Conclusions about Evolution and Extinction

Overall Synthesis of Population-size Effects The major conclusion reached in this book is that worldwide population size is inversely related to rate of evolution, whether phyletic, clado-genetic or taxonomic (morphologic rates of evolution have not been specifically discussed but would follow the same pattern as the others). All of the data from the record affording any measure of population size (number of individuals belonging to the taxa being considered) provide this same conclusion....

Silurian Devonian Community Framework

In order to assess the size of interbreeding populations in terms of the areas they occupied it is necessary to measure the area of the communities in which they occur. This section presents the bases with which such communities are defined and the newly defined Benthic Assemblage scheme employed to classify these communities systematically. Adoption of the Benthic Assemblage scheme makes possible a hierarchic subdivision of community information (Benthic Assemblages, Community Groups, Complex...

Plate

Figures 1 16 Llanoella stephensi new genus and species. Pillar Bluff Limestone. Type locality of Pillar Bluff Limestone on Pillar Bluff Creek, 5 miles southwest of Lampasas and just south of the Lampasas County line in northern Burnet County, Texas. The Barnes locality number is TF-273 (see p. 129 of Barnes, Cloud and Warren, 1947). 1. Exterior of pedicle valve (X 21). Note the rounded sulcus cross-section USNM No. 188543. 2. Exterior of pedicle valve (X 2). Note the more angular margins of the...

Notes

'Worldwide swings in available nutrient supply will, following Valentine's (1971a) deductions, certainly have an effect on rate of evolution, with increases favoring a lower rate of evolution and decreases an increased rate. During time intervals of provincialism changes in nutrient supply affecting one biogeographic unit more than another will have corresponding effects on rates of evolution in that unit. Measurement of such changes in nutrient supply are difficult. One possibility may be to...

Info

Concinnispirifer is erected to receive Paraspiriferinids having a normal spiriferoid fold and sulcus. Comparison. Paraspirifer has an inflated form and an acuminate fold and corresponding sulcus that are lacking in Concinnispirifer. Fimbrispirifer has a costate fold and sulcus as opposed to the smooth fold and sulcus present in Concinnispirifer. Exterior. Unequally biconvex shells with the pedicle valve having a greater degree of convexity than the brachial valve. The outline of the...

Devonian Biogeography

The Lower and Middle Devonian are times of high provincialism as contrasted to the preceding Silurian, particularly the Early Silurian, and the succeeding Late Devonian. The interval of highest provincialism coinciding with the highest production of new taxa per unit time and greatest development of recognizable biogeographic units is the Siegen Ems Eifel (Fig. 31). Preceding and following these times the Late Silurian andGivet (Fig.17,20,21,22) are transitional intervals of either lower rate...

Silurian Devonian Community and Biogeographic History

Sheehan (1973a, p.152) has raised the logical question concerning whether or not there was a reorganization of community associations and contents prior to the Silurian. The climactic Late Ordovician, Late Ashgill or earliest Llandovery extinction event discussed at some length here further underlines the importance of Sheehan's question. I have not carefully studied and am not very familiar with Ashgill communities. However, the general impression I possess is as follows. The Early Llandovery...

Table I

Depth-related Benthic Animal Assemblage zonal scheme employed in this book for the manyfold communities Lingula Community Eocoelia Community Lingula Benthic Marine Life Zone Benthic Assemblage 1 Eocoelia Benthic Marine Life Zone Benthic Assemblage 2 Pen tamerus Community Pentamerus Benthic Marine Life Benthic Assemblage 3 Stricklandia Community Stricklandia Benthic Marine Life Benthic Assemblage 4 Clorinda Community Pelagic Community Benthic Assemblage 5 Benthic Assemblage 6 Pelagic Community 1...

B

Benthic Assemblage 5-6 position or deeper there is no reason to assign the flysch facies in the Devonian to a shallower position. Second, the partly coeval Dicaelosia-Hedeina Community fauna of the Beck Pond Limestone occurring adjacent to flysch-type strata belongs somewhere in the Benthic Assemblage 4-5 position (Fig.7). Third, the presence of Starfish Community beds adjacent to the Beachia Community (Boucot and Yochelson, 1966) in one area indicates a deep lower range for the Beachia...

Appendix I

Descriptions of New Taxa Necessary for a Better Understanding of the Silurian and Devonian Problems Discussed in the Body of the Work Discussion. Johnson (1970a) introduced the rhipidomellinid genus Discomyorthis to receive shells possessing a large diductor field in the pedicle valve that contrast markedly with the closely related genus Dalejina. In addition to the internal character, Discomyorthis species tend to be much larger than representatives of Dalejina (except for Discomyorthis oblata...

References

Dolomitization by seepage refluxion. Bull.Am. Assoc. Petroleum Geologists, 44 1912 1920. Adey, W.H. and Macintyre, I.G., 1973. Crustose coralline algae a re-evaluation in the geological sciences. Bull. Geol. Soc. Am., 84 883 904. Aldridge, R.J., 1972. Llandovery conodonts from the Welsh Borderland. Bull. Brit. Mus. Nat. Hist. , 22 2 127-231. Amos, A. and Boucot, A.J., 1963. A revision of the brachiopod family Leptocoeliidae. Palaeontology, 6 440 457. Amsden,...

Plate Ii

Beds of probable Gedinne age. Vicinity of Ciudad Victoria, Tamaulipas, Mexico Peregrina Canyon . 1 9 Baturria mexicana, new species. 1, 2. Impression of interior of brachial valve and rubber replica of same 4 . Holotype USNM No. 181155. 3, 4. Impression of interior of pedicle valve and rubber replica of same 3 . 5. Impression of interior of brachial valve x 3 . USNM No. 181157. 6, 7. Rubber replica of exterior of pedicle valve and impression of exterior pedicle...