If there were competitions among invertebrates for size, speed, and intelligence, most of the gold and silver medals would go to the squids and octopuses. But it is not these flashy prizewinners that make the phylum Mollusca the second largest of the animal kingdom, with more than 100,000 described species. That honor has been won for the phylum mostly by the slow and steady snails, with some help from the even slower clams and oysters. The name Mollusca means “soft-bodied,” and the tender succulent flesh of molluscs, more than any other invertebrates, is widely enjoyed by humans. But many molluscs are better known for the hard shells that these slow-moving, vulnerable animals secrete as protection against potential predators. Ironically, it is for the beauty and value of these shells that many molluscs are most ardently hunted by humans, in some cases nearly to extinction.
RALPH BUCHSBAUM AND MILDRED BUCHSBAUM, ANIMALS WITHOUT BACKBONES
MISSING LINKS FOUND
The fossil record is full of amazing transformational sequences that show, for example, the evolution of horses from small four-toed ancestors and that of mammals from non-mammals (chapters 19 and 22). But many people are not satisfied with this huge mountain of evidence and ask another question: How did all the discrete phyla of animals (molluscs, worms, arthropods, echinoderms, and so on) evolve from a common ancestor? Where is the evidence for such a large-scale change in body plan, or macroevolution?
For the longest time, there was no fossil evidence to indicate how this happened, other than the clear-cut anatomical features in these creatures that show they evolved from a common ancestor. For example, the connection between the arthropods and the “velvet worms” was established by the similarity of the living animals, long before we had a fossil record to confirm this change, and the recent molecular evidence that finally proved their close relationship (
chapter 5).
Or let’s take another example: the molluscs. Today, the phylum Mollusca includes more than 100,000 described species, more than any other phylum except the arthropods. Molluscs range from such slow and simple creatures as chitons and limpets, which cling to rocks in tide pools and creep along, grazing on algae; through headless clams and oysters, which stay in one place, filter-feeding with their gills; to squids and octopi, which are extremely fast-moving and intelligent, communicate through flashing patterns on their skin, and can solve quite difficult problems. Like arthropods, molluscs have conquered most niches on Earth, including floating in the plankton and living on the seafloor bottom as well as on land (for example, land snails and slugs). Although most molluscs are small, some can be huge—such as the giant squid, which reaches about 18 meters (60 feet) in length; the giant clam, with a shell over 1 meter (3.3 feet) across; and the giant marine snail Campanile giganteum, with a huge spiraled shell over 1 meter long.
But what did the common ancestor of all this huge diversity of snails, clams, and squids look like? What kind of animal has the basic building blocks of all these body plans? And where did the molluscs come from among all the rest of the phyla of animals on Earth?
Most mollusc specialists speculated that the common ancestor of molluscs would have had a body plan based on the elements found in all the members of the phylum (
figure 6.1). They often called such a creature the “hypothetical ancestral mollusc,” based on its simple construction at the nexus of the different molluscan body plans. Such a creature would have had a fleshy layer around its body, the mantle, which secreted a simple cap-shaped shell like that of the limpets, among the most primitive of living molluscs. This creature would have had a broad fleshy “foot” along its bottom that allowed it to cling tightly to rocks for protection and to creep slowly along, feeding in safer conditions.
All living molluscs have a digestive tract that runs from the mouth to the anus and a respiratory system with feather-like gills for extracting oxygen from seawater and releasing carbon dioxide, found in a pocket in the mantle called the mantle cavity. The ancestral mollusc must have had all these features, as well as some sort of excretory and reproductive systems. So the earliest molluscs would have been very limpet-like: a simple cap-shaped shell secreted by the mantle, a broad foot for clinging to rocks and creeping, a one-way digestive tract from mouth to anus, a respiratory system, and most of the other systems found in the major molluscan groups (excretory, reproductive, and so on).
Figure 6.1
Radiation of the molluscs from the “hypothetical ancestral mollusc.” (Modified from Euan N. K. Clarkson, Invertebrate Palaeontology and Evolution, 4th ed. [Oxford: Blackwell, 1993]; from Donald R. Prothero, Bringing Fossils to Life: An Introduction to Paleobiology, 3rd ed. [New York: Columbia University Press, 2013], fig. 16.3)
Marine biologists have all the benefits of studying living molluscs. They can watch them in action, both in marine aquariums and in nature. They have all the soft tissues to dissect and study in detail. Molecular geneticists can obtain the DNA sequence of molluscs from tiny tissue samples and learn what organisms are most closely related to them. All these things give us a clear answer: the closest living relatives of molluscs are the segmented worms, such as the earthworms that live in the soil and the polychaete worms that are extremely common in almost every marine habitat. But there is still a huge gap: How does an earthworm-like creature evolve into a limpet, with its hard shell and unsegmented body?
The problem is compounded by the fact that most worms never leave fossils, except as burrows, which do not say much about the burrow maker. And the only hard parts of most molluscs are their shells, which provide only a fraction of the information offered by soft tissues. Yet paleontologists have become remarkably adept at working with the simple shells of early molluscs and finding all sorts of clues that the soft tissues leave behind.
As early as the 1880s, paleontologists began to describe simple cap-shaped molluscs from the Early Paleozoic (
figure 6.2). The fossils were not well preserved, so it was difficult to say much about them other than they had shells much like that of modern limpets, so must have lived much like a limpet as well. In 1880, the Swedish paleontologist Gustaf Lindström described a fossil shell from the Silurian of Gotland that he called
Triblidium unguis (the species name from the Latin for “hoof” or “nail,” since the shell looked like a fingernail). By 1925, this fossil had been renamed
Pilina unguis. None of the early paleontologists could say very much about this fossil except that it was very limpet-like, and thus it was thought to be a very primitive limpet. However, on the inside of well-preserved shells were two rows of scars, suggesting that the mollusc had had paired muscles. Without soft tissues, however, they could go no further with this fossil.
Over time, a number of fossils of these simple cap-shaped creatures accumulated in beds that date from the Cambrian to the Devonian. Some paleontologists thought that these fossils might those of be the earliest, most primitive molluscs, but the specimens were still too incomplete to tell. More recently, the simple cap-shaped, clam-shaped, and coiled shells found in the “little shellies” (
chapter 3) suggest that there were mollusc predecessors in the Early Cambrian (see
figure 3.2). Yet paleontologists have only the shape of the shell and some of its detailed structures on which to base this argument.
Figure 6.2
Fossil of the simple cap-shaped, limpet-like Pilina, showing the two rows of muscle scars on the inside of the shell. (Courtesy Wikimedia Commons)
GALATHEA TRANSFORMS BIOLOGY
In the late 1940s, oceanography and marine geology were enjoying a huge phase of growth. The battles with submarines during World War II had taught the nations of the world that we knew almost nothing about the 70 percent of Earth’s surface covered by oceans. Soon after the war ended, many governments (especially those of the United States, Great Britain, and Denmark) began to fund large-scale scientific expeditions to map the ocean floor, determine what lay at the bottom of the sea, and recover samples of rocks and marine life from all over the world. War-surplus destroyers were refitted and re-commissioned to the task of mapping the ocean. They carried proton-precession magnetometers originally designed to find submarines; these instruments would eventually produce the key evidence for seafloor spreading and plate tectonics. They routinely took sediment cores from nearly every part of the seafloor, bounced sound waves off the bottom to record the depth, and tossed sticks of dynamite off the fantail to bounce sound waves through the upper layers of the sea-bottom sediments and determine their structure.
Among these pioneering postwar efforts was the Second Galathea Expedition, mounted by the Danes from 1950 to 1952. The ship was named after the Greek myth of Pygmalion and Galatea. According to the story, the sculptor Pygmalion carved a perfect woman out of marble, named her Galatea, and fell in love with her. He was so enamored of his creation that the gods transformed her into a living woman, in answer to Pygmalion’s prayers. Some might recognize this plot device in the Broadway musical My Fair Lady, in which Professor Henry Higgins (Pygmalion) transforms the poor slum girl Liza Doolittle (Galatea) into an elegant, aristocratic woman. The musical, in turn, was derived from George Bernard Shaw’s famous play Pygmalion, which was based on the Greek myth.
The First Galathea Expedition had been undertaken between 1845 and 1847, using a three-masted sailing ship to explore the waters off the major Danish colonies around the world. In 1941, journalist Hakon Mielche and oceanographer-ichthyologist Anton Frederik Brunn were pushing to fund a second expedition in order to further Danish scientific and commercial interests. However, World War II and the Nazi invasion of Denmark put their planning on hold.
In June 1945, just after the war ended, the Danish scientific community resumed serious fund-raising and planning. They purchased the retired British sloop HMS
Leith, a vessel with a long and distinguished record of escorting ships back and forth across the Atlantic during the war and sinking U-boats. The Danes refitted it for oceanographic purposes and renamed it HMDS
Galathea 2. Unlike the first
Galathea, this ship was designed to do extreme deep-sea surveys, dredging sediments from and measuring depths of the deepest parts of the ocean. It visited some of the places the mid-nineteenth-century expedition had visited, but the highlights of the mid-twentieth-century voyage around the world was dredging in waters more than 10,190 meters (33,430 feet) deep in the Philippine Trench (the deepest samples ever obtained back then), as well as in many other deep parts of the ocean, yielding creatures never before seen by scientists.
Along with many spectacular and bizarre deep-sea fishes and other marine creatures was a curious-looking mollusc, brought up in 1952 from waters over 6000 meters (19,700 feet) deep in the Costa Rica Trench (
figure 6.3). When expedition zoologist Hennig Lemche got a chance to publish the specimen in 1957, he realized that it was truly revolutionary. He named it
Neopilina galatheae, in honor of the fossil
Pilina and the ship that had found it. It was indeed a relative of the mysterious cap-shaped fossils from the Early Paleozoic, and its soft tissues allowed paleontologists to interpret the mysterious marks and scars on the fossils. The prominent zoologist Enrico Schwabe called it “one of the greatest sensations of the twentieth century.”
Lemche pointed out that Neopilina is a true “living fossil,” a late-surviving genus in a class of molluscs called the Monoplacophora (from the Greek for “carrying a single shell”), which vanished from the fossil record in the Devonian. And what amazing information was revealed when the specimen was studied! As indicated by the two rows of muscle scars on the fossils, Neopilina has paired muscles that produce those scars, suggesting that it had segmented muscles just like segmented worms. Not only are the muscles segmented, but so are the gills, the kidneys, the multiple hearts, the paired nerve cords, and the gonads. In short, Neopilina shows that the mysterious monoplacophoran fossils were half mollusc, half worm: they had the segmentation of all their organ systems, like their worm-like ancestors, but they also had a mantle, a shell, a broad foot, and other features found in primitive shelled molluscs like limpets and chitons.
Since the description of
Neopilina in 1957, many more living and fossil monoplacophorans have been found. There are now 23 extant species. These “living fossils” are live mostly in waters between 1800 meters (6000 feet) and 6500 meters (21,000 feet) in depth, but a few occur in waters only 175 meters (575 feet) deep. Little is known about their life habitats, because they live in such deep water and cannot survive after they are captured and brought to the surface, where the pressure and temperature are so different from those in the deepest ocean. It is presumed that they are muddy-bottom feeders, grubbing through the seafloor muds for organic material or trapping sinking plankton, as are most creatures that live in water too deep for light to penetrate and thus for photosynthesis to occur.
Figure 6.3
The “living fossil” Neopilina, a relict of the Early Cambrian and a transitional form between segmented worms and molluscs: (A) the segmented paired gills on either side of the foot in the center of the body; there are also paired segmented retractor muscles and other organ systems; (right) a modern chiton; (B–C) living Neopilina. (Courtesy J. B. Burch, University of Michigan)
How did such an important group escape the notice of science for so long? The biggest reason was that we had almost no means of studying or collecting life in the deepest part of the oceans. The Second Galathea Expedition was one of the earliest to undertake that task. In fact, a living monoplacophoran, Veleropilina zografi, had been discovered in 1896, but it was mistakenly described as an ordinary limpet and forgotten. Not until 1983 was it restudied, and scientists realized that their predecessors had seen an extant monoplacophoran long before the discovery of Neopilina.
Not only have 23 living species of monoplacophorans been found, but the fossil record of the class has improved as well. In addition to the earliest fossils to be studied are fossils like Knightoconus, which has chambers with dividing walls, like the chambered nautilus. Some paleontologists argue that it is the transitional fossil between the primitive monoplacophorans and the cephalopods, the group that includes not only nautilus but squids and octopi as well.
The discovery of Neopilina ranks as one of the classic examples of a mysterious fossil group long thought to be extinct that was rediscovered alive and well in the deep ocean. More important, the description of many extant and extinct monoplacophorans has shown how molluscs evolved from an ancestor shared with segmented worms, and then lost that segmentation as they diversified into snails, clams, squids, and so many other groups in this important phylum. Thus the fossil record has confirmed what anatomists and molecular biologists had concluded as a result of their research: molluscs are descended from segmented worms, and members of the class Monoplacophora are the “transitional forms” that demonstrate the macroevolutionary change from one phylum to another.
FOR FURTHER READING
Ghiselin, Michael T. “The Origin of Molluscs in the Light of Molecular Evidence.” Oxford Surveys in Evolutionary Biology 5 (1988): 66–95.
Giribet, Gonzalo, Akiko Okusu, Annie R. Lindgren, Stephanie W. Huff, Michael Schrödl, and Michele K. Nishiguchi. “Evidence for a Clade Composed of Molluscs with Serially Repeated Structures: Monoplacophorans Are Related to Chitons.”
Proceedings of the National Academy of Sciences 103 (2006): 7723–7728.
Morton, John Edward. Molluscs. London: Hutchinson, 1965.
Passamaneck, Yale J., Christoffer Schander, and Kenneth M. Halanych. “Investigation of Molluscan Phylogeny Using Large-subunit and Small-subunit Nuclear rRNA Sequences.” Molecular Phylogenetics and Evolution 32 (2004): 25–38.
Pojeta, John, Jr. “Molluscan Phylogeny.” Tulane Studies in Geology and Paleontology 16 (1980): 55–80.
Runnegar, Bruce. “Early Evolution of the Mollusca: The Fossil Record.” In Origin and Evolutionary Radiation of the Mollusca, edited by John D. Taylor, 77–87. Oxford: Oxford University Press, 1996.
Runnegar, Bruce, and Peter A. Jell. “Australian Middle Cambrian Molluscs and Their Bearing on Early Molluscan Evolution.” Alcheringa 1 (1976): 109–138.
Runnegar, Bruce, and John Pojeta Jr. “Molluscan Phylogeny: The Paleontological Viewpoint.” Science, October 25, 1974, 311–317.
Salvini-Plawen, Luitfried V. “Origin, Phylogeny, and Classification of the Phylum Mollusca.” Iberus 9 (1991): 1–33.
Sigwart, Julia D., and Mark D. Sutton. “Deep Molluscan Phylogeny: Synthesis of Palaeontological and Neontological Data.” Proceedings of the Royal Society B 247 (2007): 2413–2419.
Yonge, C. M., and T. E. Thompson. Living Marine Molluscs. London: Collins, 1976.
