User:Jason Dunlop/sandbox

Chimerarachne is a remarkable genus of extinct arachnid containing a single species Chimerarachne yingi. Fossils of Chimerarachne were discovered in Burmese amber from Myanmar which dates to the mid-Cretaceous, about 100 million years ago. These fossils resemble spiders in having two of their key defining features: spinnerets for spinning silk, and a modified male organ on the pedipalp for transferring sperm. At the same time they retain a whip-like tail, rather like that of a whip scorpion. Chimerarachne is not ancestral to spiders, being much younger than the oldest spiders which are known from the Carboniferous, but it appears to be a late survivor of an extinct group which was probably very close to the origins of spiders. It suggests that there used to be spider-like animals with tails which lived alongside true spiders for at least 200 million years. The name is taken from the chimera, a monster in Greek mythology composed of parts of different animals, together with the suffix ‘arachne’ after the legendary weaver who was transformed into a spider for insulting the gods. The species name honours Mr Yanling Ying who collected one of the specimens. The legs and body of Chimerarachne has a generally spider-like. The chelicerae (mouthparts) are similar to those of a mesothele or mygalomorph spider. The fang does not have any hairs, which is another typical spider feature, but it is not clear whether or not the animals had venom. The male pedipalp has a palpal organ consisting of the tarsus (or cymbium), which is divided at the tip into two long lobes, and a simple bulbus similar to that of some mygalomorph spiders but apparently less complex than the bulbus of mesotheles. The abdomen is segmented, like that of a mesothele spider, but unlike spiders there are several short cylindrical segments at the back from which a long segmented tail (or flagellum) emerges. The abdomen also bears spinnerets on the underside, and these are especially interesting given that it was widely assumed that spiders should initially have had four pairs in the middle of the underside as in modern mesothele spiders. By contrast, Chimerarachne has two pairs of quite well developed spinnerets towards the back of the abdomen which are similar in shape to those of mesotheles and which are probably equivalent to the anterior lateral spinnerets (ALS) and posterior lateral spinnerets (PLS) of modern spiders. There are, however, no posterior median spinnerets. In the place where the anterior median spinnerets (AMS) would be expected in spiders there is instead a pair of stubby spigots which could be spinnerets in the process of formation. Four specimens of Chimerarachne yingi are known so far. Two pairs of specimens were acquired independently by two different research teams during the summer of 2017. Their results were published back to back as companion papers in early 2018 in the journal Nature Ecology and Evolution. The two publications agree on the basic anatomy and significance of these fossils, but differ slightly in the interpretation of their position of Chimerarachne in the arachnid tree of life. The Wang et al. study, which also named the fossils, placed the genus closer to gen. et sp. nov. illuminates spider origins.“ Nature Ecology and Evolution. https://doi.org/10.1038/s41559-017-0449-3

spiders. The Huang et al. study placed Chimerarachne a little more distant from spiders and as part of an extinct arachnid order known as Uraraneida which are also spider-like, and have a tail, but which were not previously thought to have spinnerets. The fossils thus raise the question whether spiders should be defined by acquiring spinnerets and a male pedipalp organ and/or be defined by having lost the tail.

References

1. Wang, B., Dunlop, J. A., Selden, P. A., Garwood, R. J., Shear, W. A., Müller, P. &Le, X. (2018). „Cretaceous arachnid Chimerarachne yingi 2. Huang, D., Hormiga, G., Cai, C., Su, Y., Yin, Z., Xia, F. & Giribet, G. (2018). “Origin of spiders and their spinning organs illuminated by mid-Cretaceous amber fossils.” Nature Ecology and Evolution. https://doi.org/10.1038/s41559-018-0475-9