monotreme - General characteristics, Physiology, Fossil monotremes

An egg-laying mammal; lays soft-shelled eggs which hatch after 10 days; suckles young for 3–6 months; no teeth as adults. (Order: Monotremata, 3 species.)

iMonotremata Fossil range: Early Cretaceous - Recent
Short-beaked Echidna
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Mammalia Subclass: Australosphenida Order: Monotremata C.L. Bonaparte, 1837
Families
†Kollikodontidae Ornithorhynchidae - Platypus Tachyglossidae - Echidnas †Steropodontidae

Monotremes (monos, single + trema, hole;

General characteristics

Like other mammals, monotremes are warm-blooded with a high metabolic rate (though not as high as other mammals, see below); It is still sometimes thought, for example, that the monotremes are "inferior" or quasi-reptilian, and that they are a distant ancestor of the "superior" placental mammals. It now seems plain that modern monotremes are the survivors of an early branching of the mammal tree;

Similarly, it is still sometimes said that monotremes have less developed internal temperature control mechanisms than other mammals, but more recent research shows that monotremes (such as the Platypus, which can maintain its body temperature even while living in an icy mountain stream) maintain a constant body temperature in a wide variety of circumstances without difficulty. Early researchers were misled by two factors: monotremes maintain a lower average temperature than most mammals (around 32°C / 90°F , compared to about 35°C / 95°F for marsupials, and 38°C / 100°F for most placentals);

Physiology

The key physiological difference between monotremes and other mammals is the one that gave them their name; In contrast to the single cloaca of monotremes, other mammal females have separate openings for reproduction, urination and defecation: the vagina, the urethra, and the anus. However, recent work suggests that monotremes acquired this form of molar independently of placental mammals and marsupials (Luo et al, 2001). The jaw of monotremes is constructed somewhat differently from those of other mammals, and the jaw opening muscle is different.

Their metabolic rate is remarkably low by mammalian standards, although the extent to which this is a characteristic of monotremes, as opposed to an adaptation on the part of the small number of surviving species to harsh environmental conditions, is uncertain. Fossil and genetic evidence shows that the monotreme line diverged from other mammalian lines about 150 million years ago and that both the short-beaked and long-beaked echidna species are derived from a platypus-like ancestor. These fragments, from species Steropodon galmani, are the oldest known fossils of monotremes. (See fossil monotremes below.)

ORDER MONOTREMATA Family Ornithorhynchidae: platypus Genus Ornithorhyncus Platypus, Ornithorhyncus anatinus Family Tachyglossidae: echidnas Genus Tachyglossus Short-beaked Echidna, Tachyglossus aculeatus Genus Zaglossus Western Long-beaked Echidna, Zaglossus brujinii Sir David's Long-beaked Echidna, Zaglossus attenboroughi Eastern Long-beaked Echidna, Zaglossus bartoni

Fossil monotremes

Excepting Ornithorhynchus anatinus, all the animals listed in this section are extinct.