Again: What is a mammal?
Of course, it's hard to draw the line. Any line is arbitrary, and doesn't reflect the incremental nature of evolution.
Taxonomists must be particularly grateful that the fossil record is gappy. The result is more harmonious: the appearance that ancestor and descendant species are somehow separate, whereas each evolutionary lineage is best represented as a continuum.
There are a number of characteristics that go towards making an animal distinctly mammal. The "canonical" signs include:
- the movement of the articular and quadrate bones from the jaw to the ear (as anvil and hammer)
- secondary palate (which enables mammals to eat and breathe at the same time).
Colin Tudge reckons homiothermy is a defining characteristic. This allows mammals to simply burn heat to keep warm, and is seen as a particular evolutionary advantage.
In a previous post, I mentioned the definition that mammals are the common ancestors (and all descendents thereof) of monotremes, marsupials, and placentals.
Other relateds are thereby referred to as mammaliformes. However, this is quite arbitrary, as the line is drawn at those currently living. Egg-laying monotremes are thus admitted in somewhat anomalously. They are non-therian mammals.
The split between mammals and reptiles occurs from the amnoite level. Reptiles (and birds and dinosaurs) are sauropsids. The mammal line of descent from amniotes runs through synapsids (from the Carboniferous period) to therapsids (mid-Permian) to cynodonts (late Permian). Synapsids through to cynodonts have been at various times in the past referred to as "mammal-like reptiles".
From there, we have the mammaliformes. The oft-cited Paleos is a fairly authoritative web site, referenced in Wikipedia (amongst many others) and with copious primary references (Paleos itself is being slowly expanded as a wiki here). The journey from cynodonts to mammals is detailed here.
Only trouble is, it's not quite in harmony with the paper on New Zealand's SB mammal (aka "waddling mouse"). The paper describes them as "more derived than morganuconodonts, and more primitive than multituberculates". However, the cladogram in the paper differs from that in Paleos. Both quote a nearly-identical set of sources, one of the main of which is a Chinese paleonotologist called Zhe-Xi Lou, who has done extensive work on early mammalian taxonomy.
More to come.