Marine Invertebrate PaleoPhylum Arthropoda

Phylum Arthropoda

Definition:1) Jointed appendages (as name implies)

i. Appendages serve locomotor functions (most groups), sensory functions (most groups), respiratory functions (some groups) and predatory functions (some group).

ii. Where both respiration and locomotion are functions of the appendages they are normally biramous (two branches) with a “gill” branch and “walking” branch

2) Heteronomous segmentation (having unlike segments)i. Body is divided into segments, termed metameres or

somitesii. Normally the dorsal and ventral surfaces of the somites

will be surface hardened or Sclerotized. These sclerotized segments are connected via musculature.

3) Growthi. Growth take place via molting or ecdysis. The

molts are called exuviae.ii. Molting introduces vulnerability to predation,

environmental shock when animal has just molted (example soft shell crabs)

4) Nervous system highly developed relative to other invertebrates.

i. Brain and ventral nerve cord with ganglia present in each somite.

ii. Development of organized nervous system has doubtlessly facilitated radiation of arthropods with elaborate feeding and social behavior (e.g., some insects)

5) Respiration by gills, tracheae, or by diffusion through the body surface

i. Very small form use diffusionii. Most aquatic forms use gillsiii. Terrestrial forms (e.g., insects) use tracheae

(inward deflected cavities or tubes in outer body wall).

6) Ecologically this phylum is extremely wide ranging encompassing ~75-80% of the extant species on the planet (mostly insects).

Fossil record consist primarily of well sclerotized formsTrilobitesHorseshoe crabsOstracodsBarnacles

ExoskeletonThe distinguishing feature of arthropods and one to

which many other changes are related is the chitinous exoskeleton or cuticle which covers the entire body.

Movement is made possible by the division of the cuticle into separate plates.

The cuticle of each segment is divided into four primary plates.A dorsal: tergumA ventral: sternumAnd two lateral: pleura

The exoskeleton is secreted by the underlying layer known as the hypodermis(= epidermis in other groups)

The exoskeleton is composed of:1) a thin outer epicuticle and2) A much thicker procuticle which can be sclerotized with

mineral salts. Epicuticle is composed of proteins and, in many

arthropods, wax Procuticle consists of an:

a. Outer exocuticleb. Inner endocuticle

Both of these layers are composed of chitin and protein bound together to form a complex glycoprotein.

In addition the exocuticle has been tanned (i.e., its molecular structure has been further stabilized by the formation of additional cross-linkages

The exocuticle is absent at the joints and along suture lines where the skeleton will rupture during molting.

Despite the locomotor and supporting advantage of an external skeleton, it posses problems for a growing animal

The solution to this problem evolved by the arthropods has been the periodic shedding of the exoskeleton, i.e. ecdysis or molting

Molting and growth are hormonally controlled and growth occurs in a step-like pattern externally

Arthropoda TaxonomySuperclass Trilobitomorpha

Class Trilobita (Cambrian-Permian) Trilobites are extinct primitive arthropods from the

Paleozoic Trilobite exhibit particular modifications for their

mode of life: Flattened body Location of the mouth on ventral surface Eyes on dorsal surface Structure and arrangement of appendages all

indicate they were benthonic animals that crept or swam along the bottom and fed on small organisms.

The body is differentiated into three regions:1. The head called the cephalon consisting of (~6)

fused segments2. The thorax with a variable number of distinct

articulated segments held together by musculature so they can move separately.

3. The tail called the pygidium which includes one or more segments fused into a rigid plate.

In addition, a pair of longitudinal furrows divide the body into three lobes the origin of “trilobite”1. Axial lobe2. Two lateral or pleural lobes

Trilobite EyesEyes of trilobite are distinctive and resemble those of

insects for they are composed of a large number of separate lenses (i.e., compound eyes)

Their compound eyes are divided into two major categories1. Holochroal eyes

Many small (30-200 mm) individual polygonal or round lenses which are mantled by a single cornea.

2. Schizochroal eyes Many large (120-750 mm) lenses with separate corneas. The

lenses are distributed in vertical strips and generally increase in size downward.

Holochroal eyes are the primitive condition among trilobites

Purpose of schizochroal eyes uncertainSchizochroal eyes occurred only in one order

of trilobite, the Order PhacopidaThey may have functioned to:

1. Concentrate light so that intensity exceeds a certain predefined minimum level. So they could see in dim light on seafloor.

2. Large schizochroal lenses oriented with greater curvature on eye than holochroal lenses, would have provided stereoscopic vision for trilobites.

Trilobite facial Suture Patterns used in order classification1) Proparian• Suture pattern begins and ends on anterior

margin of cephalon2) Opistoparian• Suture pattern begins on anterior margin of

the cephalon but ends on the posterior margin of the cephalon.

3) Hypoparian• Marginal suture pattern

Trilobite EcologyTrilobites were diversified morphologically corresponding to a

variety of life habitsMost were benthic scavengers and detritus feedersSome had spinosity to provide protection from predators and also

likely help keep them above the sediment water interface.Enrollment

Trilobites are commonly preserved as intact enrolled animals like common pill bugs.

Confirms they were probably benthic and ventral surface was most vulnerable to predation and environmental stress.

Planktonic forms A few trilobites , such as the Agnostids, are interpreted as members

of the planktonic suspension feeders Small, blind, equal sized cephalon and pygidium, found in many

facies

Trilobite EvolutionFirst appeared Lower (Basal) Cambrian fully

recognizable in all morphological detail.Lower Paleozoic was their heyday with zenith of

diversity in Middle Ordovician.Middle Paleozoic get gradual decline in overall

trilobite diversity.Late Devonian extinction extinguished most

trilobite taxa.Only single order, the Proetida survived to the

Permian, but went extinct in the Late Permian mass extinction.

Superclass ChelicerataCambrian-RecentHorseshoe crabs, Eurypterids, sea spider, and terrestrial

spidersGeneral characteristics

1) Contains the only nonantennate arthropods2) The body is usually divided into an anterior cephalothorax

or prosoma and a posterior abdomen or opisthosoma3) The first post oral appendages are a pair of food handling

Chelicerae usually followed by four pairs of legs.4) The marine origin of chelicerates is evidenced by along

fossil history, but only five marine species exits today (all horseshoe crabs “living fossils”).

Superclass ChelicerataClass MerostomataGill bearing aquatic chelicerates with two

orders.1) Order Xiphosura (horseshoe crabs)

Cambrian to Recent hence “living fossil”Limulus modern genusSoft bottom shallow water marine cheliceratesThe prosoma is covered by a large horseshoe

shaped carapace and the abdominal segments are fused together.

Posses a posterior spike-like telson Used for pushing and righting itself.

2) Order Eurypterida • Ordovician to Permian• eury= broad pterid= wing-like• Scorpion like in form• Appendages that are far more specialized than

trilobite• Eurypterid appendages (six pair)

• Four pair of walking legs• One pair of swimming paddles• One pair of Chelicerae

• Benthic predators that inhabited primarily brackish to freshwater sediments. Also hypersaline environments.

• Common in Silurian strata of Appalachian Basin.• Invaded freshwater and gave rise to Class Arachnida.

Superclass CrustaceaCrabs, shrimp, lobsters, barnacles, copepods,

ostracods, etc.Only two groups are important to geologist in the

fossil record.Ostracods and barnacles in order of importance

Class MalacostracaOrder Decapoda ( crabs, lobsters, shrimp)

Class Ostracoda (Ordovician-Recent)Bivalved arthropods with small valves of either chitin or

calcite.Small enough to be preserved in well cuttings for

subsurface stratigraphy.

Seven pairs of jointed appendages that project from the valves They include:

1) Antennules (used in locomotion)2) Antennae (used in locomotion)3) Mandibles (used for feeding)4) Maxillae (feeding or filter feeding by generating a current)5) 3 pairs of thoracic legs (walking or probing “legs”)

• Bivalved carapace• Parallel evolution to bivalve mollusca “clam shrimp”• Rounded or elliptical• Outer wall of each valve is impregnated with CaCO3• (hence good preservation)

• Dorsal hinge line with teeth and ridge• Surface of valves covered with hair-like projections or setae• Adductor muscle fibers near center of each valve

• Ecology• Widely distributed in the ocean and in all types of

freshwater habitats.• Either benthic or pelagic• Display diverse feeding habits• Carnivore• Herbivore• Scavenger• Filter or suspension feeders

• Evolution• Most extensive of any group of crustaceans (more

than 10,000 species)• Dwindled during Devonian and Permian mass

extinctions• Ostracods used in biostratigraphy from Jurassic –

Recent time.

Superclass CrustaceaClass CirripediaBarnacles (~1000 living species)Appendages modified to form cirri for

suspension feedingLack gills cirri used also for respirationExoskeleton Calcareous plates

Cemented or attached to a surfaceSome commensal or parasitism

Superclass HexapodaClass Insecta