Monotremes are the only group of extant mammals that are non-therian, i.e. non-placental. (The name refers to their peculiar single opening for intestinal/urinary/reproductory functions.)
There are five identified species: four are echidna, and one is a platypus.
I happen to be reading Richard Dawkins' Ancestor's Tale. Dawkins mentions platypus fossils (Obdurodon) older than the point at which echidna and platypus diverged, along with molecular analysis that strongly implies the echidna is derived from a platypus! - effectively, over time its ancestors gradually abandoned water, changed the form of their feeding appendage (duckbill to tubular) and acquired protective quills.
Monotremes are non-therian mammals, which would mean they lay eggs. I am casting aside the oft-used term Prototherian ('first animals') as not being truly cladistic, since it excludes descendant Eutherians ('placentals') and Metatherians (marsupials).
So to other non-therians: early mammals and their non-placental descendants.
Research is hard. The situation is not as simple as Tudge's statement (dated 2000) that there are eight types of non-therian mammals - that takes a judgment call, and a lot has happened since then.
Non-therians are typically taken to include at the least: monotremes, multituberculates, Morganucodonts, Triconondonts, Doconondonts... and a few others.
How they're grouped is a matter of current debate. It would take several posts, so I intend to lay this aside for the moment.
Most of the names above are based on characteristics of their teeth - I imagine for several reasons:
- teeth are often all that remains
- they're a useful differentiating characteristic
- they give some indication of diet, and thus niche, adaption, selectivity.
Another debate also seem to revolve around the time frames that they each occupy. Of particular interest are those mammal-like creatures (mammaliformes) that existed before the K-T boundary - the 65mya dinosaur extinction event that enabled mammals to proliferate. True mammals were all shrew-sized before then, barely eking an existence in the dominant shadow of the dinosaurs.
All this is relevant because it traces the evolution of mammals, how they developed their features over time (and how fast), and how successful they were in their environmental niches against competition from non-mammals and from later developments.
More to come on non-therians. I might take a break, however, to discuss the formation of New Zealand.
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