Planet of the Bugs: Evolution and the Rise of Insects by Scott Richard Shaw Page A
Book: Planet of the Bugs: Evolution and the Rise of Insects by Scott Richard Shaw
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Authors: Scott Richard Shaw
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still have been useful for locomotion. Even in the simplest of worms, just the smallest pair of segmental protuberances would have improved friction with the substrate, and would have been useful for burrowing. Appendages appeared in pairs, two per segment, simply because most of the Cambrian animals possess bilateral symmetry. Slice one lengthwise and you get two similar halves, like mirror images.
Jointed legs appear not just in trilobites, but also in an assortment of other Cambrian arthropods, some of them still unnamed. There were multisegmented, multilegged creatures that resemble millipedes or centipedes but lived in the oceans. There were small-shelled arthropods that appear to be early crustaceans, the forerunners of lobster and shrimp. Also, there was the four-inch-long, armor-plated
Sanctacaris
, possibly the aquatic prototype that led to scorpions and spiders. But there were no insects, not yet. How did any of these things get jointed legs? Presumably in the same way the long skinny animals gained segmentation. A long, non-jointed leg is inflexible and limited in its usefulness. Any arrangement of leg joints, however, is very useful, allowing flexibility and the ability to manipulate potential foodobjects. As the history of the arthropods demonstrates, this simple leg form can be easily modified into an astonishing array of forms and functions. Insects use their legs for walking, running, hopping, fighting, grasping food, tasting food, grooming their body, swimming, digging, spinning silk, courtship, sound-production, and even hearing. Katydids have ears on their legs.
FIGURE 2.2. A white-legged millipede illustrates some characteristics of arthropods: an external skeleton, segmentation, and paired, jointed legs. (Photo by Kevin Murphy.)
The Rise and Fall of the House of the Trilobites
If arthropods are so great, then what happened with the trilobites? Understanding the fate of the trilobites will explain not only why the realm of the trilobites rose and fell. It also reveals something fundamentally important about arthropod biology that helps to explain the later rise and success of the insects, which, after all, are the trilobites’ distant relatives. They are not derived directly from trilobite-ancestors but are more like distant cousins. As we will see, insects succeeded partly by solving some problems that the trilobites were never able to master.
I mentioned earlier that trilobite species diversity increased steadily during the Cambrian, peaking at the end of that age. At the onset of the Ordovician period, trilobite diversity started to decline, and it continued to drop until the Silurian. By the end of the Permian, they were all extinct. What caused the trilobites’ decline?
The start of the Ordovician was marked not by catastrophic environmental events but by significant changes in the communities of living organisms. New and different kinds of organisms appeared in the shallow oceans, and many of the more unusual Cambrian animals, like
Hallucigenia
, disappeared forever. From our perspective, the Ordovician most notably marks the time of the appearance and beginning of the diversification of fishes, the first obvious vertebrates with extensive skeletal features. For that reason, the Ordovician is usually highlighted as the time of the first fishes.
Once again we need to tear down some human-centrist mythology. We acknowledge the first fishes not because they dominated the animal communities; instead, we hail them as our most ancient vertebrate ancestors and assume that their appearance must have been a historical event from a human perspective. The truth of the matter is that fishes didn’t change things too much and not very quickly. Andfishes were not the most diverse or dominant animal group in the Ordovician. Trilobites started to decline in diversity over the Ordovician, but they still vastly outnumbered fishes for those sixty-two million years. By the middle Ordovician the
Jointed legs appear not just in trilobites, but also in an assortment of other Cambrian arthropods, some of them still unnamed. There were multisegmented, multilegged creatures that resemble millipedes or centipedes but lived in the oceans. There were small-shelled arthropods that appear to be early crustaceans, the forerunners of lobster and shrimp. Also, there was the four-inch-long, armor-plated
Sanctacaris
, possibly the aquatic prototype that led to scorpions and spiders. But there were no insects, not yet. How did any of these things get jointed legs? Presumably in the same way the long skinny animals gained segmentation. A long, non-jointed leg is inflexible and limited in its usefulness. Any arrangement of leg joints, however, is very useful, allowing flexibility and the ability to manipulate potential foodobjects. As the history of the arthropods demonstrates, this simple leg form can be easily modified into an astonishing array of forms and functions. Insects use their legs for walking, running, hopping, fighting, grasping food, tasting food, grooming their body, swimming, digging, spinning silk, courtship, sound-production, and even hearing. Katydids have ears on their legs.
FIGURE 2.2. A white-legged millipede illustrates some characteristics of arthropods: an external skeleton, segmentation, and paired, jointed legs. (Photo by Kevin Murphy.)
The Rise and Fall of the House of the Trilobites
If arthropods are so great, then what happened with the trilobites? Understanding the fate of the trilobites will explain not only why the realm of the trilobites rose and fell. It also reveals something fundamentally important about arthropod biology that helps to explain the later rise and success of the insects, which, after all, are the trilobites’ distant relatives. They are not derived directly from trilobite-ancestors but are more like distant cousins. As we will see, insects succeeded partly by solving some problems that the trilobites were never able to master.
I mentioned earlier that trilobite species diversity increased steadily during the Cambrian, peaking at the end of that age. At the onset of the Ordovician period, trilobite diversity started to decline, and it continued to drop until the Silurian. By the end of the Permian, they were all extinct. What caused the trilobites’ decline?
The start of the Ordovician was marked not by catastrophic environmental events but by significant changes in the communities of living organisms. New and different kinds of organisms appeared in the shallow oceans, and many of the more unusual Cambrian animals, like
Hallucigenia
, disappeared forever. From our perspective, the Ordovician most notably marks the time of the appearance and beginning of the diversification of fishes, the first obvious vertebrates with extensive skeletal features. For that reason, the Ordovician is usually highlighted as the time of the first fishes.
Once again we need to tear down some human-centrist mythology. We acknowledge the first fishes not because they dominated the animal communities; instead, we hail them as our most ancient vertebrate ancestors and assume that their appearance must have been a historical event from a human perspective. The truth of the matter is that fishes didn’t change things too much and not very quickly. Andfishes were not the most diverse or dominant animal group in the Ordovician. Trilobites started to decline in diversity over the Ordovician, but they still vastly outnumbered fishes for those sixty-two million years. By the middle Ordovician the
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