Info

111 Lingula

111 Eocoelia

110 Stricklandia

110 Pentamerus

0 0 0 Clorinda e Le

Figure 4.17 In a two-way cluster analysis, an R-mode clusters the genera (bottom) and a Q-mode clusters the community type (right). The original data matrix is in the center of the diagram. The data indicate the reality of a shallow-water biofacies (Lingula and Eocoelia communities), and mid to deep shelf (Pentamerus and Stricklandia communities) and outer shelf to slope (Clorinda community) assemblages.

building up to a climax community in equilibrium with its environment. There is still some discussion among ecologists about whether communities conform to Eltonian models of change (predictable over long periods of time), Gleasonian models (short-term, rapid change and instability) or perhaps even both. Evidence from Quaternary, mainly Holocene, communities suggests them to be rather ephemeral (Davis et al. 2005). Species may evolve, become extinct or migrate out of the immediate area during intervals of climate change thus destroying the community structure. They may, however, return and recombine to form the original communities during intervals of more favorable climate (Bennett 1997). Nevertheless, paleocommunities dominated by incumbent taxa such as the dinosaurs during the Jurassic and Cretaceous or pentameride brachiopods during the Silurian

Box 4.3 Ecological interactions

Animals and plants have participated in a wide range of relationships throughout geological time. Ecologists have classified these arrangements in terms of gain (+), loss (-) and neutrality (0). Antagonistic arrangements include antibiosis (-,0), exploitation (0,+) and competition (0,0) whereas symbiosis involves both commensalism (+,0) and mutualism (+,+).

Antibiosis is difficult to demonstrate although mass mortalities of fishes have been ascribed to dinoflagellate blooms. Some paleontologists believe that the twisted skeleton of a Late Cretaceous Struthiomimus from Alberta may show the animal died from strychnine poisoning.

Exploitation includes predation and parasitism. There are many records of bite marks, particularly by marine reptiles on mollusk shells, while the stomach contents of Jurassic ichthyosaurs have revealed a diet of belemnites. Moreover a wide variety of nibble marks have been reported from fossil leaves. The relationship between the Devonian tabulate coral Pleurodictyum and the worm Hicetes fooled many paleontologists. Was this a bizarre compound organism? In fact the worm was probably a parasite; the association is common throughout Europe and virtually every specimen of Pleurodictyum has a parasitic worm at its core.

Competition is often difficult to observe directly in the fossil record. Encrusting bryozoans, however, commonly compete for space and food resources on the seabed. Competition between the cyclostome and cheilostome clades (see Chapter 12) may have influenced the post-Paleozoic history of the phylum in favor of the latter. Encrusting bryozoans can also faithfully replicate their substrate, recording the imprint of a soft-bodied animal or aragonitic mollusk. This process of bioimmuration ("biological burial") is a useful means of preserving an organism that otherwise may have escaped detection.

Commensalism is one of the most common relationships apparent in the fossil record, where small epifauna or epibionts use larger organisms for attachment and support. Small and immature pro-ductoid brachiopods are often attached by clasping spines to crinoid stems while microconchids, previously thought to be Spirobis worms (see Chapter 12), are commonly attached near the exhalent currents of Carboniferous non-marine bivalves. Some of the most spectacular examples have been reported from the shells of Devonian spiriferide brachiopods. Derek Ager (University of Wales, Swansea) reported a succession of epifauna, commencing with Spirobis (microconchids) followed by Hederella and Paleschara and finally the tabulate coral Aulopora, clustered near the inhalent current of the brachiopod (Fig. 4.18).

Figure 4.18 Commensalism between (a) the gastropod Platyceras and a Devonian crinoid and (b) Spinocyrtia iowensis with an epifauna primarily located on the fold of the brachial valve adjacent to inhalant or exhalent currents. (Based on Ager 1963.)

can exist for tens or even hundreds of millions of years (Sheehan 2001).

Despite the fantastic potential to test models for community change through time there have been relatively few rigorous attempts. Some paleontologists have recognized a pattern of coordinated turnover followed by stasis in which many species disappear over a short time interval and are replaced by other functionally and taxonomically similar species. The new assemblages may retain their structure for 2-8 million years (Brett et al. 1996), although in some cases this appears to occur through repeated reassembly following disturbances. By contrast Ordovician marine assemblages from the Appalachians (see p. 38) show a strong relationship between environmental fluctuations and uncoordinated changes in the composition and dominance of animals in their assemblages. Even life at a small scale shows these patterns. An Ordovi-cian hardground paleocommunity constructed by encrusters, mainly bryozoans and edrioast-eroids, on cobbles shows first a low-diversity pioneer community, then a high-diversity association, and finally a monospecific assemblage characterized by a late successional dominant (Wilson 1985). Environmental disturbances, such as the tipping over of the cobbles, allowed a recolonization of the hardground, thus maintaining high diversity within the paleocommunity. Clearly in some cases an Eltonian model may be applicable and in others Gleasonian paradigms rule.

Evolutionary paleoecology_

Biodiversity trends through time for the majority of animal and plant groups have been documented in some accuracy and detail since the late 1970s (see p. 534). However, it is clear that there are a series of ecological changes underpinning this incredible taxo-nomic diversification and such changes were probably decoupled from each other. For example, there have been marked changes in the use of ecospace through the evolution of new adaptive strategies (Bambachian mega-guilds), an escalation in the number of guilds and accelerated tiering both above and below

Box 4.4 Chemosynthetic environments

Amazing new discoveries in the modern oceans have revealed some of the most bizarre living creatures that survive in the dark, cold depths, clinging onto the life support provided by hydrocarbon seeps and hydrothermal vents. Uniquely, these bizarre organisms never see the light, and their food chains are not based on sunlight and carbon, but on sulfur from hydrothermal vents. The search is on to find their fossil counterparts. Kathleen Campbell, together with a range of colleagues, has been exploring the distribution of these types of weird communities through time (Campbell 2006). She has identified 40 fossil examples, recognized on the basis of key types of faunas, specific biomarkers and their geochemical and tectonic settings. Such communities associated with Precambrian vents were populated by microbes and it was not really until the Silurian that we find our first groups of metazoan chemosynthetic organisms. These organisms are strange. Gigantic non-articulated brachio-pods are associated with large bivalves and worm tubes in a massive volcanic sulfide in the Ural Mountains (Fig. 4.19). In general terms, pre-Jurassic vent faunas were dominated by extinct groups of brachiopods, monoplacophorans, bivalves and gastropods, and post-Jurassic faunas were populated by extant families of bivalves and gastropods. The modern vent-seep fauna is endemic and may either have evolved from a collection of Paleozoic and Mesozoic relics or perhaps some invertebrate groups periodically migrated into the gloom during the Phanerozoic to set up their own communities. Although unusual, the chemosynthetic world was yet another ecosystem with its own set of rules and evolutionary paradigms, contributing to the past and present biodiversity of our planet. Perhaps, also, during times of major and rapid environmental change, this ecosystem provided a stable refugia where at least some organisms could escape fluctuations in those other ecosystems that rely on light and organic nutrients.

Francisciconcha

Figure 4.19 Selection of fossils from ancient hydrothermal vent sites. All specimens are pyritized and are contained within a matrix of sulfide minerals. (a) Gastropod: Francisciconcha maslennikovi from the Lower Jurassic Figueroa sulfide deposit, California. (b) Small worm tubes from the Upper Cretaceous Memi sulfide deposit, Cyprus. (c) Bivalve: Sibaya ivanovi from the Middle Devonian Sibay sulfide deposit, Russia. (d, e) From the Lower Silurian Yaman Kasy sulfide deposit, Russia: (d) monoplacophoran, Themoconus shadlunae and (e) vestimentiferan worm tube, Yamankasia rifeia. Scale bars: 5 mm (a, b), 20 mm (c-e). (Courtesy of Crispin Little.)

Figure 4.19 Selection of fossils from ancient hydrothermal vent sites. All specimens are pyritized and are contained within a matrix of sulfide minerals. (a) Gastropod: Francisciconcha maslennikovi from the Lower Jurassic Figueroa sulfide deposit, California. (b) Small worm tubes from the Upper Cretaceous Memi sulfide deposit, Cyprus. (c) Bivalve: Sibaya ivanovi from the Middle Devonian Sibay sulfide deposit, Russia. (d, e) From the Lower Silurian Yaman Kasy sulfide deposit, Russia: (d) monoplacophoran, Themoconus shadlunae and (e) vestimentiferan worm tube, Yamankasia rifeia. Scale bars: 5 mm (a, b), 20 mm (c-e). (Courtesy of Crispin Little.)

the substrate. Through time animals and plants have developed innovative ways to exist in new habitats. The exciting, relatively new field of evolutionary paleoecology seeks to tackle some large-scale ecological patterns and trends through geological time. Why, for example, were Cambrian food chains so short, with few guilds, and why were tiering levels both above and below the sediment so restricted? In marine environments,

Sepkoski's (1981) robust division of Phanero-zoic life into his Cambrian (trilobites, non-articulated brachiopods, primitive echi-noderms and mollusks), Paleozoic (suspension feeders such as the articulated brachiopods, bryozoans, corals and crinoids) and the Modern faunas (detritus feeders such as the echinoids, gastropods and bivalves together with crustaceans, bony fishes and sharks) has acted as template for much paleoecological research (see p. 538).

Communities and habitats through time

Most paleoecological studies attempt to recreate the dynamism and reality of past communities from environments ranging from mountain lakes (see p. 90) to the strange chemosynthetic environments of the deep sea (Box 4.4). Despite the significant loss of information through taphonomic processes, realistic reconstructions are possible, depicting the main components, their relationships to each other and the surrounding environment. During most of geological time, microbial organisms were the sole inhabitants of Earth. The Ediacara biota appeared at the base of the Ediacaran System, some 630 Ma, but most members had disappeared by the start of the Cambrian. McKer-row (1978) was first to summarize, in broad terms, the development of communities throughout the Phanerozoic (see also Chapter 20). These tableaux were necessarily qualitative but now there are a growing number of more quantitative approaches providing more accurate reconstructions of community change through time. One of the first seascape reconstructions, Sir Henry de la Beche's watercolor of Duria antiquior (1830), depicted life in an early Jurassic sea. It was an iconic painting but nevertheless scientific, illustrating, graphically, the relationships between predators and prey in the Modern evolutionary fauna. Today we know much more about the range of environments that existed during the Jurassic Period.

Jurassic Park and deep-sea worlds

Jurassic environments provide a wide range of communities and habitats showing the early stages of development of post-Paleozoic faunas. A selection demonstrating environments, life modes and trophic strategies are illustrated (Fig. 4.20). Such tableaux have been criticized for their lack of science. They are, however, based on real case histories and numerical data are now available for many of these reconstructions. For example, two spectacular deposits, the Newark Supergroup and the Posidonia Shales, provide important windows on life in continental and marine environments, respectively, during the early part of the Jurassic.

Major new finds in the Newark Supergroup and equivalent strata in eastern North America, have painted a vivid picture of life on Late Triassic and Early Jurassic arid to humid landscapes of Laurentia swept by occasional monsoons. Olsen and his colleagues (1978, 1987) have described diverse dinosaur communities of both large and small carnivorous thero-pods, at the top of the food chain, together with large herbivorous sauropods and some early armored forms. Most of the terrestrial tetrapods are preserved in volcaniclastic deposits, but adjacent fluviatile facies contain crocodiles. Lake facies have preserved diverse floras of conifers, cycads, ferns and lycopods. Fast-swimming holostean fishes patrolled the lakes and abundant insects of modern aspect, representing seven orders, populated the forests and shores or may have swum in the shallows together with crustaceans.

The Posidonia Shales crop out near the village of Holzmaden in the Swabian Alps, Germany. The shales are bituminous or tarlike, packed with fossils, generally with echi-noderms and vertebrates towards the base and cephalopods at the top. Seilacher (1985) and his colleagues showed how this rock unit with exceptionally preserved fossils, or Lagerst├Ątte, was a stagnation deposit (see p. 60) where fossils accumulated in almost completely anoxic seabed conditions, and so were hardly damaged by decomposers. Benthos is rare, and encrusting and recumbent brachio-pods, bivalves, crinoids and serpulids that could not live on the stagnant seabed attached themselves to driftwood, ammonite shells and other floating or swimming organisms to pursue a so-called psedoplanktonic life mode. The dominant animals were nektonic ammonites and coleoids together with the superbly preserved ichthyosaurs and plesiosaurs, now displayed in many European museums. Some horizons are characterized by monotypic assemblages of small taxa such as diademoid echinoids and byssate bivalves, like Posidonia itself. These benthic colonizations may have

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Francisciconcha Maslennikovi

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Figure 4.20 A cocktail of Jurassic environments. Early Jurassic: (a) sand, (b) muddy sand, and (c) bituminous mud communities. Late Jurassic: (d) mud, (e) reef, and (f) lagoonal communities. (From McKerrow 1978.)

been promoted by storms, providing fresher-water conditions for short periods of time.

Ecological patterns and trends through time

During the last 600 myr, both animal and plant communities expanded and diversified (Box 4.5). In simple terms the number of Bambachian megaguilds multiplied through the Cambrian (nine megaguilds), Paleozoic (14) and Modern (20) evolutionary faunas. The focus in the Cambrian was on marine animals that were either attached or mobile with suspension- or deposit-feeding strategies, such as the eocrinoids and trilobites. The morphologies of individual organisms were rather plastic as were their community compositions and structures. Relatively few class-level taxa were included in each ecological box (Fig. 4.21). By the Ordovician, however, the number of megaguilds had expanded, with an overall numerical dominance of suspension feeders, such as the brachiopods, bryozoans, corals and crinoids. The Paleozoic fauna was characterized by sedentary organisms. The Modern fauna, by contrast, was dominated by deposit-feeding, essentially mobile animals bound into a process of escalation, or ever-increasing competition, and the first intense arms race on the planet. The term arms race is used by ecologists to describe ever-intensifying interactions between predators and prey, for example.

Throughout the Phanerozoic there seems to have been an offshore movement in marine faunas. New communities and taxa may have occurred in nearshore, high-energy environments first, before migrating into deeper water. Thus older, more archaic groups tended to characterize deeper-water habitats. For example during the Ordovician radiation (see p. 253), typical members of the Paleozoic fauna (brachiopods, bryozoans and crinoids) expanded and migrated into deeper-water habitats, while their place in shallow water was taken by components of the Modern fauna (bivalves and gastropods). But why? Are nearshore habitats particularly harsh, driving innovative communities and taxa into deep water, or can innovative organisms arise at any depth and those in shallower-water environments are just more resistant to extinction and can readily migrate into deeper water (Jablonski & Bottjer 1990)? Perhaps it was a combination of both.

In marine environments acceleration of the height, complexity and stratification of benthic tiering was later matched by increases in the depth and sophistication of infaunal tiering as, particularly in Mesozoic and Cenozoic faunas, many more organisms adopted burrowing lifestyles and the benthos switched from filter to deposit feeding with significantly more predators. The Cambrian evolutionary fauna occupied, more or less, only the surface of the seabed, but by the Ordovician crinoids had developed tiers over a meter above the seabed and burrowing had already commenced into the sediment. Terrestrial environments, initially dominated by small green plants, various arthropods and snails, together with diverse amphibian faunas in the Mid to Late Paleozoic, changed significantly during the Mesozoic, with the diversification of vegetation and eventually flowering plants, and terminating, for now, in the high and elaborate canopies we see today in the tropical rain forests (see p. 505).

The Modern fauna was also characterized by something rather special, an arms race (Harper 2006). During the so-called Meso-zoic marine revolution, predators, such as bony fishes, crustaceans, marine reptiles and starfishes began to develop better and better ways of crushing or opening shells. The Modern world was a much more dangerous place and in order to survive, potential prey had to develop thicker, more elaborately ornamented shells with smaller apertures (Box 4.6) and devise more cunning evasive strategies such as greater mobility or deeper and deeper burrowing. Unfortunately exposure to intense predation and a much more bioturbated seafloor was no place for many groups of epifaunal animals such as the bra-chiopods, some groups of bivalves and echi-noderms. But as prey developed more armor and better evasive strategies, the hunters developed better weaponry. Together this escalation and increased tiering set the Modern fauna quite apart from those of the Cambrian and Paleozoic. Perhaps the whole ecosystem functioned in a different way, allowing biodiversity to continue to expand way beyond the plateau of the Paleozoic fauna (see p. 541).

Unlike biodiversity change, where we have numbers of taxa to count and monitor, ecological change is much more difficult to describe and quantify. Since some changes are much

Table 4.1 Hierarchical levels of ecological change and their signals.

Level Definition

Signals

First Appearance/disappearance of an ecosystem

Second Structural changes within an ecosystem

Third Community-type level changes within an established ecological structure

Fourth Community-level changes

Initial colonization of environment

First appearance of, or changes in, ecological dominants of higher taxa

Loss/appearance of metazoan reefs Appearance/disappearance of Bambachian megaguilds

Appearance and/or disappearance of community types Increase and/or decrease in tiering complexity "Filling-in" or "thinning" within Bambachian megaguilds

Appearance and/or disappearance of paleocommunities Taxonomic changes within a clade more significant than others, one way is to establish a series of levels with key, identifiable characteristics (Droser et al. 2000). Four ranks or paleoecological levels have been identified (Table 4.1) based on, for example, the appearance or disappearance of an entire ecosystem (first), the appearances and disappearances of dominant taxa (second), thickening or thinning of the Bambachian megaguilds (third) or the mere appearance or disappearance of a community (fourth). During the Phanerozoic ecological changes can be charted at all levels: the appearance of the Ediacara biota was clearly a first-order change, whereas the Cambrian explosion and the Ordovician radiation involved changes at the second, third and fourth levels. Recovery after the end-Permian mass extinction event is a textbook example involving the addition to existing Bambachian megaguilds, when the tiering of marine faunas really took off (Twitchett 2006). The major mass extinction events have been ranked ecologically too (Box 4.7).

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