Being a placental mammal, I personally find the notion of giving birth to an undeveloped young that then climbs into a pouch of the mother for a certain time after its “birth” rather incredible and completely wacky! Such is the most notable feature of marsupials, which includes famous critters such as koalas, possums, opossums, wombats, the Tasmanian devil, and the poster-child- the kangaroo. But is that the ending highlight to marsupials? Absolutely not!
Marsupials are an “infraclass” of mammals and are part of the clade that originated from the last common ancestor of extant metatherians; all mammals from Metatheria give birth to underdeveloped young as described above.
Where Do They Come From?
Marsupials from the Cretaceous period (~65-75 million years ago) have been found in North American sites, but after the success of placental mammals, who later more efficiently took up their niches, marsupials become extinct in North American 15-20 million years ago, except for the Virginia opossum, which only just came up from the south as a “re-colonization”.
Upon leaving North America, marsupials moved out to South America and Europe across land bridges. Marsupials were having a party in South America, as one normally does have down there, but in the Pliocene period (5-1.8 million years ago), a new land bridge formed to North America, causing for placental mammals, such as raccoons, cats, and bears, to once again take the niche of the larger marsupial carnivores.
The European marsupials were unexciting (sorry). They lasted for only 35 million years.
So here is the monkey wrench and the mystery: the largest number of marsupials are located in Australia, BUT we have no true idea of exactly how this happened. What is believed is that during the Paleocene period (~60-65 million years ago), Australia, Antarctica, and South America were united- yes, they were the last pieces of Gondwana (when the earth was one- literally, not in the figurative way we all wish could make happen). The issue is that though two marsupials were found in the middle Eocene period of Antarctica, which was a very toasty and humid beech tree forest, no fossils have been discovered for a 30 million year period. Supposedly a very beautiful and ideal forest re-appeared in the early Miocene in what is now present-day Australia, and marsupials reappeared.
Sadly, there were losses throughout the processes, but the total accounted number of Australian marsupials are 155 (only 30 are bound to trees), while in New Guinea, 50/83 marsupial species are strictly arboreal.
Some Anatomical Features of a Marsupial:
– Their pes (footsies) have digits (fingers/toes) with claws intact. This is considered an “unreduced state”.
– Their tails act as a 5th hand.
– The hindlimbs are a wee bit bigger than the forelimbs to reflect the behavior of often standing up and leaping, though they are considered quadrupeds, and the hind limb’s digits are completely uneven!
– They have an epipubic bone that projects from the pelvis ahead of them to support their pouch.
– Pouches can be really varied. In fact, some marsupials do not even have a pouch!
1) Rudimentary pouch: a fold of skin on either side of the nipple to protect a suckling youngin’. This method is used by quolls, antechinuses, and mouse opposums).
2) Proper pouch, used by Virginia and southern opposums, the Tasmanian devil, and dunnarts.
3) Deep pouch that opens forwards for possums and kangaroos that have smaller litters but really jump or climb a good deal.
4) Deep pouch that opens backwards, which are typical of burrowing and digging critters, are seen in bandicoots and wombats.
More to come next week on the unique reproductive methodology of marsupials, along with more structural features and omens to their future in conservation!