Geology 143

Lecture #21

April 10, 2002

Mammalian History

Recall that advanced cynodont therapsids show great reduction of accessory bones (all bones other than the dentary) in the lower jaw. These additional bones in the lower jaw would eventually (in mammals) migrate backward into the ear region. Two important bones that became reduced and specialized for the purpose of improved hearing were the articular bone (of the lower jaw) and the quadrate bone (of the base of the skull). In reptiles (non-synapsid amniotes), and in all pelycosaurs and the majority of therapsids, the articular and quadrate bones form the hinge axis for the lower jaw. In the most advanced (toward the mammalian condition) group of cynodont therapsids and the most primitive mammals, both the older "reptilian" joint between articular and quadrate, and a second, "mammalian" jaw joint between the dentary of the lower jaw and the squamosal bone were present. Within therapsids, the articular and quadrate were already transmitting vibrations to the ear as well as serving as surfaces of articulation.

Although mammalian teeth are known from the Late Triassic, the first mammal represented by numerous, realtively complete remains is Megazostrodon, a very small shrew-like insectivore from the Late Triassic and Early Jurassic. It is considered a mammal (not merely a therapsid) because of the presence of cheek teeth differentiated into premolars (which, like incisors and canines, are replaced once) and molars (which erupt only as permanent teeth). The absence of molar teeth from the milk set is a strong indication that Megazostrodon (like recent mammals) provided milk for their young. The dental formula (5-1-4-4) / (4-1-4-4) is more primitive than that found in any marsupials or placentals. Additionally, although the dentary and squamosal form a functional jaw joint, the articular and quadrate in Megazostrodon were retained as tiny nubs in the jaw and skull base, respectively. In more advanced mammals, these two bones would migrate farther back into the middle ear, where they are today called the malleus and incus. The malleus and incus join the stapes (present in the middle ear of all amniotes) in transmitting sound from the tympanum (eardrum) to the inner ear.

There are four "important" (here meaning either alive today or with an abundant fossil record) groups of mammals, all of which possess three middle ear bones (unlike Megazostrodon).

Allotheria (Jurassic to Oligocene)- the multituberculates

Prototheria (Cretaceous to Recent)- the monotremes

Theria - marsupials (Cretaceous to Recent)

...............and placentals (Cretaceous to Recent)

Multituberculates (Allotheria) were so named for the numerous bumpy protrusions on their molar teeth (termed tubercles) that were used to grind their food (indicative of herbivory). They were small rodent-like herbivores, which (like modern rodents) had a large pair of incisors up front and lacked canine teeth. However, a single lower premolar on each side had a sharp edge, suggesting occasional scavenging of meat as well. Multituberculates are the most abundant Mesozoic mammalian fossils, but their numbers declined greatly at the end-Cretaceous extinction event. They rebounded somewhat in the Paleocene and Eocene, only to be replaced by placental rodents during the Oligocene. (Note: Their narrow pelvis suggests they bore tiny live young that required extended lactation, as is the case in marsupials.)

Monotremes (Prototheria), are egg-laying mammals restricted today to Australia and nearby New Guinea. Living monotremes are the platypus and echidna (nicknamed the "spiny anteater", but not closely related to the placental anteaters of the New World tropics). These surviving monotremes are toothless as adults, but the fossil record shows that platypus relatives retained functional molars into adulthood. In the living platypus, molars are shed in adulthood in favor of hard pads at the back of the leathery "bill"). The oldest monotreme fossil is Cretaceous Steropodon of Australia, an exquisite opalized jaw fragment with molars similar to later platypus-like molars found in the Cenozoic.

The two surviving therian groups (marsupials and placentals) share common ancestry apart from the monotremes, as evidenced in part by the presence of live birth in each, as well as some skeletal and dental similarities. Therian mammals primitively have molar teeth bearing three primary cusps that are not coplanar (such teeth have been found all the way back to the Late Triassic). Cusps from the upper jaw fit into basins on the lower jaw, and vice versa, providing crushing capability as well as shearing. This is in contrast with the molars of Megazostrodon, in which three primary cusps are also present, but are all coplanar, well-adapted for shearing, but not for crushing. Although monotremes and placentals do not appear in the fossil record until the Cretaceous, therian teeth appear in the late Triassic.

Marsupial young are born tiny and helpless, crawling from the birth canal into the mother's pouch for further development and nourishment of milk (all recent mammals produce milk but only marsupials and placentals have specialized nipples for dispensing it). Although they are most numerous today in Australia, marsupials first appear in North America and South America, during the Cretaceous. Following the breakup of these two continents, the North American line ultimately perished (though not before seeing migration to Asia and Europe). The South American line persisted, and some groups migrated to Australia via Antarctica, resulting in the present distribution (primarily Australia and South America). Eocene Antarctican fossils include marsupial, but not placental, mammals. Marsupials first appear in Australian rocks from the Oligocene. The migration of marsupials from South America, however, is thought to have occurred toward the end of the Cretaceous, when the three continents (Australia, Antarctica, South America) were all much closer than at the present. Recall that Antarctica was forested and freee of ice until the Oligocene.

Placentals were the mammals that survived the Cretaceous/Tertiary boundary in the greatest numbers and that would go on to achieve the highest diversity during the Cenozoic. Their method of reproduction through long periods of gestation (residence in the mother's uterus) is accomplished through the presence of a well developed placenta (derived from the chorion and allantois of the amniotic egg), which, in addition to providing food to the young mammal and an outlet for its waste, keeps the developing young from being detected as foreign matter by the mother's immune system (and thus subject to autoimmune rejection) with the aid of a tissue layer unique to placentals, the trophoblast.

The Unique (and mostly extinct) South American Mammal fauna

For most of the Cenozoic, South America has been isolated from the other continents (just as Australia continues to be isolated today), whereas occasionally glaciations produced land bridges linking North America to Asia (and thus Europe and Africa) to create a "world fauna" that during the Miocene and beyond began to resemble the modern mammalian fauna. In contrast, Australia and South America had distinctive faunas that arose in isolation. Australia's came to be dominated by marsupials (with a very few monotremes), producing numerous famous examples of convergence with placental animals seen elswhere (for example, marsupial "cats", marsupial "moles", and marsupial "flying squirrels").

South America's fauna (now largely extinct) is generally less well known to the public (at least in North America), and was characterized by a mixture of marsupials, unusual placentals, and large, flightless birds. South American (placental) ungulates (now extinct) developed into a number of forms superficially similar to members of the Perissodactyla and Artiodactyla (namely, horse-like and camel-like litopterns and a rhino-like notoungulate[scroll down to the lower two photos of Toxodon in this Argentinian news site]). Important smaller herbivores were the caviomorph rodents (caviomorphs include the modern "guinea pig", capybara, and New World porcupine). Carnivorous mammalian roles were filled by a lineage of marsupials. South American marsupial carnivores included the sabertoothed Thylacosmilus (note that the lower jaw forms a sheath for protection of the long, stabbing canines), which was convergent upon true sabertoothed cats found elsewhere during the Cenozoic (such as the famous North American genus Smilodon). The largest and swiftest carnivores were not mammals, but large, flightless birds, the phorusracids (or "terror birds"). Marsupial opossums were important omnivores. Xenarthrans (a placental order) were important insectivores (armadillos and anteaters) and herbivores (sloths, including the now-extinct giant ground sloths, and the extinct, armored glyptodonts).

In the middle Pliocene (about three million years ago) South America and North America collided, raising the isthmus of Panama and forming a land bridge joining the faunas of the continents. Although migration was considerable in both directions, the net result (following Pleistocene extinctions, which may have been influenced by human hunting as well as climatic change and interspecific competition among members of the North and South American faunas) was widespread extinction of the South American groups, including all South American ungulates, large carnivorous marsupials, and phorusracids. Whereas northern invaders have an overwhelming presence in South America today (making up more than half the fauna), in the modern United States, the only common species derived from South American invaders are the porcupine (a caviomorph rodent), armadillo (a xenarthran), and opossum (a marsupial).