EAR 119 Arthropods

EAR 119 Arthropods

1. Arthropod Characteristics

   • Bilateral symmetry
   • Segmented bodies and joint appendages
     • Each segment = repetition of a theme (internal organs + external appearance)
     • Modification of the theme result in specialised body regions (tagmata)
       • Particularly in the head region (cephalon) appendages are modified to aid feeding
       • Main subdivision of arthropods are all based on the arrangements of these tagmata and their appendages
   • Exoskeleton
     • Armour against predation (reinforced by chitin, and in some forms by CaCO3, Ca_phosphate)
     • Protection against desiccation
     • Framework for support of soft parts and muscle attachment
   • Moulting (ecdysis)
     • Blue crabs
       • Crabs moult about 72 times in their lives (1-2 yrs). Young crabs moult very frequently. Food availability and environmental conditions impact the moutling process
       • Crab cracks its shell open from back and backs out
         • Crab is now temporarily soft shelled
       • Crab first expands its new shell by pumping water into its body
       • Takes about 3 days for soft shell to harden - crab very vulnerable during this time
         • Over 70% of arthropod mortality
     • The earliest evidence of moulting
       • Marella splendens
       • An arthropod in moult has a soft body + moulting takes place relatively quickly = excellent preservation + luck
   • Haemocoel (blood-filled cavity containing heart)
   • Respiration
     • Aquatic forms
       • Gills with vascularised, thin-walled tissue specialised for gas exchange
     • Terrestrial forms
       • Book lungs (e.g. spiders) or a network of fine tubes: tracheae air tubes + spiracles (e.g. insects)
   • Compound eyes
     • Composed of thousands of individual visual units
     • Exceptionally good at detecting movement

2. Main arthropod groups

   • Subphylum Chelicerata
     • Spiders, scorpions etc.
     • Eurypterids
     • Chelicerata
       • Cambrian - Recent
       • No antennae
       • Body divided into two tagmata (Specialised collection of segments)
         • Prosoma at the front (head + thorax) with 6 segments
         • Opisthosoma at the back (abdomen) with max 12 segments
       • Appendages
         • 1st
           • Chilcerae in front of mouth: modified for feeding
         • 2nd
           • Pedipalps (palps) e.g. pincers of scorpions
         • 3rd - 6th
           • 4 walking legs
       • 3 classes
         • Arachnida
           • Silurian - Recent
           • Includes spiders, scorpions, mites and harvestmen
             • 100,000 living species
           • Primarily terrestrial
           • Fossil arachnids >1,700 described species - well over half spiers
           • Mesozoic arachnids
             • Nephila jurassica
               • Largest known fossil spider
               • Golden orb-weaver spider from Mid Jurassic, China
               • Preserved in volcanic ash within lake deposits
               • Body 2.5cm wide - Legs 6.3cm long
               • Longest ranging spider genus known - extended range by 130 Ma
             • Amber
               • Most common inclusions = insects living on tree bark + associated arachnid predators
               • Single largest source of fossil arachnid species
               • Almost 200 known amber deposits. Oldest = Mid-Carboniferous
               • Basaltic Amber (Most known)
                 • Eocene (49 Ma)
               • Computed tomography (CT) scanning = 3D imaging used to study fossils trapped in amber
         • Merostomata
           • Cambrian - Recent
           • Subclass Xiphosura
             • Cambrian - Recent
             • Horseshoe crab
               • Compound + ocelli eyes
               • 4 living species
             • Almost 100 fossil species
             • Sparse record
           • Subclass Eurypterida
             • Mid Ordovician - Permian
             • Relatively rare
             • Sea scorpions
               • Aquatic (Marine and freshwater)
               • Carnivorous - top predators of their day
               • Up to 2.5m long - largest ever arthropods
         • Pycnogonida
           • Sea spiders
           • Cambrian - Recent
   • Trilobita
   • Subphylum Mandibulata
     • Crustacea
       • >800 families
       • Body composed of a 5-segmented head and a more or less distinct thorax and abdomen
       • Abdomen = 6 segments
       • Carapace usually present; covering head and thorax (Cephalothorax)
       • Mandibles (limbs that function as basic jaws)
       • Compound eyes present in most taxa at some point during life cycle
         • Eyes commonly raised on stalks
       • Ostracods - 'sea shrimp'
         • Abundant and widespread aquatic organisms with bivalved carapaces
         • Body almost entirely enclosed by carapace
         • Small - 0.5-1.5mm long
         • Most benthic (living at lowest water level) a few planktonic
         • Early Cambrian - Recent
     • Hexapoda
       • Insects
         • 3 tagmata - Head, thorax, abdomen
         • 6 walking legs
         • Pair of mandibles (lower jaws)
         • Single pair of antennae
       • Devonian - Recent
       • Pterygotes (with wings)
       • Apterygotes (without wings)
       • Poor preservation potential - Fossil record essentially limited to Lagerstatten
         • Exoskeletons predominantly not calcified
       • Where are they found?
         • Rhynie Chert
           • Chert outside Rhynie, Scotlant
           • Early terrestrial ecosystem: Lower Devonian
           • Earliest known insect discovered = Rhyniognatha hirsti
             • pterygote
             • Only the head is known
         • Crato Formation (NE Brazil)
           • Lower Cretaceous
           • Laminated, micritic limestones from freshwater lake
           • World's most remarkable Cretaceous insect fauna
           • Mayflies, damselfies, dragonflies, cockroaches, termites, locusts, crickets, grasshopers, earwigs, beetles, weevils, flies, wasps, bees
     • Myriapoda
       • Chilopoda = centipedes
         • Flattened bodies, more flexible; each segment has one pair of legs
         • Carnivorous - poisonous fangs
         • Prefer drier climates
       • Diplopoda = millipedes
         • Cylindrical bodies, less flexible; each segment has 2 pairs of legs
         • Detritovores/herbivores
         • Prefer moist conditions
       • Mandibles (biting jaws)
       • Head region followed by many similar segments
       • Single pair of antennae
       • Fossil record
         • Record uneven both in a chronological and taxonomic sense
         • Poor preservation potential: fossil record essentially limited to Lagersatten
         • Strongly biased toward Diplopoda (millipedes) - robust cuticle reinforced with calcium carbonate
         • Silurian - Recent
         • Carboniferous is richest, although only sampled from limited habitats
         • Only a few myriapod fossils known from Permian and Mesozoic
         • First animals to colonise on land
         • Mid Silurian from Cowie Harbour near Aberdeen
           • Millipede: Cowiedesmus
         • Most deposits associated with coal swamps
         • Capable of reaching large sizes (Arthropleura = 2.5m long)

3. Earliest Arthropods

   • Exceptionally preserved arthropods are known from a number of Cambrian strata world-wide
     • Chengjiang (Early Cambrian, China)
     • Sirius Passet (Early Cambrian, Northern Greenland)
     • Burgess Shale (Middle Cambrian, Canadian Rocky Mountains)
       • Laminated shales
       • Periodic burial by rapid sediment slides
       • Exceptional preservation = Konservat Lagerstätten

4. The Crown Group/ Stem Group Concept

   • Crown group = The last common ancestor (LCA) of a group of organisms and all of its decendants
   • Stem group = Organisms in the lineage leading towards the LCA of a crown group
   • Total group = organisms from both Crown group and Stem group

Images and files in the map