Tuesday, 24 June 2014

The Evolutionary Root Of Jaws

Mermaid's purses allow embryologists to easily
track the evolutionary development of sharks
Sharks are remarkably useful to evolutionary biologists. As the embryos develop inside translucent pouches, known as mermaid's purses, they show their development from a cluster of cells to the fully formed creature.

In turn, this gives an incredible insight into where different body structures originate, and by comparing the developmental pathways of different organisms, the evolutionary origins of those structures. This burgeoning field of study, known as evolutionary development or evo devo, makes clear that jaws have ancient origins.

The fossil of Metaspriggina walcotti from Marble Canyon
In the 1870s, the naturalist Karl Gegenbaur observed that living fish, such as sharks, have five or six pairs of bars that support the gills, and that these so-called gill bars were similar in form to jawbones. Based on this similarity, he proposed a theory, called the serial homology hypothesis, that jawbones in modern fish gradually evolved from an earlier pair of gill bars present in a creature from which all jawed vertebrates (properly called gnathostomes) would evolve. His theory fell out of favour but evo devo would eventually prove him right. All gnathostome embryos have a pair of foremost gill arches which eventually develop into the two halves of the jaw. Yet evo devo offers us only a glimpse of what occurred. The fossil record is the other side of the story.

In 2012 Jean Bernard Caron from the Royal Ontario Museum, famous for his work on the Burgess Shale which best documents the Cambrian Explosion, uncovered a hoard of fossils from a similar site at Marble Canyon in Canada's Kootenay National Park (http://prehistoricict.blogspot.co.uk/2014/02/burgess-30.html). Amongst a number of new species was a specimen of a creature called Metaspriggina walcotti. Fossils of Metapriggina had been collected from the Burgess Shale before, but were fragmentary and poorly preserved.

An artist's impression of Metaspriggina walcotti 
These new specimens are exquisitely preserved and show details of the eyes, internal organs, and most importantly, the gill arches. 'For the first time, we are able to say this is really close to this hypothetical ancestor that was drawn based on a study of modern organisms in the 19th century,' said Bernard Caron.

Barely longer than a thumb, Metaspriggina is certainly diminutive, but its anatomy places it at the root of the gnathostome family tree. What is of interest is that it suggests that lampreys and hagfish, which have a complicated basket-shaped series of gill structures, evolved from a side branch of the vertebrate evolutionary tree that diverged long after the origin of vertebrates, rather than sitting at the group's base.

Metaspriggina's overall morphology is very similar to other early vertebrates, showing that they still occupied a small area of the morphospace, but the gill arches betray their later evolutionary success. Jaws would allow gnathostomes to become the dominant group in the oceans. The overall functionality of the organ would give rise to a diverse range of teeth, enabling gnathostome vertebrates to feed on a wide variety of foods and occupy new ecological niches. Jaws are a hugely important part of the evolutionary tapestry; while evo devo, with the fossil record, sit just that little bit closer together.