Palaeontology

An insect to fill the gap

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A complete insect fossil from the Devonian period has long been sought. The finding of a candidate may improve our patchy understanding of when winged insects evolved. See Letter p.82

Insects are, in terms of species number, the most successful group of animals ever to have lived. But their evolutionary origins are a source of controversy, and will continue to be so until the fossil record finally yields up unequivocal evidence of insect beginnings. On page 82 of this issue, Garrouste et al.1 claim to have found precisely this. Although it can hardly be described as well preserved, the fossil shows a six-legged thorax, long single-branched antennae, triangular jaws and a 10-segmented abdomen (see Fig. 2 of the paper1). Insects are the only known arthropods (joint-legged invertebrate animals) with this anatomical combination, allowing the authors to make a strong case for the fossil's insectan identity.

The 8-millimetre-long fossil, which the authors named Strudiella devonica, was found in a small rock slab excavated at a quarry in Belgium. Strudiella is dated to approximately 370 million years old, which places it late in the Devonian period (Fig. 1). This was the time when terrestrial ecosystems were first assembling from their aquatic progenitors2 — the first forests were established and the earliest four-legged vertebrates were crawling out from freshwater pools onto land. So far, only suggestive traces of insects have been found in rocks of this age. The famous Rhynie chert, a sedimentary deposit in Scotland that is about 402 million years old, contains fossils of collembolans3, a class of animal that contains today's ubiquitous springtails, and which is regarded as closely related to insects. The Rhynie chert has also yielded a pair of jaw fossils called Rhyniognatha, which may be from an advanced, winged insect4. In New York state, some fossilized scraps of characteristic cuticle and the framework of a single compound eye, perhaps from a primitive, wingless insect, have been found in 385-million-year old rocks5. But these fragments more or less complete the picture of all that is known of insects at this crucial time in Earth's history.

Figure 1: Winged beginnings.
figure1

The fossil record provides ample examples of winged insects from around 325 million years ago, such as the order Palaeodictyoptera from the Carboniferous period. However, there is little evidence of insect evolution before this time; only a handful of fossils, including 402-million-year-old Rhyniella praecursor, which appears similar to extant collembolan arthropods, have been found. But none of these few examples comes from the period between 385 million and 325 million years ago, which is referred to as the Hexapoda gap (striped region) because of the lack of insect evidence. Now, however, Garrouste et al.1 report the finding of Strudiella devonica, a fossil dated to 370 million years ago that shows multiple anatomical features that are characteristic of insects.

There have also been some false alarms. For example, the fossilized head of a wingless insect found6,7 in Canadian strata somewhat older than the New York deposits is almost certainly a contaminant — a much more recent or contemporary insect lodged in a crack in the rocks. And Leverhulmia mariae, also from Scottish chert near Rhynie, could have been an insect, a close relative, or neither — it seems to have too many legs to be easily classified8.

So although its age makes it too late to be an insect ancestor, or even the earliest insect, Strudiella is nonetheless of great potential significance as the oldest complete insect fossil yet found. This is the first and primary point speaking to its importance.

We can perceive only what the fossil record permits us to perceive. From our current viewpoint, the diversification of insects and of our own terrestrial vertebrate ancestors seems to have occurred in two evolutionary bursts9. Between 425 million and 385 million years ago, both groups probably originated and underwent an initial evolutionary radiation as they began occupying the newly available subaerial realm. There then follows a long period, called Romer's gap (360 million to 345 million years ago) for the vertebrates and the longer Hexapoda gap for insects (385 million to 325 million years ago), during which few, if any, fossils of these groups can be found (Fig. 1). Then, with apparent suddenness, an explosive appearance of many new forms takes place in the second round of diversification. For the insects, large winged species of the major groups (mayflies, proto-dragonflies and others, including extinct types) show up, seemingly without precursors. The insects were off and running on their way to world domination.

These gaps, and the two bouts of evolution that they create, may or may not be real. There is evidence that a period of low atmospheric oxygen concentration coincided with the gap period, and this could have suppressed the rate of appearance of novel anatomy10. But a more parsimonious explanation is simply that we have not yet found the right rock formations to reveal fossils that would fill in the gaps. For example, most of the exposed strata for this period in Europe and North America are of marine, not land, origin.

This brings us to the second reason for the importance of Strudiella — it is dated to a time smack in the middle of the Hexapoda gap (Fig. 1). According to Garrouste et al., this significantly narrows the gap. And if, as the authors suggest, the fossil came from the young stage of an animal that would have had wings as an adult, their finding would mean that winged insects originated much earlier than fossils have heretofore told us, and that the sudden appearance of many winged kinds around 325 million years ago is deceptive. It would also suggest that the Rhyniognatha fossils could indeed be the mandibles of a winged insect, and that the diversification of winged species could have taken place at a much more leisurely pace, over some 45 million years.

Considering the crucial role of insects in present-day ecology, the number of people engaged in studying their fossil history is dismally small. Furthermore, current specialists focus mostly on events from the Mesozoic period — the 'Age of Dinosaurs' — which began some 70 million years after the time of the first known winged insect fossils, or on even more recent amber-preserved insects, which are largely indistinguishable from living forms11. The beginnings of the insects are to be found in rocks much older even than those that enclosed Strudiella, but almost no one is looking for them. The paltry few insect fossils contemporary with Strudiella — and indeed Strudiella itself — were serendipitous, not deliberate, finds, and the Hexapoda gap still looms large.

References

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Correspondence to William A. Shear.

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Shear, W. An insect to fill the gap. Nature 488, 34–35 (2012) doi:10.1038/488034a

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