Thursday, 8 February 2018

An Intermediate Spider-Scorpion!

Chimerarachne yingi, preserved in amber in exquisite detail.

Photo: Bo Wang
Part spider, part scorpion creature captured in amber | Science | AAAS

Continuing with what has turned out to be another dreadful week for creationism, we now have two papers about a 100 million year-old species, beautifully preserved in exquisite detail in amber, which has characteristics of both spiders and scorpions.

Spiders, a group of arachnids are characterised by modified body appendages called spinnerets which extrude silk, and, in males, a pair of pedipalps which are used to insert sperm into females. All but the most primitive spiders also have smooth, non-segmented abdomens. Scorpions, on the other hand have segmented abdomens and lack spinneretes and pedipalps. They also have the characteristic tail.

The two groups are believed to have diverged about 430 million years ago but some years ago palaeobiologists discovered a fossil arachnid that was 100 million years older than the earliest spiders but which had spinnerets. These were put in an archaic group, the Uraraneida, relatives of the spiders but not necessarily their direct ancestors.

These two new papers, published in Nature Ecology and Evolution shed some light on that relationship. Regrettably, they are both behind paywalls. They both deal with specimens of the same species (named Chimerarachne yingi) embedded in amber from Myanmar and reach the same conclusion - that this is a member of the Uraraneida which lived alongside true spiders for some 100 million years. Basically, they are spiders with tails. They have well-developed spinnerets, spider-like chelicerae and the males have pedipalps. One paper, by Wang Bo, et al, looked at specimens with a clear view of the underside; the other paper, by Huang Diying, et al, deals with a detailed dorsal view. (Following these links and clicking on the 'Figures' tab will give access to images of these specimens which copyright restrictions prevent me publishing here.)

What this suggests is that the early arachnid ancestors of spiders and scorpions had a mix of the characteristics of both and that evolutionary divergence consisted of loss of some characteristics and enhancement of others. Spiders lost their tails and segmentation of their abdomen and scorpions lost their spinnerets and pedipalps.

Here then we have a clear pointer to a past when the common ancestor of spiders, scorpions and these Uraraneida had intermediate characteristics between the two present day groups, spiders and scorpions and a very clear evolutionary explanation for these beautiful fossils as well as the modern groups. How's that for a cult which argues that these intermediates never existed, that evolution of new families can't occur and that the science of evolution is a theory in crisis?

Yes, it's been another dreadful week for creationism. All it needs now is creationists to have enough understanding of the subject to realise it.

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2 comments :

  1. Wonderful discovery! I have pet tarantulas, vinegaroon and amblypygids so this was bit of a fascinating read into them. Your comment on the pedipalps and chelicerae got me intrigued and looking further- turns out pincer-like chelicerae is ancestral. Cool! I knew they were all arachnids but did not fully appreciate their relation to each other they are partly because spiders had fangs while the scorpions, vinegaroons, amblypygids have the pincer-like chelicerae. Picture of fossil sure hit it home for me, that's a vinegaroon tail! And those are spider legs!

    I've been visiting your blog for a couple months, enjoying how you make publications like this easier to read and understand. Those publications can be quite heavy reads to digest.

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