Mammal

Edit links
From Wikipedia, the free encyclopedia

This is an old revision of this page, as edited by Ba'Gamnan (talk | contribs) at 11:27, 25 May 2007. The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Mammals
Temporal range: Late Triassic - Recent
File:Seehund11cele4.jpg
Harbour Seal (Phoca vitulina)
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Subphylum:
Vertebrata
(unranked):
Amniota
Class:
Mammalia

Linnaeus, 1758
Subclasses & Infraclasses

Mammals are a class of vertebrate animals characterized by the production of milk in females for the nourishment of young, from mammary glands present on most species and specialized skin glands in monotremes that seep or ooze milk. There is also presence of hair or fur, specialized teeth, three small bones within the ear, the presence of a neocortex region in the brain, endothermic or "warm-blooded" bodies, and, in most cases, the existence of a placenta in the ontogeny. The brain regulates endothermic and circulatory systems, including a four-chambered heart. Mammals encompass approximately 5,400 species (including humans), distributed in about 1,200 genera, 152 families and 28 orders, though this varies with the classification scheme.

Phylogenetically, Mammalia is defined as all descendants of the most recent common ancestor of monotremes (e.g., echidnas and platypuses) and therian mammals (marsupials and placentals). This means that some extinct groups of "mammals" are not members of the crowngroup Mammalia, even though most of them have all the characteristics that traditionally would have classified them as mammals. These "mammals" are now usually placed in the unranked clade Mammaliaformes.

Distinguishing features

Living mammal species can be identified by the presence of mammary glands in females which produce milk.

Other features are required when classifying fossils, since mammary glands and other features are not visible in fossils. The evolution of mammals from synapsids, also known as mammal-like "reptiles" was a gradual process which took approximately 70 million years, from the mid-Permian to the mid-Jurassic, and by the mid-Triassic there were many species that looked like mammals. Note that synapsids are actually not reptiles at all, but belong to a distinct lineage of tetrapods.

Paleontologists use a distinguishing feature that is shared by all living mammals (including monotremes), but is not present in any of the early Triassic synapsids: mammals use two bones for hearing that were used for eating by their ancestors. The earliest synapsids had a jaw joint composed of the articular (a small bone at the back of the lower jaw) and the quadrate (a small bone at the back of the upper jaw). Most reptiles and non-mammalian synapsids use this system including lizards, crocodilians, dinosaurs (and their descendants the birds) and therapsids (mammal-like "reptiles"). But mammals have a different jaw joint, composed only of the dentary (the lower jaw bone which carries the teeth) and the squamosal (another small skull bone). And in mammals the quadrate and articular bones have become the incus and malleus bones in the middle ear. Note: "non-mammalian synapsids" above implies that mammals are a sub-group of synapsids, and that is exactly what cladistics says they are.

Mammals also have a double occipital condyle; they have two knobs at the base of the skull which fit into the topmost neck vertebra, and other vertebrates have a single occipital condyle. But paleontologists use only the jaw joint and middle ear as criteria for identifying fossil mammals, as it would be confusing if they found a fossil that had one feature but not the other.

Mammal anatomy

Skeletal system

The majority of mammals have seven cervical vertebrae (bones in the neck); this includes bats, giraffes, whales, and humans. The few exceptions include the manatee and the two-toed sloth, which have only six cervical vertebrae, and the three-toed sloth with nine cervical vertebrae.

Respiratory system

See the section Mammalian lungs in the Lung article.

Circulatory system

The mammalian heart has four chambers: the right atrium, right ventricle, left atrium, and left ventricle. Atria are for receiving blood; ventricles are for pumping blood to the lungs and body. The ventricles are larger than the atria and their walls are thick, because muscular walls are needed to forcefully pump the blood from the heart to the body and lungs. Deoxygenated blood from the body enters the right atrium, which pumps it to the right ventricle. The right ventricle pumps blood to the lungs, where carbon dioxide diffuses out, and oxygen diffuses in. From the lungs, oxygenated blood enters the left atrium, where it is pumped to the left ventricle (the largest and strongest of the 4 chambers), which pumps it out to the rest of the body, including the heart's own blood supply.

Head and brain

All mammalian brains possess a neocortex which is a brain region that is unique to mammals.

Skin

Mammals have integumentary systems made up of three layers: the outermost epidermis, the dermis, and the hypodermis. This characteristic is not unique to mammals, since it is found in all vertebrates.

The epidermis is typically ten to thirty cells thick; its main function being to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is fifteen to forty times thicker than the epidermis. The dermis is made up of many components such as bony structures and blood vessels. The hypodermis is made up of adipose tissue. Its job is to store lipids, and to provide cushioning and insulation. The thickness of this layer varies widely from species to species.

No mammals are known to have hair that is naturally blue or green in color. Some cetaceans, along with the mandrills appear to have shades of blue skin. Many mammals are indicated as having blue hair or fur, but in all known cases, it has been found to be a shade of grey. The two-toed sloth and the polar bear can seem to have green fur, but this color is caused by algae growths.

Reproduction

Goat kids will stay with their mother until they are weaned, this is usually about one month

Most mammals give birth to live young (vivipary), but a few (the monotremes) lay eggs. Live birth also occurs in some non-mammalian species, such as guppies and hammerhead sharks; thus it is not a distinguishing characteristic of mammals. Although all mammals are endothermic, so are birds, so this too is not a defining feature.

Mammals have mammary glands, a defining feature present only in mammals. The monotremes branched from other mammals early on, and do not have nipples, but they do have mammary glands. Most mammals are terrestrial, but some are aquatic, including sirenia (manatees and dugongs) and the cetaceans (dolphins and whales). Whales are the largest of all animals. There are semi-aquatic species such as seals which come to land to breed but spend most of the time in water.

Flight

The only mammals for which true flight has been observed are bats; mammals such as flying squirrels and Colugos are more accurately classified as gliding mammals.

Evolution

Origins

Further information: Synapsid
Restoration of Archaeothyris, one of the first synapsids (or mammal-like "reptiles") to appear.

Mammals belong to a group of amniotes called the synapsids that have a single hole (temporal fenestra) low on each side of the skull on each side where jaw muscles attach. In comparison, dinosaurs, birds, and most living reptiles are diapsids, with two temporal fenestrae on each side of the skull; and turtles, with no temporal fenestra, are anapsids. The synapsids diverged from the other reptile lineages very early, in the late Carboniferous, and one of the earliest examples is Archaeothyris.

The synapsid pelycosaurs were the dominant land vertebrates of the early Permian. The therapsids probably evolved from pelycosaur ancestors, and consist of a sequence of groups which became increasingly mammal-like, especially the Triassic cynodonts ("dog-teeth"). The main article on synapsids presents a probable phylogeny ("family tree") which shows how mammals evolved from early synapsids.

Some cynodonts such as the early to mid Triassic Cynognathus had erect limbs and possibly hair or fur, and may have been warm-blooded.

Several groups of mammaliaformes ("almost mammals") arose from the mid Triassic onwards - these had non-mammalian jaw joints but almost certainly had hair and were warm-blooded. Megazostrodon is a good example. It had evolved to possess fur, be warm-blooded, and was the first animal to have mammary glands, the defining feature of mammals. It is also believed to be nocturnal, for which a warm-blooded nature is necessary. However, it still bore young by laying reptile-like leathery eggs.

Some other groups of mammaliaformes were very successful in their own right, for example the multituberculates appeared in the mid-Jurassic and became extinct about 100 million years later, in the Oligocene.

The first true mammals (from a cladistic point of view) appeared in the early Jurassic, over 70 million years after the first therapsids and approximately 30 million years after the first mammaliaformes. Hadrocodium is an example of the transition to true mammal status - it had a mammalian jaw joint but there is some debate about whether its middle ear was fully mammalian. The triconodonts may have been the earliest true mammals known so far.

Like their predecessors and modern monotremes, the first mammals laid eggs. The earliest known fossils of marsupial and placental mammals date from the mid Cretaceous, about 170 million years after the first therapsids.

Lactation is the distinguishing feature of living mammals, but scientists are not sure when it evolved. Since all living mammals (including monotremes) produce milk, their last common ancestor produced milk. Some scientists have suggested that the cynodonts produced milk, but this idea is a plausible speculation with little hard evidence to support it.

Dimetrodon, a dominant predator of Early Permian. Dimetrodon 's close relatives gave rise to therapsids, the ancestors of mammals

From the earliest synapsids (such as Archaeothyris), their temporal fenestra expanded as synapsids evolved. In cynodonts, the temporal fenestra is much larger than the pelycosaurs and the primitive therapsids. From cynodonts to mammals, the temporal fenestra has been modified, now no longer a hole. The erect posture (unlike reptiles and pelycosaurs whose posture was sprawling) evolved in the Middle Permian by therapsids. The secondary palate also evolved by therapsids at the same time (the therocephalians had both of these traits). Mammalian hair also evolved in the Middle Permian, probably evolved from scales. Pre-mammalian ears began evolving in the late Permian to early Triassic to their current state, as three tiny bones (incus, malleus, and stapes) inside the skull; accompanied by the transformation of the lower jaw into a single bone. Other animals, including reptiles and pre-mammalian synapsids and therapsids, have several bones in the lower jaw, some of which are used for hearing; and a single ear-bone in the skull, the stapes. This transition is evidence of mammalian evolution from reptilian beginnings: from a single ear bone, and several lower jaw bones (for example the sailback pelycosaur, Dimetrodon) to progressively smaller "hearing jaw bones" (for example the cynodont, Probainognathus), and finally (possibly with Morganucodon, but definitely with Hadrocodium), true mammals with three ear bones in the skull and a single lower jaw bone. Hence pelycosaurs and cynodonts are sometimes called "mammal-like reptiles", but this is strictly incorrect as these two are not reptiles but synapsids.

Mesozoic

During the Mesozoic Era, mammals diversified into four main groups: multituberculates (Allotherium), monotremes, marsupials, and placentals. Multituberculates went extinct during the Oligocene, about 30 million years ago, but the three other mammal groups are all represented today. According to the traditional view, most early mammals remained small and shrew-like throughout the Mesozoic, but rapidly developed into larger more diverse forms following the Cretaceous-Tertiary extinction event 65 million years ago.

In recent years, there has been convincing evidence that this picture is not correct.[1] It is true that certain mammals did experience a rapid period of diversification and evolution. However, most of these groups have since either died out completely, such as Andrewsarchus, or declined in diversity, such as the group containing sloths and armadillos (Xenarthra). The majority of the living mammalian orders did not arise in the wake of the Dinosaur extinction, but appeared either well before or well after the demise of the Dinosaurs.

Recent findings, based on the molecular clock method, confirms this view. According to these research, the Eutherian mammals (mammals with a placenta) experienced two rapid bursts of diversification. The first of these took place 85 million years ago, when the Dinosaurs were still in their prime. The modern eutherian lineages appeared, besides many others that apparently were more successful at that time. This is marked by the fact that fossils of extant eutherian lineages from the Cretaceous are very rare. Because of this, the molecular picture described above is not (yet) backed up by fossil evidence and is not without controversy.

The second burst of eutherian diversification did only take place 55 million years ago - well after the Dinosaurs became completely extinct (although some of them survived in the form of birds, but they are not commonly referred to as Dinosaurs). It was because of this event the extant eutherian lineages became to dominate and many others became extinct.

As a result of all of this, it seems that the Dinosaur influence on the evolution and diversification of mammals was actually not that significant. Mammals did experience a period of rapid diversification - but this happened not once but twice, both times without any relation to the fate of dinosaurs. It is however likely that the disappearance of Dinosaurs ultimately did influence the success of mammals as a group, although the effects only manifested themselves millions of years after the KT-event. It is also important to note the fact that the mammals also suffered great losses during the KT-event, as it marked the end for several groups of mammals, and all of their closest relatives (Mammaliaformes and other Synapsids) died out completely. It took the mammals millions of years to recover, but when they did they became more abundant and successful then ever before.

For a long time the evolutionairy knowledge of Mesozoic mammals has been very fragmentary, because mammalian fossils from this geological period of time are extremely rare. In recent years, however, a sudden increase in the amount of Mesozoic mammal fossils that are discovered seem to confirm the view that a major part of mammalian diversification was completed well before the K-T event. In contrary to the traditional view that Mesozoic mammals were small, nocturnal insectivores with a low diversity as a result of the dominion of Dinosaurs, recent fossils show that mammals from the late Mesozoic were indeed much more diverse and specialised.

In January 2005, the journal Nature reported the discovery of two 130 million year old fossils of Repenomamus, one more than a meter in length, the other having remains of a baby dinosaur in its stomach (Nature, Jan. 15, 2005 [1]). And the 2004 discovery in China of a 164 million year old 50 cm long aquatic mammal-like fossil of a thus far unknown species, dubbed Castorocauda, by a team led by Dr. Ji Qiang of Nanjing University and the Chinese Academy of Geological Sciences, was reported in February 2006 in the journal Science (Science, Feb. 24, 2006 [2]). Besides these two important discoveries, there are a few other examples that point to a significant mammalian diversity in the Mesozoic: the gliding, flying squirrel-like Volaticotherium and the termite-eating, armadillo-like Fruitafossor.

Cenozoic

According to the traditional view, mammals exploded into the ecological niches left by the extinction of the dinosaurs shortly after the start of the Paleocene. Recent findings, however, suggest that mammals too had to recover from the KT-event and that the evolutionary burst only took place after several million years. In contrary to the traditional view, this burst probably did not give rise to entirely new lineages of mammals. It is now believed that most extant orders of mammals already existed well into the Cretaceous (as a result of an earlier burst in mammal diversity), but that the Paleocene burst did cause a major increase in diversity within these orders (as well as leading to the appearance of some new ones).

This resulted in the appearance of forms that were ancestral to modern forms and also led to the appearance of the first truly large mammals (although even the largest forms never approached the size of the largest dinosaurs). As a result, the ecological niches left by the dinosaurs and other extinct reptiles did became occupied by mammals during the Paleocene, but this was a gradual process and certainly not something that happened overnight. But no matter how it happened, during the Paleocene mammals became the dominant terrestrial lifeforms and the rightful successors of the dinosaurs. On some continents the mammals did experience some competition from large flightless predatory birds (such as Gastornis), a group of animals that can be seen as direct descendents of dinosaurs. These forms ultimately became extinct, leaving the land to the mammals (while birds remained very successful in the air).

Some examples of forms that evidently existed before the KT-event, but only became truly successful during the Paleocene are:

  • Ungulates as the major large herbivores.
  • Primates as arboreal insectivores and later frugivores.

Some groups of mammals only appeared in the fossil record after the KT-event and diversified during the Paleogene, such as:

  • Rodents as small herbivores specialised in seeds, before developing an incredible diversity.
  • Cetaceans (whales, dolphins and their ancestors).
  • Chiropterans (bats) as flying insectivores (although bat-like teeth have been found in late Cretaceous rocks).
  • Various orders of carnivorous mammals - first mesonychids, then creodonts, and finally Carnivora.

Molecular data suggests that most orders of mammals already developend in the Cretaceous, but this view is not (yet) backed up by relevant fossil evidence.

The names "Prototheria", "Metatheria" and "Eutheria" expressed the theory that Placentalia were descendants of Marsupialia, which were in turn descendants of Monotremata, but this theory has been refuted. However, Eutheria and Metatheria are often used in paleontology, especially with regards to mammals of the Mesozoic.

A series of vertebrate animal lineages is listed below. All of these groups predate mammals, and are close relatives.

Mammals appear in the mid-Jurassic period, and persist to the present (as Monotremes, Metatheria, and Eutheria).

Convergent evolution

As a result of the molecular and genetic research that have been rewriting the picture of mammalian classification, it is now suspected that convergent evolution has been an even greater factor in the evolution of mammals than suspected before. It was already known for a long time that Eutherian mammals were almost absent on some continents (such as Antarctica and Australia). On these continents the mammals were represented by the marsupials instead. The marsupials evolved the same way as Eutherians did, resulting in many forms that mimmic Eutherian counterparts on other continents. The marsupials of today are no exception to this phenomenon. However, it now seems that the evolution of Eutherian mammals was based on convergent evolution as well.

After the Eutherian mammals appeared, several lineages became separated from the others as a result of continental drift and other factors, resulting in each continent having a distinct genetic stock of Eutherian mammals. Later on, each of these developed in similar ways, resulting in forms that mimic Eutherians on other continents. As a result, there has never been "one" group of ungulates, or one group of insectivores, or one group of carnivores. Instead, each continent developed an own, distinct "version" of the body plan of ungulates and other forms.

Some of these groups already came in contact with each other very early on, while others remained separated for many millions of years. Ultimately, these continents did become reconnected with other continents, while others became separated again. All of this resulted in multiple periods of biological exchange between continents, until all the original core groups of Eutherian mammals could be found on many continents and their evolutionary heritage became completely obscured.

Classification

Main article: Mammal classification
Over 70% of mammal species are in the orders Rodentia (blue), Chiroptera (red), and Soricomorpha (yellow)

George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" (AMNH Bulletin v. 85, 1945) was the original source for the taxonomy listed here. Simpson laid out a systematics of mammal origins and relationships that was universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics. Though field work gradually made Simpson's classification outdated, it remained the closest thing to an official classification of mammals.

Standardized textbook classification

A somewhat standardized classification system has been adopted by most current mammalogy classroom textbooks. The following taxonomy of extant and recently extinct mammals is from Vaughan et al. (2000).

Class Mammalia

McKenna/Bell classification

In 1997, the mammals were comprehensively revised by Malcolm C. McKenna and Susan K. Bell, which has resulted in the "McKenna/Bell classification".

McKenna and Bell, Classification of Mammals: Above the species level, (1997) is the most comprehensive work to date on the systematics, relationships, and occurrences of all mammal taxa, living and extinct, down through the rank of genus. The new McKenna/Bell classification was quickly accepted by paleontologists. The authors work together as paleontologists at the American Museum of Natural History, New York. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa that reflects the historical genealogy of Mammalia.

The McKenna/Bell hierarchical listing of all of the terms used for mammal groups above the species includes extinct mammals as well as modern groups, and introduces some fine distinctions such as legions and sublegions (ranks which fall between classes and orders) that are likely to be glossed over by the layman.

The published re-classification forms both a comprehensive and authoritative record of approved names and classifications and a list of invalid names.

Extinct groups are represented by a cross (†).

Class Mammalia

Molecular classification of placentals

Molecular studies based on DNA analysis have suggested new relationships among mammal families over the last few years. Most of these findings have been independently validated by Retrotransposon presence/absence data. The most recent classification systems based on molecular studies have proposed four groups or lineages of placental mammals. Molecular clocks suggest that these clades diverged from early common ancestors in the Cretaceous, but fossils have not been found to corroborate this hypothesis. These molecular findings are consistent with mammal zoogeography:

Following molecular DNA sequence analyses, the first divergence was that of the Afrotheria 110–100 million years ago. The Afrotheria proceeded to evolve and diversify in the isolation of the African-Arabian continent. The Xenarthra, isolated in South America, diverged from the Boreoeutheria approximately 100–95 million years ago. According to an alternative view, the Xenarthra has the Afrotheria as closest allies, forming the Atlantogenata as sistergroup to Boreoeutheria. The Boreoeutheria split into the Laurasiatheria and Euarchontoglires between 95 and 85 mya; both of these groups evolved on the northern continent of Laurasia. After tens of millions of years of relative isolation, Africa-Arabia collided with Eurasia, exchanging Afrotheria and Boreoeutheria. The formation of the Isthmus of Panama linked South America and North America, which facilitated the exchange of mammal species in the Great American Interchange. The traditional view that no placental mammals reached Australasia until about 5 million years ago when bats and murine rodents arrived has been challenged by recent evidence and may need to be reassessed. These molecular results are still controversial because they are not reflected by morphological data, and thus not accepted by many systematists. Further there is some indication from Retrotransposon presence/absence data that the traditional Epitheria hypothesis, suggesting Xenarthra as the first divergence, might be true.

Citations

  1. ^ Fox News: Rise of Modern Mammals Occurred Long After Dinosaurs' Demise

Further reading

  • Bergsten, Johannes. February 2005. "A review of long-branch attraction". Cladistics 21:163–193. (pdf version)
  • McKenna, Malcolm C., and Bell, Susan K. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp. ISBN 0-231-11013-8
  • Nowak, Ronald M. 1999. Walker's Mammals of the World, 6th edition. Johns Hopkins University Press, 1936 pp. ISBN 0-8018-5789-9
  • Simpson, George Gaylord. 1945. "The principles of classification and a classification of mammals". Bulletin of the American Museum of Natural History, 85:1–350.
  • William J. Murphy, Eduardo Eizirik, Mark S. Springer et al., Resolution of the Early Placental Mammal Radiation Using Bayesian Phylogenetics,Science, Vol 294, Issue 5550, 2348-2351 , 14 December 2001.
  • Springer, Mark S., Michael J. Stanhope, Ole Madsen, and Wilfried W. de Jong. 2004. "Molecules consolidate the placental mammal tree". Trends in Ecology and Evolution, 19:430–438. (pdf version)
  • Vaughan, Terry A., James M. Ryan, and Nicholas J. Capzaplewski. 2000. Mammalogy: Fourth Edition. Saunders College Publishing, 565 pp. ISBN 0-03-025034-X (Brooks Cole, 1999)
  • Wilson, Don E., and Deeann M. Reeder (eds). 1993. Mammal Species of the World, 2nd edition. Smithsonian Institution Press, 1206 pp. ISBN 1-56098-217-9
  • Jan Ole Kriegs, Gennady Churakov, Martin Kiefmann, Ursula Jordan, Juergen Brosius, Juergen Schmitz. (2006) Retroposed Elements as Archives for the Evolutionary History of Placental Mammals. PLoS Biol 4(4): e91.[3]
  • Wilson, D. E., and Reeder, D. M. (eds) Mammal Species of the World, 3rd edition, Johns Hopkins University Press. ISBN 0-8018-8221-4.

See also

External links

Wikispecies has information related to Mammalia.
The Wikibook Dichotomous Key has a page on the topic of: Mammalia

Template:Link FA Template:Link FA Template:Link FA

Retrieved from "https://en.wikipedia.org/w/index.php?title=Mammal&oldid=133392598"