Tetrapods Flashcards
Define tetrapod?
Any animal descended from an ancestor with four limbs. Fins only gradually transformed into limbs so defined by phylogeny.
Define tetrapoda?
Last common ancestor of all living tetrapods
When did tetrapods evolve?
Late Devonian (360 mya). Temperatures were around 5 degrees warmer. Oxygen levels had dropped sharply mid Devonian leading to one of 5 mass extinctions.
Early tetrapods were mainly aquatic. It is thought that they searched for food and dispersed their juveniles on land.
What do we know about Devonian tetrapod distribution?
They must have been able to disperse around coastlines as they are found in many areas. Can’t infer too much from where their remains are found though.
What is an Ichthyostega?
The first Devonian tetrapod to be discovered in 1932. Primarily terrestrial. Probably had gills, and ears which could hear in both environments. Limited vertical abdominal flexion and seal like movements.
Wasn’t involved in main line of tetrapod evolution.
Until finds of other early tetrapods and closely related fishes in the late 20th century, Ichthyostega stood alone as the transitional fossilbetween fish and tetrapods, combining a fishlike tail and gills with an amphibian skull and limbs.
What is an Acanthostega?
Entirely aquatic, but was the first vertebrate animals to have recognisable limbs. Dismissed the idea that limbs evovled as an adaptation to terrestrial life.
Existed at the same time as Ichthyostega.
What is a Tiktaalik?
Possibly represents the evolutionary transition from fish to amphibian. Developed adaptations to the oxygen poor shallow water habitats of its time.
Had a pectoral fin which could bend in the middle to prop the body up, so it could lift its head in the air. Wasn’t yet a tetrapod as had well-developed gills.
What are the Coelacanths?
The oldest known living lineage of Sacropterygians.
What was the first tetrapod character to evolve?
The internal nostrils. Allowed for breathing throught the nose, whilst fish had 2 pairs of external nostrils which ventilate the nasal cavities and are used for olfaction.
Also early evolution of a tetrapod neck. Isolated head from the movement of the body. Provides motility to the head in an aquatic animal. The bones in the back of the head were more firmly integrated and the bony operculum was lost. All allowed for muscles to raise the head relative to the body.
Bones in the snout became more interconnected allowing for the nose to be lifted into the less supportive air.
What is the old view of tetrapod evolution?
Tetrapods evolved from fish which had to haul themselves between shrinking pools of water to avoid drying out. This was unlikely as a lot of changes would have had to occur simulataneously.
What is the current view of tetrapod evolution?
Limbs were originally an adaptation for moving around in shallow water. This was required as warmer shallow water holds less oxygen.
Gave the animals an advantage over the larger fish which couldn’t access the newly flourishing plants in the shallow waters of the Devonian.
When did the tetrapods radiate masssively?
In the Carboniferous. Lineage leading to Amphibia emerged during this time period.
Necessary changes to allow for terrestrialisation?
Endothermy due to a less stable environment. However, due to low heat conductivity in air, terrestrial animals can maintain body temps which are massively different from the air temp.
Tetrapods have remodelling capabilities in bone which allow broken bones to bend. Joints also contain synovial fluid for lubrication due to living in a drier environment.
Circulation system. Blood is weightless in water so doesn’t require big pressures to pump it around. Double circulation system necessary to allow for differences in pressure.
