Tech UPTechnologyMongolarachne jurassica: the giant Jurassic spider

Mongolarachne jurassica: the giant Jurassic spider

 

It was originally described, in 2011, as a species in the genus Nephila , known as golden silk spiders because of the color of their threads. They are the largest weaver spiders today with a wingspan including their legs that can reach 15 centimeters and they usually inhabit tropical and subtropical regions, where they are capable of producing a spider web of up to 1.5 meters in diameter. However, the comparison with another fossil found later, made the researchers decide to change the genus of the fossilized spider.

Well-preserved spider fossils are rare

The holotype fossil is that of an adult female preserved almost completely in a compressed way in sedimentary rocks resulting from the ash expelled by volcanic activity.

It lived 165 million years ago, that is, in the middle of the Jurassic , so it lived with the dinosaurs. With this chronology, it is also the oldest known spider fossil . Its body measures about 24.6 millimeters and its front legs reach 56.5 millimeters in length. A 2013 study analyzed the fossil of a male of the same species, it was then that doubts arose about the correct classification of the spider between the genera Mongolarachne and Nephila . This second fossil had a body of 16.54 millimeters and legs of up to 58.2 millimeters.

It is fortunate that researchers were able to get their hands on two such extraordinarily well-preserved fossils of extinct spiders, since their delicate bodies often decompose before fossilizing:

“You don’t just see the hairs on the legs, but little things like the whipworms, which are very, very fine. They are used to detect air vibrations. There is a very different group of them and they are a very different size.”

This was expressed by Professor Paul A. Selden , from the Paleontological Institute and Department of Geology at the University of Kansas, who heads the studies on this species of spider that has had to be changed gender.

Changes in the interpretation of the species

The female fossil was found in 2005 in the Jiulongshan Formation in Ningcheng County, northwest China. It was spotted by farmers in Inner Mongolia, who alerted experts.

However, it was not until 2011 when the results of the research were published in the journal “Biology Letters”, due to the great difficulty involved in analyzing and classifying the spider fossil . It was then that they saw a greater resemblance to the Nephila group of spiders, so they named the new species Nephila jurassica given the period to which the fossil belongs.

Two years after publication, in 2013, researchers found another fossil of the same species, this time a male. The new finding then made it possible to compare the fossil of the female with another of the same species , which is of great help to specialists. Although, the evidence observed in the second fossil only made life difficult for the research team.

The male is very similar in size to that of the female, a characteristic that does not coincide with the current spiders of the Nephila genus, which have a notable sexual dimorphism since the female spiders are different and much larger than the males:

“The male cannot be accommodated in Nephilidae due to its pedipalp morphology, thus the new genus Mongolarachne and the family Mongolarachnidae are erected for the species.”

Common mistakes in paleontology

The history of paleontology is full of examples similar to this one, in which the original interpretations are modified or totally annulled by the discovery of new materials that provide data and evidence that were impossible to know before . Specifically, sexual dimorphism has already played a trick on the researchers in charge of classifying the giant moas of New Zealand. The size of the females was so superior to that of the males that, initially, they were taken for different species that today are considered the same. Normally it takes years for science to find new information that allows changing and improving this type of classification, but in the case of these prehistoric spiders, it was curious to experience the change of gender thanks to the discovery of fossil remains that occurred in the same decade.

The new classification proposes these fossilized spiders as a genus closer to Deinopis , known as the ogre-faced spiders. These animals have a carnivorous diet and hunt with a curious method . Instead of weaving a web into which their prey falls, they wrap the web around their own legs to go hunting, unfurling their sticky thread to catch their prey in flight.

We cannot rule out at all (in fact it would be ideal) that new fossil materials found provide more information and the description of these Jurassic spiders changes again. There is no doubt: patience is the mother of science.

References:

Selden, P. 2011. A golden orb-weaver spider (Araneae: Nephilidae: Nephila) from the Middle Jurassic of China. Biology Letters, 7, 5, 775-778. DOI: 10.1098/rsbl.2011.0228.

Selden, P. 2013. A giant spider from the Jurassic of China reveals greater diversity of the orbicularian stem group. Naturwissenschaften 100, 1171-1181. DOI: 10.1007/s00114-013-1121-7.

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