Trilobites.
A = Isotelus from the Ordovician of New York state
B = Dalmanites from the Silurian of Indiana
C = Ogygopsis from the Cambrian of British Columbia
Insects, spiders, shrimp, crabs, lobsters, barnacles, ostracodes, trilobites, eurypterids
Name: means "jointed" (arthro) + "foot" (pod).
Chief characteristics: Segmented body with a hard exterior skeleton composed
of chitin (organic material).
Paired, jointed legs.
Highly developed nervous system and sensory organs.
Geologic range: Cambrian to Recent.
Mode of life: Arthropods inhabit a wide range of environments. Most fossil forms are found in marine or freshwater sediments.
(Trilobites)
Name: Trilobite means "three" (tri) + "lobed" (lobus).
Chief characteristics: Body has a three-lobed appearance; two long grooves running from the head to
the tail divide the body into three lobes.
Skeleton composed of chitin, strengthened by calcium carbonate in places not requiring flexibility.
Body is divided into three segments:
Geologic range: Cambrian to Permian - all extinct.
Mode of life: Exclusively marine. Most were bottom dwellers living in shallow shelf environments.
Trilobites.
A = Isotelus from the Ordovician of New York state
B = Dalmanites from the Silurian of Indiana
C = Ogygopsis from the Cambrian of British Columbia
Diagram showing the major body parts of a trilobite.
A. CLASS XIPHOSURA (horseshoe crabs)
Geologic range: Silurian to Recent.
Horseshoe crab fossil from the Jurassic Solnhofen limestone, Germany.
Photo courtesy of Pamela Gore.
B. CLASS EURYPTERIDA (eurypterids)
The eurypterids are extinct scorpion-like or lobster-like arthropods which were for a time the dominant predators in the Paleozoic. Some were nearly 10 ft long. They have a semicircular head with compound eyes, and a segmented rear section ending in a long projection called a telson. Two swimming legs are prominent near the head. Also have pincers called chelicerae.
Geologic range: Ordovician to Permian. Most are found in the Silurian and Devonian.
Mode of life: Inhabited brackish estuaries.
Fossil eurypterid.
Photo courtesy of Pamela Gore.
Diagram showing the major body parts of a eurypterid.
C. CLASS ARACHNIDA (scorpions, spiders, ticks, and mites)
Scorpions are the oldest arachnids with a fossil record.
Scorpions had evolved by the Late Silurian. The earliest ones appear to have lived in the water, because their fossils have gills.
Scorpions, spiders, and mites are found in Devonian rocks.
Geologic range: Late Silurian to Recent.
CLASS OSTRACODA (ostracodes, also spelled ostracods)
The ostracodes are mainly microscopic in size. They consist of a tiny bivalved shell encasing a shrimp-like creature.
Geologic range: Cambrian to Recent.
Mode of life: Both marine and freshwater.
Scaning electron microscope image of an ostracode from the Pennsylvanian.
A. CLASS ONYCHOPHORA
Living and ancient velvet worms, including Asheia from the Cambrian Burgess Shale.
Onychophorans share many characteristics of segmented annelid worms and arthropods, and are considered to be intermediate in evolution between the two groups.
Geologic range: Cambrian to Recent.
The onycophoran, Aysheaia.
B. CLASS HEXAPODA (insects - six legs)
The insects are among the most diverse living group on Earth, although they are rarely found as fossils.
Body is divided into three parts, head, thorax, and abdomen. Thorax has six legs. Wings may be present or absent.
The earliest insects were wingless. Winged insects appeared by the Pennsylvanian.
Geologic range: Middle Devonian to Recent.
Fossil dragonfly, Pennsylvanian period.
January 5, 2006