4-5: The Cambrian Explosion, Bilateria Flashcards
What’s the main group of diversity?
Bilateria
Features of bilateria
Bilateral symmetry (left and right side), triploblastic (endoderm, ectoderm and mesoderm), have cephalisation (concentration of sensory structures in a head)
What are the three groups in bilateria?
Lophotrochozoa (molluscs, annelids, brachiopods), Ecdysozoa (nematodes, arthropods), Deuterostomata (echinoderms, chordates)
When did fish, molluscs and coral appear?
Silurian
Who discovered the Burgess shale?
Charles Doolittle Walcott
What is special about the Burgess shale?
Exceptional preservation of soft tissue, lots of invertebrate fossils eg trilobites, arthropods
How many modern phyla are there?
Around 30
How many uninterpreted phyla have been found in the Burgess shale?
15-20
Total group definition
Living representatives and all extinct relatives of a group
Crown group definition
Just the living representatives of a group and closely related
Stem group definition
Extinct organisms only, more closely related to the most recent common ancestor than any living representative
Are there more stem or crown group representatives in the Burgess shale?
Stem- very few are crown
Waptia features
Antennae, eyes, specialised legs, a crustacean- a crown group!!
Anomalocaris features
Large claws, disc mouth, swimming fins, a ferocious predator, stem arthropod
Ancient Deuterostomata (earliest evidence for us)
Pikaia and Metaspriggina
Pikaia features
Notochord, muscle bands
Metaspriggina features
Closer than pikaia, notochord, muscle band, paired eyes, Gill arches
How does the Burgess shale overcome preservation bias?
Can see soft animals too, not just biomineralising ones with a hard shell/skeleton, so can see more complex food webs and work out relative species balance based on how many specimens of which type we find
What is Chengjiang?
Similar to the Burgess, but in China, different species found (eg. no nematodes in Burgess), but some overlap
What is disparity?
The variation of species
Why was Burgess so good for preservation?
Deep sea bed was rapidly buried by fine sediment, possibly by the collapse of a sea cliff. This starved the organisms of oxygen and preserved carbon films, which metamorphosed. Low sulphur and high alkalinity enabled preservation. The window for preservation closed after the Cambrian. Different places give rare insights of an era
What’s the current sister phylum to all animals?
Ctenophora
What symmetries do bilaterians have?
Left/right, anterior/posterior, dorsal/ventral
What is gastrulation of the blastula?
One layer of cells becomes more
What is a blastula?
A recently fertilised ball of cells
What is the cnidarian endoderm homologous to?
The bilaterian mesoderm
3 main superphyla of bilateria
Spiralia (including lophotrochozoa), Ecdysozoa (arthropods etc), Deuterostomia (verts etc)
Which main superphyla are Xenacoelomorpha in?
None- they don’t fit into any!
What make up Xenacoelomorpha?
Composed of acoela (flatworm type things) and xenoturbella (simple sacs, difficult to place into anything).
Xenacoelomorpha features
Have nerves, but no anus, respiratory/circulatory system, or nephridia (kidney type organs). Have a proper mesoderm, but no proper coeloms like most bilaterians
What is a coelom?
A body cavity filled with water, basically a gap. It is a more complex body plan
What is acoelomate? + Example
Lacking a coelom cavity eg. Flatworms
What is pseudocoelomate? + Example
Fluid filled cavity around the gut, eg. Nematodes
What is coelomate? + Example
Fluid filled cavity within the mesoderm, eg. Annelid worms
2 divisions of bilateria based on development of the gastrula
Protostomia (Spiralia and Ecdysozoa) and Deuterostomia
Protostomia features
Mouth is the first invagination during gastrulation, anus is second. Removal of one cell = cell death (called determinate and mosaic). Spiral cleavage of the blastomere. Eg. Wasps, octopus
Deuterostome features
Anus is the first invagination during gastrulation, mouth is the second. Removal of one blastomere cell = 2 embryos… Twins! (Indeterminate and regulative). Radial cleavage of the blastomere. Eg. Fish, starfish
What are most Spiralia?
Lophotrochozoa
What are platyhelminthes?
Flatworms, simple Spiralians. Hermaphrodites, do penis fencing where they try and be the male when mating
Distinguishing features of lophotrochozoa
Lophophore (ring of ciliated feeding tentacles), trochophore (larvae with characteristic bands of cilia)
Examples of lophophorata
Bryozoa, Brachiopoda
Ecdysozoa features
Lack a spiral cleavage, periodic moulting of the cuticle, three layered cuticle. Include arthropods and friends
Deuterostomia features
Defined by blastula development, notochord, include us!
3 types of Deuterostome
Hemichordates, Echinoderms, Chordates
Hemichordate features
Burrowing marine worms with gill slits and a stomochord, eg. Acorn worms
Echinoderm features
Pentaradial symmetry, vascular water system, eg. Starfish
Chordate features
Vertebrates and our invertebrate relatives, eg. Us
What are Anbulacaria?
Echinoderms + hemichordates
Homologous features between chordates and hemichordates
Body axis formation and organisation, pharyngeal gill slits
