Thursday, 2 August 2012

An Incredible New Development In The Evolution Of Insects

The fossil record of our planet is vast and ancient, spanning over 3 billion years of geological history. It contains a truly incredible range of organisms, from the primordial single celled organisms which clung to life on the young planet, giant marine predators which stalked the seas for hundreds of millions of years, armored mammals built like tanks to creatures so bizarre that they are like nothing which exists or has existed on the face of the planet.

The gems of the fossil record are the evolutionary firsts which represent the origins of a group of organisms. They are extensive, including the first ever hominids, the first fish and the first reptiles. Indeed, almost all groups of living creatures have their earliest members preserved in rocks somewhere on Earth. However, the fossil record is by nature an incomplete one. Occasionally palaeontologists will have unearthed the first members of a group and their distant descendants, but nothing inbetween.

A timeline marking the Hexapod Gap, with the first ever
insect, Rhyniella praecursor, at the base, the giants of the
Carboniferous at the top and the positions of Strudiella
in the middle
These are the blank spots on the fossil record. Some are quite famous such as Romer's Gap which marks a parsimonious point in the Devonian some 370 million years ago between the first amphibians and their later and more advanced descendants. However there are others. One such exists in the insect family. While it has been left with the rather bland name of the Hexapod Gap, it is a rather important one.

Insects are one of the most diverse groups of arthropods on the planet. Yet where are the fossils from their early days? The oldest and almost certainly the first come from the Rhynie Chert in Scotland where their remains are preserved down to the cellular level within silica. We have specimens from the Carboniferous period, when arthropods, especially insects, were at their largest ever due to increased levels of atmospheric oxygen, but nothing between the two until recently.

Unearthed in a quarry in Belgium by palaeontologists led by Andre Nel from the National History Museum of France, the fossil in question is of an 8 millimeter long, soft bodied larva which falls straight into the gap at the 370 million year old mark (the gap's boundaries are 385 and 325 million years ago). Incredibly, apart from being the least likely candidate for preservation owing to its soft bodied nature, the fossil is complete and in prime condition.

The area itself was a swamp 370 million years ago. Other fossils from the same locality show that predatory shrimps were common, making the survival of the larva fossil even more unlikely. Yet against all the odds, it did. Named Strudiella devonica, the specimen displayed a six legged thorax, long, single branched antennae, triangular jaws and a 10 segmented abdomen making it, without a shadow of a doubt, an insect.

The remarkable new fossil of Strudiella devonica
'Insects are an extremely ancient group, but we know very little about the earliest among them,' said Nel. 'This find enables us to confirm our molecular dating, it's a palaeontological marker.' Until now, the only other evidence consisted of two uncertain and poorly preserved mandibles from Scotland. The researchers believe that, based upon the shape of the mandibles, the creature was the larva of a creature similar in shape to a grasshopper.

If this is the case, it pushes the origins of insect wings much further back in time than previous pieces of fossil evidence suggests. However, the fact that it is a larva and the fossil is not so well preserved means that we can be certain. Over the years, Romer's Gap has become smaller and smaller, as more and more fossils have been uncovered, giving us a more detailed picture of the origins of amphibians. While this tiny larva is but one fossil, it is the first step on the road to closing the Hexapod Gap, making it, one of the more important discoveries in the past few years.