W3 LECTURE 8: From the Cambrian to the Cretaceous

Marine radiation of fishes

• Cambrian saw the origin of Chordates

• Fossil Agnatha – jawless fish, simple mouths and head morphology

• Hagfish/lamprey of today

• Benthic browsers/parasites

Silurian Diversification of fishes

• 450-420 Mya: jawed fish radiation ‘Gnathostomata’

• Chondricthyes

  • Cartilaginous fish (Sharks and Rays) - No bones (cartilage instead)

• Osteichthyes

  • Bony fish, Complicated structures - head/jaw

  • Ray-finned fish (Actinopterygii) - Teleosts

  • Lobe-finned fish (Sarcopterygii)- Coelocanth, lung fish, big

Evolutionary transition in fishes

Enabling feature: Genome duplication

• Early chordates had two genome duplication events

  • four Hox gene clusters (allow position of body parts to be developed)

  • Enabled morphological complexity (jaws, limbs)

 Algal life cycle

• Haploid phase is dominant - one set of chromosomes

• Haploids get together and create gametes

• Gametes are fertilised to form a zygote

• Zygote undergoes meiosis to form haploid cells

Land plant life cycle

• Diploid phase started persisting

• Zygote experienced mitosis

• Mitosis in haploid and diploid

Byrophytes

• Liverworts and Mosses

  • Long haploid phase, short diploid phase

  • No vascular tissue (xylem, phloem)

  • Don’t require soil

• Evidence by fossilized sporangia

• Mosses also evolved stomata – gas/water regulation (no vascular tissue)

Land plants accompanied by terrestrial fungi

• Evolution of desiccation resistant fungal hypha from aquatic form

• Hypothesized fungi enabled plant life on land through symbiosis

  • Water and ion gathering (origin of roots)

• Fungal-plant interactions essential for many bryophytes

• Fungi may have preceded plants into terrestrial environments

Plant death leads to organic-bearing terrestrial soil

• Soil = breakdown of plant matter (fungal, microbial action)

• Bryophytes do not ‘need’ soil but do create soil

  • ‘paleosoil’ in the Devonian

• Soil enables larger plants to grow higher

• Soil stores water and mineral

Cooksonia

First vascular land plant (from soil development)

Phloem/Xylem transport system

• (transport water/solutes)

• Bifurcating (branching) growth

• Increased size through Lignification (rigidity)

Leaves & fronds

• Above ground photosynthetic surfaces

• Evolved on multiple occasions - easy to evolve

Vascular plants life cycle

• Extension of the diploid (sporophyte) phase

• Short haploid phase

• Thousands of mitotic divisions

• Get long-lived organisms (trees)

Vascular plants to Richer soil

• Deeper roots

  • Degradation of rock

  • Mineral release

• PO4, NO3 are fuel for plant growth but run off may have ‘poisoned’ oceans via eutrophication

• Leads to Devonian mass extinctions

• (Mass extinction – many taxa disappear concurrently > 75% in 3 Mya)

 

What evolved from the carboniferous

All major plant taxa except flowering plants

Land plant evolution accompanied by terrestrial arthropods

• First insects – flightless

• Derived from Crustacea (in Silurian)

• Devonian: Winged forms + Millipedes, Spiders (Aranaea)

Age of coal

• Carboniferous rocks - Appearance of organic carbon rich deposits

• Coal beds/measures - Shows plant death

Change in plant life

• Coal beds have abundant plant (and other) fossils

• Large, woody (lignified) plants appear alongside large ferns

• Derived from lowland marsh/wetlands (anaerobic - no oxygen)

Why has the deposition of organic carbon increased

• Tall Woody plants had become dominant

  • Wood is rigid and plants can be tall because of lignin (many C-C bonds)

• Lignin = complex carbohydrate

  • Decomposes much more slowly, hard to break down

• Slow decomposition was probably very slow when lignin first became ‘a thing’

• This lead to more organic carbon being buried and coal strata forming

Consequences of increased organic carbon deposition

• Rise in global O2

• Oxygen cannot go back in respiration and carbon is stored

• Rise in atmospheric oxygen

Biological consequences of increased organic carbon deposition

• Insect gigantisms

• Giant millipedes/cockroaches

Oxygen limiting body size

• Amount of oxygen in water depends on temperature and salinity (cold/less saline holds more oxygen)

• Colder and more saline environments hence produce larger organisms

O2 enables fire

• Very common wildfires

• Charcoal deposition from the coal means that carbon sink remains

• Ecosystem stabilised by wet conditions

 

Carboniferous also saw radiation of tetrapods

• Warm, wet climate, green terrestrial biome enabled movement out of water

• Amphibian lifestyle – still linked to laying eggs in water

• Closest ancestors = the lobe finned fish

• Transitional form - Tiktaalik

• Early forms: Ichthyostega, proto-legs

Giganticism seen in amphibia

• Sole tetrapod group – dominant predators of the Carboniferous

The Permian

• Rise of reptiles

• End of the Carboniferous/early Permian saw a period of drying alongside a hot climate

• One large land mass – Pangea/Gondwana

Drying makes a poor environment for amphibians

• Selection for water retention

• Reptiles form (first fossil, Hylonomus 318Mya)

Amniotic egg

• Zygote in fluid filled capacity

• Chorion and shell - able for gas exchange but retains water

• Egg can be free from water

Reptile adaptations to live away from water

• Keratinous scales - Water impermeable skin

• Alteration in excretion

  • Uric acid

  • Utilize arid landscape areas

• Internal fertilisation

  •  Animals in water cam fertilise internally or externally

  • Reproduction on land leads to internal fertilisation

 

Evolution of metamorphis

• Permian saw rise of metamorphosing insects

• Drivers of metamorphosis are unsure

Permian mass extinction

• End of the Permian saw largest mass extinction event in fossil record

• ‘The Great dying’

Causes of mass extinction

• Occurred at same time as very high Volcanic activity in Siberia

• Massive SO2, CO2 release

• Ocean acidification

• Reduced primary productivity

• Death of consumers

• Did not hit evenly and some species survived better

Reconstructing the biology of dinosaurs

• Scientists study the new remains to determine relationships to other known species & genera

• Comparisons are made between these and living animals reconstruct skeleton and lifestyle

• Aspects such as size, movement, weight and shape can also be determined

Understanding the biology of extinct biology

• Extend from understanding extant organisms to extinct ones:

  • Movement, running speed

  • trackways of footprints

  • Principles of anatomy, biomechanics

Thermal biology

• SA/Volume ratio = retained heat

• But can’t tell behaviour, e.g. basking, or metabolism

• Or can we? Growth rates accelerate in endothermy

What has been discovered about dinosaurs

• Diet is clear from teeth

• Brain size scales with body size as for other reptiles

• Likely did not have complex cognitive capacity of birds and mammals

• Unclear how they reproduced

Origin of mammals

• Mammals: c. 220 Mya

• Derived from synapsids

• (non-dinosaur reptiles, no living examples)

• Remained a minor taxa during the age of dinosaurs

Hadrocodium

• Hadrocodium - an extinct mammalia form

  • Lived during the Early Jurassic approximately 195 million years ago in what is now the Yunnan province in south-western China

  • Considered the closest relative of the class Mammalia

• Very small

• Insectivores, nocturnal (enabled by endothermy)

• Maleus-incus-stapes derived from jawbone

  • Very good Hearing

Birds

• c. 160 Mya

• Evolved from theropod dinosaurs during the Jurassic (around 165–150 million years ago)

• Classic small, lightweight, feathered, and winged body plan

• Evolved gradually over tens of millions of years of evolution rather than one burst of innovation

• Dinosaur-like

  • Tail with bones

  • Teeth

• Bird-like

  • Feathers

  • Large brain

Descendants of the theropod group of dinosaurs

• Birds are the only living dinosaurs, descended from the theropod group of dinosaurs

• Theropods were two-legged, meat-eating dinosaurs that first appeared 231 million years ago

Angiosperm

• Flowering plants

• First fossils – 130 Mya

• Fast growing compared to Gymnosperms (conifers)

• Initially wind pollinated

• Insect pollination c. 110Mya

Plant and insect diversification

• Massive plant Diversification @ 70- 100 million years ago –> 250K species today

• Loss of Gymnosperms in tropics

• Diversification of beetles