All arthropods have a segmented body and paired, jointed appendages (hence the name) Appendages become specialized as mouth parts, pincers, legs, etc
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Most successful phylum
Most diverse – over 85% of all species
Occupy all habitats
Arthropods - Morphology
Owe much of their success to their ability to increase their numbers quickly in good conditions All arthropods have a segmented body and paired, jointed appendages (hence the name) Appendages become specialized as mouth parts, pincers, legs, etc. They also have a hard exoskeleton which accounts for most of their preservation in the fossil record. The exoskeleton doesn’t grow with the animal, but is shed as growth takes place. It is these molts that are often preserved. In general arthropods are small animals. Among the larges living forms are king crabs with bodies of the largest reaching barely .5m. In the fossil record, however, Eurypterids reached over 3 m in length. Arthropods - Systematics Subphylum – Cambrian – Permian; most recognized fossil arthropod Trilobitomorpha Most diverse group in the Cambrian – over 300 genera. THE index fossil of the Cambrian Hard hit by the Late Ord. extinction and again in the Late Devonian. Only a handful of genera survived into the late Paleozoic. They disappeared at the end of the Permian Trilobites Three body segments – Cephalon (head), thorax, and Pygidium (tail) Name actually comes from the three longitudinal lobes that divide the body The characteristics of the exoskeleton and appendages are what define the Orders of trilobites Subphylum Trilobitomorpha Class Trilobita Order Agnostida – Cambrain- Ordovician Very small; cephalon equal to pygidium in size; only two or three thoracic segments. Most were blind. Widespread distribution suggests pelagic lifestyle. Order Redlichiida – Cambrian Most primitive forms; Pygidium segments are not completely fused; large cephalon; long genal spines; large eyes; many thoracic segments; some species with telson; Olenellus and Paradoxides Box-like glabella; subparallel facial sutures; 7-8 thoracic segments Order Ptychopariida – Cambrian – Ord. Typically considered to be all that don’t fit into other groups; many segments; small pygidium; simple glabella; Elrathia Order Asaphida – Mid Cambrian – Ord. Cephalon and pydigium are very smooth and nearly equal in size; large eyes; 6-9 thoracic segments; pleural lobes rounded at the ends; Isotelus Order Illaenida – Ordovician – Devonian Smooth cephalon and pygidium; pygidium often larger than cephalon; Bumastus Order Trinucleida – Ordovician – Mid Sil. Very small pygidium and short thorax; long genal spines and knobby or decorated cephalic brim; most were blind; glabella expands anteriorly; Cryptolithus Order Harpida – Late Cambrian – Late Dev. Very broad cephalic rim giving a horseshoe appearance; many thoracic segments; Harpes – Ordovician – Devonian only group with proparian facial sutures; Order Phacopida Suborder Calymenida – many thoracic segments, highly lobed, furrowed glabella; rolled their pygidium into the glabella; Calymene, Flexicalymene Suborder Phacopidea – glabella broadly expanded anteriorly; distinctive eyes; Dalmanites and Phacops Order Lichida – Ordovician – Devonian glabella extends to the anterior border of the cephalon and has elongated furrows; large pygidium Order Odontopleurida – Ord.- Devonian rare; long spines on the cephalic brim, occipital ring, along the thorax and pygidium Order Proetida – Late Cambrian – Permian large vaulted glabella; large eyes; furrowed pygidium; no spines; Kaskia Arthropods - Ecology Many forms are believed to have been free swimming in their juvenile forms becoming benthic bottom feeders in the adult form. Some were burrowing Trace fossils have been identified as trilobite tracks and trails Spines on some forms were apparently for protection while others used spines for locomotion. Evolutionary Trends Trilobites have along and excellent fossil history so their evolutionary trends have been well studied Ability to “rollup” as a protection method Increase in size and proportion of the pygidium As an overall generality: The number of thoracic segments tend to decrease and the cephalon and pygidium become more equal in size Arthropods - Systematics Subphylum Trilobitomorpha – Cambrian – Permian Subphylum Eurypterida – Ordovician – Permian; best known from Silurian and Devonian; “sea scorpians” with large pincers and a pair of paddle-like legs; large – 3m long; over 25 genera; first prominent predators in the fossil record; marine to fresh water Crustaceans and insects while interesting are not especially important or plentiful in the fossil record.