Lecture 22. Evolution of Multicellularity, Colonization of Land.

1. Lecture 22. Evolution of Multicellularity, Colonization of Land. reading: Chapter 5

2. Vendian Period 650-543 Ma before 600 Ma fossils are rare, simple mostly trace fossils worm-like creatures moving across sediment surfaces radially-symmetric impressions of animals 575-544 Ma wider diversity of fossils radially symmetric fossils and bilaterally symmetric fossil (bilaterians) Biological interpretation controversial. Several distinct body plans, many don’t exist today. Vendian=Ediacaran Diverse algae, simple animals.

3. Vendian Fauna Cyclomedusa - most common and widespread Vendian fossil - few mm to m in diameter - bottom-dwelling, like a sea anemone Cnidaria stinging cells sea anemones, jellies, sea pens radial symmetry simple body cavity • Charnia • - one of the largest Vendian fossils • - up to a m in length • disk shaped holdfast attaching to the bottom • similar to “sea pens”

4. Vendian Fauna, cont. Eoporpita - thick tentacles surrounding central body - 6 cm across - cnidarian? (jellyfish) Nemiana - simplest of all Vendian fossils - sac-like body - never found alone, always in colonies - alga or sea anemone?

5. Vendian Fauna, cont. Pteridinium - not known what it once was - usually found squished flat - bottom-dweller Echinoderms sea urchins, sea stars exoskeleton with plates Arkarua - small disk-like fossil - echinoderm?

6. Vendian Fauna, cont. Spriggina - soft-bodied, only found in the Precambrian - 3 cm long - annelid worm or arthropod Arthropod- segmented body appendages on segments molting exoskeleton Annelid worm earthworms, leaches complex body cavity Dickinsonia - annelid worm or a cnidarian?

7. Vendian Fauna, cont. Tribrachidium - bizarre, disk-shaped fossil - three-part symmetry - may be cnidarian or echinoderm Mollusc- shells body cavity, muscular foot mantle and gills Kimberella - tough shell that stuck up out of the sediment covering the organism - box jellyfish or mollusc?

8. Duoshantuo Formation 570 Ma Sponge sessile, tube shaped filter feeders no true tissues Exceptionally preserved fossils in phosphate minerals. Preserves soft-tissue fossils, cellular structures. Nuclei and cytoplasmic contents (organelles) preserved! Well-preserved algae/seaweed. Sponges most primitive metazoans - multicellular animals with differentiated tissues few occurrences in Precambrian Preserved animal embryos - bilaterian animals, although no adults are seen!

9. Terminal Proterozoic Small shelly fossils unknown what they were Microdictryon Cloudina

10. Cambrian 543-490 Ma • First 10-12 Ma: • - few animal taxa • - some Vendian organisms survived • - diverse sponges, rare cnidarians • - small skeletal fossils • - complex tracks, trails, & burrows (more complex behaviors) • After that: • - abundant crown group animals • - diversification of crown group animals • - wide expansion of mineralized skeletons • arthropods the most abundant fossils • - > 80 skeletal taxa • - some with 3-fold symmetry • - mollusc spiral shells, arthropods, bivalve shells of brachiopods

11. Burgess Shale Yoho National Park, Canada 40 Ma after start of Cambrian. Exceptional preservation of soft-bodied organisms. Wide diversity of fossil invertebrates. Anomalocaris large >60 cm arthropod-like predator

12. Burgess Shale, cont. Marrella small arthropod similar to a trilobite one of the most common fossils Trilobites several species some soft appendages preserved

13. Burgess Shale, cont. Vauxia sponge with branches Tuzoia bivalve crustacean similar to brine shrimp Wiwaxia not sure what it is

14. Burgess Shale, cont. Ottoia common worm carnivorous burrowed Hallucigenia one side has tentacles one has spines tentacles have claws at the end, so they were probably the “feet” don’t know which end is the head, which the tail

15. Snowball Earth Episodes Early evolution of animals coincides with global glaciations. Possibly 3 glaciation events. Duoshantuo animal embryos and sponges at 570 Ma.

16. What Factors Contributed to the Cambrian Explosion? • Increasing oxygen concentrations in the Neoproterozoic - limits size due to energy yields - larger organisms need more oxygen - early animals limited by diffusion - animals need at least 50% PAL O2 • Decreased temperatures from cold T’s - global glaciations 800-550 Ma • - disruption in biosphere • - carbon isotopes disrupted • Origination of body armor • - arms race of predatory relationships • Origination of developmental pathways - metazoans have modular body plans • Abundant food sources - microbial mats and stromatolites

17. Land Plants Are Related to Green Plants hornworts mosses vascular plants liverworts Chara Coleochaetae

18. Colonization of Land • Need to obtain traits • to deal with lack of water • to deal with gravity • how to obtain nutrients • new reproductive strategies

19. Traits Needed For Animals to Colonize Land First land vertebrates retained a tail fin, suggesting they spent most of their time in water. First land animals are amphibians. A strong support system - bones, vertebrae Locomotion - limbs (adapt fins to legs) Able to breathe air - get rid of gills, lungs evolved from the swim bladder Prevent drying out - need tear glands to wet eyes - thick skin, scales, keratin coating to prevent drying out Mechanism for hearing - fish have hair cells that sense vibration - evolution of the ear (fluid filled channel with hair cells) Lay eggs on dry land ~310 Ma - amniotic egg

20. Traits Needed For Plant to Colonize Land • Waxy cuticles - keeps plant from drying out • Vascular tissue - transports water to tissues • Woody tissue - supports the structure against gravity • Stomata - allow for gas exchange, minimizing drying out • Pollen, seeds, flowers, fruit - new reproductive strategies, • wind pollination (then insect, animal pollination) • Early land plants (liverworts, hornworts, mosses) • water is a critical part of the life cycle • no cuticle, vascular tissue, woody tissue • reproductive cells swim • can’t grow tall, restricted to wet environments

21. Lecture 23. Mass Extinctions, K-T boundary, Buckyballs. reading: Chapter 5