The Origin of Tetrapods and the Transition to Life on Land Flashcards
Describe the Osteichthyes
- bony fishes
- comprise two clades:
- Actinopterygii
- Sarcopterygii
Actinopterygii
ray-finned fishes
Sarcopterygii
- lobe-finned fishes
- tetrapods, lungfish, and coelacanths
Define the Tetrapoda
- clade containing the last common ancestor of all living tetrapods, and all its descendants
- crown-group tetrapods
Define the Tetrapodomorpha
- sister group to lungfish
- crown-group tetrapods plus all their stem-group relatives
Describe the Devonian
- 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.
Describe Ichthyostega
- 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
Describe Acanthostega
- 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.
Describe Acanthostega
- 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
Define Parmastega
- 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
Describe Tiktaalik
- 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
Describe Brittagnathus
- recently described from the same deposit as Acanthostega
- adult jaw length of just 45 mm
dorso-ventrally flattened form, dorsally-placed eyes, and complete lack of dorsal fins all suggest
surface- skimming or bottom-dwelling existence.
List some features of crown-group Tetrapoda
limbs, digits, ribs, and a neck
Describe polydactylous tetrapods
- 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
Tetrapods must have originated by the
Middle Devonian
Describe the choanae evolution
- internal nostrils
- present in tetrapodomorph fish
- first key character of tetrapods to evolve
Describe extant choanae
allow breathing through the nose, but this would not have been their original function
Describe fish nostrils
- two pairs of external nostrils
- ventilate the nasal cavities
- used for olfaction
- posterior nostrils homologous to the choanae
- migrated inside the mouth
Describe choanae migration inside the mouth
- 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
Describe the evolution of the middle ear
- 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.
hyomandibula bone
supported the palate from the back of the skull
Describe evolution of tetrapod limbs
- 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
Describe evolution of tetrapod digits
- 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
List some tetrapod limb bones
humerus, radius, ulna
Describe evolution of the tetrapod limb
originated as an adaptation for moving about in shallow water – not as an adaptation for life on land
Contrast Tetrapoda and Tetrapodamorpha evolution rates
- 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
Describe evolution of the tetrapod body plan
- 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
Describe Tetrapodomorph body plans
persisted for tens of millions of years alongside more derived forms; document evolutionary changes that occurred long beforehand
Describe the origin of tetrapods
not so much an evolutionary event, as an episode in evolution
Describe the environment of the original tetrapods
- swamps, rivers, estuaries, or lagoons
- locally high tidal ranges in the Devonian might have left large tetrapodomorphs in estuarine environments intermittently stranded
First evidence of complete terrestrialisation of the tetrapod lineage only appears in
the Carboniferous, with the origin of amniotes.
Describe Romer’s Gap
- 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.
Describe Pederpes
- 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
Describe Palaeozoic “amphibia”
- secondarily legless
- mostly fall into two (not necessarily monophyletic) groups: the Temnospondyli and Lepospondyli.
Describe amniote origins
first amniote body fossils appear late in the Upper Carboniferous
