The Fossils of Monte San Giorgio

Transcription

1 SCIENCE IN PICTURES The Fossils of Monte San Giorgio An ancient sea provides a rieh assemblage of vertebrates from the Trias sie period by Toni Bürgin, Olivier Rieppel, P. Martin Sander and Karl Tschanz TONI BÜRGIN, OLIVIER RIEPPEL, P. MAR- TIN SANDER and KARL TSCHANZ, listed here in alphabetical order, are members of a fossil vertebrate study group at the Paleontological Institute and Museum at the University of Zürich. Bürgin, who has a Ph.D. from the University of Basel, is currently studying the fishes of Monte San Giorgio for the Swiss National Science Foundation. Rieppel, whose Ph.D. is also from the University of Basel, is a privatdozent, who teaches vertebrate paleontology and supervises research on the vertebrates of Monte San Giorgio. Sander has an M.A. from the University of Texas and a Ph.D. from the University of Zürich. Tschanz has a Ph.D. from the University of Zürich. A little more than 200 million becn excavated from Monte San Giorgio over the years have provided pa- /\s ago, in a part of the world JLJL that is now the border between Switzerland and Italy, there existed a small sea. Although this body of water eventually dried up and disappeared, the remains of its animal inhabitants a diverse assemblage of fishes and rcptiles were preserved after their death in the muddy sediments at the bottom of the basin. Over the course of millions of years the Sediments hardened, were upliftcd by tectonic activity and eventually were transformcd into a mountain called Monte San Giorgio. Today Monte San Giorgio, which rises above Lake Lugano in the southern Alps, is something of a tourist attraction, not so rnuch for its geological past äs for the panoramic view it offers those who succeed in reaching its peak. Yet it is the richness of the fossils embedded in its sedimentary rock that gives the site its real distinction. The thousands of fossils that have leontologists with an unparallcled, if somewhat puzzling, vlcw of the past. The fauna represents a pivotal time in the history of lif'e on earth known äs the Triassic period, which lasted from 210 to 250 million years ago. It was during this relatively short time span that several major groups of terrestrial vertebrates made their debut, including the ancestors of most of the modern reptiles, the dinosaurs, the mammals and possibly the birds. The flrst fossils frorn Monte San Giorgio were discovered in 1863, when Antonio Stoppani, then director of the Natural History Museum of Milan, stumbled on several vertebrate fossils (some fishes and a few reptiles) at the base of the mountain near Bcsano, a small vlllage in northern Italy. On returning to Milan, he gave the fossils to Francesco Bassani, who published brief descriptions of them and classified them taxonomically. Although Stoppani returned to the sitc again in 1878 and collcctcd several more specimens, the brief anecdotal accounts of bis findings stimulated scant interest. It was not until the turn of the Century, some 40 years later, when the bituminous shales (sedimentary rocks rieh in organic matter) of Monte San Giorgio were exploited for an oily compound called saurol (believed to be an effective remcdy for rheumatism), that the area's fossils were once again the focus of attention. The year was 1919 and Bernhard Peyer, a young zoologist frorn the University of Zürich, was skimming through a pile of discarded shale near the saurol extraction site; äs he sorted through the pieces, bis hammer struck a piece of shale on which was preserved the paddlelike limb of an extinct marine reptile called an ichthyosaur. No cornparable fossil of this age had been found elsewhere in Europe, and not surprisingly, Pcyer's discovcry gencrated a grcat dcal of excitement. Spurred by the importance of his find, Peyer devoted much of the rest of his life to a systematic search for addi- 74 SCIENTIFIC AMERICAN June. 1989

2 tional fossil material from the region. When he retirod in 1956, the task of collccting and cataloguing fossils was passed to his successor Emil Kuhn- Schnyder, who in turn added thousands of new specimens to the collection at the University of Zürich. Today, largely because of the efforts of these two men, the Paleontological Institute and Museum at the University of Zürich is considered to have the world's finest collection of Triassic marine EXTINCT REPTILE, the nothosaur Ser piano saurus, is presentin large numbers in the shales of Monte San Giorgio. The animal superficially resembied a lizard but was amphibious, spending some of its titne in the water and some on land. This species, which reached about 70 centimeters in length, had a long, flexible neck and probably fed on small fish. Near the base of its tail, and at right angles to it, is the backbone of another individual. SCIENTTFIC AMÜRICAN June

3 MONTE SAN GIORGIO rises near Lake Lugano in the southem Alps, not far from the border between Switzerland and Italy. Most of the fossils come from exposed beds on the southern slope of the mountain, north and northwest of the Swiss villages ofmeride and Serpiano. vertebrates and is home to more than 4,5 00 specimens from Monte San Giorgio alone. By analyzing both the nature and stratigraphy of its Sediments, geologists have determined that in the Triassic period the San Giorgio basin was not very big: it is estimated to have been from six to 10 kilometers in diameter and approximately 100 meters deep. Careful analysis of fossil and stratigraphic data indicates that the basin was not far from the coast of an ancient ocean called the Tethys Sea, a predecessor to the now existing Mediterranean. Although the basin itself was surrounded by algal reefs and dry land, the presence of pelagic (opensea) species indicates that it was accessible, at least intermittently, from the open sea. According to Hans Rieber, currently the director of the Paleontological Institute, the bottom of the basin consisted of fine-grained mud; when the animals that lived in the basin died, they sank to the bottom, where conditions were anoxic (without oxygen), and so their remains, which would normally be broken down by aerobic bacteria and other scavengers, were protected from decay. Over the course of many thousands of years the mud accumulated and turned to stone, compressing and flattening the animals' skeletons äs it petrified. In some instances the, force of compression crushed the skeletons so severely that Interpretation of fine anatomical detail is difficult, if not impossible. Most of the fossils are well preserved, however, and even 200 million years after their death delicate bones and fine details such äs tiny spines and scales are distinctly visible. It is readily apparent even at the glance of an untrained eye that the San Giorgio basin supported a wide variety of animal species. In terms of fish diversity alone, the basin can be considered analogous to a modern coral reef. Five species of sharks have been identified so far; four of them are small, yet robust in shape, with crushing teeth that suggest a diet of shellhsh. The fifth species, represented by a rather large fin spine and some teeth, is estimated to have been from two to three meters long and to have fed on shellfish. Like fossil sharks from other localities, the San Giorgio specimens are represented mostly by teeth and an occasional fin spine or backbone (sharks, unlike bony fish, have skeletons made of cartilage and deteriorate rapidly after death). Yet several almost complete shark specimens have been found, providing further testimony to the exceptional preservation of the site. Lobe-finned fishes are present in limited numbers, but the specimens that have been found tend to be complete because their heavy, enamellike scales resist decay. These fishes, members of a mostly extinct order called the Actinistia (the group to which the living coelocanth Latimeria belongs), have been the focus of much attention because they are considered to be related to the stem group from which all land vertebrates descended. So far more than 20 specimens have been collected from the San Giorgio shales, all of which fall into one of three distinct size categories. Such DIVERSE ARRAY OF RAY-FINNED FISHES has been excavated from the bituminous shales of Monte San Giorgio. More than 550 specimens in some 30 genera have been collected so far. Representatives, shown here drawn to scale, fall into three distinct size classes. In one category are the large predatory fishes, such äs Birgeria stensiöi (a), which grew to more than a meter in length, Colobodus bassanii (b), which was about 70 centimeters (or 27.5 inches) long, and Saurichthys cu- 76

4 EXQUISITE PRESERVATION of the San Giorgio fossils is exemplified by some of the ray-finned fishes. Fine details such äs the tail fins and elongate snout of the lizard fish Saurichthys are visible (a); even more remarkable is the presence oftwo embryos inside the abdomen of a female (b), evidence that these fish gave birth to live young. Just below the tail of the small, heavily armored Peltopleurus (c), a highly modified anal fin can be seen. The fin bears vertical hooklike rays that may have played a role in courtship by enabling males to hold onto females. One, äs yet undescribed, species (d) is informally called "big teeth" in reference to the animal's unusual dentition; its diet remains a mystery. discrete size distribution leads us to speculate that the size groupings may reflect ecological perhaps dietary specialization. The largest ones measure about 70 centimeters in length and may have fed on small fish. In contrast, the smallest ones were only 20 centimeters long, but tbeir eating preferences are not known. More than 550 well-preserved specimens of ray-finned fishes (the group to which the majority of living fishes belong) have been catalogued. Although most have yet to be studied in great detail, we expect that once they are carefully studied, they will shed new light on the overall evolution and relationships of ray-finned fishes. Already these fishes have provided valuablc Information about the paleoecology of the San Giorgio basin. The presence of ocean-dwelling, f'astswimming forms, for example, provides some of the evidence that the basin was connected intermittently to the Tethys Sea. One such indicator fish is Birgeria, which was widcsprcad during the Triassic and is comrnon in many deposits of that period. In contrast to the fishes, which have becorne the focus of intense study only recently, the reptiles of San Giorgio have been the objects of paleontological curiosity since Peyer published his first account of them. The most abundant and perhaps best studied are the amphibious nothosaurs, members of a large and common order of Triassic reptiles called the Sauropterygia. Nothosaurs are descendants of a branch of terrestrial sauropterygians and retain many of the features of their land-dwelling relatives; their limbs, for example, were not specially adapted for aquatic locomotion. Unlike many aquatic reptiles, including ichthyosaurs, whose limbs were modified into paddles and therefore played an important role in propelling the animals through water, nothosaurs are thought to have moved forward by lateral undulations of the trunk and tail. In addition to having elongate, flattened tails, most had long, flexible rionii (c), approximatety 60 centimeters long. In another category are the medium-size fishes such äs Ptycholepis barboi (d), which was about 30 centimeters long and superficially looked like a modern mackerei, and Bobasatrania (e), an inhabitant of coral reefs. In the smallest category are fishes such äs Peltopleurus (f) that were flve centimeters or less in size; they are preserved in the shales of Monte San Giorgio in large numbers. SCIENTTFIC AMERICAN June

5 SMALLEST yet most abundant reptile at Monte San Giorgio is Neusticosaurus (above). The anitnal's feet are better adapted for paddling through water than for walking on iand. A thin cross section of a Neusticosaurus leg bone (left) reveals the presence of annual growth rings. The animal from which this seclion of bone was taken was five years old when it died; the diameter of the bone is 4.1 millimeters. necks and wert prohably adept at catching small fish äs they swam. The San Giorgio nothosaurs varied in shape and size from Ceresiosaurus, an animal That grew to äs much äs three meters in length, to Neusticosaurus, the dwarf lizard, which averaged only about 30 centimeters (a foot) in length. Altogether some 400 specimens representing each stage in the life cycle, from embryo to adult have been documented. Development in these animals can be studicd becausc nothosaurs, likc all cold-blooded reptiles, grew by adding ncw bone in the form of annual growth rings. Thin cross sections of their bones (aboul 50 micrometers thick) can be examined undcr a microscope and the number of rings counted. Extensive analysis of the growth rings (made possible by the sheer number of nothosaur specimens in the Institute's collection) reveals that the animals became sexually rnature at about three or four years of age and lived to a maximum of six years. Related to the nothosaurs but fully aquatic are the placodonts: a group of short, stout marine reptiles that had large, flattened teeth and are believed to have fed primarily on bivalves and other mollusks found along the edge of the basin. The two genera of placodonts known from Monte San Gior- 78 SCIENTIFIC AMERICAN June 1989

6 gio are Cyamodus and Paraplacodus. Anothcr well-represented group of reptiles at San Giorgio is the Archosauromorpha. The fossils in that category constitutc a largc and poorly un* derstood asscmblage but are of interest becausc they are members of the group that eventually gave rise to the dinosaurs. One of the more bizarre archosauromorphs indccd perhaps the strängest reptile ever described is Tanystropheus. Known äs the giraffeneck saurian, Tanystropheus has become somcthing of a mascot for the San Giorgio fossil fauna. The animal, which measures äs much äs 4.5 meters from head to tail, is famous for its absurdly long neck, which is more than twice the Icngth of its trunk. We have concluded that the animal must have been aquatic because it is impossiblc to think that it could have supported the weight of its neck on land. Closely related to Tanystropheus but much smaller in size {no more than 80 centimeters from head to NOTHOSAUR Ceresiosaurus, shown here surrounded by eight small Neusticosaurus specimens, is exquisitely preserved. It is 2.3 meters (7.5 feet) long and was the best adapted among the nothosaurs to an aquatic habitat. Its lony feet were somewhat paddlelike, and its tail was large and lateraily flattened for better propulsion through the water. SciENTinc AMERICAN June

7 tail) was Macrocnemus. The animal is thought to have been fairly mobile on land, where it may have spent considerable time chasing insects and grabbing them with its mouth. The large number of specimens of different age classes of bothmatrocnemusand Tanystropheus indicates that their grossly elongated necks are a function of allometry, that is, the necks grcw at a rate faster than the rest of the body. It is difficult to believe that the exaggerated neck of Tanystropheus could have provided its bearer with any kind of advantage; indeed, we think that the animal survived for several million years not because of its extraordinary neck but in spite of it. The only true archosaur (the group to which dinosaurs belong) found at the site is Ticinosuchus ferox. The animal, which was about 2.5 meters long, is believed to have been terrestrial, and a ferocious carnivore. Thalattosaurs, another enigmatic group of marine reptiles, are also represented in the San Giorgio shales. These animals belong to the Diapsida, the group from which all living reptiles, except turtles, descended. The thalattosaurs are a poorly known group of marine reptiles. Three genera from San Giorgio have been described: Askeptosaurus, a fish-eating animal that measures about 2.5 meters in length and has a long, narrow skull; Clamzia, about one meter long with crushing teeth (with which it probably fed on mollusks); and Hescheleria, an incomplete specimen whose dietary habits are completely unknown. Ichthyosaurs, a group of marine reptiles that were similar in size and shape to modern dolphins, having paddleshaped limbs and distinctive snouts, were also inhabitants of the San Giorgio basin. Abundant and widely distributed during the Triassic, they are nonetheless a puzzling group; their relationship to other reptiles, for example, is still highly conjectural. Some specimens have been found that con- BIZARRE REPTILE from the San Giorgio basin was Tanystropheus, a 4.5-meter-long animal, whose neck was twice äs long äs its trank. The relative weight of the neck was so great that the animal could probably have supported it only in water. For that reason the species is thought to have been aquatic. The head, which is missing from the fossil (above), is known from other specimens and was unusually small relative to the rest of the body (below). Related to Tanystropheus but much smaller was Macrocnemus (above right), a highly mobile reptile that probably spent considerable time on land and may even have assumed an upright position while hunting insect prey. 80 SCIENTIFIC AMERICAN June 1989

8 tain the remains of unborn young; it is surmised that the cggs must have devcloped within the mother's body and that the young wcre born live, a strategy that seems clearly advantageous for an aquatic reptile unable to come out on land to lay its cggs. Three genera of ichthyosaurs have been found at San Giorgio: Mixosaurus, the most common and best known of Triassic ichthyosaurs, Cymbospondylus, and possibly Toretocn&mus. In summarizing the fossils of Monte San Giorgio it can be said that they offer an exquisite glimpse at a now extinct fauna. Yet the anatomical similarities and confusing traits of many of the reptilcs make it difficult to fit them into the framework of existing classifications. The diversity of the San Giorgio vertebrate fossils sccms only to enforce the prcvailing notion that relationships among the major groups of fishcs and rcptiles are difficult to untangle. Why, given the wc'alth of fossil material, should the evolution of the reptiles prove so elusive? 1s it simply because the major groups have not been properly defined? And, if that is the case, is it because convergence (the evolutionary process whereby unrelated organisrns in similar habitats come to superficially resemble each other) is more common than previously recognizcd? Do the animals look alikc because there are unknown structural constraints that mandate their size and shape? The answers to such questions are currently the focus of an intensive research program, which makes it clear that deciphcring the past is not an easy task. One comment is certain: the exquisitcly preserved San Giorgio fossils have at least enabled paleontologlsts to ask thought-provoking questions about the nature of life and the processes of evolution. FOSSIL ASSEMBLAGE from San Giorgio includes the thalaltosaur Askeptosaurus (a), a marine reptile about 2.5 meters lang that fed on /Ksfr/Ticinosuchus ferox (b), a fierce terrestrial species about 2.5 meters in length that was closely related to true dinosaurs; and ichthyosaurs such äs this adult Mixosaurus (c), a meter-long marine reptile that bears a superficial resembiance to a dolphin. Careful examination reveals that the mixosaur specimen (d) is a pregnartt female and is carrying several tiny embryos inside her abdomen. FURTHER READING A NEW GENUS or SIIARK EROM TUE MID- DLE TRIASSIC OF MONTE SAN GIORGIO, SWITZERLAND. O. Rieppcl in Palaeontology, Vol. 25, No. 2, pages ; ALLOMETRY AND HETEROCHRONY IN THE GROWTH OF THE NECK OF TRIASSIC PRO- ].ACi:k'i'iroRM REPTII.ES. Karl Tschan/ in Palaeontology, Vol.!U, No. 4, pages ; A FOSSIL REPTILE EMBRYO FROM THE Mm- DLL TRIASSIC OF THE ALPS. P. Martin Sander in Science, Vol. 239, No. 4841, pagcs ; VERTEBRATE PALEONTOLOGY AND EVOLU- TION. Robert L. Carroll. W. H. Frccman and Company, SciKNTiHC AMKRICAN June