The earliest mammals are shrewlike animals known from the Late Triassic and Early Jurassic.  By this point in earth’s history, dinosaurs were already dominating the world’s ecological niches.  Just as modern mammals are currently limiting the evolution of modern reptiles throughout the world, advanced reptiles once did the same to early mammals.  All known Triassic mammals weighed between 20-30 grams (Lillegraven).

     Early mammals are classified as mammals on the basis of their jaws and teeth.  This is not only because the lower jaw was the site of many important mammalian modifications, it is also because these are the easiest and sometimes the only parts to fossilize.  In mammals, the jaw joint is between the dentary and squamosal bones as opposed to the quadrate-articular jaw joint in reptiles and primitive synapsids.  In mammals, canines, premolars, and incisors are replaced once while molars are not replaced at all.  Since cheek teeth are permanent, it is an advantage to have matching surfaces on upper and lower teeth (occlusion); the occlusion of mammalian molars and premolars are an important characteristic.  Other mammalian characteristics such as hair, mammary glands, and placentas obviously do not fossilize well.

    Several groups of early mammals are known; the morganucodontids are the best known.


    Morganucodon, Sinocodon, Megazostrodon and Dinnetherium have the tooth structure of cynodonts.  Some feel that this group is ancestral to all mammals while others feel that they had diverged from ancestral stock and were the ancestors of the monotremes.  Morganucodon is known from thousands of elements from 4 continents.  Sinocodon may be more primitive than Morganucodon or may not even be a mammal.

     The braincase of Morganucodon was larger than that of the cynodonts (and 3-4 times the same relative size of a therapsid braincase) and its head is large compared to its body.  Its occipital and sphenoid bones consisted of smaller ancestral bones which fused together during development (as in modern mammals).  As in reptiles and monotreme mammals, the cochlea of its inner ear was not coiled.  There was a slight reduction in the size of ear bones.  The lower jaw still included tiny remnants of the ancestral prearticular, angular, surangular, and coronoid bones.  The two halves of lower jaw are not yet firmly joined and could move relative to each other.  The dental formula was 5,1,4,4/5,1,4,4 (5 incisors, 1 canine, 4 premolars, 4 molars in both jaws) (Kemp, 1982; Carroll, 1988; Hopson). 

morganucodon morganucodon

   In many of these early mammals, and in the most primitive mammals alive today, the cervical ribs were not yet fused to vertebrae as in later mammals.  In the embryos of higher mammals, cervical ribs form which fuse to the bodies of cervical vertebrae.


cervical ribs

      These early mammals possessed a number of primitive skeletal traits.  In Morganucodon, the proatlas and the articulation with neural arch of the axis was lost, allowing rotation of the head. The atlas’s neural arches were still paired and not yet fused with the intercentrum of the atlas. The centra still retained traces of notochord pits.  The procoracoid bone was still present but it no longer formed part of shoulder joint.  There was not yet any spine on the scapula (shoulder blade).  The clavicle and interclavicle were still large.  The humerus was still primitive in its appearance, the pubis was smaller, and the lower margin of the hip socket was incomplete.       

       The earliest mammals diversified into a number of forms.  A group of mammals belonging to the Order Haramiyida lived from the Late Jurassic to the Early Tertiary.  They had a unique style of chewing, which can be described as “puncture crushing” (as opposed to the shearing and grinding styles used by other Mesozoic groups) (Jenkins, 1997; Kemp, 1982; Carroll, 1988).    

    Kuehneotherium had a tooth cusp pattern that differs from these others and is probably closer to the ancestors of the therian mammals (marsupials and placentals). Unfortunately, its fossils are incomplete.  

hadroconium      Hadrocodium was a small mammal known from the Early Jurassic about 195 million years ago  Its braincase is enlarged in the alispheniod and parietal regions (wider in each case).  Its addition of the malleus and incus as ear bones is inferred from the absence of the postdentary trough which would otherwise contain them in the lower jaw. This trough is present in other early mammals such as Morganucodon, kuehneotherids, and Haldanodon. The swelling which contains the inner ear in the petrosal bone is larger than in other mammaliforms.  It estimated body weight would have been about 2 grams, about the same size as the smallest living bats (2 grams) and insectivores (2.5 grams).  The advanced features of Hadrocodium put it closer to the main mammalian lineage than Morganucodon, Sinoconodon, Adelobasileus, and Haldanodon.  It is the sister group to mammals with living representatives and triconodontids (Luo, 2001).


In the Late Jurassic, primitive mammals were replaced by eutriconodonts, symmetrodonts, dryolestids, and multituberculates. Most of these mammals were insectivores or omnivores with unspecialized skeletons. The Late Jurassic mammal Fruitafossor represents a basal mammalian lineage which evolved shortly after the monotreme lineage diverged from that of the therian mammals. It seems to have been specialized for digging and feeding on insects (Luo, 2005).


    Most remains of mammals from the late Mesozoic are little more than jaws and teeth.  These fossils possessed some modern mammalian characteristics such as a dentary-squamosal joint, the replacement of teeth only once, and molars with a specific occlusion pattern.  A variety of nontherians (mammals that are not marsupial or placental mammals) are known from the Jurassic and Cretaceous.  Most were small; the largest were about the size of an opossum.

     The docodonts from the Middle and Upper Jurassic consist of 4 genera that left no descendants. They were mouse-sized, had long snouts, and unique teeth. The docodonts evolved tooth occlusion early in their evolution and independently of other groups of mammals.  They still possessed a trough in their lower jaws which might have held vestiges of ancestral jaw bones.  As a result, there is some discussion as to whether they should be considered as mammals.  Some feel that animals whose major jaw joint was the dentary-squamosal should be considered as mammals even if the quadrate and articular bones had not become part of the middle ear (Lillegraven; Kemp, 1982; Carroll, 1988; Hopson).     The Middle Jurassic docodont mammaliaform Castorocauda possessed middle ear bones (articular, surangular, and angular) located on its mandible. Its teeth and broad and scaly tail suggest it was adapted to a semi-aquatic lifestyle. At 164 million years of age, it represents the oldest known fossil to possess fur. Its skull measured about 6 cm and it would have weighed about ½ kg (Ji, 2006).


     The amphilestids from the Mid Jurassic to Early Cretaceous were about the size of opossums and were perhaps related to the therians because of similarities of the shoulder and sphenoid bone.

     Triconodonts existed from the Late Triassic through the Cretaceous.  The earliest weighed 10-20 grams while some later forms were the size of oppossums but more robust (Lillegraven).  The triconodontids, such as Gobiconodon above, were similar to Morganucodon.  In the triconodontid Jeholodens, the rear limb was abducted as in reptiles but their arms and shoulders were more mammalian in their mobility and orientation (Zimmer, 1999).  Vincelestes was a Late Cretaceous mammal closely related to therians.
vincelestes ptilodus

     One family of Cretaceous mammals known as gondwanatheres were known throughout the Southern continets (Krause, 1997). The discovery of grasses in late Cretaceous dinosaur coprolites supports the hypothesis that the specialized teeth of gondwanatherian mammals represent an adaptation for grazing on grass (Prasad, 2005).

      The multituberculates were the most diverse and numerous Mesozoic mammals.  They varied in the types of teeth present in their mouths--some had 2,0,1,2/1,0,0,3 tooth pattern (notice that some tooth types such as canines and lower premolars were absent).   Most are known only from fossils of teeth and jaws.   They ranged in size from that of a woodchuck to that of a mouse and probably were omnivorous.  Multituberculates possessed a mix of primitive and derived features.  Bulganbaatar, for example, retained the primitive interclavicle bone in its shoulder girdle (as do monotremes) but its shoulder was much more mobile and was held closer to its body rather than angling away from it (abducted) (Sereno, 1995).  Taeniolabis is the largest known multituberculate, about the size of a beaver. There is some disagreement over whether the middle ears of multituberculates evolved derived characterisitcs separately from those of other mammals and there is also disagreement over whether monotremes and multituberculates are closely related to therian mammals.   Multituberculates persisted into the Cenozoic and perhaps competition from rodents led to their extinction (Kemp, 1982; Carroll, 1988; Meng, 1995; Rougier, Montellano, 2000).         Multituberculates are sometimes referred to as the “rodents of the Mesozoic” and about a dozen families are known.  In the Northern Hemisphere, multituberculates typically compose half of the mammalian fossils and seemed to spread with the rise of flowering plants.  (Mesozoic fossils from the southern hemisphere are almost unknown.)  Many multituberculates became extinct in the Cretaceous.  Although there was some diversification of their lineages in the Paleocene, their range was limited to North America during the Eocene.  Multituberculates possessed large lacrimal bones, the tabular and ectopterygoid bones were still present, the jugal bones were absent, there was no supraspinous fossa on the scapula, and the cochlea was straight (Lillegraven).   Eomaia, the earliest known placental mammal, was preserved with a coat of fur and fur is known from multituberculate fossils as well (Ji, 2002).

multituberculate multituberculate


     All modern mammals share a number of characteristics in addition to the skeletal characteristics already mentioned: they possess at least some hair, they are endothermic, and they produce a fatty, protein rich sweat called milk which nourishes their young.  A mother cat nursing her kittens is pictured below.


There are three groups of modern mammals:



     There are three living genera of primitive mammals from Australia classified as monotremes: the duck-billed platypus and 2 genera of echidna.  Monotremes are classified with triconodonts, docodonts, and multituberculates in the subclass Prototheria.  Monotremes are known since the early Mesozoic in Australia.  This subclass of mammals has survived 200 million years, far longer than the therian mammals (Musser, 2003). 

A basal therian mammal with a greater similarity to marsupials than to placental mammals is known from China from strata dating from 125 million years ago. Sinodelphys szalayi, possessed a number of anatomical features linking it to other metatherians and marsupials such as modifications of the wrist (an enlarged hamate, scaphoid, and triqetrum and a smaller trapezium), ankle, arm, and teeth (Luo, 2003).

Evidence indicates that the placental and marsupial lineages had diverged by the Early Cretaceous by about 125 million years ago (the date of the metatherian Sinodelphys and the eutherian Eomaia). The most primitive metatherians and a sequence of more derived forms are known from Asia. The divergence of the modern orders of marsupials is thought to have occurred in the Late Cretaceous (Luo, 2003).

There is one monotreme lower jaw from the Cretaceous (Steropodon) known from Australia (which was from a badger-sized animal.  One monotreme fossil is known from South America and the fossil of teeth of an Australian Miocene platypus ancestor (Obdurodon) is known in which the teeth of adults resemble vestigial teeth found only in immature modern platypuses (Pascual, 1992; Archer, 1985).   Australian platypuses are known since the Oligocene and echidna fossils are known in Australia since the Miocene (Musser, 2003). 

 Modern monotremes still possess a number of primitive features: they lay eggs, have a more reptilian structure of their reproductive tract, possess a cloaca (common opening for urinary, reproductive, and digestive tracts), and lack a scrotum in males.  The shoulder is similar to that of cynodonts and primitive Mesozoic mammals in the retention of both coracoid bones and the interclavicle; there no supraspinous fossa of the shoulder blade.  The cervical ribs are not fused to vertebrae.  They have a somewhat sprawling posture and low metabolic rate. In monotremes, body temperature is lower and more variable than in therian mammals. Monotremes do possess a number of derived mammalian characteristics: they have a mammalian jaw joint, 3 inner ear bones, hair, and mammary glands (although there is no nipple and milk is secreted onto females chest like sweat and is licked off of tufts of hair (Kemp, 1982; Carroll, 1988; Grutzner, 2004). The shape of sperm and their formation in monotremes is more similar to reptiles than to therian mammals.  Several characteristics of monotreme chromosomes (such as the lack of imprinting and X inactivation) differ from therian mammals (Grutzner, 2003; 2004). Male platypus possess a poisonous spur (Grutzner, 2004).


THERIANS (marsupials and placentals)

   Modern placental and marsupial mammals are classified as therians because they give live girth to their young.  Kuehneotherium had dental characteristics (such as molar occlusion patterns) of therian mammals.  A few species in the Cretaceous seem to predate the split between marsupials and placentals such as Deltatherium (tooth formula 4?,1,3,3-4/1-2,1,3,4-3), Kielantherium.  Some extinct groups left no descendants.  Symmetrodonts were rare but last from the late Triassic to the Cretaceous (Rougier, 1998).  Mitochondrial data tend to support grouping marsupials and monotremes together while most nuclear genes support a therian grouping (Belov, 2003).




     Marsupials are the second most common group of mammals today.  They differ from the placentals in their teeth and a unique marsupial bone in pelvis.

Their embryos but they are born at about the same developmental stage at which monotreme mammals hatch from eggs. Kangaroo infants weigh 1 gram at birth.  Marsupial newborns are ectothermic (cold blooded) when they are young due to their size.        Marsupials fetuses possess a thin eggshell membrane and a vestigial egg tooth. This shell allows for diffusion but no contact between the mother and fetus which might trigger the mother’s immune system. (Lillegraven, p. 262-4)  Marsupial development, which is completed during lactation in a pouch, takes longer and has a higher energy cost than placental development.  The malleus and incus (middle ear bones) are retained in a jaw joint when marsupials are born and only after they leave the pouch to they become incorporated into the middle ear (Lillegraven, p. 265).

     The earliest marsupials are known from the Upper Cretaceous of North America and were probably in living in South America at this point as well.  Marsupials were already diverse in South America by the Paleocene.  These North American fossils belong to the group Didelphoidea, an early group that contains the modern opossum and survived in North America through the Miocene.  Marsupials migrated from North America to Europe (with one specimen known from Asia as well) and all became extinct in the Miocene. Marsupials migrated from South America to Antarctica (by the Eocene) and then to Australia (by the Oligocene).    Pucadelphys is a didelphoid marsupial from the Paleocene.

pucadelphys skulls
     Why were marsupials so successful in South America and Australia?  North and South America were separated from the Upper Cretaceous until about 3 million years ago and therefore modern placental mammals reached South America only recently.  Australia, obviously, still lacks direct land contact with any continent where placental mammals dominate.  Today there are 17 genera of marsupials in South America and at least 76 fossil genera are known.  While placental mammals did not thrive in Australia, fossil evidence suggests that a few were present, including the condylarth-like Tingamarra (Godhelp, 1992; Rich, 1997; Wuethrich, 1997). The opossum is known from North America since about 2 million years ago (Kurten, 1988).
     The family Didelphidae were/are oppossum-like, omnivorous and have a tooth formula 5,1,3,4/4,1,3,4.  Most can climb and have a prehensile tail.  They range in size from mouse- to cat- sized.  Twelve genera are known from the Paleocene. 
     The microbiotherids were marsupials which were very different from other groups and had a huge bony case around their ear bones.  The borhyaeinoidea included at least 35 genera which evolved from the didelphids.  They were carnivores that bore superficial similarities to medium sized placental carnivores.  Borhyaena resembled a wolf, Thylacosmilus a tiger sized saber tooth cat, and one was bear-sized.  Lycopsis was a coyote-sized predator (de Mulzon, 1997).

       Caenolestids were small insectivorous and omnivorous animals and the most abundant marsupials of Miocene.  The group Polydolopoidea includes the only marsupial known from Antarctica.  The groups Groeberoidea and Argrolagoidea were rodent-like.

    In Australia, marsupials are first known from Late Oligocene.  Since they were already diverse by this point, they probably arrived earlier.  The Dasyuroidea are small to mid sized insectivores, omnivores, and carnivores that evolved from the didelphids (Akotarinja was very similar to the didelphids).  There are 14 modern genera (including the Tasmanian devil, the Tasmanian wolf Thylacynus which became extinct in 1934, and the numbat or Australian anteater).  There are 6 fossil genera.  The group Perameloidea includes modern bandicoots and some fossil species.  They actually have a placenta but the young are still born very immature and must mature in the mothers pouch.

Diprotodon (below) measured 10 foot long and was the largest known marsupial.   Simostenurus was an unusual kangaroo which had only one toe on each foot.  The fossil kangaroo Procoptodon stood 2.6 m (8 ft.)


--after Martin, 1984

     The phalangeroids include wombats, koalas, kangaroos, gliding and pygmy opossums, squirrel-like species, and a number of extinct groups which could be sheep to hippo sized.  Thylacoleo may have been carnivorous.