Exercises In Evolution

Chapter 18: From Water to Land to Air: Amphibians and Amniotes

Jennifer Clack is a paleontologist from Cambridge University, England, who has discovered and supervised the reconstruction of Acanthostega. Hall and Hallgrimsson (2008) point out that evolving from fish to four-legged animal ocurred in "a relatively short geological interval of no more than 15 or 20 My."
View this tree of life description of the accurate skeleton, the body based upon it, and imagined (but possibly realistic) coloration. Notice that other genera of stem tetrapods have been discovered. Correlate the figures in your textbook to those in this website to the text descriptions of "tetrapod adaptations and diversification". Review the definition of homology, the diagrammed lineages associated with homology (Hall and Hallgrimsson, 2008, Chapter 11), and compare the Chapter 18 comparisons of osteolepiform (including rhipidistian) fish to labrinthdont amphibians (ibid., Fig. 18-4). Notice that Acanthostega is not the only "stem" tetrapod known and that (Box 18-1) some consolidation of individual specimens into fewer species will be the tasks of taxonomists viewing the fossils. This section of Strickberger's Evoluton has a wealth of taxonomic terms to keep straight unless you make the choice to make much more qualitative comparisons.
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The "Devonian Times" is a very useful website. This section describes circumstances that led the great comparative anatomist, Alfred Sherwood Romer, to conclude that the transition from water to land occurred because of climatic and drying conditions. As ponds dried in severe weather, fish with the ability to, even if clumsily, struggle up to land and breathe, they could have a naturally selected advantage. Acanthostega and related fossils suggest that evolution of limbs that are separately articulated from the skull did occur in water habitats. The drying pond scenario has given way to an "escape from aquatic predators" scenario wiht more mobile animals as Acanthostega.
Read of the "fish out of water/drying pond" theory for acquisition of land by animals and of "an alternative, and frustration" in the link provided. Then, run down the list of genera in the left hand column to get an idea of the array of specimens known. Notice the useful coding in the body's profile of the bones actually found on which the genus was proposed. Continue until you find Tiktaalik roseae and follow links within that website to the University of Chicago website on this remarkable specimen. Reflect on the reality that science's most powerful ethic or feature is its willingness to be self-correcting, as an institution. Likely, there will be more to write of how and where the transition from water to land occurred as new data are discovered.
Fish out of water: the drying pond and Tiktaalik roseae
Most students will be familiar with the egg - tadpole - adult metamorphosis of common frogs but the large genus Eleutherodactylus includes tree frogs that are direct-developing; they have no tadpole stage. The website offers an impressive range of reproductive biologies and it is that impression of astounding diversity for which it is offered. A recent and nearly 300-page proposal (Bulletin of the American Museum of Natural History, volume 297, 2006) was published to revise the phylogeny and taxonomy of amphibians as there are some taxa that are not monophyletic. If you are studying at a site that that has institutional access to the on-line Journal BioOne, you might be able to obtain the 9+ Mb and see what effort is involved in a major, major taxonomic revision.
Beginning students often underestimate the abundance of species that have evolved. Simply scrolling down this website and viewing the number of species will help to give an impression of the amazing productivity that evolution has attained. Be sure to check Eleutherodactylous coqui, the species illustrated in Fig. 18-14.
Eleutherodactylous: a direct-developing frog
Strickberger's Evolution (2008, Figs. 18-18, 18-19) raises the issue of evolution within the internal organs. Other examples are found in Figures 3-7 and 3-16. This website offers a glimpse into "comparative anatomy", determining how diverse taxa achieve the same ends as, for example, circulation of blood, removal of urine, or release of sperm.
Review the summaries of anatomy of animals. The organization of the summaries, by systems, is very effective. With the new insights that you have acquired, return to Chapter 16 and review the invertebrate taxa remembering (given the lesson of exercise 3 on direct-developing frogs, that there can be substantial variations, some generalizations can be made.
Anatomy of animals

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