Fascinating Fossils of Florissant

Fossil Insects and Spiders

Topic Overview
Fossil insects and spiders are incredibly rare in the fossil record, but abundant at the Florissant fossil beds. Insects and spiders do not normally preserve as readily as other macrofossils, so their presence at Florissant indicates that the conditions for preservation were ideal. The exceptional preservation of these organisms is due to frequent volcanic activity in the area of Florissant 34 million years ago. 

Finding Insect and Spider Fossils
Periodic volcanic eruptions and diatom(a kind of alga) mats formed thinly layered rocks referred to as paper shale in the ancient Florissant lake. This shale is split along its layers to look for macrofossils (fossils visible to the naked eye). Most bodies decay, erode or are scavenged before they can become fossils. Softer body material is more fragile, so it is less likely to preserve. Animals with a thick cuticle (hard covering) preserve more frequently than animals with a soft body, which is why beetles are one of the most common fossil insects and butterflies are one of the least common. [Click here to learn more about the geology of Florissant.]

What's the difference in an Insect and a Spider?
Insects have six legs and three distinct parts to their bodies: the head, thorax, and abdomen. 
1.) Head, 2.) Thorax, 3.) Abdomen, 4.) Legs

Spiders have eight legs and two distinct parts to their bodies: the cephalothorax and the abdomen.
1.) Cephalothorax, 2.) Abdomen, 3.) Legs

Identifying Fossil Insects and Spiders
Unlike with plants and vertebrate fossils, insects and spiders are usually fossilized as the entire organism. Because of this, you might think that identification is easy, and in some cases it is; however, paleoentomologists have another problem when identifying species- life cycle stages. Many insects have four different life stages (nymphs, larvae, pupae, and adults). Often the life cycle stages within one species look very different from each other, so it can be difficult to determine which of the life cycles stages belong to the same species of insect. Trace fossils from insects like larval casts and leaf-feeding traces can also be difficult to associate with the species that left them. Because of these difficulties, the different components we've talked about are sometimes given different names, confusing understanding. Paleoentomologists, just like paleobotanists, have developed diagnostic identification techniques such as identification based on wing venation.
Fossil Examples: 
        Scientific Classification                                                    Common Name
          Arachnida-------------------------------------------------> spiders
          Lepidoptera-----------------------------------------------> butterflies and moths
          Odonta----------------------------------------------------> dragonflies and damselflies
         Hymenoptera ---------------------------------------------> bees, wasps, sawflies, ants  
Isoptera----------------------------------------------------> termites         
-------------------------------------------------> beetles    
          Orthoptera-------------------------------------------------> crickets, grasshoppers, locusts      
Fun Facts
  • Spider legs typically curl under the body when a spider dies, making the extended legs of most Florissant spider fossils unique. This phenomenon is hypothesized to be due to either warm or acidic lake water caused by volcanism. 
  • Spiders do not metamorphose, but emerge from their eggs looking like miniature adults. Spiders molt several times through their lives in order to accommodate new growth in their exoskeletons.
  • The most important work on Florissant fossils was done in 1922 by Petrunkevitch. He reclassified some of the spiders originally described by Samuel Scudder. There is not much agreement on the higher-level classification of fossil spider, which makes Florissant classifications a bit tenuous.
  • Florissant has more fossil butterfly species than any other paleontological site in the whole world.  
  • Adult insects are the most commonly preserved fossil insects at Florissant. Some of these species has incredibly    short-lives adult lives, some as short as only 2 days. 
  • Some species of Florissant insects were highly social and had different roles within groups. We know from modern insects that some species develop different forms depending on their job - think about physical differences between queen bees and worker bees. This difference in physical appearance is how scientists know some of the fossil groups were socially similar to modern groups.