The Origin of Tetrapods Flashcards
What is a tetrapod
A vertebrate descended from an ancestor with four limbs
Which extant groups are tetrapods
Living amphibians and amniotes
Why is defining tetrapods problematic when considering transitional forms
Because fins transformed gradually into limbs, creating ambiguity
What are key derived characters of tetrapods besides limbs with digits
A neck, ribs for weight support, internal nostrils (choanae), and more
What are the two clades of Osteichthyes (bony fish)
- Actinopterygii (ray-finned fishes)
- Sarcopterygii (lobe-finned fishes)
Which extant animals are sarcoterygians
Tetrapods, lungfish, and coelacanths
What is Tetrapoda
The clade containing the last common ancestor of all living tetrapods and its descendants
What is Tetrapodomorpha
Crown-group tetrapods + all their stem-group relatives
What is meant by a “tetrapod” in a broader sense
Any animal primitively possessing four limbs, including stem tetrapods
When did tetrapods originate
During the Devonian (~416–360 million years ago)
What were Early Devonian global conditions
~5˚C warmer than today, with a major oxygenation event
What happened to O₂ levels in the Middle Devonian
They decreased sharply, possible contributing to a major extinction event
What are the key traits of Ichthyostega
Gills, dual-purpose ear, seal-like movement, ~365–359 Mya
What are the key traits of Acanthostega
Aquatic lifestyle, paddle-like limbs, large caudal fin, gill chamber, lateral line system
What is notable about Parmastega
Older than Ichthyostega, crocodile-like, lived in lagoon environments
Is pentadactyly ancestral for tetrapods
No, early stem tetrapods were polydactylous (e.g. Ichthyostega with 7, Acanthostega with 8 digits
What are choanae and where do they originate
Internal nostrils, first appear in tetrapodomorph fish
What does Kenichthys show about choana evolution
Posterior nostril interrupts the tooth arcade - intermediate stage between external and internal nostrils
What is the evolutionary origin of the middle ear
Enlarged spiracle from Late Devonian forms; hyomandibula shortened and became the stapes
What shift occurred in the use of limb girdles
From “front-wheel” to “rear-wheel” drive; pelvic girdle becomes dominant in tetrapods
What role did fins play in early locomotion
Anchoring during body-flexion propulsion in shallow water
Are digits homologous with any fin structures
Yes, they are homologous with distal radials in lobe fins, supported by fossils and hox gene expression
What was the original function of tetrapod limbs
Movement in shallow water, not terrestrial locomotion
What adaptations came first in the tetrapod body plan
Limb mobility for aquatic limb-substrate interaction; weight-bearing adaptations came later
