The origin of skeletons

The fossil record had caused Darwin more grief than joy. Nothing distressed him more than the Cambrian explosion, the coincident appearance ofalmost all complex organic designs.

Stephen Jay Gould, The Panda's Thumb (1980)

The appearance of skeletons in the fossil record some 540 million years ago has long been a puzzle. It is not perhaps such a puzzle that scientists throw in the towel, as creationist critics gleefully report on their websites, but a real problem to be resolved. The fact is that, shortly after the beginning of the Cambrian period, currently dated at 542 million years ago, and some time after the extinction of the Ediacaran organisms, a broad diversity of animals with skeletons appeared in the sea. A skeleton to a biologist is any kind of mineralized, or partly mineralized, structure that acts as a support or framework for an organism. So our internal skeleton of bones fits the bill, but so too do the calcareous shells of molluscs and corals, the outer cuticles of insects and crabs, and even arguably the woody stems of trees.

The Ediacaran fossils of the Neoproterozoic did not have shells or skeletons of any kind we would recognize today. Perhaps, as Dolf Seilacher suggests, they had a quilted pneumatic structure that stiffened their bodies and allowed them to reach reasonable body size. Then, in Lower Cambrian rocks around the world, a diversity of shelly fossils appears. It is the fact that skeletonized organisms seem to appear suddenly, geologically speaking, and all at the same time, that is the puzzle. Why, for example, don't we first find sponges with skeletons of spicules, then corals with their tube-like houses, then perhaps shellfish with their encapsulating valves, and so on? Of course, when looking back over half a billion years, it's not easy to date every rock formation precisely, but every study seems to suggest a rather coordinated appearance of animals with skeletons about 542 million years ago. This dramatic event has been called the Cambrian Explosion.

The debate revolves around the reality of this event. Most palaeontologists and evolutionists, including Darwin, have suggested that the Cambrian Explosion was real and that what you see actually happened. Others, however, urge caution and suggest that we might be seeing something artificial, the result perhaps of

5 incomplete preservation of the fossils. It could be, for example, o t that there are major gaps in the rock record at the end of the

| Neoproterozoic, or that the sediments that were deposited

¡= through that interval were not the right ones to preserve mineralized skeletons. In this chapter we will explore what skeletons are, what the fossil and rock record shows, new molecular evidence, and the rather heated debates about whether the Cambrian Explosion is real or not.

Skeletons

Skeletons are not just for physical support, although that is a major, often the major, function. They also provide sites for the attachment of muscles and a mineral store. So, for example, in humans, we rely on the framework of our skeleton to be able to walk and eat. The muscles attach at both ends to bones in the skeleton, and muscle contractions make the arms and legs work. In feeding, jaw muscles pull the lower jaw up and down against the skull, and the jawbones carry the teeth, all essential in feeding.

Bone is composed of two main components, the protein collagen and spicules of apatite, a form of calcium phosphate. Collagen is the primary component of cartilage. We have cartilage in our noses and ears, and it is a bendy kind of unmineralized bone. Among living vertebrates, the backboned animals, sharks have almost entirely cartilaginous skeletons that only occasionally become mineralized (and of course their teeth are mineralized), and it seems that the Cambrian predecessors of modern fishes also mostly had cartilage skeletons.

Our bones also act as mineral stores. When we are young and growing, the body has to scavenge large amounts of calcium and phosphorus from our food and it passes through the blood vessels to the bones. If a person is starved at a young age, their bones cannot grow properly, and they become stunted. Later in life, calcium and phosphorus may be mobilized from within the bones h e when they are needed. Bone is living, laced through with blood i vessels, and other tissues. If food is short, calcium and phosphorus g are absorbed from the bone back into the blood supply and passed e to the cells where it is needed. The minerals can be replaced later g when food is abundant. So if you were to cut through any of your s bones, you would see evidence for how it grew to its present size during your childhood. You would also see evidence for episodic extraction and replacement of calcium and phosphorus in the form of channels that are widened as minerals are extracted, and that fill up in layers as minerals are replaced, rather like a water pipe furring up in an area of hard water.

Other animals have different kinds of skeletons. Skeletons may be composed from inorganic mineralized materials, such as forms of calcium carbonate, silica, phosphates, and iron oxides. Calcium carbonate makes up the shells of microscopic foraminifera, some sponges, corals, bryozoans (colonial creatures), brachiopods ('lamp shells'), molluscs, many arthropods (trilobites, crabs, insects), and echinoderms (sea urchins, sea lilies). Silica forms the skeletons of radiolarians (planktonic organisms) and most sponges, while phosphate, usually in the form of apatite, is typical of vertebrate bone, as we have seen, and the shells of certain brachiopods and the tiny toothed jaw structures of certain worms. There are also organic hard tissues, such as lignin, cellulose, sporopollenin, and others in plants, and chitin, collagen, and keratin in animals, which may exist in isolation or in association with mineralized tissues.

The simplest skeletons are seen in the sponges, which are composed of loose aggregates of spicules, pointed microscopic structures made from calcium carbonate or silica. Most other animals have an external skeleton, or exoskeleton. (Humans, and other vertebrates, have an internal skeleton, or endoskeleton.) In corals, brachiopods, and molluscs, the exoskeleton is a layered structure, built up year by year, or month by month, with growth lines often visible on the outer surface and in cross-sections.

5 Other animals shed their exoskeletons - animals such as o t arthropods, nematode worms, and some rarer groups. Indeed, o

| skeleton-shedding may be a unique feature of this particular

The diversity of skeleton types, and the fact they are constructed in so many different ways - some are internal, some external, some are shed, and others are not, they may be made of different mineral constituents - makes it hard to understand how skeletons seemingly evolved at the same time in all these animal groups, and everywhere in the world. What does the fossil record show us, if we follow it step by step through the transition from the latest Precambrian into the Cambrian?

Small Shelly Fauna

The first step is represented by the time of the 'Small Shelly Fauna', so called, perhaps not surprisingly, because it is a fauna that is composed of small shells. The term 'small shells', however, hides a great deal of ignorance: small shells they may be, but the affinities of many of them are unclear.

The Small Shelly Fauna (SSF) has been identified in the latest Precambrian, but is best known in Lower Cambrian rocks, dating from perhaps 542 to 530 million years ago. The importance of the SSF is that it comes before the appearance of larger fossils with skeletons, and so marks the first phase of the Cambrian Explosion.

It has proved very hard to understand the biology of the SSF animals, and they are generally named simply according to their a

Precambrian Tube
10. Fossils from the Early Cambrian. A: A selection of Small Shelly Fossils from the Siberian Precambrian-Cambrian boundary strata; B: Microdictyon

a shapes (Fig. 10A). Two major groups are the hyolithelminthids with phosphatic tubes, open at both ends, and the tomotiids with phosphatic cone-shaped shells, usually occurring in pairs. Other animals were tube-builders that secreted carbonate walls, organic-walled tubes possibly of an unsegmented worm, and phosphatic plates, or sclerites, from larger but unknown animals.

The sclerites give clues to a whole array of animals we barely understand. Mostly their bodies have gone, and all we have are the minute, microscopic leaf-shaped sclerites. It is assumed that these fitted together as some kind of flexible armour over animals that may have looked roughly like pine cones. Some exceptionally preserved specimens from China, called Microdictyon, suggest that some of the sclerite-bearers at least were worm-like animals (Fig. 10B), which carried oval plates arranged in pairs along the length of the body which may have provided a base for muscle

5 attachment associated with locomotion. What is intriguing is that o t some of the sclerites might have come from quite large animals o

| that are otherwise entirely unknown, and may never be known

¡= other than by these intriguing exuviae.

The Cambrian Explosion

The Small Shelly Fauna of the Early Cambrian was a precursor of the Cambrian Explosion proper. Towards the end ofthe Early Cambrian, and overlapping in time with the SSF, a dozen or more major animal groups appeared. At one time, it was thought that they all appeared at once, but more careful study suggests a rather more orderly procession, with one group appearing after another. Some of the evidence comes from fossils of the organisms themselves, and other steps along the way are indicated at present only by trace fossils, tracks, and trails. This may seem rather uncertain evidence, but many tracks and trails can be quite diagnostic of their makers, especially if they show foot or leg marks, for example.

So, in sequence, the first evidence for the radiation of animals in the sea, and the first step of the Cambrian Explosion, is represented by tracks and trails dating from 555 million years ago, at the end of the Neoproterozoic. These tracks were made by elongate bilaterally symmetrical animals, mostly worms of one sort or another. Then, the trace fossil record shows the first evidence of arthropods, the 'jointed limbed' animals, in the earliest Cambrian, some 540 million years ago. Then the Cambrian Explosion proper began about 530 million years ago, with the appearance of the first skeletal fossils of trilobites and echinoderms, and it lasted for perhaps 10 million years, during which time global diversity burgeoned. Groups such as molluscs and brachiopods, which possibly appear in the SSF, are represented by unequivocal fossils. Sponge spicules are abundant in places.

If you had gone back to this time in the Early to Mid Cambrian, h

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