Monday, December 01, 2008

from globigerina ooze to megafauna

I learn something new every day, at least that's something. 

From my reading today I learned about the globigerina ooze which at first I thought was a science fiction construct, but no it's what much of the ocean floor is covered with. The globigerina are a genera of globigerinida, a group of forams, as the experts call them. They're plankton, essentially, and their shells form this ooze. one of many titbits from The Blind Watchmaker. More important, though, is convergent evolution, the observation that, on isolated land masses, organisms will develop to suit certain trades, as they're called, though the variety within trades is considerable. For instance, the principal grass-eating herbivores in Africa are the hoofed animals such as zebra and antelope, gallopers, ungulates. In Australia, the kangaroo fills this niche, though it has developed so very differently. Differences and similarities would be worth pursuing. Herbivores tend to have complicated, bacteria-infested guts, to break down cellulose. The development of this evolutionary pathway would obviously be different in the Australian and African mammals [and among, say, Australian herbivorous mammals], each one finding unique solutions to the problems raised. Their place in the food chain, too would make speed a priority. Herbivores are rarely at the top of the food chain - presumably the kangaroo was prey for some other mammal until recently? Their speed was an important attribute in escaping predators, and they must have developed this speed in response, not to dingoes, who were a relatively newly introduced animals, but maybe thylacines - who knows how plentiful the thylacines were? Certainly, as a marsupial, they will have evolved along with the kangaroo. But the Wikipedia article claims that it wasn't so much predation that brought about the highly developed mobility, but the need to travel long distances in reasonably quick time in a land in which water was scarce. I'm sure they know what they're talking about, but consider that herbivorous ungulate, the camel, Camelus dromedarius [actually camels can probably run quite fast]. They evolved another way of surviving in dry conditions, why such vastly different solutions? 
Certainly there would have been predators of the kangaroos before the introduced species. The marsupial lion, Thylacoleo carnifex, not of course a lion but a carniverous diprotodont marsupial, which seems to have been wiped out by the advent of humans 40 to 50 thousand years ago, was one. Diprotodonts, extant or extinct, are almost all herbivores, and they are restricted to Australasia. The Thylacoleo is distinct inter alia for its retractable claws. It is thought to have been a slow animal, catching its prey by stealth and perhaps also falling on it from above like the proverbial drop bear. In those days there were also various giant herbivorous marsupials including giant kangaroos, though I suppose the Thylacoleo would've preferred smaller prey. It also probably shared its range with  another species of megafauna, the goanna Megalania prisca. Though there's much debate about its actual dimensions, Megalania is regarded as the largest terrestrial lizard known. However, fossil remains are rarer than those of Thylacoleo, and its habits are a matter of speculation. 

Anyway, all of this opens up a whole new world of Australian paleontology never visited before by me. Fascinating.  



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