Saturday 20 August 2016

Through The Lamprey's Eye

Lampreys and hagfish are not the most attractive creatures. Eel-like in form, living unsavoury lifestyles as carcass scavengers or parasites, they are not creatures you would wish to keep as pets. Yet they are prized by scientists. Hagfish slime has remarkable hydroscopic properties, and immensely strong for its weight, is a potential source for new synthetic polymers.

These creatures are also useful to evolutionary biologists looking to examine the early phases of vertebrate evolution. Sitting at the base of the vertebrate family tree, they are the most primitive members of the group and so offer insight into the life and morphology of the extinct and truly basal vertebrate species. One thing which marks out lampreys and hagfish is their simplicity.

Their bodies are little more than a tube with a circular, jawless mouth at one end and a rod of cartilage down the back. Their eyes are similarly primitive - little more than light sensitive spots - suggesting that eyes in vertebrates were a gradual acquisition and that the lamprey-like early group members had little more than a basic construct. A recent fossil discovery, however, has overturned this classic view of vertebrate evolution.

The 300 million year old specimen of Mayomyzon, showing the
preserved lens and retina, and the coloured banding along its back
The Mazon Creek fossil locality in Illinois has yielded remarkably well preserved specimens of lampreys and hagfish, showing soft tissue preservation. Just this year, an enigmatic species known as Tullimonstrum gregarium was identified as the former after decades of confusion over its true identity.

By examining preserved eyes within 300 million year old fossils of Mayomyzon (lamprey) and Myxinikela (hagfish) an international team of researchers, led by Professor Sarah Gabott from the University of Leicester, were able to show that eyes were much more complex in ancestral members of both groups, rather than primitive as was once suspected. The fossils showed in beautiful detail that the eyes were not mere photosensitive patches, but similar to sophisticated camera eyes of other vertebrates, complete with lenses and retinas.

'Sight is perhaps our most cherished sense but its evolution in vertebrates is enigmatic and a cause célèbre for creationists,' said Professor Gabott. 'We bring new fossil evidence to bear on an iconic evolutionary problem: the early evolution of the vertebrate eye. We will now scrutinize the eyes of other ancient vertebrate fossils to see if we can finally build a picture of the sequence of events that took place in early vertebrate eye evolution.'

Sight is important not just in and of itself. It opens up investigations into new behavioural possibilities, including camouflage, mating displays and warning colouration; analysis of the fossils revealed the presence of striped bands along the animals' lengths. Combined with newfound evidence of the complexity of nervous systems in stem group members of the animal phyla during the Cambrian, it suggests the possibility that organisms on the budding branches of the tree of animal life possessed similarly complex behaviours.