Chapter 25 (2)
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Starting with the earliest traces of life, teh __ opens a window into the world of long ago and provides glimpses of the evolution of life over billions of years.
__ is based primarily on the seuences in which fossils have accumulated in sedimentary rock layers called __. Useful info is also provided by other types of fossils, such as insects prevserved in amber (fossilized tree sap) and mammals frozen in ice.
The __ shows that there have been great change s in the the kinds of organisms that dominated life on Earth at dif. points in times. Many past organisms were unlike today's organisms, and many organisms that once were common are now extinct. __ also document how new groups of organisms arose from previously existing ones.
As substantial and significant as the __ is, it is an incomplete chronicle of evoultionary change. many of Earth's organisms prob did not die int he right place at the rght time to be preserved as fossils. Of those fossils that were formed, many were destroyed by later geologic processes, and only a fraction of the others have been discovered.
As a result, the known fossil record is biased in favor of species that existed for a long time, were abundant and widespread in certain kinds of environments, and had hard shells, skeletons, or other parts that facilitated their fossilization. Even with its limitations, however, the __ is a remarkably detailed account of biologica change over teh vast scale of geologic time. Furthermore, gaps int eh __ cntinue to be filled by new discoveries.
fossil record x2
Fossils are valuable data for reconstructing life's history, but only if we determine where they fit in that. While the order of fossils in __ tells us the seqence in which the fossils were laid down- their ages- it does not tell us their actual (absolute) ages. You can determine the sequence in which the layers were applied, but not the year each layer was added.
One of the most common techniques of determining the absolute age of a fossil is __, which is based on teh decay of radioactive isotopes. A radioactive "__" isotope decays to a "__" isotope at a constant rate. THe rate of decay is expressed by teh __, the time required for 50% of the parent isotope to decay. Each variety of radioactive isotope has a characteristic __, not affected by temp., pressure, or other such environmental variables.
- radiometric dating
- half-life x2
Fossils contain __of elements that accumulated in teh organisms when they were alive. By measuring the ratio of carbon -14 to carbon-12 in a fossil, we can determine the fossil's age. This method works for fossils up to about 75 G years old; fossils older contain too little carbon-14 to be detected with current techniques. Radioactive isotopes with longer half-lives are used to date older fossils.
What are two reasons determining the age of old fossils in sediemtaryt rocks can be challenging.
hence, we usually cannot date old fossils directly. However, geologists can apply an indirect method to infer teh absolute age of fossils that are sandwiched between two layers of volcan rocks.
- 1- organisms do not use radioisotopes that halve long half-lives, such as uranium- 238, to build their bones or shells.
- 2- sedimentary rocks themselves tend to be composed of sediments of differign ages.
The __ of rocks can also provide dating info. During the formation of __ and __ rocks, iron particles in the rock align themselves with Earth's magnetic field. When teh rock hardens, the particles' orientation is frozen in time. Measurements of the __ of various rock layers indicate that earth's N and S magnetic poles have reversed repeatedly in the past. Because these magnetic rehearsals affect the entire planet at once, reversals in onee location can be matched with corresponding patterns elsewhere. This approach allows rocks to be dated when other methods are not available. It also can be used to corroborate ages estimated in other ways.
Some fossils provided a detailed look at the origin of new groups of organisms. Such fossils are central to understanding of __; they illustrate how new features of organissms arise and how long it takes for such changes to occur.
Along with amphibians and reptiles, mammals belong to the group of animals called __, named for having four limbs. Mammals have a number of unique anatomical features that fossilize readily, allowing scientists to trace their orignin.
For example, the lower jaw is composed of one bone (the __) in mammals but several bones n other tetrapos. In addition, the lower and upper jaw hinge between a different set of bones in mammals than in other __. Mammals also have a unique set of three bones that transit sound in th e middle ear (__,__,__), whereas other tetrapods have only one such bone (the __). Finally, the teeth of mammals are differentiated into __ (for tearing), __ (for piercing), and the mulit-pointed __ and __ (for grinding)> In contrast, the teeth of other tetrapods usually consist of a row of undifferentiated, single-pointed teeth.
- hammer, stirrup, anvil
- the stirrup
The __ shows that the unique features of mammalian jaws and teeth evolfe as a series of gradual modifications. If all the known fossils in teh sequence were arranged by shape and placed side by side, their features would blend smoothly from one group to the next, reflecting how the features of a new group, the __, gradually arose in a previously existing group, the __.
- fossil record
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