Evolution and systematics

The first reaction of many taxonomists on examining the monotremes was to classify them as an unusual group of fur-bearing reptiles. It took almost 200 years for monotremes to be declared unequivocally as mammals—indeed, some people remain unconvinced. The characteristics that qualify them as mammalian include a single bone in the lower jaw, three small bones in the middle ear, a high metabolic rate and warm blood, a body covering or hair, and the ability of females to produce milk to feed their young. This last feature is the most important. Milk is secreted by mammary glands; hence, the class name, Mammalia.

In order to emphasize the differences between monotremes and other mammals, the order is often placed in its own subclass, the Prototheria. The name, meaning "first beasts," is unfortunate, since it implies that monotremes are in some way

The platypus (Ornithorhynchus anatinus) is one of only three egg-laying mammals. (Photo by J. & D. Bartlett. Bruce Coleman, Inc. Reproduced by permission.)

ancestral to other mammals. This is a common misconception. Many people still insist on describing monotremes as primitive, or inferior. This is demonstrably not the case. However, the Monotremata are certainly an ancient group—they split from the main branch of the mammalian phylogenetic tree sometime during the Cretaceous period, probably about 125-130 million years ago. This was before the divergence of marsupials and placental mammals, but at least 80 million years after the split between reptiles and mammals. Monotremes are no less advanced than any other living mammal group. Far from being ancestors of other mammals, they are cousins that have simply evolved in a different direction. They have retained many characteristics attributed to mammal ancestors, but they have also developed sophisticated adaptations lacking in other mammals. For example, they possess a remarkable "sixth sense" that enables them to sense the minute electric fields generated by other animals.

Seemingly, the monotremes have never been a particularly large or predominant group. The fossils identified to date include just eight extinct species, most of which have been found in Australia. However, these are quite diverse, including two species representing extinct families, the Kollokodontidae and Steropodontidae. There are also three extinct species of echidna and three long-dead species of platypus. The earliest known monotreme is also the earliest known mammal in Australia, a fossil known as Stenopodon galmani. It was discovered in rocks about 110 million years old during the excavations of an opal mine at Lightning Ridge in New South Wales. Only a fragment of jaw and a few teeth have been recovered, but this is enough to show that monotremes were as much a feature of Cretaceous Australian landscape as dinosaurs. Another important fossil find includes a 62-million-year-old platypus tooth from Patagonia. This fossil, known as Monotrematum su-damericanum, the South American monotreme, is significant because it suggests that the ancestors of modern monotremes may once have been widespread on the prehistoric southern landmass of Gondwana.

The most informative monotreme fossil discovered to date comes from the extraordinarily rich fossil beds of Riversleigh in Queensland, Australia. Estimated at 13 million years old, the specimen is an almost perfectly preserved skull that closely resembles that of a modern platypus, except that the jaw is full of developed teeth. The modern platypus only has baby teeth (milk teeth), which are, in the adult, replaced by flat horny pads that are used like millstones to crush and grind food before swallowing. The long-extinct relatives of the duck-billed platypus probably had a broad insectivorous diet much like that of modern-day hedgehogs and shrews. The Riversleigh platypus was given the generic name Obdurodon, meaning "enduring tooth." It is considered one of the mammalian fossil finds of the century, and is one reason River-sleigh has been designated a World Heritage Site.

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