L3- Origin of Vertebrates

Vertebrates phylogeny:

• Vertebrates are in the phylum chordates

• Chordates also have sea squirts (urochordates) and cephalochordates

  • are both invertebrate chordates

• Vertebrates closest relatives are the urochordates

• Vertebrates are in the group deuterostomes

Features of chordates (at some point in their lives):

1. Notochord→ flexible rod for support, is often lost

2. Nerve Cord→ is dorsal and hollow, develops into the brain and nerve cord

3. Pharyngeal (gill) Slits→ within the pharynx in the early part of the gut, is a deuterostome feature: invertebrates use this for filter feeding, vertebrates use this for respiration

4. Post-anal Tail→ muscular, used for movement

5. Endostyle→ in the pharynx, produces mucus to aid filter feeding, is homologous to the vertebrate thyroid gland which concentrates iodine

   • Lamprey metamorphasise their endostyle into a thyroid gland

Vertebrate chordates-

1. have vertebrate instead of notochords

2. not all have fully formed vertebrate

3. have a cranium

4. have an endoskeleton

5. have duplications of the hox gene complex

6. have a developed neural crest

7. have placodes

8. brain is larger and has 3 parts

9. are larger and have a higher metabolic rate

10. can have short periods of anaerobic respiration

1. have vertebrate instead of notochords

• made of cartilage or bone

• centrum→ centre

• neural canal→ where the spinal chord goes through

• the notochord can remain as intervertebral discs

2. not all have fully formed vertebrate

• Arcualia→ vertebral precursors, in the jawless vertebrates, have notochords and arcualia for support

• Lampreys have arcualia along the body and above the notochord (dorsal)

• Hagfish have arcualia in the tail and below the notochord (ventral)

• Ancestral vertebrae have arcualia above and below the notochord (dorsal and ventral)

3. have a cranium

• can be bony, cartilaginous or fibrous

• e.g. the skull

4. have an endoskeleton

• made up of the vertebrae and the cranium

• was cartilaginous in early vertebrates

5. have duplications of the hox gene complex

• developmental genes that control anterior-posterior patterning

• duplications:

  • the origin of vertebrates→ first duplication into 2 clusters

  • the evolution of the jaw→ second duplication into 4 clusters

  • the evolution teleosts→ third duplication + loss into 7 clusters

  • the evolution of teleosts into salmonids→ fourth duplication + loss into 13 clusters

    → duplications add to the complexity of vertebrates

6. have a developed neural crest

• neural crest cells are migratory (can move and leave) and are multipotent (can form most cells in the body)

• neural crest cells are responsible for new structures, mainly in the head→ add another germ layer to the (quadroblastic)

• there are precursors of the neural crest cells in invertebrate chordates→ similar genes expressed when neural plate forms, tunicates (sea squirts) have migratory cells that form pigment cells

7. have placodes

• complex sense organs, homologs to invertebrate chordates

8. brain is larger and has 3 parts

• forebrain, midbrain, hindbrain

• brain of invertebrates is not divided but has similar genes except from in the forebrain→ link between vertebrates and invertebrates

9. are larger and have a higher metabolic rate

• need organ systems as they cannot do diffusion/ciliary action

10. can have short periods of anaerobic respiration

• small bursts of energy, produces lactic acid

Vertebrates have transitioned from…

filter feeding to active predators

Mineralised Tissues-

• arose later in vertebrate evolution

• are made up of collagen fibres, proteinaceous tissue and hydroxyapatite

  • mineral made up of calcium and phosphorus

  • is more resistant to lactic acid so does not dissolve

Types of mineralised tissue:

1. mineralised cartilage

2. bone

3. enamel, enameloid, dentine

4. cementum

1. mineralised cartilage

• cartilage that has minerals added to it, main skeletal tissue of sharks and cartilaginous fish

• 70% mineralised

2. bone

• internal skeleton of bony fish and tetrapods

• has a blood supply→ can be repaired

• dermal bone→ in the skin e.g. skull

• endochondral bone→ in the body e.g. cartilage precursor replaced by bone

• 70% mineralised

3. enamel, enameloid, dentine

• teeth, exoskeleton of ancient vertebrates, dermal scales

• 90-96% mineralised→ is more tough than bone

4. cementum

• keeps teeth inside its socket

what did early vertebrates have instead of teeth:

odontodes

• tooth-like structures on the outside of animals

• grouped together on the outside to form an armour of mineralised tissue

Development of mineralised tissue in vertebrates:

• Lampreys→ no mineralised tissue

• Ostracoderms→ have an armoured outside tissue made of odontodes

• Bony fish→ retain the exoskeleton through fins and scales but also have an endoskeleton

• Tetrapods→ retain the exoskeleton only in the head region, retain the endoskeleton

Theories of why mineralised tissue evolved:

1. Defense

2. Protecting electroreceptors→ used by some groups to detect prey

3. Store and regulate phosphorus and calcium→ hydroxyapatite

The Earliest Vertebrates→ believed to be:

• conodonts

  • arose ~500mya

  • fossils look like mineralised tissue and had tooth-like structures in their throats

  • could have evolved convergently

The Earliest Vertebrates→ may have been:

• ostracoderms

  • armour of odontodes

The Earliest Vertebrates→ actually was…

• Myllokunmingia

  • fossil found to be from ~530mya

  • has current vertebrate features e.g. notochord, dorsal fin…

  • has early vertebrate features e.g. skull and skeletal elements made of cartilage

The Earliest Vertebrates→ also found…

• Haikouichthys

  • ~530mya

  • can see the sensory structures too→ implies a neural crest

  • may be the same as Myllokunmingia

The earliest vertebrates originated in what environment?

• marine environment:

  • earliest fossils are in marine sediments

  • closest vertebrate relatives are marine (invertebrate chordates and hagfish)

  • the body fluids of vertebrate closest relatives and hagfish are isotonic to marine water (same concentrations)