Arthropods belong to the largest of the invertebrate groups, living forms numbering into the millions. Animals such as crabs, lobsters, barnacles, shrimp, centipedes, millipedes, spiders, scorpions, and insects comprise this fantastically diverse group. In addition to the still-living forms which are represented by sparse fossils, there are two important extinct groups; the trilobites and eurypterids. Generally, the arthropod body is elongate and segmented, with jointed legs. Like other animals with external skeletons, trilobites molt their skins with growth. Fossil arthropods are rare, due to their comparatively fragile skeletons and to their habit of living in environments in which fossilization infrequently occurs. Members of the group are known from the Cambrian, or even Precambrian, to the present.
CLASS
TRILOBITA
Hey, I'm seriously thinking about it!
Trilobites, an extinct group of Paleozoic arthropods, is one of the most fascinating of all animal groups. Much now is known of the soft parts of these organisms because of exacting collection techniques and exceptionally well-preserved specimens. Because of their age, diverse forms, and because of an ability to be preserved in near-perfect condition, the trilobite has become the hallmark of paleontology as a science. Many fossil collectors started out by finding a trilobite, and then they were hooked for life.
Usually only the arthropod's external shell, composed of calcium carbonate that has replaced chitin plates, is found fossilized, all articulate with each other to allow flexing movement in life like a knight in armor. Characteristic of trilobites and many other arthropods is a three part body plan; the cephalon, thorax, and pygidium. On the cephalon, or head region, two large compound eyes composed from two to hundreds of individual lenses, enable some sense of sight. The thorax is the mid-section. The pygidium or tail, is commonly fused together, making it more rigid and preservable. From rare, well preserved specimens, we know that trilobites had numerous legs.

The trilobites are generally less than two or three inches long, but some giants reached up to eighteen inches or longer in length. Trilobites were exclusively marine, with swimming, crawling, burrowing, or planktonic habits. As with all arthropods, trilobites molt the skeleton to accommodate increase in body size. Trilobites appeared in early Cambrian seas, already highly developed, leading to the belief that the group must have had its origins in the Precambrian. They were dominant until cephalopod evolution reached its climax in the Ordovician. After the Ordovician, trilobites steadily declined in number and variety, until the end of the Paleozoic, finally becoming extinct in the Permian period. Gone forever, with no known descendants.

Arthropods in general rarely occur as good, complete fossils. Because of this, a good fossil of a trilobite, eurypterid, insect or crustacean are highly prized in any collection and is well worth extra effort in collection

Trilobites showing preserved appendages. So Cool!
Some of Arizona's Cambrian trilobite species
Olenoides sp.
Agraulos sp.
Tricerepicephalus sp.
Glossoplura sp
Albertella sp.
Olenellus sp .
Biiceratops sp.
SUBPHYLUM CHELICERATA
HORSESHOE CRABS, EURYPTERIDS, SPIDERS AND SCORPIONS
This large group of arthropods is characterized by their lack of antennae and in the development of one pair of legs into large pincers. Most chelicerates live on land, this explains their rarity as fossils. Differing from Crustacea in that they lacked gills (except for the Class Merostomata), and had different types of legs, some members of the group resemble the extinct trilobites. Some paleontologists believe the chelicerates developed from the trilobites while others believe the reverse. The subphylum is divided into two classes, the Merostomata and the Arachnida. Only the tracks of the merostomats are significant as fossils in Arizona.

SUBPHYLUM CRUSTACEA

This division of the arthropods is characterized mainly by the form of the legs. The shrimp, crabs, lobsters, ostracods, and barnacles belong to this subphylum. Because members of the group are arthropods, with jointed lets, frequently the appendages come un-glued at the limbs' joints before burial and are separated in to their various parts.

Personality? Charisma?
Fossil Crabs can make beautiful display specimens
Most often, fossil collector will find a leg, pincer, or shell isolated in the rocks. Finding an entire specimen, like the one shown above, is a moment to celebrate, and the specimen is a real paleontological treasure. These days, preparators leave the specimen mounted on a block of the original matrix for display purpos

CLASS OSTRACODA

A strange and important group of crustaceans are the ostracods. Externally the shells of these microscopic and near microscopic animals resemble mollusk shells. But the animals internal structure betrays their true identity as arthropods having segmented bodies and jointed legs, characteristics which all arthropods share.

A living ostracod
Fossil ostracod X5
Ostracods live in all aquatic environments but favor marine waters. Fossils of this group have been found from the Upper Cambrian to the present and have been much studied because, as microfossils with tough shells, they are well-preserved, common, and escape destructio
SUBPHYLUM INSECTA
Everybody is familiar with the winged insects, many of which are pests which live anywhere and eat almost anything. Their reproductive capacity is so great that man has much difficulty controlling them. Some say we are actually living in the "Age of Insects" because these mostly little creatures make up the bulk of lifeforms on the planet today
What an irony that as fossils, insects that virtually account for more life-forms on earth than any other animal groups, are not likely to be preserved and are extremely rare. Their soft body disintegrates rapidly and only under special conditions are they preserved. Many insect fossils come from classic localities producing fossil tree resins (amber) which trapped and entombed the insects, while others are preserved in volcanic ash deposits of fine-grained shales in which burial was fast. The earliest insect fossils come from the Middle Devonian and the group has expanded since then into its present size
COLLECTING FOSSIL ARTHROPODS
The arthropod's exoskeleton is composed essentially of a chitinprotein structure (much like our own fingernails) which rapidly disintegrates after the animals death, and only in ideal sedimentary conditions do the remains of these animals have a chance of being preserved. Arthropod fossils occur at, or on, the bedding planes of sedimentary rocks. The chitinous shells, carbonized residue, or molds and casts, weaken bedding planes allowing rock to split so as to expose fossil specimens.
The moment of truth! Arthropod fossils occur at or on the bedding planes of sedimentary rocks. The chitinous shells, carbonized residue, or molds and casts, weaken bedding planes allowing rock to split so as to expose fossil specimens.
In Arizona? Anything's possible
Fossil chitin and carbonaceous residues tend to chip and spall when exposed to air. Although not recommended by many paleontologists, the use of a thin layer of lacquer is often needed to preserve a specimen from rapid disintegration. This author has found that a thin coat of hair spray will be of great help. It is very important that a collector not overdo this technique as too thick a film of preservative may change color over time or even shrink and pull away from a specimen, causing irreparable damage.
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