Part V: The Part of Tens in .NET

Draw QR Code JIS X 0510 in .NET Part V: The Part of Tens

Just as important as the trilobite-specific information that these fossils offer is what they show about the time frame of evolutionary transitions in lineages with a plentiful fossil record We have so few human fossils, for example, that it s reasonable to assume we will miss transitional forms because the hominid fossil record is poor Even though trilobite fossils are numerous, we still see sudden (in geological time) transitions and the rapid appearance of new forms in this lineage Where speciation events occur in a small, localized area, the probability of finding intermediate forms decreases, and the fossil record appears to contain more sudden transitions as species appear and go extinct Head to 8 for info on speciation

Perhaps you ve seen cartoons depicting the transition of vertebrates from water to land animals Usually, these drawings show some sort of fish with

stubby little legs, which isn t too far off the truth Scientists recently found a fossil quite similar to the type that s famous from all those evolution cartoons: Tiktaalik rosea Tiktaalik has a lot of fishlike structures, such as fins, scales, and gills But its fins have a leg-like structure with wrists and parts like a hand or fingers (see Figure 20-3) This critter could do pushups! It s not clear to what extent Tiktaalik left the water, if it did at all Some scientists hypothesize that this new fin structure would have been advantageous for maintaining position in fast-flowing shallow water

Other interesting characteristics of this fossil are its neck and the eyes on top of its head more like a crocodile than a salmon What scientists have discovered from this fossil is that the anatomical modifications associated with the transition to land may have evolved first in the water

The Burgess Shale is a concentration of fossils in the Canadian Rockies containing large numbers of morphologically distinct species that at first glance don t seem much like the organisms that are around today The fossils found in this shale are cool for two reasons: Their excellent degree of preservation often reveals intricate structures of the softer body parts The large diversity of forms that were new to science raises interesting questions about the evolutionary history of life on Earth

For the most part with fossils, it s possible to identify the major group to which the organism belongs For example, you might know that a particular fossil is a clam, perhaps not like any clam we have today, but a clam nevertheless Before the discovery of the Burgess shale, scientists could identify fossil animals as earlier members of particular branches of the tree of life Trilobites (see the preceding section), for example, were recognizable as members of the arthropods branch But when the animals in the Burgess Shale were examined, it wasn t clear immediately what groups many organisms belonged to, and in some cases, such as the organism hallucigenia, it wasn t even immediately clear which end was up (see Figure 20-4) Creatures like hallucigenia make one think that perhaps many such interesting and unusual creatures have evolved and subsequently gone extinct

On subsequent analysis, and with the passing of considerable time and effort, scientists have been able to assign many of the fossils found in the Burgess Shale to existing groups But some fossils still defy placement and may indeed be examples of unique body plans that turned out to be evolutionary dead ends

Stromatolite fossils dating back more than 3 billion years are among the oldest fossilized evidence of life on Earth Stromatolites are pillars formed by

layer upon layer of microorganisms, such as cyanobacteria (see Microfossils, later in this chapter) These layers trap sediments that become incorporated into the structure, or in some cases, the organisms themselves are responsible for producing calcium carbonate, which becomes a component of a structure Either way, when the structure is fossilized, the layers are visible as bands in the columnar structure Stromatolites are common in the fossil record; however, it wasn t until the discovery of living stromatolites that we could be sure that these structures were the result of biological activity While there certainly seem to have been a fair number of stromatolites formed billions of years ago, they are a lot less frequent now Living stromatolite structures are currently found only in areas where there are no predators to feed on the layers of microorganisms In the days before anything else had evolved to eat them, being eaten wasn t a problem Today, they re just out of luck in most places