The beauty of paleontology is its power to resurrect and recreate. In what other discipline can one resurrect a life form that has been dead for millions of years, reconstruct its habitat, and track its evolutionary history through space and time? In Time Machines: Scientific Exploration in Deep Time, Peter D. Ward, author of The End of Evolution and On Methuselah's Trail: Living Fossils and the Great Extinctions, recounts the methods and procedures paleontologists use to make long-extinct creatures "live" again and to enhance our understanding of their long-gone world and actions.

The author uses as a case example a series of relatively unexplored (geologically speaking) islands that bridge the international boundary between Canada and the United States in the Pacific Northwest. The islands offer seemingly anomalous rocks and fossils, primarily invertebrate, from the Cretaceous Age (80 to 65 million years ago). Recounting the history of scientific efforts to assess the age of rocks, Ward explains how, after generations of bickering, the introduction of radiometric dating finally allowed the disparate theories of the age of the earth to be confirmed or refuted. The tool of this trade, and the first component of the time machine Ward constructs for readers, is the mass spectroscope, which assesses with high precision the rate of decay of one element into another.

Unfortunately, mass spectroscopy cannot be applied to every rock sample. Proper readings are impossible for altered or transported sediment. In such instances, there is an alternative method that involves assessing the magnetic polarity of sedimentary rocks, determined from the magnetization of their constituent minerals. This information can be used to date when the rock first formed, because throughout the earth's history, the magnetic poles have flip-flopped at numerous - and now pinpointed - times.

Paleomagnetism can indicate not only the direction of polarity in the sediment from which the rocks formed, but also the sediment's relative geographic position at the time. Coupled with the theory of continental drift and plate tectonics ("one of the most profound scientific revolutions of our time, rivaling those sparked by DNA in biology and by relativity and quantum mechanics in physics," according to Ward), the paleomagnetic record reveals the global forces that determined much of evolution's course.

In the second half of Time Machines, Ward discusses sea level changes and turns his attention toward the creatures found in Pacific coast sediments. The fossils of this area are marine and, together with the sedimentary record, can be used to decipher the sea levels of the late Cretaceous. This period was characterized by widespread extinction of species, marking the famous Cretaceous-Tertiary boundary, whose possible cause has recently been a popular topic for speculation among paleontologists and in several books written for the general public. Was there a reduction in sea level near the end of the Cretaceous, and could this have caused a mass extinction? Any decrease in sea level - and there have been many fluctuations over time - would trigger climatic changes, altering ocean currents and exposing what was once sea bottom to the sun's intense rays. In his example, Ward compares the sedimentary record of the Pacific Northwest to that of a well-known region in northern Africa, which also marks the last few million years of the Cretaceous. Study of the strata vividly shows that sea level did not fall, but rose, and that the changes were global: the African sediments, though different in some ways, are generally comparable to those of the Pacific Northwest.

The fossils in such sediments are time machines themselves, "for they as much as any other information can transport us back into deep time." Ammonites, nautilus-like creatures abundant in the Pacific Northwest deposits, became extinct at the Cretaceous-Tertiary boundary, along with the dinosaurs and other creatures. Although we can observe ammonites only as fossils, we know they struggled in an unforgiving environment. Voracious mosasaurs, 30-foot long swimming killers of the deep, would have preyed upon ammonites. Well known in paleontological circles, and often illustrated in museum exhibits, are the round holes found on a number of ammonite shells. These were long interpreted as punctures made by mosasaurs. But as Ward pieces together from the literature, conferences, and interviews with paleontologists, this is probably not true. Modeling mosasaur jaws, and mechanically having them bite on invertebrates with similar shell structures, has vividly illustrated that if a mosasaur bit into an ammonite, the shell would simply shatter.

Another question raised about the integrity of ammonite shells concerns the design of the septum, the point of separation between chambers. The complexity of septa design seemed to indicate an increase in the shell integrity over time. But was this true? Computer models, another component of the time machine, show that an increase in chamber complexity would actually weaken, not strengthen, the shells. Bioengineers have found that septum complexity has more to do with considerations of buoyancy than of shell integrity. The more complex the septa, the faster the chambers refill, thus countering any buoyancy problems. In turn, the more control over buoyancy, the better the chances of escaping predators.

The only living relative of the ammonites is the nautilus, which is somewhat similar in design. "No paleontologist is immune to the call of evolutionary study," Ward sighs. "Even we stratigraphic paleontologists, . . . quickly hear the siren song of evolutionary changes as we dig fossils from rocks." The author goes on the hunt for both the fossil and living forms of the nautilus. Are these creatures a recent evolutionary innovation, as most have thought, or does their history date back to the age of dinosaurs - to the islands of the Pacific Northwest? Did they, unlike the ammonites, survive the mass extinction? Together with other components of the time machine, cladistic analysis and comparative anatomy indicate that this is actually a rather primitive, old group, contemporary to the more bountiful ammonites.

Ward's concluding chapter is novel and vivid: a fictional trip in time by a future explorer dropped in the sea some 80 million years ago. The habitat, the inhabitants, and their behavior are recreated in an H.G. Wellsian fashion. "Near the 30-foot level [of the dive] he began hearing sharp cracking noises, and he pirouetted to scan the water around him" eventually coming upon a mosasaur in full consumptive force. "[T]he marine lizard seemed unconcerned with him, if it had seen him at all; it dashed again into the crowd of far slower ammonites, taking the body chamber of one of the larger ammonites shells in its jaws." With this account, the author reveals a predisposition shared by many paleontologists to be a storyteller of the past.

Time Machines is an informative, easy trek across the rocks, sea, and time, and a wonderful embodiment of the interactions between the physical and biological sciences.

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