Paleontology - Bennington

Basic Paleoecology

Paleoecology is a very broadly defined branch of paleontology that includes just about any study that concerns itself with understanding how fossils functioned as living organisms within a habitat or environment. As such, most paleoecological studies require that fossils be studied and placed within the context of the sediments from which they came. Paleoecologists must be well versed in sedimentology as well as in paleontology.

There are two main theoretical divisions of paleoecology:

Ecology of individuals

Paleoautecology is the study of how a particular species functioned in its environment. Paleoautecological studies usually involve trying to reconstruct the function of different aspects of an organism's morphology in relation to the environment and conditions present in the habitat in which that organism was inferred to live.

Vocabulary related to autecology

Morphology: the physical form and structure of an organism.

Ontogeny: the process of growth through the life of an organism.

Related to these are some concepts of growth and development:

Solitary: organisms that exist and function independent of other members of their species for most of their existence.

Colonial: organisms that live in large, interdependent groups (colonies).

Clonal: clones are organisms that are genetically identical having been reproduced asexually. Many clonal organisms are also colonial.

Animals that move around under their own power are called motile. Animals that remain stuck in one place through their adult lives are called sessile.

An example of a paleoautecology study is Richard Grant's famous (1966) paper on the Permian brachiopod Waagenoconch in which he argued for a particular mode of life and life history based on several aspects of the brachiopods morphology and the characteristics of the sediments it was entombed in.

Small spines in juveniles for attachment above the mud bottom.

Large spines on pedicle valve in adults functioned as 'snowshoe' to anchor the brachiopod in the mud and prevent its sinking downward.

Another famous example is Steven Stanley's (1968) study of bivalve life habits in which he studied the relationship between shell shape and mode of life in living bivalves in order to elucidate the life habits of fossil bivalves.

Many of the interpretations of evidence from fossil organisms are based on comparisons with living forms- called analogies. Without recourse to living examples, many fossil organisms such as ammonites, bivalves, turtles, etc. might have been very difficult to understand in an ecological sense. In many cases, the functioning of extinct organisms must be deduced using physical and engineering principles or by experiments with models. This type of study is also called functional morphology.

Ecology of Communities

Organisms live in ecological groups in the natural world. Although there are no firmly established scales or boundaries for naming ecological groups, ecologists and paleontologists generally refer to local groups of co-occurring organisms as a community. On a larger scale, a community or communities that occupy a single broadly defined habitat is called an ecosystem.

A group of fossil species found together in a particular rock stratum is called an assemblage or a paleocommunity. There is a subtle shade of difference in the meaning of these two terms. Because of the way fossil accumulations form, it is often uncertain whether all the fossils found preserved together actually lived together, a condition implied by the term paleocommunity. Paleontologists who wish to remain agnostic on this point employ the term fossil assemblage.

Paleosynecology is the study of how groups of coexisting species or communities interacted and functioned in particular habitats. Paleosynecology is often called community paleoecology.

Community paleoecology can be very similar in many ways to community ecology as practiced by modern biologists. Both biologists and paleontologists strive to understand why a particular group of organisms lives together in a particular habitat, how the organisms interact with each other and with the environment.

Paleoecologists are more limited than neoecologists in many ways for the obvious reasons that so much information available to a neoecologist is lost to the paleoecologist. In fact, while a neoecologist can directly observe an ecological community and choose the aspects of that community that are of interest, the paleoecologist must first reconstruct the extinct community from the available fossils. These 'community reconstructions' encompass the activities of many paleoecologists and while they are restricted both in geographic extent and in detail, they provide the snapshots from which the overall history of life is filled in.

Evolutionary Paleoecology

More recently, paleoecologists have shifted their focus to take advantage of the fact that they are able to study the fate of ecological communities and systems over much longer periods of time than can be studied by the neoecologist. Many paleoecological studies work to unravel the pattern of change in communities through time, or on a broader scale, the pattern of change in the functioning of ecological systems in general.

The marine story (big picture)

The patterns of evolution and extinction in the marine realm have been extensively studied over the past two decades, thanks largely to the efforts and inspiration of Jack Sepkoski and David Raup of the University of Chicago.

Sepkoski has tabulated an extensive database of the names and stratigraphic durations of marine animal families through the Phanerozoic. The overall pattern seen in marine animal families is one of increasing diversity. However, it should be noted that the diversity increase has not been continuous.

Diversity seems to increase rapidly from the Cambrian into the Ordovician, slows but continues into the Devonian, and then levels off through the end of the Permian. A sharp drop in diversity is occurs at the end of the Permian in what is now recognized as history’s largest mass extinction. However, diversity began increasing again in the Triassic and continued to increase up to the Recent.

Can we explain this diversity history?

The initial rise in diversity from near zero at the base of the Cambrian is clearly the initial adaptive radiation of the metazoa - a evolutionary filling of the available ecological space.

Similarly, the leveling off of diversity after the beginning of the Ordovician suggests that there is some upper limit to how many different families of organisms can be supported by the finite number and size of environments in the world’s oceans.

Extinctions such as occurred at the end of the Ordovician and Devonian temporarily depress diversity, but it seems to rebound back to earlier levels, but no higher.

However, something different happens with the end Permian extinction. Diversity continues to rise after this extinction to levels far above those seen in the Paleozoic. Why?

3 explanations (all may contribute to the truth).

1. Diversity rise is partly an artifact of increasing rock record and increasingly good preservation approaching the recent.

2. Richard Bambach of Virginia Tech has shown that with each rise in overall diversity, more ecological ‘guilds’ were being occupied by animals. An ecological guild can be defined as a particular method of exploiting a food resource. In this view, overall diversity is only allowed to increase when evolution events a new way of making a living.

3. Related to number two - post Triassic marine organisms may have had a greater supply of food to exploit than Paleozoic faunas. This is explained by the evolutionary radiation of land plants and the rise of extensive primary productivity on the land. Organics and nutrients produced by plants on land are washed into the sea where they become available to marine organisms. Increasing the marine food supply may permit the coexistance of more species within ecosystems, or it may simply provide additional novel opportunities for feeding - new guilds.

The nature of the three faunas

One more element needs to be factored into this picture. The same types of animals are not evolving uniformly to produce new families through the history of the Phanerozoic.

Sepkoski has recognized three distinct groups of organisms that are dominant ‘evolvers’ at different times in the Phanerozoic. He calls these three groups ‘evolutionary faunas’. They are recognized statistically as groups of classes of organisms that wax and wane in diversity in concert with one-another.

Cambrian fauna: Trilobita, Inarticulata, Eocrinoidea, Archaeocyatha, Hyolitha

This fauna diversified very rapidly from the latest Vendian into the Early Cambrian and was the principle constituent of the ‘Cambrian Explosion’. Its maximum diversity was attained in the late Middle and early Late Cambrian. Beginning in the latest Cambrian it began a long, gradual decline that was accentuated by the mass extinctions at the end of the Ordovician and Devonian.

Paleozoic fauna: Tabulata, Rugosa, Crinoidea, Articulata, Stenolaemata, Cephalopoda, Graptolithina

The Paleozoic fauna began expanding as the Cambrian fauna began its decline. What is particularly interesting about this is that there is no mass extinction to blame for eliminating the Cambrian fauna. It appears that the Paleozoic groups steadily replaced the Cambrian fauna, perhaps due to ecological displacement or else passive displacement as the older group went into decline.

In the Ordovician the Paleozoic fauna underwent a series of rapid radiations that tripled global diversity over a span of 50 million years.

The Paleozoic fauna reached its maximum diversity from the Late Ordovician to the Devonian and then began a long, very slow decline to the end of the Permian.

In the end Permian extinction the Paleozoic fauna takes a massive hit from which it never fully recovers, although it rediversifies twice and remains a relatively constant component of marine ecosystems to the Recent.

Mesozoic and Cenozoic (Modern) fauna: Pelecypoda, gastropoda, cephalopoda, gymnolaemata, Scleractinia, Echinoidea, Malacostraca, Vertebrata

The members of the Modern fauna increased in diversity very slowly throughout the Paleozoic. However, at the Permo-Triassic boundary the members of this fauna were much less affected by the mass extinction and appear to diversify at a much higher rate after the extinction.

This suggests that the trend of slow replacement of the Paleozoic fauna by the Modern fauna (which might have gone on the mimic - in a much more drawn out fashion - the replacement of the Cambrian fauna by the Paleozoic fauna) was greatly accelerated by the Permian mass extinction. It seems that the Permian extinction cleared the world of the older fauna and opened up ecological room for the new fauna to diversify into and occupy.

Thus, the faunal composition of modern marine ecosystems seems to be partly a result of evolutionary ‘inevitability’ (the slow replacements seen through the Phanerozoic) and partly the luck (or lack of it) associated with mass extinctions.

The Persistence of Ecosystems Through Time

EEUs

Paleontologist Arthur Boucot (U. Oregon) and later Peter Sheehan (Milwaukee Museum) have noted that within the intervals of the 3 great marine faunas are smaller intervals of relative evolutionary and ecological similarity, wherein species and species associations remain similar. These intervals were named EEUs by Boucot (Ecological-Evolutionary units). Many EEUs appear to be structured by the extinction events that define period boundaries, with short recovery EEUs immediately following mass extinctions followed by much longer EEUs between extinction events.

Coordinated Stasis

Several paleontologists, notably Carlton Brett (formerly of the U of R, now at U. Cincinnati) and Gordon Baird (SUNY Fredonia), have observed that paleocommunities in stages of the Silurian and Devonian tend to recur consistently with the same approximate species composition and relative abundances, with very little evolutionary turnover of species. This repetitive reassociation of species occurs in spite of repeated disruptions of habitat due to changes in sea level. When change does come, it is relatively severe, with widespread replacement of species and the formation of new community associations. Brett and Baird have termed this phenomenon coordinated stasis and have raised questions as to its cause.

Coordinated stasis may be primarily an evolutionary phenomenon, where most species are simply resistant to extinction for some interval of time, before a threshold level of environmental stress is passed and many go extinct simultaneously.

Coordinated stasis may be primarily an ecological phenomenon. Some paleoecologists see in coordinated stasis evidence for tight ecological control over species survival and community composition (referred to as ecological locking). The evidence for this idea comes from the observation that the same numerical groups of species (paleocommunities) appear to recur throughout an interval of coordinated stasis. The formation of tightly interconnected marine communities might inhibit the evolution or invasion of new species, until something happens to disrupt the existing communities.

Recently, paleontologists, notably Bret Bennington (Hofstra) and Richard Bambach (Virginia Tech) have begun to test the pattern of coordinated stasis and have argued that there is a lot of variability in these supposedly recurring paleocommunities that is revealed through careful sampling and statistical analysis. This suggests that coordinated stasis is nothing more than species repeatedly reinvading the habitats that they are adapted to, and that as long as those habitats exist in some form, species will persist and will appear to associate in similar groups.

Other paleontologists have looked for and failed to find any evidence for the coordinated stasis pattern in paleocommunities from the Ordovician and the Triassic. It remains to be seen how common this phenomenon is and if, indeed, it is a phenomenon at all.