A remarkable tailed arachnid found in the mid-Cretaceous (approximately 100 million years ago) Burmese amber of Myanmar documents a key transition stage in spider evolution.
Despite its appearance, the new arachnid is not a direct ancestor of modern day spiders.
Named Chimerarachne yingi, the animal resembles a spider in having fangs, male pedipalps, four walking legs and silk-producing spinnerets at its rear.
However, it also bears a long flagellum or tail. No living spider has a tail, although some relatives of spiders, the vinegaroons, do have an anal flagellum.
Four specimens of Chimerarachne yingi were found, and all are tiny, about 2.5 mm body length, excluding the nearly 3-mm-long tail.
“Any sort of flagelliform appendage tends to be like an antenna,” said Dr. Paul Selden, a researcher in the Paleontological Institute and the Department of Geology at the University of Kansas.
“It’s for sensing the environment. Animals that have a long whippy tail tend to have it for sensory purposes.”
The find confirms a prediction made a few years ago by Dr. Selden and colleagues when they described a similar tailed arachnid, which resembled a spider but lacked spinnerets. These animals, from the much older Devonian (about 380 million years ago) and Permian (about 290 million years ago) periods, formed the basis of a new arachnid order, the Uraraneida, which lies along the line to modern spiders.
“The ones we recognized previously were different in that they had a tail but don’t have the spinnerets,” Dr. Selden said.
“That’s why the new one is really interesting, apart from the fact that it’s much younger — it seems to be an intermediate form.”
“In our analysis, it comes out sort of in between the older one that hadn’t developed the spinneret and modern spider that has lost the tail.”
Chimerarachne yingi lies one step closer to modern spiders on account of its possession of spinning organs.
“Little of the tiny spider’s day-to-day behavior could be determined,” Dr. Selden said.
“We can only speculate that, because it was trapped in amber, we assume it was living on or around tree trunks. Amber is fossilized resin, so for a spider to have become trapped, it may well have lived under bark or in the moss at the foot of a tree.”
“While the tailed spider was capable of producing silk due to its spinnerets, it was unlikely to have constructed webs to trap bugs like many modern spiders.”
“We don’t know if it wove webs. Spinnerets are used to produce silk but for a whole host of reasons — to wrap eggs, to make burrows, to make sleeping hammocks or just to leave behind trails. If they live in burrows and leave, they leave a trail so they can find their way back.”
“These all evolved before spiders made it up into the air and made insect traps. Spiders went up into the air when the insects went up into the air. I presume that it didn’t make webs that stretched across bushes. However, like all spiders it would have been a carnivore and would have eaten insects, I imagine.”
The scientists don’t know whether Chimerarachne yingi was venomous.
“We do know that the arachnid ancestor probably had a tail, and living groups like whip scorpions also have a whip-like tail. Chimerarachne yingi appears to have retained this primitive feature,” said Dr. Jason Dunlop, from the Museum Für Naturkunde in Berlin, Germany.
“Taken together, Chimerarachne yingi has a unique body plan among the arachnids and raises important questions about what an early spider looked like, and how the spinnerets and pedipalp organ may have evolved.”
The research is published in the journal Nature Ecology Evolution.
Bo Wang et al. Cretaceous arachnid Chimerarachne yingi gen. et sp. nov. illuminates spider origins. Nature Ecology Evolution, published online February 5, 2018; doi: 10.1038/s41559-017-0449-3