Comparative Anatomy Test 1 Lecture Notes

Homologous vs. Analogous features

Homologous: equivalent structure, common ancestor; Analogous: equivalent structure, convergent evolution,

Synapomorphy definition + examples

Exclusive shared derived character from most recent common ancestor; Ex: Mammals (hair, mammary glands), Tetrapods (four limbs with digits),

Skeleton classification

Postcranial: Axial (skull, vertebral column, ribs, sternum), Appendicular (girdles, appendages); Cranial: Splanchnocranium, Chondrocranium, Dermatocranium,

Mesenchyme

Undifferentiated stem

Neural crest

Migratory stem cells → teeth, cartilage, smooth muscle, endocrine tissue,

Endoderm

Innermost germ layer → GI tract + organs (pancreas, etc.),

Mesoderm

Middle germ layer → skeletal, muscular, excretory systems,

Ectoderm

Outer germ layer → epidermis, hair, enamel, nervous/endocrine tissue,

What is the somite

Mesodermal blocks → dermatome, myotome, sclerotome,

Dermatome

Forms dermis,

Myotome

Forms skeletal muscles,

Sclerotome

Forms vertebrae + ribs,

Dermatocranium

Outer skull covering, from dermal bone,

Chondrocranium

Brain/support structures,

Splanchnocranium

Gill support → jaws,

Jaw articulation Paleostyly

Arches separate from skull,

Jaw articulation Euautostyly

Jaws attach directly to skull,

Jaw articulation Amphistyly

Double connection (palatoquadrate–braincase + hyomandibula),

Jaw articulation Hyostyly

Lower jaw attached via hyomandibula (sharks),

Jaw articulation Metautostyly

Lower jaw attaches via quadrate,

Jaw articulation Craniostyly

Upper jaw fused to braincase, lower jaw suspended by dermatocranium (mammals),

Cranial kinesis

Skull bone mobility for feeding (snakes, birds),

Modes of cranial kinesis and what they are

Metakinesis (Whole upper jaw), Mesokinesis (Middle of upper jaw), Prokinesis (Front of upper jaw), Streptostyly (Entire lower jaw moves)

Jaw–ear connection

Quadrate and articular bones become incus and malleus of the middle ear.

This illustrates how jaw elements were repurposed for hearing

Temporal fenestrae evolution

Anapsids: none; Synapsids: one low opening; Diapsids: two; Euryapsids: one high opening,

Atlas

Holds head upright,

Axis

Allows head to move side to side

Cervical vertebrae

Support head, increase mobility,

Thoracic vertebrae

With ribs, aid respiration + trunk stability,

Lumbar vertebrae

Rib-free for locomotion and mobility

Sacral vertebrae

Fused, connect pelvis to axial skeleton,

Caudal vertebrae

Tail, locomotion, balance,

Amphicoelous vertebrae

Both ends concave (fish), flexible, weak under compression,

Procoelous vertebrae

Anterior concave, posterior convex (reptiles), stable, flexible,

Opisthocoelous vertebrae

Anterior convex, posterior concave (amphibians/mammals),

Acoelous vertebrae

Flat ends (mammals), resist compression, support weight,

Heterocoelous vertebrae

Saddle

Arcualia

Primitive incomplete vertebral elements (lamprey); precursors to vertebrae,

True ribs

Connected to sternum,

False ribs

Connect to another rib, not sternum,

Floating ribs

No anterior articulation,

Cranial

Toward head,

Caudal

Toward tail,

Rostral

Toward nose,

Anterior

Toward front,

Posterior

Toward back,

Dorsal

Toward back,

Ventral

Toward belly,

Proximal

Closer to origin,

Distal

Away from origin,

Palmar

Toward palm,

Plantar

Toward foot sole,

Pectoral girdle evolution

Originated earlier, supported fins, connected to skull then separated, diverse functions,

Pelvic girdle evolution

Appeared later, anchored to vertebral column, conservative, locomotor support,

Stylopodium evolution

One large proximal element (humerus/femur), conserved,

Zeugopodium evolution

Paired bones (radius/ulna, tibia/fibula), reduction/specialization,

Autopodium evolution

Early polydactyly, standardized pentadactyly, specialized reduction (horses, birds),

Gill theory

paired fins evolved from gill (branchial) arches.

Fin-fold theory

paired fins originated from continuous lateral folds of skin along the body.

Pneumatic bone

Air spaces, lighter (birds, dinosaurs),

Apneumatic bone

Solid, heavy, stronger under compression,

Epidermis

Outer epithelial layer, keratinized,

Basement membrane

Anchors epidermis to dermis,

Dermis

Connective tissue; vessels, glands, follicles,

Hypodermis

Fat storage, insulation, connection to muscle,

Hair

Keratinized, insulation, sensory,

Nails/claws/hooves

Keratinized tips, protection, locomotion,

Beaks/horns

Keratinized coverings, feeding/defense/display,

Glands

Sebaceous, sweat, scent, mammary,

Teeth

From dermal armor, enamel + dentine,

Muscle structure hierarchy

Myofibril → fiber → fascicle → muscle; CT = endomysium, perimysium, epimysium,

Flexion

Decreases angle at joint,

Extension

Increases angle at joint,

Elevation

Up,

Depression

Down,

Abduction

Away laterally,

Adduction

Toward midline,

Pronation

Radius crosses ulna (palm down)

Supination

Radius uncrosses ulna (palm up)

Lateral flexion

Bending sideways,

Internal rotation

Toward body,

External rotation

Away from body,

Epaxial musculature

Dorsal: with vertebrae, ribs, skull base,

Hypaxial musculature

Ventral: locomotion, respiration, prey capture,

What features unite all chordates?

Notochord, dorsal hollow nerve cord, pharyngeal slits, endostyle/thyroid, postanal tail

What additional features define vertebrates?

Cranium, vertebral column, neural crest cells

What chordate like traits are found in echinoderms?

Larvae have bilateral symmetry and are free floating

What chordate like traits are found in hemichordates?

They have pharyngeal slits, some bilateral symmetry, and some are free floating but lack a notochord and dorsal hollow nerve cord

Why is metamorphosis important for understanding chordate origins?

Metamorphosis – Tunicate larvae show chordate traits (notochord, dorsal nerve cord, postanal tail, pharyngeal slits). These traits are lost in adults, suggesting early chordates came from a larval form that retained these features into adulthood (paedomorphosis).

Why is embryology important for understanding vertebrate origins?

Embryology – Embryonic development shows how chordate body plans form. Neurulation creates the dorsal hollow nerve cord; the notochord marks the midline. Somites form in paired blocks (bilateral symmetry). Pharyngeal arches function in filter-feeding in primitive chordates and become jaws/ear structures in vertebrates. Vertebrates also develop a head with specialized sense organs from unique embryonic tissues.

What is dorsoventral inversion in chordates?

Definition: In protostomes the nerve cord is ventral and the heart is dorsal. In chordates this is reversed: a dorsal hollow nerve cord above the notochord and a ventral heart.

Significance: Explains the unique chordate body plan, uniting traits around a dorsal CNS. Shows how a major evolutionary shift in body axes produced efficient, directed locomotion and distinguishes chordates from protostomes while using the same developmental toolkit.

Explain how body symmetry evolved and developed in early chordates and vertebrates.

Radial → Bilateral: Early animals (cnidarians) radial; bilaterians evolved bilateral symmetry for cephalization and movement.

Chordates: Bilateral body plan with notochord midline, paired somites, larval symmetry for swimming.

Vertebrates: Further refinement—cranium, vertebral column, paired appendages; external symmetry maintained for locomotion.

Significance: Bilateral symmetry supports efficient movement, nervous system coordination, and flexibility in paired structures.