Tuesday, 5 November 2013

On The Nervous System Of The Megacheirans

The fossil record is not only biased, but superficial. The shells and bones palaeontologists unearth from rocks around the world only give us an indication as to what a creature looks like. The internals are nearly always lost to us. Yet occasionally we get a glimpse of what lies beneath the surface. Earlier this year, I reported on a discovery of a near complete fossilised nervous system from a creature known as Chenjiangocaris, linking the eyes, antennae and many feeding tentacles to a central ganglion.

The fossil of Chenjiangocaris I reported on in early 2013
While an incredible find, the nervous system was primitive. Yet a new fossil discovery shows that some creatures were far more neurologically complex then previously thought.

Like Chenjiangocaris, it was unearthed from 520 million year old rocks in Chenjiang, China. Known as a megacheiran, the creature had two large, scissor-like claws, a three centimetre-long segmented body and 12 pairs of legs. Today, its closest living relative is the scorpion.

The species, called Alalcomenaeus, has been known for decades. Yet this specimen is special. Using iron deposits created during the fossilisation process, a team led by Nick Strausfeld, from the University of Arizona, used chemical imaging to investigate its physiology. They used a CT and then a laser scan to map the distribution of different elements within the fossil.

When they superimposed the two images and removed background extraneous colour from the CT, the magenta from the iron in the chemical scan and the green from the CT lined up to reveal something incredible: the lines of the complete nervous system in white. By removing all but the white and then inverting the colour, the structures now showed up as black on a white background, perfectly contrasted and ready for examination.

Different images of Alalcomenaeus. (a) is the fossil. (b) is an outline of iron desposits. (c) is a CT scan. (d) is the previous two overlaid on each other. (e) is the nervous system.
The analysis revealed the presence of three nerve clusters known as ganglia. These were fused together to form a brain and a dense bundle of nerves running along the length of the back, with smaller neurons branching out into the appendages and two pairs of eyes; all in all a highly developed and complex nervous system. This also allowed the team to accurately re-position the megacheirans within the tree of life. The structure of the brain showed that Alalcomenaeus was most closely related to modern chelicerates, like the horseshoe crab.

'The prominent appendages that gave the megacheirans their name were clearly used for grasping and holding and probably for sensory inputs,' said Strausfeld. 'The parts of the brain that provide the wiring for where these large appendages arise are very large in this fossil. Based on their location, we can now say that the biting mouth-parts in spiders and their relatives evolved from these appendages.

This is an illustration of the nervous systems of the Alalcomenaeus
fossil (left), a larval horseshoe crab (middle) and a scorpion (right

Our new find is exciting because it shows that mandibulates (to which crustaceans belong) and chelicerates were already present as two distinct evolutionary trajectories 520 million years ago, which means their common ancestor must have existed much deeper in time.'

The fossil record is a vital part of our understanding of evolution. Yet it does not give up its secrets easily. For decades we have been hampered by what we could see on the surface only. New technologies, however, are beginning to redress this balance.