Objectives: The modern world is dominated by mammals. Mammals have evolved, adapted and currently thrive in diverse habitats, including the oceans, lakes, rivers, tropical and temperate forests, mountain regions, caves, deserts, grasslands, the arctic tundra and polar regions. How did mammals become so diverse on the planet? Why did they remain small and a relatively insignificant part of faunas for the first two-thirds of their extraordinary 200 million year history? Where and when did modern mammal groups originate? In this module, we will explore the extensive fossil record of mammals in order to gain insights to these and other questions concerning the history of this critically important animal group.

Background:

What distinguishes mammals from reptiles, their closest vertebrate relatives?There are three main anatomical characteristics that mammals share:

1.Three middle_ear bones (incus, malleus, stapes) (More!)

2. Mammary_glands

3. Hair

In addition, most mammals share the following features that also distinguish them from reptiles:

Stance Features: The skeleton of a mammal reflects improvement in stance, with the legs positioned more directly below the body. Associated with upright stance are: 1) the Epiphysis = a cartilage junction for growth, 2) Spines on the scapula, 3) Twisting Facets on vertebra and 4) an Acetabulum developed on the pelvis.

The skull of the mammals reflects improvements in chewing and increases in brain size. Specific features associated with these changes are: 1) the development of Dual occipital condyles (Example: dog_basicranium), 2) Large Brains 3) Secondary Palates (Example: dog pallantine bones), 4) Single jaw bone Single Jaw Bone and 5) Diverse Molarization of teeth.

Origin of Mammals (the transition from reptiles to mammals)

In order to understand the development of these features, one has to look back to the mammal-like reptiles of the late Pennsylvanian Period (280 mya = million years ago). At that time reptiles had evolved into 4 major groups characterized by the position and number of holes in the back of their heads for muscles (recall the importance of skull holes from a previous module). Of particular significance was a group of synapsid reptiles called the pelycosaurs (320-258 mya), that began to diversify and evolve mammal-like features.

Pelycosaurs

The earliest pelycosaurs are similar to modern lizards Examples: Archaeothyris, Varanosaurus (picture). Later pelycosaurs are characterized by their sail-backs and include both herbivorous and carnivorous types (Edaphosaurus and Dimetrodon, picture ). The Pelycosaurs dominate the terrestrial world for about 50 million years!! Some become very large, 7-13 feet in length (Ophiacodon, picture). Virtually all of the pelycosaurs, as well as 90% of earth's species, became extinct about 245 mya at the Permian-Triassic extinction. From the pelycosaurs two other important groups evolved, the therapsids and the cynodonts.

Therapsids

An advanced carnivorous group of pelycosaurs, the therapsids, are the next great synapsid group to diversify into many carnivorous and herbivorous forms during the Permian and Triassic Periods (258-208 mya). The therapsids show a chain of evolutionary steps towards mammals: differentiated teeth and skull development, jaw articulation, consolidation of the jaw bones, development of mammal ear bones, palate, elvis consolidation, scapula with spine, and leg positioning.

Cynodonts

The Cynodonts, are a very advanced type of therapsid, that show anatomical features even closer to the mammalian condition:

Example: Procynosuchas (the "first" cynodont) S. Africa. A terrier-sized, carnivorous cynodont showing an adductor muscle fossa, double occipital condyle, frontal & nasal bone enlargement, palatine bones, and enlarged braincase.

Example: Thrinaxodon showing lumbar vertebrae development and a semi-erect hindlimb.

Example: Probelesodon, S. America. Shows whisker pits (!!!) suggesting warm-bloodededness and hair.

TEETH TEETH TEETH = you are what you eat with. Two of the great innovations of Cynodonts which lead directly to mammals are the development of occlusion and single replacement of teeth (i.e. two sets). Occlusion = the meeting of molars and premolars to grind over one another to process food more efficiently. Single-replacement = two sets of teeth, probably to ensure accurate occlusion and suckling.

Example: Diademodon Diademodon from the middle Triassic, S. America. A herbivore with narrow lower teeth that come in contact with broad upper teeth. The lower jaw could pull back to tear food.

The First "true" Mammals

By the late Triassic, (220 mya), there were three major groups of Therapsids: Tritheledonts, Tritylodonts and the First Mammals. The late Triassic, a fabulous time on earth, marks the beginning of mammals as well as the Age of Dinosaurs. It is the dinosaurs that diversify and dominate the landscape for the next 160 million years!! By the middle Jurassic period (160 mya) only the true mammals are left of the original major therapsid groups. The early mammals likely spend their time keeping out of the way of the dinosaurs.

The first mammals (late Triassic) are considered to be the"Morganucodontids". An example is Moganucodon-found in ancient cave deposits in S. Wales. These early mammals are often represented by a few isolated teeth, These mammals were very small with body lengths around 1 inch! They were weasel-like and had a complete secondary palate suggesting high food intake and endothermy. They had advanced teeth, whiskers, and expanded visual and auditory areas and were probably nocturnal.

Long Twilight of the Mammals

From the Triassic to the Late Cretaceous (208-65 m.y.a.), representing 2/3 of mammalian history, the fossil record of mammals is poorly known (mostly teeth) although new discoveries are happening all of the time. A well-preserved fossil mammal, Crusafontia (a dryolestid), from the late Jurassic of Portugal is an exception. It has three main cusps on the lower and upper molar in a triangular pattern. It is about 8" long and is presumed to have been an insectivore. During the Jurassic and Early Cretaceous there were several lineages of mammals evolving based on isolated fossil teeth and jaws. The main groups were: 1) Docodonts 2) Triconodonts 3) Symmetrodonts 4) Dryolestids 5) Monotremes (e.g., Steropodon) 6) Multituberculates, a tremendously successful group during the late Jurassic to Oligocene = 130 mya (e.g., Ptilodus) , and 7) Therians (the ancestors of the modern mammal placental and marsupial groups).

Rise of the Major Modern Mammalian Lineages

During the middle to late Cretaceous period 80-65 m.y.a., the three modern living groups of mammals (placentals, marsupials, and monotremes) evolve.

A. Monotremes (prototherians) = lay eggs, have underdeveloped young that move to a fold of skin of the adult, suckle on primitive nipples anatomically closer to sweat glands. Modern Examples = the platypus and the echidna (the only living forms), toothless. (Fossil example. Steropodon).

B. Marsupials (metatherians) = young born undeveloped, move to a pouch, 3 premolars/4 molars Modern Examples = kangaroos, opossum, wombats, koalas. Late Cretaceous fossil record = Deltatheridium of Mongolia, Alphadon of North and South America.

C. Placentals (eutherians) = young born well-developed, nourished in the womb via a placenta which transfers nutrients, 4-5 premolars/3-4 molars. Modern Examples = look at your dog or cat! Late Cretaceous fossil example = Zalambdalestes.

This initial diversification occurs during the later stages of the age of dinosaurs. However mammals remain small and relatively minor players in the terrestrial world. With the great dinosaur extinction at the end of the Cretaceous (K-T) all medium to large sized animals are absent. Only small birds, reptiles and mammals are present in the early Paleocene (65-63 mya). Within 10 million years of the dinosaur extinction, nearly all modern placental groups have emerged on the scene. It appears that mammals rapidly take advantage of the diverse niches that dinosaurs have left behind

Example: Crazy Mountain Mammals (Fort Union Formation, Montana) 63-58 m.y.a. with 79 species. Mammals of the Crazy Mountain locality suggests that mammals are beginning to radiate very rapidly into a variety of new niches. The fauna includes.......

- mostly multituberculates 10-15% e.g.,the arboreal Ptilodus

- lemur-like primates, e.g., Plesiadapis

- shrew-like insectivores

- cat-like carnivores

- pig-like taeniodonts

Additional Mammalian Diversification During the later Cenozoic

During the Miocene and Pliocene (24 - 2 million years ago) the world gets colder, polar ice begins to become prominent, tropical areas diminish and move to the more central latitudes, and grasslands and prairies flourish. In order to survive with the changing climate and corresponding habitats, many herbivores, and particularly the ungulates had to evolve new strategies. This interval is called the Age of Ungulates.

Living in a "world of grass" is done most effectively if: 1) you have teeth that can resist the abrasiveness of the silica in grass (Example: Horses = "ever-growin" hypsodont teeth" or 2) you develop a gut that can process the grass rather than wear your teeth down. This latter strategy is employed by the "Regurgitators". Examples of regurgitators include the Ruminants (bovids = cattle, sheep, goats and antelope) as well as deer and giraffes. Cattle have four stomach chambers with the first called the "rumen" where bacteria break down food. As food passes to the second chamber it is regurgitated chewed again and sent into further chambers maximizing the nutrition obtained. Getting larger and faster to escape predators in the open grasslands is also helpful.