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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The Arthropods

  • 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 Trilobitomorpha – Cambrian – Permian; most recognized fossil arthropod

  • 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


  • 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

  • Phylum Arthropoda

  • 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

Order CorynexochidaEarly-Mid Cambrian

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

Order Phacopida – Ordovician – Devonian only group with proparian facial sutures;

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

  • Eyes became more complex, but reduced in size and eventually were lost

  • 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

  • Phylum Arthropoda

  • 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

  • Phylum Arthropoda

  • Crustaceans and insects while interesting are not especially important or plentiful in the fossil record.

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