2-5: Early Animals + Bilateria

Question 1

Explain “Darwin’s Dilemma” and name some breakthroughs that helped to resolve it

Answer 1

Darwin was vexed at how suddenly animals seemed to appear during the Cambrian (e.g., Treptichnus and Trilobites), with a lack of significant fossil record beforehand - this caused him to doubt his entire theory of gradual evolution

In 1955, two landmark papers were published about 2-billion-year-old microfossils from the Gunflint chert (Tyler + Barghoorn, then Cloud)

In 1958, Ford discovers Pre-Cambrian fossils in Charnwood Forest - frond-like (genus Charnia) and disc-like (genus Charniodiscus)

In 1961, Glassener discovers Ediacaran fauna

Question 2

Describe the main fossil evidence for the kind of life that existed in the Pre-Phanerozoic world (Archaean era)

Answer 2

Fossilised stromatolites from 2700mya are evidence of cyanobacterian photosynthesis

Fossilised evidence of endosymbiosis (eukaryotic cells)

Fossil evidence of multicellularity - including Metazoan stem

Question 3

Describe the main NON-fossil evidence for Archaean life?

Answer 3

2-methylhopanes (biomarker for photosynthesis) found in 2.7 billion year old shale

Question 4

Describe some examples of (possible) Ediacaran motility

Answer 4

Kimberella has been associated with bifid scratch traces, suggesting mat grazing

Dickinsonia and Yorgia show resting/feeding traces in a definite order, and possibly even evidence of muscle contraction

Helminthoidichnites avoidance behaviours suggest Bilaterian

Question 5

Describe the three main morphological forms of Ediacara described in the lecture

Answer 5

Branching Forms (Rangeomorphs) - have “frond” morphology and fractal growth; include Charmodiscus (looks like Cnidarian Sea Pen)

Radial Forms - have 3, 4, 5 even 8 degrees of symmetry (very odd) - some were interpreted to be jellyfish/Cnidarians, but unlikely

“Bilaterian” Forms - E.g. Dickinsonia, Kimberella; have glide symmetry like rangeomorphs rather than true symmetry; also no mouth/legs/gut

Question 6

Evaluate Dickinsonia as a possible Metazoan

Answer 6

“Footprints” suggest mat digestion - osmotrophy consistent with large surface area; possible Placozoan; also ANIMAL-LIKE MOLECULES (cholesteroids based on Gas Chromatography)

Question 7

Evaluate Kimberella as a possible Metazoan

Answer 7

Mollusc-style feeding grazing traces suggest motility

This makes it more convincing than Dickinsonia (which just flop/attach)

Question 8

Describe Haootia and evaluate it as a possible Metazoan

Answer 8

Bundled fibres resemble actual MUSCLES - which would make it definitively metazoan; Cnidarian-like morphology

Question 9

Describe the Biomarker evidence for metazoan life before the Ediacaran period

Answer 9

24-isopryl (a unique biomolecule synthesised by demosponges) recorded in fossil record BEFORE EDIACARAN

Question 10

Summarise the ecology of most Ediacaran biota

Answer 10

Large, flat bedding planes with epibenthic and sessile organisms - high SA:Vol

Complex multigenerational life cycle with asexual reproduction (based on geospatial analysis of specimens)

Question 11

Describe and evaluate the three (mentioned) possible causes for the appearance of the Ediacaran biota at that time

Answer 11

Deglaciation - about 9my before Ediacarans appeared (BUT many previous glaciations did not cause this)

Oxygen - increasing atmospheric oxygen (due to tectonic activity) allows larger body size? AND Ediacarans have high SA:Vol (BUT not a 1:1 correlation so hard to tell)

Carbon Excursion - Shuram C isotope excursion may have shifted carbon cycle?

Question 12

Describe the “end” of the Ediacaran biota

Answer 12

Likely they went extinct, as none seen in Burgess shale or other well-preserved Cambrian biota

HOWEVER, some, e.g. Gehling, suggested they still existed for a while, but weren’t preserved due to new conditions

Question 13

What defines the beginning of the Cambrian - and what possible issues are there with this?

Answer 13

The trace fossil Treptichnus, a worm burrow

Criticised for ichnotaxonomical confusion, unclear behavioural interpretations, etc. but generally accepted

Question 14

What was the major subphylum that appeared in the Cambrian and dominated for a long time (and when did they first appear)?

Answer 14

TRILOBITES (three-lobed arthropod form with jointed appendages) - appeared around 521mya

Question 15

Describe the groups of Molluscs seen in Cambrian Fossils

Answer 15

Helcionellida - univalved molluscs
Also some Gastropods - FOOT AND RADULA
Paragastropods?

Question 16

What was the initial reaction to the discovery of the Burgess shale

Answer 16

General dismissal - related to modern groups or “Walcott’s shoehorn”

Question 17

Describe the role Morris played in progressing understanding of the Cambrian explosion

Answer 17

He returned to the Burgess Shale and identified 15-20 unknown phyla, and drew attention to the significance of this rare preservation of soft-bodied tissue such as fragile, thin-cuticle skeletons

Question 18

Define the terms “Stem Group” and “Crown Group”

Answer 18

A crown group is all living members of a clade, as well as the common ancestor and all its descendants

Stem groups are extinct groups outside of the crown group, but are on the lineage to the common ancestor

Question 19

Name a stem group Arthropod and a Stem group Vertebrate

Answer 19

Arthropod: Anomalocaridids
Vertebrate: Metaspringgina

Question 20

How do the stem- and crown-group concepts help to resolve the concerns surrounding the sudden explosion of diversity seen in the Burgess Shale?

Answer 20

Many of the fossil taxa observed in the Burgess Shale are interpreted as stem groups of modern phyla (thus representing a less abrupt radiation than “Gould’s Weird Wonders”)

Question 21

Discuss the reasons for the Cambrian explosion

Answer 21

A complex set of interactions between both biotic and abiotic factors:
- Bilaterian development + Hox genes allowed new body plan experimentation
- More complex food webs -> evolutionary arms race
- Origin of burrowing -> ecosystem engineering (oxygenation of substrates)
- Defensive AND predatory hard tissues appeared simultaneously (calcium biomineralisation)

Abiotic:
- erosion -> low-relief continental interiors
- sea level rise -> flooding of interiors -> ions (e.g., Calcium, Phosphate) released into oceans
- Exposed continental shelf = new habitat

Question 22

What are Bilateria, and which phyla are included therein?

Answer 22

Animals with ONE plane of symmetry, as opposed to none (asymmetry) or multiple (radial symmetry)

All animal phyla except Porifera, Ctenophora, Placozoa, Cnidaria are Bilaterians

Question 23

Are starfish bilaterians or not?

Answer 23

Adult starfish, like many echinoderms, show pentameral symmetry. However, echinoderms are thought to have originated from Bilaterian ancestors, which is reflected in their development - their larvae are bilateral

Starfish also possess Hox genes (which control symmetrical development)

Question 24

What is cephalisation?

Answer 24

The concentration of nerves and sensory organs at the anterior end of an animal - a common evolutionary trend

Question 25

What are diploblastic and triploblastic animals, and which phyla do they include?

Answer 25

Diploblastic = have only 2 germ layers (ectoderm and endoderm) in development

Triploblastic = have 3 germ layers (ectoderm, endoderm and MESODERM) in development - at least in the gastrula stage when blastula invaginates

All Bilaterians are triploblastic, whereas Cnidarians/Ctenophorans/Poriferans/Placozoans are diploblastic

Question 26

What tissues do the 3 germ layers in triploblasts develop into?

Answer 26

Ectoderm: Skin, Exoskeleton (external layers)

Mesoderm: Organs, Internal Muscles, Tubules, Erythrocytes, etc

Endoderm: Internal layer, e.g. gut lining

Question 27

In what way has our understanding of the evolutionary origin of mesoderm changed recently?

Answer 27

(Steinmetz et al 2017) used TF expression profiles and cell type combinations to test the traditional hypothesis about mesoderm origins

Traditional Hypothesis:
- Cnidarian Ectoderm is homologous with our Ectoderm
- Cnidarian endoderm (“mesendoderm”) -> our Endoderm AND Mesoderm

Alternative Hypothesis:
- Cnidarian Ectoderm -> Our Ectoderm
- Cnidarian Endoderm -> Our Mesoderm
- Cnidarian Pharyngeal Ectoderm -> Our Endoderm

Question 28

What is a coelom and what are the three categories of animals in relation to the coelom?

Answer 28

The coelom is a fluid-filled cavity within the mesoderm, found in most Bilaterians

Acoelomates (e.g., flatworms) have NO coelom

Pseudocoelomates (e.g., nematodes) have a Pseudocoel - a fluid-filled cavity surrounded by muscle layer on the outside, but not the inside

Coelomates have a true coelom - the cavity is completely within the mesoderm, and the internal organs are suspended from a peritoneum, not the coelom

Question 29

What are the two main divisions of Bilateria (and which major groups are included within each)?

Answer 29

Deuterostomia: anus forms from Blastopore first in gastrulation, mouth after- includes Chordata and Ambulacrania (Echinodermata and Hemichordata)

Protostomia: mouth forms from Blastopore first in gastrulation, anus after- includes Ecdysozoa and Lophotrochozoa/”Spiralia”

Question 30

What are some notable features that are different in Deuterostome + Protostome development (apart from the defining one)

Answer 30

Protostomes:
- Development is Determinate and Mosaic - removal of one blastomere cell at 4-cell stage is lethal
- Spiral cleavage of blastomere
- Nerve cord is ventral to gut

Deuterostomes:
- Development is Indeterminate and Regulative - removal of one blastomere cell at 4-cell stage -> 2 full embryos
- Radial cleavage of blastomere
- Nerve cord is dorsal to gut

Question 31

What features do Hemichordates share with Chordates?

Answer 31

Many Hox gene expression patterns, and pharyngeal gill slits

Question 32

What are the four Bilaterian superphyla according to the lecture (and what is the controversy with this)?

Answer 32

1. Ecdysozoa (arthropods and allies which moult - ecdysis)
2. Spiralia (includes Lophotrochozoa and some other obscure phyla - spiral cleavage in development)
3. Deuterostomia (chordates and Ambulacrania)
4. Xenacoelomorpha (sister group to Nephrozoa (all other Bilateria), obscure groups e.g. Acoela - simple, flatworm-like acoelomates - and Xenoturbellida - blind gut, sac-like phylum with only one genus)

Controversies:
“Spiralia” - term has a confused history and is often conflated with Lophotrochozoa. Also development is more varied than previously thought

“Xenacoelomorpha” - may be a clade WITHIN Deuterostomia (though more robust phylogenetic analyses based on large transcriptome dataset has provided support for the sister group hypothesis)

Question 33

What are Metazoa - what are the defining features?

Answer 33

(Surprisingly difficult to define succinctly) they are eukaryotic, multicellular heterotrophs

They lack rigid cell walls - but a lack of a feature isn’t a great synapomorphy

Migration of cells through mesenchyme (CT) is unique to metazoans

MOTILITY - whole-organism powered motility is seen in at least one stage of life cycle for all animals

Question 34

What is the larger group (containing multiple kingdoms) that includes Metazoa?

Answer 34

OPISTHOKONTA (includes Fungi, and more closely related to animals, Choanoflagellates)

Question 35

What are Porifera and what are their defining features?

Answer 35

SPONGES: Sessile filter-feeders with no organs/nerves/muscles/symmetry

Spongy connective tissue containing cells and microscopic SPICULES (silicious, calcerous or spongin)

Choanocytes - flagellated cells to create internal water currents

Ostia (ring of choanocytes) -> Internal Cavities -> Osculum

Question 36

What are Cnidaria and what are their key features?

Answer 36

A diverse phylum of stinging, diploblastic species including jellyfish, anemones, corals

Cnidocysts (stinging cells), can be colonial or solitary

Anthozoa are sessile, while Medusozoa have a Medusa (motile + sexual) stage

Question 37

Describe the life cycle of Medusozoa

Answer 37

Motile PLANULA larval stage -> Sessile Budding Stage -> Motile, Sexual Medusa Stage

Question 38

What are Ctenophora and what are their key characteristics?

Answer 38

“Comb jellies” - radial, diploblastic

Motile - use combs (Ctenes) of Cilia

Recently confirmed as having a proper through-gut

Question 39

What are Placozoa and what are their key characteristics?

Answer 39

“Flat Animals” - only three species

Ciliated Epithelium for movement - digests detritus on ventral surface

No tissues or symmetry

Mostly asexual budding

Question 40

What is the major debate about the root of animals?

Answer 40

Whether Porifera or Ctenophora are sister to all other animals

The traditional view based on morphology was Porifera = Sister, but a large study by Whelan et al (2015) proposed Ctenophora, followed by a scathing rebuttal by Pisani et al (2015^

Question 41

What are the arguments and evidence for Porifera/Ctenophora being sister taxon to other animals?

Answer 41

1. If Ctenophora were sister taxon, this would require extreme gains and losses (e.g., Ctenophora have nervous system while Porifera/Placozoa do not)
2. Pisani et al criticised the methodology of Whelan et al - they may have failed to account for LBA of Ctenophora and Fungi, and site heterogeneity
   -> they said a CAT (multiple categories of site) model should have been used
3. Another possible bias was not accounting for genes that are only present in one species, or genes that have been lost in all sample species - underestimated lost genes in Ctenophores
4. A large dataset from Simion et al 2017 again supported Porifera as basal - accounting for LBA and SH
5. Other papers since have questioned some of Pisani’s arguments, including whether the CAT model is superior in this context - also they didn’t acknowledge that convergent evolution could explain Nervous Systems issue, especially since those seen in Ctenophores are compositionally and structurally different from others

Ultimately, question is still open and there is evidence to support either hypothesis

Question 42

What are the three main factors that make the sister taxon debate difficult to resolve?

Answer 42

1. The time period being debated/studied was in the extremely distant past (approx 540 mya)
2. Rapid evolution and loss of features in Ctenophora
3. Lack of informative intermediates before the radiation of modern species