What is the most primitive (ie most akin to earlier forms- least developed) mammal currently existing on earth?
From what I understand, our friend the platypus has spent the last 150 million years doing its own thing while remaining unchanged. So it gets my vote.
Your conditions are rather difficult to answer objectively.
The only living family we are certain was extant before the K-T event is the Didelphidae, the opossums of the Americas. Didelphis itself, the familiar 'possum, dates IIRC from the Eocene. The duckbilled platypus and the spiny anteaters are adapted for particular econiches, but preserve a lot of “primitive” traits. Among placental mammals, the one most likely to be closest to the ancestral placental mammal would be the solenodon, found in two species inhabiting Cuba and Hispaniola.
Monotremes (platypuses and echindas) are usually thought to be the most primative animals.
Reading further, I’m wondering if 150,000,000 is about half again too many (i.e. the website lied to me).
Nature.com says that:
On the other hand, the platypus could have been wandering about long before they became a distinct group as well so the two don’t contridict per se. I was trying to find more sites saying exactly how long they’ve been around, but little luck so far.
Ok, [urk=http://www.platypus.org.uk/det-ancest.html]Platypus.org – who I assume has all your platypus needs – says the oldest known fossil of a “modern” platypus is 100,000 years old. On the other hand, they don’t seem to have changed a heck of a lot from their forefathers which I guess gets you points in the “most primitive” catagory.
No arguement that it, and its spiny friends, developed in special circumstances, but I read the OP to mean that if we call it a mammal and it’s really, really old, it qualifies.
The monotremes are the only group of mammals too primitive to have developed lactose as the sugar of choice in their milks. They also do not have true nipples - babies lick the milk off the mother’s fur. And then there is that egg-laying business.
Unquestionably, the monotremes preserve unmodified some traits that they inherit from the most primitive early mammals. However, there seems little doubt that their precursors were not adapted for the particular roles they occupy today. (Australia’s annoying nearly total lack of early Tertiary fossils means we have very little evolutionary evidence on everything that’s Aussie-endemic.) Contemplate the scenartio:
“Hey, I know what I’m gonna do when I grow up – I’ll be a fierce carnivore!”
“But dude – you ain’t got no teeth, you have a friggin’ duck bill, a beaver-tail for swimming, little short legs, and your only weapon is that spur on your big toe. What are you gonna do – jump out from hiding and wait while they laugh themselves to death?”
In very broad and misleading terms, evolutionary biologists tend to array the creatures they study on a spectrum from specialist to generalist. A specialist has evolved to take maximum advantage of a single common econiche, cannot live without that niche but can outcompete anything else within that niche. A generalist, on the other hand, has few or no specializations, and its adaptations are in the direction of making use of whatever econiche is not being exploited fully.
The two great niches that the dinosaurs and other reptiles of the Mesozoic did not fill were insect-predator and fruit-and-nut-eater. (Segnosaurs may have filled the giant-anteater role, but anything smaller that lived on insects was vacant.) These roles were the premier ones for mammals throughout the later Mesozoic. A real Jurassic Park would have far more mammals than dinosaurs in it, but they’d all be small shrew-, mole-, and lemur-like types. These creatures tended to be generalists, living on whatever the environment provided that they could feed on.
So we have the contradictory premises that primitive traits are best preserved in creatures that are very much specialists, but the primitive creatures themselves were generalists.
The opossum is, next to man, the ultimate generalist – it can make a living in almost any niche available for a creature of its size. And hence it has had almost no evolutionary pressure to evolve since marsupials emerged in the late Cretaceous. (There are specialist marsupials in South America: the yapok, a 'possum that works the otter role; one adapted with fluffy fur for the Andean heights; etc.) Likewise, the most primitive placental mammals were almost definitely insectivores, whose skeletons were quite similar to that of the solenodon.
Therefore, a case can be made for all three of my suggested answers, depending on what “most primitive” and “least developed” mean in the OP.