The forests of the Late Carboniferous period (about 300-320 million years ago) harbored a great variety of arachnids. In addition to the familiar spiders, harvestmen, and scorpions, there were other, stranger kinds of spider-like animals. In a new paper published this month in the Journal of Paleontology, a duo of paleontologists described Douglassarachne acanthopoda, a large spider-like arachnid with very spiny legs (presumably to deter predators), from the world-famous Mazon Creek fossil localities of Illinois, United States.
Douglassarachne acanthopoda. Image credit: Paul Selden & Jason Dunlop, doi: 10.1017/jpa.2024.13.
“Douglassarachne acanthopoda comes from the famous Mazon Creek locality in Illinois and is about 308 million years old,” said Dr. Paul Selden, a paleontologist at the University of Kansas and the Natural History Museum of London.
“This compact arachnid had a body length of about 1.5 cm and is characterized by its remarkably robust and spiny legs — such that it is quite unlike any other arachnid known, living or extinct.”
“Carboniferous Coal Measures are an important source of information for fossil arachnids, representing the first time in Earth’s history when most living groups of arachnids occurred together. Yet, the fauna was still quite different to today.”
“Spiders were a rather rare group, only known at that time from primitive lineages, and they shared these ecosystems with various arachnids which have long since died out,” said Dr. Jason Dunlop, a paleontologist at the Museum für Naturkunde Berlin.
“Douglassarachne acanthopoda is a particularly impressive example of one of these extinct forms.”
“The fossil’s very spiny legs are reminiscent of some modern harvestmen, but its body plan is quite different from a harvestman or any other known arachnid group.”
Douglassarachne acanthopoda doesn’t belong in any of the known arachnid orders, the researchers found.
“Unfortunately, details such as the mouth parts cannot be seen, which makes it difficult to say exactly which group of arachnids are its closest relatives,” Dr. Selden said.
“It could belong to a wider group, which includes spiders, whip spiders and whip scorpions.”
“Whatever its evolutionary affinities, these spiny arachnids appear to come from a time when arachnids were experimenting with a range of different body plans.”
“Some of these later became extinct, perhaps during the so-called ‘Carboniferous Rainforest Collapse,’ a time shortly after the age of Mazon Creek when the coal forests began to fragment and die off. Or perhaps these strange arachnids clung on until the end-Permian mass extinction?”
The specimen of Douglassarachne acanthopoda was discovered in a clay-ironstone concretion in the 1980s by Bob Masek.
Masek deployed a common method for splitting a concretion by leaving it outside in water through the winter, so that frost penetrates the natural crack in the concretion along the plane containing the fossil.
A sharp hammer blow split the concretion along the plane, revealing the fossil.
Around 1990, David Douglass acquired the specimen from Bob, at which time it became part of The David and Sandra Douglass Collection and was displayed in the Douglass family’s Prehistoric Life Museum.
In 2023, when it became apparent that this specimen represented a new species, David Douglass donated the specimen to the Field Museum of Natural History so it could be researched.
“The genus name Douglassarachne acknowledges the Douglass family,” Dr. Dunlop said.
“Then, acanthopoda refers to the unique and characteristic spiny legs of the animal.”
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Paul A. Selden & Jason A. Dunlop. A remarkable spiny arachnid from the Pennsylvanian Mazon Creek Lagerstätte, Illinois. Journal of Paleontology, published online May 17, 2024; doi: 10.1017/jpa.2024.13
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