Tachyglossus aculeatus, the short-nosed echidna, is one of three extant members of Monotremata, the others being Zaglossus bruijni and Ornithorhynchus anatinus. Monotremes, or the egg-laying mammals, are named for the single common opening for the urogenital and digestive systems. Most phylogenetic analyses based on morphological and molecular data place Monotremata as the sister taxon to Theria (Placentalia + Marsupialia) among the major clades of living mammals. An alternative hypothesis based on molecular and morphological data places Monotremata as the sister taxon to Marsupialia (Gregory, 1947; Penny and Hasegawa, 1997).
Tachyglossus is native to Australia, Tasmania, and central and southern New Guinea. The fossil record of echidnas is poor only extending back to the Pleistocene of Australia and New Guinea (Murray, 1978; Griffiths et al., 1991).
The short-nosed echidna lives in a variety of terrestrial habitats and shelters in burrows or caves. Tachyglossus is nocturnal and crepuscular in activity. It rarely enters torpor during cold weather but rather uses shivering for thermoregulation. Ants and termites comprise the majority of the diet of this animal (Nowak, 1991).
Echidnas, also known as spiny anteaters, are covered by hollow spines that are essentially modified hairs. The body is rounded and dorsoventrally compressed ending in a short tail. The legs are short and stout each ending in feet with five digits that terminate in elongated claws. The most prominent feature on the head is the elongate, hairless snout. The mouth is toothless and contains a long, sticky tongue.
The skull of Tachyglossus is characterized by an elongate, rounded snout and a laterally bulging braincase. The palate extends back to the level of the ears. The ectotympanic is oriented horizontally, and the external auditory meatus is directed ventrally. The lower jaw is reduced and has poorly developed coronoid and angular processes. Internally, the most prominent features are the turbinates. They extend far posteriorly in the skull (see 3D model), underlying the olfactory and cerebral cavities of the braincase.
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Tachyglossus aculeatus on the Animal Diversity Web (University of Michigan Museum of Zoology)
The brain of Tachyglossus aculeatus on the Comparative Mammalian Brain Collections
Tachyglossus aculeatus page by Dr. Ellen K. Rudolph