Phylum Chordata and Early Vertebrate Evolution

- subphylum Vertebrata
- Infraclass Gnathostomata (jaw vertebrates)
- Class chondrichthyes (cartilaginous fishes)

1. Sarcoptergyii (Teleostomi=> Ostrichthyes)
2. Actinoptergyii (Teleostomi=> Ostrichthyes)
3. Acanthodii (extinct)
4. Chondrichthyes
5. Placodermi (extinct)

- appear at some point in their life, or only in the embryo
- bilateral symmetry
- dorsal nerve cord (=spinal cord) (runs down the dorsal side, connecting the brain with the muscles and other organs)
- Notochord (stiff cartilaginous rod which supports the nerve cord)
- myotomes = muscle blocks
- pharyngeal gill *slits: a series of openings that connect the inside of the throat to the outside of the neck
- post-anal tail: an extension of the body past the anus
- Blood that circulates forward in a main ventral vessel and backward in a dorsal vessel

- the ancestor was considered to be *Pikaia
- had tentacles
- 5 cm in length
- not a vertebrate
- recent fossil Haikouichthyes is now considered to be among the oldest possible true craniate, as it has a defined skull (an ancestor of all vertebrates?)

- fishes, reptiles, amphibians, birds, mammals
- all have a vertebral column, or a chain of bony elements that:
+ runs along the dorsal surface from head to tail and forms the main skeletal axis of the body
+ surrounds and more or less replaces the notochord as the chief 'stiffener 加強筋' of the body in locomotion
- general body plan: head, truck, 2 pairs of appendages, and a post-anal tail (but highly modified in many vertebrates and sometimes the tail is absent altogether)
- later animals developed a cranium, or brain case (Craniata)
- the first vertebrates were *fishes

- small, slow-moving jawless fishes were around filter-feeding 500mya: the *ostracoderms (shell skin)
- then evolution of jawed fishes ~ mya, including the armoured placoderms (plate-skin).
- all extinct by the late Devonian( The Age of Fishes)

- Approx. 400mya in the Devonian period
- all 5 major lineages of fishes co-existed for these 48 million years
1. Ostracoderms (extinct)
2. Placoderms (extinct)
3. Chondrichthyes (cartilaginous fishes)
4. Acanthodii (extinct)(cartilaginous fishes)
5. Osteichthyes
- some early Osteichthyes also have some cartilage

- are the cartilaginous fishes
- the chondricthyan lineage survived to the present day with ~850 extant species, which includes chimaeras 吐火银鲛, sharks and rays鰩
- they have a mainly cartilaginous endoskeleton that is not strongly calcified (like in bone)
- the only bony parts are their teeth and scales
- Osteichthyes are the bony fishes

- sharks, rays, skates and chimaeras are characterized by a skeleton of tough, light, flexible cartilage lined with hard tissue which provides enough structural support to allow these fish to grow to large sizes
- cartilage (deposited by cartilage cells) is different from skeleton of other fishes which is composed of bone (also cartilage has no blood supply
- but cartilage does not fossilize well, unlike bone, and so there are not many fossils of Chondrichthyes except for the bony teeth
- important in Chondrichthyes as they do not have a swim bladder( its lighter than bone and helps keep the shark from sinking, also allows the shark to turn in a tighter radius than other fish)
- cartilage found in jaws and backbones of sharks require more strength than cartilage found in fins. such areas are strengthened with calcium salts forming a calcified *cartilage which has similar strength to bone and is hard and brittle but without the added weight, while the other is softer and weaker

- a cartilaginous braincase and cartilaginous endoskeleton are adaptations for weight reduction
- note that bone actually preceded cartilage in fishes in evolutionary terms, e.g. ostracoderms.
- A cartilaginous skeleton results from a secondary loss of bone
- Embryos of Chondrichthyes start with a notochord that is then surrounded by cartilage.

- chondrichthyes and Osteichthyes (bony fishes) share a clade, but had evolved separately and in parallel with 同時 each other
- chondrichthyes arose ~380mya in the Devonian Period
- among the earliest chondrichthyans was the genus Cladoselache (extinct)

- chimaeras (also called ratfish or rabbitfish) are typically deep water species
- their upper jaw is fused to the brain case, hence their name holocephali means 'whole head'
- have a single pair of gill openings and no scales

- the elasmobranchs (meaning Strap gills) are characterized by having multiple gill *slits
- includes sharks, skates and rays
- sharks:
+ modern sharks are highly evolved
+ their body has changed relatively little in the last ~150 million years because they have a successful design of their body.

- upper jaw is not fused to the braincase (cranium)
- teeth are replaceable (continually growing)
- multiple gill slits
+usually 5, but 6 or 7 in some species
- unsegmented fin rays
+ called ceratotrichia, contrast bony fishes)
- the skin of sharks are covered in placoid scales, which have the same origin as teeth (with enamel and dentine)
- lack swimbladders (gas bladders), instead have large, oil-filled livers for buoyancy浮力 (up to 25% of body fluid)
- the spiral valve intestine increases surface area for digestion
- male sharks have a pair of claspers situated at the pelvic fins (are intromittant organs 插入器官 used in mating for internal fertilization)
- heterocercal 歪尾的 caudal fin(tail fin): asymmetrical, with upper lobe larger, provides uplift when swimming
- Ampullae of Lorenzini: small pits on the head that are used for electro-reception sense (好似比人用針吉左好多個lone咁)
+ are most concentrated on the ventral side of the snout, used for detecting the weak electro-magnetic waves given off by *prey
- generally large in size (>1 m)
- Carnivorous, mostly predator fishes
- most Chondrichthyes are marine, but there are some freshwater rays and only very few freshwater sharks

- river sharks: little known
- Asia and Australia
- bull shark

1. superorder Selachimorpha
2. superorder Batoidea

- there are over 350 species of sharks in the world
- the largest is the whale shark
1. pelagic sharks
2. demersal sharks

- swim in the water column and usually have a streamlined body shape for speed as predators
- large, stiff pectoral fins act as hydrogoils, providing lift
- some pelagic sharks must keep swimming in order to obtain enough *oxygen through their gills

- skates and rays
- are dorsoventrally flattened
- mouth and gill slits are on ventral side
- are believed to have evolved from sharks
- over 500 species found around the world
- many are benthic species, some are pelagic 遠洋的 (e.g. manta ray) and swim using their pectoral fins胸鰭

- morphologiclly no single distinguishing feature to differentiate between them, but rays give birth to live young while skates lay eggs
- *rays belong to 3 orders: Pristiformes, Myliobatiformes, Torpediniformes
- Skates are classified in the order Rajiformes (includes skates, guitarfish)

- 2 basic modes
1. egg laying (40%)
+ oviparity
2.give birth to live young (60%)
+ ovoviviparity
+ lacental viviparity

- typically characterized by:
+ slow growth and long life-span (70-100 years)
+ late maturity (most in 6-18 years)
+ low reproductive rate (low *fecundity多產)
+ low natural mortality rates
+ a typical K-strategy, or K-selected, life hisotry (to grow slowly, live close to the carrying capacity of their habitat and produce a few progeny each with a high probability of survival)
- sharks and rays are therefore extremely vulnerable to overfishing.
- As apex predators (at the top of a food chain) in the marine ecosystem, they occur in low numbers
- they have evolved in an environment where they have few or no natural predators

- the practice of cutting off the shark's fins and dumping the body, is banned in many countries
- shark without its fins cannot survive and dies a slow death