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http://animaldiversity.ummz.umich.edu/chordata/mammalia/


Order Monotremata

(2 Families; 3 genera)

Monotremes probably split from the lineage leading to other mammals sometime in the Mesozoic. They are often placed in a separate subclass from other mammals, Prototheria. They retain many characters of their therapsid ancestors (for example, a complex pectoral girdle, laying of eggs rather than bearing live young, limbs oriented with humerus and femur held lateral to body, and a cloaca). The skulls of monotremes are almost birdlike in appearance, with a long rostrum and smooth external appearance. Modern monotremes lack teeth as adults; sutures are hard to see; the rostrum is elongate, beak-like, and covered by a leathery sheath; and lacrimal bones are absent. Monotremes have several important mammalian characters, however, including fur (but they lack vibrissae), a four chambered heart, a single dentary bone, three middle ear bones, and the ability to lactate.

Besides the absence of teeth, lacrimals, and obvious sutures, monotremes share a number of skeletal characteristics. On the skulls, the jugals are reduced or absent, the dentary is a slender bone with only a vestige of a coronoid process, the angle of the dentary is not inflected medially (unlike that of marsupials), auditory bullae are missing (part of the middle ear is enclosed by tympanic rings), and much of the wall of the braincase is made up by the petrosal rather than the alisphenoid (unlike all other modern mammals). Postcranially, the skeleton of monotremes is also unique among mammals. It is a fascinating mosaic of primitive characteristics inherited from therapsids but found in no other living mammals, and modifications probably related to the burrowing habits of modern monetremes. Their shoulder girdles are complex, including the standard components of modern mammals (scapula and clavicle), but also additional elements including coracoid, epicoracoid, and interclavicle. The scapula, however, is simplified, lacking a supraspinous fossa. The shoulder girdle is much more rigidly attached to the axillary skeleton than in other mammals. Femur and humerus are held roughly parallel to the ground when the animal walks, more in the fashion of therapsids and most modern reptiles than like modern mammals. Ribs are found on the neck (cervical) vertebrae as well as the chest (thoracic) vertebrae; in all other modern mammals, they are restricted to the thoracic region.

Another interesting skeletal characteristic of monotremes is the large epipubic bones in the pelvic region. Epipubic bones were originally thought to be related to having a pouch, but they are found in both males and females. They also occur in all species of marsupials, whether a pouch is present or not (not all marsupials have a pouch). It is now thought that epipubic bones are a vestige of the skeleton of therapsids, providing members of that group with extra attachments for abdominal muscles to support the weight of the hindquarters.

Monotremes are endothermic, but they have unusually low metabolic rates and maintain a body temperature that is lower than that of most other mammals.

The eggs layed by monotremes are small (13-15 mm diameter) and covered by a leathery shell. The number of eggs laid is small, usually 1-3, and they are placed in the mother's pouch. They contain a large yolk, which is concentrated at one end of the egg very much like the yolk of a bird's egg. Only the left ovary is functional in the platypus, but both produce eggs in the echidna. Like the eggs of birds, monotreme eggs are incubated and hatched outside the body of the mother. Incubation lasts about 12 days. The young, which are tiny and at a very early stage of development when they hatch, break out of the eggs using a "milk tooth. They are protected in a temporary pouch in echidnas but not platypuses. They are fed milk produced by mammary glands; the milk is secreted onto the skin within the pouch and sucked or lapped up by the babies. Weaning takes place when the young are 16-20 weeks old.

All male monotremes have spurs on their ankles that are presumed to be used in fighting and in defense. In one family (Ornithorhynchidae), a groove along the spur carries poison secreted by adjacent glands.

Monotremes are restricted to Australia and New Guinea. Their fossil record is very poor; the earliest fossil attributed to this group is from the early Cretaceous. A fossil from Argentina suggests that the monotremes were more widely distributed early in their history.

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References and literature cited



Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. 1999. Mammalogy. Adaptation, Diversity, and Ecology. WCB McGraw-Hill, Boston. xii+563pp.

Marshall, L. G. 1984. Monotremes and marsupials. Pp. 59-115 in Anderson, S. and J. Knox Jones, Jr., eds. Orders and Families of Recent Mammals of the World. John Wiley & Sons, New York. xii+686 pp.

Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vi+576 pp.

Vaughan, T. A., J. M. Ryan, N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia. vii+565pp.

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Click on the name of a family to learn more:



Family Ornithorhynchidae
Family Tachyglossidae


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Ornithorhynchidae

(platypus)

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This family consists of a single genus and species, the duck-billed platypus. Often considered the most unique and bizarre of mammals, the duck-billed platypus uses receptors sensitive both to tactile stimulation and weak electrical fieldsto sense prey when digging under water. These sense organs are located in its "bill." This species is highly aquatic. It has webbed feet; dense woolly, water-repellant fur; and furrows along the sides of its head to protect the eyes and ears when it swims under water. The external auditory meatus is tubular and the ears lack pinnae.



The bill of a platypus is soft, flexible, and leathery, unlike a bird's beak. Nostrils are located at its tip. While young platypuses have molars, adults are toothless. They grind their food between horny (keratinous) plates located over the gums.

Young platypuses have teeth, but these are lost in adults. Food is masticated between horny plates located on each jaw. The anterior part of these plates is ridged and is used to chop food; the posterior part is expanded and flat and used for crushing.

Male platypuses have a sharp spur attached to each ankle. The spurs are grooved and connected to venom glands; these weapons may be used in combat between males for mates.

A platypus feeds primarily on aquatic crustaceans, insect larvae, and some plants.

Duck-billed platypuses live in burrows along the banks of water, including lakes, rivers, and even mountain streams. They have well-developed claws, but these are not as large as the claws of echidnas. Platypuses are excellent diggers. When a female is about to lay her eggs, she builds a deep burrow (which may be as much as 20-30 m in length), plugs the entrance, and incubates the eggs for 10-12 days. There is no pouch, and the mother curls her body around the eggs to keep them warm. Young are nursed for about five months.

Technical characters

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References and literature cited



Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. 1999. Mammalogy. Adaptation, Diversity, and Ecology. WCB McGraw-Hill, Boston. xii+563pp.

Marshall, L. G. 1984. Monotremes and marsupials. Pp. 59-115 in Anderson, S. and J. Knox Jones, Jr., eds. Orders and Families of Recent Mammals of the World. John Wiley & Sons, New York. xii+686 pp.

Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vi+576 pp.

Vaughan, T. A., J. M. Ryan, N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia. vii+565pp.


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Tachyglossidae

(echidnas or spiny anteaters)

This family includes only two genera, each with one species. Zaglossus, the long-nosed echidna, is found in New Guinea, and Tachyglossus, the short nosed echidna, is native to Australia. Echidnas have spines covering their stout bodies. They curl up into a spine-covered ball in a rather effective method of defense. Echidnas are powerful diggers and can wedge themselves into a burrow or crevice with their spines so that they are difficult to remove.

In general, echidnas dig for food, which consists of termites, ants, and assorted invertebrates. Food is located with the help of special electroreceptors located in the rostrum. Echidnas have long, protrusible, mucous-covered tongues that aid in the capture of prey. The sticky mucous coating is produced by enlarged submaxillary salivary glands. Spines at the base of the tongue grind against spiny ridges on the palate to masticate food.

Echidnas are moderately large animals (up to 16 kg for Zaglossus). They have narrow, slender rostra, not at all expanded like that of the platypus. Their skeletons are heavily built, perhaps to accomodate the powerful muscles used for digging. Unlike platypuses, echidnas lack webbing and instead have large, shovel-like claws are present on all feet. Spurs, the function of which is unclear, are located on the ankles of all males and some females.

Echidnas lay a single leathery egg that is kept in the pouch 7-10 days, until the young hatches. The young remains in the pouch another 6-8 weeks, until its spines begin to harden.

Technical characters

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References and literature cited



Feldhamer, G. A., L. C. Drickamer, S. H. Vessey, and J. F. Merritt. 1999. Mammalogy. Adaptation, Diversity, and Ecology. WCB McGraw-Hill, Boston. xii+563pp.

Marshall, L. G. 1984. Monotremes and marsupials. Pp. 59-115 in Anderson, S. and J. Knox Jones, Jr., eds. Orders and Families of Recent Mammals of the World. John Wiley & Sons, New York. xii+686 pp.

Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vi+576 pp.

Vaughan, T. A., J. M. Ryan, N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia. vii+565pp.

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Species included in the Animal Diversity Web:


  Tachyglossus aculeatus (Short-Nosed Echidna) -pta- 
  Zaglossus bruijni (Long-Nosed Echidna) -t-