THE largest marsupial and rodent ever to walk the earth have turned up in two fossil studies. The two extinct behemoths are raising questions about why no giants from each group exist today.
The marsupial (Diprotodon optatum) resembled a massive wombat and died out in Australia around 35,000 years ago. Few fossils exist, so researchers had been unable to estimate its size accurately. Now Stephen Wroe at the University of Sydney and his colleagues have developed a new method for putting a figure on the animal鈥檚 bulk. They calculated the relationship between limb bone measurements and total size in 18 living marsupials and 32 true mammals. By applying the same equation to the leg bones of Diprotodon, they worked out its vital statistics. Their figure of over 2.5 tonnes is double the previous estimate (Biology Letters, DOI: 10.1098/rsbl.2003.0095).
That casts doubt on the accepted wisdom that Australia鈥檚 poor soils prevented it from supporting really large animals. 鈥淥ur work obviously undermines this interpretation of Australia鈥檚 mammalian fauna as the poor runts of the continental litter,鈥� Wroe told 快猫短视频.
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An unconnected fossil find in Venezuela has thrown up another monster, this time a distant relative of the guinea pig. The 8-million-year-old Phoberomys pattersoni had been known only from fossil teeth and bone fragments, but the discovery of an almost complete skeleton has allowed researchers to measure the giant rodent for the first time.
Marcelo S谩nchez-Villagra at the University of T眉bingen in Germany says the cow-sized beast weighed in at 700 kilograms 鈥� 10 times the size of today鈥檚 largest rodent, the capybara (Science, vol 301, p 1708). He speculates that the giant rodents of the past lost out to more fleet-footed ungulates.