Calcium glut may have hardened marine life.
George
Sea-shells might be the product of a geological accident that flooded the
oceans with calcium, say US researchers. And this could have helped to drive the
extraordinary diversification of species and body shapes known as the Cambrian
explosion.
Sean Brennan and colleagues from the US Geological Survey in Virginia,
have found evidence that the amount of calcium in sea water shot up between the
end of the Proterozoic era (about 544 million years ago) and the early Cambrian
period (about 515 million years ago).
They suggest that this change allowed soft-bodied marine organisms to
create hard shells or body parts made from insoluble calcium minerals. Calcium
carbonate is the main component of sea-shells today, and is also used by
microscopic sea creatures such as radiolarians to make exquisitely patterned
'exoskeletons'.
"If they're correct, it's a fantastic result", says Tony Dickson, a
geologist at the University of Cambridge, UK. But he stresses that making
deductions about the chemistry of ancient sea water is a very tricky task,
riddled with uncertainties.
Salted away
The team, who published their results in Geology1, studied the chemical
composition of the briny liquid trapped in cavities within salty rocks called
halites from the late Proterozoic and early Cambrian. The rocks are thought to
have formed when sea water evaporated, and the pockets of liquid that remain are
samples of these prehistoric oceans.
The brines from the Proterozoic halites have calcium concentrations that
are similar to those in today's oceans, but the liquids in the younger Early
Cambrian rocks contain about three times as much calcium.
The researchers think that geological processes in the Early Cambrian
could have triggered this sudden increase in calcium. Molten rock pushed through
the cracks between tectonic plates up to ten times faster during the Early
Cambrian than it does today. More cracks in the rocks would allow more sea water
to percolate through, dissolving minerals from below the surface and then
delivering enormous amounts of extra calcium to the ocean.
Waste disposal
Although hard shells and exoskeletons are generally regarded as defences
that make soft organisms hard for predators to eat and digest, some researchers
think these mineral shells might have originally served a different purpose.
They may have been simply a way for organisms to get rid of the calcium
dissolved in their tissues when its concentration became dangerously high. In
other words, the shells are a form of solid waste.
The formation of mineral shells might then have prompted organisms to
become more diverse, making new body shapes possible, fostering new designs in
predators and helping to trigger the Cambrian explosion.
"This work is very intriguing", says Simon Conway Morris, a
palaeobiologist at Cambridge University. But he stresses that not everyone
thinks organisms have to be so responsive to changes in their environment. After
all, marine organisms have sophisticated mechanisms for keeping their internal
calcium concentrations much lower than that of sea water, he says.
References
a.. Brennan, S. T., Lowenstein, T. K. & Horita, J. Geology, 32, 473 -
476, doi:10.1130/G20251.1 (2004). |Article|
