lecture 2: vertebrate relationships

what are the three subphyla of the phylum chordata

• subphylum vertebrata

• plys non vertebrate chordates: subphylym cephalochordata and subphylum urochordata

which phyla are deuterostomes

chordata, echinodermata and hemichordata

what are deuterostomes

anus forms before the mouth

what are the five chordate characteristics

1. notochord

2. dorsal hollow nerve cord

3. postanal tail

4. pharyngeal (gill) slits

5. endostyle/thyroid gland

what is the notochord

• semi rigid rod for support and muscle attachment

• replaced by vertebrae in most vertebrates

what is the dorsal hollow nerve chord

• forms spinal chord

• appears to be induced by presence of notochord

• ectodermal cells grow upward at dorsal midline forming a hollow tube

what is the post anal tail

• segmented, muscular tail

• extending beyond gut region

• present in all chordates during embryonic development

what are the pharyngeal (gill) slits

• paired openings in walls of anterior pharynx

• likely evolved as filter feeding mechanism

• modified for respiration in fish

• present in tetrapod embryos but modified during development

what is the endostyle/thyroid gland

• endostyle in adult non vertebrate chordates and larval lampreys

• groove of ciliated glandular tissue on pharynx floor

• secretes mucus to trap food

• homologous to thyroid gland of vertebrates

• larval lamprey endostyle metamorphoses into adult thryoid

what is cephalochordata and what are common examples

a subphylum of around 25 small, marine, superficially fishlike animals; includes lancelets

what is the lifestyle and habitat of cephalochordates

mostly burrowing and sedentary marine animals

what role does the notochord play in cephalochordates

the anterior elongation of the notochord helps with burrowing

what is the function of pharyngeal slits in cephalochordates

used for filter feeding, not for gas exchange

what are myomeres and how do they function

myomeres are blocks of striated muscle separated by connective tissue; their sequential contraction bends the body side to side for movement

what is urochordata and what organisms does it include

uro=tail, includes tunicates or sea squirts; around 2000 species, all marine

what are the key adult characteristics of most tunicates

sedentary adults with large ciliated pharynx for filter feeding, encased in a fibrous “tunic”

what are the characteristics of tunicate larvae

free swimming larvae that possess all five chordate characteristics (notochord, dorsal nerve, post anal tail, pharyngeal slits, endostyle)

what happens during tunicate metamorphosis

larvae transform into adults and lose the tail, notochord and dorsal nerve chord

what are larvaceans and how are they different from other tunicates

they do not undergo metamorphosis; they reach sexual maturity in larval form and retain the notochord, tail, and dorsal nerve cord as adults

how has our understanding of tunicate evolution changed

older classifications grouped cephalochordates and vertebrates as sister taxa, but molecular evidence shows the sessile adult tunicate stage is derived, involving secondary loss of segmentation and sharing derived traits with vertebrates

why was the term craniata used in older classifications

because hagfishes lack true vertebrae but do not have a skull so they were separated from vertebrata

how are hagfishes classified in modern taxonomy

in current classifications hagfishes are included within the subphylum vertebrata

why might hagfishes appear “primitive”

some traits (e.g., reduced eyes, lack of vertebral elements) are likely due to secondary loss, not ancestral conditions

what does molecular evidence show about hagfishes and lampreys

hagfishes and lampreys form a monophyletic group, meaning lampreys are not sister to jawed vertebrates

what are the 7 synapomorphies of vertebrates

1. cranium (skull that protects the brain)

2. tripartite brain

3. complex sense organs

4. pharyngeal arches respiratory

5. vertebrate (secondarily lost in hagfishes)

6. dorsal fin (secondarily lost in hagfishes)

7. pituitary

how do vertebrates differ from other chordates in size and activity

vertebrates are generally larger and more active

why do vertebrates require complex organ systems

their larger size and activity mean diffusion and ciliary action are insufficient, so specialized organ systems are needed

what physiological and structural features support vertebrate activity

higher metabolic rate, well-developed muscles and skeleton, and a flexible but protective outer covering

what unique mineral is found in some vertebrate tissues

hydroxyapatite (calcium + phosphorus), found in bone enamel and bone mineral

why is hydroxyapatite advantageous compared to calcium carbonate

it is more resistant to acid (e.g., lactic acid) than calcium carbonate found in mollusk shells

do all vertebrates have mineralized tissues

no, some vertebrates, such as hagfishes and lampreys, lack mineralized tissues

what are the three main types of bone that evolved in vertebrates

dermal bone, perichondral bone, and endochondral bone

what is dermal bone and where does it form

bone that forms in the skin

what is perichondral bone

bone that forms around cartilage or existing bone within the body

what is endochondral bone

bone that develops from a cartilaginous precursor within the body