Evolution of the Tetrapods Flashcards
What was Panderichthyes?
A type of Elpistostegalians
A more derived form of fish that gave rise to the tetrapods
From around 385 MYA
Lived in shallow water
Had no dorsal and anal fins
Was dorsoventrally flattened
Had large ribs that were lateral and pointed ventrally
Eyes found on top of the head so could possibly look above water
Had a reduced tail
What was the spiracle a precursor for in the fish that gave rise to the tetrapods?
Beginnings of the middle ear
What was Tiktaalik?
A more derived form of basal fish that gave rise to the tetrapods
From around 383 MYA
Had reduced gill covers - therefore had reduced water flow through gills
Had longer neck and could raise its head above the water - also had long snout so could snap up prey
Ribs were large and could lift its chest and move head - therfore more flexible than a fish
Had flexible pectoral fins - could be used as a prop possibly
Had wrist bones (homologous to those found in the tetrapods) and fin rays but had no digits
Really important discovery as had features of both the fish and tetrapods
What was ichthyostega?
More derived from than Tiktaalik
Dated around 365 MYA
Primarily aquatic
Had forelimbs and hindlimbs
Hindlimbs had contact with the vertebral column - not seen in Tiktaalik
Still had an operculum so used gills for respiration
More robust skeleton for movement
Had an ear region for hearing underwater
Give some examples of the basal tetrapods and their features
Parmastega:
Nares (nostrils) were underwater so must have taken water through nares and therefore mainly used gills to respire
Had a spiracle to breath air
Had large eyes to look above water surface
Had large teeth so probably didnt feed on insects - possibly other vertebrates
Colosteids
Nares still underwater
No spiracle so only used gills for respiring
Temnospondyls
Nares above the water and so they breathed air
Spiracle has been modified into an ear
This lineage gave rise to the amphibians
Seymouriamorphs and Embolomeres gave rise to the amniotes
Were early tetrapods terrestrial or aquatic?
Aquatic
Features were initially advantageous to animals in the water - improving locomotion, feeding at the surface/edge
Then these features became advantageous on land - feeding, dispersal and basking
The evolution of tetrapods and evolution of terrestrial life or two SEPARATE events
What is a recent discovery that has confused scientists on the evolution of the tetrapods?
They have found tracks but they date 10 MY before Panderichthys and the more derived forms
Some scientists tried to say they arent tracks but just marks (as no other fossils have been found) - BUT have found other similar tracks elsewhere and more inland
Maybe some tetrapods evolved before others did?
Evolution of tetrapods maybe happened in waves?
How many toes did early tetrapods have?
They were polydactyl
Acanthostega had 8 toes on front and hind feet
Ichthyostega had 7 toes on hind feet and the forefeet are unknown
What are the development stages of tetrapod limb development?
Preaxial development = development of parts of the limb closest to the body
Postaxial development = development of the digits
Order of digit development = 4, 3,2,1,5
Digit 5 = little finger
Digit 1 = thumb
Digit 4 = ring finger
Why did basal tetrapod lineages have more digits?
Branching of digits went on for longer which led to more digits being formed
Time of branching decreased through the evolutionary lineage so number of digits also decreased
When did digits start forming?
When acanthostega evolved
What development gene is responsible for the development of digits?
Hoxd13
Overexpression causes the formation
What is reduction of digits in the vertebrates associated with?
Specialisation for locomotion
Fast-moving species vertebrates tend to lose digits
What are the two mechanisms for digit loss?
Reduced gene expression
Cell death
What groups gave rise to the amphibians and the amniotes? When did they split?
Temnospondyli split into the amphibians
Reptiliomorpha split into the amniotes
Split 355 MYA
What are some transitions that happened to move from water to land?
Increased solar radiation - chromatophores in skin to absorb light rays
Less support and resistance to movement - strong skeleton and musculature
Light and sound waves travel differently - fish-like eyes transform during metamorphosis. Evolution of tympanic membrane
Air has lower thermal stability - ectotherms have low metabolism. May hibernate or aestivate
Oxygen more abundant but gas exchange without water - skin is vascularised. Lungs become primary site for gas exchange. Have cutaneous or buccopharyngeal respiration
Water is limited - keratin and mucous in skin. Live in damp places. Excrete urea or some excrete uric acid
Reprouction - Most return to water to breed
Evolved a tongue for feeding (either for catching prey or manipulating food) - evolved from branchial arches and associated muscles
What can the amphibians be grouped into?
Urodeles = newts and salamanders
Anurans = frogs and toads
Caeceillians = legless amphibians
What are common characteristics of the amphibians?
Permeable skin for respiration
Mostly carnivores
Many have tongue specialisation
Give some features of the urodeles
Most generalised
Usually 4 limbs
Some have gills as adults - external gills
Most have internal fertilisation
Elongated body
Give some features of the anurans
Large and diverse
Adapted for jumping and swimming
Fertilisation is usually external
Usually lose their lateral line during metamorphosis - except for Xenopous
Have decreased number of vertebrae
Large pelvis
Urostyle (for support) - from the fusion of the vertebrae
Lost one digit
Large eyes for binocular vision
Have fusion of radius and ulnar (to form radio-ulna) and tibia and fibia (to form tibio-fibula) - these allow strengthened bones for movement
How does anuran limb length change their movement?
Those with longer forelimbs and hindlimbs are more adapted for jumping, ambushing prey e.g. common frog
Those with shorter forelimbs but longer hindlimbs are more adapted for hopping and swimming e.g. common toad
Those with short forelimbs and hindlimbs are more adapted for walking-hopping and burrowing
Those with longer forelimbs but shorter hindlimbs are more evolved for walking-hopping
