BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 43. PP. 875-916. PL. 24. 2 FIGS.. DECEMBER 30. 1932 GONDWANA LAND BRIDGES 1 BY CHARLES SCHUCHERT (Read before the Geological Society December 29, 1932) CONTENTS Page Introduction.................................................................................................................. 876 Summary........................................................................................................................ 877 Part I.—Geological evidence...................................................................................... 878 On permanency..................................................................................................... 878 Barbados as evidence of continental marginal oscillations.............................881 History of the theory of the Gondwana land bridge....................................... 882 How can Gondwana Land be submerged into oceanic depths?..................... 885 When did the Gondwana land bridge break down?........................................ 886 Conclusions............................................................................................................ 887 Part II.—Biogeographic evidence.............................................................................. 889 Summaries of biogeographic literature.............................................................. 889 Recent and Cenozoic invertebrates of the tropical Atlantic.......................... 890 Living brachiopods............................................................................................... 892 Living crustacea.................................................................................................... 893 Mesozoic ammonites............................................................................................ 894 Other Mesozoic evidence..................................................................................... 898 Echini.............................................................................................................. 898 Land molluscs................................................................................................ 898 Marine mammals.......................................................................................... 899 Land mammals of South America.............................................................. 899 Triassic land evidence in South America.......................................... 899 Permian evidence.................................................................................................. 900 Devonian marine evidence.................................................................................. 901 Silurian marine evidence...................................................................................... 902 Part III.—Evidence from larval life.......................................................................... 903 Distribution of larvae in general........................................................................ 903 Dispersal of marine invertebrates...................................................................... 904 Duration of larval life.......................................................................................... 906 General............................................................................. .............................. 906 Corals.............................................................................................................. 907 Echinoderms.................................................................................................. 907 General................................................................................................... 907 Echinoids (sea-urchins)........................................................................ 907 Starfishes................................................................................................ 908 Ophiurans (brittle-stars)...................................................................... 908 Bryozoa.......................................................................................................... 909 1 Manuscript received by the Secretary of the Society June 25, 1932. (875) LVII— B ull. Geol. Soc. Am., Vol. 4 3 ,1 9 3 2 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/43/4/875/3414951/BUL43_4-0875.pdf by guest on 27 September 2021 876 CHABLES SCHUCHERT---- GONDWANA LAND BRIDGES Page Brachiopods . 909 Mollusca.... 910 Crustacea... 911 Bibliography. 913 I ntroduction Nearly a century ago one school of Geology taught that every part of the land has once been beneath the sea, and every part of the oceans has once been land (Lyell). The theory of another school was: once a con­ tinent, always a continent; and its natural corollary: once an oceanic basin, always an oceanic basin (Dana). Later came an intermediate school which held that no proof exists that individual continents have always remained the same, and that parts of the ocean or even of the dry land may tomorrow sink to form new depths (Suess). The teach­ ing of the first-mentioned school has gone out of fashion, but the other two have much in common, one being strictly conservative and the other liberal in its interpretation of the permanency in the earth’s grander features. The writer belongs in the last-named school, holding that both continents and oceanic basins are, in the main, permanent features of the earth’s surface; but that they have not necessarily always had their present shape and area. The theory that land bridges once existed across the Atlantic and In­ dian oceans, postulated in the beginning by biogeographers' to explain their facts of life distribution, is popular in Europe but is not as gen­ erally accepted in America. One great difficulty with the theory is that thus far no method has been found by geologists or geophysicists to ex­ plain satisfactorily the making of such bridges, or their foundering. Having long held that the land bridges came into existence with the con­ tinents, the writer had not been concerned with their origin, but realized the difficulty of finding an explanation for their foundering. This problem formed the subject of many discussions with the late Professor Barrell, who came to the rescue of the theory by postulating that the dis­ integration of radioactive elements in the basaltic shell supplies that slow increment of heat necessary to generate new molten rocks locally, which work their way into and through the lithosphere, overloading it, so that under the principle of isostasy a mid-Atlantic bridge would sub­ side into the depths.2 This theory met with objections from geologists and geophysicists. 2 Barrell died in 1919 and among his unpublished papers was one written in 1917, en­ titled “The genesis of the earth,” a part of which was published in 1918 in “The evolution of the earth and its inhabitants” (Yale University Press). The part referred to above was published in 1927 with C. R. Longwell as editor. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/43/4/875/3414951/BUL43_4-0875.pdf by guest on 27 September 2021 BULL. GEOL. SOC. AM. VOL. 43, 1932, PL. 21 SYNTHETIC PALECGEOGRAPHIC MAP OF ALL PERMIAN TIME On Goode’s Homalographic Projection Oceans—white. Inland and shelf seas, and mediterraneans—green. Late Carboniferous in northern South America introduced to show geosynclines—green and dotted. Lands—brotvn. Land bridges and isthmian links—yellow. A, A, a probable isthmian link of Mesozoic time Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/43/4/875/3414951/BUL43_4-0875.pdf by guest on 27 September 2021 INTRODUCTION 877 After the death of Barrell, the writer’s attention was drawn more and more to other fields, and it was not until the publication of Bailey Willis’ book, “Living Africa” (1930), with its interesting tectonic ideas, that the old question of the mechanics of foundering came back strongly to his mind. Correspondence with Willis revealed that he had distinct ideas about continental fracturing, and the writer urged him to try to apply these to the explanation of land bridges. To illustrate the fact that the Schuchert bridge across the Atlantic from South America to Africa was much narrower than that of other authors, a Permian world map traveled out to the West Coast and came back to New Haven with western Gondwana still further shrunken to a series of “isthmian links.” The discussion of which the present papers are an outgrowth was based on this map, here published as plate 24, to show to what extent our somewhat divergent, but in a measure harmonious, views have approached agreement. To accompany the map the writer has again summarized some of the already familiar biogeographic evidence, added new data, and brought together evidence to show that the larval life of marine in­ vertebrates is far too short to explain their distribution across stretches of open ocean from one continent to another. In a second paper, Pro­ fessor Willis has stated his views of the dynamic problems and attempted to show how land bridges were raised, and how they subsided. S u m m a r y In geological literature the theory of a land bridge across the Atlantic appears to have had its origin with Marcou in 1860; but was first estab­ lished by Neumayr in 1887 and earlier, and placed in its world setting by Suess between 1885 and 1909. Neumayr called this great bridge (including South America and Africa) the Bmzil-Ethiopian continent, with