L3- Origin of Vertebrates

  • 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:

  • Have vertebrae 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

  • Not all vertebrates have fully formed vertebrae

    • 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)

  • Vertebrates have a cranium

    • can be bony, cartilaginous or fibrous

    • e.g. the skull

  • Vertebrates have an endoskeleton

    • made up of the vertebrae and the cranium

    • was cartilaginous in early vertebrates

  • Vertebrates have duplications of the hox gene complex

    • developmental genes that control anterior-posterior patterning

    • 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

  • Vertebrates 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

  • Vertebrates have placodes

    • complex sense organs, homologs to invertebrate chordates

  • Vertebrate brains are larger and made up of 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

  • Vertebrates are larger and do more activity (higher metabolic rate)

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

  • Vertebrates 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

  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 and Dentine→

    • teeth, exoskeleton of ancient vertebrates, dermal scales

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

  4. Cementum→

    • keeps teeth inside its socket

  • early vertebrates had odontodes instead

    • 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

  • May have been ostracoderms

    • armour of odontodes

  • 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

  • Also found Haikouichthys

    • ~530mya

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

    • may be the same as Myllokunmingia

  • Early vertebrates originated in a 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)