The Australosphenida is a group of mammals that has been studied fairly extensively in recent years, which is not bad going when one considers that, at most, less than twenty species have been assigned to it and some authors are of the opinion that the majority of those should not be regarded as australosphenidans at all.
The undoubted Australosphenida (or, more correctly if dealing with the restricted grouping, Ausktribosphenida) are five small Mesozoic insectivores (Rougier et al., 2007) - Asfaltomylos patagonicus and Henosferus molus from Jurassic South America, Ambondro mahabo from Jurassic Madagascar, and Ausktribosphenos nyktos and Bishops whitmorei from Cretaceous Australia. Despite their probably being fairly unprepossesing animals in life (as far as we can tell - so far, ausktribosphenids are only known from teeth and jaw bones), ausktribosphenids have provoked a fair amount of interest because of the resemblance between their teeth and those of modern marsupials and placentals. All three groups possess an arrangement called the tribosphenic molar, in which the lower molars each have a large posterior depression that contacts with a large cusp in the corresponding position on an upper molar, facilitating the grinding of food ("like a mortar and pestle", is the comparison that has been used in print).
The discovery of tribosphenid mammals in Gondwana earlier than they had been found in northern continents (where they appear in the early Cretaceous) therefore led to the suggestion that tribosphenid mammals may have evolved in the Southern Hemisphere and only later spread to the North (modern marsupials, despite their current Southern Hemisphere distribution, were derived from Northern Hemisphere Mesozoic ancestors). However, further phylogenetic analyses lead to the alternative suggestion (Luo et al., 2001) that ausktribosphenids evolved the tribosphenic molar independently from Northern Hemisphere tribosphenids. Instead, Luo et al. (2001) placed ausktribosphenids as related to modern monotremes, which lack tribosphenid molars but share with ausktribosphenids a distinct shelf (the cingulum) around the front of the molars. This ausktribosphenid + monotreme grouping is what Luo et al. (2001) dubbed the Australosphenida. Later analyses (e.g. Rougier et al., 2007) make the 'ausktribosphenids' paraphyletic with regard to monotremes. Alternatively, some analyses have continued to support a monophyletic tribosphenid clade uniting ausktribosphenids, marsupials and placentals that excludes monotremes (Rowe et al., 2008). Things are not made easier by the point that, while ausktribosphenids are known from little else than teeth, known monotremes, both living and fossil, mostly have teeth that are vestigial, absent or just plain wierd (Kollikodon, I'm looking at you).
Living monotremes are, of course, restricted to Australia, though it wasn't always so - Monotrematum sudamericum is a monotreme known from the Palaeocene of South America. Other known fossil genera are all Australian. I won't bore you with the things everybody already knows about monotremes - the presence of venomous ankle spurs in platypuses, the four-headed penis of echidnas, or the fact that baby echidnas (which are held in a pouch on the mother's underside) are known as puggles. Some things I will mention - if you've never seen a live echidna, they're a lot bigger than you think they are (I don't know how big you think they are, but I can assure you that they're bigger). According to Wikipedia, Tachyglossus aculeatus, the short-beaked echidna (the species found in mainland Australia), reaches about a foot and a half in length, while the New Guinean Zaglossus species are even bigger. The extinct mainland Australian species known as 'Zaglossus' hacketti (probably not a Zaglossus, but unrevised) would have been as large as a sheep. Echidnas when disturbed are able to dig with their fore-feet in such a way that they effectively sink into the ground while remaining horizontal, meaning that they retain full protection from their spines. In some areas (best known on Kangaroo Island in South Australia; see here) echidnas may form trains - shortly before a female becomes sexually receptive, a train of up to ten males will begin to follow her around in single file, waiting for her to give them the go-ahead*. Echidnas mate lying on their sides dug into a trench made by the male.
*If humans were to do this, it would be regarded as creepy. Echidnas don't seem to have this problem.
And in case you were wondering, I have been informed that the best way to deal with the spines when cooking an echidna is to roast it whole with the spines still on; after it's finished cooking, the spines can be pulled out fairly easily.
Luo, Z.-X., R. L. Cifelli & Z. Kielan-Jaworowska. 2001. Dual origin of tribosphenic mammals. Nature 409: 53-57.
Rougier, G. W., A. G. Martinelli, A. M. Forasiepi & M. J. Novacek. 2007. New Jurassic mammals from Patagonia, Argentina: a reappraisal of australosphenidan morphology and interrelationships. American Museum Novitates 3566: 1-54.
Rowe, T., T. H. Rich, P. Vickers-Rich, M. Springer & M. O. Woodburne. 2008. The oldest platypus and its bearing on divergence timing of the platypus and echidna clades. Proceedings of the National Academy of Sciences of the USA 105 (4): 1238-1242.