Arthropod Origins and Evolution
Trilobita and Chelicerata

• Eucoelomate protostomes: 3 major (molluscs, annelids, arthropods) phyla and several minor phyla
• Phylum Tardigrada
• Phylum Onychophora - the missing link?
• The arthropod body plan
• Synapomorphies, constraints, and consequences
• Three views of how arthropods evolved
• Probable evolutionary relationships of the 4 subphyla of arthropods: 2 main issues
• Subphylum Trilobita
• Subphylum Chelicerata
• Class Pycnogonida
• Class Merostomata
• Class Arachnida

Coelomate Protostome relationships

• Monophyletic
• Synapomorphy
• True coelom via Schizocoely (and other embryological traits — review)
• Three major phyla (molluscs, annelids, arthropods); annelids and arthropods united by segmentation

Coelomate Protostome relationships

• Three major Phyla plus several minor phyla, including:
• Onychophora
• Tardigrada
• Share segmentation

Phylum Onychophora — the missing link between annelids and arthropods?

• Velvet worms
• About 70 species
• Terrestrial, Southern hemisphere
• Fossils (marine) dating to Cambrian
• "Intermediate" between annelids, arthropods

Phylum Onychophora

• Annelid-like features
• Cuticle soft, permeable
• Well developed muscles in body wall
• Muscles, ganglia, neprhidia arranged metamerically down the length of the body
• No joints
• Little cephalization

Phylum Onychophora

• Arthropod-like features
• Appendages*
• Molting
• Reduced coelom
• Tracheae/spiracles*
• Large brain
  *Homology disputed

The Arthropod Bauplan

• General characteristics
• Segmentation/Metamerism/Tagmosis
• Open circulatory system
• Well developed
  • Appendages
  • Respiratory systems (trachea or gills)
  • Sensory organs
• Metamorphosis common

Phylum Arthropoda

• Synapomorphies
• Arthropod eyes
  • Compound

Phylum Arthropoda

• Synapomorphies
• Arthropod eyes
• Hard cuticle
• Jointed appendages

Phylum Arthropoda

• Synapomorphies
• Arthropod eyes
• Hard cuticle (chitin)
  • Waxy epicuticle
  • Mineralized procuticle
  • Secreted by epidermis
• Jointed appendages
  • Cuticle thin at joints (and where body segments join)

Phylum Arthropoda

• Synapomorphies
• Arthropod eyes
• Hard cuticle (chitin)
  • Waxy epicuticle
  • Mineralized procuticle
  • Secreted by epidermis
• Jointed appendages
  • Cuticle thin at joints (and where body segments join)
• Consequences of exoskleton:
• Must molt
  • How
  • Significant mortality, evolutionary constraint

Phylum Arthropoda

• Synapomorphies
• Arthropod eyes
• Hard cuticle
• Jointed appendages
• Consequences of exoskeleton:
• Must molt
  • How
  • Significant mortality, evolutionary constraint
• Size limitations!
  • Tubular construction good when small
  • Difficult to accommodate large size - largest examples in Paleozoic
  • Small body size associated with high rates of evolution (think why)

Arthropod evolution - three views

• 1) Polyphyly
• Arthropodization (hard exoskeleton and joints) evolved several times independently
• Some arthropods are more closely related to non-arthropods than they are to other arthropods

Arthropod evolution - three views

• 1) Polyphyly
• 2) Monophyly/parallelism
• Arthropodization evolved independently in the descendants of a single non-arthropodized ancestor

Arthropod evolution - three views

• 1) Polyphyly
• 2) Monophyly/parallelism
• 3) Monophyly/single origin
• Arthropodization evolved only once in a single common ancestor of all arthropods

Arthropod evolution - three views

• 1) Polyphyly
• 2) Monophyly/parallelism
• 3) Monophyly/single origin
• #2,3 tough to distinguish, but monophyly/single origin is the prevailing view

Arthropods - four subphyla

• Trilobita
• Chelicerata
• Crustacea
• Uniramia

Arthropod relationships - two main issues

• Evolution of biramous appendages
• Y- shaped
• Shared by Trilobita and Crustacea
• Lost in some crustaceans

Arthropod relationships - two main issues

• Evolution of mandibles
• Shared by Crustacea and Uniramia
• Derived from same segment
• Some differences in detail
• Monophyly of the Mandibulata widely accepted
• Relative position of Trilobites and Chelicerates is debated.

Subphylum Trilobita

• All extinct by the end of Paleozoic
• Flourished in Cambrian, declined but still abundant until middle and decline in last third of Paleozoic
• They are major motile predators on the benthic sessile and crawling forms of the early Paleozoic
• Decline due to predation by cephalopods, chelicerates?
• Replaced by crustaceans?
• Climate change?

Subphylum Trilobita

• Three tagmata:
• Cephalon
• 5-6 fused segments
• Pair of antennae
• 4-5 pairs biramous appendages
• Thorax
• More biramous appendages
• Pygidium
• Still more biramous appendages

Subphylum Trilobita

• Defenses against predation
• Spines
• Rolling

Subphylum Trilobita

• Primitive characteristics
• No jaws - use gnathobases (bases of legs) to process food (similar to Paleozoic chelicerates and early crustaceans)
• Appendages not specialized
  • Numerous
  • Not specialized for feeding
  • No chelae

Subphylum Trilobita - Lifestyles

• Crawling
• Fairly broad
• Well developed eyes

Subphylum Trilobita - Lifestyles

• Crawling
• Burrowing
• Often streamlined
• Trunk reduced
• Eyes on stalks

Subphylum Trilobita - Lifestyles

• Crawling
• Burrowing
• Planktonic?
• Eyes sometimes lost
• Spines

Subphylum Chelicerata

• Three classes

Chelicerate History

• Present in early Paleozoic seas, reach greatest diversity and abundance in mid-Paleozoic (some of the most important predators in Paleozoic [especially mid-Paleozoic] seas); some lineages move out onto land in the Silurian (early mid-Paleozoic)
• Marine eurypterids and most merostomatans decline in the late Paleozoic (replaced by crustaceans?), but persist until the end of Paleozoic, then are eliminated by mass extinction (except for a tiny thread of merostomatans that persists to the present)
• Terrestrial chelicerates (scorpions, offshoot of eurypterids) evolve out onto land in Silurian, diversify as Arachnids in the late Paleozoic coal swamps, make it through the end-Paleozoic and end-Mesozoic mass extinctions and remain diverse today.

Subphylum Chelicerata Synapomorphies

• Two tagmata:
• Cephalothorax
• Abdomen

Subphylum Chelicerata Synapomorphies

• Two tagmata
• First pair of appendages form chelicerae
• Four pairs of walking legs (usually)

Class Pycnogonida

• All marine
• About 1000 species
• Body plan
• Tiny abdomen (gonads extend into legs -- male often broods eggs)
• Sucking proboscis
• Eight legs (usually)
• No special respiratory or excretory organs

Class Merostomata

• Two Orders
• Eurypterida - Sea scorpions -- important Paleozoic predators
• Xiphosura - Horseshoe crabs -- prominent group in Paleozoic, a thin thread today
• Synapomorphy
• Book gills
• Specialized appendages!
• Gnathobase feeding (bases of legs)
• Larva similar to trilobites and sea spiders

Order Eurypterida

• Could be large, predatory
• Up to 3 meters!
• Many with pincers (chelae)
• Specialized swimming appendages probably scooted, sailed up off bottom but probably not pelagic

Order Xiphosura

• Five living species
• Walking legs with chelae
• Reproduce in intertidal (refuge from Paleozoic marine predators?)

Class Arachnida

• Very successful
• Terrestrial!!!
• >> 50,000 species
• Respiration via book lungs and/or tracheae (convergent)
• Numerous orders, including
• Scorpionida - scorpions
• Acari - ticks and mites
• Araneae - spiders

Order Scorpionida

• About 1200 species
• Four pairs of walking legs
• Venom of most not dangerous
• But, 5000 people/year die from stings!
• Aquatic fossils, similar to eurypterids -- first arachnids out on land (Silurian)
• Live birth, parental care

Order Araneae

• About 35,000 species
• Active predators
• Chelicerae form fangs
• Some have good vision, possibly image-forming eyes
• Four pairs of walking legs
• Cephalothorax and abdomen

Order Araneae

• Abdomen unsegmented in most
• Spinnerettes modified appendages (otherwise appendages lost on abdomen)

Order Araneae

• Highly diverse styles of prey capture

Order Acari

• About 30,000 species
• Very important effects on human health, economy
• Allergans - Dust mites
• Disease vectors - Lyme disease
• Crop pests
• Bee parasites
• Some live in human hair follicles!