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SARCOPTERYGII | TETRAPODA |--Acanthostega `--+--Ichthyostega `--+--Crassigyrinus `--+--Colosteidae | `--Greererpeton `--+--Baphetidae |--Eucritta `--Tetrapoda* |--TEMNOSPONDYLI `--+--LEPOSPONDYLI `--REPTILIOMORPHA
It is customary to celebrate the advent of the Tetrapoda* with a long paean of praise to their talents and abilities -- something like the death song of a barbarian warrior reciting the deeds of his ancestors. For the moment, we are forced to make do with the following table. ATW020329.
|Caudal vertebrae||Development of atlas-axis complex||Allows head to be moved independent of forelimbs. Probably multiple convergent adaptations|
|Ribs||Numerous, divergent "experiments" including big thick ribs (Ichthyostega), virtually absent ribs (numerous examples), ribs light, but tightly bound together (colosteids)||A difficult balancing of different needs: to support the forelimbs and gut against gravity, to reduce weight, to reinforce the vertebrae, to permit the trunk to flex, to provide a rigid cage for the lungs and heart, etc.|
|Dermal skull||Becomes more tightly integrated, with less kinesis, but often less massive||Since many later tetrapods (e.g. snakes) have very kinetic skulls, the significance is not entirely clear.|
|Dermal skull||Simplification, with loss of some bones||Probably related to loss of kinesis|
|Limb girdles||Loss of many elements; development of "standard" tetrapod limbs and girdles||Gradual, and perhaps rather late, acquisition of the ability to walk on land on a regular basis. But footprints are already known from the Famennian of Australia.|
|Hip||Ilium contacts and partially fuses with elements of vertebral column to form true sacrum||A clear necessity to allow the legs to support the body|
|Hindlimbs||Reverses orientation||Early tetrapods had the feet facing posteriorly, as in fish.|
|Hands/feet||Complete transition from fins to limbs||Walking probably predates walking on land|
|Water balance||unknown||Very important soft tissue changes must have taken place, but the timing and nature of the transition is not known.|
|Senses||Loss of electrical senses and lateral line, probable loss of most hearing, development of flexible lens in the eye||Senses don't work the same out of water. A "fish" out of water is nearly deaf and blind, and it can't use its pressure or electrical senses. Consider how much neural re-wiring had to be accomplished!|
Tetrapoda*: Rana + Rambo. The last common ancestor of living amphibians and amniotes and all of its descendants. This is the "crown group Tetrapoda" that many paleontologists complain about. The reasons for this terminology are discussed at What is a "Tetrapod?"
Range: from the Early? Carboniferous.
Characters: see Overview.
Links: Definition of the Taxon Tetrapoda; Terrestrial Vertebrates; Herpetology- Phylogeny and Tetrapods; Phylog. Syst. of Odonata - Taxonomy; First International Phylogenetic Nomenclature Meeting Paris, July ... (see sample abstract); PII- S0169-5347(99)01780-2; PII- S0169-5347(00)01927-3 (contrasting viewpoints); Should we embrace the PhyloCode as the future of taxonomy (Best on the Web); PhyloCode Preface. ATW031013.
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