Mammalia (mam-A-li-a) is derived from a single Latin root that means breast (mamma).  The name refers to the occurrence of milk glands, a synapomorphic character of this group.  The formal name was coined by Linnaeus (1758).


The mammals are synapsids and a sister group to the "mammal-like reptiles", the Eosynapsida (see Figure 1A.  In general, the mammals have a suite of synapomorphic characters that define them as a monophyletic group.  They have hair, and a single lower tooth-bearing jaw bone (the dentary) that articulates with the squamosal. The teeth are replaced one time and show a differentiation between incisors, canines, and double-rooted cheek teeth (premolars and molars).  The brain case is enlarged in the parietal region of the skull and the occipital condyles are large and separated by a notch.  The mammary glands are sweat glands that have been modified to deliver milk, a protein and lipid-rich substance, to newborn mammals before they can ingest food on their own. Though not unique to mammals, they are endothermic and capable of high levels of activity.  Mammals have diversified to occupy almost all terrestrial and aquatic environments.  



FIGURE 1.A. Major lines of mammalian evolution within the context of all tetrapod groups after Benton (2005).  Note that taxa in red are entirely extinct.














FIGURE 1.B. Major Clades of the Mammalia.  The relationships of the major groups of mammals after Pough et al. (2009), Benton (2005), Rose and Archibald (2005), and Kemp (2005).  Descriptions of the clades by name and number can be found in the text below.


Protomammals and Early Mammaliaformes

The earliest synapsids considered to be mammals appeared in the upper Triassic.  These Protomammals like Sinoconodon displayed a mosaic of advanced and primitive characters.  For example, although their cheek teeth seemed to have been replaced just once, the canines and incisors were replaced multiple times (Luo et al. 2004).  This seemed to correlate with the similarly indeterminate growth of the skull.  Multiple tooth replacement and continued skull, and presumably similar growth of the post cranial elements are reptilian characters like those of the Eosynapsids.  

The genus Morganucodon (Figure 2) was one of the earliest mammaliaformes.  Still, they retained the extradentary bones in the lower jaw which, even there, appeared to function in hearing.  Morganucodon had developed the mammalian diphydont or single tooth replacement.  Like the other mammals, the cheek teeth were differentiated between premolars and molars and all cheek teeth occluded. The lower jaw was narrower than the upper jaw, a condition common to most mammals.  Ribs occurred only in the thoracic region, a trait also common to the "higher" mammals.  Although the forelimbs were still somewhat sprawling, the hind legs were erect.  They seem to have had determinate growth (mature animals were of one size).  Also, they seem to have been nocturnal (inferred from the architecture of the brain case which emphasized parts of the brain responsible for hearing and smell) and enthothermic (inferred from the small size and adaptations to rapid respiration).  Most of them were insectivorous and very similar to modern-day shrews.


Holotherian Clade (2)

These animals possess the suite of characters that we generally associate with mammals.  In addition to the mammalian features of the Protomammals and the early Mammaliaformes, the lower jaw is made of a single bone, the dentary.  Furthermore, their teeth are diphydont.  This group of mammals appeared in the Jurassic and largely died out in the Cretaceous.  During their heyday they were quite diverse with at least 8 orders in the Mesozoic.  The Multituberculates were, by far, the most successful group of mesozoic mammals though members of only one group, the Monotremes, survives today.  Because the living Holotherians are egg-bearers, we can infer that the others must have been also.  


The multituberculates (e.g. Ptilodus, Figure 3) were the largest group of Mesozoic mammals.  They were small and rodent-like; presumably, they were omnivores.  Like rodents, their incisors were long and separated from the cheek teeth by a gap.  There were no canines, but the first premolar had a large shearing surface).  Like the monotremes and other Mesozoic mammals, the multituberculates had two small epipubic bones.  At least one species seems to have had a prehensile tail and hind feet that swiveled backwards like present-day squirrels.

Living Holotherians: Monotremes

The approximately 4,700 extant mammals can be divided into three very unequal groups: the monotremes, the marsupials, and the placentals.  The monotremes are the only surviving members of the Holotheria.  Those alive today are highly modified burrowing animals that are restricted to the continent of Australia and represented by only three extant species: the platypus, echidna, and spiny anteater.  All are toothless; furthermore, the platypus has an unusual horny beak (see Figure 4).  They retain the primitive character of laying eggs rather than gestating the young within their bodies.  

Hurum et al. (2006), in a study of the venom glands and delivery of the venom by a spur on the hind feet of Platypus, suggest that comparable but reduced structures exist on the hind feet of echidnas.  Furthermore, similar skeletal evidence from multituberculates also implies similar structures on mesozoic mammals.  If so, the mesozoic mammals may have been venomous, a trait that was lost in the marsupials and eutherians.




Marsupial Clade (4)

Marsupials are characterized by having a marsupium, a protective pouch in which the almost fetus-like young resides and feeds attached to the nipple from the time it is born until it can move about on its own.  Their teeth have a dental formula similar to other mammals, but they replace only the premolars and molars.  The southern continental distribution of marsupials is a consequence of their having evolved on Gondwanaland during the Cretaceous period.  The South American marsupials are called the Ameridelphids and are represented primarily by opossums (see Figure 5), mainly omnivorous animals that have prehensile tails.  The exchange of animals between North and South America after the completion of the Central American land bridge around 5 million years ago, led to the mass extinction of many different South American marsupials.  Only the Virginia possum (Didelphis virginiana) of all the marsupials has made its way into North America where it has been very successful.

Australia also broke away from Gondwanaland, but it never again became reconnected to a major land mass.  Thus, its marsupial fauna remained diverse and rich until the introduction of placental mammals by humans (who also are placental mammals).  Perhaps, the most renowned members of the australidelphid marsupials are the kangaroos and wallabies (Figure 6).  Large kangaroos are the ecological equivalents of gregarious herbivores like the ungulates of North America (e.g. Bison) and Africa (e.g. antelope).  Aside from terrestrial mammals, the Australian marsupials include arboreal and fossorial species.  They also range from carnivores to omnivores as well as herbivores.  Many taxa have specialized to eat nectar, insects, and eucalyptus leaves.



Eutherian (Placental) Clade (5)

Placental mammals, also called the eutherians, comprise more than 90% of all extant mammals.  They are characterized by the occurrence of a placenta, a connection between the uterine wall and the developing young.  Placental mammals, because they allow the gestation of a fairly well-developed offspring before birth, also have a relatively large birth canal.  Though they appeared much earlier in the Cretaceous period in which much of their diversification occurred, the successful exploitation of almost all continents and oceans by the placental mammals has occurred only over the last 65 million years.  Much of the organization of eutherians seems to be associated with plate tectonics and is reflected in the names of the four distinct clades: the Afrotheria, Xenarthra, Laurasiatheria, and Euarchontoglires (Rose and Archibald 2005).


Afrotherian Clade (6), including Clades 7 and 8

Six orders form a monophyletic group of very different animals that evolved in Africa at a time when the continent was somewhat isolated and, therefore, are called the Afrotheria.  Molecular evidence suggests a strong, ancient relationship between these taxa.  Unfortunately, structural synapomorphies characteristic of the Afrotheria have not been identified although the snouts of most members of this group are tactile (except in hyrax) and mobile (except in tenrecs), characters that likely are superficial (Hedges 2001, Tabuce et al. 2008).  Also, the males do not have scrotums.

The recognition of the afrotherian clade was also the recognition that well-established morphologically-based classification systems were wrong.  The most significant of the changes was the demise of the Insectivora, a group formerly considered to include all of the shrews and shrew-like placental mammals.  The insectivorous animals called tenrecs, otter shrews and golden moles emerged from the Afrotherian Clade while the shrews, moles, and hedgehogs emerged from the Laurasiatherian Clade. Thus, the "Insectivora" was shown to be polyphyletic.

Afrosoricida (Tenrecoidea)

The afrotherian insectivores (tenrecs, otter shrews, and golden moles) are now in an order called Afrosoricida.  Tenrecs (Figure 7) are insectivores of Madagascar and western Africa and range in size from very small shrew-like animals to cat-sized animals.  Some are spiny and resemble hedgehogs. They occur in almost all habitats: aquatic, terrestrial,  fossorial, and arboreal.  The golden moles are fossorial and superficially resemble moles (laurasiatherians), 

Otter shrews are aquatic animals of western and central Africa.  They superficially resemble small otters (laurasiatherians) with smooth fur.  They have long muzzels, reduced eyes and ears, and laterally-flattened tails.  Otter Shrews are sometimes placed within the Tenrecidae as a subfamily.

Golden moles are fossorial insectivores of south and east-central Africa.  They are adapted to a burrowing lifestyle in that they have large claws on the forefeet, a leathery pad over the nose, and small ears.  Unlike most eutherians, they have a cloaca. 


The elephant shrews (Figure 8) are terrestrial leaping mammals are from south and east-central Africa.  They have dentition that resembles rodents (long incisors) and a body that resembles shrews.  They have long, prehensile snouts, together with large eyes and ears. Though not obvious in Figure 8, elephant shrews are well-adapted for leaping.


Aardvarks (Figure 9) are terrestrial animals of Africa .  They are large (pig-sized) with a tubular snout and a reduced dentition.  They are adapted to eating termites with a long tongue, thick skin, and strong digging claws.  The earliest record of this order is the Miocene.


The remaining afrotherian orders are grouped together in a clade called the Paenungulata.  This group was first recognized by Simpson (1945) using traditional morphological methods.  Molecular work has confirmed the relationships Simpson suggested (Kemp 2005, Rose and Archibald 2005). 


Hyraxes (Figure 10), dassies, and conies are herbivores from sub-Saharan Africa and the Middle East.  Hyraxes are rabbit sized animals with incisors like those of rodents.  In addition, their soft-padded feet have sweat glands that help them to have adequate traction in the rocky places that they inhabit.  Extinct members of this group were much larger and some may have been aquatic.


Sirenians (sea cows, dugongs, and manatees, Figure 11) are fully aquatic mammals with paddle-like forelegs and a flattened tail.  They occur in the coastal areas of tropical and subtropical areas where they eat aquatic plants.  The dentition is reduced mainly to cheek teeth; however, the dugongs do have two pair of upper incisors, one of which serves as a tusk.  Also, they have a short, elephantine proboscis.  Early forms had short legs.  Later, the forelegs became paddle-like and the hindlegs became reduced to a few bones that do not emerge from the animal and acquired a whale-like tail. 


The proboscidians include the living elephants and many other extinct forms that developed long prehensile trunks, which allowed them to have strong massive heads supported by strong necks that did not have to bend to the ground to drink or feed.  In addition, they evolved very large size with pillar-like legs, similar to the sauropods.  The dentition was distinctive in that the second incisors tended to project from the mouth (as in the sea cows) and the rest of the dentition was reduced to a premolar and a few large molars. The earliest proboscidians (Figure 12) appeared in the Eocene and were semi-aquatic hippo-like animals with elongated incisors projecting from the upper and lower jaws.  Gomphotheres (Figure 13) had incisors emerge from both upper and lower jaws, some had lower incisors modifed as giant shovels.  The deinotheriids, of the Eocene to the mid-Pleistocene) were elephant in form, but the recurved incisors emerged from the lower jaw only.  The more modern of the proboscidians include the living African and Indian Elephants (Figure 14) as well as Mammoths and Mastodons, whose incisors projected as massive tusks from the upper jaws.  Mammoths, enormous hairy animals, many of which were adapted to the cold of the northern continents (Figure 15), became extinct at the end of the Pleistocene.  However, evidence suggests that one dwarf mammoth species survived on Wrangel Island (Alaska) until around 4,000 BCE (Vartanyan et al. 1995). 


















Xenartheran Clade (9)

Xenarthra is a monophyletic group that has been recognized for some time (formerly the order Edentata) and appears to be sister to the afrotherians (Hallstrom 2007).  These animals have a suite of unique characters including extra joints between some of the trunk and tail vertebrae.  Also, the pelvic bones ischium and the ilium are fused to the first caudal vertebra.  As the old ordinal name (Edentata) implies, the xenarthrans have reduced teeth or no teeth at all.   Current views (e.g. Wilson and Reeder 2005) separate the xenarthrans into two orders: Cingulata (armadillos) and Pilosa (sloths).


Armadillos (Figure 16) are terrestrial animals that range from the central US south through South America.  They are distinctive in that their bodies are covered by imbricate (overlapping) or articulated bony scutes with horny plates on the outside.  The head and tail are similarly armored.  The eyes are small.  All have muscular forelimbs that terminate in strong digging claws.  They have a very simple dentition that lacks canines.  An extinct group (Pliocene to Pleistocene), the glyptodonts (Figure 17), were similar to the armadillos, but many grew to very large size and were much more heavily armored.  They had continuously growing cheek teeth.  Some had a spiked club at the end of the tail.


These are the sloths and American anteaters.  South American Anteaters (Figure 18) are remarkable animals of Central and South America.  They have no teeth, but have elongate, tubular snouts from which they can extend a very long tongue that is tipped with sticky saliva with which they gather ants and termites.  Tails are long and prehensile.  The front legs are strong with powerful claws that they use to tear into ant and termite nests.  

Living sloths (Figure 19) are arboreal animals that range from Central America to southern Brazil, where they feed on leaves.  They have hair which serves as a substrate for algal growth giving the animals cryptic coloration in the tree canopies where they live.  They have elongated curved claws by which they hang on limbs.  They do have reduced cheek teeth, but no incisors or canines.  The two-toed sloth has a body temperature that is quite variable for a mammal (24-33C).

Unlike the extant pilosans, the giant sloths lived as terrestrial animals of open forests and grasslands, and could assume a bipedal gait.  Like the living sloths, giant ground sloths ate leaves.  However, the leaves that made up their diets were tough (e.g. grass, yucca, and agave).  So, their mouths had as many as 100 teeth (Benton 2005).  We know much about their appearance because mummies of Megatherium have been found in the desert southwest of the US.  The animals were truly massive.  When bipedal, Megatherium stood much taller (up to two times) than an elephant (Figure 20).  They also weighed nearly as much.  The giant ground sloths became extinct during the Pleistocene mass extinction






Boreoeutheria (10)

The Euarchonotoglires and Laurasiatheria together are in a clade called the Boreoeutheria (animals of the northern continents), which appeared in Laurasia during the mid-Cretaceous (molecular estimate) or near the K-T boundary (paleontological estimate) according to Kitazoe et al. (2007).  The basal taxa of the Boreoeutheria comprise eight extinct orders that flourished from the Paleocene to the Oligocene.  These were quite diverse and represent an early radiation that included small insectivores, piscivores, and herbivores.  The diversity in size was great and ranged from animals the size of shrews to others as large as a rhinoceros. 

The insectivores were represented by two orders of small mammals.  The leptictids, represented by Leptictidium (Figure 21), were small bipedal animals that resembled elephant shrews by having long snouts and small sharp teeth.  The apatimyids were larger with dentition that was more like that of a rodent.

Herbivores dominated the basal boreoeutherians and included five of the eight orders.  Primarily, they were browsers or diggers and the dentition of the animals indicated that they fed on tough plant material.  Some of them looked like rabbits, pigs, bears, and ground sloths.   The most impressive of them were the rhinoceros-sized Dinocerata (Figure 22).  These animals, also called uintotheres, had bony proterberances, some of which resembled horns.  Also, they had very long canine teeth, about as long as saber-toothed cats.  The oversized horns and teeth may have been used for ritual combat or defense.  Othniel Marsh made them famous in a book called the Gigantic Animals of the Dinocerata (1885).  However, it was caught up in the great war between Cope and Marsh and declared to be a monumental plagiarism (Holt 2006).  Because the book was published by the U. S. Geological Survey and very expensive to run a printing, it caught the attention of congress, which then began to strip the USGS of its funding.  It would be nearly a century before the US Geological Survey would regain its standing in the scientific community.

The pantolestids (see the image of Pantolestes, Figure 23) were semi-aquatic animals that resembled otters.  However, they had very large molars which implies that they may have fed on shellfish.  Fossil remains of their stomach contents indicate that they also fed on fish.  




Laurasiatheria Clade (11)

Animals of the Laurasiatheria evolved on the northern supercontinent of Laurasia after the breakup of Pangea.  The diversity in form, habitat, and size in this group is astounding and contains the largest and among the smallest of all mammals.  The Laurasiatheria includes 7 or 8 orders depending on how the cetaceans and artiodactyls are lumped together into a group called the Cetartiodactyla.  This group (here treated as a superorder) is an unlikely collection of animals like pigs, cows, camels together with whales and dolphins.  

As remarkable as the collection seems, the relationship has been confirmed by fossil, molecular, and anatomical evidence (Thewissen and Williams 2002).  Furthermore, the emerging story of whale evolution has become one of the most exciting demonstrations of macroevolution to have emerged in vertebrate paleohistory.


The former order Insectivora, which turned out to be a polyphyletic assemblage of taxa (Douady et al. 2002), contained tenrecs, ottershrews, and golden moles (see afrotherian insectivores) as well as diverse groups of laurasiatherians: shrews, moles, and hedgehogs (of the order Eulipotyphla).  These are small animals with elongate muzzles and superficially similar to the earliest mammaliformes like Morganucodon (see Figure 2).  The order is represented by three extant families and extinct groups that push its initial radiation well into the Cretaceous.  

The shrews (Figure 24) occur on all continents except Antarctica and Australia.  Shrews form two major groups: red-toothed shrews (Europe, northern Asia and North America) and white-toothed shrews (Africa and southern Asia).  These tiny animals have a very high metabolic rate and must eat nearly their own body weight in insects each day.  

Moles (Figure  25) and desmans are fossorial and aquatic animals of North America and Eurasia.  Moles are adapted to burrowing with large digging claws and weak hind legs; small eyes and ears.  Desmans look like fat shrews (they are aquatic) with webbed feet.  

Hedgehogs (Figure 26) and rat shrews are terrestrial mammals of Africa , and Eurasia to Borneo .  They are plantigrade and some have enlarged incisors.  Early forms were dog-sized and covered with stiff hairs.  Later forms evolved the spines characteristic of today’s hedgehog. Like moles, they are are good diggers and are omnivorous.  Some are good climbers and swimmers.


The bats (Chiroptera) are the second-most speciose order of mammals with more than 1,100 living species (~22% of all living mammals).  Bats use fingers 2-5 as the frame for their wings.  The shoulder girdle together with the ribs, scapula, and sternum, is adapted for the attachment of large flight muscles.  Also, their hind feet can swivel so that they are turned backwards. The origin of flight in bats remains something of a mystery.  They separated from the other mammals, most likely a common ancestor to the Eulipotyphla, around the end of the Cretaceous (Teeling et al. 2005, Cao et al. 2000).  The earliest known fossils of bats are from the upper Paleocene, and they clearly are bats with elongated digits of the manus, the hallmark of powered flight in that order.  The discrepancy was explained by Teeling et al. (2005) who determined that the fossil record of bats underestimates their fossil history by 61%.  The relatively sudden elongation of bat digits was explored by Sears et al. (2006) who noted that although bat digits begin development much smaller than even those of mice, the expression of Bmp protein causes the elongation of the digits (artificial addition to mouse embryos also promoted digit elongation).  Hockman et al. (2008) also observed that Sonic hedgehog, which controls Bmp, makes a second round of expression causing the forelimb digits to elongate.  This evolution through developmental changes (now referred to as evo-devo) could explain the very rapid and profound change in digit length in bats.  However, Cooper and Tabin (2008) suggest that we should be cautious about being lured by such a siren's song because we do not have early fossils and, therefore, do not know what utility early minor elongations in bat digits might have had.

Aside from flight, the most distinctive character of many bats is echolocation, a sonar system that seems to have evolved more than once in the order (Teeling et al. 2002, Eick et al. 2005).  Echolocating bats emit high-frequency sounds using the larynx (14,000 to 100,000 hz) through the mouth or the nose.  The integration of the echoes with the emitted sound allows the flying animal to navigate through complex terrain with minimal light while locating prey items.   Aside from the sound-emitting and neural modifications, (Eick et al. 2005), other morphologies like relatively large and elaborate ears and many with nose-leaves also function in echolocation.  The earliest microbats (e.g. Gunnell et al. 2003), indeed the earliest placental mammals found in sub-Saharan Africa, had an enlarged cochlea, which showed that they likely possessed echolocation in the Eocene.

Traditionally, the order is divided between two suborders, commonly called megabats and microbats (after Dobson 1875).  The megabats are the flying foxes (Figure 27) and Old World fruit bats, which occupy a single family, the Pteropodidae.  They are large bats of the tropics and subtropics of Africa, southern and central Asia, Australia, and the Pacific islands.  Some are quite large and have wingspans up to 1.7m.  Most, however, are much smaller.  Pteropodids have a suite of unique characters that include: a second finger with a claw, elongated bony palate, no more than four upper and four lower incisors, relatively simple ears and small cochlea, tail absent or, if present, much reduced.  They eat fruit or nectar.

The microbats (see Figure 28) tend to be insectivores, but some are fruit-eating, blood-eating, or fish-eating.  Distributed into 16 families, the microbats contain 84% of all bat species.  Mainly, they are small animals, an index finger without a claw and a relatively long tail.   Microbats are found from the tropics through temperate latitudes.  Teeth broad with many cusps.  Most are sociable and some hibernate.

During the past 30 years molecular and anatomical work have not supported the traditional subordinal classification.  Pettigrew et al. (1989) and Pettigrew (1991) saw a strong relationship between the brain anatomy of primates-flying lemurs and the megabats.  They proposed that bats and mammalian flight were polyphyletic.  That is, the microbats were related to the insectivores while the megabats were related to the primates and similarities between the two groups was superficial.  Simmons et al. (1991), in a response to Pettigrew's assertions outlined the need for more anatomical and molecular work coupled with phylogenetic analysis.  The initial molecular work (e.g. Baker et al. 1991, Hucheon et al. 1998) supported the monophyly of the Chiroptera; however, they also suggested that the microbat-megabat dichotomy was a false one.  Furthermore, Eick et al. (2005), Hutcheon and Kirsch (2004, 2006), and Teeling et al. (2000, 2002, 2003, 2005) demonstrated that five superfamilial groupings of bats occurred.  Also, in the emerging subordinal structure, megabats (Pteropodidae) clustered within a clade that included five formerly microbat families.  So, molecular phylogenetics confirmed the monophyly of bats, but restructured the organization at the subordinal level.

Now the standard classification of bats is that of Simmons (2005) as modified by Hutcheon and Kirsch (2006).  Springer et al. (2001) named the two suborders Yinpterochiroptera and Yangochiroptera.  Hutcheon and Kirsch (2006) objected to such names because they had little information and were taxonomically inconsistent and suggested the names Pteripodiformes (superfamilies Pteropodoidea and Rhinolophoidea) and Vespertilioniformes (superfamilies Emballonuroidea, Noctilionoidea, and Vespertilionoidea).  


The odd-toed ungulates (Perissodactyla) include two major groups: Hippomorpha (the horse forms) and Ceratomorpha (horned forms).  The living hippomorphs (horses, asses, and zebras) are relatively large herbivores of open habitats, particularly grasslands, through the Old World.  They also have a single digit (The modern domestic horses were derived from wild stock like the surviving Przewalski's Horse, see Figure 29).  However, the Eocene horses, like Hyracotherium (Figure 30), were woodland browsers with 4 toes.  As they began to make use of more open habitats, the animals became larger and reduced the number of toes to one.  At the same time, the legs grew longer, which made them very fast.  The browsing molar was small while the grazing molar became large and able to withstand the abrasion of the silica in grass.  

Ceratomorpha taxa were and remain much more diverse than the hippomorphs.  Though the name means "horned forms", some of the ceratomorph taxa do not have horns.

Among the first to appear in this line were the enormous brontotheres (also called titanotheres, Figure 31) during the Eocene.  These were large animals that resembled rhinoceros with a catapult-shaped horn on the nose.  They were not true odd-toed animals.  Although their hind feet had three toes, their front feet had four.  Though massive, they likely could not the tougher vegetation, especially the grasses, that began to appear in open habitats in the Eocene.

Chalicotheres (Miocene to Pleistocene) looked like a large ape with hooves and a horse's head (Figure 32).  Their forelegs were very long and the hind legs were short.  They may have been able to walk bipedally or knucle-walk.  

Tapirs (Figure 33) are terrestrial animals that range from southern Mexico to Venezuela.  A fourth species occurs in Burma, Thailand south to Sumatra.  The tapirs radiated from animals that looked like Eocene horses.  Then, the surviving lineage developed a short proboscis, which evolved independently to the proboscis of the afrotherians.

Living rhinoceroses are restricted to open areas of south-central Asia and sub-Saharan Africa.  They are large animals with thick skin and one or two horns made of compressed hairs.  Rhinos evolved from animals that resembled Eocene horses.  By the Oligocene, the rhino line had produced the largest terrestrial mammal that ever evolved, Paraceratherium+ (Figure 34; syn: Indricotherium; Baluchitherium).  Horned rhinoceroses radiated in the Miocene.

Carnivora + Pholidota

The carnivora have a particular dental formula that includes: a pair of carnassial teeth.  They are best known for the canines, which are long and adapted for piercing or puncturing.  The feet are modified as clawed paws.  The carnivores are divided into two modern natural groups: Feliformia and Caniformia, cat forms and dog forms, respectively.  They can be recognized by the type of tympanic bulla: feliforms have an ossified tympanic bula that has an internal septum, the caniforms do not have an internal septum.  The earliest carnivores were the miacids (Figure 35), semiarboreal cat-like or weasel-like animals that appeared in the Paleocene perhaps allied with a line of early ungulates (Wesley-Hunt and Flynn 2005).  They did not have an ossified tympanic bulla.  Recognizable members of the carnivora appeared in the Eocene and radiated to all continents and continental margins.

The feliforms are the civets, mongooses, aardwolves, hyneas, and cats (Figure 36).   Most of them are digitigrade and striped or spotted.  The felids (the cat family) have retractable claws.  The caniforms are more diverse and include dogs, wolves, bears, raccoons (Figure 37), mustelids (otters, weasels, etc., Figure 38) and the pinnipeds (Figure 39).  The results of Arnason et al. (2006) indicate that the seals are sisters to the walrus + sealion clade and that the sealions separated from the seals before the middle Oligocene.

Pangolins (Pholidota, Figure 40) make up a group of anteaters called scaly anteaters, sister group to the carnivores.  These animals are toothless and covered with armored overlappingd scales.  They, too seem to have appeared in the Eocene.


This group that includes the odd-toed ungulates (Artiodactyla) and the whales (Cetacea) was identified as a clade by Thiewissen et al. (2001).  The unlikely grouping of animals as different as pigs, deer, and whales was a major insight into the origin of whales, one of the great enigmas of vertebrate evolution.  The earliest members of the clade were the zhelestids, arctocyonids and the mesonychids.  The zhelestids were present in the upper Cretaceous but their position is uncertain.  Though accepted as placental mammals (Archibald et al. 2001), the zhelestids seem to be paraphyletic and basal to both the glires and the ungulates.  The zhelestids persisted into the Paleocene.

Arctocyonids (Figure 41) were Paleocene herbivores that superficially resembled sheep-sized dogs (Benton 2005).  The molars, which were very large and adapted to crushing, indicated their herbivorous nature.  They were somewhat diverse with some taxa becoming arboreal, having muscular limbs and even the suggestion of a prehensile tail.

Mesonychids (Figure 42) also resembled large wolves, but these, unlike the arctocyonids were carnivores (Benton 2005).  They had a plantigrade stance with paw-like feet  ending in toes, each of which had a small terminal hoof.  These hoofed carnivores had teeth like certain early cetaceans and toes like ungulates.  Though once considered the ancestors of the cetaceans, mesonychids are now thought to be sisters to the cetacean + artiodactyl clade (Thewissen and Williams 2002).  They appeared in the Paleocene and died out in the Eocene.


The artiodactyls, the even-hooved ungulates, are divided into two extant groups: the Suiformes (pig-forms) and Selenodontia (moon-teeth).  The sister groups derived from a basal ungulate group, the dichobunids, which were herbivorous browsers.  These basal artiodactyls had multiple digits on the front and hind feet (4-5 toes).  Each digit terminated in a small toe; however, only digits 3 and 4 bore weight, a condition seen in all artiodactyls.  Also, the dichobunids had a diagnostic double-pulley astragalus (heel bone) that is characteristic of all artiodactyls.  They appeared in the lower Eocene and disappeared in the Oligocene.

Suiformes appeared in the upper Oligocene and persist today as pigs, peccaries (Figure 43), and hippos (Figure 44).  In general, they are omnivores with bulbous cusps on their molars and large canines that emerge from their mouths.  Their stomachs may have 2 chambers (pigs) or 3 chambers (peccaries and hippos), but they are never ruminating (regurgitate food to chew as a cud).

Selenodonts appeared in the Eocene and underwent several diversifications.  In general, all members of this group tend to be animals of open habitats, usually grasslands or open woodlands as grazers or browsers, respectively.  The key character is the occurrence of a lunate pattern on the surfaces of certain cheek teeth.  Also, the upper incisors are very reduced or missing altogether and the lower incisors bite against a horny pad in most.  There are two clades within the selenodonts: tylopods and ruminants.  Only camels (Figure 45), which have a 3 chambered ruminating stomach, represent the tylopod line today.  The ruminants, well represented today as deer (Figure 46), antelope, giraffes, and bovids, etc., have 3-4 chambered ruminating stomachs and true horns or antlers.

Cetaceans (whales and dolphins)

Cetaceans either are sisters to the artiodactyls or emerged from within the artiodactyls and are sisters to hippos.  The distinction is important.  If whales are sisters to the artiodactyls at large, then there could be an order Cetacea and a related order Artiodactyla.  However, if the whales emerged from within the recognized artiodactyls, particularly the suiforms, then the clades would become much more complicated with the selenodonts as an isolated group and the suiforms separated into pigs and peccaries as one clade and hippos and whales as a second clade.  Until this is sorted out, we will retain the more traditional classification.  Most morphologically-based phylogenetic analyses show the cetaceans as sisters to the artiodactyls (e.g. Benton 2005, Thewissen et al. 2007, and Gatesby and O'Leary 2001).  However, molecular analyses (e.g. Price et al. 2005, and Nikaido et al. 2001) seem to point to the hippos as the closest living relatives to the cetaceans.

The whales appeared in the Eocene, much earlier than the earliest known remains of hippos, as semiaquatic carnivores, similar to crocodiles in lifestyle (summarized in O'Leary et al. 2004).  Though the mesonychids were considered the likely precursors of cetaceans (e.g. O'Leary et al. 2001), information from an impressive set of fossil animals from the Eocene of Pakistan and India across to North Africa, show a relatively complete series of animals that transitioned from terrestrial carnivore/scavengers to fully aquatic leviathans in relatively short time. Among the earliest known in the line was Pakicetus (the whale of Pakistan, Figure 47), a most unlikely whale ancestor.  It was an amphibious animal that clearly was able to move effectively over terra firmaAmbulocetus (the walking whale, Figure 48) was more transitional in its form and lifestyle.  Thewissen et al. (1994, 1996), Thewissen and Fish 1997, and Madar et al. (2002) describe an animal that was at home in the water and on land.  It had a long crocodilian-like head and inhabited fringing swamps and marshes of the Tethys Sea.  Because it was a carnivore but equally slow on land and the water, likely lived as an ambush predator.

The transition to a more whale-like body with nostrils moving higher on the head, the pectoral limbs becoming paddle-like, the reduction and loss of the hind limbs seems to have occurred somewhat rapidly.  By the middle Eocene, animals that would be recognized as whales (e.g. Dorudon, Maiacetus, and Basilosaurus, Figure 49) had appeared (Gingerich et al. 2009).  They had small hind legs, and long tails that probably had flukes.  The further transition to modern whales involved elongation of the skull and the development of the melon organ, a structure involved in echolocation.  Also, the hind limbs either disappeared or remained as bony elements that were not attached to the vertebral column.

Modern whales are divided into two groups: the toothed whales, which may not be monophyletic and the baleen whales, which seem to be monophyletic.  Though Price et al. (2005) treat them as separate clades, fossils of toothed whales come from deposits as old as the Miocene and the baleen whales seem to have arisen more recently in the Oligocene (Benton 2005).  All modern whales have well-developed echolocation abilities and seem to be quite social with various forms of communication.  

All toothed whales are carnivores and feed on fish, other mammals (including other whales), and squids.  The Sperm Whale (Figure 50), which can grow to be up to 18m long, have a head that occupies about one-third of the body and an enormous melon organ.  They feed primarily on giant squids for which they can dive up to 1,000m.  This group also includes Orcas, Dolphins, and Pilot Whales.  Some of the toothed whales are native to brackish and fresh waters.

The baleen whales include the largest animals that have ever lived on the earth (e.g. Blue Whale, Figure 51).  All of these animals are adapted to straining their food from water or suspended mud through overlapping sheets of baleen in their mouths.  The Blue Whale fills its enormous pleated mouth cavity with plankton-containing water and then forces it through the baleen with its tongue.  This way, the animal can strain out hundreds of kilograms of krill, a planktonic crustacean, with each mouthful.  Other whales like Right Whales have equally large heads but no throat pleats and strain out much smaller crustaceans like copepods.  Gray Whales are somewhat unusual in their method of feeding.  They scoop up bottom mud from which they strain out crustaceans.




































Euarchonotoglires (12)

The Euarchonotoglires (Clade 12), sometimes called the Supraprimates, have two major monophyletic sister groups: Glires (rodents, rabbits, and hares) and Euarchonta (primates, tree shrews, and flying lemurs).  They are sisters to the Laurasiatheria in a clade called Boreoeutheria (Clade 10).  


This group seems to have appeared in the late Cretaceous, when it separated from the Laurasiatherian line (Benton 2005).  The Plesiadapiformes, an early group of arboreal mammals, were the earliest known members of the euarchontoglires and appeared to be sisters to the primates.  Animals like Purgatorius (Figure 52) had bodies that resembled primates, even to the extent of having nail-like claws, but their heads were long and squirrel-like with the eyes on the sides rather than the front.  Although many aspects of these animals seemed basal, their teeth were highly modified with distinctive incisors and molars.  The derived nature of their dentition suggests that they did not give rise to any particular groups in the euarchontoglires.  Indeed, Wible et al. (2007) show them as non-placental mammals altogether.


According to Benton (2005), the primates have three major sets of adaptations: agility in the trees, large brain and acute diurnal vision, and parental care.  Clearly, these are not synapomorphies, nor are they restricted to the primates, but they can explain the features of large forward-set eyes and grasping manus and pes.  

The primates are roughly divided into three major groups: Strepsirrhini, Haplorhini, and Anthropoidea.  The strepsirrhini (the curly nostrils, also referred to as the Lemuriformes) include somewhat primitive adapiformes and lemurs.  The adapiformes were lemur-like animals and among the most abundant of the early primates (lower Eocene to the upper Miocene).  They had a relatively long snout, small orbits, and dentition that indicated herbivory. 

Lemurs are alive today and restricted to Madagascar, southeast Asia, and Africa south of the Sahara.  In general, they have an elongate fox-like face and all members of the group have a tooth comb made of forward pointing incisors and canines (feeding and grooming) and a toilet claw on the second toe.  Though most are nocturnal, some like the Woolly Lemur (Figure 53) are diurnal and move about on the ground in social groups.  Lemurs and their relatives tend to be omnivores, but a few like the Aye-Aye (Figure 54) have become specialists and have elongate fingers for probing wood for insects.

The Haplorrhini (the simple noses, also referred to as the dry noses) as defined by Wilson and Reeder (2005) has two major groups: the Tarsiformes and the Simiiformes.  The tarsiforms include the tarsiers (Figure 55), which are insectivores with relatively large heads, small snouts, and large orbits.  They likely are related to the Omomytidae, which were small tree-dwelling mammals and among the oldest primates (Eocene).

The Simiiformes (the monkey forms) form two great monophyletic groups: Platyrrhini (flat noses) and Catarrhini (the hook noses).  The platyrrhines are New World Monkeys which occur in the tropics and subtropics of North and South America.  They usually live in troops and are omnivorous. Generally  arboreal, the platyrrhines have prehensile tails and nails.  Examples include Spider Monkeys and Howler Monkeys (Figure 56), both of which can move through the forest canopy with great agility using their tails as fifth hands.  All of them except the Marmosets and Tamarinds, have have 36 teeth, 4 more teeth than other primates.

The catarrhines are the Old World Monkeys and Apes, which in general have 32 teeth.  Unlike many of the platyrrhines, none of the catyrrhines have prehensile tails.  This line seems to have appeared in the upper Eocene.  The early catarrhines like Aegyptopithecus appear to have been close to the point of divergence between the Old World Monkeys and the Apes.  Animals that clearly were Old World Monkeys appeared by the late Miocene.  Today, they occur throughout warm regions of the Eastern Hemisphere except Australia and Madagascar.  Although at home in the trees, the Old World Monkeys do not have prehensile tails.  Examples include well-known groups like baboons (Figure 57) and macaques (Figure 58).

The Old World Monkeys tend to be somewhat omnivorous with a general diet of fruit, leaves, and small invertebrates. They have cheek pouches that aid in the immediate collection and transportation of bulky food items.  The old world monkeys seem to have diverged from the ape ancestors by adaptations toward a more foliverous (leaf-eating) diet.

The apes appeared in the lower Miocene and the earliest (Proconsul) walked on all four feet, which were ape-like.  The earliest apes also had small molars and very large canines.  They were somewhat similar to the tree-dwelling gibbons today.  Most of the living apes are in the Hominidae, the family in which our species occurs.  The basal hominids also appeared in the Miocene and seemed to be equally at home in trees and on the ground.  The hominid apes include two living subfamilies: the Ponginae (orangutans) and the Homininae [humans, chimps (Figure 59), and gorillas].  Chimpanzees and gorillas are poorly represented in the fossil record; however, the human (bipedal) line has quite an elaborate record with at least 15 species.  Read more about Hominin Evolution.

Tree Shrews and Flying Lemurs

Living sister groups to the primates include the tree shrews and colugos or flying lemurs.  Both groups appeared by the Eocene and persist as line of low diversity, mostly of southeastern Asia.  Tree shrews (Figure 60) occupy deciduous forests of central and southeastern Asia.  They are small and very squirrel-like (but with 5 toes).  Unlike squirrels, they have complete auditory bullas and complete zygomatic arches.

Flying Lemurs or Colugos (Figure 61) are arboreal herbivores of southeastern Asia to the Philippines.  They have an expandable flap of skin that connects the shoulder to the forefeet, hindfeet, and tip of the tail.  With the expanded membrane, they can glide up to 100m.  Colugos are about the size of a squirrel and feed by scraping leaves with their incisors from which they gather material with their long tongues. 


The rodents and hares are very speciose and comprise more than 40% of all living mammals.  The basal rodents may have appeared in the upper Cretaceous with small rat-like animals like Zalambdalestes (Figure 62) and Kulbeckia.  Archibald et al. (2001) affirm the relationship of these taxa in a group called the glires.  That view is not universally accepted, though.  Misawa and Janke (2003) reject the monophyly of the glires and claim that the hares and rabbits (Lagomorpha) are part of the clade that includes humans, dogs, and cows.  With that in mind, I present the views of Benton (2005) and Wilson and Reeder (2005), who propose that the glires are formed by two great orders: Rodentia (rodents) and Lagomorpha (hares and rabbits).


Rodents are characterized by having deep-rooted incisors which grow continuously through their lives.  The incisors are followed by a gap (diastema), a premolar and the molars. The typical rodent digit plan (4 on the forefeet and 5 on the hind feet) can be variable with fewer digits on both feet.  Typically, rodents are small animals that are found on most continents.  They range in habitat from fossorial, arboreal, or semi-aquatic environments.  Traditionally, the rodents have been divided into two large suborders (Sciurognathi and Hystricognathi) divided according to the structure of the jaw, the insertions of jaw muscles, and the structure of the zygomatic arch.  Molecular phylogenetics, however, discovered a somewhat hidden diversity, especially within the former Sciurognathi, which Wilson and Reeder (2005) divide into three groups that they interpret as suborders: Sciuromorpha (squirrel forms), Castorimorpha (beaver forms), and Myomorpha (mouse forms).  The former Hystricognathi is almost unchanged with most of the families of the hystricognaths now found in the Hystricomorpha (porcupine forms).  The major difference is the removal of two families in the Anomaluromorpha (the scaly-tailed forms) as a separate suborder.  So, in all, Wilson and Reeder (2005) recognize five rodent suborders.

Sciuromorphs include squirrels and dormice.  Squirrels (Figure 63) occur on all continents except Antarctica, Australia, and southern South America.  They range from arboreal (e.g. Gray Squirrel) to fossorial (e.g. Chipmunk) animals, they eat nuts and seeds, but supplement their diets with other things like fungi, small birds, sap, etc.  Dormice are mainly herbivorous, but do eat small invertebrates and other mice; found from northern Africa to Europe across to central Asia.  They resemble chipmunks with short limbs and broad feet tipped by curved claws (4 foredigits and 5 hinddigits).  

Castorimorphs include an unlikely mix of taxa including beaver, pocket gophers, and kangaroo rats.  Beavers are found throughout the northern temperate zone of the Northern Hemisphere.  Semiaquatic herbivores (they feed on bark and leaves and fell small trees) with a coat of long outer fur and dense underfur.  They build lodges of sticks and mud and their activities modify depth and flow rates of lakes and streams.  Beavers have a strongly-built skull that is flattened in profile; jaws are especially powerful.  Their epiglottis lies above the soft palate and can be sealed off with the tongue; so, they can gnaw underwater without getting water in their lungs.

Pocket Gophers (Figure 64) are fossorial animals of North and Central America.  A distinguishing character is fur-lined cheek pouches (pockets).  They are stout with short neck and powerful digging legs with feet (5 digits) that are broad tipped with strong claws.  They have short, almost hairless tails, small eyes and ears.

Kangaroo Rats and Pocket Mice are fossorial herbivorous (mainly seed eaters) animals of deserts and dry grasslands of North America and also of wet and dry tropical forests of northern South America.  These animals are adapted to leaping and hopping with very long hind legs (Kangaroo Rats are almost bipedal).  Like Pocket Gophers, they have large fur-lined cheek pouches.  

The Myomorphs include about a fourth of all living mammals.  They include mice, rats, gerbils, hamsters, lemmings, and voles. Most members of this group are small animals.  They have exploited almost all terrestrial habitats on earth and are found on all continents except Antarctica.  Myomorphs have two very unequal groups: Dipodidae (two-feet) and Muroidea (mouse-like).

The dipoids include a single family of small kangaroo-like jumping mice.  These include most of the jumping mice of the Northern Hemisphere.  Jerboas (Figure 65) show features of the family almost to an extreme.  They can move about by leaping, hopping or walking on their hind legs, a true bipedal stance.

The muroids include most of the common rodents including rats (Figure 66), mice, hamsters, voles, gerbils, lemmings, and muskrats.  The relationships between humans and members of this groups are mixed.  Rats and mice can be extremely destructive to agriculture and human dwellings.  They have been implicated as carriers of the vectors of disease such as Bubonic Plague.  Once confined to Eurasia, the Brown Rat has been carried over land and sea as an invasive in many areas.   The Brown Rat also is used in scientific research and can make nice pets.  Other murids like the Deer Mouse are important components of natural communities in North America.  So, all in all, the group gets mixed reviews.

Amomaluromorphs are the Scaly-Tailed Squirrels and Springhares (Figure 67) of Africa.  The two groups are very different from each other.  The Scaly-Tailed Squirrels are  arboreal herbivores of the forests of central Africa.  These animals resemble flying squirrels, and almost all have gliding membranes.  The underside of the tail has two rows of scales, a character that gives the suborder its name.  Springhares (Springhaas) are rabbit or kangaroo-like nocturnal herbivorous rodents of southern Africa.  They hop and also move in a quadrupedal gait.  The pinnae of the ears are sparsely-haired and they have large eyes.  The animals are good diggers and are somewhat fossorial. 

Hystricomorphs have a dentary with a distinctive broad flange for the insertion of jaw muscles.  Among the largest of the rodents occur in this group which includes Pacas, Mole Rats, Guinea Pigs, Chinchillas, Pacaranas, Porcupines (both New World and Old World families), Capybaras, and Nutrias. The following descriptions are not exhaustive of the group but serve to introduce the diversity of the suborder.

Pacas (Figure 68) are animals of tropical forests in Central and South America.  These animals are very large (up to 12kg).  They have stout bodies with long legs and no tails.  The forefeet have 4 toes and three toes on the hindfeet, and the claws are almost hoof-like.  Their skulls are massive with large zygomatic arches (the jugal does not connect with the lacrimal).  They are terrestrial animals and good diggers.

Mole Rats (Figure 69) are fossorial animals in Africa, south of the Sahara Desert.  Their legs are short and strong with small ears and eyes.  Their incisors are massive, procumbent, and lack the yellow pigment characteristic of rodents.  The animals are highly fossorial and herbivorous.  They range from solitary to colonial animals with social structures reminiscent of the social bees.  Some are naked, but most have a thick fur.

Cavies (Figure 70) are diurnal animals which occur over most of South America.  They occur in two major groups the cavies with compact bodies (like Guinea Pigs) and almost rabbit-like (Patagonian Hares).  They have rootless incisors, and flat crowns.  They are herbivores with many areas of specialization (e.g. leaves, fruits, grasses).  They do not hibernate.

Chinchillas are herbivores of the central and southern Andes, Patagonia, and  Southern South America.  Chinchillas have a fine fur with compact bodies and pronounced tails.  Their hind legs are longer than the front legs, and their paws have fleshy pads.  The forefeet have 4 toes that are highly mobile and can be used for grasping.  They jump bipedally but generally move about quadripedally.  All are colonial and have large behavioral repertoires.

Agoutis (Figure 71) are animals of the New World tropics.  They have slender bodies, relatively large heads, long legs, and short tails.  The hindlegs are longer than front legs and the feet are tipped with sharp, hoof-like claws.  Fur is long and coarse with a distinctive sheen.  Normally, they are solitary.  They are good runners.

Pacaranas are animals of the foothills and slopes of the Andes in northern South America.  They have broad, massive heads with very long vibrissae and short legs with a plantigrade stance.  The incisors are large and powerful allowing them to eat a variety of stems, leaves, and fruits.  Little is known of the living animals because they are rare.  Josephhoartigasia (Figure 72) lived about 2-4 million years ago in the area of Uruguay and weighed nearly 1000kg making it the largest known rodent that has ever lived (Rinderknecht and Blanco 2008).

New World Porcupines (Figure 73) occur from the Arctic coast of North America through Central America to northern Argentina.  All have hairs modified into spines with barbs.  They are solitary nocturnal animals with weak eyesight and good sense of smell.  Some have prehensile tails.

Capybaras (Figure 74) are large rodents found in the tropics of South America.  They have stout bodies with short and deep heads.  Their ears and eyes are small, and their nostrils sit high on the rostrum, apparently adaptations to a semi-aquatic lifestyle.  Their legs are not particularly long, but the forelegs are shorter than the hind legs; also, they have 4 digits on their front legs and 3 on their hind legs.  Capybaras feed on aquatic plants.

Old World Porcupines are terrestrial animals of Africa and Asia and are distinguished from the New World Porcupines by having quills without barbs.  Their bodies are stout and their legs are short terminating in feet with plantigrade stance (5 digits each foot).  They are good diggers and construct burrows.  Generally omnivorous, they eat many kinds of plants and carrion.

Nutrias (Figure 75) are semi-aquatic animals of Central and South America, but they have become distributed over much of the world, where they are destructive exotic invasive animals.  They are robust for rodents and have numerous adaptations to an aquatic life: webbed hind feet (5 digits and 4 on the forefeet), small ears, small eyes, the tail is long, almost naked and round.  Their pelts have a thick underfur.  Nutrias are herbivores and feed on land and in the water.  Also, they are good diggers, making burrows in the banks of streams or lakes.  They are social and usually live in groups of 10-15.

Lagomorphs (rabbits, hares, and pikas)

Rabbits, hares, and pikas.  Like rodents, the lagomorphs have incisors that are deep rooted and grow continuously.  However, they have a second small pair of incisors in the upper jaw.  They appeared in the middle Eocene.

Rabbits (Figure 76) and hares are globally distributed animals of grasslands, forests, and tundra.  Usually their ears are quite long (much longer than wide).  The hindlimbs are long and longer than the forelimbs.  The tail is quite short.  Skulls are distinctive and arched with a broad zygomatic arch.  They may be solitary or live in small groups.  Females are larger than males.  Many species flourish in the presence of humankind and can become agricultural pests.

Pikas (Figure 77) are animals of open plains and steppes of western North America and central and northern Asia.  They are compact animals with a very short tail and long, fine fur.  The skull is flattened with a short rostrum.  The zygomatic arch is delicate.  The first incisors appear like those of rodents, but the teeth are surrounded by enamel. 

































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By Jack R. Holt.  Last revised: 12/26/2012