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 somites ii. Normally the dorsal and ventral surfaces of the somites will be surface hardened or Sclerotized. These sclerotized segments are connected via musculature.

3) Growth i. 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 diffusion ii. Most aquatic forms use gills iii. 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 forms Trilobites Horseshoe crabs Ostracods Barnacles

Exoskeleton The 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: tergum A ventral: sternum And 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 and 2) 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 exocuticle b. 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 Taxonomy Superclass 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 segments 2. 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 lobe 2. Two lateral or pleural lobes

Trilobite Eyes Eyes 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 categories 1. Holochroal eyes Many small (  m) individual polygonal or round lenses which are mantled by a single cornea. 2. Schizochroal eyes Many large (  m) 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 uncertain Schizochroal eyes occurred only in one order of trilobite, the Order Phacopida They 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 classification 1) Proparian Suture pattern begins and ends on anterior margin of cephalon 2) 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 Ecology Trilobites were diversified morphologically corresponding to a variety of life habits Most were benthic scavengers and detritus feeders Some 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 Evolution First 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 Chelicerata Cambrian-Recent Horseshoe crabs, Eurypterids, sea spider, and terrestrial spiders General characteristics 1) Contains the only nonantennate arthropods 2) The body is usually divided into an anterior cephalothorax or prosoma and a posterior abdomen or opisthosoma 3) 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 Chelicerata Class Merostomata Gill bearing aquatic chelicerates with two orders. 1) Order Xiphosura (horseshoe crabs) Cambrian to Recent hence “living fossil” Limulus modern genus Soft bottom shallow water marine chelicerates The 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= broadpterid= 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 Crustacea Crabs, 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 Malacostraca Order 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 Crustacea Class Cirripedia Barnacles (~1000 living species) Appendages modified to form cirri for suspension feeding Lack gills cirri used also for respiration Exoskeleton Calcareous plates Cemented or attached to a surface Some commensal or parasitism

Superclass Hexapoda Class Insecta