Friday, 21 March 2014

Ancient Stick Insect Shows Evolutionary History

Early Cretaceous stick insect (Cretophasmomima melanogramma)
Image: Olivier Béthoux
The stripy stick insect that walked with dinosaurs - life - 19 March 2014 - New Scientist:

In my previous blog I touched on how what we see today in a species may be the result of ancient evolutionary forces which no longer apply, so it may not be immediately obvious what adaptive advantage there is to something in particular.

It's easy to overlook the fact that everything alive today is carrying the descendents of genes which survived through the Cambrian, the Devonian, the Triassic, Jurassic and the Cretaceous periods and were selected not by today's environment but maybe a very different one with different species and different ecosystems, even in the absence of orders like birds or mammals or in the presence of dinosaurs, pterodactyls, ammonites or giant tree ferns.

None of today's living species has an ancestor which did not live on an Earth where these extinct species once lived.

So, it should be no surprise to find ancient species like this early cretaceous stick insect which had adapted to live alongside early cretaceous plants by protective mimicry. This fossil was found in Inner Mongolia, China, in the Yixian Formation which has been dated between 122-130 million years ago. This rock formation which was laid down probably as volcanic ash from a catastrophic eruption, so capturing and preserving in exquisite detail, delicate structures like this insect with its wings, feathered dinosaurs and proto-avians as well as plant leaves. This is known collectively as the Jehol biota.

The significance of this fossil of Cretophasmomima melanogramma is that it is the earliest known example of stick insect mimicry. Stick insects today are renowned for their evolved mimicry to resemble the plants on which they feed but this example mimics a plant which was common then, but is extinct today. It was a species related to the maidenhair tree or Ginkgo biloba, now the only known representative of the Ginkgophyta division of the plant kingdom. The dark straight veins in the wings seen in this fossil, when the wings were folded over the back, would have closely resembled the leaves of Ginkgophyta, Membranifolia admirabilis also found in the Jehol biota.

Abstract


Background

Fossil species that can be conclusively identified as stem-relatives of stick- and leaf-insects (Phasmatodea) are extremely rare, especially for the Mesozoic era. This dearth in the paleontological record makes assessments on the origin and age of the group problematic and impedes investigations of evolutionary key aspects, such as wing development, sexual size dimorphism and plant mimicry.

Methodology/Principal Findings
A new fossil insect species, Cretophasmomima melanogramma Wang, Béthoux and Ren sp. nov., is described on the basis of one female and two male specimens recovered from the Yixian Formation (Early Cretaceous, ca. 126±4 mya; Inner Mongolia, NE China; known as ‘Jehol biota’). The occurrence of a female abdominal operculum and of a characteristic ‘shoulder pad’ in the forewing allows for the interpretation of a true stem-Phasmatodea. In contrast to the situation in extant forms, sexual size dimorphism is only weakly female-biased in this species. The peculiar wing coloration, viz. dark longitudinal veins, suggests that the leaf-shaped plant organ from the contemporaneous ‘gymnosperm’ Membranifolia admirabilis was used as model for crypsis.

Conclusions/Significance
As early as in the Early Cretaceous, some stem-Phasmatodea achieved effective leaf mimicry, although additional refinements characteristic of recent forms, such as curved fore femora, were still lacking. The diversification of small-sized arboreal insectivore birds and mammals might have triggered the acquisition of such primary defenses.

Wang M, Béthoux O, Bradler S, Jacques FMB, Cui Y, et al. (2014)
Under Cover at Pre-Angiosperm Times: A Cloaked Phasmatodean Insect from the Early Cretaceous Jehol Biota.
PLoS ONE 9(3): e91290. doi:10.1371/journal.pone.009129
Copyright: © 2014 Wang et al. Published under open access permission.

C. melanogramma being hunted amongst Membranifolia admirabilis foliage
Artists impression.
It seems very likely that these stick insects would have been the prey of the same early birds and mammals which are also found in the Jehol biota and evolved mimicry as a defence against them.

Of course, we can't know for sure if any of today's stick insects evolved directly from C. melanogramma but if they had they would carry genes that were once selected by the presence of early birds and late Ginkgophytes and they might well still have wings with dark straight veins having no apparent adaptive purpose today.

Evolution can't even look forward to tomorrow but it harks back to life on Earth hundreds of millions, even billions of years ago. We all carry the results inside us embedded in our genome, some of which was shaped by species and ecosystems of which we are not yet aware.

Dismissing this as magic, as creationist frauds and pseudo-scientists do, denies those who fall for their deceptions the pleasure of finding out how they were shaped by these forces and how they are connected to every other living thing through the process of genetic evolution by descent with modification from a common ancestor. It's like denying people the wonder of looking up into a night sky and being inspired by the awesome majesty of the cosmos.

It is denying people a heritage which is literally their birthright - the right to know who they are and where they came from.

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