Megarachne, the Giant Spider That Wasn't

Imagine that you are are standing in a massive junkyard with the remains of cars strewn all about you. A few are relatively complete, but most of the heap is made up of bits and pieces of models from the entire history of automotive innovation. If you were to reach down and pick up one of the scraps, would you be able to tell the make and model of the car it came from?

The challenges a paleontologist faces in reconstructing the life of the past are not much different. Complete, articulated remains of prehistoric organisms are rare. More often than not, paleontologists must turn their attention to scraps; a piece of skull, a broken tooth, an isolated leaf, a shard of shell, and so on. It takes years to build up the mental catalog of characteristics necessary to properly identify these petrified bits and pieces, and even then paleontologists are sometimes shocked to learn that fossils thought to belong to one kind of creature actually belonged to another. Such was the case with Megarachne, the giant spider that wasn't.

In 1980 paleontologist Mario Hunicken made a startling announcement; he had found the remains of the largest spider to have ever lived. Discovered in the approximately 300 million year old rock of Argentina, this prehistoric arachnid appeared to have a body over a foot in length and a leg span of over 19 inches. It was given the name Megarachne servinei, and its status as the biggest (and hence scariest) spider of all time made museums eager to include reconstructions of it in their displays.

Yet something was not right about Megarachne. The partial remains that Hunicken had described seemed generally spider-like, yet the specimen lacked specific traits that a spider would have been expected to posses. Further study was needed to understand what Megarachne truly was, but the original specimen was sequestered in a bank vault, out of the reach of most paleontologists. It would not be until 2005 that these remains, as well as a new specimen of Megarachne, would come under the scrutiny of other paleontologists.

The announcement was made by Paul Selden, Jose Corronca, and Hunicken in the pages of Biology Letters. Megarachne did not belong among the spiders, but among a related group of extinct arthropods called eurypterids, more commonly known as the "sea scorpions". The points (mucrones) and crescents (lunules) of its carapace, especially, identified it among the aquatic arthropods, though due to the standardized rules of taxonomy it had to retain the name Megarachne.

Despite this reanalysis, however, the public was introduced to the spider-version of Megarachne in the BBC documentary Before the Dinosaurs: Walking With Monsters. Any restoration of the world 300 million years ago would not have been complete without including the largest spider of all time, but at the 11th hour the true identity of the spider became known (though this was before the release of the Biology Letters paper). It was too late to change the program, and so the show's spider was cast as a species of Mesothelae, a true spider that was much smaller and looked quite different from the TV monster. Such are the perils of reconstructing ancient life. We lost a gigantic spider, but we gained a very strange eurypterid.

Selden, P., Corronca, J., & Hünicken, M. (2005). The true identity of the supposed giant fossil spider Megarachne Biology Letters, 1 (1), 44-48 DOI: 10.1098/rsbl.2004.0272