The Ducks of Death

Under no circumstances should readers tease or otherwise annoy ducks.  Australian palaeontologists have unearthed fossil evidence that a family of enormous, flightless birds – the dromorthinids or ‘thunder birds’ – which roamed Australian rain forests from 24 Ma to as recently as 50 ka, were not related to emus as previously thought, but were ducks.  “Fine”, you might think.  “Pretty big ducks”.  “Quack, quack”.  This is an unwise attitude.

Newly discovered in Queensland, 15 Ma old fossils of the giant and fondly named Bullockornis – estimated at 3 m tall and weighing a third of a ton – include its beak.  This is not akin to the beak of Daffy Duck – it was other anatomical details that placed dromorthinids among the anseriforms – but a serious pair of biting shears with immense musculature, fronting a head about the size of a horse’s.  The even taller, though lighter moas of New Zealand had small heads in proportion to body size, and, like ostriches and emus, were undoubtedly herbivores.  Bullockornis was either a fearsome predator or a pretty awesome scavenger.  The only Australian mammalian predator that might conceivable have been a competitor was the 15 Ma old marsupial lion, Wakaleo vanderleueri; about Rottweiler size, but better equipped in terms of fangs.

Full proof of its predatory habits awaits discovery of remains that preserve the stomach contents of this dangerous duck.  Should that materialize, and the excellence of preservation in the Miocene limestones of Queensland suggests that it is possible, Bullockornis would have been the largest land predator since the demise of the dinosaurs.

Source:  Stephanie Pain, The Demon Duck of Doom.  New Scientist, 27 May 2000

Life sneaked through ‘Snowball Earth’

The awesome magnitude of glacial epochs in the late-Precambrian from about 850 to 590 Ma was first brought to popular attention by the late Preston Cloud in his book Oasis in Space.  More recent work than his centred on the position of the continental masses that underwent repeated glaciation at that time.  One puzzle was the close association in time and place of glacigenic sediments with thick sequences of biogenic carbonates, as well as the fact that every continent preserves evidence for glaciations during this lengthy episode.  Carbonates today are manufactured at tropical latitudes, but that cannot be certain for all geological time.  So the key technique in checking for low-latitude ice sheets was using magnetic field evidence, in particular the inclination of remanent magnetism preserved in rocks of that age.  This gives a good approximation for their latitude at the time.

Repeatedly, investigators found evidence that large Neoproterozoic ice sheets able to extend to sea level did indeed occur on continents straddling the equator at that time.  That presents a major climatic problem.  Ice reflects incoming solar energy extremely well – and at that time solar power was probably somewhat less than its present value.  Ice at the equator implies ice everywhere and runaway cooling, so that the oceans would freeze over too.  This would seem to be a  situation from which there could be no thermodynamic escape, except by slow build up of volcanic carbon dioxide to give global warming by the ‘greenhouse’ effect.  Clearly, the Earth did emerge from a ‘snowball’  state, but even a short period of complete ice cover would annihilate marine life forms dependent on photosynthesis.  The whole of the Eucarya would quickly disappear, though bacterial forms depending on chemical and thermal  energy sources could have survived in the depths, kept liquid by geothermal energy.  Eucarya did survive, at least some did, for following the so-called ‘Cryogenian’ period the fossil record properly begin with a vengeance in the Cambrian Explosion.  Quite possibly the enormous stress placed on primitive, small Eucarya by repeated long periods of global glaciation helped accelerate the pace of evolutionary change.  But that demanded at least some ice-free parts of the oceans.

William Hyde, Thomas Crowley, Steven Baum and Richard Peltier (25 May 2000,Nature  vol. 405, p 425) have modelled the climate when Earth had its continents clustered mainly in the southern hemisphere in the late Precambrian.  For the first time they build into a late-Precambrian climate model the effects of ice sheets themselves, as well as the mathematics of energy balance and general air and ocean circulation.  Even with reduced solar input and no build-up of CO2 they found that air temperatures could have been high enough to sustain a permanent belt of open water at tropical latitudes, while clustered continents were ice bound.  A spin-off from this result is that isolated, ice-free continental fragments in the tropics of the time may preserve fossils of those few metazoa that did make it through the big freezes- the long sought missing ancestors for the Cambrian Explosion of life as we know it.


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