Spiders are amazingly sophisticated animals, and probably the premiere complex adaptation of modern spiders is the ability to spin silk. They have multiple internal glands that can produce multiple kinds of silk — webs contain different kinds, from structural strands to adhesive strands, and other kinds are used for spinning egg cases and for wrapping prey — and they are sprayed out through small spigots mounted on swiveling spinnerets, which are modified opisthosomal (abdominal) limbs. Obviously, these detailed features did not spontaneously appear all at once, but had to have evolved progressively. A couple of fossils have recently been described that reveal a) silk spinning is ancient, from at least the Permian, but that b) these early spiders did not have the full array of modern adaptations.
Here is a pair of fossils: Permarachne novokshonovi, from the Permian in Russia, and a more recent specimen, and Palaeothele montceauensis, from the Carboniferous in France. Both are eight-legged arthropods, and if you saw one scuttling about now you wouldn’t hesitate to call them spiders. There are some differences, though: Permarachne in particular shows a little less tagmosis, or fusion and specialization of segments, than we usually see in spiders, and it also has that prominent flagellum (which is completely different from a bacterial flagellum!), a long segmented ‘tail’ covered with sensory hairs that was probably a sense organ; it has no sign of a web-spinning function.
What about the production of silk and webs in these old spiders? Here’s another specimen, Attercopus fimbriunguis, a 376 million year old fossil. It’s a little less dramatic because these are fragments of cuticle that have been carefully extracted by dissolving the rocky matrix with acid; it means, unfortunately, that it is more fragmented, but the advantage is that now we can zoom in microscopically and see far more detail in the structure. What we can now see in pieces of the ventral plates of the opisthosoma are small spigots, and in a few cases, there are even strands of spider silk still extended from these pores. In F, there’s also a nice shot of a chelicera (fang) from the spider — it’s wicked sharp, but the small holes seem to be preservation artifacts, and there’s no sign that venom secretion, another important spider adaptation, has evolved yet.
One of the critical observations here is very simple: no spinnerets. These spiders did not have the modified limbs with sets of spigots that we see nowadays, but instead, had a series of spigots arrayed across the bottom of the abdomen. They almost certainly were not able to make webs: what they could have done was produce sheets of silk, of the kind that could be used to make egg cases or wrap around prey. These are another example of a transitional fossil, forms that have only some of the capabilities of a later organism.
(via Cheshire, who promises to have his own post on this paper soon.)
Selden PA, Shear WA, Sutton MD (2008) Fossil evidence for the origin of spider spinnerets, and a proposed arachnid order. Proc Nat Acad Sci USA 105(52):20781-20785.