Monday, 7 April 2014
As is the case for all vertebrates, the ancestors of today's mammals were jawed fishes (or Gnathostomata). Among these, some would one day use four of their fins like primitive legs to allow locomotion, and they would lead to the development of the tetrapods, or four-limbed animals. The gorgeous fossil Tiktaalik is an example of a fish with four rudimentary limbs; it is a perfect example of a transitory form between fishes and land animals. Tiktaalik's name come from Inuktitut since it was discovered in Nunavut; it means "burbot", a large freshwater fish that looks like cod. Please note that not all extant tetrapods actually have four limbs; some, like snakes, lost them along the way (but are descended from four-limbed animals).
Tetrapods include amphibians, reptiles and birds, as well as mammals; the amphibians were the first among them to walk the earth. A primitive trait among amphibians is that they still lay their eggs in water to keep them from dehydrating; this allows naturalists to group fishes and amphibians in one informal group called anamniotes. This distinguishes them from the later reptiles, birds and mammals who are collectively referred to as amniotes, animals that see their embryos develop in an amnios, a protective pouch found either in a solid egg that can be laid on land or in a uterus.
Early amphibians could get around on land, unlike their fish forebears, and some among them developed characteristics that would set them apart and allow the eventual appearance of reptiles. The amphibian Proterogyrinus is an example of such a proto-reptilian amphibian.
As reptiles diversified, they and their descendants accumulated differences that set them apart one from the other; they can be grouped according to the presence and/or position of certain skull cavities called fenestrae. We personally belong to the group called synapsids, defined by the presence of a temporal fenestra behind the eye, on the side of the skull. All mammals are synapsids, and the synapsid reptiles are now extinct so there's no way to get it wrong! The other reptiles still living today (and the birds!) are sauropsids, which can be further distinguished as anapsids (without a fenestra, and to which belong the turtles and tortoises) and as diapsids (all other extant reptiles as well as birds, with two fenestrae on the side of their skull, separated by the postorbital and squamosal bones. Dinosaurs were diapsids).
Perhaps you've once seen a reconstituted dimetrodon. This spectacular animal is often mistaken for a dinosaur because it is a very ancient reptile (it goes back to the Permian) but it is actually a synapsid, and therefore more closely related to mammals than it is to actual dinosaurs. Before mammals proper appeared, there were lots of such synapsids known as mammal-like reptiles.
The mammals that would evolve from mammal-like reptiles had new distinguishing features: they produced milk, they had hair, three auditory ossicles in the middle ear and a neocortex. They would then diverge along several ways, leaving us today with three types of mammals: the prototherians or monotremes, the metatherians or marsupials, and the eutherians or placentals.
Monotremes are mammals that still lay eggs, just as our reptilian forebears did. There aren't that many left today, and they're found only in Australia and New Guinea: all we have are four species of echidnas and the ever sympathetic duck-billed platypus, very likely my favourite animal of them all because it looks so unlikely. And it has a badass venomous ankle spur. Monotreme babies hatch out of an egg like reptiles but then rely on their mother's milk, which does not come out of a nipple but through pores in the skin.
Marsupials no longer lay eggs, but their embryonic development does not occur entirely in a uterus. The marsupial embryo comes out of its mother's womb after just a few weeks of gestation and crawls all the way to her abdominal pouch, where it will find a nipple to which it will stay attached as it finishes its development. Kangaroos, koalas and wombats are all marsupials. There are few marsupials outside of Australia and New Guinea; it seems that they fare poorly when having to compete with placentals. (Australia was already isolated from other land masses when placentals took over the rest of the world). The only marsupials left in America are a few types of opossums.
Placentals are the last mammals on the list; their embryos develop entirely in the mother's womb. They can be found pretty much everywhere on the planet.
The water opossum featured above, also called Chironectes minimus or yapok, is an American mammal with the great honour of being the only aquatic marsupial, and the dubious honour of being the only extant marsupial with a marsupial pouch in both males and females. Alas, the tasmanian tiger or thylacine (of which you can see an echo here), who also used to have a pouch in both males and females, was made forcibly extinct by man in the early XXth century. The last thylacine died in Hobart zoo, in Tasmania, in 1936.