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ZOO4 HONOURS

ARTHROPOD ORIGINS AND RELATIONSHIPS. ZOO4 HONOURS. Mark Blaxter 2007-2008 www.nematodes.org. Arthropod origins and relationships. Three main questions: Is Arthropoda a monophyletic group? Which non-arthropod groups are most closely related to Arthropoda?

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ZOO4 HONOURS

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  1. ARTHROPOD ORIGINS AND RELATIONSHIPS ZOO4 HONOURS Mark Blaxter 2007-2008 www.nematodes.org

  2. Arthropod origins and relationships Three main questions: Is Arthropoda a monophyletic group? Which non-arthropod groups are most closely related to Arthropoda? What are the relationships among different arthropod groups? NB: these questions are inextricably interlinked…

  3. Arthropod origins and relationships Three main questions: Is Arthropoda a monophyletic group? Which non-arthropod groups are most closely related to Arthropoda? What are the relationships among different arthropod groups? Why do these questions matter? • Inform models of metazoan evolution - what is the pattern of evolution of morphological and physiological novelty? Are there general evolutionary ‘laws’? • Arthropods are ~80% of described diversity.

  4. Is Arthropoda Monophyletic? Monophyly: a natural group that includes ALL the descendents of a particular ancestor So: Reptilia is only MONOPHYLETIC if we include Aves The taxon “marine mammals” is not monophyletic within Mammalia (as it excludes groups that last shared a common ancestor with some non-marine mammals since the last common ancestor of all marine mammals)

  5. Is Arthropoda Monophyletic? According to ‘traditional’ systematic taxonomies, there are four extant classes within Arthropoda CHELICERATA spiders and allies MYRIAPODA centipedes and allies CRUSTACEA crabs and allies HEXAPODA insects and allies There is one extinct class TRILOBITA the trilobites

  6. Trilobites Ancient Marine No jaws Gills on limbs

  7. Chelicerates Spiders Ticks and Mites Solifuges Pycnogonids Scorpions Pseudoscorpions Amblypygids Horseshoe crabs Ancient, originally marine, chelicerae, book gills/lungs

  8. Myriapods Ancient originally marine Mandibles Tracheae

  9. Crustacea Ancient Originally marine Mandibles Gills on limbs

  10. Hexapoda Insecta Dipleura Ellipleura Young Terrestrial Mandibles Tracheae

  11. Is Arthropoda Monophyletic? Monophyly: a natural group that includes ALL the descendents of a particular ancestor What defines “Arthropoda”? Bilaterally symmetrical, triploblast, protostomious Coelomate, but coelom reduced to reproductive and excretory systems Body cavity is a haemocoel; circulatory system is open

  12. Is Arthropoda Monophyletic? Monophyly: a natural group that includes ALL the descendents of a particular ancestor What defines “Arthropoda”? Bilaterally symmetrical, triploblast, protostomious Coelomate, but coelom reduced to reproductive and excretory systems Body cavity is a haemocoel; circulatory system is open

  13. Is Arthropoda Monophyletic? Monophyly: a natural group that includes ALL the descendents of a particular ancestor What defines “Arthropoda”? Bilaterally symmetrical, triploblast, protostomious Coelomate, but coelom reduced to reproductive and excretory systems Body cavity is a haemocoel; circulatory system is open

  14. Is Arthropoda Monophyletic? Monophyly: a natural group that includes ALL the descendents of a particular ancestor What defines “Arthropoda”? Segmented internally and externally Teloblastic growth Tagmosis (minimally cephalon and trunk) Cephalon includes nonsegmental acron and labrum; Cephalon with lateral faceted eyes and median ocelli Each segment with appendages with proximal protopod and distal telopod Gut complex and regionalised

  15. Is Arthropoda Monophyletic? Monophyly: a natural group that includes ALL the descendents of a particular ancestor What defines “Arthropoda”? Segmented internally and externally Teloblastic growth Tagmosis (minimally cephalon and trunk) Cephalon includes nonsegmental acron and labrum; Cephalon with lateral faceted eyes and median ocelli Each segment with appendages with proximal protopod and distal telopod Gut complex and regionalised

  16. Is Arthropoda Monophyletic? Monophyly: a natural group that includes ALL the descendents of a particular ancestor What defines “Arthropoda”? Segmented internally and externally; Teloblastic growth Tagmosis (minimally cephalon and trunk) Cephalon includes nonsegmental acron and labrum; Cephalon with lateral faceted eyes and median ocelli Each segment with appendages with proximal protopod and distal telopod Gut complex and regionalised

  17. Is Arthropoda Monophyletic? Monophyly: a natural group that includes ALL the descendents of a particular ancestor What defines “Arthropoda”? Musculature metameric; No circular somatic musculature; Dorsal and ventral longditudinal muscles present Dorsal CNS ganglia (protocerebrum is ocular, deuterocerebrum antennal) Paired ventral nerve cords and ganglia

  18. Is Arthropoda Monophyletic? Monophyly: a natural group that includes ALL the descendents of a particular ancestor What defines “Arthropoda”? Musculature metameric; No circular somatic musculature; Dorsal and ventral longditudinal muscles present Dorsal CNS ganglia (protocerebrum is ocular, deuterocerebrum antennal) Paired ventral nerve cords and ganglia

  19. Is Arthropoda Monophyletic? Monophyly: a natural group that includes ALL the descendents of a particular ancestor What defines “Arthropoda”? Musculature metameric; No circular somatic musculature; Dorsal and ventral longditudinal muscles present Dorsal CNS ganglia (protocerebrum is ocular, deuterocerebrum antennal) Paired ventral nerve cords and ganglia

  20. Is Arthropoda Monophyletic? Monophyly: a natural group that includes ALL the descendents of a particular ancestor What defines “Arthropoda”? Cilia absent except in some sperm Growth by moulting (ecdysis) mediated by ecdysone Cuticular exoskeleton with chitin and resilin/cuticlin, NOT collagen; sometimes calcified

  21. Is Arthropoda Monophyletic? Monophyly: a natural group that includes ALL the descendents of a particular ancestor What defines “Arthropoda”? Could these characters have arisen multiple times, independently? Could the arthropod “grade” have evolved more than once from a common ancestor with the Annelida? (HOMOPLASY - the convergent evolution of shared characters)

  22. Which non-arthropod groups are most closely related to Arthropoda? (if all arthropods share the same ancestors, they will be monophyletic) ?

  23. Which non-arthropod groups are most closely related to Arthropoda? The common ancestor of all Arthropods was probably a ‘lobopod’, similar to extant onychophorans and tardigrades

  24. Onychophora the velvet worms Terrestrial Soft-bodied Segmented Lobopod legs Rare Viviparous Smallish (cm) patchy fossil record, but present in Burgess Shale and other faunas (marine)

  25. Tardigrada the water bears Terrestrial and marine Oviparous Soft-bodied Segmented Lobopod legs Common Tiny (~1 mm or less) essentially absent from the fossil record

  26. Is Arthropoda Monophyletic? S. Manton and D. Anderson suggested that Arthropoda was polyphyletic Tracheal system Hexapods Uniramous limbs Myriapods 1 Onychophorans Biramous limbs Crustacea 2 No mandibles Trilobites 3 Chelicerates These groups also differ in patterns of segmentation in the head 3 independent origins of the arthropod “grade”

  27. Is Arthropoda Monophyletic? Polyphyletic origin hypotheses have been tested and are less likely than a monophyletic origin Support for monophyly comes from Molecular phylogenetic data (that excludes Onychophora and Tardigrada) Gene arrangements in the mitochondrial genome Patterns of expression of HOX genes during development Analysis of limb development and evolution Close analysis of “tracheal” systems

  28. Which non-arthropod groups are most closely related to Arthropoda? Tardigrada Onychophora Arthropoda Panarthropoda

  29. Which non-arthropod groups are most closely related to Arthropoda? Onychophora Tardigrada Arthropoda Panarthropoda

  30. Do fossils help? Are there “intermediate” fossils that will help in clarifying the relationships of arthropods and other animals?

  31. Do fossils help? Lagerstätten - fossilifierous sediments with exquisite preservation even of soft-bodied fauna and soft parts of those with shells. see http://palaeo.gly.bris.ac.uk/Palaeofiles/Lagerstatten/ Burgess Shale, Canada 505 MYa Sirius Passet, Greenland 518 MYa Chengjiang, China 530 MYa phosphatic sediments Doushantuo, China 590-540 MYa Ediacaran assemblages Ediacara, Australia 560-540 MYa Mistaken Point, Newfoundland Charnwood Forest, UK

  32. The Burgess Shale location, Jasper National Park, Canada

  33. Burgess Shale animals were killed and covered by the collapse of a marine reef

  34. reconstructions...

  35. Cambrian arthropods from the Burgess Shale Some are ~easily~ slotted into “standard” places in Arthropoda Chelicerate Crustacean Trilobite Aysheaia Hallucigenia Onychophorans

  36. Cambrian arthropods from the Burgess Shale ...but for others, it is much less easy to shoehorn them into current taxa Marella Opabinia Anomalocaris Sidneya

  37. Sirius Passet deposits are older, but have some similar taxa Lagannia, an anomalocarid ...and some unique ones: Kergymachela

  38. Chienjiang deposits are still above the origin of arthropods

  39. Doushantuo fossils are small The animal fossils are EMBRYOS

  40. The Ediacara “fauna” is, at best, enigmatic Spriggina, a trace fossil, or an arthropod, or an annelid? Pteridinium Dickinsonia Tribrachidium

  41. Do fossils help? Are there “intermediate” fossils that will help in clarifying the relationships of arthropods and other animals? Current available fossils post-date the separation of the major animal phyla, or are too enigmatic to really tell us... ... but careful analysis can yield useful information.

  42. end of part one ...

  43. Arthropod origins and relationships Three main questions: Is Arthropoda a monophyletic group? Which non-arthropod groups are most closely related to Arthropoda? What are the relationships among different arthropod groups?

  44. Which non-arthropod groups are most closely related to Panarthropoda? ? “lobopod” intermediate

  45. Which non-arthropod groups are most closely related to Panarthropoda? ARTICULATA hypothesis: ARTHROPODA plus ANNELIDA “lobopod” intermediate

  46. Which non-arthropod groups are most closely related to Panarthropoda? Some of the arthropod “defining” characters are SHARED with ANNELIDA(ragworms, earthworms, leeches and allies) Segmented Appendages @ one per segment CNS with ventral cords and segmental ganglia Dorsal tubular heart 4 or 5 bands of longditudinal muscles Ontogeny of segments (teloblastic) including engrailed expression ? Coelomic cavity origins/development ARTICULATA hypothesis: ARTHROPODA plus ANNELIDA

  47. Coelomata/Pseudocoelomata hypothesis of metazoan relationships This hypothesis groups Arthropoda with Annelida and Mollusca Annelida+Panarthropoda = ARTICULATA Articulata+Mollusca+others = PROTOSTOMIA Protostomia+Deuterostomia= COELOMATA

  48. BUT Arthropods differ from Annelids in important ways • Annelids do not moult

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