Evolution of Vertebrates and their features

Question 1

Origin of bone and other mineralised tissues

Answer 1

• early verts didn’t have any mineralised tissues - at start of vertebrate evolution
• hagfish and lampreys do not have mineralised tissues (jawless)
• early verts had odontodes - dermal, mineralised tissues within skin of jawless verts “dermal armour”
• still seen today - denticles of sharks, tooth-like structures, homologous with teeth

Question 2

Evolution of bone and mineralised tissues

Answer 2

• Initially had unmineralised endoskeletons - hagfish & lampreys
• Ostracoderms (armoured fish) mineralised tissue found as an exoskeleton - would have had a cartilage skeleton (not mineralised tho)
• The bony fish retain the mineralised ectoskeleton (scales and fins have rays of ectoskeleton origin), but a mineralised endoskeleton too
• Tetrapods have lost their exoskeleton in the trunk region - retained dermal mineralised tissue in head and mineralised endoskeleton

Question 3

Why did mineralised tissues evolve?

Answer 3

Defensive structure - not sure though, as its embedded in skin, might not be that helpful (e.g. very different to shells)
Protected electroreceptors - in cartilaginous fishes (prey detection)
Regulation of P and Ca - good source -storage if needed

Question 4

Early verts

Answer 4

1. Conodonts - not sure of relationship with other, date from 500mya
2. Ostracoderms - date from 500mya
3. Fossils from China date back to 530mya - oldest fossil
   - Myllojunmingia believed to be vertebrate as has chordate features, skull and skeletal features of cartilage (and dorsal fin) - no mineralisation though

Question 5

Conodonts

Answer 5

Likely to be vertebrates…
•have well-developed head
•similar features like the neural crest and fins
•conodont elements - tooth-like structures of these animals, believed to be mineralised tissues
•earliest conodonts didn’t have these features, evolved later
•not very clear where they fit in with other verts - side branch?

Question 6

Ostracoderms

Answer 6

500mya
•odontodes - from external coverings
•oldest vert?

Question 7

Where did vertebrates evolve?

Answer 7

Originated in the marine E evidence…

1. Marine sediments where earliest fossils are found
2. Comparative physiology - invert chordates and other deuterostomes (echinoderms), all marine animals and have body fluids isotonic to marine water
   - early verts like hagfish, are isotonic with marine water (only vertebrate) and are marine