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What fossils reveal about the spider family tree is far from horrifying | Susannah Lydon

An illustration of the Cretaceous arachnid Chimerarachne yingi found in Myanmar.
An illustration of the Cretaceous arachnid Chimerarachne yingi found in Myanmar.
Illustration: Dinghua Yang/University of Kansas/Reuters

The discovery of a 100m-year-old spider ancestor with a whip-like tail, bearing a more than slight resemblance to everyone’s favourite parasitoid alien – the facehugger – gained a lot of media interest last week. Some arachnologists were upset by both the language of fear in the coverage (“creepy” and “horrifying” were popular descriptions) and by some folks expressing a desire to nuke it from orbit. It seems that despite (or perhaps because) of the intense responses that spiders evoke in people, there is always an interest in where and how they evolved.

The newly described species, Chimerarachne yingi, was based on two specimens found in amber of about 100 My old from Myanmar. Unusually, the new find was revealed in two simultaneously-published papers in the journal Nature Ecology & Evolution. The rules for the naming of species mean that only one of the papers, by Bo Wang and colleagues, gets to be the formal description and naming of the species (and new genus, the next level up in classifying organisms). Both Wang’s paper and that of Diying Huang and colleagues, aimed to place the new find in terms of the spider family tree. The new species has features of modern spiders, known as the Araneae: a male pedipalp (sensory appendage) modified for sperm transfer, and well-defined spinnerets for silk spinning. But, it also has its distinctive tail, a feature not found in modern spiders, but associated with an ancient grouping of “almost-spiders” known as the Uraraneida.

Amber is the ideal medium for the preservation of small land animals like spiders, and the vast majority of fossil spider species are known from amber. More than thirty families of spiders have been recognised in Baltic amber (around 44 My old), including one family, the Archaeidae, which was initially thought to be extinct, until members of the family were found alive and well in South Africa, Australia and Madagascar.

If you are seeking fossil spider paradise, then the Crato Formation in north-east Brazil might be for you. Fine-grained limestones laid down about 108 My ago, so a tad older than the new Myanmar species, preserve extraordinary detail of all manner of arachnids which sank to the bottom of a salty lagoon. Modern araneomorph spiders (the vast majority of spiders today, including the orb-webs, wolf spiders and jumping spiders) are present, as well as mygalomorphs, the arachnid group including tarantulas, bird-eating spiders and trap-door spiders. Scorpions are present and even tiny fossil parasitic acariform mites. Other, more niche, arachnids are also present: whip scorpions, whip spiders and camel spiders.

Further into the past, true spiders can be tracked all the way back to the Carboniferous Period, about 300 My ago. Paleothele montceauensis has the spinnerets required to place it in the modern Mesothelae group (which today line their burrows with silk), while another arachnid found in the same deposits in France, Idmonarachne braisieri, has true spider legs and jaws, but no spinnerets. It seems that different types of spider-like arachnids, with different combinations of features, have co-existed on Earth over deep time.

Spider-like animals were even there at the beginning of life on land. The Rhynie Chert, which preserves so many wonderful plants, animals and fungi from the move onto land around 400 My ago, also bears trigonotarbids. Trigonotarbids are an extinct group of arachnids, with eight legs and book lungs for breathing on land, just like spiders, but without silk-producing organs. Around 70 different species of trigonotarbids have been recognised, mostly from the Carboniferous period, but some representatives are even older than the Rhynie Chert, pushing the origins of spider-like land animals back into the Silurian Period.

The trigonotarbid group includes heavily-armoured spiny forms, which would no doubt be the stuff of nightmares for the arachnophobic readers of media reporting on spider evolution. Whether we should fear these animals or not (short answer: most of the time, we really shouldn’t), spiders and their near relatives have been a part of the story of life on land since the very beginning, and they deserve our respect, if not our love, on Valentine’s Day.

This article titled "What fossils reveal about the spider family tree is far from horrifying" was written by Susannah Lydon, for on Wednesday 14 February 2018 01.09pm


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