1. Phylum Arthropoda: stuff to know
Classification
Superclasses, classes, orders
Stratigraphic and paleoenvironmental ranges
Morphology (bold terms in text)
Genera:
Elrathia, Isotelus, Phacops, Cryptolithus, Peronopsis
Fossils & Evolution—Arthropoda

2. Arthropoda—Phylum overview
Highly specialized invertebrates
Possess a true coelom (reduced)
Bilaterally symmetrical
Body divided into discrete segments, including head region with nervous and sensory features
Paired serial appendages (some highly specialized)
Body covered with a chitinous exoskeleton (sometimes secondarily calcified)
Fossils & Evolution—Arthropoda

3. Arthropoda—Phylum overview
Extremely diverse: trilobites, insects, crabs, ostracodes, barnacles, spiders, scorpions, ticks, centipedes, limulids, eurypterids
Specialization and rapid rates of evolution make certain groups extremely useful in biostratigraphy and paleoecology
Fossils & Evolution—Arthropoda

4. Arthropoda—Phylum overview
Probably evolved from annelid worms
Conspicuous segmentation links the two phyla
Main evolutionary innovation was “sclerotization” — hardening of outer cuticle to form an exoskeleton
Exoskeleton is secreted by epidermis; serves for protection and locomotion (muscle attachment)
Fossils & Evolution—Arthropoda

5. Arthropoda—Phylum overview
Body plan
Groups of segments form body regions (tagmata) that perform specialized functions
Each segment possesses a pair of jointed appendages
Uniramous = pair of single appendages
Biramous = pair of double appendages
Growth is periodic (successive instar stages facilitated by molting of exoskeleton)
Fossils & Evolution—Arthropoda

6. Fossils & Evolution—Arthropoda
Body regions—Tagmata
Fossils & Evolution—Arthropoda

7. Fossils & Evolution—Arthropoda
Paired appendages
biramous
uniramous
Fossils & Evolution—Arthropoda

8. Fossils & Evolution—Arthropoda
Instars
ostracode
trilobite
Fossils & Evolution—Arthropoda

9. Fossils & Evolution—Arthropoda
Molting
Fossils & Evolution—Arthropoda

10. Fossils & Evolution—Arthropoda

11. Arthropod diversity through time
Fossils & Evolution—Arthropoda

12. Fossils & Evolution—Arthropoda
Trilobites
Dorsal surface divided into three lobes
Axial lobe and two pleural lobes
Tagmata include cephalon, thorax, pygidium
Appendages:
One pair of uniramous antennae
Many pairs of biramous limbs
Fossils & Evolution—Arthropoda

13. Trilobite skeletal morphology
Cephalon
Cranidium, glabella, eyes, fixed cheek, free cheek, facial sutures, border, genal spine/angle
Thorax
Axial rings, axial furrow, pleura
Pygidium
Fused axial rings and pleura
Fossils & Evolution—Arthropoda

14. Fossils & Evolution—Arthropoda
Skeletal morphology
Fossils & Evolution—Arthropoda

15. Fossils & Evolution—Arthropoda
Cephalon
Fossils & Evolution—Arthropoda

16. Cephalon—Facial sutures
Genal angle
Pygidium
Genal spine
Fossils & Evolution—Arthropoda

17. Fossils & Evolution—Arthropoda
Trilobites (cont.)
Trilobite eyes are the most ancient visual system known
Eyes are compound (many lenses) and highly variable in shape and position
Some even on ventral side of body!
Each lens (or half-lens) is a single crystal of calcite
Optical c-axis oriented normal to visual surface
Filters polarized rays; eliminates double vision
Fossils & Evolution—Arthropoda

18. Fossils & Evolution—Arthropoda
Trilobite eyes
Fossils & Evolution—Arthropoda

19. Fossils & Evolution—Arthropoda
Trilobites (cont.)
Order Polymerida
Eyes, facial sutures, five or more thoracic segments
Lower Cambrian to Upper Permian
Order Agnostida
No eyes, no facial sutures, two thoracic segments
Cephalon and pygidium about same size (“isopygous”)
Lower Cambrian to Upper Ordovician
Fossils & Evolution—Arthropoda

20. Fossils & Evolution—Arthropoda
Order Polymerida
Fossils & Evolution—Arthropoda

21. Fossils & Evolution—Arthropoda
Order Agnostida
Fossils & Evolution—Arthropoda

22. Trilobite paleoecology
Occur most abundantly in normal marine shelf environments with good water circulation
Mostly benthic detritus feeders
Some may have been pelagic; even back-floaters
Fossils & Evolution—Arthropoda

23. Lithofacies distribution
Fossils & Evolution—Arthropoda

24. Trilobite evolution and diversity
Early Cambrian trilobites radiated very rapidly into previously unoccupied ecospace
Diversity declined dramatically during end-Ordovician mass extinction
Rarely found in post-Devonian rocks
Fossils & Evolution—Arthropoda

25. Fossils & Evolution—Arthropoda
Trilobite diversity
Fossils & Evolution—Arthropoda

26. Fossils & Evolution—Arthropoda
Other Arthropods
Crustaceans
Ostracodes (microfossils)
Decapods (shrimp, lobsters, crabs, etc.)
Chelicerates
Eurypterids
Limulids (horseshoe crabs)
Fossils & Evolution—Arthropoda

27. Fossils & Evolution—Arthropoda
Ostracodes
Mineralized, bivalved carapace
Cambrian-Recent
“Kidney bean” shells; some highly ornamented
Found in marine and freshwater environments (eurytopic)
Very useful in biostratigraphy and paleoecology
Fossils & Evolution—Arthropoda

28. Fossils & Evolution—Arthropoda
Ostracodes
Fossils & Evolution—Arthropoda

29. Fossils & Evolution—Arthropoda
Ostracode trivia
Male ostracodes posses two copulatory organs and the largest gonads in animal kingdom
Female ostracodes also possess two copulatory organs
Fossils & Evolution—Arthropoda

30. Fossils & Evolution—Arthropoda
Chelicerates
Spiders, ticks, scorpions, eurypterids, limulids
Marine, fresh water, terrestrial
Two tagmata
Anterior tagma (prosoma) contains six pairs of major appendages
Including a single pair of chelicerae (preoral pincer limbs)
Fossils & Evolution—Arthropoda

31. Fossils & Evolution—Arthropoda
Chelicerates
Eurypterids
Two tagmata
Small prosoma; 12 segments on opisthosoma
Mixed benthic and nektic lifestyle
Occur in marine, marginal-marine and freshwater environments
Ordovician-Permian, but mainly Middle Paleozoic
Gigantic forms up to 2 m long
Fossils & Evolution—Arthropoda

32. Fossils & Evolution—Arthropoda
Eurypterid
Fossils & Evolution—Arthropoda

33. Fossils & Evolution—Arthropoda
Chelicerates
Limulids
Two tagmata
Large highly convex prosoma; opisthosoma with ten or fewer segments (some fused)
Benthic lifestyle
Occur in marginal marine environments
Silurian-Recent
Good example of “living fossils”
Fossils & Evolution—Arthropoda

34. Fossils & Evolution—Arthropoda
Limulid
Fossils & Evolution—Arthropoda