Monotremata - AccessScience from McGraw-Hill Education http://accessscience.com/content/monotremata/433900

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Article by: Szalay, Frederick S. Department of Vertebrate Paleontology, American Museum of Natural History, New York, New York. Publication year: 2014 DOI: http://dx.doi.org/10.1036/1097-8542.433900 (http://dx.doi.org/10.1036/1097-8542.433900)

Content

Morphology Ornithorhynchidae Additional Readings Tachyglossidae Bibliography

The single order of the mammalian subclass Prototheria. Two living families, the Tachyglossidae and the Ornithorhynchidae, make up this unusual order of quasi-mammals, or mammal-like reptiles.

The known species are highly specialized for their mode of life, and monotreme anatomy is therefore a curious mixture of reptilian and mammalian features along with highly diagnostic monotreme features. The fossil record of the group is virtually unknown, and Pleistocene forms can be classified within the living genera. Although ties with Jurassic multituberculates, docodonts, and other groups have been advocated, monotreme relationships are obscure. It is likely that they represent a divergent form of therapsid reptiles, independent of all other known groups of mammals, without fully attaining a truly mammalian grade of biological organization.

Morphology

The rostrum on the birdlike, toothless (in the adult) skull is covered with a leathery beak; lacrimals and auditory bullae are absent; cervical ribs are present; all vertebrae, except caudals, are without epiphyses; and large epipubic bones are characteristic. The scapula spine is restricted to the anterior ridge and the supraspinous fossa is absent; large precoracoids, coracoids, interclavicles, clavicles, and scapulae form the pectoral girdle. Monotremes are oviparous with telolecithal and meroblastic eggs. The poorly developed uteri are unfused. A cloaca is present, and the ureters open into a papilla in the urogenital sinus. The mammae are teatless; the penis is attached to the ventral wall of the cloaca; the prostate and the seminal vesicle are absent; the testes are abdominal; and there is one pair of bulbourethral glands. There is no corpus callosum, and the non glandular stomach is lined with stratified epithelium. Endothermy is poorly developed.

Tachyglossidae

The echidnas (spiny anteaters) have relatively large brains with convoluted cerebral hemispheres. The known genera, Tachyglossus and Zaglossus, are terrestrial, feeding on termites, ants, and other insects. They are capable diggers, both to obtain food and to escape enemies. Like hedgehogs, they can erect their spines and withdraw their limbs when predators threaten. The long, sticky tongue can be extended 6 or 7 in. (15–18 cm) past the snout. Echidnas have well-developed senses of smell and hearing but their vision is poor. Commonly one egg, but occasionally two or even three, is laid directly into the marsupium (pouch) of the mother where it is incubated for up to 10 days. When the young hatch, they are poorly developed and remain in the pouch for about 2 weeks.

Species of Tachyglossus live in rocky areas, semideserts, open forests, and scrublands. They are found in Australia,

1 of 3 3/14/2016 12:26 PM Monotremata - AccessScience from McGraw-Hill Education http://accessscience.com/content/monotremata/433900 Tasmania, New Guinea, and Salawati Island. Species of Zaglossus are found in mountainous, forested areas.

Ornithorhynchidae

The duck-billed platypus has a relatively small brain with smooth cerebral hemispheres. The young have calcified teeth, but in the adult these are replaced by horny plates which form around the teeth in the gums. The snout, as the name indicates, is duck-billed; the tongue is flattened out to work against the palate. The pelage is made of soft hairs, and the well-developed tail is flattened to aid in swimming.

The semiaquatic platypus is a capable swimmer, diver, and digger. When it is diving, its eyes and ears are closed by folds of fur. The bulk of its diet consists of aquatic vegetation, worms, aquatic insect larvae, crustaceans, and mollusks. Two eggs are usually laid by the female into a nest made of damp vegetation. After incubating the eggs for about 10 days, the female leaves, and returns only when the eggs are hatched.

The platypus is found in Australia and Tasmania in almost all aquatic habitats, ranging from muddy streams and ponds to clear, rapid mountain streams. See also: Mammalia (/content/mammalia/402500); Prototheria (/content/prototheria /551700)

Frederick S. Szalay

Bibliography

A. F. DeBlase and R. E. Martin, A Manual of Mammology, 2d ed., 1980

R. M. Nowak and J. L. Paradiso (eds.), Walker's Mammals of the World, 2 vols., 6th ed., 1999

Additional Readings

N. Gust and J. Griffiths, Platypus (Wildlife Res., 38(4):271–289, 2011 DOI: 10.1071/WR10162 (http://dx.doi.org/10.1071 /WR10162)

E. C. M. Parsons, An Introduction to Marine Mammal Biology and Conservation, Jones & Bartlett Publishers, Burlington, MA, 2013

T. Vaughan, J. Ryan, and N. Czaplewski, Mammalogy, 5th ed., Jones & Bartlett Learning, Sudbury, MA, 2011

I. Werneburg and M. R. Sánchez-Villagra, The early development of the echidna, Acta Zool., 92(1):75–88, 2011 DOI: 10.1111/j.1463-6395.2009.00447.x (http://dx.doi.org/10.1111/j.1463-6395.2009.00447.x)

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