Monotremata - monotremes

The order Monotremata is the only extant group within the subclass Prototheria - the oldest living taxon of theclass Mammalia. Whilst there has never been much doubt that monotremes split off from other mammal group at an early stage, due to their many ancestral reptilian characters (see below), the mid 20^th-Century saw much dispute regarding the evolutionary positioning of the other two major groups of mammal - the metatherians (marsupials) and the eutherians (placental mammals) - with respect to the monotremes.

The question was whether the metatherians were the sister clade of the monotremes or of the eutherians. This proved a difficult question to answer, as much of our understanding of mammalian phylogeny has been gained through comparisons of tooth morphology, yet monotremes are toothless - a character that is certainly not ancestral.

However, much study on the ancestral amniote features of monotremes, as well as a recent complete analysis of their genome, has shown that they split off from other mammal groups around 200 million years ago, at the Triassic-Jurassic boundary. Therefore, Metatheria and Eutheria together form the clade Theria (live-bearing mammals), to the exclusion of Prototheria. The cladogram below summarises these relationships:

Diversity and Lower Taxonomy:

The monotremes are a group of highly specialised egg-laying predatory mammals, containing the platypus and echidnas. There are only five living species of monotreme, contained within two families:

• Family Ornithorhynchidae: the platypus, a single species in a single genus, Ornithorhynchus anatinus.
• Family Tachyglossidae: the echidnas. Four species divided into two genera:

1. Tachyglossus: one species of short-beaked echidna (Tachyglossus aculeatus).

2. Zaglossus: three species of long-beaked echidnas. (One of these, Zaglossus attenboroughi, a species from the Papua province of the island of New Guinea named after British naturalist Sir David Attenborough, has never been seen in the wild and is only known to science through a single museum specimen from 1961! In 2007, researchers at the Zoological Society of London confirmed that Sir David's echidna is not extinct, by finding burrows and tracks made by the species)

Distribution and Habitat:

Endemic to Australasia - an important point to note as it means that they represent a whole subclass of extant mammalian life in a single geographic region. While the platypus is semi-aquatic, the echidnas are all terrestrial, and their respective distributions and habitats are as follows:

• Platypus - Confined to Eastern Australia and Tasmania; freshwaters streams, rivers, and some lakes.
• Short-beaked echidna - Australia and New Guinea; most habitats, from semi-arid to alpine.
• Long-beaked echidna - New Guinea; mountainous terrain.

Conservation Status (IUCN):

• Platypus - Least Concern (LC) - However, although the platypus is common, it has a fairly narrow, specific habitat range and, consequently, is distributed in localised regions. It is therefore vulnerable to local extinction.
• Short-beaked echidna - Least Concern (LC)
• Long-beaked echidna - Critically Endangered (CR)

Features:

• Males have a spur on their ankles, which bears poison in the platypus.
• Toothless - platypuses have a leathery electrosensory bill, with crushing horny plates to break through the tough exoskeleton of arthropods; echidnas have an elongate horny rostrum with a long sticky tongue for collecting insects.
• A range of mammalian characters:
  • Produce milk (lactate) from mammary glands. However, while therians have nipples, monotremes do not, and consequently the young suck milk from patches of mammary hairs - specialised areas of fur positioned around the ventral openings of the mother's mammary glands.
  • Epipubic bones - two thin rod-like bones extending anteriorly from the pubic bones of the pelvic girdle.
  • Lower jaw (mandible) made up of a single bone, the tooth-bearing dentary.
  • A middle ear formed of three bones: the incus, malleus, and stapes. While the stapes is present in the middle ear of all living tetrapods, the incus and malleus are modified bones from the typical amniote jaw joint. The jaws of non-mammalian amniotes articulate via the quadrate of the upper jaw, and the articular of the lower jaw; in mammals, the quadrate migrated to form the incus, while the articular became the malleus, leaving a jaw joint formed of the dentary articulating with the squamosal (the angular bone of the non-mammalian amniote lower jaw is used as a bony support for the eardrum in mammals). The following diagram illustrates these differences:

• A range of ancestral reptilian characters:
  • Egg-laying (oviparity); however, these soft-shelled eggs are short-lived, the young hatching after around ten days and being dependent on mother's milk for up to six months after.
  • Sprawling gait (although it is possible that this is a derived feature of monotremes, relating to specialisations for swimming in the platypus, and for digging in the echidnas).
  • A single common opening for the digestive, urinary, and reproductive tract, called the cloaca (the general amniote condition). In therian mammals (marsupials and placentals), there are two openings: one for the digestive system, and one for the urogenital tract.
  • Presence of the coracoid - the ventral bone of the shoulder girdle seen in all non-mammalian amniotes. This is the general amniote condition, where the humerus articulates with the shoulder girdle at the junction between the coracoid and the scapula (commonly termed the shoulder blade).
  • Not entirely homeothermic. While monotremes are - meaning that they regulate their body temperature using heat produced during endothermic metabolism - they are fairly poor at maintaining a constant body temperature during extreme enviromental conditions.