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Chapter 8 Conclusions
CHAPTER 8
Conclusions
I have tried t o summarize the relation between population size (number of individuals) and rates of evolution and extinction for Silurian-Devonian shallow-water, marine benthos. Population size has been estimated by comparing the relative areas occupied by various taxa and assuming that population size will be a relatively direct function of area occupied. Areas have been estimated by studying the area and the distribution of communities in which the taxa are known t o occur, and the area of the biogeographic entities in which the communities themselves are known to occur. The information for Silurian and Early Devonian marine benthic communities is as complete as present data permits. Limited information has been presented for the Middle Devonian. The available zoogeographic data for both the Silurian and Devonian has been summarized. Because inadequate attention has been given t o the distribution patterns of most marine invertebrate groups during this time interval, whereas data are available for the overall abundant brachiopods, the conclusions are based presently almost entirely on the latter. This situation obviously should be rectified as soon as practicable. I have also summarized the available data on numbers of brachiopod genera currently recognized during each stage of the Silurian-Devonian (almost 100 million years) and have tried to outline their occurrence, either endemic or cosmopolitan, in the biogeographic units recognized for each stage. A summary of the community distribution of these same brachiopods in terms of Benthic Assemblage occurrence of the communities has also been made for the entire time interval. Preliminary and unsatisfactory as much of the data may be it still permits some conclusions to be made as follows.
Evolution Rate Conclusions
( I ) The entire time interval Silurian through Devonian shows a good inverse correlation between size of potentially interbreeding population (as measured by area occupied) and rate of change of form (i.e., organic evolution). The size of the area may be judged either as the area of the
344
CONCLUSIONS
Benthic Assemblage during a time of relative cosmopolitanism (for only a few entities can we estimate the area occupied by each community within a Benthic Assemblage), or by the size of the zoogeographic entity plus the area of contained Benthic Assemblages during times of great provincialism. Therefore, on a worldwide basis there is a higher rate of taxonomic evolution (more new taxa per unit of time) during times of provincialism than during times of cosmopolitanism. There is a general tendency for the biogeographic units covering smaller areas (Fig. 2, 41) to have a more diverse fauna than do the larger units (Tasman Region of the Old World Realm and Cordilleran Region of the Old World Realm as opposed to the Rhenish-Bohemian Region, and Eastern Americas as opposed t o the Old World Realm), although this tendency is complicated by the problem of trying t o ascertain whether or not the various units occur in identical but isolated environments (if they represent greatly differing environments it is very likely that differences in diversity may be partly due to these same differences rather than to pure area because of the difficulty or impossibility of occurrence of certain biotopes that might have supported diverse communities. The absence of limestone and reef environments in the Malvinokaffric Realm is an excellent example paralleled today by the absence of reef environments in polar regions; both examples have far lower taxonomic diversities than those which include the spectrum of reef environments). This appears to be a first-order effect overshadowing environmental factors. (2)Any study of the factors influencing rates of evolution among marine invertebrates should consider the worldwide size of the potentially interbreeding populations. In other words, natural selection is concluded to play the usual important role in determining what organisms develop, but not a first-order role in determining the rate at which they evolve, unless it can be shown that natural selection has determined the size of the interbreeding populations. ( 3 ) There is a tendency for lower rates of evolution to be produced during time intervals when fewer communities (Fig. 22H) are present because of the population-size effect,. Fewer communities may be present during a particular time interval because of either physical factors such as climatic uniformity or of biologic factors such as evolution in the direction of greater niche breadth. ( 4 ) Cosmopolitan taxa have a greater time duration (Fig. 29) than provincial taxa, and also are more abundant at any one locality in terms of individual specimens. Thus, cosmopolitan taxa tend to be cosmopolitan in both time and space. Cosmopolitan taxa evolve far more slowly than do provincial taxa. Cosmopolitan taxa occur in a variety of communities (from Benthic Assemblage 2 through 5). There is good evidence that
EVOLUTION RATE
345
cosmopolitan taxa are not restricted, or even more abundant, in so-called “unstable” environments. ( 5 )There is good evidence for rate of evolution not being well correlated with either the so-called “stable” or “unstable” environments. (6) Intervals characterized by abundant and widespread reef environments (Fig. 22M) correlate well with increased rates of evolution, presumably owing to the multiplicity of isolated, small, reef-related, rapidly evolving populations. (7) It is concluded that during times of either provincialism or cosmopolitanism the evolutionary effects of the environment have far less impact in terms of rate of evolution than has the size of the interbreeding population. (8) There is no 1:l correlation between rate of evolution and either intervals of transgression or regression, although both phenomena during certain specific time intervals in certain specific areas may be interpreted to act in concert with other factors (geography, climate, marine current location) to help produce barriers to faunal migration that in turn will affect the potential size of interbreeding populations. However, if other factors are equal times of regression should increase rate of phyletic evolution. (9) Competition for food and place does not appear t o be a first-order control over rate of evolution. (10) The high inverse correlation between population size and rate of evolution applies to both marine and non-marine organisms, and t o “low” groups and “high” groups. The small residue of organisms that show a marked departure from this correlation (small populations characterized by slow evolution of the bradytelic type; large populations characterized by rapid evolution of the tachytelic type) indicate the importance of additional factors involved as rate controls below the first-order level. (11) The seeming correlation between higher rate of evolution in terrestrial and “higher” groups of organisms as contrasted with marine and “lower” groups of organisms is easily interpreted as a population-size effect with terrestrial organisms tending t o occur as smaller populations than marine organisms, and many “lower” groups as larger populations than many “higher” groups. Thus it is evident that many of the “lower” groups are marine whereas many of the “higher” groups are terrestrial. There are so many exceptions to the earlier generalizations about the correlation between “high” and “low”, marine and terrestrial as related t o rate of evolution (the ammonites and fusuline foraminifers as prime examples) that the importance of the population-size effect is easy to grasp s
’.
346
CONCLUSIONS
Silurian-Devonian Biogeographic Conclusions (12) The trend is from relatively cosmopolitan distribution t o more provincial patterns (established after the termination of the more provincial Late Ordovician) for Llandovery age marine benthos (Table 111). (13 ) This benthos shows increasing provincialism during the WenlockPridoli time interval (Table 111). (14) The marine benthos show high provincialism during the Early and Middle Devonian (highest during the latter half of the Early Devonian). This provincialism terminates in the Late Devonian, essentially absent by Frasne time, and is replaced by cosmopolitanism (Table 111, Fig. 31). (15)There is no correlation between times of high provincialism and times of maximum orogeny during the Silurian-Devonian. Evidence from other time intervals strongly suggests that orogeny does not correlate with provincialism or with rate of evolution (Fig. 221). Rate of Extinction (16) Rates of terminal extinction increase at the end of some times of provincialism for the marine benthos, suggesting that competition among similarly adapted forms plays an important role in some cases. (17) Major extinction events may be preceded by the occurrence of either highly cosmopolitan or highly provincial faunas, and may be followed by the occurrence of either highly cosmopolitan or provincial faunas. There is no 1:l correlation between the factors causing major extinction events and those controlling cosmopolitanism or provincialism.
Hypersaline Water as a Biogeographic Barrier (18)The occurrence of large areas of hypersaline water in the more central parts of the Northern Hemisphere and Australian Platforms in the Late Silurian-Middle Devonian (as shown by the presence of evaporites and hypersaline communities) and of faunal provincialism (Fig. 37-41) possibly suggests that large bodies of hypersaline water on the platforms during this time interval had the potential t o act as one of the factors in erecting faunal barriers that produced provincialism. It is possible that the production of hypersaline water was the result of a climatic regime in concert with restricted circulation in which the platform central regions were affected by a greater evaporation rate than input of rainfall necessary to maintain normal, sea-water salinity (in the absence of large riverine input).
ENVIRONMENTAL STABILITY
347
Unimportance of Dolomitization as a Biogeographic Barrier
(I 9) Sedimentary secondary dolomite replacing subtidally deposited limestones on the Cambro-Silurian platforms in such great abundance (as contrasted with the Precambrian and post-Silurian) is thought to have been formed by biotic dolomitization. Biotic dolomitization calls on poor circulation with the oceanic reservoir across vast platforms. The poor circulation makes possible a cycle in which biogenic activity removes calcium which in turn permits the excess magnesium to slightly replace calcareous bottom detritus; the calcareous bottom detritus is being continuously subject t o solution; solution affects the differing solubility phases so as t o preferentially leave some dolomite undissolved and free to accumulate over an interval of time as a long-term residue. This process of dolomitization does not appear to have any biogeographic or environmental effects on the co-occurring benthos. Species Diversity
(20) Brachiopod species diversity in a community decreases as dominance of any species increases. (21) Maximum species diversity is evidently reached during a geologically almost instantaneous interval of time. This effect appears to hold for both reef and level-bottom communities. The time-stability concept, if it actually operates, occurs over a geologically very small time interval before leveling off to a steady state that may be maintained for many tens of millions of years. Environmental Stability of Taxa during Evolution
(22) Consideration of the Benthic Assemblage position of the SilurianDevonian brachiopods, and of their community associates, leads one to conclude that the majority have not changed their position during the interval (Fig. 35). A few have evolved in the direction of a narrower Benthic Assemblage spread, a few have evolved in the direction of a broader Benthic Assemblage spread, a few have evolved t o form lowdiversity communities as opposed t o earlier high-diversity associates, and a few have evolved t o form part of high-diversity associations as opposed t o earlier low-diversity associations, but the majority appear t o have stayed put. From this information I have concluded that there will be an overall tendency for most taxa to undergo little change in environmental requirements during the course of organic evolution during moderate portions of geologic time. This conclusion is critical in any attempt at evaluating the
CONCLUSIONS
348
similarities and differences between the taxa thought t o occupy functionally similar positions in different biogeographic units. (23) Finally, the overall conclusion is that it is the resultant interplay in time of all these factors observable in the geologic record (provincialism versus cosmopolitanism, regression versus transgression, high abundance of reefs as opposed t o absence of reefs, highly differentiated climatic regime as opposed t o a uniform climate, high number of animal communities as opposed to a low number of animal communities) plus others that we cannot easily measure or estimate (including such factors as food supply) from the geologic record at this time, which determines the size of interbreeding populations in time and thus the first-order control over rates of evolution (Fig. 33). The review of the Silurian-Devonian benthic record presented here makes it unlikely that extraterrestrial radiation influences on rate of evolution have had widely differing magnitudes during this time interval although cometary collisions might be investigated. After we have been able to normalize the Silurian-Devonian data for different population sizes we may be in a position to analyze the importance and effects on rate of evolution of various second- and lower-order factors 8 2
.
P.S. All of the conclusions arrived at in this treatment are based on two assumptions. If these assumptions are seriously in error the conclusions are probably of little value. ( 1) The fossil record of the brachiopods is a reasonable approximation of their real record. (2) The ease with which brachiopods have become fossilized and preserved in the geologic record has not varied widely during the Phanerozoic. That is, we assume that brachiopods during one time interval are as likely to have been preserved as fossils as those during other time intervals.
Conclusions
I have tried t o summarize the relation between population size (number of individuals) and rates of evolution and extinction for Silurian-Devonian shallow-water, marine benthos. Population size has been estimated by comparing the relative areas occupied by various taxa and assuming that population size will be a relatively direct function of area occupied. Areas have been estimated by studying the area and the distribution of communities in which the taxa are known t o occur, and the area of the biogeographic entities in which the communities themselves are known to occur. The information for Silurian and Early Devonian marine benthic communities is as complete as present data permits. Limited information has been presented for the Middle Devonian. The available zoogeographic data for both the Silurian and Devonian has been summarized. Because inadequate attention has been given t o the distribution patterns of most marine invertebrate groups during this time interval, whereas data are available for the overall abundant brachiopods, the conclusions are based presently almost entirely on the latter. This situation obviously should be rectified as soon as practicable. I have also summarized the available data on numbers of brachiopod genera currently recognized during each stage of the Silurian-Devonian (almost 100 million years) and have tried to outline their occurrence, either endemic or cosmopolitan, in the biogeographic units recognized for each stage. A summary of the community distribution of these same brachiopods in terms of Benthic Assemblage occurrence of the communities has also been made for the entire time interval. Preliminary and unsatisfactory as much of the data may be it still permits some conclusions to be made as follows.
Evolution Rate Conclusions
( I ) The entire time interval Silurian through Devonian shows a good inverse correlation between size of potentially interbreeding population (as measured by area occupied) and rate of change of form (i.e., organic evolution). The size of the area may be judged either as the area of the
344
CONCLUSIONS
Benthic Assemblage during a time of relative cosmopolitanism (for only a few entities can we estimate the area occupied by each community within a Benthic Assemblage), or by the size of the zoogeographic entity plus the area of contained Benthic Assemblages during times of great provincialism. Therefore, on a worldwide basis there is a higher rate of taxonomic evolution (more new taxa per unit of time) during times of provincialism than during times of cosmopolitanism. There is a general tendency for the biogeographic units covering smaller areas (Fig. 2, 41) to have a more diverse fauna than do the larger units (Tasman Region of the Old World Realm and Cordilleran Region of the Old World Realm as opposed to the Rhenish-Bohemian Region, and Eastern Americas as opposed t o the Old World Realm), although this tendency is complicated by the problem of trying t o ascertain whether or not the various units occur in identical but isolated environments (if they represent greatly differing environments it is very likely that differences in diversity may be partly due to these same differences rather than to pure area because of the difficulty or impossibility of occurrence of certain biotopes that might have supported diverse communities. The absence of limestone and reef environments in the Malvinokaffric Realm is an excellent example paralleled today by the absence of reef environments in polar regions; both examples have far lower taxonomic diversities than those which include the spectrum of reef environments). This appears to be a first-order effect overshadowing environmental factors. (2)Any study of the factors influencing rates of evolution among marine invertebrates should consider the worldwide size of the potentially interbreeding populations. In other words, natural selection is concluded to play the usual important role in determining what organisms develop, but not a first-order role in determining the rate at which they evolve, unless it can be shown that natural selection has determined the size of the interbreeding populations. ( 3 ) There is a tendency for lower rates of evolution to be produced during time intervals when fewer communities (Fig. 22H) are present because of the population-size effect,. Fewer communities may be present during a particular time interval because of either physical factors such as climatic uniformity or of biologic factors such as evolution in the direction of greater niche breadth. ( 4 ) Cosmopolitan taxa have a greater time duration (Fig. 29) than provincial taxa, and also are more abundant at any one locality in terms of individual specimens. Thus, cosmopolitan taxa tend to be cosmopolitan in both time and space. Cosmopolitan taxa evolve far more slowly than do provincial taxa. Cosmopolitan taxa occur in a variety of communities (from Benthic Assemblage 2 through 5). There is good evidence that
EVOLUTION RATE
345
cosmopolitan taxa are not restricted, or even more abundant, in so-called “unstable” environments. ( 5 )There is good evidence for rate of evolution not being well correlated with either the so-called “stable” or “unstable” environments. (6) Intervals characterized by abundant and widespread reef environments (Fig. 22M) correlate well with increased rates of evolution, presumably owing to the multiplicity of isolated, small, reef-related, rapidly evolving populations. (7) It is concluded that during times of either provincialism or cosmopolitanism the evolutionary effects of the environment have far less impact in terms of rate of evolution than has the size of the interbreeding population. (8) There is no 1:l correlation between rate of evolution and either intervals of transgression or regression, although both phenomena during certain specific time intervals in certain specific areas may be interpreted to act in concert with other factors (geography, climate, marine current location) to help produce barriers to faunal migration that in turn will affect the potential size of interbreeding populations. However, if other factors are equal times of regression should increase rate of phyletic evolution. (9) Competition for food and place does not appear t o be a first-order control over rate of evolution. (10) The high inverse correlation between population size and rate of evolution applies to both marine and non-marine organisms, and t o “low” groups and “high” groups. The small residue of organisms that show a marked departure from this correlation (small populations characterized by slow evolution of the bradytelic type; large populations characterized by rapid evolution of the tachytelic type) indicate the importance of additional factors involved as rate controls below the first-order level. (11) The seeming correlation between higher rate of evolution in terrestrial and “higher” groups of organisms as contrasted with marine and “lower” groups of organisms is easily interpreted as a population-size effect with terrestrial organisms tending t o occur as smaller populations than marine organisms, and many “lower” groups as larger populations than many “higher” groups. Thus it is evident that many of the “lower” groups are marine whereas many of the “higher” groups are terrestrial. There are so many exceptions to the earlier generalizations about the correlation between “high” and “low”, marine and terrestrial as related t o rate of evolution (the ammonites and fusuline foraminifers as prime examples) that the importance of the population-size effect is easy to grasp s
’.
346
CONCLUSIONS
Silurian-Devonian Biogeographic Conclusions (12) The trend is from relatively cosmopolitan distribution t o more provincial patterns (established after the termination of the more provincial Late Ordovician) for Llandovery age marine benthos (Table 111). (13 ) This benthos shows increasing provincialism during the WenlockPridoli time interval (Table 111). (14) The marine benthos show high provincialism during the Early and Middle Devonian (highest during the latter half of the Early Devonian). This provincialism terminates in the Late Devonian, essentially absent by Frasne time, and is replaced by cosmopolitanism (Table 111, Fig. 31). (15)There is no correlation between times of high provincialism and times of maximum orogeny during the Silurian-Devonian. Evidence from other time intervals strongly suggests that orogeny does not correlate with provincialism or with rate of evolution (Fig. 221). Rate of Extinction (16) Rates of terminal extinction increase at the end of some times of provincialism for the marine benthos, suggesting that competition among similarly adapted forms plays an important role in some cases. (17) Major extinction events may be preceded by the occurrence of either highly cosmopolitan or highly provincial faunas, and may be followed by the occurrence of either highly cosmopolitan or provincial faunas. There is no 1:l correlation between the factors causing major extinction events and those controlling cosmopolitanism or provincialism.
Hypersaline Water as a Biogeographic Barrier (18)The occurrence of large areas of hypersaline water in the more central parts of the Northern Hemisphere and Australian Platforms in the Late Silurian-Middle Devonian (as shown by the presence of evaporites and hypersaline communities) and of faunal provincialism (Fig. 37-41) possibly suggests that large bodies of hypersaline water on the platforms during this time interval had the potential t o act as one of the factors in erecting faunal barriers that produced provincialism. It is possible that the production of hypersaline water was the result of a climatic regime in concert with restricted circulation in which the platform central regions were affected by a greater evaporation rate than input of rainfall necessary to maintain normal, sea-water salinity (in the absence of large riverine input).
ENVIRONMENTAL STABILITY
347
Unimportance of Dolomitization as a Biogeographic Barrier
(I 9) Sedimentary secondary dolomite replacing subtidally deposited limestones on the Cambro-Silurian platforms in such great abundance (as contrasted with the Precambrian and post-Silurian) is thought to have been formed by biotic dolomitization. Biotic dolomitization calls on poor circulation with the oceanic reservoir across vast platforms. The poor circulation makes possible a cycle in which biogenic activity removes calcium which in turn permits the excess magnesium to slightly replace calcareous bottom detritus; the calcareous bottom detritus is being continuously subject t o solution; solution affects the differing solubility phases so as t o preferentially leave some dolomite undissolved and free to accumulate over an interval of time as a long-term residue. This process of dolomitization does not appear to have any biogeographic or environmental effects on the co-occurring benthos. Species Diversity
(20) Brachiopod species diversity in a community decreases as dominance of any species increases. (21) Maximum species diversity is evidently reached during a geologically almost instantaneous interval of time. This effect appears to hold for both reef and level-bottom communities. The time-stability concept, if it actually operates, occurs over a geologically very small time interval before leveling off to a steady state that may be maintained for many tens of millions of years. Environmental Stability of Taxa during Evolution
(22) Consideration of the Benthic Assemblage position of the SilurianDevonian brachiopods, and of their community associates, leads one to conclude that the majority have not changed their position during the interval (Fig. 35). A few have evolved in the direction of a narrower Benthic Assemblage spread, a few have evolved in the direction of a broader Benthic Assemblage spread, a few have evolved t o form lowdiversity communities as opposed t o earlier high-diversity associates, and a few have evolved t o form part of high-diversity associations as opposed t o earlier low-diversity associations, but the majority appear t o have stayed put. From this information I have concluded that there will be an overall tendency for most taxa to undergo little change in environmental requirements during the course of organic evolution during moderate portions of geologic time. This conclusion is critical in any attempt at evaluating the
CONCLUSIONS
348
similarities and differences between the taxa thought t o occupy functionally similar positions in different biogeographic units. (23) Finally, the overall conclusion is that it is the resultant interplay in time of all these factors observable in the geologic record (provincialism versus cosmopolitanism, regression versus transgression, high abundance of reefs as opposed t o absence of reefs, highly differentiated climatic regime as opposed t o a uniform climate, high number of animal communities as opposed to a low number of animal communities) plus others that we cannot easily measure or estimate (including such factors as food supply) from the geologic record at this time, which determines the size of interbreeding populations in time and thus the first-order control over rates of evolution (Fig. 33). The review of the Silurian-Devonian benthic record presented here makes it unlikely that extraterrestrial radiation influences on rate of evolution have had widely differing magnitudes during this time interval although cometary collisions might be investigated. After we have been able to normalize the Silurian-Devonian data for different population sizes we may be in a position to analyze the importance and effects on rate of evolution of various second- and lower-order factors 8 2
.
P.S. All of the conclusions arrived at in this treatment are based on two assumptions. If these assumptions are seriously in error the conclusions are probably of little value. ( 1) The fossil record of the brachiopods is a reasonable approximation of their real record. (2) The ease with which brachiopods have become fossilized and preserved in the geologic record has not varied widely during the Phanerozoic. That is, we assume that brachiopods during one time interval are as likely to have been preserved as fossils as those during other time intervals.