Tetrapods (Lec 11)

Question 1

Tetrapod origins

Answer 1

Animals had to transition from life in water to life on land

Transition included:

• -Challenges
• -Opportunities

A diversity of Denovian FW fish lineages had highly vascularized lung-like structures
–Allowed them to take advantage of atmospheric oxygen

Origin of double-circulation pattern
–Dedicated systemic and pulmonary circulation

Origin of limbs and digits

• -Likely evolved in relation to movement in shallow ephemeral waters
• -Modifications facilitating mobility on land in a variable environment

Question 2

Opportunities of Life Transition

Answer 2

Oxygen is 20x more abundant on land and diffuses faster

Oxygen easily acquired on land

HUGE variety of habitats on land
–Abundant shelter (vegetation) and food (especially inverts)

Question 3

Challenges of Life Transition

Answer 3

Air is ~800x LESS dense than water (at standard temp and pressure) = no buoyancy -> must deal with gravity

Must evolve new ways to support body:

• -Girdles and limbs
• -Skeletal remodeling

Terrestrial habitats have more frequent and extreme cycles of freezing/thawing, drying/flooding, etc.
–Water provided INERTIA against fluctuations

Question 4

Life in the Denovian

Answer 4

Period began >400 mya (419-359 mya)

Many land masses near equator, swampy with FW, low elevation

Mild and consistent temps BUT many FW environments unstable due to alternating wet and dry periods
–Perhaps one of the catalysts to make transition to land

Question 5

Why invade land? DROUGHT Hypothesis

Answer 5

Denovian ponds dried intermittently: going to land would allow transfer to other water sources

LIMITATION: Animals at this time still dependent on water and led aquatic existence

Question 6

Why invade land? Competition and Predation Release Hypothesis

Answer 6

Many fishes were occupying the same aquatic niches = competition; most predators = predation

NO land verts in Denovian

• -Lack of predators
• -Lots of prey (i.e. arthropods)
• -Recall driving forces for ADAPTIVE RADIATION

Question 7

Tetrapodomorpha

Answer 7

Includes Tetrapoda and all EXTINCT sarcopterygians sharing a more recent common ancestor with tetrapods than any extant sarcopterygian

SYNAPOMORPHIES:

• -Choanae (internal nostrils):
• —Fish have 2 pairs of external nostrils with olfactory apparatus between
• -Humerus with convex head articulating with glenoid fossa

Question 8

Sarcopterygians and Early Tetrapods

Answer 8

Tetrapoda nested within Tetrapodomorpha, Tetrapodomorpha within Sarcopterygia, Sarcopterygia within Osteichthyes/Eutelostomi…

EARLIEST “amphibian-like” creatures date from late Denovian (Acanthostega, Ichtyostega)

Includes:

• Eusthenopteron
• Panderichtys

Question 9

Eusthenopteron

Answer 9

Able to move along bottom of pools using lobed fins (“Walked” along bottom)

Pectoral girdle attached to skull = NO neck = FISH characteristic

Early origins of limbs

HOMOLOGIES:
-Between lobed pectoral fin and major elements of tetrapod forelimb

Question 10

Panderichtys

Answer 10

Eusthenopteron BASAL to this group

Shallow water dweller:

• -Depressed skull w/ dorsal eyes (like early tetrapods)
• -Dorsal and anal fin LOST and caudal fin REDUCED
• -Pectoral girdle attached to skull (NO neck)

Question 11

Tiktaalik

Answer 11

Found in 2004, described in Nature in 2006

Late Denovian (~375mya)

Dubbed “fishapod”
–Good transition stage between Eustenopteran/Panderichtys and Tetrapods

Fish-like:

• -Fins
• -Scales
• -Gills

Tetrapod-like:

• -Skull shape
• -NECK (synap)
• -Ribs (synap)
• -SHOULDER, ELBOW, WRIST elements (synap)

Fin rays instead of digits radiating from wrist

Skull NOT attached to pectoral girdle = MOST BASAL “necked” vert lineage known

Distal components of appendages mix homologs to elements in sarcopterygians and tetrapods

MOST compelling ex. of animal close to fish/tetrapod transition

RETAIN lateral line system

Question 12

Tetrapoda

Answer 12

EXTANT + EXTINCT Tetrapods

SYNAPOMORPHIES:

• Digits on front and rear paired appendages
• Various cranial and post-cranial skeletal modifications

Question 13

CROWN Tetrapoda

Answer 13

Only EXTANT tetrapods

SYNAPOMORPHIES:

• Most recent common ancestor of all extant tetrapods and all descendants from that ancestor
• 5 digits on front and rear paired appendages

Question 14

Acanthostega

Answer 14

Early example of sturdy girdles and well-formed limbs

Many digits

• -REDUCTION and LOSS
• -Usually 5 in front and back in CROWN tetrapods

Primarily aquatic

Limbs too weak for long walks on land
–Could get out of water at shoreline and move between close pools

Stapes:

• Most BASAL lineage where hyomandibula is called stapes
• -Homologies:
• –Hyomandibula (from hyoid arch)
• Suggests animal could detect airborne sounds

Question 15

Ichthyostega

Answer 15

More developed girdles and sturdier limb bones than Acanthostega

Supporting vertebral column, rib cage, and associated muscles

Still had ancestral characters (also Acanthostega) of sarcopterygian ancestors:

• Gills
• Opercular bones over gills
• Rays supporting tail fin
• Lateral line system (aquatic HABIT)
• Prominent notochord

Question 16

Acanthostega and Ichtyostega had ________________________________

Answer 16

A mix of fish-like and tetrapod-like characteristics

Question 17

Carboniferous Radiation of Tetrapods

Answer 17

Time of warm, moist, swampy environment with many insects on land and invertebrates in waters

Opportunity for tetrapods to invade new terrestrial habitats and diversify (adaptive radiation)

Radiation gave rise to diverse lineages: 2 major lineages have extant representatives:

1) Lissamphibia
2) Amniota

Question 18

Temnospondyli

Answer 18

Several EXTINCT lineages _ Lissamphibia

SYNAPOMORPHIES:

• Tympanic membrane like most extant amphibians (capable of detecting airborne sounds)
• 4 digits on forelimb (like extant amphibians)

Question 19

Lissamphibia

Answer 19

3 EXTANT amphibian clades (Anura, Caudata, Gymnophiona)