The Origin of Tetrapods and the Transition to Life on Land

Question 1

Describe the Osteichthyes

Answer 1

• bony fishes
• comprise two clades:
• Actinopterygii
• Sarcopterygii

Question 2

Actinopterygii

Answer 2

ray-finned fishes

Question 3

Sarcopterygii

Answer 3

• lobe-finned fishes
• tetrapods, lungfish, and coelacanths

Question 4

Define the Tetrapoda

Answer 4

• clade containing the last common ancestor of all living tetrapods, and all its descendants
• crown-group tetrapods

Question 5

Define the Tetrapodomorpha

Answer 5

• sister group to lungfish
• crown-group tetrapods plus all their stem-group relatives

Question 6

Describe the Devonian

Answer 6

• c. 416-360 Mya
• 400 days lasting 22h each in a year
• landmasses forming the two great supercontinents of Gondwana and Euramerica
• global temperatures ~ 5 ̊C warmer in the Early Devonian than today but cooled in the Middle Devonian
• major oxygenation event occurred in the Early Devonian
• O2 levels decreased sharply in the Middle Devonian
• time of immense biotic and abiotic change, witnessing the radiation of land plants, from small simple lycophytes in the Early Devonian to the first true forests of large progymnosperms by the Late Devonian.

Question 7

Describe Ichthyostega

Answer 7

• a stem tetrapod
• first Devonian tetrapod to be discovered
• found in strata from Greenland dating to 365-359 Mya
• gills
• ear specialised for hearing both underwater and in air
• restricted hip and shoulder mobility suggests a seal-like movement on land

Question 8

Describe Acanthostega

Answer 8

• another stem tetrapod
• More gracile than Ichthyostega and known from the same strata (365-359 Mya), but never found together in the same deposit, presumably indicating ecological differentiation. Description of the first complete material in the 1990’s exploded the old notion that limbs evolved as an adaptation to terrestrial life.
• Probably almost completely aquatic, Acanthostega had paddle-like limbs, a large caudal fin, a large gill chamber, an extensive lateral line system, and a feeding apparatus specialised for aquatic feeding. Still the most significant of the Devonian tetrapod finds to date, but not the earliest.

Question 9

Describe Acanthostega

Answer 9

• another stem tetrapod
• more gracile
• almost completely aquatic
• paddle-like limbs
• large caudal fin
• large gill chamber
• extensive lateral line system
• feeding apparatus specialised for aquatic feeding

Question 10

Define Parmastega

Answer 10

• stem tetrapod
• older than either Ichthyostega or Acanthostega (~372 Mya)
• Russian specimen
• resembles a snub-nosed crocodile
• eyes sticking up above its head
• cruising ecology

Question 11

Describe Tiktaalik

Answer 11

• stem tetrapodomorph
• ~375 Mya
• considered a bona fide “fish”
• fins, rather than limbs
• distinctly croc-like appearance given by its elongate snout, flattened head, and dorsally-located eyes
• neck separating head from body
• presumably used either to raise its head above the surface to breathe, or to snap sideways at prey in its estuarine environment

Question 12

Describe Brittagnathus

Answer 12

• recently described from the same deposit as Acanthostega
• adult jaw length of just 45 mm

Question 13

dorso-ventrally flattened form, dorsally-placed eyes, and complete lack of dorsal fins all suggest

Answer 13

surface- skimming or bottom-dwelling existence.

Question 14

List some features of crown-group Tetrapoda

Answer 14

limbs, digits, ribs, and a neck

Question 15

Describe polydactylous tetrapods

Answer 15

• pentadactyl limbs ancestral for crown-group Tetrapoda
• Ichthyostega had 7 digits on its hindlimbs
• Acanthostega had 8 digits on its forelimbs
• pentadactyly is not the ancestral state for tetrapods

Question 16

Tetrapods must have originated by the

Answer 16

Middle Devonian

Question 17

Describe the choanae evolution

Answer 17

• internal nostrils
• present in tetrapodomorph fish
• first key character of tetrapods to evolve

Question 18

Describe extant choanae

Answer 18

allow breathing through the nose, but this would not have been their original function

Question 19

Describe fish nostrils

Answer 19

• two pairs of external nostrils
• ventilate the nasal cavities
• used for olfaction
• posterior nostrils homologous to the choanae
• migrated inside the mouth

Question 20

Describe choanae migration inside the mouth

Answer 20

• basal tetrapodomorph fish Kenichthys
• posterior nostril is in the upper jaw, interrupting the tooth arcade
• perfect intermediate between the ancestral (external) and derived (internal) states

Question 21

Describe the evolution of the middle ear

Answer 21

• Late Devonian
• tetrapodomorphs had enlarged notch-shaped spiracles
• might have permitted air-breathing when the head was above water
• enlargement of the spiracle associated with shortening of the hyomandibula bone
• enlarged spiracle would later become the middle ear of tetrapods, and the hyomandibula the stapes.

Question 22

hyomandibula bone

Answer 22

supported the palate from the back of the skull

Question 23

Describe evolution of tetrapod limbs

Answer 23

• in tetrapods, pelvic girdle is usually larger than the pectoral girdle
• opposite in tetrapodomorph fish
• tetrapod locomotion: body-flexion propulsion
• pelvic fins played only a minor part in anchoring the body

Question 24

Describe evolution of tetrapod digits

Answer 24

• homologues of the limb bones of tetrapods in the lobe fins of tetrapodomorph fish (e.g. Panderichthys)
• amoung the last features of limbs to evolve
• homologous with the distal radials of lobe fins

Question 25

List some tetrapod limb bones

Answer 25

humerus, radius, ulna

Question 26

Describe evolution of the tetrapod limb

Answer 26

originated as an adaptation for moving about in shallow water – not as an adaptation for life on land

Question 27

Contrast Tetrapoda and Tetrapodamorpha evolution rates

Answer 27

• early Tetrapoda and closest relatives among the stem tetrapods show elevated rates of morphological evolution through the last 30 myr of the Devonian (associated with the origin of the tetrapod body plan)
• Tetrapodomorpha show very low rates of morphological evolution associated with stabilizing selection on their “fish” morphotype

Question 28

Describe evolution of the tetrapod body plan

Answer 28

• involved modifications to all the major organ systems and body parts: only some documented in the fossil record
• changes highly interrelated, and highly contingent (e.g. origin of the middle ear was made possible by changes to the breathing apparatus)
• most key features evolved in water

Question 29

Describe Tetrapodomorph body plans

Answer 29

persisted for tens of millions of years alongside more derived forms; document evolutionary changes that occurred long beforehand

Question 30

Describe the origin of tetrapods

Answer 30

not so much an evolutionary event, as an episode in evolution

Question 31

Describe the environment of the original tetrapods

Answer 31

• swamps, rivers, estuaries, or lagoons
• locally high tidal ranges in the Devonian might have left large tetrapodomorphs in estuarine environments intermittently stranded

Question 32

First evidence of complete terrestrialisation of the tetrapod lineage only appears in

Answer 32

the Carboniferous, with the origin of amniotes.

Question 33

Describe Romer’s Gap

Answer 33

• few complete tetrapod fossils from the first 15My of the Carboniferous
• when tetrapods do begin to be found in large numbers, they already have all the derived characters of crown-group Tetrapoda.

Question 34

Describe Pederpes

Answer 34

• currently the only articulated fossil from Romer’s gap
• stapes primitive in form,
  resembling that of Acanthostega (hearing not very well developed)
• pentadactyl hindlimb
• polydactyly cannot be ruled out on the forelimb
• metatarsal asymmetry may indicate a forward pointing foot typical of a walking (rather than sprawling) gait

Question 35

Describe Palaeozoic “amphibia”

Answer 35

• secondarily legless
• mostly fall into two (not necessarily monophyletic) groups: the Temnospondyli and Lepospondyli.

Question 36

Describe amniote origins

Answer 36

first amniote body fossils appear late in the Upper Carboniferous