Arthropod Appreciation An Overview of the History and Diversity of the Phylum

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Arthropod AppreciationAn Overview of the History
and Diversity of the Phylum

• Christopher E. Carlton
• Louisiana State Arthropod Museum

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But, first a review of metazoan origins and body
plans
Genesis 120 And God said, "Let the water teem
with living creatures, and let birds fly above
the earth across the expanse of the sky." 21 So
God created the great creatures of the sea and
every living and moving thing with which the
water teems, according to their kinds, and every
winged bird according to its kind. And God saw
that it was good.
Day 4, 3,500,000,000 ybp. One of several
explanations of the origin of animal life.
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Day 5, 700,000,000 ybp. Metazoans evolved.
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How metazoans originated is a bit of a mystery.
From a blastea or gastrea?
From a syncytium?
Or maybe a plankula?
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As is the evolution of a coelom.
Or from gonocoels?
Enterocoelic pouching?
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Dominant metazoan phyla. Monophyletic or
polyphyletic?
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And the beginning of entomology, worms!
Body plan of an annelid in transverse section
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Body Plan of PhylumArthropoda--external

• Bilateral symmetry
• Body segmented with a tendency towards
  consolidation into tagma
• Well-developed, typically rigid exoskeleton
• Multiple jointed legs (highly variable)

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Body Plan of PhylumArthropoda--internal

• Coelom reduced, body cavity an open hemocoel
• Circulatory system open (lacunar)
• Gut complete
• Nervous system annelid like
• Growth through molting of old cuticle (ecdysis)

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Before the Arthropods
NOW
The ediacarian soft bodied fauna
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Origin of Arthopods During
NOW
Walcotts Burgess Shale Quarry, Banff, British
Columbia
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The Cambrian Explosion
Branchiocharis
Canadaspis
Sancticaris
Marrella
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Progression of Arthropod Body Forms the Paleozoic
NOW
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Dominance of Trilobites
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Trilobites the First Megafauna
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Predators and Competitors
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Invasion of the Land
NOW
What is the major challenge of water- land
transition?
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Respiration!
Book lung respiration (Chelicerates)
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Tracheal respiration (myriopods, insects)
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Carboniferous Diversification of Insects
NOW
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Predominance of Primitive Forms and Evolutionary
Experiments
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Emergence of Modern Arthropods the Mesozoic
NOW
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Major Groups of Extant Arthropods Terrestrial
Chelicerates
Scorpion (Scorpionida)
Black Widow Spider (Aranea)
Pseudoscorpion (Pseudoscorpionida)
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Major Groups of Extant Arthropods Marine
Chelicerates
Horseshoe Crab (Merostomata)
Sea Spider (Pycnogonida)
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Major Groups of Extant Arthropods Crustaceans
Lobster (Decapoda)
Primitive Crustaceans (Remipedia)
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Major Groups of Extant Arthropods Myriopods
Centipede (Chilopoda)
Millipede (Diplopoda)
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Major Groups of Extant Arthropods Insects
Lace bug (Hemiptera)
Swallowtail butterflies (Lepidoptera)
Fulgorid plant hopper (Hemiptera)
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Rhinocerus beetle (Coleoptera)
Pselaphid beetle (Coleoptera)
Nitidulid beetle (Coleoptera)
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Extant Arthropods Insect larvae(the key to
success for the sophisticated insect)
Hawkmoth larvae (Lepidoptera Sphingidae)
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• Conceptual issues
• Blauplan and body plan-architectural and
  functional commonalities of related organisms
• Archetype-a construct that displayed the unity of
  composition that can be found among related taxa
• Hypothetical ancestor-a common ancestor that must
  have displayed most features shared by related
  organisms
• Phylotype-morphological similarity early in
  development, diverging as development progresses
• Cladistic ancestors-a taxon from which two sister
  taxa diverge
• Genome and developmental ancestors-an extension
  of the ancestry concept that encompasses the
  genotype in the organization of developmental
  cellular fields

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The emergence of cladistic methodology
Willi Hennig 1913-1976 (from Wikepedia)
German Diptera systematist and the father of
modern phylogenetic systematics. Hennis, W. 1966.
Phylogenetic Systematics.
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Are Arthropods monophyletic? Yes!
Apomorphies in support of this 1. Exoskeleton
formed from chitinous cuticle. 2. Appendages
with claws 3. Components of the primary coelom
form metanephridia, ostiate heart, and mixocoel
body cavity 4. Segmentation, with intrinsic
segmental musculature 5. Cephalon comprising a
number of fused segments
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Are Arthropods monophyletic? No!
Arguments in support of this 1. Compound eyes
have involved numerous times and obviously
unrelated phyla (arthropods and annelids) 2.
Jointed appendages might have been present in
some extinct non arthropod organisms 3. Common,
chemically driven physiological mechanisms in
non-arthropods 4. Molecular studies have not
solved the problems of determining relationships
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http//www.peripatus.gen.nz/Taxa/Arthropoda/Index.
html
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Sources and suggested reading
Brusca and Brusca Invertebrates. Gould, S. J.
Wonderful Life. Briggs, Erwin, Collier. The
fossils of the Burgess Shale. Fortey and
Thomas. Arthropod Relationships. Margulis and
Schwartz. Five Kingdoms, an illustrated Guide to
the Phyla of Life on Earth. Hyman, L. Treatise
on Invertebrate Zoology http//www.peripatus.gen.
nz/Taxa/Arthropoda/Index.html Available for
study in LSAM library