretaceous. During the Cretaceous, around 100 million years ago, the placentals themselves split into several groups, of which four survive today. As the southern continents began to split apart, the 'Xenarthra' were restricted to South America, and the 'Afrotheria' to Africa. In the northern hemisphere the ancestors of the rodents and primates, and those of the 'Laurasiatheria' also diverged, giving rise to the four major groups that exist today.
The DNA analysis doesn't help us understand how many of the extinct animals we find in the fossil record. Many of these are so similar in some ways to modern animals and their ancestors that we can confidently trace their origins. Others, though, are quite unusual and it will not be until we find more fossils that we can understand their relationships with other groups of mammals.
Monotremes
The monotremes, the egg laying mammals, have a very poor fossil record. They have never been abundant, and today there are only two groups surviving: the duck-billed platypus and the echidnas, found in Australia and New Guinea. Like their reptilian ancestors they lay eggs, but the young are then nurtured in a pouch like marsupials.
Recent DNA analysis has given confusing results as to whether the monotremes are closely related to the marsupials or not. It is not yet clear exactly when the three groups diverged, but it was probably in the Jurassic or Early Cretaceous, before Australia split from South America.
Marsupials
The early marsupials often looked very much like modern opossums. They seem to have evolved on the great continent of the Southern Hemisphere, Gondwanaland, when it split from the Northern Hemisphere continents in the Jurassic. Gondwanaland was made up of South America, Africa, Australia and Antarctica all joined together. In the Late Jurassic, Australia split away from the rest of Gondwanaland, and took with it its own collection of dinosaurs and marsupial mammals.
During the Cretaceous, North America joined briefly to the rest of Gondwanaland, and marsupials moved into the Northern Hemisphere, and placental mammals invaded the south. At the end of the Cretaceous the marsupials seem to have suffered an extinction, along with the dinosaurs although the fossil record around this time is restricted to just one place (Hell Creek in North America) and so this pattern may not reflect what was happening globally.
The opossums, however, did well in Europe, and spread from there to Africa and Asia in the Oligocene as the separate continents came together again. During the Miocene, though, the forests began to give way to grassy plains, and the forest-adapted marsupials in Africa, Europe and North America became extinct. The Antarctic marsupials also became extinct as global temperatures fell and the continent froze over. South American and Australian marsupials, however, successfully adapted to the changing conditions and gave rise to two great lineages.
Today they still hold sway in Australia. In South America the opossums are successful and are in fact invading North America once again as the two continents have joined.
Afrotheres
The afrotheres were once the ‘native' animals of Africa when the continent was an island. The group includes elephant shrews, sea cows, hyraxes, tenrecs and elephants and has a fascinating history.
The afrotheres evolved from an ancestor which was probably also the ancestor of the xenarthrans, on the great continent of Gondwanaland during the Cretaceous period. As Africa split away from the rest of the continent, it took with it this ancestral afrotherian, which was probably an insectivorous tenrec-like animal. The descendants of this insectivore diversified over the millennia to give rise to a great diversity of animals ranging from elephants to golden moles, elephant shrews to sea cows.
These ancient African groups still form the basis of the continent's mammals, but now they live alongside incomers from their Northern Hemisphere counterparts, the Laurasiatheres.
Elephant Shrews (or Sengi)
The elephant shrews are small members of the Afrotheria, and live only in Africa. Today there are 15 species, in two groups: the giant elephant shrews and the soft-furred elephant shrews. They are all quite small insectivores living in the forests or wooded savannah of Africa. During the Miocene, though, about 23 million years ago, there were many more. Some of these were herbivores and looked rather like the modern hyrax, which are also Afrotheres. Despite their name, the elephant shrews are not at all closely related to shrews, which are Laurasiatheres. As with many animals, it is only recent DNA analysis which has shown which animals are Afrotheres and which are Laurasiatheres.
Aardvark
The aardvark is the only member of its group (the tubulidentates), and is a bizarre, highly-specialised termite eater. It has a poor fossil record, but fossils are known from the Miocene when there were three or four different kinds. When Africa and Europe joined, some aardvarks moved out of Africa to colonise Europe and Asia, but now there is just the one species left. Recent DNA analysis has confirmed that it is a member of the Afrotheria - perhaps more fossils will help piece together its ancestry.
Tenrecs
The tenrecs are a group of insectivores found mainly on the island of Madagascar, with a few species on mainland Africa. Although they are members of the Afrotheria there are tenrecs which resemble many of our more familiar Laurasiatherian insectivores such as hedgehogs and moles, and the three tenrec species found on mainland Africa are all otter-like. They are therefore a very good example of ‘convergent evolution'. This means that animals from different groups come to look like each other because they are adapted for the same niche and not because they are related. This has caused much confusion in the past. It is only recent DNA analysis which has shown which animals are Afrotheres and which are Laurasiatheres.
Hyraxes
Long before the recent confirmation through DNA analysis, it has been suspected that the elephant and the hyrax are closely related. This may seem unlikely at first glance since all 11 modern species of hyrax are small, furry animals. However, their living representatives do not do the diversity of fossil hyraxes justice. During the Oligocene, about 35 million years ago, the hyraxes were very common herbivores. Some looked like tapirs and others like small hippopotamus, the size of a pig. They were the major herbivorous animals within the Afrotheria, but as the Laurasiotherian hoofed animals invaded Africa and diversified as the grasslands opened up, all but the rodent-like hyraxes became extinct.
Elephants
The elephants are probably the best known of the living Afrotheres. Despite the fact that there are only a few living species, in the past members of the elephant family have been much more diverse.
From their early, tapir-like ancestors in the forests of Africa, members of the elephant family have travelled as far afield as South America and Siberia, and lived in habitats from frozen tundra to desert.
Sea cows
The sea cows (dugongs and manatees) show that the Afrotheres also adapted to the water - they are fully aquatic, just like the whales (which are Laurasiatheres). Fossil sea cows, already looking very similar to modern dugongs, are found in the Late Eocene Fayum deposits of Egypt. Just like their modern counterparts, they were slow swimmers, feeding on sea grass in the shallow coastal waters. From their African origin the sea cows spread not only to the tropical waters of the Americas and India but also to the Arctic. The giant Steller's Sea Cow of the Arctic was sadly hunted to extinction only a few hundred years ago.
Xenarthrans
The Xenarthra are a very ancient group of placental mammals, based in South America. Some of the most impressive extinct animals belong to this group but today there are only the anteaters, tree sloths and armadillos.
The Xenarthra are a very ancient group of placental mammals, originating in the Cretaceous period on the great southern continent of Gondwanaland, and eventually became restricted to South America. In the Messel shales of Germany, though, an Eocene fossil anteater has been found, which is a xenarthran, and shows that these animals were not always exclusively South American.
The name 'xenarthra' refers to extra articulations between the vertebrates which all the animals in this group share. These articulations give the back of the animal greater strength (which allowed the giant glyptodonts to bear such enormous armour), but they are less able to twist and turn.
Some of the most impressive extinct mammals belonged to this group, but now there are only the anteaters, tree sloths and armadillos. The armadillos, though, are beginning to invade new territory - North America.
Anteaters
The true anteaters, the family Myrmecophagidae (which doesn't include unrelated animals which are similarly adapted for eating ants such as the pangolins of Africa and Asia), are part of the South American group of animals, the Xenarthra. A fossil from the Eocene Messel shales from Germany, though, is confusing. It is of an animal very similar to the modern tamandua, or collared anteater. Its vertebrae have the extra articulations which show that it is a true anteater, a member of the Xenarthra, making it the only member of the group known outside South America. Fossil anteaters look very similar to living animals, having all the specialisations to their diet such as long claws to break into ant nests, and long toothless jaws, probably to house a very sticky tongue.
Ground sloths
The ground sloths first appear in the fossil record about 35 million years ago, in the Oligocene. Like the anteaters they have very long claws. Unusually for herbivores, their teeth lack protective enamel, and it is possible that they evolved from an ant-eating ancestor. As with the anteaters, their long claws mean that they have to walk on the sides of their feet. This makes it even more strange that the giant ground sloths, weighing several tonnes, should actually walk on their hind legs. Preserved footprints, though, show that they certainly did.
When North and South America became joined together, during the Pleistocene, ground sloths moved north and were successful colonisers until only about 10,000 years ago. They probably became extinct because of quite sudden climate change during the ice ages. Only the small tree sloths survived, in South America.
Tree sloths
The living sloths are famously slow-moving and strange-looking tree climbers, quite unlike their impressive extinct cousins, the ground sloths. They probably evolved from a ground-living ancestor, but little is known of their past as fossils are rare. Today only a few species of tree sloth remain, living in the forests of South America.
Glyptodonts
In the Early Miocene, as grasslands began to become dominant, the grazing glyptodonts appeared - probably from an armadillo-like ancestor. These animals, some of them growing to enormous sizes, flourished during the Pliocene and Pleistocene, and crossed into North America when the two continents joined up. They became extinct at the end of the Pleistocene, probably due to dramatic climate changes as the ice ages began, sparked off by the closing of the gap between the two American continents and the diverting of ocean currents.
Armadillos
The armadillo family is first found fossilised in the Late Palaeocene, about 60 million years ago. By the Late Oligocene, many of them looked like modern armadillos, with their typical horny protective armour. Some grew to enormous sizes and may have been hunters or scavengers. The glyptodonts are thought to have evolved from the armadillos as grazers of the new grass plains of the Miocene. The smaller armadillos carried on as insectivores and are now successfully invading North America, increasing their range every year.
Laurasiatheres
The Laurasiatheres are probably the most diverse group of living mammals. It includes the hoofed animals, the familiar carnivores as well as the bats and whales!
The Laurasiatheres are probably the descendants of a small, insectivorous mammal which lived during the Cretaceous on one of the continents of the Northern Hemisphere. This group, though, now contains the greatest diversity of mammals. Bats exploit the air, whales the sea. On land, the hoofed animals graze the plains, pursued by the 'true carnivores' like dogs and cats. Although they still dominate North America, the Laurasiatheres have also come to share the continent of Africa with the Afrotheres, their Southern Hemisphere counterparts. In South America they now dominate the remaining Xenarthrans - the remainders of the once much more impressive group of placental mammals.
Insectivores
The insectivores have always been a rather rag-bag assortment of animals whose relationships with each other have been unclear. Recent DNA analysis has suggested that within the Laurasiatheria there is indeed a group of 奏rue insectivores' (粗ulipotyphla') which are genuinely closely related. These include the familiar European insectivores such as the hedgehogs, shrews and moles. The bats, although related to these insectivores, form their own separate group. In the Afrotheria, though, there are a number of very similar, insectivorous animals which evolved to fill the same niches in Africa - golden moles, elephant shrews and tenrecs. There are also similar insectivores in the Xenarthra as well, like the armadillos and in the 喪odents & primates' group, like the tree shrews. In each case the insectivores are probably most similar to the ancestors of the great placental groups as a whole. Since insects are a very ancient food source (having evolved in the Early Carboniferous) insectivores have had to change very little over the millions of years.
Bats
The bats have always been a difficult group to place within the mammal family tree. Despite having small, delicate bones which usually do not fossilise well, amazingly preserved fossils from sites such as the Eocene Messel shales show us that even 50 million years ago they were already looking like the bats we recognise today. In the past they have been linked to the primates, rodents, and flying lemurs but recent DNA analysis has shown that they are in fact Laurasiatheres, closely related to the ‘true insectivores' such as shrews and hedgehogs.
There are two major groups of bats - the small, insectivorous ‘microbats', and the large, fruit-eating flying foxes or ‘megabats'. There has been some debate about how closely they are related, but the DNA evidence currently suggests that they are very close - possibly the megabats are derived from microbat ancestors.
Creodonts
The creodonts are a now-extinct group of animals which were once the dominant mammalian hunters. Since they are long extinct we have no DNA to help us place them in the mammal family tree, but it seems most likely that they are somehow related to the living ‘true carnivores'. It is also possible that there were many unrelated animals that we have grouped under the term ‘creodont' just because they have similar adaptations for meat eating.
The first fossil creodonts are in the Cretaceous, and have teeth designed to slice meat. Throughout the Eocene and Early Oligocene they were highly successful, but then the ‘true carnivores', who had a slightly different arrangement of slicing teeth, began to be more successful. This is perhaps because the creodonts' slicing equipment was right at the back of their jaws, leaving no grinding teeth behind them which could be used to diversify their diets. The creodonts finally became extinct about 8 million years ago.
True Carnivores' or Carnivora
The Carnivora are a huge group today. Far from being exclusively carnivores, many of them are omnivores, and one (the giant panda) is a vegetarian. They all share slicing 祖arnassial' teeth, and molars behind these which can be used for grinding food other than meat. This has allowed them to adapt to a range of diets and probably partly explains their success.
About 60 million years ago, the Carnivora were a group of rather weasel-like hunters in the lush forests of the time. They were called miacids, but there were two types, and these types eventually gave rise to the two great branches of Carnivora alive today. One, the 'Dog branch' was based in North America. The other, the 'Cat branch', developed mainly in Europe.
Carnivora - ‘cat branch'
The cat branch of the Carnivora includes not just the cats, but also the viverrids (civets and genets), the hyaenas, and the mongooses.
The cat branch developed in the Old World (Africa and Eurasia), filling each available predatory niche. The viverrids are the small, lithe forest predators, and the mongooses are their open-country counterparts. The cats are mostly ambush hunters, and the hyaenas are their long-distance running relatives. The exact relationships between the four cat branches isn't clear at the moment, although further DNA analysis should help clarify that.
About 20-30 million years ago, the Bering landbridge between North America and Eurasia allowed the cat and dog branches of the Carnivora to mix. The cats and hyaenas successfully colonised North America, but the other cat branches are still only found in the Old World and the North American hyaenas are now extinct.
Carnivora - ‘dog branch'
The dog branch of the Carnivora is made up of the dogs, the bears, the mustelids (weasels, badgers etc) and the racoons. Until about 8 million years ago there was a fifth group, the amphicyonids, or bear-dogs, but these are now extinct. The seals, sealions and walruses are also descended from the dog branch but at the moment it is not clear how they are related to the other members of the branch. They are probably most closely related to the bears or the mustelids.
The dog branch of the Carnivora developed in North America. The mustelids and racoons are their generalists and small sleek killers, whilst the dogs and bears are pursuit predators. In the past there were cat-like dogs, dog-like bears and the amphicyonids (bear-dogs), but when the Bering land bridge connected North America and Eurasia about 20-30 million years ago, the two groups met and had to adapt to each other. Many lines became extinct, but others successfully crossed over into cat territory, adapting to live alongside their cousins without competing too much.
Perissodactyls (odd-toed ungulates)
The perissodactyls are first found in the Eocene. The tapirs, the horses, the chalicotheres and the brontotheres were all early members of the group. The chalicotheres and brontotheres became extinct, and the rhinos evolved from a tapir-like ancestor.
All the perissodactyls are herbivores. The tapirs remain faithful to their old habits of browsing on the forest floor, but members of the horse and rhino groups have adapted to move out onto the grasslands. The mighty brontotheres were also plains-living, but browsed bushes before grass had become such a dominant plant. The chalicotheres had two major groups. One lived in forests, sitting on its haunches and pulling vegetation down with its clawed front legs. The other consisted of more goat-like animals which lived in open areas, browsing on shrubs and trees.
Artiodactyls (even-toed ungulates)
The earliest artiodactyls are known from the Eocene. These were very similar to the earliest perissodactyls, being forest-living animals a bit like small deer or chevrotains foraging in the undergrowth. The descendants of these animals, though, would eventually give rise to a huge range of animals. The camels, the pigs, the (now extinct) entelodonts, the antelope, giraffes, cows and deer are all artiodactyls. The artiodactyls also had some unusual cousins - the carnivorous ungulates - whose only remaining representatives are the whales.
Whales
Recent molecular analysis suggests that the ancestors of the whales were from the same stock as the other artiodactyls. In the palaeocene there was a group of carnivorous ungulates, the mesonychids, and from these, whales evolved. During the Eocene there were otter-like animals descended from the mesonychids, and not long afterwards there were entirely aquatic forms.
The early whales were all fearsome, toothed animals. At the end of the Eocene, ocean current changes caused the formation of fertile polar waters, filled with plankton. Then some whales adapted to exploit this source, losing their teeth and instead developing filtering baleen plates, and huge layers of blubber to keep themselves warm in the icy water.
South American Ungulates
The early Palaeocene fossil record in South America is poor, but in the mid to the late Palaeocene it is clear that the continent (now separated from North America) has developed its own hoofed animals, probably from the same early ancestors as those around the world. These were the litopterns and not ungulates. Because they were filling the same niches, these animals often tended to look like our more familiar hoofed animals such as camels and rhinos, but sometimes with strange differences - such as the 'ose nose' of Macrauchenia.
Rodents & primates
The rodents and primates are very closely related, and have always been a very widespread group - mice were the first placental mammals to reach the island continent of Australia.
The ancestor of both the rodents and the primates was probably an animal which looked rather like a small squirrel or tree shrew. From that start, back in the Cretaceous on one of the continents in the Northern Hemisphere began a huge radiation. Today rodents are one of the most successful groups of animals on earth - and the greatest mammalian enemies of their close allies, the primates! Mice and rats have a great ability to live alongside us primates, eating the same food and living in our houses.
Although they originated in the Northern Hemisphere, the rodents and primates have both colonised the Southern continents. Here they have managed successfully to live alongside the Afrotheres and Xenarthrans. In the shape of humans, the primates have also invaded the marsupials' great stronghold, Australia - bringing with them their rodent relatives.
Rodents
The rodents are a huge group of animals, but because they are all relatively small (the largest rodent is the capybara) they have not got a particularly good fossil record and their evolutionary history is not all that well studied. Recent DNA analysis has confirmed that all the South American rodents are closely related, and that the lagomorphs (rabbits and hares) are related to the rodents but form a group of their own.
The fact that there are rodents (and primates) in South America has always been a slight mystery as it was separated from the rest of the world since about 100 million years ago. Either the ancestors of the rodents and primates crossed into South America before it was separated, or they somehow colonised it whilst it was separate. Only further fossils will reveal the answer to the mystery of the missing mice...
Lagomorphs (rabbits/hares)
Until relatively recently, rabbits and hares were classified as rodents because they had the same constantly growing front teeth. Then they were recognised to be a group of their own - the lagomorphs. Recent DNA analysis has confirmed this, but also shown how closely related the lagormorphs and rodents are. The fossil evidence suggests that rabbits and hares had split off from the rodents by the end of the Eocene, and were already becoming specialist grass eaters by the Oligocene, when grass was still only found in small tufts and clumps. Most lagomorphs have the recognisable rabbit shape, but little short-eared scurrying pikas are also lagomorphs. This is probably what the earliest rabbits looked like too.
Tree Shrews
The tree shrews are a small group of animals which look rather like squirrels and live the tropical forests of South East Asia. Despite their name, they are not related to shrews (which are Laurasiatheres) and many of them live on the ground! There is very little fossil material for the tree shrews, but what there is suggests that they evolved from animals which lived mostly on the ground and had a shorter snout - the ancestors of tree shrews, primates and rodents.
Prosimians
The prosimians are a diverse group of primates, today including the lemurs, bushbabies and tarsiers. All the earliest primates were prosimians, with many of them resembling modern lemurs. A lot of work has gone into piecing together the relationships between fossil and modern primates, and the picture is still far from clear.
New World Monkeys
The monkeys of South America - the ‘platyrrhine' monkeys (meaning ‘flat nosed') - are a bit of a mystery at the moment. They include the marmosets, howler monkeys and spider monkeys. DNA analysis confirms that they are all closely related, and share a common ancestor, but how that ancestor came to be in South America, which had been separated from the other continents since 100 million years ago, is a mystery. It is possible that the monkeys managed to cross to South America from Africa (where the other monkeys were found), or perhaps the ancestor of the platyrrhines was already present in South America 100 million years ago. It is certainly one of the great mysteries of mammalian evolution at the moment, and only more fossils will help us solve it.
Old World Monkeys
The monkeys of Africa and Eurasia - the Old World or ‘catarrhine' monkeys (meaning ‘hanging nosed') have a much better understood history than their South American counterparts. They appear to have evolved in North Africa at the end of the Eocene, and spread to Europe and Asia. They gave rise to a group of larger, tail-less animals - the apes, including ourselves.
Apes
The apes evolved in Africa, probably in the early Oligocene. They are generally larger than their ancestors, the monkeys, and don't have tails. They are adapted for living on the ground rather than in the trees, and evolved to exploit this relatively new niche as the climate dried out, and the forests began to open up. It was from these apes that we, the hominids, evolved.
Hominids
The hominids are now represented by only one species, ourselves. In the past, though, they were much more diverse. They differ from the other apes in being adapted to walk on their hind legs, although some of them were probably highly agile in the trees as well. The hominids lived in patchy forest or entirely open savannah.
The first hominids are found about 4 million years ago. They are rare finds as fossils, despite the enormous effort put into searching for them. Most of the earlier hominids were either entirely herbivorous, or mostly so. More recent species have tended to eat more meat, culminating in the Neanderthals, which were almost exclusively carnivorous. This species of human became extinct only very recently, leaving our own species the only remainder of the group.
