I present to you a simple proposition: when it comes to animal form, an external skeleton is better. It’s certainly more likely to develop in the first place, because it’s better to shunt toxic excretory byproducts to the outside rather than storing them on the inside, and it’s obviously far more likely to succeed because of immediate defensive advantages. Just look at the vast diversity of trilobite species that lived at the end of the Cambrian period versus our little Pikaia relative hiding in the sediments. Then also consider our modern world, where all vertebrate species combined present only a small fraction of seemingly astronomical arthropod diversity. Success might be measured in various ways. We are ever so proud of our oversized contemplative brains. But with those brains we must ponder the sheer improbability of our existence and our constant vulnerability through the ages. The humble insects cannot contemplate their own measure of success: the numerical dominance of arthropod species through all ages of animal life. If this were to all play out again on another planet, it seems to me highly improbable that soft-bodied creatures with internal skeletons would develop first or become successful over the long run. Hard-covered creatures with external skeletons would almost certainly hold the advantage over time, in most contests of soft versus hard-shelled players.
Looking back on the earliest Cambrian trace fossils, those worm burrows, we know that even those simple animals could tunnel in sediments. We don’t suppose that they had any legs yet, because there are no fossil footprints. So how did they move? We must assume they had muscles arranged in body segments, allowing them to contract segments and wiggle their bodies, as with modern earthworms. Segmentation is a common body form, but it is an ancient one as well. All modern insects are segmented animals; hence the name “insect,” which means “in sections.” But that characteristic is not unique to insects. It is an inherited trait from earlier ancestors. All arthropods, including the trilobites, were segmented creatures, and so were simpler creatures, such as the annelid worms. That ancient burrowing creature Trichophycus pedum was probably also segmented, precisely because it could tunnel, but did not have apparent legs.
The origins of segmentation clearly reside in the earliest multicellular animals. Just as single cells became multicellular aggregations by building duplicates of themselves, early multicellular creatures became segmental by building duplicates of their cellular arrangements and linking them in a chain. Segmentation is an excellent trait for an animal, not just because the components are easiest to build and link together, but also because the shock is less when you lose a part to an accident. If you have seen the science-fiction movie The Core, you may recall the following example: explorers used a multisegmented craft to travel deep into the earth, losing parts along the way but surviving. Trilobites and insects can lose body parts and survive more easily than we can.
It’s no mistake that the earliest skeletal parts were on the outside. As I already mentioned, it makes more sense to shunt waste products to the exterior than to pile them up on the inside. If an ancient wormlike creature were evolving a hard outer skeleton, it is only logical that the skeleton would form in segmental plates, just as in all arthropods. They already had segments with muscles. Body flexibility could only be maintained if the segmental parts remained flexible, with membranes at the edges. Any attempt at a fully hardened exterior would be useless and maladaptive, because a completely hardened creature could not move at all.
The other unique feature of arthropods, and another key to their success, is their multijointed legs. Take a moment to imagine a delicious plate of steaming Alaskan king crab legs, and you know what I’m talking about. Jointed legs in hard shells, that’s the feature that defines the arthropods and the characteristic was passed along to the insects. The name “arthropod” translates to “jointed foot.” Here’s an easy way to remember that. You recall that when you have problems with your joints, we call it arthritis. For problems with your foot, you see a podiatrist. So, arthropods have a jointed foot.
Among the Cambrian fossils there are several indications of leg origins. Some Early Cambrian annelid worms like Burgessochaeta have short paired protuberances on each segment. These appendages were not jointed but had long bristles. Clearly they were not legs but could 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 food objects. 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. And fishes 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 trilobites declined to about thirty-five families, but there were still only five families of fishes. At the same time
there were about fifty families of cephalopods, large predatory squids with coiled shells. We might well have combined the Ordovician time with the Cambrian and called it all the “age of trilobites.” Or if you want to reflect the changes in the biological communities, then we should call it the “age of the cephalopods.” But calling attention to the appearance of the first fishes is just a manifestation of our egos. If you looked at Early Ordovician communities, you would still see lots and lots of trilobites but not many fishes.
What really happened then is that some descendents of little Pikaia sprouted gills and fins and started swimming. Several million years spanned between the early Pikaia fossils and fishes’ appearance, so we must suppose that however vulnerable that modest little creature may have been, she still had established a way of life that allowed her to survive for millions of years among the Late Cambrian arthropod communities. Then, at the start of the Ordovician, and with the appearance of fishes, there is no longer any trace of little Pikaia. Never again do we see soft little worms with notochords or vertebrae. Maybe the lineage of Pikaia was transformed entirely into fishes. Or, perhaps, those descendents that became fishes turned right around and gobbled up the last Pikaia.
Since the start of the decline of the trilobites coincides with the appearance of fishes, it’s tempting to think that fishes played an important role in their disappearance. Probably they were a contributing factor, but it’s obvious that nothing changed very quickly. Trilobite diversity started to decline after the Cambrian, but they didn’t disappear entirely until the end of the Permian, about 250 million years later. Fish diversity increased only slowly and at first the fish were jawless, no doubt nibbling in the sediments. It took tens of millions of years before fish developed bone-crushing teeth to crack shells. The trilobites retained their advantage of hard external skeletons, and they were vulnerable to predators only during their small planktonic stages, and during their soft-shelled molting phase. Also, from the Ordovician to the Permian times, while trilobites were in gradual decline, the family diversity of cephalopod squids was always much greater than the diversity of fishes, and marine crustaceans were on the rise. Another predatory arthropod group made its first appearance in the Late Cambrian, the eurypterid sea scorpions. These nasty creatures had large armor-crushing claws, so they probably could consume large trilobites far more efficiently than either fish or cephalopods, which may have consumed more small planktonic forms of the trilobites. Finally, even the trilobites may have contributed to their own demise by evolving predatory species. During the Ordovician time there appeared the giant predatory trilobite Isotelus, which reached sizes of 16 to 28 inches. This ferocious monster has been dubbed the “Tyrannosaurus of the Ordovician” and has been declared the state fossil of Ohio. So although increasing predation pressure was a perhaps factor in the decline of the trilobites, it is clear that the fishes were not the main reason for their decline, and probably fishes were not the dominant trilobite predators.
To truly understand the disappearance of the trilobites, we need to look beyond the predators and examine some aspects of trilobite biology. Danita Brandt, a trilobite biologist at Michigan State University, has been studying the molting processes of trilobites, and she may have uncovered an important clue regarding their decline. Ironically, it seems that the key to trilobite’s initial success, their external skeleton, may also hold the secret of their demise. It seems that although trilobites were among the first to successfully develop an arthropod skeleton, they never fully perfected the process of living inside it.
Danita Brandt has also been examining fossils of immature trilobites that died during the molting process. She found that the trilobites had a very irregular and inefficient method of molting their skeleton. Modern arthropods, like insects, have mastered the art of escaping from their skeletons by evolving an ecdysal suture: a line of weakness along the upper side that allows them to “unzip” the old skeleton. Insects do not molt until they have extensively recycled materials from the old skeleton and built a flexible new skeleton underneath. With the insects, the new skeleton hardens quickly, enabling them to regain normal functions within a matter of hours. Trilobites, by comparison, lacked both of these innovations. The process of breaking and escaping the old skeleton was irregular, even within a particular species it happened in various ways, and there were many deaths during this process. Even more troublesome for the trilobites was the extended vulnerable period between molts. It seems they did not have an efficient method for recycling skeletal materials, so they needed to regrow a hard skeleton after each molt. For a trilobite, the tender time between molts may have lasted days, or even weeks.
Brandt also studied the trilobites’ survival rate relative to their segmentation and spinal patterns. She found that trilobites with fewer body segments and fewer complex spines were more successful at molting and survived over longer periods of time. In the end, her work helps to explain not only the decline of the trilobites, but also the ultimate success of other arthropod groups, such as the crustaceans and the insects. All arthropods show varying degrees of body segment fusion, brought about by the process called tagmosis. The general pattern seen across all arthropod groups is that multisegmented ancestors fused their body segments into functional regions, or tagma, and the arthropods with less complex body forms were more successful at molting, and therefore survived better over geological time. Although trilobite diversity rose during the Early Cambrian when all animals were first evolving skeletal forms, trilobites started to decline at the end of the Cambrian, as other arthropods such as crustaceans and multilegged myriapods developed more efficient molting procedures, and while new predatory groups such as fishes, squids, and sea scorpions appeared in the waters.
Now you have a picture of life in the ancient shallow oceans until about 444 million years ago. Arthropods, especially in the various forms of trilobites, skittered about in the sediments, paddled, and floated in the balmy waters. Increasingly diverse faunas of shelled squids, fishes, sea scorpions, and crustaceans continued to feast on the tender molting stages of the trilobites. Then, during the Silurian time, life did something it had never done before in more than 3 billion years of the earth’s history. Animals finally set foot on land. Plants sprouted up toward the sun. Terrestrial ecosystems were established. What brought about this remarkable change?
3
Silurian Landfall
All things have beauty, just not all people are able to see it.
ANONYMOUS (fortune cookie wisdom)
If strength and size were everything, then the lion would not fear the scorpion.
(more fortune cookie wisdom)
People of my age vividly remember the events of July 1969 when humans first walked on the moon. We regard them as historically important, and justifiably so. For the first time in nearly four billion years, individuals of a species from earth set their feet in another place entirely, a place so distant and hostile that the challenges of surviving there, even for a short visit, were enormous. Like many of my generation, I remember sitting in front of our grainy black-and-white television, waiting for Neil Armstrong to step off his ladder onto the dusty gray lunar surface. For those of you who are unfamiliar with the term “black-and-white TV,” isn’t it even more noteworthy that we accomplished this feat at a time when most earthbound viewers didn’t have color on their screens? Armstrong’s boot prints are so ingrained in our cultural psyche, I’d bet you could sketch their picture. We’ve all seen them time and again, in books, magazines, posters, and on television.
I propose that there was another day in our history, this one lost in the depths of time, when another set of equally historic footprints were made. But we seldom celebrate or hear about this day in the news. It took place 443 million years ago or more, and like the big bang or a supernova explosion, it was a singular event—the moment when a living organism, an animal, first stepped on the earth.
These earthly footsteps were far more monumental than going to the moon. Fo
r the first animals emerging from the oceans and moving onto land, the dry earth was harsh and forbidding. They needed a structural vehicle capable of making the trip: a skeletal system able to sustain the stresses of the terrestrial environment and a locomotion system able to carry them there and back again. They also needed the necessary life-support systems to keep them alive: surface protection from solar radiation as well as extremes of heat and cold, and to prevent water loss, and a respiratory system capable of functioning in a gaseous as well as a liquid environment. Finally, they needed a reason to go there. Life was comfortable enough in the oceans for a long time. What factors motivated animals to move into what seems to have been an impossibly hostile place?
Planet of the Bugs: Evolution and the Rise of Insects Page 5
