Ancient spider relative hints at origins of web spinning

Garwood et al. (2016)/Muséum National d’Histoire Naturelle, Paris

Ancient spider relative hints at origins of web spinning

The postage stamp–sized fossil of a close relative of the earliest spiders provides new insight about how today’s web-spinners evolved. Using high-energy x-rays to produce an MRI-like scan of the palm-of-your-hand-sized rock that holds the 305-million-year-old fossil, researchers were able to discern anatomical details as small as 5 micrometers across. The team placed the creature (artist’s sketch, above) in a new genus dubbed Idmonarachne (named after Idmon, who in Greco-Roman mythology was the father of Arachne; she, in turn, was a talented weaver who lends her name to the group of invertebrates called arachnids, to which spiders belong). Idmonarachne can’t be placed within previously known groups of arachnids, the researchers report online today in the Proceedings of the Royal Society B. It has limb proportions and mouthparts very similar to those of true spiders, lending it the spooky appearance that’s so disturbing to arachnophobes. But unlike true spiders, the rearmost portion of Idmonarachne’s abdomen is divided into three distinct segments (left, right, and center). Nor does the newly described species sport a taillike segment called a telson that’s often seen in earlier and more primitive arachnids. Also, importantly, Idmonarachne doesn’t sport spinnerets, small structures on its abdomen that help control the direction at which material emerges from the arachnid’s silk glands. (So although Idmonarachne could likely excrete silk, it would have formed strands and sheets of the material rather than intricate webs.) This previously unseen set of traits supports the idea that the ancestors of spiders lost their telsons before they developed the spinnerets that set the stage for their modern relatives’ silk-spinning prowess. Some of the earliest true spiders lived at the same time as Idmonarachne (and some even lived about 10 million years earlier), but Idmonarachne likely retained the primitive traits of its evolutionary ancestors, the researchers say.