Mary L Droser, Richard A Fortey, Xing Li. American Scientist. Volume 84, Issue 2. March 1996.
World geography has changed and changed some more. Deserts have not always been desolate, for ancient seas have left their legacy of vanished life in the rocks that now bake under an uncompromising sun. The Great Basin area of Nevada, Utah and California is one of the drier areas in the United States. But in the high mountain ranges thick piles of Ordovician rocks, dating back 440 to 510 million years, teem with evidence of past life that thrived in warm, sunny tropical waters. This rock record provides some of the best clues about an explosion of marine invertebrates that took place at that time, one of the most significant radiations in the history of metazoans—complex multicellular animals with differentiated body parts. Major groups of animals, such as bryozoans and rugose corals, first appear in the fossil record of this time. Other pre-existing forms—trilobites, brachiopods, mollusks and graptolites—all underwent important changes and diversifications.
During a radiation existing life forms give rise to new variations, which eventually become new species. Of all the major radiations, the Ordovician is the one whose underlying dynamics scientists have the best hope of understanding. The marine record is continuous through this period, and skeletonized fossils exist from long before and after the radiation. In addition, the Ordovician does not follow any major extinction in the previous Cambrian era, which makes the record less complicated to follow.
The Ordovician radiation was complex and took place over millions of years. It is not as dramatic as the radiation at the base of the preceding Cambrian period, approximately 540 million years ago. Rather than being known for its seemingly bizarre forms, the Ordovician radiation is characterized by more familiar fossils. Some, such as trilobites, were common in the Cambrian seas. Others, such as bivalves, were rare in the Cambrian but are abundant in today’s oceans. The Ordovician is not a time of the first appearance of new phyla, but is marked largely by changes in species, genera and families. Nevertheless, the significance of the Ordovician radiation should not be underestimated. This period represents one of the largest major turnovers in the history of life and marks the appearance of groups that came to dominate marine ecosystems for the next 250 million years. As with other significant events in life’s history, the magnitude of this radiation has been identified as a function of the increase in biodiversity: The Ordovician radiation records a near tripling of marine biological families.
Typically, the Ordovician radiation has been studied one of two ways. Some scientists have looked at diversification within a taxonomic group. Others have done analyses of data bases looking for evidence of large-scale ecological and environmental changes. These data bases are compiled primarily from reports on fossil occurrences in a specific rock unit—data that were not compiled specifically to address questions about the radiation.
The recognition of large-scale patterns either taxonomically or paleoecologically sets the stage for more rigorous questions addressing the nature of the diversification event. We are poised to go back and specifically collect data from the field to test these patterns as well as to address the questions that they raise. What is the timing of this radiation? Was it sudden (geologically speaking)? Do different taxonomic groups, called clades, actually radiate in concert or independently? Are there pivotal turning points? Were there paleoecological shifts? Is there a biogeographic pattern? The answers to these questions are important, not only in telling us something about the nature of biological innovation, but because they teach us something about the context prompting these events. For example, if just one clade is diversifying at a time, then we can conclude that a biological trigger or some very local physical changes were prompting this diversification. If, however, many clades were radiating at the same time, we might infer that some large-scale environmental event was behind it.
In order to describe the nature and the patterns of diversification so that the dynamics may be better understood, we have been examining geological strata dating back to the Ordovician from the Great Basin areas of California, Nevada and Utah in detail. We are examining a number of aspects of the paleontological and stratigraphic record, in keeping with the current aim to integrate alternative sources of paleontological data with traditional taxonomic approaches. Here we focus on three quite different aspects of the paleontological record. We take the traditional taxonomic approach and focus on the trilobite record during this period. To learn whether and how the dominance and/or abundance of particular shell-bearing creatures are changing during the same periods, we also examine the nature and distribution of shell beds, the taphonomic approach. In addition, we take an ichnological approach and document the history of those animals that lived in or on the sediment by analyzing preserved burrows, tracks and trails to see whether they, too, are undergoing any changes at about the same time as are the other groups.
The rocks revealed some startling facts. Although the radiation spanned tens of millions of years, we find major changes occur in a variety of groups in concert, over a geologically short time span during the overall radiation. This is surprising because each group originated at different times, but many of the forms that came to dominate the next 215 million years all appeared and were common at almost the same point in the Ordovician. Because so many different types of unrelated groups underwent a major turnover at the same time, we believe an extrinsic event affected all groups. We find evidence for large-scale ecological and environmental changes during this time, but rather than causing large-scale extinctions, we believe these changes to have resulted in the pattern of radiation we see in the Ordovician fossil record.
Record of Radiations in the Great Basin Ordovician strata outcrop throughout the Great Basin. The Great Basin has a thick succession of well-exposed Ordovician rocks that contain a rich fossil record and represent deposition in a variety of environments. In addition, the Ordovician strata of the Great Basin have a relatively good biostratigraphic framework so that rocks can be correlated across the Great Basin, as well as between continents. The two stages of the Lower and Middle Ordovician used in North America, the Ibexian and the Whiterockian, were both originally described from and have their standard stratigraphic reference sections in the Great Basin. Thus these strata provide an excellent laboratory to examine the Ordovician radiation. During the Ordovician, the land area that is now the United States spanned the equator and was part of a land mass called Laurentia. Although Ordovician paleogeography is currently the subject of some debate, the latitudinal position of Laurentia is relatively well established.
The western part of the current United States was covered with a shallow carbonate sea, rather like some tropical areas of today, such as the Bahamas. At times, sand deposition was locally prevalent. Throughout the Ordovician, muds and silts were commonly deposited with the shallow marine carbonates. In the deeper-water outer-shelf and basinal settings, muds accumulated.
There are several ways to study faunal radiations. In the first approach, paleobiologists like to determine the first appearance of new taxonomic groups. A new group, or taxon, may first appear as quantitatively insignificant members of the fauna. Yet the same taxon may grow to include many species and have a very long life. And what is “first appearance?” Most groups have sister taxa that are both older and more primitive, so that a purely taxonomic definition of faunal radiation becomes difficult and arbitrary. Fortunately there is one criterion that can clearly distinguish a major shift for taxonomic purposes: the sudden immigration of a clade over a broad area—such as the Ordovician Laurentia—when major evolutionary events have taken place elsewhere.
Biogeography, the geographic distribution of organisms, is therefore important in fully comprehending the changes that took place through the Ordovician. Evolutionary changes may accompany major changes in geography. If the ancient continent were suddenly flooded, animals that had formerly lived on the fringes might suddenly spread widely over the continent’s interior and encounter new opportunities for colonization as they invade. Conversely, the regression of the seas might cause a diminution in the area that shallow-living fauna can occupy. At the same time, offshore islands may actually expand as their shorelines extend. To understand such subtleties it is necessary to have a fine time control, preferably based on organisms that both evolved fast and were sensitive to environmental change, such as trilobites.
It is less difficult to study the second aspect—quantitative changes during a diversification. This includes the increase in number of species, genera or families of a particular clade. These types of changes have been documented effectively by T. John Sepkoski, Jr., of the University of Chicago. Quantitative measures can also include changes in dominance of one group over another—in essence, abundance or commonness. For this type of work, taphonomic studies are often very informative, where paleontologists analyze the composition of fossilized shell beds, as has been demonstrated by Susan Kidwell of the University of Chicago. The changes in dominance of one shelly component of a fauna as opposed to another can be recorded from bulk sampling from the field. Shell beds and the like are recruited from local populations of living animals and reflect broadly the population structure of living animal communities.
Paleobiologists can also take an ichnological approach and study patterns of sediment use and exploitation by animals. These patterns reflect ecological change in the absence of taxonomic data for the animals that produced the change. By examining all three kinds of change—taxonomic, taphonomic and ichnological—we hope to come close to examining the real dynamics of a fundamental faunal change.
Trilobites in the Great Basin
Trilobites are some of the most exciting fossils to be found in the Ordovician of the Great Basin. The remains of these extinct arthropods are scattered throughout the limestone and mudstone beds in Nevada, California and Utah. When alive, trilobites would have swarmed over the sea floor, or swum among the sponges in the shallow seas in search of food, much as crabs and shrimp do today. It is unusual today to find whole remains, as trilobite bodies tended to fall into separate pieces. In the fossil record, one finds isolated heads, or even parts of heads, which had a tendency to dissociate into cranidia and cheeks and tails (pygidia) before fossilizing. Furthermore, one finds a mix of head and body parts from several species.
The importance of these Ordovician fossils was recognized in the 1950s by Reuben J. Ross Jr., with the United States Geological Survey, and Lehi Hintze of Brigham Young University. These investigators subdivided the Ordovician rocks into distinct zones based on the changing stratigraphic record of trilobite and brachiopod species. Their zonal scheme has been used across the rest of the United States ever since.
Our studies of trilobite fossils have concentrated on one of these intervals in particular—the base, or the earliest part, of the Middle Ordovician, as defined by Ross and Hintze, which is found in the strata corresponding to the base of the Whiterock Series in North America. We discovered ten new species and one new genus from the lowermost Whiterockian in the Great Basin strata. These new species come from several families of Ordovician trilobites, including Bathyuridae, Pliomeridae, Raphiophoridae, Dimeropygidae, Cheiruridae and Telephinidae. This made us recognize that the base of the Middle of the Ordovician was a time of exceptional biological change. Many of the new trilobites belong to genera known from somewhat younger rocks in the Great Basin, or from western Newfoundland; a few are more familiar from older Ordovician formations. But the commonest genus, Psephosthenaspis, was originally described in 1963 by Harry B. Whittington of Cambridge University from a glacial boulder in Quebec, whose existence was known since the 1860s. Putting an age on the fossil itself proved difficult for Whittington since the boulder is isolated from the geological context so necessary for dating rocks and the fossils they contain. Beginning at the base of the Whiterock, three different species of Psephosthenaspis, all new ones, occur in a distinct and consistent order of appearance. This sequence not only allows a much refined biostratigraphy of the zone, but also solves Whittington’s mystery about the age of the boulder.
Many of the new species are highly distinctive. A spiny trilobite of the family Pliomeriae is named Pseudomera arachnopyge (Greek for “spider tail”). One of the few species known elsewhere is a form, Ischyrotoma stubblefieldi, originally discovered from central Scotland in the Dounans Limestone. Its discovery there provides a graphic illustration that in the Ordovician, Scotland was part of a North American, rather than European, continent. Most exciting of all is a completely new genus of trilobite with a smoothed-out, almost “bald” head, which we have named Madaraspis, or “bald shield.” This genus is representative of the family Bathyuridae.
The components of the trilobite radiation are twofold. Animals native to the Laurentian craton, which included, in addition to North America, northwest Ireland, Scotland, Greenland, the north slope of Alaska and the Chukotsk peninsula of northeast Russia, continued to diversify. At the same time, animals that had previously only been found on other continents started to show up there, too. Once the new transplants made their home on the shallow, tropical shelves, they too diversified and added to the richness of the native Laurentian faunas. Not all of the invaders arrived at the same time. Trinucleids, for example, which are of great use in dating European Ordovician successions, seem to have arrived later than calymenid trilobites. However, many of the outsiders seem to appear at the base of the Middle Ordovician.
The new trilobites from the shallow shelf outcrops in Utah and Nevada, are, for the most part, a new phase of the endemic North American radiation. The pliomerid trilobites, such as Pseudomera arachnopyge, are close relatives of species that lie stratigraphically below, in Ibexian strata. Ectenonotus is another Laurentian endemic, representative of an entire subfamily of pliomerids that were exclusively North American. Dimeropygids were probably derived from shallow-shelf hystricurids in the Ibexian. The family Bathyuridae, which, as long ago as 1953, Harry Whittington identified as the “fingerprint” family of the North America Ordovician craton, continued to diversify and produce new body plans as exemplified by Psephosthenaspis, Pseudoolnoides and Madaraspis. The bathyurids survived to almost the end of the Ordovician, always within the compass of the Ordovician tropics.
The family Proetidae appears at the base of the Middle Ordovician in Laurentia quite suddenly and yet without obvious antecedents either on North America or elsewhere. This particular group outlasted all other trilobites into the Carboniferous and Permian periods, finally disappearing 250 million years ago, so its appearance really marks the inception, in a sense, of the last trilobite faunas. The group radiates into several families even through the Ordovician.
The appearance of two other groups—the lichid and pterygometopid trilobites—happens widely at the Middle Ordovician of Laurentia. The earlier history of both these groups as revealed by cladistic analysis is written on the Ordovician rock of Gondwana, a supercontinent that included the modern land masses of South America, Africa, Madagasgar, India, Antarctica and Australia. Lichid trilobites are related to rare species from the Lower Ordovician of two areas, Bohemia and northern Germany, both of which lay toward the edge of the vast Gondwana continent in the Ordovician. Since this continent was also close to the South Pole at the time, the shelf waters surrounding it were presumably much cooler than they would have been in Laurentia, near the paleoequator. Pterygometopid trilobites are known from earlier strata in Morocco and South France, also Gondwanan localities. Calyminid trilobites follow suit. Although these trilobite clades have their origins earlier in the Ordovician or in the Cambrian, the first global spread of typical examples of these groups corresponds closely with the base of the Middle Ordovician.
Hence the spread of trilobite species over the great Laurentian area signals an important biogeographic shift—one that left a permanent impression on all subsequent faunas in North America. So does the appearance of these species in North America at the base of the Middle Ordovician signal a sudden climatic change? Or was the change stimulated by other changes in sea level, permitting arrival on the shelf of species that may have previously been confined to deeper, cooler off-shelf sites? The answer to these questions lies in a continued detailed examination of the rock record in the Great Basin and elsewhere.
Most important, these finds, along with documented occurrences of other trilobites, demonstrate that the radiation of endemic North American clades as well as the first North American appearance of other major clades of trilobites took place at or near the base of the Whiterock—that is, at the beginning of the Middle Ordovician. These are the trilobite clades that come to dominate the Paleozoic era for the next 250 million years. This pattern indicates a complex diversification with both a large-scale biogeographic diversification as well as a phylogenetic diversification of endemic faunas, all at or near the base of the Middle Ordovician.
A snorkeling expedition in a shallow marine setting, or even a walk on the beach, reveals piles of empty shells or layers of shells on top of the sediment surface. These shell beds may eventually become preserved in the stratigraphic record where they are found in rocks deposited in marine environments from the intertidal to the slope and deep basin. Paleobiologists can read this shell record and learn something about the history of marine life through time.
Shell beds (shell concentrations) can range from millimeters to meters in thickness, and can measure from centimeters to kilometers across. Shell concentrations result from different physical processes and can be organized into four broad types based on their stratigraphies and inferred histories of accumulation. These include event concentrations, composite or multiple-event concentrations, hiatal or condensed concentrations and lag concentrations. Although the conditions under which shell beds form differ, a comparison through a time interval of shell concentrations with similar origins and patterns of preservation is a useful proxy for recognizing the possible changing dominant community elements.
Shell concentrations are common in the shallow marine Ordovician strata of the Great Basin, where they are composed of brachiopod shells, trilobite remains, echinoderm debris, ostracod valves, gastropod shells and bryozoan skeletons as well as bivalve and cephalopod shells. The fossils are found with a sedimentary matrix consisting primarily of muds, intraclasts (pebbles of broken-up sea floor), silts and sands.
Different combinations of the skeletal grains and sedimentary matrix result in a variety of taxonomic types of Ordovician skeletal concentrations. Each taxonomic type of shell bed is distinct.
Cambrian shell beds are dominated by trilobites, but there is an increase of common echinoderm-dominated shell beds as one moves into Upper Cambrian strata. In Lower Ordovician shallow marine carbonate, strata, presenting the beginning of the Ordovician, this pattern continues. Although brachiopods, for example, are beginning to diversify, brachiopod shell beds are relatively uncommon, and shell beds remain dominated by trilobites and echinoderms. Indeed, ostracods, gastropods and bivalves all turn up in lower Ordovician strata, but only rarely do they form shell beds.
Then, at the base of the Middle Ordovician, there is a dramatic change in shell concentrations. The number of shell beds increases and their taxonomic composition changes and becomes more diverse. Even the physical dimensions of the shell beds expand. Although echinoderm-dominated shell beds remain common (about 25 percent of recorded shell beds), trilobite-dominated shell beds are rare and occur primarily as thin pavements or lenticular beds. Shell beds dominated by brachiopods become very common and include nearly monospecific beds over decimeters in thickness that extend over tens of kilometers. Ostracod shell beds also become common during this time. Polytaxic shell beds as well as concentrations of bivalves, bryozoans and gastropods are also present.
Shell beds containing mixed species are much more common in Middle than in Lower Ordovician strata. In addition, Middle Ordovician shell beds include forms representing different life habits. For example, many shell beds in the Kanosh Formation of the Confusion Range of western Utah are composed of sessile brachiopods, sessile bryozoans, vagrant trilobites and cephalopods, which are active swimmers.
The taxonomic shift of the Middle Ordovician is particularly intriguing, since it demonstrates that a taxonomic group might be diverse without being particularly abundant, as is the case with the trilobites during this period. Although the trilobites showed their greatest diversity of form in the Ordovician, thick trilobite beds are not present in Middle Ordovician strata and trilobites are relatively uncommon in other shell beds. Thus, although the Middle Ordovician is a pivotal point for the trilobites, generating clades that come to dominate the Paleozoic periods that followed, data from the shell beds suggest that they are not as common as they were in the Cambrian and Early Ordovician.
Some investigators have suggested that the trilobites did not decrease in number, but rather their constant numbers became diluted in the explosion of animal forms of the time. Our studies comparing Cambrian and Ordovician shell beds, however, suggest otherwise. We calculated the thickness, lateral extent and abundance of species from sites representing deposition under similar environmental conditions. Thus these changes likely reflect real changes in shell beds through time rather than environmental changes.
The Ichnological Record
There are two main sources for evidence of metazoan life. The diversity and abundance of shelly organisms is best known from body fossils such as those that form shell beds. The activity of both shelly and soft-bodied organisms in sediment can also be understood by studying the burrows, tracks and trails that they create. When preserved in the rock record, these structures are called trace fossils, and their study is called ichnology. Trace fossils represent the activities of once-living organisms, rather than the organisms themselves. Because up to 80 percent of all animals are soft-bodied—and preservation of these creatures is rare—evidence of behavior is particularly important for understanding the history of soft-bodied marine life.
Burrowing animals mix and modify the sediment as well as create new structures as they move through in a process called bioturbation. The total record of sedimentary rock fabric resulting from bioturbation is known as ichnofabric. Ichnofabric includes individual trace fossils as well as mottled bedding produced by bioturbation, where discrete trace fossils are not readily identifiable. By studying both ichnofabric and trace fossils, paleobiologists can gain an understanding of the faunal community that cannot be gleaned from the record of body fossils alone.
The ichnological record reveals a differential environmental history in the development of the infaunal habitat—the habitat below the interface between the water and the sediment. This is to be expected, as different environments support different infaunal communities.
In Ordovician near-shore sandstones where trilobites and other body fossils are not well preserved, trace fossils provide the best evidence of past life. Although these near-shore sandstones in the Great Basin are only of Middle Ordovician age, we see here the type of ichnofabric that is characteristic of Paleozoic (540-250 million years ago) near-shore sandstones all over the world. The ichnofabric most commonly results from Skolithos, a simple vertical burrow, and Diplocraterion, a vertical U-shaped burrow. These two trace fossils dominate the near-shore sandstones and produce a type of ichnofabric known as piperock. Additionally, other vertical trace fossils may appear commonly in a cross-cutting relationship with the deeper Skolithos and Diplocraterion. Data collected by our group and others from similar Cambro-Ordovician strata in Australia suggest that by the earliest Ordovician, a complex infaunal community had already developed. In this setting, then, the characteristic ichno-fabric of the Paleozoic is already established by the earliest Ordovician and shows no significant changes throughout the Ordovician.
The Ordovician ichnofabric in strata representing deposition in a carbonate system reveals a different history of development. Ordovician carbonate ichnofabric is relatively simple. The most common trace fossil is Thalassinoides, a branching burrow that has two main shapes: a maze-like structure that is preserved primarily on a single layer and a three-dimensional boxwork structure. In modem environments, a Thalassinoides-type burrow is made by callianassid shrimp. In the Great Basin, mazelike Thalassinoides dominate the Lower and Middle Ordovician. The depth of bioturbation of these trace fossils averages a few centimeters. However, the boxwork structure of Thalassinoides dominates Upper Ordovician strata. The depth of bioturbation recorded from these strata ranges from 30 centimeters up to one meter. Thus there is a significant change in the style and depth of bioturbation between the Middle and Upper Ordovician.
What are we to make of these changes? Bioturbation disrupts the original physical sedimentary structures, so changes in the amount of bioturbation, when examined in strata deposited under similar physical conditions, may reflect changes in the amount of mixing or even the extent to which the infaunal ecospace was being used. Analysis of extent of bioturbation throughout the Ordovician reveals a significant increase from the Middle to the Late Ordovician. However, there are no significant changes from the Lower to Middle Ordovician strata. The increase in bioturbation during the Upper Ordovician is thought to have been due to a soft-bodied animal. Interestingly, this increase lags behind the major “burst” of the Ordovician radiation as well as significant changes in shell concentrations. The increase corresponds with the appearance of boxwork Thalassinoides and thus may be due almost entirely to this one type of burrow structure.
A Time of Change
A number of patterns emerge from this work. Most significant is the importance of the transition from the Early to Middle Ordovician. Although it was previously recognized as a significant biostratigraphic boundary, it has become clear that the boundary is not simply a turnover of species. It is a pivotal point in the Ordovician radiation for trilobites and in the appearance of new kinds of organisms in the record. In addition, the base of the Middle Ordovician marks a major change in shell beds, which are not tied to any specific taxonomic groups and represent a completely different type of paleontological data. This also proves to be an important point in the development of bivalves, graptolites, cephalopods and brachiopods. Work in the Great Basin demonstrates a significant mid-Ordovician turnover in graptolites, a group of extinct pelagic hemichordates, a group with some resemblance, but no biological relationship, to salps.
Thomas Guensberg at Rock Valley College in Rockford Illinois and James Sprinkle of the University of Texas at Austin have recently examined the early history and paleoecology of echinoderms, in particular crinoids (popularly known as sea lilies), in the Great Basin. They have demonstrated a two-phase diversification. Echinoderms appear to have an earlier history of expansion on hardgrounds that continues throughout the Middle Ordovician as they adapt to soft substrata. Corals, on the other hand, have a later history, dating to the later Middle and Late Ordovician. The later increase in bioturbation may be due to the appearance of a single group of deep burrowers who appeared later in the Ordovician.
The data suggest that, although the radiation spanned millions of years, most groups display at least one pivotal point. For many groups, but not all, this point is at or near the base of the Middle Ordovician. This short interval also records a major biogeographic shift. Rigorously detailed collection of other fossil groups will test whether these groups merely appear to act in concert with the trilobites and shell beds because of the coarse time scale used, or whether in fact a bona fide biological event took place at this time. If an actual biological event took place, it suggests that extrinsic factors may have been important in structuring or restructuring marine life during, at least, this radiation. Extrinsic events have, of course, long been invoked for extinction events, and although they have been suggested for the Cambrian radiation, they have not been thoroughly documented for major radiations.
A major drop in sea level has been well documented for the period around the base of the Middle Ordovician. In the Great Basin, this sea-level drop is recognized by shoaling in many stratigraphic sections. Sea-level changes have previously been linked to many types and scales of faunal change. In these studies, however, the scale of faunal change is local and commonly involves species migration from other environmental settings.
The Ordovician radiation also coincided with a period when mountains were actively being forged throughout the world. Arnie Miller and Shuguang Mao of the University of Cincinnati have demonstrated that genera were far more diverse in areas close to formation of new mountains than they were in areas more distant. This suggests that an enriched mosaic of habitats, a possible increase in nutrients and changing substrates associated with mountain-building might have played a part in the evolutionary story.
In the transition from Lower to Middle Ordovician, the actual relationship between extrinsic factors and the innovation in life forms, which appears to be global, must be further refined. Once the timing is firmed up, explanations for a global biological diversification and geographic expansion during, or just after, a major drop in sea level remain to be determined. Future work will concentrate on the possible links between global environmental changes and this important time in life’s history