Lecture 20. Evolutionary trends in animals: invertebrates

Reading: Chapters 32 and 33.

1. Plants and animals evolved together, evolution of animals depended on evolution in plants

a. production of oxygen by photosynthetic bacteria and marine algae facilitated radiation and diversification of early animals

b. evolution of vascular plants, bryophytes Ë gymnosperms Ë angiosperms created new ecological opportunities and environments for animals

2. Major evolutionary trends in invertebrates: key innovations (note location of these on suggested phylogeny of major invert phyla at end of handout, also Fig. 32.3 in book). NOTE: all the major body plans were around 550 MYA-- since then, fine tuning!

a. Cellular organization

i. change from 2 embryonic cell layers (endoderm and ectoderm--diploblastic) to 3 embryonic cell layers (endoderm, mesoderm and ectoderm--triploblastic)

ii. this permits increase in organization and specialization of cells

cells -----> tissues -----> organs -----> organ systems

iii. symmetry of animals: from radial to bilateral

b. Cephalization (evolution headfirst)

i. in bilateral symmetry, possible to identify an anterior (front) and posterior (behind) of an organism

ii. sense organs and feeding organs begin to be concentrated in the anterior

iii. permits the evolution of concentrations of nerve cell -----> ganglia ----->brain, and increasing complexity of neural processing

c. Evolution of the coelom (Fig. 32.5)

i. coelom = space between digestive tract and body wall, lined with mesoderm

ii. in early coelomates, a fluid filled cavity that acts as a hydrostatic skeleton.

iii. cavity also provides place for internal organs

d. Evolution of segmentation

i. similar repeated units permit larger organisms

ii. in annelids (segmented worms), segments are similar

-- form fluid filled compartments that can be stretched or compressed separately-- worm locomotion

iii. segmentation permits specialization of different segments

e. Evolution of the exoskeleton (Arthropods)

i. cuticle makes rigid segmented structures possible, flexible cuticle permits bendable joints. Permits complex locomotion, good barrier to desiccation

ii. accompanying changes in development: molting

iii. changes in respiration: gills (crustacea), book lungs (arachnids) tracheal system (insects)

3. Diversity in animals

a. 97% of all animals are invertebrates!! 27 phyla

b. all vertebrates are in a single phylum-- only 3% of all animal phyla

c. the major phyla (number in parentheses are figures in your text). You will need to know these, and the major key innovations during animal evolution.

i. Porifera: sponges. Specialized cells, no tissues (33.1, 33.2)

ii. Cnidaria = Coelenterata: hydra (33.4), jellyfish haveboth polyp and medusa stages (33.3); sea anemones (polyps only) and coral (medusa only, see fig 33.5)

iii. Platyhelminthes: flatworms. Bilaterally symmetrical, free-living or parasitic (33.8-33.11)

iv. Nematoda: roundworms, not segmented, often parasitic (33.13) have a pseudocoelom

v. Mollusca(33.16): snails (33.18, 33.19), bivalves (33.20, 33.21), squid and octopus (33.22). open circulatory system, coelom,well developed nervous system with anterior ganglia

vi. Annelida: segmented worms. Many are marine (polychaetes), some terrestrial, complete digestive system and other organs (33.23, 33.24)

vii. Arthropoda: Trilobites (extinct), Crustacea (33.25, 33.35), Chelicerata (horseshoe crabs, Arachnids (spiders and mites, 33.28, 33.29, 33.30), Insects (Table 33.6, for interest, not for memorizing). fused and specialized segments, exoskeleton for support, sophisticated gas exchange, well developed nervous systems, digestive system (see other figures in this section)

viii. Echinodermata: sea stars, sea urchins, sea cucumbers (33.36) marine. Tube feet and water-vascular system (33.37)

4. History of animal diversity

a. all phyla present by 490 million years ago (MYA)!!

b. Cambrian explosion (550-500 MYA)-- evolution of all body plans present on earth today, plus a lot more that are no longer found!

c. Burgess Shale-- fossil beds from the Cambrian in British Columbia, amazing preservation of even soft-bodied forms.

-- 60% of organisms found in Burgess Shale cannot be assigned to any known phylum! Many body plans were tried, then went extinct

Next time: Evolutionary trends and key innovations in the vertebrates