Vertebrate characteristics and early vertebrates

History of the earth

Phanerozoic [EON]

Paleozoic [ERA] : 1.Cambrian 2.Ordovician 3.Silurian 4.Devonian 5.Carboniferous 6.Permian

Mesozoic [ERA]: 1.Triassic 2.Jurassic 3.Cretaceous

Cenozoic [ERA]: 1.Paleogene 2.Eogene 3.Quaternary

What makes vertebrates

1. Prominent head with sense organs: Cranium (bony, cartiligious or fibrous surrounding the brain). Aquatic craniates have evolved gills for respiration (over pharyngeal clefts). Large brain in 3 parts: fore, mid and hind (with Telencephalon)

2. Spinal column : made up of vertebrae that form round the embryonic notochord and nerve cord. Vertebrae replace notochord after embryonic phase.

3. Tend to be larger

4. Neural crest: 4th germ later, involved in production of pigment cells and several structures around the head

5. Placoids: tissue formed by thickening epidermis that gives rise to sensory organs.

6. Hox genes and micro RNAs: hox genes cluster has multiplied twice in vertebrates compared to invertebrates. The number of micro-RNAs is also higher

Vertebrate Tissue

1. Epithelial: covers all body surfaces

2. Connective: organic material (collagen + inorganic substance + living cells) bind structures together, store fat, transport, tissue repair and protect against disease

3. Vascular: Blood

4. Muscular: muscle fibers-apply force by contraction

5. Nervous: coordination, control, cognition, memory and emotion

Fundamental importance of collagen

Softer tissues of organs

Organic matrix of hone

Tough tissue of tendons and ligaments

May be combined with protein elastin which can stretch and recoil

Fibrillar unique to vertebrates

Skin

Keratin= found in epidermis of tetrapods.

15-20% of vertebrate weight. Include scales, hair and dermal armor. Also secretary glands, melanocytes. Vital for protection and exchange with environment

Mineralized tissue

1. Cartilage: Produced=chondrocytes, mineralized found in jawed vertebrates. Main internal skeleton in sharks and other cartilaginous fishes. 70% mineralized

2. Bone: produced=osteocytes. Vascularizsd= it can regrow/repair itself. Can also change shape to adapt to changes in mechanical stresses. Old bone removed by osteoclasts, new bone laid down my osteoblasts. 70% mineralized

3. Enamel : 96% mineralized

4. Dentine: 90% mineralized

5. Enameloid

6. Cementum

Hydroxyapatite

Compound of Ca+P. More resistant to acid than CaCO3 may have played a role in its evolution. Mineralized tissues are made up of collagen protein secreting cells and hydroxyapatite crystals. Crystals are aligned with alternating directions, structure allows strength while remaining relatively light

Bone evolved

From tooth like odontodes probably. Highly complex structure from the outset. Unlikely it was only for protection: mineral deposits, insulation hydrodynamics, acid resistance.

Two types of bone

1. Dermal: from skin, like exoskeleton. Endoskeleton only the brain case.

2. Endochondral: from cartilage that was later replaced by bone, 1st seen in bony fish, endoskeleton, modern skull bones still mostly dermal bone

Sense and sense organs

1. Taste (contact) and smell (distance) -both chemosensory

2. Vision: via cones and rods on a retina

3. Electroreceptors: only in water, some fish and monotremes

4. Balance and orientation:

5. Water vibration

6. Pressure

7. Hearing

8. Immune system: adaptive unique to vertebrates

Notochord

Original backbone, lost in adult tetrapids but vestiges remains part of inter-vertebral disc

Muscles

Composed of myomeres, complex folding. Sequential blocks overlap to produce body undulations as muscles contract. In jawed vertebrates, divided into dorsal and central portions by septum

Chordate locomotion

Serial contraction of muscle bands in trunk and tail. Notochord/spinal cord stiffen the body preventing compression

Respiration and circulation

Ancestral chordates relied on diffusion for gas exchange (some modern still use cutaneous respiration) but its not suitable for large active animals. Lungs/gills therefore better.

Blood is pumped by the heart. The heart is a folded muscular tube divided into 3 chambers (the sinus venous, atrium and ventricle) blood carries O2, nutrients and hormones to the tissues and removes waste. All vertebrates have closed system via capillaries

Nervous system

Made up elongated axon covered in myelin sheath.

1. CNS: many functions controlled by spinal cord. Nerves running between CNS and other parts of the body =PNS

2. Somatic : muscles that we move consciously. Information form sensations that we are made aware of.

3. Involuntary: muscles that are not consciously moved. Sensations that we are not consciously aware of. Sympathetic and parasympathetic in advance vertebrates

Living agnathans

Paraphyletic origin. Hagfish and lampreys more primitive than exist.

Hagfish

Lack jaw and paired fins. Lack mineralized tissue. No specialized reproductive ducts. From Carboniferous deposit. 75 species and 2 genera. Elongated no scales ~0.5m long. Entirely marine, cold water usually in deep sea. Mucous gland protection against predation. No vertebrate, simple kidney. Single semi circular canal on each side of the head. Single nasal opening connects to pharynx and gill slit. Low blood pressure- has auxiliary hearts in liver and tail. Skin used to make eelskin. Lawless mouth surrounded by 6 tentacles (help search for food). Keratin ‘teeth’ on two plates in the mouth. Scavengers (strong sense of smell and degenerate eyes) feed on dead/dying animals, attach to animal tie a know in their tails, pass the knot forward, eat into colonic cavity and then feed on soft tissue. Odd sex ratio (100;1)

Lampreys

From Devonian and Carbonferous periods. 40 species 2 genera. Similar shape and size to lampreys. Skeletal elements, shared with jawed fish, unique singer nasal opening on top of head. Well developed eyes and pineal gland. Two semi-circular canals on each side of the head. Heart is worked by parasympathetic nervous system. 7 pairs of gills-tidal ventilation. Feeding (almost all parasitic, mouth at the bottom of oral hood, attach to host by suction scrape a wound through skin, tidal ventilation, teeth keratin conical spines, also has spines on tongue like structure, feeds mostly on bodily fluids. Secretes anti-coagulant).

Reproduction: Anadromous. Males and females construct nests by thrashing. Females produce 1000s of eggs. Males fertilize them as they are laid. Larvae hatch in 2 weeks. Leave the next in about a week. Carried downstream. Spend years in a burrow in sheltered banks as filter feeders. Adult like 2 years or less

Myllokunmingia

Early Cambrian period (500mya). Fossil beds in China. 3cm long. Soft bodied. Dorsal fins. Paired sensory structures. Cranium. Mouth, gill pouches, ventral fins, myomeres.

Conodonts

Late Cambrian to late Triassic. Tooth like elements were 1st mineralized fossils. Whole animal fossile found later, proving vertebrates with mineralised tissue. Apparently used for feeding. Anterior and posterior have different functions

Ostracoderms

From early ordovician period. Globally diverse about 400 Mya. Extinct by end of Devonian. Shell skin. Bony plates on head below skin. Bony scales on body. Up to 35cm long. Several gill openings. Diverse vertebrates by this time.