Thursday, 15 January 2015

On The Origin Of Jaws In Extant Fish

At 535 million years old, Myllokunmingia is thought
 to be one of the oldest chordates on the planet
The first chordates on the planet were little more than tubes of tissue with a stiffening rod running along the length of the back. More advanced forms, which may represent the earliest fish, developed skulls and robust skeletons, but with one key difference compared to modern fish, they did not have jaws, merely a sucker mouth used to harvest nutritious detritus from the seabed. Then around 460 million years ago, during the middle Ordovician, fish evolved jaws. This opened up an incredible range of ecological niches to exploit.

The earliest structures which can be truly classed as jaws exist in a group of fish known as the placoderms. Heavily armored, with some species growing up to five metres in length, they soon came to dominate the ocean. Yet these pioneers became extinct at the end of the Devonian. Their evolutionary cousins, the acanthodians, also possessed jaws but they too became extinct at the end of the Permian period 250 million years ago. Today there are just two groups of fish on the planet: the bony osteichthyes and the cartilaginous chondrichthyes.

Genetic evidence suggests that the two lineages split from their common ancestor 420 million years ago, but the early diversification and evolution of both groups is unclear from an anatomical perspective. Yet, now a fossil discovery from Siberia maybe the transition, giving us an understanding of the development of modern jawed fishes from their primitive jawed ancestors. The fossil, which is just two centimetres in length, was unearthed in the 1970s and classified as a species of bony fish, osteichthyan. Re-examinations of the fossil conducted by researchers from Oxford University showed that it actually blended characteristics from both the osteichthyes and chondrichthyes.

A phylogram showing the placement of
Janusiscus schultzei in relation to other fish groups
By taking detailed 3D scans of the fossil using computer tomography they showed the presence of a sensory line running down the length of the body, an anatomical feature involved in predator detection through pressure changes in the water, which is only found in osteichthyans. However, the scans also showed that the arrangement of blood vessels in the brain case and around the jaws was much closer to that of chondrichthyans.

'There are over 60,000 species of living jawed vertebrates, and they encompass pretty much everything you can think of [with a backbone] that lives on land or in the sea,' said Sam Giles, the study leader. 'But we don't really know what they looked like when they split.'

This species which was given the name Janusiscus schultzei, after Janus the two faced Greek god and in honor of the fossil's discoverer Hans-Peter Schultze, gives us a clue as to what that 420 million year old common ancestor looked like. Indeed the two are most likely very closely related.

Ironically the jaw itself is missing in the specimen. 'It's frustrating for most of us paleontologists that we just have only the brain case and part of the skull roof,' said John Long, a world expert on fossil fish. It would be nice to know about its jaws, teeth and cheek plates, but that must await further discoveries to fully understand the anatomy of Janusiscus.'

Moreover, researchers have long believed that the common ancestor of bony and cartilaginous fish had more cartilage than bone. This would mean that vertebrates with cartilaginous skeletons, such as sharks, would have evolved less over the ages than bony creatures. 'But what this animal tells us is that actually the last common ancestor of the two groups had lots of bone, So rather than sharks being primitive, sharks are actually very highly evolved in their own way, and just as highly evolved as we are.'