David answered on Nov 30 2019
EVOLUTION OF TETRAPODS FROM SARCOPTERYGIANS
Sarcopterygian fishes (lobe finned fishes) constitute a large group comprising of Coelacanths, tetrapods, lung fishes and their fossils. They have central appendage in their fins having many bones inside them. So, Due to central boney skeletal structure they are thought to be ancestors of modern tetrapods. Presence of a central pectoral and pelvic fleshy girdle also resembles to four limbs in tetrapods. Many other characteristic features exhibit a gradual transformation from aquatic to te
estrial mode of life evolving through many millions of years.
The essay explains all the features and factors responsible for such a drift of aquatic animals towards land. It elaborates all the intermediate stages in between the transition, challenges faced by the animals during transition and significant adaptations that made the transformation happen. The study also elaborates Dry pond Hypothesis which was a major pushing factor for animals in the transition. The study is based on characteristic features of animals in Devonian, Permian and Triassic periods to find all the relevant changes in body.
At last the essay explains why amniotes emerged out to be the most successful of all the animals.
Sarcopterygii are lobe finned fishes from which tetrapoda have evolved. Living lobe finned fishes like Coelacanths and Lungfish have symmetric tail with two basal fins. The fishes have single large bone in their fins of the body. Fins support movement, locomotion, and positioning of body on the land. Sarcopterygii also possess enamel in their teeth (Zimmer, 2017).
In late Devonian era, there occu
ed a massive transition of verte
ate animals (for millions of years) from water to land. Aquatic animals had to face a lot of struggle for existence and competition from other water animal species thereby they were forced to move onto land in order to find better te
estrial niche opportunities (Hickman, Roberts & Keen,2015).
The species undergoing this transition underwent many physical changes throughout the process. For example fins changed to limbs, act of movement changed from swimming to walking, style of feeding transformed to biting from suction, and reproduction mode changed from larval development to metamorphosis.
One of the fish species that underwent this transition was Acanthostega. It had 8 digits in each limb and used to thrive in weed filled swampy areas. The gill bars of this species had a supportive
ace that helped it in recognizing sound vi
ations inside the water, just like an underwater ear. Te
estrial animals retained these gill bones to recognize sound waves in air. Later on the bone transformed into middle ear, as can be seen in tetrapod mammals (Hickman et al. 2015).
Fig.1. Acanthostega (Source: http:
There are other species like Ichthyostega, which used to live in water during early phase of life and later on during the adult phase, it used to live on land near water bodies. It had lungs and limbs to enable movement to swampy areas. This adaptive feature of the species shows its transition from water to land. It owned a tail and gills like fishes, and skull with limbs like amphibians (Newdinasaurs, 2017).
Fig.2. Ichthyostega (Source: https:
One of the most prominent but extinct Sarcopterygian species (lobe finned fish) showing characteristic features like those of Tetrapods is Tiktaalik. It can be seen as a transitional species between fish and amphibians due to following features (Bioweb, 2017):
1. It has fins like fishes, but there are bones in the fins, which is a feature of tetrapods possessing boney limbs.
2. Its fins have bones like wrist and shape like fingers. These features help it to stick firmly with land during high ocean waves.
3. It has no boney plates in gill area, due to which its lateral head movement is restricted. This feature gives rise to neck having separate pectoral girdle.
4. Flat skull, a pair of fontal bones and no anterior dorsal fin.
5. Breathing through lung like organs.
6. Eyes at the top of head like tetrapods.
Fig.3.Tiktaalik (Source: http:
The transition from aquatic to te
estrial life took millions of years. During this time many driving forces like struggle for existence inside water, competition from larger species expelled the species out of water. Such activities kept on creating possibility of better niche out of water. Ultimate outcome of these driving forces made the verte
ate transition possible (Raade &Wojtowicz,2007).
Factors that pushed the species out of wate
For millions of years, there were unfavorable biotic and abiotic factors for some aquatic animals. Conditions like low oxygen levels, high temperature and increased salinity in water compelled them to move to shallower waters.
According to Joseph Ba
ell, an American geologist, during late Devonian period there was presence of substantial amount of oxidized sediments across Europe and North America which show the conditions of drought pertaining that time. The drought caused many water bodies to dry out due to which many aquatic animals stepped out to nea
y land shores to find water. Survival of the fittest and Natural selection caused only winner animals to succeed in living on land (Yahya, 2017).
Factors that pulled the species towards land niche
The niche and resources available outside water which attract aquatic animals are included in this category. Such factors include good amount of oxygen levels, appropriate temperature, food, water, fewer predators (competitors) and abundance of prey (Yahya, 2017).
Challenges faced by Aquatic animals in transition
Aquatic animals had many features adapted to aquatic niche. As they sought to move to land niche, their features also changed with time to adapt te
1. Catabolism of waste nitrogenous products in the body also differs a lot in te
estrial and aquatic animals (Dejours, Bolis, Tailor & Weibel, 1987).
2. Sense organs of aquatic animals are well suited to perceive darker environment under water. And hearing is also better suited for listening under water.
3. Homeostasis (stability of physiological processes) under water is very different than that on land.
4. Anatomical features like development of lungs, strong muscles and bones to withstand gravity on land.
5. Body movement and...