Early Mammals

Mammals differentiated from other synapsids in the Triassic, but the fossil record for the earliest mammals is sparse. They existed from the Triassic through the Cretaceous without becoming large or abundant. The mammals that lived at that time differ in many respects from the mammals of today.  Note that fossil mammals are studied largely on the basis of the morphology of the teeth.

Mammals are distinguished by a larger brain size, Their back teeth are not replaced, and their front teeth are only replaced once. The teeth have a specific pattern of occlusion so that the teeth in the upper jaw fit precisely into the teeth of the lower jaw. Different groups of mammals have somewhat differing patterns of occlusion. The primitive appearance of the teeth is that they have three large cusps oriented front to back in the mouth (the triconodont condition).  The jaw is all dentary and the articular-quadrate articulation has been converted into the incus-malleous ear bones. The vertebral column is articulated such that the back bends up and down instead of side to side. The nature of the teeth that can efficiently chew food in the earliest mammals indicates that they were warm blooded, and the small size would have necesitated that they have fur. Hair is associated with sweat and oil glands, which in turn are directly related to the mammary glands, so it is thought that even the earliest mammals suckled their young.

Triconodonts

(Late Triassic - ) Late Jurassic to Late (but not latest) Cretaceous. These retain most of the primitive features of the first mammals, and were no larger than shrews. They retained the primitive sprawling posture (like mice and shrews). Some of the animals included in the triconodonts during the Late Triassic gave rise to other groups of mammals, so cannot be properly included in a monophyletic group, but they retain the same features.
 

Multituberculates

Late Jurassic to Miocene (Middle Cenozoic). These are the most abundant of the Mesozoic Mammals. They are characterized by a very distinctive shearing molar tooth and a superficially rodent like dentition. They ranged from rat to possum sized and appeared to be arboreal (tree dwelling).
 about multituberculates
fossils of the hell creek

"Pantotheres"

This refers to the group of mammals that gave rise to the modern mammals, both marsupials and placentals - Middle jurassic to Early Cretaceous. They are distinguished by the development of tribosphenic teeth, which are designed for "sideways" chewing.

Monotremes and the nature of early mammals.

  • Known from rocks as old as the Cretaceous to the present.
  • These include the modern Spiny Anteater (Echidna) and the duck-billed platypus, both of Australia.
  • Both creatures lay eggs and have primitive mammary glands. That these mammals lay eggs suggests that most of the early mammals laid eggs, yet suckled their young. In fact, the monotremes babies hatch at a very immature stage and begin suckling from that point. The echidna has a pouch into which the young are brought and raised, and in that way shows a link between the egg laying and the marsupial mammals.
  • The early hatching is thought to be an adaptation to allow the earliest mammals to be small, yet warm-blooded (endothermic). The food needed to be provided by the mother to allow rapid growth and high metabolism and the mother could also keep the babies warm, so they did not waste energy on heating such tiny bodies. Monotremes are not tiny, but they have inherited what must be the primitive reproductive method.
  • Affinities are unknown, Some recent work puts them with multituberculates, others have placed them as descendents of the pantotheres.
Introduction to the monotremata
More on monotremes

Marsupials

Marsupials are the most common mammals of the Cretaceous. Many of the Cretaceous forms bear remarkable resumblance to the modern opposum.
They are distinguished by having live-born young which crawl into a pouch (marsupium) that is equipt with mammary glands. The young are born immature, much as the hatchlings of monotremes. Brains are typically smaller than placental brains.

Placentals

Late Cretaceous to present. The first placentals were like the living insectivores, including shrews and hedgehogs. The placentals are characterized by the presence of the placenta which allows the mother to supply the food needed by the developing embryo without forcing the embryo to be born early.  The brains are larger than marsupials, and placental types are represented by most of the familiar mammals today.

Cenozoic Mammals

Many small inconspicuous mammals are nocturnal. It is thought that these nocturnal mammals retain the primitive condition-an adaptation to living in the presence of the dinosaurs for over 100 million years. It is presumed that the dinosaurs were diurnal, so it would be the sensible thing to do, afterall. It was only with the extinction of the dinosaurs that mammals could see the light of day, so to speak.
The diversification and expansion of mammals after the extinction of the dinosaurs is an illustration that newer forms rarely outcompete the older forms, but that most things diversify to fill an ecological vacuum, such as was left by the dinosaurs.
You are familiar with the major groups of mammals. The most conspicuous are the carnivores, adapted to eating meat, and the ungulates or hoofed mammals. Other groups include the insectivores (shrews, hedgehogs), the primates (monkeys), subungulates (elephants and related types), rodents, lagomorphs (rabits: no they are not rodents), and edentates (sloths and anteaters).
 

Archaic (Placental) Mammals

insectivores retain most of the characteristics of the earliest placentals

Carnivores

Creodonts Early carnivore types . Paleocene to Early Miocene. Developed the typical teeth--blade like incisors, retained stabbing canines, and developed a shearing carnassial.
Fissipeds also start in the paleocene and dominate today. These include the feliformia (Cats, haenas, civits, and mongoose) and the Caniformia (dogs, racoons, bears, Sea Lions, and Weasels)
 

Ungulates

  • Characterized by more advanced foot structure. Walk on their toes.
  • Condylarths Paleocene (just about it) fairly undifferentiated from the primitive type
  • Pantodonts Started in the Paleocene as smallish types and got to be cow sized about the Eocene about when they went extinct.
  • Uintatheres Eocene Very large like rhinos. The had bony knobs all over their heads.
  • Notoungulates Common in the Paleocene, persisted in South America through the Pliocene. South american forms developed into all sorts of forms paralleling the evolution of other ungulates and even rodents on other continents. Horses types, camel types etc.
  • Perissodactyls Paleocene to present. odd toed ungulates Horses, Rhinos, tapirs. Early forms typified by Hyracotherium (AKA Eohippus).
  • Artiodactyls Paleocene to present. Even toed ungulates (cattle, deer, sheep, goats, antelope, pronghorns, pigs, camels).
Uintatherium
south american mammals
 

Proboscidians (Elephants and their kin).

Whales

Modern Mammals

Carnivores

  • Major radiation in later cenozoic
  • Saber-toothed cats
  • Seals
Saber toothed cats

Ungulates

  • artiodactyls and perissodactyls
  • tendency to increase size
  • tendency to reduce toes
  • larger teeth and longer jaws
  • ruminant success story (cattle, camels, deer)

Elephants

Special Events in Mammalian History

 

Evolution of the horse: centers of Radiation

Spifier version of Evolution of the Horse
evolution of the horse

Isolation of South America

Mammal Paleontology of South America
Fossil Horses and the Great American Intercange

Isolation of Australia

Australian Fauna
Australian fossil research
AUSTRALIAN WORLD HERITAGE NARACOORTE Cave
Tasmanian Devil