Arthropoda

• Most diverse and speciose clade of organisms known
• Protostomians
• Ecydosozoans (have a cuticular skeleton and hence must molt to grow (ecdysis; also in Rotifera, Nematoda, Nematomorpha, Cephalorhyncha, and non-arthropod panarthropods)
• Panarthropoda
  • Arthropoda, Onychophora, Tardigrada, and their extinct allies
    • Have paired limbs running in series down the body; limb surface is series of bands, ending in a pair (typically) of claws
    • Have eyes, compound in some fossil forms
    • Specialized feeding appendages modified from walking limbs
    • Fossil panarthropods include armored lobopods like Hallucigenia; Opabinia; and the predatory Anomalocaridida (see also here)
    • Cuticular skeleton of most panarthropods was not calcified, so only preserved in rare cases (e.g., Burgess Shale)
    • Some forms ("armored lobopods") did have calcified plates and spikes; may be represented in Small Shelly Fauna of Tommotian Age, Early Cambrian

Arthropoda is united by the following synapmorphies:

• Jointed appendages
  • In primitive forms most limbs are similar, but in most various limbs are highly specialized
  • Ancestrally limbs seem to be biramous: one branch for locomotion, the other (dorsal) branch being a gill
  • Various derived forms lose the gills to become uniramous
• Complete body segmentation
  • Body of primitive forms is homonomous (all body segments are very similar), but in vast majority there is some degree of tagmatization (specialization and fusion of joints)
  Compound eyes
• Highly cephalized

Possible Arthropoda Phylogeny (combines some molecular and some morphological studies:

Arthropoda

• Myriapoda
• Schizoramia
  • Arachnomorpha
    • Marrellamorpha
    • Advanced arachnomorphs
      • Chelicerata
        • ?Pycnogonida
        • Xiphosura
        • Advanced chelicerates
          • Eurypterida
          • Arachnida
      • Trilobita
  • Mandibulata
    • Crustacea
    • Hexapoda (may be nested inside Crustacea)

Myriapoda

• Traditionally allied with hexapods into Uniramia (based on shared presence of uniramous limbs, terrestrial habit, head capsule, and tracheae
• However, recent molecular studies put Hexapoda within Crustacea, and Myriapoda near base of Arthropoda
• Myriapods are VERY homonomous
• Terrestrial
• Includes Diplopoda (millipedes: mostly herbivores and detritivores) and Chilopoda (centipedes; primarily carnivores)
• Body fossils of myriapods back to late Silurian; possibly older trace fossils
• Among the first arthropods (and thus first animals) to become terrestrial
• Fossils present but rare; include the spectacularly large Arthropleuridea

• Allied with Trilobita and early Paleozoic marrellamorphs by shared presence of head shield followed by segmented (ancestrally) thorax
• Anterior segments fused to form prosoma (bearing the head and thorax, with six pairs of appendages); posterior form opisthosoma (primitively segmented); often followed by a telson, or tail spine
• First appendage of chelicerates: chelicerae, claws
• Second appendage: pedipalps, of a variety of functions
• Breath by book gills (book lungs in the terrestrial arachnids, and even trachaea in some specialized small-bodied arachnids)
• Several major groups:
  • Pycnogonida (sea spiders)
    • Possible chelicerates, highly reduced anatomically
    • Have chelicerae, but may have evolved them independantly
    • Very little fossil record
  • Xiphosura (horseshoe crabs)
    • Aquatic (most marine, but a few fossil brackish and freshwater forms), but can come onto land for short periods of time
    • Fossil record from Silurian onward
    • Early forms and larvae of modern forms are VERY trilobite-like
    • "Xiphosura" sensu lato may be paraphyletic with respect to other chelicerates
    Eurypterida (sea scorpions)
    • Ordovician through Middle Permian, but heyday is Silurian and Devonian
    • Predatory: chelicerae were powerful claws
    • Anteriormost six segments of opisthosoma typically very broad: form mesosoma
    • Opisthosoma flexible dorsoventrally, with tergites (half-bands) of cuticle above and below
    • Posteriormost appendages modified into paddles: some were probably very good swimmers
    • Reached large sizes (more than 2 m long)
    • Trace fossils suggest some could survive on land for short times
    • Traditionally lumped with Xiphosaura into "Merostomata", but this is probably a paraphyletic assemblage (essentially "marine chelicerates" or "non-arachnid chelicerates")
    • Seem to have been more heavily mineralized than xiphosurans or arachnids, possibly because of larger body size
    Arachnida (arachnids: scorpions, whip scorpions, pseudoscorpions, mites, ticks, daddy longlegs, spiders, and their extinct allies)
    • Terrestrial chelicerates with book lungs (although early scorpions had gills)
    • Silurian (scoprions) onward: among the first terrestrial animals
    • Pedipalps end in maxillae; posterior four pairs of limbs are walking limbs
    • Highly diverse: carnivores, hebivores, detrivores, parasites, etc.
    • Some mid-Paleozoic forms show origins of spiders (prior to fusion of opisthosoma segments)

Trilobita (trilobites)

• Culturally, are to the Paleozoic Era and to Invertebrate Paleonology what dinosaurs are to the Mesozoic Era and to Vertebrate Paleontology!
• Extremely common fossil makers, particularly in early and mid-Paleozoic
• Range from Early Cambrian to the Permo-Triassic Extinction
• Exoskeletons were highly calcified, and thus well preserved
• Body divided into three lobes, two different ways:
  • Antero-posteriorly into a cephalon (head, containing most of the viscera), thorax (generaly with many segments, containing the limbs), and pygidium (fused "tail" segments)
  • Medio-laterally into a central axial lobe (containing the nervous and digestive systems) and a pair of lateral pleural lobes (overlying the spread of the limbs, including their gills)
• Very diverse feeding habits, including:
  • Predation/scavenging: primitive state, includes the largest forms
  • Particle feeding: most common forms (numerically and taxonomically), kicking food into their mouths
  • Filter feeders
  • Chemosymbiotic forms: found in deep water black shales
  • Pelagic planktonivers: small-bodied, large-eyed swimmers
  • Plankton themselves: the tiny eyeless agnostids

Crustacea

• Extemely morphologically diverse
  • Only non-reversed synapomorphy is the order of the cephalic appendages: antennule, antenna, mandible, maxilla (if Hexapoda lies WITHIN Crustacea, then even this falls apart)
  • Many have fused cephalic segments with cephalic shield, or fusion of cephalon and thorax into cephalothorax or carapace
• Extremely taxonomically diverse (second only to Hexapoda)
• Mostly aquatic, but some (isopods, some crabs, etc.) are terrestrial and many freshwater
• Remipedes are very homonomous; represent basalmost living crustaceans
• Carapaces of many groups can be moderately to heavily calcified, yielding good fossil record
• Groups with best fossil record:
  • Cirripedia (barnacles), from Silurian onward (with relatives in Cambrian)
  • Ostracoda (ostracods), BEST crustacean record, from Ordovician onward
  • Branchiopoda (water fleas and clam shrimp), from Silurian onward
  • Malacostraca (shrimp, lobsters, crayfish, crabs, etc.), from Devonian onward

Hexapoda

• Includes basal forms and true Insecta (11 abdominal segments)
• Have three pairs of thoracic limbs
• Within Insecta is Pterygota, the winged insects (first organisms to fly)
• Within pterygotes are Neoptera, the folding-wing insects
• Hexapoda first appears in Devonian, Pterygota in Pennsylvanian
• Almost all are terrestrial; remaining ones are almost all freshwater
• Some primitive pterygotes in Pennsyvlanian had 75 cm wingspans
• Insect fossil record is actually quite good, but largely overlooked (specimens recovered by same techniques as pollen)
• Recent molecular studies place Hexapoda WITHIN Crustacea, making them "land shrimp"!

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