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The History of Life
Communities through Time
INTRODUCTION AND OBJECTIVES
The science of paleontology, as was stressed in previous labs, contributes much more than a simple description and classification of fossils. Among other things, we can not only gain insight into what ancient organisms looked like, but also what they did during their lifetimes: where they lived, what they ate, how they moved, and what other animals they potentially interacted with. We can also "watch" as different groups of organisms replace each other in particular environments through time. A Paleozoic lagoon, for example, was inhabited by very different animals than a Mesozoic one, which in turn is very different from one of today. This lab exercise takes the organisms you learned about in the first lab and places them in a temporal and ecological context. We will be tracking three different environments through time: reef, marine soft-bottom, and terrestrial, and we will be looking at how the taxonomic makeup of organisms living in these environments has changed during the course of the Phanerozoic Eon. You should come away with a set of mental images of the characteristic organisms that lived together at different periods in Earth history. Once again, please read this handout before coming to section. Also, bring your copy of Lab 1 with you , for you will be looking at many of the same organisms again, and you will need to be able to recognize them in a different context. A copy of the geologic time scale (on the cover of your sourcebook) will also be helpful.
PALEOECOLOGY
"Behind the history of every sedimentary rock there lurks an ecosystem..." (Edward Deevey)
In last week’s lab, you learned how to determine the depositional environment in which a sedimentary rock formed. Once the environment of deposition is determined, paleontologists can then look at the fossils preserved in the sedimentary rock and try to determine how those organisms fit into their physical surroundings. The modern science of ecology examines the relationships of organisms with their environment and with other organisms in the same community - a group of organisms that live together in the same environment. The field of paleoecology applies modern ecological principles to the fossil record in an attempt to reconstruct ancient communities. Because it is far more difficult (though not impossible) to address questions of organism interactions in the fossil record, paleoecology can be more realistically defined as simply the study of the relationships between ancient organisms and their environments. The raw data of paleoecological research are the fossils preserved together within individual layers of rock. One can never be quite sure, however, that these organisms actually comprise a true community. There are many other reasons that could account for associations observed. The transportation of mollusk shells, for example, from many different areas onto a beach where they accumulate together results in a false association of species. This particular source of error is relatively easy to identify with a bit of experience, but others are not so
obvious. For this reason, paleocommunities are more conservatively defined as recurring groups of fossil organisms that may or may not correspond to real communities. An even more encompassing term is the assemblage , which only implies a collection of organisms that lived in roughly the same environment in roughly the same time frame. This is probably the more accurate word to use when describing the majority of ancient "communities". The most common questions asked by paleoecologists revolve around the mode of life of ancient organisms. Where did they live? What did they do? How and what did they eat? These questions are addressed using a combination of methods described in Lab 1 (e.g., comparison with modern relatives or functional analogs). There are several different ways to describe and group organisms according to their modes of life, and these methods are usually done simultaneously. The first and most fundamental is the distinction between mobile and immobile. (Another is the distinction between solitary and colonial organisms, life modes that were discussed in the first lab.) Mobile animals are those that are able to move around - swim, crawl, burrow, etc. Immobile organisms, called sessile , are restricted to living in the same spot for their lifetimes either because they are physically rooted or cemented, or because they have no means of locomotion. Oysters, for example, are sessile animals because they are permanently cemented to a substrate, while snails are mobile because they crawl or glide along a surface. One can also address the question of where the organism lives with respect to the substrate - the sediment accumulated at the bottom of the body of water. Organisms may either live in the water column or on the bottom, and those that live on the bottom can live on the surface of the sediment or buried within it. Among benthic organisms (those that live on the bottom), we can further distinguish between those that prefer hard substrates (barnacles grow on rocks) and those that prefer soft substrates (some clams burrow in the sand). Another useful means of characterizing organisms relies on the feeding strategies they employ. Animals that eat other animals are called carnivores or predators , while those that eat plant matter are herbivores or grazers. Many animals do not fit into either of these categories, relying instead on bits of dead organic matter ( detritus ) for their main food source. Animals that eat detritus mixed in with the sediment are called deposit feeders. Conversely, suspension feeders filter suspended particles of detritus out of the water column and ingest them. There are other categories that can be defined also, depending on what environment you are looking at. A final concept that has proven useful in paleoecologic analyses is that of the guild. A guild is a group of organisms that are in competition for the same resource. This resource is usually food, but could also be space or light, particularly for plants. A familiar example of a guild system based on feeding strategies for the African savannah could be as follows: photosynthesizers (which produce their own food) include a variety of grasses and shrubs, herbivores are the antelopes, zebras, and giraffes, predators are the leopards, lions, and hyenas, and scavengers are vultures, hyenas, and lions. Membership in a particular guild is independent of taxonomy (e.g., zebras and giraffes are not closely related, but both eat the same food), and a given species may be included in more than one guild at the same time (e.g., the hyena is both a carnivore and a scavenger). It should become apparent throughout this lab that the concept of guilds can be rather plastic and subjective, yet affords a very useful approach to comparing communities through time.
It appears from this material that all the traditional feeding guilds of the later Phanerozoic were occupied already, even the predators, as evidenced by the 2 foot long Anomalocaris. Later Paleozoic soft bottom communities were characterized by a great diversity of brachiopods, trilobites, crinoids, solitary rugose corals, bryozans, and infrequent bivalves and gastropods. Benthic animals lived predominantly on the surface of the sediment as opposed to buried within it. Evidence suggests that burrowers were not particularly common, and those that did exist were restricted to shallow depths. Predators were mostly active swimmers and included the giant eurypterids during the Silurian, uncoiled nautiloid cephalopods during the Early and Middle Paleozoic, coiled nautiloids and ammonoids later on, and fish. Fossils of these animals are found preserved in sediments with bottom dwellers, and so paleontologists need to be able to recognize them as transitory members of bottom communities.
Mesozoic The close of the Paleozoic brought about a dramatic change in soft-bottom dwellers. Most notably, the trilobites became and the once numerous brachiopods dwindled significantly. Mesozoic assemblages are characterized by a growing diversity of molluscs and crustaceans, with brachiopods, crinoids, and bryozoans still present but subordinate. Some Mesozoic bivalves grew asymmetrically (like the rudists) so that one valve became large, heavy, and cup-like and the other became a flat lid. Some animals (e.g., the oyster Exogyra ) lay free in the sediment with the lid on top, filtering food particles from the water. Such a shape was successful unless the clam was overturned, for they had no means of righting themselves. This way of life was common until the Cretaceous, when "biological bulldozers" like burrowing bivalves, worms, and crustaceans became more active and abundant. These animals churned the sediment more completely and to greater depths so that large, free-lying organisms were no longer stable in the soft sediment. Because of constant overturning by bioturbators, this life mode is no longer represented today. Classic Mesozoic fossils are the ammonites, found almost ubiquitously in a wide range of marine depositional environments. Again, many of these are swimming, open- ocean animals that are only preserved in soft-bottom assemblages when they die and fall to the sea floor.
Cenozoic The rise of molluscs and crustaceans that began in the Mesozoic continued to accelerate in the Cenozoic. Predatory forms became more common, and the diversification of fast, deep burrowing bivalves and gastropods with thick shells and spines is presumed to be in response to the rise in predators. Fish are also important components of Cenozoic soft-bottom ecosystems. Sponges, bryozoans, and assorted worms comprise the rest of the bottom dwellers. Flowering plants in the form of seagrasses entered the marine realm late in the Cretaceous and spread during the early Cenozoic. Their appearance created a new habitat for marine animals, making them important members of shallow-water, soft-bottom communities
REEF ASSEMBLAGES THROUGH TIME Although most people are aware that tropical areas today are characterized by coral reefs, few realize that there have been reefs of some kind present on the earth for nearly all of the history of life. Even before the origination of large animals, bacterial reefs formed in shallow
tropical seas. The term reef is here defined as a marine "framework" community with topographic relief (i.e., a large structure that stands above the ocean floor). They are constructed by the rapid growth of closely packed, calcareous organisms, and thus the resulting rock type is biogenic (biologically produced) limestone. The physical structure of a modern reef itself is nothing more than layer upon layer of abandoned coral skeletons, thus the only living part of the reef is the thin skin of coral animals on the outer surface. Most substantial reef buildups are found in relatively shallow, warm water areas removed from the influence of large rivers, which deliver clay that dilutes the carbonate and clogs the organisms. Because this rock is precipitated in place by the upward growth of coral or other animals, it is not bedded. Instead, the structural details of the reef animals themselves are preserved in a massive framework, and the outline of the original reef can often be seen in the surrounding rock. Al Fagerstrom (1987), in an effort to better understand reef communities, identified five different guilds that could be traced and compared through time. In his scheme, the resource for which organisms are competing is space. His groupings are as follows, with examples given from modern reef communities:
- Constructors - These are the organisms that build the actual framework structure of the reef. When you visit tropical reefs today, the big coral heads you see first are the constructors: massively calcifying, scleractinian corals.
- Bafflers - The chief role of the bafflers is to block or slow the flow of water over the reef and cause the deposition and stabilization of loose grains. These animals usually wave back and forth in the current on stalks attached to the constructors below. Because of their delicate, fan-like nature, bafflers are often not preserved as intact, whole organism fossils, but rather as broken pieces in the surrounding sediment. Common examples are the sea fans, colonial animals in the same class as corals.
- Binders - This is the glue that holds it all together. These organisms are encrusters that often exhibit rapid lateral growth as they cover surfaces and fill the interstices of the reef. Members include sponges (which may also be members of the constructor guild) and calcifying algae.
- Dwellers - Animals that live on or in the reef simply because they prefer a hard substrate are the dwellers. This is a diverse assemblage of solitary, mobile or sessile organisms (e.g., sea anemones, gastropods, crustaceans, fish, echinoderms, brachiopods, etc.).
- Destroyers - These are animals that bore into, bite off, or otherwise destroy the structure of the reef. These include solitary, mobile animals like parrotfish, sea urchins, and starfish that prey on the coral polyps, and also again include some types of sponges that bore into the reef. The end result of the actions of destroyers is copious fine carbonate mud, which fills in the crevices of the reef or is transported to nearby environments.
As you read about and examine the reef material from different time periods, try to imagine which of the guilds various animals belong to. Some will be much more obvious than others
meter and a half! Other rudists grew both their valves into fantastic, coiled shapes resembling a set of buffalo horns! Rudists are very poorly understood creatures. In fact, it is still an area of active debate among rudist workers as to whether or not rudist assemblages should actually be considered reefs. Rudist assemblages are atypical of most other reefs in part because they exhibit a paucity of dwelling and binding organisms. It is possible that the absence of the ammonites, brachiopods, and echinoids indicates a more fluctuating physical environment (temperature, salinity, and oxygen) than these dwellers could tolerate. Other researchers suggest that flourishing growth of the rudists choked out other organisms that could potentially have lived with them. Scleractinian corals, not yet large or diverse, filled the scant baffler guild. Evidence for destroyers is present in rudist reefs mostly in the form of boring sponges (and that doesn’t mean dull and uninteresting!), echinoids, and boring "lithophagid" bivalves.
Cenozoic A wealth of information is available about the more modern, scleractinian coral reefs because they can be easily observed today. Cenozoic reefs have reached the zenith of calcification, growing at fantastic rates. The photosynthetic action of algal symbionts in the coral tissue is responsible for this ability to deposit massive amounts of carbonate. A major question in paleobiology is whether or not the main constructors of fossil reefs also contained such symbionts. The best potential so far seems to lie with the rudists, for their massive buildups are difficult to account for with normal bivalve growth rates. Guild membership in modern reefs was touched upon in the discussion of Fagerstrom’s paper, so we will not repeat it here. We have provided samples from both modern and fossil reefs for you to compare. The fossil material is from the 125,000-year-old Key Largo Limestone, which makes up much of the Florida Keys. See if you can identify some of the modern corals in the Key Largo limestone. Are there any other organisms present besides the coral? What are the spaces between corals filled with? How do these samples differ from older ones? A spectacular exposure of this unit occurs on Big Pine Key, where huge coral heads are clearly visible in a quarry wall. Some heads had evidently been tipped over during large storms, and some show evidence of regrowth in the new upward direction. Smaller corals, binding calcareous algae, numerous dwellers, and abundant destroyers are also present in the rock.
TERRESTRIAL ASSEMBLAGES THROUGH TIME
Terrestrial assemblages are more difficult to discuss in a coherent ecological framework because there are so many different subenvironments within the terrestrial realm. In lieu of this, we will instead summarize some of the major innovations acquired during the evolution of land-based organisms, both plant and animal. You should also keep in mind that the fossil record of terrestrial organisms is far less complete than that for marine (remember the marine bias discussed in Lab 2); therefore, evolutionary relationships among animals are often sketchy and unclear.
Paleozoic Prior to the appearance of the first land plants in the Silurian, the terrestrial world was not completely barren, but inhabited by a diverse suite of microbial organisms. Some were
photosynthetic, others heterotrophic (eating other microbes), and all contributed to the development of very ancient soils preserved in the rock record. The first steps toward a terrestrial existence by multicellular life were taken by the green algae and several groups of specialized fish and arthropods. The subsequent story of terrestrial life during much of the Paleozoic is one of invasion, diversification, and evolution, rather than extinction, with some organisms becoming progressively better adapted to life out of the water. The first land plants were small, simple, diminutive plants that colonized the muddy fringes of coastal swamps. The main innovation that led to their further diversification and success was the appearance of a vascular system -- a network of minute vessels that carry water, nutrients, and the products of photosynthesis throughout the plant. This system allowed plants to become more complex during the Devonian by evolving specialized parts for different functions: leaves for photosynthesis, stems for structure and rigidity, and roots for the absorption of water and nutrients. Though the earliest land plants were quite simple, they changed the surface of the earth considerably by accelerating the formation of soils and anchoring those soil particles in place. The colonization of land by plants was probably the key to subsequent colonization by animals. Some of the earliest land animals were arthropods that lived within the shelter of plants. The majority of these arthropods were probably carnivorous on other invertebrates, but others may have fed on the plant remains, algae, and decomposing fungi. In any case, we know that the first insects lived in close association with swamp plants since their remains are often found fossilized together. Though soft-bodied invertebrates are missing from the fossil record, it is likely that they too were present in late Silurian/early Devonian swamps. By the end of the Devonian, plants had evolved leaves, roots, and wood. Many of the plant groups whose modern representatives are small and insignificant grew to giant proportions during this time. Trees like Lepidodendron - related to the tiny ground pines of today, Calmites - related to modern horsetails, and tree ferns flourished. (Look for the specimens provided of parts of these common fossil plants). The first seed plants also appeared during the Devonian. The seed is important for plants because it (1) provides a protective capsule for the plant embryo and (2) houses a rich food supply to nourish the young plant as it grows its first roots and leaves. These early seed plants gave rise to lineages important in the Paleozoic (seed ferns), Mesozoic (conifers and cycads), and today (flowering plants). Other than ferns, which produce spores, most of the plants you are familiar with are seed plants. Meanwhile, the arthropods continued to diversify, both taxonomically (137 genera collected at 1 site!) and ecologically. By the end of the Devonian, insects had evolved wings, which facilitated dispersal, feeding, radiation into new habitats, and led to some jumbo-jets of the arthropod world. Carboniferous dragonflies reached lengths of 60 cm! Other large arthropods such as spiders and centipedes were important predators, evolving new hunting tactics like webs, while still others specialized to feed on leaves, sap, roots, or stems. Other than the arthropods, the only other invertebrate groups to successfully invade the terrestrial realm were assorted "worms" and the gastropod molluscs. Even today, out of more than 30 invertebrate phyla, these are the only groups represented on land. The transition between the Devonian and the Carboniferous also saw the earliest terrestrial vertebrates: the lobe-finned fishes (e.g., lungfish). These were soon followed by early amphibians like Ichthyostega. Although amphibians live a more or less terrestrial life, they still need to return
radiation coincided with an explosion in insect diversity: the bees, wasps, ants, butterflies, and moths (important pollinators) also first appear in the Cretaceous. The subsequent diversification of insects and angiosperms is thought to have occurred by co-evolution: the highly specific, synergistic relationships that evolved between particular species of plants and insects. Eventually, the angiosperms displaced conifers as the dominant trees in most low-latitude forests, which is a position they retain today.
Cenozoic The extinction events that marked the end of the Mesozoic and the Age of Dinosaurs was not felt markedly by terrestrial plant communities. Angiosperms (and insects) continued to radiate, with the loss of only a few groups at the boundary. Important new additions were the grasses, well adapted to thrive in the cool, dry interiors of continents. The expansive grasslands present today are thus a relatively recent phenomenon, but they have played pivotal roles in the evolution of many large mammals, including us. The Cretaceous/Tertiary boundary was a crucial turning point for terrestrial vertebrates. The giant impact event and ensuing devastation that defines the boundary (see Alvarez et al., 1980, in your sourcebook) caused the demise of the dinosaurs and permitted the radiation of mammals, which until this time had been hiding in the nooks and crannies of the dinosaurs’ world. (As you ponder the contingent nature of life’s history, consider what the world might be like today had that asteroid not fallen and prematurely snuffed out the reign of the dinosaurs.) The entire range of Cenozoic mammals evolved from small, shrew-like ancestors, producing forms that filled roles previously occupied by dinosaurs. The Eocene and Pliocene were times of particularly large size for mammals, with huge relatives of elephants, cows, rhinos, deer, armadillos, and sloths that were hunted by giant cats, dogs, and bears. Though the first primates originated in the Cretaceous before the major diversification of mammals, a side branch of the primate lineage adopted an upright posture during the Pliocene and moved from the forest into the grasslands. Descendants of some of these primates would persist through the Pleistocene Ice Age and go on to affect the natural world in a more significant way than any other animal.
