Eomaia is a eutherian mammal known from the Late Jurassic. Given that its skeletal features separate it from two other fossils known from about the same time, Murtoilestes and Prokennalestes, the evolution and diversification of eutherian mammals must have begun by the Late Jurassic. Eomaia possessed a number of primitive characteristics (including an epipubic bone) and derived characteristics (including a patella). The fossil indicates that it was covered with fur and modifications of the limbs are consistent with a more arboreal lifestyle (Ji, 2002).

    Since the End Cretaceous extinction, placental mammals have been the dominant terrestrial vertebrates (except in Australia and Antarctica

).     Four basal genera are known from the Upper Cretaceous.  Kennalestes, Ukhaatherium, and Asiorcytes were ancestral forms of all placentals and similar to shrews.  In their skulls (3 cm in length), the squamosal bone contributed to the braincase, the lacrimal bones extended outside of the eye socket, the nasal bones expanded towards the back of the skull, and a large part of the maxillary bone making up a portion of the eye socket.  There was no auditory bulla (bony covering around middle ear) and the angular bone still supported eardrum.  There was the first appearance of a mastoid process behind ears.  The olfactory bulbs in the brain were large.  Asiorcytes and Ukhaatherium had 5/4 incisors (upper/lower; 5 upper incisors is a primitive trait) as in early marsupials and the last premolar beginning to become molar-like.  Kennalestes had 4/3 incisors; adults possessed 4 premolars but infants retained 5.  There was still a trace of the coronoid bone in lower jaw.  The atlas was still united by sutures in Asiorcytes (rather than being one single bone).  Zalambdalestes and Barunlestes may be more derived species closer to the ancestry of rodents (Novacek, 1997).  If Montanalestes keebleri (known from

North America), is a eutherian mammal, then eutherians had spread across the Northern continents by the Early Cretaceous (Cifelli, 1999).

kennalestes asiocytes

Symmetrodonts include 4 families of primitive mammals.  The basal family Kuehneotheridae includes the oldest known therian mammals.  The symmmetrodonts and early therian mammals retained the primitive condition of a straight cochlea.  It seems that this group originated before the therian lineage split into placental and marsupials. Eupantotherians evolved from symmetrodonts. Although symmetrodonts are therian mammals, primitive members such as Akidolestes possessed a number of primitive traits which are retained in modern monotremes. These traits include short lumbar ribs, a primitive hip, and an enlarged process on the fibula (Li, 2006).

      Many Mesozoic mammals, modern monotremes, and modern marsupials, have small bone extending from the pubis called epipubic bones.  Epipubic bones existed in many primitive mammals (mutlitberculates, eupantotheres, and even some therapsids) of both sexes.  These bones may have served more for muscle attachment than for a pouch.  Although modern placental mammals lack epipubic bones, at least two early species, Zalambdalestes and Ukhaatherium (and probably Barunlestes as well) had them.  Epipubic bones seem to be homologous to the bones which exist in the penis and, more rarely, the clitoris, of many modern placental mammals (Lillegraven; Hu, 1997).

      A number of mammals are classified as “mammals of the metatherian-eutherian” grade.  They may not be true placentals, but they are at about the same level of complexity in early mammalian lineages.

     Placental species of the latest Cretaceous and Early Cenozoic had already diversified into 3 groups of placental mammals: insectivores (3 genera), primates (Purgatorious), and herbivores called condylarths.  More than 30 placental families are known by the Mid Paleocene and nearly all modern placental orders are known by the Early Eocene.  The relationships between the orders of mammals remain unclear.  Molecular studies may undermine some widely accepted relationships, such as therians (if marsupials are closer to monotremes than placentals) and rodents (which may be polyphyletic, DeJong, 1998).

     Several groups of the early Cenozoic left no descendants and most were extinct by the end of the Eocene.  Even though many look as if they are relatives to rhinos, cattle, and other ungulates, they belong to unrelated groups of mammals.  Lepticids lived in the early Paleocene and Eocene and left no descendants (Storch, 1992). arsinothere

Aptemyids were small.  Taeniodonts were climbing and digging herbivores.  Tillodonts and pantodonts were large herbivores and some were bear-sized.  Unitatheres were a group of 15 large genera of North America and Asia.  Prouintatherium links the early Paleocene forms to more primitive mammals; Probathyopsis links the Paleocene forms to the larger species of the Eocene (Dorr, 1958).
sinclairella leptictis



In some areas of the world, oil from previously marine sediments is exposed on land. As lighter elements evaporate, the remaining material forms a sticky tar. In some places, such as Peru and the famous La Brea tar pits of California, this tar became a natural animal trap. Animals which became trapped (perhaps because they thought the tar would provide a drink) attracted insects, predators and scavengers (including saber tooth cats, dire wolves, golden eagles, and giant teratorn birds). The bones of carnivores far outnumber other groups of animals. Dating techniques indicated that different tar pits were active during different time periods and some could be active during intervals over a course of thousands of years (Kurten, 1988).


It is possible to estimate global climatic conditions from the past 650,000 years from the ice and air trapped with ice sheets. The isotopes ratios of certain elements in rocks (such as oxygen) can allow estimates of more ancient time periods. Since ice sheets leave traces on underlying rock, it is possible to estimate the extent of past glaciations. Combined data indicate that the earth has been warmer and completely devoid of ice sheets for most of the past 500 million years (IPCC, Document I, 2007).

Louis Agassiz first proposed the possibility of Ice Age in 1837. A series of Ice Ages occurred over the past 1.5 million years. In North America, two ice sheets formed: one originating in the Northwest Rocky Mountains (the Cordilleran ice sheet) and another originating around the Hudson Bay (the Laurentide ice sheet). When the glaciations reached their greatest extent, these two ice sheets joined and formed an ice area of 6 million square miles—an area greater than the ice of Antarctica and possibly representing the largest ice sheet to ever form in earth’s history. Ice sheets also covered parts of Europe, Asia, Greenland, Antarctica, and the southern regions of South America, Africa, and Australia. Because so much water was locked up in ice sheets, sea level dropped an estimated 300 feet. As a result, many regions which are currently covered by ocean were exposed such as an ice-free land bridge which connected Alaska and Siberia (the land bridge is referred to as Beringia) (Kurten, 1988).

The Late Tertiary was cooler than the earlier part had been and a variation of cooler and warmer periods occurred although the magnitude was less than the more recent glaciations and interglacial periods. In the past 1.5 million years, a series of glaciations were interrupted by interglacial periods of comparable warmth. The Nebraskan Glaciation began 1.5 million years ago and ended about 1.1 million years ago. After the Aftonian interglacial period, the Kansan Glaciation began about 900,000 years ago and ended about 600,000 years ago. Following the Yarmouthian Interglacial, the Illinoian Glaciation began about 600,000 years ago and ended 100,000 years ago. After the Sangamonian interglacial period, the Wisconsonian Glaciation began about 100,000 years ago and ended only recently (about 10,000 years ago). The last interglacial period lasted 11,000 years and the prior interglacial period lasted 16,000 years. Since it has been 10,000 years since the last glaciation ended, the current period might be another interglacial period which will end in another Ice Age. The alternation between glacial and interglacial periods is largely due to variations in the earth’s orbit (which changes regularly from elliptical to circular over a period of about 100,000 years), the tilt of the earth’s axis (which changes over a period of about 41,000 years), and the variation of the time period in which the earth is closest to the sun (the precession of the equinox) (Kurten, 1988).

During the last interglacial period (130 to 116 thousand years ago), there was less ice on earth than there is today. During the glacial and interglacial periods, more than 20 abrupt climatic changes have been recorded. For example, the Dansgaard-Oeschger events included an 8 degree Celsius temperature increase in Greenland over a few decades. The Heinreich events were rapid periods of warming in which ice sheet melting in the North Atlantic led to sea level rises of up to 15 meters and dramatic changes of the major currents. Sea level during the last glacial maximum was 140 feet below modern sea level. About 8,200 years ago, a sudden drop of temperature of 2 to 6 degrees Celsius apparently resulted from a reorganization of the Atlantic’s MOC following the massive release of 10 14 cubic meters of water from melting glaciers in the Hudson Bay area (the postglacial “Lake Agassiz”) (IPCC, Document I, 2007).

The earth’s climate of the period prior to 2.6 million years ago was typically warmer than it is today. Three million years ago, the temperature is estimated to have been 2 to 3 degrees warmer than modern times (IPCC, Document I, 2007).


More than fifty New World sites contain remains of humans which have been dated as older than 12,000 years and some have been dated at 32,000 to 40,000 years. From the period of 12,000 to 8,000 years ago, more than 25 species of large herbivorous mammals became extinct. Overhunting and the climate change associated with the end of an Ice Age were probably both contributing factors. A number of mammoth kill sites are known in North America. Several examples of mastodon hunting, in both North and South America, are also known (Kurten, 1988).



     Insectivores are known from Upper Cretaceous and the oldest members of modern groups (moles, shrews, hedgehogs) are known from the Late Eocene.  Because of the retention of a number of primitive characteristics, insectivores are considered the most primitive placental mammals alive today.   These primitive characteristics include an auditory bulla (the case over the middle and inner ears) which is rarely ossified, the small size of the brain, the smooth surface of the cerebrum is smooth, the lack of cerebral expansion over the cerebellum, abdominal or inguinal testes in some males (as opposed testes which descend to the scrotum), a cloaca in some species (a common opening for the urinary, digestive, and reproductive tracts), the lack of opposable digits, and the primitive number and shape of teeth.

      Some of the mammals classified as insectivores are probably not closely related to each other.  Tenrecs and golden moles (both of which exist only in Africa) seem to be more related to elephants, hyraxes, sirens, the aardvark, and elephant shrews (all of which originated in Africa).  Tenrecs and golden moles should be classified with these other groups in a superordinal clade of African mammals rather than with Insectivores (Stanhope, 1996; Springer, 1997).  The fossil insectivore Batodonoides vanhouteni is the smallest mammal known which an estimated weight of 1.3 grams, less than the 2 grams of the modern bumblebee bat (Monastersky, 1998). The modern species of beaver and two species of shrew (least and masked) appeared in North America about 2 million years ago. The eastern mole appeared in North America about 2 million years ago (Kurten, 1988).

     Placental mammals have been classified into four superorders, two of which (Xenartha and Afrotheria) are considered to be more primitive.  The superorder Xenarthra originated in South America and includes armadillos, anteaters, and sloths.  The superorder Afrotheria originated in Africa and includes elephants, sea cows, extinct demostylans, hyraxes, aardvarks, elephant shrews, and tenrecs (although some feel that golden moles and tenrecs should be classified in their own group).  The superorder Euarchontoglires includes the Glires (rodents and rabbits) and Euarchonta (tree shrews, flying lemurs, and primates).  The last of the superorders to evolve is the group Laurasiatheria, which includes whales artiodactyls, perrisodactyls, carnivores, and bats (Carter, 2004). Molecular evidence supports Boreoeutheria as a clade composed of (Euarchontoglires + Laurasiatheria). Within Laurasiatheria, the Eulipotyphla lineage diverged first, followed by Cetartiodactyla. Carnivora, Perissodactyla, and Chiroptera form a clade which has been named Pegasoferae (Nishihara, 2006).



Bats compose one fifth of the species of mammals (Sears, 2006). There are two major groups of bats: the megachiropterans (the largest of which have wingspans approaching 5 feet) and the more diverse group of microchiropterans.  Microchiropterans evolved from megachiropterans.  A number of bat fossils are known from the middle Tertiary through recent times. The oldest bats from Europe and North America date to the Mid-Eocene. Since these bats are microchiropterans capable of flight, the evolution of flight in bats must have occurred prior to the Mid-Eocene (Kurten, 1980). The earliest known bat fossil dates from the Early Eocene, Icaronycteris, and was a microchiropteran which was virtually modern in appearance.  Unfortunately, fossils which would link this michrochiropteran to megachiropterans or bats in general to more primitive mammals have not yet been found.  In Icaronycteris, the teeth are primitive showing a link to insectivore ancestors.  The ribs, sternum, and the number of phalanges (finger and toe bones) are also primitive.  Bats have a number of characteristics (such as the unique innervation of the wing by the facial nerve) which indicate that they descended from a common ancestor (Thewissen, 1991).

Since the first bat fossils (about 50 million years old) there have not been significant increases in the lengths of the digits which compose the bat wing mammals (Sears, 2006). During embryological development, the digits of the bat hand begin with lengths comparable to those in the hand of a mouse. Later in development the third, fourth, and fifth digits increase due to increased production of cartilage in these developing digits. The embryonic signalling protein Bmp2 is not only capable of stimulating the growth of cartilage and elongation of developing bat digits, this signal is produced in the bat hand in levels much higher than those of the bat leg or of the mouse hand. An increased production of Bmp2 in the embryonic bat hand was probably a major factor in its specialization mammals (Sears, 2006).

Bone morphogenetic proteins induce apoptosis in the cells uniting digits in tetrapods as diverse as chickens in mice. Webbed digits, such as those of ducks, result from the inhibition of Bmps. In bats, Bmps cause the loss of interdigital tissue in the feet but the expression of Fgf8 and the Bmp inhibitor Gremlin result in the retention of this tissue in the hands (Weatherbee, 2006). Bat wings are composed of both this interdigit tissue and a membrane which develops from the trunk which joins the hand. Flying lemurs develop a gliding membrane from both interdigit tissue and a membrane developed from the trunk but they lack the lengthened digits of the bat wing (Weatherbee, 2006).

The large cochlea of the Eocene bat Tanzanycteris inciates that echolocation had evolved by the Eocene (Gunnell, 2003).  Archerops  was a Miocene bat intermediate between other fossil bats and modern Old World leaf-nosed bats (Hand, 2003).




squirrel chipmunk

--after Martin, 1984


      About half of all modern species of placental mammals are rodents (and an additional third are bats).  More than 1700 species are existed since the Paleocene and 1/4 of the known rodent families are now extinct.  The earliest rodents (family Ischyromyidae) are known from North America and Europe and they lack the feeding specializations that characterize later rodents.  The order Rodentia includes rabbits, which are distantly related to the other groups.  The Oligocene rodent Anagale still retained many primitive features (for example, it lacked a postorbital bar but a postorbital process was present).  The Eurymyloidae was a Paleocene group that shared characteristics with both rodents and rabbits and was close to the ancestry of rodents.  Rabbits have been known since the Paleocene.

     The first squirrels are known from the Late Oligocene of North America and Europe.  In the Miocene, there was a great radiation of squirrels, including a variety of flying squirrels in Europe.  Four extinct genera are known from the Oligocene (and one possible representative of the modern genus Sciurus), 17 extinct genera are known from the Miocene (in addition to 4 modern genera) (Black, ).  The pig-sized modern capybaras are the largest rodents but their fossil relatives, such as Neochoerus were 40% larger.  The fossil beaver Castoroides of North America could reach ten feet in length (including a 3 foot tail).  The fossil rodent Telicomys was about the size of a hippo, measuring more than 2 meters in length.  Protohydrochoerus in South America could reach the size of a tapir.  Horned gophers such as Ceratogaulus measured 2 feet and had horns.  The fossil beaver Paleocastor could dig burrows 2.5 meters deep (and the remains of these burrows have been called ‘Devil’s Corkscrews’) Fossil guinea pigs could reach 10 feet in length and weights of 1,500 pounds.  The early rodent Epigaulus hatchery possessed a pair of horns (Giant Guinea pig, 2004; Alderton, 1996).  The Pliocene rodent Actenomys possessed arms intermediate between burrowing and non-burrowing rodents and flared zygomatic arches (Fernandez, 2000).  Mutant mice can produce vestigial premolars which have been absent in the rodent lineage for 50 million years (Peterkova, 2005). During the period of 1.8 to 2 million years ago, the species which migrated to North America from South America included capybaras, porcupines, ground sloths, and giant armadillos. Muskrats evolved gradually over the past 3 million years from small forms with few dental specializations (Kurten, 1988).



beaver skulls

--after Martin, 1984

     The group Hystricomorpha is a group of rodents, which includes porcupines; this group differs from other rodents.  Guinea pigs are in this group and there is some evidence that they might not be rodents at all, but their own separate group (Graur, 1991). Laonastes aenigmamus is an Asian rodent which is the only living survivor of the family Diatomyidae which was previously thought to have gone extinct 11 million years ago (Huchon, 2007).

The following photos are of capybaras, the largest living rodents.

capybara capybara 2
capybara 3