When spiders had tails
A recent discovery from the ca. 100 million year old Burmese amber of Myanmar sheds important light on where spiders may have evolved from.
The paper in Nature Ecology & Evolution is here: http://go.nature.com/2FTehbd
Spiders are one of nature’s success stories with more than 47,000 living species. A recent discovery from the ca. 100 million year old Burmese amber of Myanmar sheds important light on where spiders may have evolved from. Chimerarachne yingi is an extraordinary fossil species described in the journal Nature Ecology and Evolution by an international research team from China, Germany, the USA and the UK. Chimerarachne looks like a spider and preserves several important spider features such as spinnerets for laying down strands of silk and a modified male pedipalp which was used to transfer sperm during mating. Uniquely, it also has a long whip-like tail. It is either the most primitive spider known, or else belongs to a group of extinct arachnids which were very close to spider origins. Either way, the implication is that there used to be a time when spiders still had tails.
Figure 1. The holotype of Chimerarachne yingi.
Figure 2. The paratype of Chimerarachne yingi.
Spiders evolved several key innovations: spinnerets producing silk used in webs, as well as for other purposes like egg wrapping, a male pedipalpal organ which is unique for each species and used by the male to transfer sperm to the female during mating, and venom for immobilizing prey. Chimerarachne yingi looks on first appearances like a member of the most primitive group of living spiders – the mesotheles – which still have a segmented abdomen. Based on the anatomy of mesotheles, we would have expected a spider ancestor to have had four pairs of spinnerets, all positioned in the middle of the underside of the abdomen. Chimerarachne only has two pairs of well-developed spinnerets, towards the back of the abdomen, with another pair apparently in the process of formation. The pedipalp organ appears quite simple and more like that of a mygalomorph spider (tarantulas, etc.) than a mesothele spider. We don’t know whether Chimerarachne 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 appears to have retained this primitive feature. Taken together, Chimerarachne 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.
Figure 3. Reconstruction of Chimerarachne yingi and its web. The silk can be used for egg wrapping or for lining tunnels. Modern mesotheles all build tunnels although the new fossils don’t really let us answer this question.
Chimerarachne was not the ancestor of modern spiders. Spider fossils go back even further to the Carboniferous, more than 300 million years ago. Chimerarachne belongs instead to an extinct group from which modern spiders evolved, some of whom survived into the mid-Cretaceous of Southeast Asia. The name Chimerarachne comes from the way in which the new fossil seems to combine features of different arachnids, and is based on the chimera: a monster in Greek mythology composed of different animal parts. The species name yingi honours the collector of one of the specimens, Yanling Ying.
Figure 4. Evolutionary history of spiders and spider-related arachnids.