are the most successful and diverse of all animal phyla.
Of the known million species of animals, approximately
900,000 are arthropods. One order of insects, Coleoptera
has as many species as the entire plant kingdom (Arnett
& Jacques, 1981, p. 16). A few acres of tropical
forest can contain over a billion arthropods.
Ant colonies may house over one million individuals.
A cubic meter of ocean water may be home to millions
(Prothero, 1998, p. 248). Arthropods are very versatile
and have evolved adaptations that allow them to live
in all habitats from the deepest oceans to the highest
the coldest ice
to the hottest deserts. In the history of life on Earth
arthropods lay claim to many evolutionary
it was argued that arthropods share a common ancestor with
annelid worms. Annelids and
Arthropods have a coelom and a segmented body. However, molecular
phylogentics analyses comparing DNA and RNA indicate that
arthropods are more closely related to the roundworms or
nematodes (Prothero, 2007, p. 193). Arthropods were the
jointed appendages. Joints
permit powerful movement, aid in locomotion and can be modified
to serve many functions. The name arthropod means jointed
foot. Trilobites were the first animals with eyes that were
capable of forming images (Johnson & Raven, 2001).
Arthropods were the first animals to invade land during
and eventually insects) (Prothero, 1998, p. 248). Arthropods
were the first organisms to evolve wings. Insects took to
the skyies 100 million years before the flying reptiles of
Jointed Appendages & The Exoskeleton
success and diversity is due to mainly two factors. First,
arthropod jointed appendages are susceptible to adaptive
Legs, mouthparts antennae, claws pincers, swimming paddles,
and gills represent some of the specializations. Second,
their exoskeleton provides protection and mechanical advantage.
The exoskeleton serves as an external armor for protection
against the environment and predators. Leverage is increased
in arthropods as the muscles pull from the inside of
the exoskeleton instead of muscles pulling from the
as in vertebrates. The exoskeleton is not a living tissue
and does not grow. Therefore, the exoskeleton must
be shed or molted in order for growth to
After molting an arthropod can make radical body changes
before the new skeleton dries and hardens, allowing for the
evolution of life cycles that include metamorphic processes.
your skeleton on the outside can also make you vulnerable
at times as
well as place limits on size. After molting the soft
exposed body is vulnerable to predators. Arthropods must
hide from predators during molting. In molting its exoskeleton
the arthropod has lost most of its structural support.
If the body were too large it would lose its integrity
molt "dissolving" into a blob.
with an external skeleton can evolve
also depends on the buoyancy provided by its surroundings.
The largest arthropods to evolve did so in the marine environment.
On land oxygen levels may also limit arthropod size as
respiration through book lungs and tracheae depend partly
exoskeleton is made of chitin (polysaccharide and protein).
In some groups, such as trilobites and ostracodes, the cutile
is also mineralized with calcite. Arthropods with chitin
reinforced with calcite leave a good fossil record. Unfortunately,
most arthropods have exoskeletons made only of chitin, which
explains why overall the arthropod fossil record is poor.
Luckily, fossil lagerstatten that preserve nonmineralized
such as Baltic amber, the Burgess Shale, and the Florissant
fossil beds provide important windows into arthropod evolution.
can be divided into four subphylums.
The subphylum Tracheata or Uniramia (millipedes, centipedes,
and insects) inlude arthropods
with jaws. Arthropods with
fangs or pincers belong to the subphylums Trilobitomorpha
(trilobites), Crustacea (lobsters, crabs, shrimps, crayfish,
barnacles, ostracodes, and pill bugs) and Chelicerata (scorpions,
mites, spiders, horseshoe crabs, and eurypterids).
range from the Cambrian to the Permian. Next to ostracodes
they are the most common arthropod fossil. Trilobite means
three-lobed and refers to the three longitudinal divisions
of their calcified exoskeleton. An axial lobe running down
the center is attached to pleural lobes on either side.
The head or chephalon is separated from the tail or pygidium
by thorasic segments. As juveniles, trilobites were pelagic,
swimming in the plankton. As adults, most trilobites were
detritus feeders, although there are some exceptions such
as the agnostids, which are thought to have lived a pelagic
life due to their world-wide distribution. Trilobites were
one of the first organisms to have eyes and may have been
the first with
eyes that focused images. Many trilobite species lost their
eyes through selection such as the agnositds and Cryptolithus.
have compound eyes made of calcite crystals. Two important
types of compound eye can be found in trilobites. Holochroal
eyes consisted of hundreds (up to 15,000) of closely
packed elongated prisms of calcite arranged in a hexagonal
2000, p. 99). The lenses of these eyes must be viewed
with the aid of magnification. Holochroal eyes produced
composite images of the world, but with little resolution. Phacops
trilobites possessed schizochroal eyes. Schizochroal
eyes are made of larger spherical-shaped lenses numbering
in the hundreds (up to 700) and separated by exoskeleton.
Each lens is made of two calcite crystals
arranged as a
doublet lens. Ironically, the doublet lens was designed
by Dutch scientist Christian Huygens (1629-1695) and the
French philosopher Rene Descartes (1586-1650) to correct
for spherical aberration. Through selection nature had
anticipated the same design 400 million years earlier.
lens produced larger images bringing them into sharp focus
(Fortey, p. 106).
enjoyed a great adaptive radiation during the Cambrian
their peak in abundance and diversity. In fact, Trilobites
are the biostratigraphic standard for the Cambrian. Trilobites
declined during the Ordovician and were only a minor part
of the seafloor fauna in the Silurian and Devonian. Trilobites
did enjoy some increase in diversity during the Devonian;
however, they never fully recovered from the late Devonian
and vanished during the
rana is the state fossil for Pennsylvania. Isotelus is
the state fossil for Ohio. Calymene celebra is the state
fossil for Wisconsin.
include spiders, scorpions, mites, ticks, horseshoe crabs,
and eurypterids. The chelicerate body is divided into a
cephalothorax and the abdomen. They do not have antennae.
The mouth parts are composed of chelicerae(small claws
or fangs) and pedipalps. Pedipalps are modified into pincers
in scorpions, eurypterids,
and some spiders. The thorax has 4 pairs of legs. The subclass
Arachnida includes spiders, scorpions, ticks, and mites.
Trigonotarbids are spider-like arachnids that lack poison
glands and silk-producing organs, but they do have book
lungs. Trigonotarbids make their first appearance in the
Silurian. The first true spider, pseudoscorpions, scorpions,
are an extinct group of arthropods in the subclass Eurypterida.
The eurypterid is a
chelicerate arthropod and looks like a cross between
a scorpion and lobster. Eurypterids range from the
Ordovician to the Permian. Eurypterids were major marine
predators during the Silurian. The Eurypterid Eurypterus
remipes is the state fossil for New York. Horseshoe crabs
belong to the subclass Xiphosura. Primitive horseshoe
their first appearance in the Silurian. In the early Paleozoic
horseshoe crabs evolved into a variety of body forms. Horseshoe
similar to the present day Limulus do not appear
until the Pennsylvanian.
range from the Cambrian to recent times. Today, crustaceans
are the most successful arthropods in marine environments.
Crustaceans also inhabit freshwater environments and a
few are terrestrial. Familiar crustaceans include shrimp,
lobsters, crabs, crayfish, barnacles, water fleas (Daphnia),
and pill bugs. The body and legs of crustaceans are enclosed
in a chitinous shell, which in some species is reinforced
with calcium containing minerals. Crustaceans have two
pairs of sensory antennae, three pairs of limbs used to
the mouth, and biramous walking legs that may have gills.
The general body plan of crustaceans consists of a head,
thorax, and abdomen. The head and throax are often fused
into a cephalothorax and the abdomen often ends in a tail-like
structure. All crustaceans develop from a distinctive
larva type known
as nauplius. Many taxa below the class level leave no fossil
record; however, two classes, Malacostraca and Maxillopoda
have a substantial fossil record.
class Malacostraca includes crabs, shrimps, lobsters, krill,
pillbugs, and amphipods. Some of the earliest crustaceans
from this class included shrimp-like organisms called phyllocarids,
which first appear in the Cambrian. The familiar order
of Decapods (crabs, lobsters, crayfish and shrimps) did
not appear until the Devonian period. The Decapods underwent
a great adaptive radiation during the Mesozoic. Crabs
make their first undisputed appearance in the Jurassic.
Predation during the Mesozoic from crabs and lobsters may
have forced mollusks to burrow and many brachiopods to
1998, p. 265).
class Maxillopoda includes ostracodes, copepods, barnacles,
and many shrimp-like forms. Barnacles and ostracodes have
calcified skeletons that fossilize well. Barnacles (subclass
Cirripedia) are strictly marine organisms and range from
Ordovician to recent times. The barnacle nauplius larva
changes into a shrimp-like,
The shrimp-like organism adheres its head to a surface,
casting off its bivalved shell. After a profound metamorphosis
the adult form becomes cemented to the substrate and the
mantle, which encloses its body, secretes a calcareous
shell. The calcareous shell is made of side plates and
The lid plates are needed for precise identification, but
are rarely fossilized. Most barnacles attach to hard surfaces
and are filter feeders. Some species burrow into the shells
of mollusks and corals. Still others are parasitic and
burrow into other organisms. Ostracodes (subclass Ostracoda)
are the most common fossil arthropods and range from early
Cambrian to recent times. Ostracodes are usually microscopic
crustaceans with a pair
calcareous shells hinged over their back. Ostracodes are
rapidly evolving, abundant microfossils, making them very
useful for biostratigraphy. Ostracodes are sensitive to
depth and water conditions, which makes them useful tools
for studies in paleoecology.
include millipedes, centipedes, and insects. Millipedes
and centipedes belong to the class Myriapoda. Millipedes
have two pairs of legs per segment. Millipedes have a long
tubular body covered with a calcified cuticle coated with
wax and equipped with up to 200 pairs of legs. Millipedes
live among rotting vegetation and are mostly scavengers
or detritus feeders. Possible millipede trace fossils are
found in the Ordovician, which would make them the first
animals to invade land. Millipede body fossils first appear
in the Silurian. Centipedes have a more flattened body
with fewer segments and only one pair of legs per body
segment. Centipedes have at most a few dozen pairs of legs.
Centipedes are carnivores equipped with poison claws and
fangs. Centipedes first appear in the Silurian.
are the most successful animal in terms of numbers and
diversity. Insects belong to the epiclass Hexapoda and
the class Insecta. Hexapoda consists of entognathous hexipods
and the true insects. Entognathus hexipods have their mouthparts
recessed within their head, springtails are a familiar
insect body plan consists of a head with antennae, thorax
with six legs, and an abdomen. The insect cuticle
does not fossilize well. At the species level the insect
fossil record is poor; however, at higher taxonomic levels
the resolution improves. At the family level 63% of living
families are represented in the fossil record. At the
order level 100% are represented. Primitive wingless insects
first appear in the Devonian. Insects with wings
(Pterygota) make their first appearance in the Pennsylvanian.
There is evidence to suggest that insects with fixed
(Paleoptera), like mayflies, came before those with folded
wings (Neoptera), like cockroaches. Herbivorous insects
make their first appearance in the Carboniferous. Insects
up through the Carboniferous undergo incomplete metamorphosis
(superorder Exopterygota or Hemipterodea). Insects with
complete metamorphosis (superorder Endopterygota or
Holometabola) make their first appearance in the Permian.
Social insects make their first appearance in the Cretaceous.
Fleas, which are parasites to mammals, do not appear
in the fossil record until the Cretaceous.
ichnofossils (trace fossils) can be helpful in determining
what types of insects were present at a particular time
and provide information about the nature and persistence
of past plant-insect associations.
for herbivory in insects appears in the Carboniferous.
Like vertebrates, the first insects were carnivores and
detritivores. Herbivory requires hosting cellulose-digesting
bacteria through a symbiotic relationship within the gut.
The oldest examples of marginal and surface feeding are
on Carboniferous seed fern leaves of Neuropteris and Glosspteris (Grimaldi & Engel,
2005, p. 52). It is estimated that only 4% of the leaves
in Carboniferous deposits exhibit damage from feeding.
Herbivores do not make a significant impact on plant life
until the Permian (Kenrick & Davis, 2004, pp. 166-167).
are excessive growths on stems, leaves, cones, and flowers
caused by insect feeding or egg laying. The earliest fossil
galls are found on the petioles of Psaronius tree
ferns of the Late Carboniferous. Insect gall fossil diversity
and abundance takes off with the advent of flowering plant
evolution in the Cretaceous (Grimaldi & Engel, 2005, p. 53).
produce tunnels in wood known as borings or galleries.
Some insects eat the
cambial layer while others eat fungus that grows within
the galleries, still others eat the wood itself. The
oldest borings and galleries in wood, attributed to mites,
known from the Carboniferous. The
first definitive beetle borings are from the Triassic.
There are some borings in permineralized Triassic-aged
wood from Arizona that are attributed to
termites or bees; however, they may be beetle borings (Grimaldi & Engel,
2005, p. 54 & 55).
mines are meandering tunnels produced by the feeding larvae
of some beetle, fly, and sawfly species. The first definitive
leaf mines first appear in the leaves of Triassic conifers
and pteridosperms. Interestingly, the abundance and diversity
of fossil leaf mines coincides with the radiation of flowering
plants (Angiosperms) during the Cretaceous. Leaf mines
have been used to establish the peristence of insect and
plant associations. For example, the larvae of certain
moth families have been eating the leaves of Quercus (oak)
and Populus (poplars) for 20 million
years and hispine beetles have been
eating the leaves of Heliconia for
70 million years (Grimaldi & Engel,
2005, p. 52).
larvae live in lakes, ponds, and rivers. Many build distinctive
protective cases from bits of sand, shells and vegetation.
Fossil caddisfly cases can often be identified to the
family or even genus level. The oldest larval caddisfly
cases (Trichoptera) are found in the
(Grimaldi & Engel,
2005, p. 51).
a fossil bee nest (in the form of subterranean excavations)
that is first found in Late Cretaceous deposits. Celliforma is
found from the Cretaceous to the Pliocene (Grimaldi & Engel,
2005, p. 51). Termite borings appear in the Cretaceous
and represent the oldest undisputed fossil nest for social
insects (Grimaldi & Engel, 2005, p. 54). Coprinisphaera is
the fossil burrow of a scarabaerine dung beetle, which
makes its first apperance during the Paleocene. Coprinisphaera lived
from the Paleocene to the Pleistocene and had a wide geographic
range being found in South America, Antarctica, Africa
and Asia. Coprinisphaera coincide with the evolution
of the first ecosystems to have abundant mammalian herbivores. Evidence
for the first scarab tunnels are found in the coprolites
dinosaurs from the Late Cretaceous of Montana (Grimaldi & Engel,
2005, p. 50).
Science Olympiad Fossil Event
The 2016 Science Olympiad Fossil List includes trilobites,
eurypterids, insects, and crustaceans.
The following trilobite genera are listed: Phacops, Isotelus, Cryptolithus,
Calymene, and Elrathia. Crustaceans mentioned
on the list include: shrimp, lobster,
crabs, and barnacles.