Chimerarachne yingi, preserved in amber in exquisite detail.
Photo: Bo Wang
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.)
Abstract
Spiders (Araneae) are a hugely successful lineage with a long history. Details of their origins remain obscure, with little knowledge of their stem group and few insights into the sequence of character acquisition during spider evolution. Here, we describe Chimerarachne yingi gen. et sp. nov., a remarkable arachnid from the mid-Cretaceous (approximately 100 million years ago) Burmese amber of Myanmar, which documents a key transition stage in spider evolution. Like uraraneids, the two fossils available retain a segmented opisthosoma bearing a whip-like telson, but also preserve two traditional synapomorphies for Araneae: a male pedipalp modified for sperm transfer and well-defined spinnerets resembling those of modern mesothele spiders. This unique character combination resolves C. yingi within a clade including both Araneae and Uraraneida; however, its exact position relative to these orders is sensitive to different parameters of our phylogenetic analysis. Our new fossil most likely represents the earliest branch of the Araneae, and implies that there was a lineage of tailed spiders that presumably originated in the Palaeozoic and survived at least into the Cretaceous of Southeast Asia.
Wang, B., Dunlop, J.A., Selden, P.A. et al.
Cretaceous arachnid Chimerarachne yingi gen. et sp. nov. illuminates spider origins.
Nat Ecol Evol 2, 614–622 (2018). https://doi.org/10.1038/s41559-017-0449-3
© 2018 Springer Nature Ltd.
Reprinted under the terms of s60 of the Copyright, Designs and Patents Act 1988.
AbstractWhat this shows 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.
Understanding the genealogical relationships among the arachnid orders is an onerous task, but fossils have aided in anchoring some branches of the arachnid tree of life. The discovery of Palaeozoic fossils with characters found in both extant spiders and other arachnids provided evidence for a series of extinctions of what was thought to be a grade, Uraraneida, that led to modern spiders. Here, we report two extraordinarily well-preserved Mesozoic members of Uraraneida with a segmented abdomen, multi-articulate spinnerets with well-defined spigots, modified male palps, spider-like chelicerae and a uropygid-like telson. The new fossils, belonging to the species Chimerarachne yingi, were analysed phylogenetically in a large data matrix of extant and extinct arachnids under a diverse regime of analytical conditions, most of which resulted in placing Uraraneida as the sister clade of Araneae (spiders). The phylogenetic placement of this arachnid fossil extends the presence of spinnerets and modified palps more basally in the arachnid tree than was previously thought. Ecologically, the new fossil extends the record of Uraraneida 170 million years towards the present, thus showing that uraraneids and spiders co-existed for a large fraction of their evolutionary history.
Huang, D., Hormiga, G., Cai, C. et al.
Origin of spiders and their spinning organs illuminated by mid-Cretaceous amber fossils.
Nat Ecol Evol 2, 623–627 (2018). https://doi.org/10.1038/s41559-018-0475-9
© 2018 Springer Nature Ltd.
Reprinted under the terms of s60 of the Copyright, Designs and Patents Act 1988.
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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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!
ReplyDeleteI'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.
Thank you.
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