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Field Guide





Michael Wall, Ph.D.
Director of the Biodiversity Research Center of the Californias

Crab, Icriocarcinus xestos, Cretaceous, Point Loma Formation
Crab, Icriocarcinus xestos, Cretaceous, Point Loma Formation

Fossil   Insects and Crustaceans
Armored beasts in the San Diego Natural History Museum's Paleontology Collection

N. Scott Rugh, Collections Manager, Invertebrate Fossils
Photographs by Torrey Nyborg and Delle Willett

Little black ants have invaded my kitchen again. As they march along in a straight line, it would take little imagination to see them as cars on a highway in the country, as observed from high up in an airplane. They won’t find anything to eat on the clean countertop, even though they’re very persistent. As I sit down to write on this warm first evening of fall, I can hear the dreamy chorus of crickets outside my screen door. If I were in a hot, wooded area of the country, at the right time in their life cycle, it would be cicadas making the sound, but it would be a louder, tenser sound like some kind of electrical device resonating without end. On the east coast, it would not be a symphony of sound outside, but a of show of light, with fireflies flickering on and off among the trees and bushes.

Mantis Shrimp, Eocene, Santiago Formation
Mantis Shrimp, Eocene, Santiago Formation

These are the insects. Ants, flies, termites, cockroaches, lady bugs, and bees are just a few examples of the largest class of animals on the planet. Insects make up more than 75% of all known animal species with some 950,000 different kinds. The insects are one class, Class Insecta, within the Phylum Arthropoda. Also in this phylum are the crustaceans, in the Class Malacostraca. This class includes crabs, shrimp, and lobsters (three of my favorite sea foods), barnacles, and a number of other less familiar forms. One other arthropod class is the Arachnida, including spiders, scorpions, and ticks. Other arthropod classes include horseshoe crabs, centipedes, and millipedes.

If you observe a crab walking among the rocks, you can see the obvious resemblance to the closely related insect. Like insects, crabs have jointed legs; insects have three pairs of legs, whereas crabs have five pairs of legs. The jointed legs of crustaceans and insects are one of the principal characteristics of the arthropods. It is this characteristic that gives this phylum the name. Arthro means “joint,” and pod means “foot.” Thus, arthropods are the phylum of joint-legged animals. These unique appendages are enclosed in the exoskeleton, another important characteristic of Phylum Arthropoda. This outer crusty shell, which provides support for the soft internal body, is compared to the armored knights of medieval times. This is the perfect comparison, but it is doubtful that the knights of old ever moved with the grace and speed of arthropods such as centipedes and scorpions.

Fly, Eocene, Florissant Formation
Fly, Eocene, Florissant Formation

Arthropods were among the first animals on Earth and existed long before the dinosaurs and primitive mammals, more than 400 million years ago. Most arthropods today are very small; few are over several inches long. In prehistoric times however, some fossil species were giants. The largest insect known was a prehistoric dragonfly, Meganeura monyi, with a wingspan of up to 75 cm long which lived during the Permian Period about 280 million years ago. This giant dragonfly was found in the 1880s in Stephanian Coal Measures, a historically famous area of Commentry, France. A giant scorpion, Pulmonoscorpius kirktonensis, 70 cm in length, known from the East Kirkton Limestone of Scotland, lived during the early Carboniferous, around 320 million years ago. This giant scorpion was small, however, by comparison to its relative, the sea scorpion, Euypteurus remipes, which lived during the Silurian Period about 420 million years ago and grew to two meters long—as big as a full grown human! The giant sea scorpions were the largest arthropods that ever lived. This arthropod monster is known from the Bertie Dolostone, a limestone deposit from New York State, including the Niagara Escarpment.

No, the San Diego Natural History Museum does’t have any fossil remains of giant scorpions, giant dragonflies, or sea scorpions in any of our collections. But our invertebrate fossil collections do include the remains of insects from two famous fossil deposits: the Green River Formation of Wyoming, Colorado, and Utah, and the Florissant Fossil Beds of Colorado. Each of these collections in our Museum includes a specimen of a beautifully preserved fossil fly. Both of these locations are Eocene-aged deposits that were formed at the bottom of ancient lakes, and both are famous for their fossil remains of tropical and temperate insects, plants, fish, and other vertebrates. The oldest known bat was found in the Green River Formation, and fossil tree trunks have been found in the Florissant Beds, including the fossil tree stump which is on display in Disneyland in California.

We have not found any identifiable remains of insects in San Diego County. This is because the conditions for preservation of fossil insects did not exist in our region during prehistoric times. Ideal conditions for the preservation of insects as fossils include being embedded in amber or buried in fine sediments in calm bodies of water such as lakes. Neither of these conditions occurred to a significant degree in the region surrounding present-day San Diego.

By contrast, we do have a number of species of fossil crustaceans in our paleontology collection. They were abundant because the ocean bottom environment was the most common environment in our region during most of the time periods from which we find fossils—the Cenozoic Era, Eocene Epoch, Pliocene Epoch, and the Pleistocene Epoch. It was the combination of the common presence of crustaceans and the ideal conditions for their preservation that resulted in the fossilization of these species.

As a result, we have large parts of the bodies of crabs and hermit crabs, along with smaller pieces like claw and leg segments, from these and other species from the Pliocene, 2 to 5 million years ago, and numerous, nearly complete bodies from two crab species, and parts of ghost shrimp from the Cretaceous, 90 to 70 million years ago, are also part of our collection. Several species of well-preserved crustacean fossils have been found in material from the Eocene, about 45 million years ago, including a type of stone crab, swimmer crabs, three species of ghost shrimps, a comb-like leg segment of a mantis shrimp, and small, nearly whole mantis shrimps. Within the last three years, a half dozen new species and genera were described from Eocene fossil crustaceans recovered in the field by our PaleoServices staff from various projects in San Diego County. The crustacean fossils from our area are a valuable scientific resource, and will enable us to learn even more about the fascinating arthropods and the kinds of environmental conditions they once lived in.

As I conclude this article, I think about getting some sleep and wonder what should I have for dinner tomorrow. For some reason, shrimp and crab pasta sounds really good…

To learn more about Eocene crustaceans, see the article on the Museum website about a fascinating new ghost shrimp found by paleontology field collectors a few years ago.