THE picture above is of one of the five known specimens of Chimerarachne yingi, a newly discovered arthropod that lived 100m years ago, during the Cretaceous period. It is preserved in amber and was found in the Hukawng Valley amber mines in northern Myanmar. It, and one of the other specimens, are described in a paper that has just been published in Nature Ecology and Evolution by Wang Bo of the Nanjing Institute of Geology and Palaeontology, in China, and his colleagues.

Dr Wang thinks Chimerarachne yingi is a spider, albeit an unusual one in that it has a tail. Two further specimens are reported simultaneously in a different paper in the same journal, by a team led by Huang Diying, a colleague of Dr Wang in Nanjing, and Gonzalo Giribet of Harvard University. They think the critter is part of an extinct group, related to but different from spiders, called the Uraraneida, of which tails are characteristic.

Dr Wang points to the well-defined spinnerets for handling silk that Chimerarachne yingi possesses (a feature of spiders, but not of Uraraneids), and also to certain of its mouthparts, called pedipalps. These have been modified in a way that makes them look like the pedipalps of male spiders, which are used to transfer sperm to a female’s genital orifice during mating. This would imply that, by chance, all four reported specimens are male, an assumption that worried the authors of both papers. But a fifth specimen has now turned up, without the modified pedipalps, so presumably she is a female.

Dr Huang and Dr Giribet acknowledge these spiderlike features, but think that a wider statistical analysis, which takes account of other body parts as well as spinnerets and pedipalps, shows that Chimerarachne yingi is actually a Uraraneid. In their view the features Dr Wang sets store by must have evolved in species not yet found, which predate the split between spiders and Uraraneids.

Whoever is right, Chimerarachne yingi is clearly descended, more or less unmodified, from something that existed near the point of that split, which happened more than 200m years before these specimens were alive, during the Carboniferous period. The fossil record of the Uraraneida peters out in rocks laid down 275m years ago, during the Permian period, leaving a 175m-year gap before the appearance of Chimerarachne yingi. Whatever label modern palaeontologists finally decide to apply to the species, that gap is a timely reminder of just how patchy the fossil record is, and how hard it is to reconstruct what was really going on in the past.