Saturday, 31 January 2015

On The Origin Of Flying Fish

A modern species of flying fish
Flying fish have developed one of the most remarkable defence strategies seen in the animal kingdom. With a powerful flick of the tail they launch themselves out of the water and unfold wing-like pectoral fins, enabling them to glide for 50 meters on average. At speeds of 75 kilometers an hour, this can rise to 400 metres if they catch the wave's updraft.

In so doing, they place themselves out of reach of predators save for when they briefly re-enter the water. The oldest known examples of this survival strategy come from a 235 million year old fish fossil called Potanichthys xingyiensis. 

Part of a family of now extinct fish known as the thoracopterids, this creature is unrelated to modern flying fish, but new specimens belonging to the group, have elucidated how this flying mechanism first evolved. They were recovered from the Middle Triassic Zhuganpo Member of Falang Formation near the town of Wusha, Xingyi County, Guizhou Province and analysed by researchers from the Institute of Vertebrate Palaeontology and Palaeoanthropology. They constitute a new species, Wushaichthys exquisitus, which appears to be a primitive member of the thoracopterid family.

A female Wushaichthys exquisitus
'Our comparative studies resulted in a reinterpretation of the previously alleged 'peltopleurid' Peripeltopleurus as a primitive thoracopterid. Peripeltopleurus and the new fish from China share with the type genus Thoracopterus of this family the specialization of the skull pattern and scale cover, but both taxa retain many primitive characters unknown in Thoracopterus and other derived thoracopterids. Studies of these taxa provide new insights into the early evolution of the thoracopteridae,' said Dr. Xu Guanghui from the IVPP.

This later species of thoracopterid possessed a bottom heavy tail,
indicating that it was capable of propelling itself out of the water.
Its anatomy showed that it lacked the bottom heavy tail fin required to propel the flying fish, both modern and ancient, out of the water. This is a feature shared by all other species of thoracopterid, but its absence in Wushaichthys and Peripeltopleurus suggests that they were early members of the group and incapable of flight. Additionally, both species were covered in large, thick scales which would have generated drag and increased weight. Later thoracopterids and the modern flying fish group gradually lost these scales to increase gliding efficiency and maneuverability. Moreover, the primitive species lacked the wing-like pectoral fins required to glide in the first place.

The shared skull morphology between Wushaichthys and later thoracopterids, however, shows the former was already adapting to life at the surface of the oceans - the habitat of flying fish when not gliding through the air. This suggests that the evolution of flight was a gradual process starting with skulls to enable the group to live in surface waters. This was followed by the evolution of a bottom heavy tail designed to launch out of water, then the wing-like pectoral fins enabling them to glide and finally, the eradication of scales to optimize flight mechanics.

At two inches long Wushaichthys was diminutive and so would have been easy prey for larger predators. Gliding adaptations seen in later thoracopterids were most likely a response to this selection pressure. Hooklets on the anal fins of the males resemble those found in a group of live-bearing teleost fish. This suggests that the early thoracopterids may also have birthed live young which would have also reduced the risk of population damage through predation.