The discovery was made during a palaeontological field study authorised by the Department of Cultural Heritage of the Autonomous Government of Aragon, and the fossils are kept in the collection of the Museo Fundación Conjunto Paleontológico of Teruel-Dinópolis.
Spiders are arthropods that have played an important ecological role in the terrestrial ecosystem since their origin in the Devonian, approximately 385 million years ago. They are abundant in almost all terrestrial habitats – there can be more than 100 per square metre – and are one of the major predators of insects. Their evolutionary success is due largely to a particular innovation: the use of silk threads and the construction of different types of webs to catch flying and jumping insects. Until now, the oldest known example of this particular characteristic of spiders was a silk strand with sticky droplets found in Lebanese amber (between 138 and 124 million years old). For now, little is known about the origins of spiders’ webs, although older fossil spiders have been found preserving the structure of fine strands used for weaving webs. Amber, a fossilised resin, is almost the only medium in which the fine strands of a spider’s web can remain preserved.
The new fossil from Teruel is the oldest direct evidence of a spider’s web used to trap insects. According to experts, the web was constructed by a spider of the Araneae group. The amber section contained the remains of the web with a variety of prey: a fly, a beetle and a small parasitic wasp that fed on the eggs of beetle species that became extinct more than 80 million years ago. It seems that the insects were a source of food for the spider: their abdomens are broken and full of fossil resin and the spider probably sucked out the internal tissue after injecting digestive juices. Once dead and left empty, the insects remained submerged in the resin. The wasp is also caught by a collection of strands that firmly trap its leg, no doubt a strategy used by the spider to keep it more strongly attached to the web.
The web found in the amber section from Teruel is of great scientific interest for two other reasons: it exhibits droplets that would once have been sticky and the geometry of one fragment shows that the strands making up the web were arranged in a regular pattern. Specifically, the web was circular and formed by a sticky spiral on a system of radial strands. These strands, as in the case of current spiders’ webs, had elastic properties: when broken, after coming into contact with the resin stalactite, some remained in the amber, but became twisted or contracted into balls.
Another interesting point is that the fossil dating coincides with the sudden diversification of flowering plants (angiosperms) and pollinating insects. The oldest fossil flowering plants are also from the Early Cretaceous, a period in which they became an important part of terrestrial mediums, which were inhabited principally by gymnosperms (conifers and similar groups). The great success of flowering plants can be attributed to their symbiotic relationship with insects, which act as efficient pollinators. According to the authors, “thanks to this fossil, we can be certain that spiders influenced the early evolution of pollinating insects”.
Rosa Martínez | alfa
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