Tuesday, 9 July 2013

Old Five Arms

Sea urchins (echinoids) can often be quite menacing in their own odd way. Set against the pale limestones and clear blue water of the Mediterranean, the spiny, dark huddled masses can be seen, denying would-be swimmers access to the shallows. At a glance they are not the nicest of objects. Yet a closer look will reveal their beauty. Unlike many animals, they do not possess bilateral symmetry. Instead, they have pentaradial symmetry composed of delicately patterned plates and scales.

The fossil of Helicocystis moroccoensis
Underneath a forbidding coating of spines, they are remarkably well organised. Yet when did their unusual forms originate and why? Last year, a team led by Andrew Smith from the Natural History Museum in London, uncovered several fossils from 520 million year old sediments in the Anti Atlas Mountains, Morocco. According to the researchers, they are the remains of a creature which represented the most primitive members of pentaradial sea urchins and therefore the oldest ancestor of all modern echinoids.

Named Helicocystis moroccoensis, it was just four centimetres high and had a cigar-shaped body composed of a calcite skeletal lattice supporting a calyx of checkered plates, with a short stem at the bottom and a mouth at the top. Based on its physiology, it would have been able to expand and contract, thus making itself larger than its fossilized four centimetres.

What makes it important is that it lies between pentaradial sea urchins and the first echinoids on the planet. The oldest known echinoderm, a creature from the White Mountains in California named Helicoplacus, had a spiral body but was asymmetrical in form. Helicocystis also has a spiral body by pentaradial symmetry. What is interesting is that all echinoids develop from a bilateral larva rather then possessing the adult form of symmetry from birth.

Helicocystis moroccoensis, the oldest sea urchin with
pentaradial symmetry, coming in at 520 million years old
This suggests that the urchin family was undergoing a rapid period of diversification 520 million years ago which resulted in their bizarre form of symmetry. Various other echinoid fossils with equally mad body plans were found in the same formation at Helicocystis, showing that the number of different species during the Cambrian period was high. Exactly what was driving this new wave of evolutionary innovation is still unknown but is most likely intimately linked with the Cambrian Explosion.

Echinoids and the wider group of the echinoderms which includes sand dollars and starfish have been highly succesful since they first evolved. They appear all through the fossil record, a diverse panoply of forms which have endured throughout hundreds of millions of years, a story whose origins and success can be traced all the way back to Helicocystis. The next task is to pin down exactly where, in the first instance, echinoids came from.