4. The Cambrian Explosion Flashcards
(39 cards)
1
Q
Summarise ediacaran biota. 6
A
- variety of odd organisms 2. vendobionts - frond like 3. some evidence of sponges, but largely thought they occurred after ediacaran and before cambrian 4. some evidence of cnidarian-placozoan grade animals 5. little evidence for real bilaterians 6. ediacaran biota appears to only cover metazoa and eumetazoa, before bilateria split off
2
Q
Define clade. 1
A
- common ancestor plus linear descendants - a tree branch
3
Q
define grade. 1
A
- area of phylogenetic tree but not a true clade
4
Q
what are the characteristics of the bilateria? 8
A
- Symmetry - 2 layers, left and right sides 2. triploblasty - a developmental feature 3. 3 body tissue layers - ectoderm, endoderm, mesoderm 4. often have a coelum 5. these features allow more complexity 6. cephalization - process of development of a head 7. concentration of sensory apparatus 8. cnidarians and sponges lack heads
5
Q
What are the 3 bilaterian clades? 6
A
- lophotrochozoa - eg. molluscs, annelids, brachiopods 2. some feed using lophophore 3. ecdysozoa - shed external layer 4. nematodes and arthropods 5. dueterostomata - anus forms before mouth 6. echinoderms, chordates and us
6
Q
How was Darwin’s problem of lack of evidence solved? 3
A
- Darwin saw fossils from the Silurian period - 443m yr ago 2. this includes some Cambrian-style fossils, along with trilobites (arthropods), fish etc 3. We have found cambrian and ediacaran fossils since
7
Q
What are phyla? 5
A
- One of the biggest taxonomic groups 2. all within phyla have a unique body plan 3. about 30 modern phyla 4. Huge diversity within a phylum 5. Burgess shale animals were interpreted as 15-20 unknown phlya
8
Q
Do the bilateria family tree. 3
A
9
Q
Describe the stem group concept. 4
A
- Terminology to describe clades and interpret fossils
- total group - living representatives and all extinct representatives
- Crown group - living representatives and closely related
- stem group - extinct organisms only. more closely related to common ancestor than anything alive today
10
Q
How was the Burgess Shale reinterpreted? 4
A
- many of these animals look stange as they are so old, and were originally incorrectly identified
- the stem group concept is useful here
- many are stem group representatives of major phyla
- few are crown group
11
Q
Who was Charles Walcott and what did he do in 1909? 3
A
- Geologist who found unusally well preserved trolibites in the Canadian Rockies
- . these cambrian arthropods had soft tissue preservation eg. legs
- walcott explored until he found the source - the burgess shale
12
Q
How were the early discoveries made in the burgess shale classified? 41
A
- A VARIETY OF INVERTEBRATE FOSSILS WERE FOUND
- WALCOTT DECIDED THEY WERE MOSTLY RELATED TO MODERN GROUPS
- THIS BECAME KNOWN AS WALCOTT’S SHOW HORN, AS HE WAS FORCING THE FOSSILS INTO TEXONOMIC GROUPS THAT WERE BASED ON MODERN CLAIMS
- AFTER HIS ATTEMPTS, THE FINDINGS WERE LARGELY NEGELECTED
13
Q
How were the burgess shale fossils reclassified? 3
A
- in the 1970s, a group from Cambridge revisited the shale and made additional collections and reanalysis
- the fossils were carefully taken apart to see other layers and reconstructed
- the unique anatomies were recognised rather than viewing the fossils as ancient representatives of modern claims
14
Q
Describe burgess shale fossils that belonged to the metazoa. 5
A
- Porifera
- calcerae spicules found
- Tough spongin of demospongiae
- ctenorhabdatus - 24 comb rows, different body plan but relation to ctenophores. probably pelagic
- Thumaptilon - similar to vendobiont, but fits patterns of the sea pen, so in terms of growth, making it a cnidarian
15
Q
Describe possible annelid and mollusc fossils that were found in the burgess shale. 11
A
- Canadia
- has setae bristles and interpreted as active swimmer/walker
- may swim in a pelagic fashion
- has a head with tentacles
- may be annelid
- Wiwaxia
- has sclerites and radula (sclerites are like scales)
- almost certainly a mollusc
- Odontogriphus
- unsure of this for a long time
- Radula and large muscular foot, therefore mollusc
16
Q
What are priapulida and onychophora? 4
A
- 2 living phyla
- ecdysozoa
- relatively minor today
- evidence suggests they were abundant in the cambrian
17
Q
Describe the priapulida and onychophora fossils that were found in the burgess shale. 14
A
- Ottoia
- Evertable proboscis
- U shaped burrows to live in
- We have preserved guts of these
- Seem similar to penis worm - priapulida
- Aysheaia
- Lobe limbs
- Head tentacles
- Found with sponges - may feed on them
- Similar to velvet worms - onychophera
- Hallucigenia
- Lobed limbs and spikes. Onychophera
- unknown for a long time which way up
- flexible legs and defensive spines
18
Q
Describe the possible arthropoda fossils that were found in the burgess shale. 12
A
- Anomalocaris
- found in pieces
- up to 1m
- large claws
- disc mouth
- swimming fins
- interpreted as a ferocious predator
- stem arthropod
- waptia
- looks like modern crustacean
- Has specialised compound eyes, like flies - which are specialised crustaceans
- Has antennae
19
Q
what is the preservation like of arthropods from the burgess shale? 3
A
- some are very good
- show chelicerate like brain
- There have been claims that the digestive system, neural system and vascular system have been preserved
20
Q
Describe the fossils that have been collected from the burgess shale that are not arthropods, but have been interpreted as deuterostomes. 13
A
- echinoderm or coral
- cone shaped
- 9 feeding arms
- Pikaia
- muscle bands
- potentially a human ancestor
- stem-hemichordate
- proboscis with collar
- similar to acorn worms
- metaspriggina
- Muscle bands
- paired eyes
- gill arches
21
Q
Describe the cambrian fossils that have been found at chengjiang. 12
A
- Fossil deposits in china that are older and mroe diverse than burgess shale
- Cnidarians eg. anemone
- Nematodes - very rarely fossilised
- arthropods
- labopods - stange worms with legs
- myllokunmingia/haikouichthys
- muscle blocks
- tailfins
- paired eyes and nose
- notochord but no skull
- Strong candidate for stem vertebrate, crown chordate
- distinctly fishy
22
Q
Describe Orsten microfossils. 5
A
- dissolve rock with acid to produce them
- less than 1mm big
- compared with modern plankton eg. bivalved crustaceans and crustacean larva
- Precise interpretations difficult as not fully grown so don’t have full anatomy
- can use particle accelarations to see structures inside
23
Q
Describe burgess shale ecology. 4
A
- preservation bias overcome, as soft tissue preserved as well as hard shells
- allows better picture of relative specimen % - mostly arthropods and porifera, with few chordates, annelids and molluscs
- foodwebs are vey complex compared to ediacarab - can view this in more detail
- more predation
24
Q
What are the traditional, gould and stem group approaches to diversity and how do these relate to the cambrian? 6
A
- Darwin would expect the traditional, gradually expanding core
- The weird wonders appraoch suggests Gould’s early maximum, as cambrian represents new phyla
- More likely to be an early narroe and rapid early expansion
25
How were the burgess shale fossils preserved so well?4
1. deep sea bed rapidly burried by fine sediment as sea cliff erodes and falls on top
2. area is starved of oxygen so less decay
3. preservation of carbon films and clay minerals as they are under pressure, therefore metamorphosed
5. this may not represent a true evolutionary signal, couold be due to filtration by fossil record
26
How important are fossilisation windows when studying cambrian fossils?4
1. fossilisation changes between ediacaran and cambrian as animals begin to burrow and change substrate
2. low sulfur and high alkalinity allowed better preservation of soft tissue at burgess shale
3. window appears to have closed, but there are some rare post-cambrian insights
4. morrocan ordovician and german devonian fossils show very good preservation of combination of ancient and modern fauna
27
What was special about the rate of cambrian evolution? 5
1. time based tests of genetic and morphological data carried out
2. algorithms used to see speed of evolution
3. both begin fast, then slow down
4. 5.5x (Arthropods) and 4x faster evolution during cambrian than afterwards
5. very high rate but not impossible
28
Did environmental changes contribute to the cambrian explosion? 3
1. more relevant for ediacaran
2. oxygen and glaciation occured too early to really affect cambrian
3. but geological plate arrangement meant more shallow marine habitats
29
How did eoclogy contribute to the cambrian explosion? 3
1. more predation/protection than ediacaran
2. arms race?
3. 3D eco-spaces rather thgan sessile
30
How did genes and development contribute to the cambrian explosion? 4
1. Hox genes - evolution lead to rapid morhpological changes
2. duplication of hox genes - cnidarians had 2 copies, but deuterstomes and ecdysozoa have more copies so more body plan possibilities and back ups in cases of mistakes
3. canalisation - end of evolutionary experimentation so more difficult to make changes
31
Explanation of the cambrian 'explosion' of animals. 2006. c. marshall.
describe body fossils of the cambrian. 3
1. small, shelly fossils that are difficult to interpret
2. larger and more diverse ones
3. increase in diversity explosion occured in early cambrian, but crown group taxa found in mid-late
32
Explanation of the cambrian 'explosion' of animals. 2006. c. marshall.
How reliable is the fossil record? 2
1. molecular clock evidence suggests very imcomplete
2. generally, clock and fossil evidence match
33
Explanation of the cambrian 'explosion' of animals. 2006. c. marshall.
Describe disparity and diversity in the cambrian fossils. 4
1. Disparity = distinctness of species
2. cambrain has hufe increase
3. also increase in diversity, this lasted longer than increase in disparity
4. increase in these things is unrivalled
34
Explanation of the cambrian 'explosion' of animals. 2006. c. marshall.
What were the limitations of diversity in the cambrain explosion? 2
1. lower overall diversity than the rest of phanerozoic
2. no terrestiral or aerial species
35
Explanation of the cambrian 'explosion' of animals. 2006. c. marshall.
What is the significance of the increase in atmospheric oxygen levels before the cambrian?3
1. increase in oxygen to level that could support larger like may have caused explosion
2. some think this was case long before explosion, as many large ediacarans
3. conditions necessary before it happpens, may have provided enough energy for predation
36
Explanation of the cambrian 'explosion' of animals. 2006. c. marshall.
What is the significance of the carbon isotopic anomly at the boundary of pre-cambrian/cambrian? 3
1. lots of negatice carbon isotope suggests an unknown environmental change
2. mau have led to extinction of ediacaran biota, allowing cambrian radiation
3. doesn't explain disparity and diversity increase
37
Explanation of the cambrian 'explosion' of animals. 2006. c. marshall.
How may the origin of the bilaterian developmental system have contributed to the diversity increase of the cambrian? 5
1. Many believe this (hox genes etc) responsible or explosion
2. gene duplication may have been important
3. similar genes shared across species, but functions may have been different
4. common ancestor to bilaterians is common ancestor to all model organisms we use today so can make good inferences about their genome
5. canalization shows a weakness in that things may be due to other functional/ecological changes
38
Explanation of the cambrian 'explosion' of animals. 2006. c. marshall.
Explain's Kauffman's 1993 theory of cambrian explosion. 2
1. hihg disparity initally, then decline as species find optimal forms
2. explosion represents exploration of forms
39
Explanation of the cambrian 'explosion' of animals. 2006. c. marshall.
explain the significance of realised vs unrealised morphologies in the cambrian explosion. 2
1. evolution has found all optimal solutions of being a plant
2. explored within one geological period
3. cambrian may represent bilaterian/animal equivalent
