A Short Note On: Fossil Spiders from Florissant

The order of Araneae (true spiders) are represented as a large and diverse group in the 34.1 million-year-old Florissant Formation. Spiders were among the earliest animals to live on land. They are thought to have evolved about 400 million years ago from primitive ancestors that emerged from water to live on land. Spiders are arachnids—not insects, however both spiders and insects belong to the largest group of animals on Earth, the arthropods.


Florissant is well-known for its fossil spiders. More than 150 specimens of spiders have been found in the Florissant shales.


This Eocene-age fossil spider is a male, based on the swelling of the pedipalps. Many fossil spiders are impressions that are barely discernable.  Toni Clare, owner of the commercial quarry north of the Florissant Fossil Beds National Monument, collected the specimen shown above in 1997. Florissant Fossil Beds National Monument specimen number 2971A. Photo: R. Wolf.

When spiders die their legs normally curl under their body. Florissant spiders, like the image of one above, have their legs extended—rather than curled up. Scientists argue that the extended legs of Florissant’s spiders suggest the waters of ancient Lake Florissant, during the latest Eocene, were warmer or more acidic than normal; the likely cause may have been from thermal vents associated with area volcanism or from ash falls.
It is difficult to assign Florissant’s fossil spiders to a genus and species based on their external features observed in the paper shales. Since microscopic characteristics cannot be seen in the fossil impressions at Florissant, an outline morphometric study, using carapace (dorsal exoskeleton) shape and leg characters, has been effective in making family placements of fossil spiders.

All spiders have two well-divided body sections: the cephalothorax (prosoma) followed by an abdomen (opisthosoma). The abdomen contains the digestive and reproductive systems and on the ventral surface near the apex are spinnerets that deliver small threads of silk.

Silk has many functions such as making intricate webs that capture prey, encasing eggs, and building elaborate nests or burrows. Spiders with a distinctive silk organ, called a cribellum, are included in a special group called the Cribellatae. The fossil spider in figure 1 has a cribellum and belongs to this group.

Spiders have eight walking legs, all attached to the cephalothorax. On the front of the cephalothorax are the mouth, fangs to bite prey with, and eyes. The first pair of appendages—the chelicerae—are used for piercing, handling prey, and injecting venom. The second pair of appendages, the pedipalps, are used for mating and are much larger in male spiders than in females.

All species of spiders are predatory—spiders that do not spin webs, such as wolf spiders and tarantulas, stalk or ambush their prey. Spiders feed by a process known as external digestion. When spiders catch an insect, they inject venom that paralyzes their prey. The spider’s venom also contains digestive enzymes. These enzymes liquefy most of its victim’s insides so the spider can feed on this mixture of nutrients.

Spiders were once a part of an ancient ecosystem at Florissant that has long since vanished; the only record of it is held in the fossil beds. Some of the spiders that lived there built elaborate webs; several built tunnel-like lairs under rocks or under the dead leaves littering the primeval forest floor, while others lived on rocks or trees. Some of the spiders ultimately turned into fossils. More exciting discoveries of these remarkable fossils will no doubt occur and add to our understanding of this prehistoric ecosystem and creatures.