April 21, 2011
Just in time for Halloween 2008, several gruesome spider photographs popped up in the news. The shots recorded two incidents—both of which took place in Queensland, Australia—of huge golden orb weaver spiders eating birds that had flown into the webs of the arachnids. Birds aren’t exactly a staple of the golden orb weaver diet, but the spiders make the most of the occasional bad luck of small birds.
I was reminded of the photos by a discovery announced just this week in Biology Letters. Not only have paleontologists discovered the fossil of a large orb weaver spider, but it comes from Jurassic fossil beds well-known for small, feathered dinosaurs.
Named Nephila jurassica by Paul Selden, ChungKun Shih and Dong Ren, the newfound orb weaver dates to about 165 million years ago and is the largest fossil spider yet discovered. Its leg span stretched about 6 inches, which makes it comparable in size to females of some living golden orb weaver species. The fossil specimen was also a female—indicated by the presence of an organ called the epigyne—but, unlike modern-day golden orb weavers, males may have been just as big.
In living species, male golden orb weavers are much, much smaller than females. This difference in anatomy between the two sexes is known as sexual dimorphism. Based on a 130-million-year-old fossil of a male golden orb weaver—called Cretaraneus vilaltae—found in Spain, however, it seems that this was not always a characteristic of the group. The fossil male spider from Spain was large and probably did not differ in size from the female. If this is true, then the larger Jurassic species described by Selden and colleagues probably had males and females of equal size, too.
These spiders wove their silken nets in warm Jurassic forests filled with insects, but I wonder if they ever caught dinosaurs by accident. Nephila jurassica was found in the Daohugou Beds of northeastern China. These rocks have also yielded the finely-preserved remains of several feathered dinosaurs—Pedopenna, Epidexipteryx and Scansoriopteryx. Of these, both Epidexipteryx and Scansoriopteryx were small dinosaurs—between the size of a sparrow and a pigeon—that may have spent at least some of their time in the trees. They could not fly and so would not accidentally flutter into a golden orb weaver web, but I wonder if unlucky Scansoriopteryx or Epidexipteryx sometimes blundered into the spider webs and became prey for the arachnids. Barring the discovery of a truly spectacular fossil, we will probably never know, but, if it did occur, the frightening photos of Australia are glimpses of interactions between feathered dinosaurs and spiders that stretch back a very long way.
Selden, P., Shih, C., & Ren, D. (2011). A golden orb-weaver spider (Araneae: Nephilidae: Nephila) from the Middle Jurassic of China Biology Letters DOI: 10.1098/rsbl.2011.0228
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