Question

All the vertebrates are chordates – and all tetrapods are vertebrates – and all amniotes are tetrapods. Each of these leading to the amniotes resulted in major morphological change.

a) What three morphological/functional changes separate the groups in bold above?

b) For the amniotes, what are the primary parts (membranes) and the function of their morphological change?

c) How do you explain the major differences in terms of evolutionary selection for each of these groups?

d) Why are intermediate organisms (like the Tiktallik) often species which don't survive for long periods in fossil record?

Answer 1

Answer:

(a) The origin and early diversifications of tetrapods are major features in the evolution of vertebrates. As we now understand, the first tetrapods evolved in a fully aquatic environment, and subsequent morphological transformations allowed later tetrapods to invade more terrestrial environments and produce remarkable changes in body form.

For example, whereas there is a reasonable consensus regarding the crownward end of the amniote stem group, the identity of the Palaeozoic tetrapods that gave rise to modern amphibians remains controversial. Still, our understanding of the phylogenetic history of this interesting period of vertebrate evolution is becoming quite detailed, despite the fact that no species survive as evolutionary relicts that are at all closely related to the earliest tetrapods.

With a phylogeny in hand, one can meaningfully search for ancestral states, and for patterns of evolutionary change accompanying this period of major transition in lifestyle, and, of particular interest in this study, to learn to what degree the evolution of various functionally related bones is linked.

(b) The amniotes (reptiles, birds, and mammals) are distinguished from amphibians by their terrestrially adapted egg, which is protected by amniotic membranes (fluid-filled membranes which function in embryonic development). The evolution of amniotic membranes meant that the embryos of amniotes were provided with their own aquatic environment, which led to less dependence on water for development and thus allowed the amniotes to branch out into drier environments.

This was a significant development that distinguished them from amphibians, which were restricted to moist environments due their shell-less eggs. Although the shells of various amniotic species vary significantly (from hard, to leathery, to using internal fertilization and development instead), they all allow retention of water.

The amniotic egg is the key characteristic of amniotes. In amniotes that lay eggs, the shell of the egg provides protection for the developing embryo while being permeable enough to allow for the exchange of carbon dioxide and oxygen. The albumin, or egg white, provides the embryo with water and protein, whereas the fattier egg yolk is the energy supply for the embryo, as is the case with the eggs of many other animals, such as amphibians.

However, the eggs of amniotes contain three additional extra-embryonic membranes: the chorion, amnion, and allantois. Extra-embryonic membranes are membranes present in amniotic eggs that are not a part of the body of the developing embryo. While the inner amniotic membrane surrounds the embryo itself, the chorion surrounds the embryo and yolk sac.

The chorion facilitates exchange of oxygen and carbon dioxide between the embryo and the egg’s external environment. The amnion protects the embryo from mechanical shock and supports hydration. The allantois stores nitrogenous wastes produced by the embryo and also facilitates respiration. In mammals, membranes that are homologous to the extra-embryonic membranes in eggs are present in the placenta.