If the island is particularly small, of course, the animals in question are likely to die out, if not immediately, then dwindling over a few generations until they lack the genetic diversity to sustain themselves. On the other hand, if the island is a large one (such as, say, Britain) then there may not be a problem at all, and nothing happens beyond the usual genetic drift between isolated populations.
At some sweet spot in between, however, the animals can survive, but because their food supply is limited, the smaller members of their kind have a competitive advantage. Over the generations, the average size of the animals shrinks. Given long enough, the isolated population can become an entirely new species, noticeably smaller than its mainland kin. It's something I've discussed before, for example, when describing the miniature buffalos of Sulawesi and Mindoro.
The largest land animals on Earth today are elephants. As such, we might expect them to be prime candidates for insular dwarfism; it wouldn't take much for them to face a restricted food supply. And, indeed, while all three living species of elephant are, of course, big (and continental) several dwarf island forms did once exist. It may help that elephants are better swimmers than we tend to think, which may be partly how they got themselves to the islands in the first place.
Perhaps the best-known of these is Falconer's dwarf elephant (Palaeoloxodon falconeri) which lived on Malta and Sicily during the Ice Ages. Related to the straight-tusked elephant (P. antiquus) of the mainland, it was probably the smallest of all the dwarf elephants, although others are known from Crete and Cyprus. Dwarf mammoths, however, are less well-known, perhaps partly because they appear to have been less common. Currently, we know of three species of pygmy mammoth, including one each from Santa Rosa Island in California and from Crete in the Mediterranean.
The remaining one is the Sardinian mammoth (Mammuthus lamarmorai), living on the only large Mediterranean island that was never home to true elephants. In a sense, of course, mammoths are elephants, since they descend from the last common ancestor of the three living elephant species, being more closely related to the Asian one than to the other two. However, they are distinctive, so our purposes today, I'll stick to the narrower term.
The Sardinian mammoth was first named by Charles Forsyth-Major in 1883, although he wasn't the first to describe bones we now assign to the species. In fact, the second half of the name comes from Alberto della Marmora, a soldier and amateur naturalist who, while governor-general of Sardinia, had briefly written about recently uncovered fossil remains of a miniature elephant in 1857. (Those specific remains have long since been lost, but it seems almost certain that they are the same species we now recognise).
Forsyth-Major described his new fossil species as a member of the genus Elephas, to which today's Asian elephant belongs. However, not everyone agreed. Some argued that it wasn't anything new at all, but just another example of one of the species of dwarf elephant already known from the Mediterranean islands. Others disagreed with it being an Asian elephant; in 1942, for example, Henry Osborn placed it in Palaeoloxodon, as a close relative of Falconer's dwarf elephant, while others continued to place it in Elephas until at least the 1970s.
Only in this century has it really been confirmed to be a mammoth, not a regular elephant, mainly on the basis of the shape and microstructure of its teeth and tusks. This prolonged confusion is at least partly due to the fact that we have only limited remains of the animals. Sardinian mammoth fossils are known from six sites along the western side of the island, but most of these have revealed nothing but a few teeth. The most complete skeleton we have consists of parts of the legs, shoulders, and hips, a few fragments of ribs, some bones from the neck and tail, and part of a tusk.
Interestingly, close by to the site that yielded the skeleton is a second that provided not bones, but fossilised footprints, pressed into what was once a beach, and is now a sandstone cliff. These are no more than 130,000 years old, and include the impressions of both front and hind feet.
Having most of the legs and shoulders allows us to make a reasonable estimate for how high the animal would have stood in life - approximately 150 cm (5 feet) - but it is much harder to determine what it would have weighed. Here, we can only make guesses based on the size of various bones and (to an even more limited extent) of the footprints. Trying to apply different formulae gives us widely differing answers, which probably goes to show how limited these formulae are, but the best guess is that it was somewhere between 750 and 950 kg (0.8 to 1.0 US tons). Not exactly small, then, but considerably less than any living elephant, let alone the 9-ton estimated weight of the largest woolly mammoths.
Having said which, it's unlikely that the Sardinian mammoth does descend from its most famous relative. That's because the oldest woolly mammoth remains date back 400,000 years, whereas the oldest Sardinian mammoths are 50,000 years older. Instead, the most plausible ancestor, based both on the dates and some similarities of the teeth, is the steppe mammoth (Mammuthus trogontherii). This is thought to be the immediate ancestor of its woolly relative, and lived across most of Europe and northern Asia between about 1.7 and 0.2 million years ago.
It was also noticeably larger than the woolly mammoth, with the males estimated to have weighed around 11 tons on average. So, even at the most conservative estimate, the Sardinian mammoth is thought to have been about 90% smaller by weight than its ancestor. Which is a lot.
The question remains as to how, and when, mammoths reached Sardinia. Sea levels were lower during the Ice Ages, but, while Sardinia was almost certainly united to Corsica, it was never connected to the mainland by a landbridge. The Corsica Channel would, however, have been narrower than it is now, so it is possible that the mammoths managed to cross that before heading south to the more hospitable pine forests that occupied Sardinia at the time. (Corsica, for what it's worth, has few geological deposits of the right age, so we don't know for sure that the 'Sardinian' mammoth didn't live there, too). This likely happened well before the oldest fossils that we have, not least because it must have taken thousands of years for the mammoths to shrink to the size we find in their remains. Some point between 650,000 and 450,000 BC seems the most likely, well before our own species arose in Africa.
We also don't know quite when they went extinct. There are no known fossils younger than about 30,000 years, but the dating is a little uncertain and it's certainly possible that they survived until more recently. It was probably climate change and competition from newly arrived deer that killed them off, although, again, we can't absolutely say that the last Sardinian mammoth didn't meet the first Sardinian human.
With, no doubt, predictable consequences.
[Image from Palombo et al. 2024, available under the CC-BY 4.0 licence.]
How did the deer get there? And perhaps more interestingly, why only then, about 30k BP IIUC? Simeito do with the Glacial Maximum?
ReplyDeleteIt would be close to the right time, so that's possible. But we're talking Megaloceros, so it isn't exactly a /small/ deer...
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