Animal diversity and relationships

Living forms At least 30 phyla But only x “important” ones Importance = numerous, ecologically important, and fit into our conceptions of evolution

Porifera = sponges Assymetrical, 2 cell layers, internal silica spicules = skeleton.

Cnideria, jellyfish, corals Radially symmetrical, 2 cell layers, one ended digestive system. Often with algae inside to provide nourishment Light sensitivity.

Ctenophora – comb jellies Bilateral, one ended gut, marine

Platyhelminthes = flat worms. Three cell layers, no body cavity, one ended digestive system, brain, nerves, many are parasitic.

Two ended digestive system Head and tail, have an internal cavity – not solid. Very numerous in soil.

Annelida = segmented worms; bigger most free living. Have nerves, three cell layers, true body cavity = coelom, circulatory system, excretory system. Skin breathing and many have legs. Some carnivorous, soil eaters or leaches – blood suckers

Mollusca: clams, snails, squid, octopus, slugs Two ended gut, nerves, circulation, filter feeders or carnivorous. Some (squid, octopus) with eyes, brain, etc. Remnants of segmentation – chitins.

Arthropods – joint leg = insects, crustacea Segmented, brain, sense organs, excretory system

Echinoderms – pentaradiate, Starfish, urchins, sea lillys.

Chordata, including vertebrata. Backboned organisms.

The fossil record How to determine relationships – try fossils first.

Burgess Shale in British Columbia, a unique preservation at the right age.

Segmented worms (with legs?)

arthropods

Chordate – similar to early members of this group

sponge

????

Result of fossil record: all major groups plus some other odd ones appear almost simultaneously = no sequental appearance.

How to make sense out of diversity without fossils. Rules: simple to complex symmetry (assymetrical, radial, bilateral) Cell layers – 1 to 2 to 3 Embryology - (old ontogeny recapitulates phylogeny statement of Haeckel) Digestive system design; one ended to two ended

Ernst Haeckel, 1866; “ontogeny recapitulates phylogeny” = early stages of development mirror evolutionary changes. Note – gill clefts in all embryos, tail present, etc. Basis; development is often incremental – add new stages to old – harder to change the beginning stages.

Classic example of this, the aortic arches in vertebrates. Start with a complete set; need gill arches to deposit them. All embryos have gill clefts and a complete set of arches To get to adult stages, loose some of the arches.

Phylum porifera; sponges. assymetrical Because of assymetry, doesn’t fit into any neat story – so viewed as a development separate from everything else.

Symmetry, bilateral or radial

Jellyfish, corals, radial cnideria Radial symmetry plus one ended digestive system = primitive

Bilateral symmetry; all other groups.

Clues from development 1.Hollow ball of cells 2.Then 2 cell layers, one opening,radial symmetry= cnidaria 3.Then three cell layers – new opening bilateral symmetry = all higher forms

Cell layers 2 vs 3. sponges and cnideria = 2

Digestive system – one ended vs. two ended

Flatworms – bilaterial but with one openning to digestive system

Other ‘worms’ – two ended digestive system.

So: 1 – no symmetry, followed by radial and then bilateral - porifera oldest then cnidaria, then everything else 2. one ended gut, followed by 2 ended cell layers, followed by three

Now what? Use embryology – the great Protostome – Deuterostome split

Protostomes vs deuterostomes First opening = mouth Determinate clevage Spiral clevage Mesoderm = 4d cell Schizocoel coelom First opening = anus Indeterminate clevage Radial clevage Mesoderm = infolding Endocoel coelom

Protostomes – how to organize Classical method mesoderm – solid to pseudocoelom to true coelom one ended gut to two ended no segments to segments

Body cavity (coelom) in relation to mesoderm Solid mesoderm = no coelom = flatworms acoelomate Coelom partially lined with mesoderm = roundworms pseudocoele Coelom completely lines with mesoderm = Segmented worms, vertebrates, etc. true coelom

Result = Flatworms – one opening dig system, solid (acoelomate) Round worms; two openings dig system pseudocoel Seg. Worms (annelids) segments true coelom Arthropods – trilobite = segmented and legs Molluscs = chitin, segmented. Odd –legs??

Trilobite = arthropoda Chitin = mollusca Marine worm = annelida All segmented.

Deuterostome, protostome split Protostomes groups by segmentation. Link segmentation together Note: Classical classification ((it’s wrong))

New phylogeny Lophotrochozoans Filter feeders Ecdysozoans Shell shedders

Two phylogenies: left based on hypothesized relationships, right based on Both genetic similarity and time. Right is correct in that all major groups appear almost simultaneously (brushpile evolution)

Note: flatworms,mollusc and annelid together// no arthropods

Arthropods and round worms in this group.

Central position of flatworms as ancestral. Question marks. separate Ecdysozoans Shell shedders segmented NEXTNEXT

How come the old phylogeny wrong? 1.Segmentation arose more than once – not a unifying trait 2.Some organisms, especially parasites (flatworms, roundworms) may have gotten simpler in structure through time. 3.A poor choice of “unifying” characters.