University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln

USGS Staff -- Published Research US Geological Survey

1965

Miocene and Central American Seaways

Frank C. Whitmore Jr. U.S. Geological Survey

Robert H. Stewart Panama Canal Company

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Whitmore, Frank C. Jr. and Stewart, Robert H., " Mammals and Central American Seaways" (1965). USGS Staff -- Published Research. 235. https://digitalcommons.unl.edu/usgsstaffpub/235

This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Science, New Series, Vol. 148, No. 3667 (Apr. 9, 1965), pp. 180-185 times have missed the target in his M. Planck, Forlesungen iiber die 7heorie der his paper (12), changing his prediction to speculations, as, for example, in his Wiirmestrahlung (Barth, Leipzig, 1906), pp. the statement that either diamond will ab- 163, 220. As a matter of fact, Planck rec- sorb at 11 microns or it will have cannot no ab- hypothesis of light quanta, really ognized the constant h as a fundamental sorption peak in the infrared, thus allowing be held too much against him, for it is natural constant even before he had intro- for the possibility that the vibrations are not possible to introduce really new ideas, duced the idea of energy quanta, at a time due to neutral atoms. See A. Einstein, Attnn. when h was simply a particular combina- even in the most exact sciences, without Physik 22, 800 (1907). tion of the constants in the Wien distribu- 25. F. A. Lindemann, Physik. Z. 11, 609 (1910). sometimes taking a risk. tion law. See M. Planck, Ann. Physik 1, Preiss. Akad. Wiss. Ber- 120 I thank 26. W. Nernst, Sitzber. (1900); Dr. Joseph Agassi, who Z. Elek- first pointed this out to me. lin, Kl. Math. Phys. 1911, p. 306; References and Notes trochemn. 17, 265 (1911). The quotation is 14. P. Ehrenfest, Antn. Physik 36, 91 (1911). from p. 275 of the latter paper. 1. The proceedings of the first Solvay 15. See, for example, W. Nernst, Theoretische this, 27. W. Nernst, Sitzber. Preuss. Akad. Wiss. Ber- Congress on Physics, were published under Chemie (Enke, Stuttgart, ed. 3, 1900), p. lin, Ki. Math. Phys. 1911, 65 The the editorship of P. Langevin and M. de 175. (1911). quotation is from p. 86. La Thleorie du Rayonnement et les 16. J. W. Strutt, Lord Rayleigh, Phil. Mag. 49, Broglie, F. A. Z. Elek- Quanta (Gauthier-Villars, Paris, 1912). 98 (1900). Rayleigh's remark is quoted by 28. W. Nernst and Lindematin, trochetm. 817 2. This letter is quoted in full in an unpub- Kelvin in his famous lecture "Nineteenth 17, (1911). 29. A. Annt. 170 (1911). lished manuscript by J. Pelseneer, "Historique century clouds over the dynamical theory of Einstein, Physik 34, des Instituts Internationaux de Physique et heat and light," in W. Thomson, Lord Kel- 30. WV.Sutherland, Phil. Mag. 20, 657 (1910). de Chimie Solvay." I should like to thank vin, Baltimore Lectures (Clay, London, 1904), 31. E. Madelung, Physik. Z. 11, 898 (1910). John Heilbron of the project, "Sources for p. 527. 32. A. Einstein, Ann. Physik 35, 679 (1911). History of Quantum Physics," for the loan 17. Optical absorption frequencies were cldeter- 33. W. Nernst and F. A. Lindemann, Sitzber. of this manuscript, and Prof. Pelseneer for mined by the method of "residual rays" de- Preuss. Akad. Wiss. Berlin, KI. Math. Phys. permission to quote from it. veloped by H. Rubens. See, for example, 1911, p. 494. 3. Solvay's letter and Nernst's original German 0. W. Richardson, The Electrotl Theory of 34. The detailed theory of the spectrum was idraft are to be found in the Lorentz Col- Matter (Cambridge Univ. Press, Cam- begun by M. Born and T. von Karman, lection at the Algemeen Rijksarchief in the bridge, 1914), p. 157. Physik. Z. 13, 297 (1912), and almost simul- Wiss. Geittin- Hague. 18. W. Nernst, Nachr. Kgl. Ges. taneously an eminently successful approxi- Kl. 1. For a 4. M. Planck, Verhandl. Deut. Physik. Ges. 2, getn, Math. Phys. 1906, p. mate theory was given by P. Debye, Aisnn. review of Nernst's work see F. Simon's 789 Both refer 237 (1900). See M. J. Klein, Arch. Hist. Physik 39, (1912). papers Guthrie Lecture in Yearbook of the Physical Exact Sci. 1, 459 (1962), for a detailed dis- to Einstein's two articles of 1911. London 1. cussion and bibliography. Society of 19556, p. 35. N. Bohr, Essays 1958-1962 otn Atoomic and Chemistry 5. M. Planck, Ann. Physik 31, 758 19. W. Nernst, Thermodynamics Physics anid Hzttuman Knoswledge (Interscience, (1910). Univ. New Haven, 1913), pp. 6. See M. Planck, Scientific Autobiography (Yale Press, New York, 1963), pp. 31, 83. New York, 63-64. 36. L. de Broglie, Physicien et Peniseur (Edi- (Philosophical Library, 1949). W. Sitzber. Preluss. Akad. Wiss. Ber- 7. Quoted by J. Pelseneer 20. Nernst, tions Albin Michel, Paris, 1953), p. 458; (2). Kl. Math. Phys. 1906, 933. 8. A. Z. 10, 185, 817 (1909); lin, p. see also p. 425 for Maurice de Broglie's Einstein, Physik. 21. See, for example, J. Dewar, Proc. Roy. Soc. see also L. Rosenfeld, Osiris 2, 149 (1936). comments, as well as M. de Broglie. Les Lotidon Ser. A 76, 330 (1905). The new Premiers de 9. F. Reiche, The Quantumi Theory (Dutton, at Nernst's Congres Physique Sol'ay (Edi- technioue was developed sugges- tions Albin Michel, Paris, 1951). New York. 1922). tion A. Eucken, Physik. Z. 10, 586 (1909). by 37. F. W. F. Smith, Earl of Birkenhead, The 10. A. Einstein, Annt. Physik 17, 132 (1905); 22. W. Sitzber. Preutss. Akad. Wiss. Ber- Nernst, and the Primne Minister see also M. J. Klein, in The Natural Philoso- Kl. Math. see Professor (Houghton lin, Phys. 1910, p. 262; par- 43. pher, D. Gershenson and D. Greenberg, ticularly pp. 276 and 282. Mifflin, Boston, 1962), p. Hist. Sci. Eds. (Blaisdell, New York, 1963), vol. 2, 23. W. Nernst, J. Phys. Theor. Appl. 9, 721 38. T. Kahan, Arch. Intern. 15, 337 p. 57. (1910). (1962). Kahan quotes the German text and 11. For a detailed analysis see M. J. Klein, in 24. Actually Einstein originally thought that gives a French translation. An English ver- The Natural Philosopher, D. Gershenson and only the vibrations of positive ions were in- sion is included in C. Seelig, Albert Einstein D. Greenberg, Eds. (Blaisdell, New York, volved, and on this basis predicted that (Staples, London, 1956), pp. 144-146. 1964), vol. 3, p. 1. diamond should have an absorption peak at 39. This work was supported in part by the 12. A. Einstein, Ann. Physik 22, 180 (1907). 11 microns. He recognized his error several National Carbon Company, a division of 13. M. Planck, Antn. Physik 4, 564 (1901); months later and published a correction to Union Carbide Corporation.

one or more straits of some size be- tween North and South America, we are uncertain as to the number of straits, their location, and the time of their existence. It is almost certain that Miocene Mammals and there were straits across the Isthmus of Panama (3), and the distribution Central American of Tertiary marine deposits indicates Seaways a major seaway in northwestern Co- lombia, the Bolivar Trough (Fig. 1). Fauna of the Canal Zone indicates separation of Thus, the separation of North from South America did not depend on the Central and South America during most of the Tertiary. opening of a single strait. In Central America as a whole, however, the dis- tribution of land and water during Ter- Frank C. Whitmore, Jr., and Robert H. Stewart tiary time can be only approximately delineated, because geologic observa- tions, as well as finds, are rela- tively few and scattered .over a large The fossil faunas of North the geology of this large area, makes tropical region where there are few and South America indicate that the reconstruction of the Tertiary zooge- rock outcrops. A difficulty that faces two continents were separated from ography of the region between the con- the paleogeographer studying the isth- Paleocene or earlier time until the late tinents an exercise involving consider- mian portion of Central America is Pliocene (1, 2). Unfortunately our al- able speculation. While the differences most complete ignorance of the Terti- between the faunas of the two con- Mr. Whitmore is a paleontologist on the staff from and of the U.S. Geological Survey, Washington, D.C. ary land mammals of Central America, tinents, resulting complete Mr. Stewart is a geologist with the Panama together with incomplete knowledge of long separation, prove the existence of Canal Company, Balboa, Canal Zone.

180 SCIENCE, VOL. 148 the large amount of tectonic activity their patterns of distribution on the mammals in Middle America. The col- that characterizes it to this day. In islands) to have come from Central lection also includes bones of turtles, this the student of the Isthmus of America, and they include , es- part of the skeleton of a large croco- Panama has problems in common with pecially hystricomorph rodents, of sorts dile, and innumerable coprolites. The the paleogeographer who tries to un- which would probably not have been mammals in the fauna are of un- ravel the history of an island arc. We in Central America if it had been con- doubted North American affinity. Four know that faulting and folding were nected to North America." Darlington mammalian families are represented: practically continuous in the isthmian thus regards Central America as hav- the Equidae, the Rhinocerotidae, the area for a long time, thus it is risky ing been an island in Tertiary time, Merycoidodontidae, and probably the to postulate the stability required for from which animals migrated eastward Protoceratidae. the existence of a land bridge (we use into the West Indies. Simpson, on the This locality is the southernmost the term in its literal sense). It is other hand, is of the opinion (1, p. point in the Western Hemisphere at more likely that the area of the Isth- 388) that "with local and geologically which members of the last three of mus of Panama was characterized brief interruptions for part of it, Cen- these families have been found (the through most of the Tertiary by a tral America has been continuous with Equidae reached South America dur- shifting pattern of island groups and the North American land mass ing the Pleistocene). It must be nearly of peninsulas attached to one continent throughout the Cenozoic." This con- the southernmost point in the West- or the other. clusion is also based largely upon analy- ern Hemisphere reached, in Tertiary An attempt to reconstruct the pale- sis of the living fauna of the region. times, by land mammals moving across ogeography of the Isthmus of Panama Both these authors regard Central (or, a continuous land area, as contrasted almost inevitably starts with considera- more broadly, Middle) America as an with forms, such as rodents, primates, tion of the Canal Zone. As a result evolutionary center. and raccoons, that reached South of the construction and maintenance America by island hopping (1). The of the Canal, the geology of the Zone fossil material now available indicates Fossil is better known than that of the rest Mammals of the that the mammals of the Cucaracha of Central America (4, p. 2). Marine Cucaracha Formation Formation were undifferentiated mem- and brackish-water beds of Eocene to bers of a well-established North Amer- Pliocene age have been described. From In contrast to the abundance of Ter- ican fauna; four of the five recogniz- their distribution across the narrow tiary invertebrates in the Isthmus of able genera in the collection are con- isthmus it is reasonable to assume that Panama, only a few scraps of terrestrial generic with forms found widely in these beds were deposited in a Terti- mammal bones had been found there North America, and the fifth belongs ary seaway that can appropriately be until recently, despite many years of to a North American family. As re- called the Strait of Panama. The in- intermittent prospecting (4, p. 37). In gards the larger herbivores, at least, fauna found in these beds 1962 Stewart discovered fragmentary this information lends no support to permits an age definition of the strait but identifiable mammal bones in an Simpson's suggestion (1) that "Middle that embraces the period during which area of Miocene terrestrial deposits America must have been an important the North and South American mam- that had been exposed in the course center of regional faunal differentia- mal faunas were separated from each of engineering operations along the tion within the North American gen- other and thus pursued divergent evo- right-of-way of the Panama Canal. As- eral fauna," or to Darlington's conten- lutionary paths. However, when one sisted by Joanne Allen, he collected tion (5, p. 285) that Central America attempts to define very closely the time the first of about 100 fragments of was an island throughout much of the of opening or closing of this strait, or mammal bone from the Cucaracha Tertiary. The concept that North and to determine whether it opened and Formation, a unit consisting mainly of Central America were continuously closed more than once, or even to de- bentonitic clay and siltstone. The fos- connected during the Tertiary is rein- termine precisely where it was, lack of sil bones are found over an area about forced by the presence of a Pliocene data prevents the formulation of a pre- 2/5 kilometer long, parallel to the mammal fauna of North American cise answer. Thus, isolated fossil finds, Canal in Gaillard Cut; the belt of type in Honduras, described by Olson even of poorly preserved material, and outcrop is about 90 meters wide. In and McGrew (6). correlations over long distances assume this area the Cucaracha Formation dips The most common mammal at the greater in importance studying the Pan- about 30? to the southwest. The fossil Cucaracha locality is a medium-sized ama area than would be accorded such bones are weathered out on the sur- selenodont artiodactyl here tentatively information in most geologic studies. face or in the slightly weathered clay assigned to the Protoceratidae. Its low- Because of this paucity of , near the surface of the outcrop. They er premolars are unreduced and show students of the terrestrial zoogeography appear to occur in several bands 1.2 to no sign of modification and increasing of Central America have been forced 1.5 meters thick distributed through a resemblance to the molars. This is a to reconstruct the history of the area stratigraphic thickness of some 45 primitive characteristic in selenodont on the basis of the largely distribution meters. artiodactyls and indicates a stage of of the modern fauna. Darlington (5, The Cucaracha bones are the first evolution that can roughly be called believes p. 285) that Central America identifiable Tertiary mammal remains . (These fossils bear a fa- was cut off probably completely by wa- found between Honduras and Colom- milial resemblance to Protoceras, a ter from North as well as South bia and the first Miocene mammal fos- well-known artiodactyl of the late America through much of the Terti- sils found between southern Mexico Oligocene of the High Plains; the pre- ary. "My reason for he thinking this," and Colombia, a distance by land of molars of Protoceras are more reduced, says, "is that of the older many West some 2000 kilometers. They are the and thus the genus is probably more Indian seem from (judging second discovery of Miocene land advanced, than is the Panama genus.) 9 APRIL 1965 181 The rhinoceros material from the cene of Florida and the late Lower either late Lower or early Middle Mio- Cucaracha consists of a partial lower Miocene of the Texas Gulf Coastal cene deposits; the tooth of Anchi- jaw of a young individual, a separate Plain. therium resembles most closely its late lower molar, one and one-half upper The family Merycoidodontidae Lower Miocene relatives to the north. molars, a lower premolar, a first up- (oreodonts) is represented by the low- Diceratherium is found throughout the per incisor, and a few fragments of er jaw of a very large adult and by Miocene. The only member of the limb bones. The upper incisor, an the badly crushed skull and lower jaw fauna that is seriously out of line with anteroposteriorly elongated tooth, is of a young . this restricted age span is also the typical of the true rhinoceroses (fam- The adult specimen is 275 millime- most common-the unnamed pro- ily Rhinocerotidae) as opposed to the ters long, exceeding in length all but toceratid, which is probably a relict extinct families Hyrachyidae, Hyra- a few of approximately one hundred form surviving, at the periphery of the codontidae, and Amynodontidae. The known species of the family. It is as- range of its family, from the Oligo- conformation of the upper incisor and signed to the genus Merycochoerus, cene fauna. the degree of complication of the up- hitherto known from early middle Mio- Assignment of an age for the Cu- per molars are consonant with a Mio- cene beds in Colorado, Nebraska, caracha fauna must depend upon those cene age. The specimens are assigned South Dakota, and Wyoming. members that resemble the geological- to the genus Diceratherium, which was The immature skull has well-pre- ly youngest forms found in dated se- widespread in the North American served molar teeth; the third molars quences elsewhere. The Cucaracha ore- Miocene. had not yet erupted. The incisors and odonts fall in this category, and are Two upper cheek teeth of horses canines are present but are badly brok- thus the basis on which we regard the are present in the collection. Both are en. A fragment of the maxillo-pre- Cucaracha Formation as early Middle brachydont (low-crowned), and there- maxillary region shows a longitudinal Miocene. This fauna was peripheral to fore come from horses that subsisted depression in the premaxillary surface, the North American mammal fauna; by browsing rather than by grazing. as in the oreodont Brachycrus of the therefore migration must have been to, Two genera are represented. The larger middle and late Miocene of California, rather than from, the Cucaracha area. tooth belongs to Anchitherium, known Montana, Nebraska, New Mexico, and Accordingly, any animals of pre-Mid- from the Lower and Middle Miocene Wyoming. The lower jaw, however, dle-Miocene aspect in the Cucaracha of North America, the Middle and Up- does not appear to be deep posteriorly, fauna are regarded as relicts. per Miocene of Europe, and the Mio- as is typical of Brachycrus; it has a In contrast to the age assignment cene of Asia. The Florida tooth is in more gently curved ventral border and based upon mammals, Woodring (4, an early stage of wear. A constriction a lower ascending ramus, as does p. 39) assigns the Cucaracha Forma- separates the protocone from the pro- Merycochoerus, which may be ances- tion to the early Miocene because he toconule. The hypocone is separated tral to Brachycrus. This combination considers both the underlying Culebra from the metaconule, but would be of characters is interpreted to mean Formation and the overlying La Boca connected with it after additional tooth that the immature Panama skull rep- Formation (9) to be of that age. He wear. The hypostyle is triangular and resents a transition stage between these bases his view that the Culebra is of appressed against the posterior side of two genera. It can thus be placed in early Miocene age on the resemblance the metaloph. In these characteristics an evolutionary series of known range of its corals and mollusks to those of the Panama Anchitherium represents an in the North American stratigraphic the Anguilla Formation of Anguilla in evolutionary stage similar to that of sequence (7). Using the evolutionary the Lesser Antilles and to corals and A nchitherium agatense (Osborn) of stages of oreodonts of the High Plains mollusks of other formations of the the Lower Miocene of Wyoming and of the United States as a standard for same age, including the Tampa Lime- Nebraska. It appears to be slightly comparison, we find the immature Cu- stone of Florida. The fauna of the La more primitive than A. clarencei Simp- caracha oreodont to represent the same Boca Formation, including Foramini- son of the late Lower Miocene of stage as late Marsland or early Sheep fera, corals, echinoids, and mollusks, Florida and A. navasotae (Hay) of Creek faunas (8). This places it in differs little in age, according to Wood- the late Lower Miocene of Texas. the early middle Miocene-that is, ring, from the Culebra, but represents The smaller of the two horse teeth early time in the North a somewhat different facies. is almost unworn. It is assigned to American provincial age sequence. The five mammalian genera of the Archaeohippus, a small browsing horse Cucaracha fauna, all browsing ungu- found in Miocene deposits of the Pa- lates, represent an herbivorous fauna cific coast, Rocky Mountain region, Age of the Cucaracha Formation that was spread during Miocene time High Plains, Gulf Coast, and Mary- from coast to coast in what is now land. In the course of evolution of Comparison of the Cucaracha mam- the United States. This fauna is well Archaeohippus additional folds of the mals with their nearest known rela- known in the High Plains and Pacific tooth enamel, especially of the hypo- tives in other faunas leads to the con- Northwest but is represented in Coastal style and crochet, tended to close the clusion that they are of early Middle Plain deposits only in a few widely enamel lakes (pre- and postfossettes) Miocene age. The most advanced evo- scattered localities (Fig. 1). The wide that would eventually become a ma- lutionary stage, by comparison with range and apparent homogeneity of the these jor feature of the horse tooth. In the North American faunas, is seen in the fauna are, however, indicated by Panama Archaeohippus this process Panama oreodonts, both of which ex- occurrences, even though the speci- had reached about the same stage as hibit characteristics typical of Middle mens known from some of the ex- so that seen in A. blackbergi (Hay) of Miocene members of their family. The tremes of the population range are little the late Lower to early Middle Mio- Archaeohipptus tooth could be from fragmentary that they contribute VOL. 148 182 SCIENCE, to the morphology or taxonomy of the The oreodonts (Merycoidodontidae) the northeast as the island of Martha's group they represent. are remarkably numerous and wide- Vineyard, Massachusetts. The Protoceratidae compose a small spread in North American deposits of The browsing horses A nchitherium family (four or five genera) known Oligocene through Pliocene age. Until and A rchaeohippus probably ranged from the Oligocene and Miocene of oreodonts were discovered at the Cu- over much of what is now the United the High Plains of the United States caracha locality they were unknown States in Miocene time; both have and the Miocene of the Texas Coastal outside of North America, yet they been found at localities on the West Plain and of Florida (Fig. 1, N1, N2). constitute an important element of the Coast, in the Rocky Mountain area, and Their low-crowned teeth, light body fauna in the areas where they occur. in the High Plains, as well as on the build, and tendency to develop horns In Miocene time they were especially Gulf Coast, in Florida, and, in the case give the impression that they may have numerous in the High Plains, and in of Archaeohippus, in the Calvert For- occupied an adaptive zone similar to the John Day region of eastern Ore- mation of Maryland. Anchitherium that of the modern Cervidae. They gon; they are known from California also lived, in Miocene time, in Mon- differ from the latter, however, in and have recently been found in Big golia and, in the middle and late Mio- that their horns are bony out-growths Bend National Park, Texas (10) (Fig. cene, in Europe. It is believed to have of the skull rather than antlers. The 1, N3), but none has been identified migrated from North America to the few known specimens of this family with certainty from the Gulf Coastal Old World. exhibit an amazing variety of grotesque Plain or from the eastern United The presence of Diceratherium, An- horn forms, suggesting that the family States. chitherium, and Archaeohippus in may have included a range of types The rhinoceros Diceratherium is ex- Florida, Texas, and Panama hints at similar in extent to that of the modern tremely abundant in the Miocene of the presence of a homogeneous Mio- antelopes. Their rarity probably results South Dakota and Nebraska, and in cene mammal fauna on the North and from their having lived in a woodland the John Day region; it has been Central American coasts of the Carib- habitat, where bones are unlikely to be found in the Gulf Coastal Plain and bean Sea. It is probable that this fauna preserved. in Florida (11); it occurs as far to inhabited a subtropical environment.

Fig. 1. Miocene land-mammal localities in the circum-Caribbean area. N, faunas of North American type; S, faunas of South Ameri- can type. Hatched areas show distribution of Miocene clastic deposits (19). (N1) Five North Florida localities, early- to late-Mio- cene (20). (N2) Oakville, Burkeville, Cold Spring, and Lapara faunas, early- to late-Miocene, Texas Gulf Coastal Plain (11). (N3) Early Miocene fauna, Big Bend National Park, Texas (10). (N4) El Gramal fauna, late Miocene, Oaxaca, Mexico (21). (N5) Cucaracha fauna, early middle Miocene, Panama Canal Zone. (SI) Le Venta and Carmen de Apicala faunas, late Miocene, Colombia (17). (S2) Penfiatafauna, late Miocene or early Pliocene, Colombia (18). (S3) Two occurrences, one late Miocene, Pliocene, or possibly Pleistocene, and one probably Miocene, Falcon Province, Venezuela (22, 23). (S4) Single Miocene occurrence, western Portuguesa Province, Venezuela (24). (S5) Single Miocene occurrence, northeastern Guarico Province, Venezuela (25). (S6) Springvale beds, late Miocene, Trinidad (23). 9 APRIL 1965 183 Cucaracha Vertebrate Paleoecology

The only associated bones of one individual found in the formation com- pose the partial, headless skeleton of a large crocodile. Fecal pellets, prob- ably derived from crocodiles, are very numerous. Turtle shell fragments are common. The mammal bones are brok- en and scattered. A disproportionately large number of them are from young animals. One partial lower jaw of a protoceratid, removed from a concre- tion, has been deeply etched, possibly by digestive acids; even the tooth en- amel has been affected. The Cucaracha sediment contains a considerable of detrital The Cu- amount quartz. Fig. 2. Southern Central America and northwestern South America in early middle caracha Formation at the mammal lo- Miocene time. (N5) Cucaracha Miocene mammal site. [Modified from Lloyd (3) and cality probably was deposited in a Jacobs, Biirgl, and Conley (26)] swamp, the mammal remains deriving from nearby forest (source of the Pro- toceratidae and horses) and savanna marine Miocene deposits north of the marine sediments in the region (4, p. (source of oreodont and rhinoceros). northwest end of the Talamanca Range 10) indicates that the San Blas Area Associated with the vertebrate remains (Figs. 1 and 2) may mean that cen- was an island in both early and late were a few fossil fragments of the tral Costa Rica, as well as the Panama Miocene time; in early middle Mio- wood of Schwartzia, a legume that still Canal Zone region, was intermittently cene time, when the Cucaracha mam- grows in the area. submerged during the Miocene. The mal fauna lived, it was connected to Nicaraguan Depression (Fig. 2) may the stable area to the west (Fig. 2). also have been a transcontinental In discussing the Bolivar Trough Paleogeography of Panama trough during part of Miocene time. (Fig. 1), Olsson (13) points out that in Cucaracha Time Mammal migration from North Amer- Miocene mollusks of distinctly Carib- ica to Panama may have occurred in bean character are found as far south The presence in Panama of wide- several stages, as water barriers ap- as northern Peru, and that faunal in- ranging members of the North Ameri- peared and disappeared in front of and termingling between Atlantic and Pa- can ungulate fauna probably indicates behind the migrants, but the similarity cific took place through the Miocene that, in middle Miocene time, this por- of the Cucaracha mammals to mam- along this geosynclinal area, which tion of the isthmus was connected to mals of North America indicates that was 100 to 160 kilometers wide. B. F. North America by a land area of con- such barriers were short-lived. Uhl (14) has not yet found evidence siderable size and stability. This is an- The presence of North American of intermingling of Caribbean and Pa- other argument against the sometimes mammals at the Cucaracha locality cific forms in the foraminiferal faunas postulated existence of a Tertiary sea- also requires reconsideration of the of the upper middle Miocene of the way across the Isthmus of Tehuantepec paleogeography of the area east of the Bolivar geosyncline. Partly on the basis in southern Mexico (12). On the other site. Lloyd (3) and most earlier work- of lack of such evidence, Nygren (15) hand, by contrast with nuclear Central ers have regarded the arc of eastern concludes that the Bolivar Trough was America to the west (Fig. 2), the Ca- Panama from the Colombian border closed from upper Miocene to Recent nal Zone area was unstable. Marine to Gatun Lake as a part of the South time. As a result of his study of the deposits in the area where the Cu- American continent that did not partici- distribution and thickness of marine caracha malmmal fauna was found in- pate in the instability which marked and terrestrial sediments in the Bolivar dicate inundation by the sea both be- the Tertiary history of the isthmian geosyncline, Nygren is of the opinion fore and after the time when the mam- strip. However, the existence of the that migration of terrestrial animals mals arrived there by land from North Bolivar Trough through most of Ter- could have taken place through the America. Thus, the area now occupied tiary time indicates that this area, pre- area between upper and by the Canal Zone probably was the viously called the Panama Spur, was middle Eocene time; during the mid- site of narrow seaways which were not connected to the South American dle Oligocene, the lower Miocene, and closed several times during the Terti- continent until the very late Tertiary. the middle Miocene; and between the ary by uplift of the land. It existed, rather, as a large stable is- upper Miocene and the Recent. Such The age of the Cucaracha mammal land which was at times connected to conditions in the Bolivar area may fauna seems to indicate that, by early the land mass to the west, the Talaman- have facilitated migration of the few middle Miocene time, extensive emer- ca Range (Fig. 2). Since the term spur mammals designated by Simpson (1) gence had connected nuclear Central emphasizes the attachment to South as ancient immigrants (early Paleo- America with the area of the present America, this stable region is here des- cene), old island hoppers (late Eocene Canal Zone. A possible migration route ignated, instead, the San Blas Area and Oligocene), and late island hoppers is shown in Fig. 2. The presence of (Figs. 1 and 2). The distribution of (middle Miocene through Pliocene); 184 SCIENCE, VOL. 148 but faunal evidence, both fossil and Summary 5. P. J. Darlington, Jr., Zoogeography (Wiley, New York, 1957). Recent, indicates that general move- 6. E. C. Olson and P. 0. McGrew, Bull. Geol. ment of fauna between the continents The presence of Miocene mammals Soc. Am. 52, 1219 (1941). 7. C. B. Schultz and C. H. Falkenbach, Bull. did not take place during these in- of North American affinity in the Pan- Am. Museum Nat. Hist. 77, 213 (1940). tervals. ama Canal Zone indicates that Central 8. H. E. Wood, 2nd, R. W. Chaney, J. Clark, E. H. Colbert, G. L. Jepsen, J. B. Reeside, During periods of emergence of America was attached to North Amer- Jr., C. Stock, Bull. Geol. Soc. Am. 52, 25 the Bolivar its northern and ica. That this attachment was a broad (1941); ibid., p. 31. Trough, 9. W. P. Woodring, U.S. Geol. Surv. Profess. southern extremities appear to have and stable land mass is shown by the Paper 306-C, 244 (1964). last to rise close relation of the Panama Miocene 10. J. A. Wilson, personal communication. been the areas above sea 11. -, in Geology of the Gulf Coast and level (16). Thus, the trough could herbivores to the widely distributed Central Texas (Houston Geological Society, Houston, Texas, 1962), pp. 348-350. have been a barrier to intercontinental Miocene herbivore fauna of North 12. J. W. Durham, A. R. V. Arellano, J. H. migration even during the last stages America. A continuous connection Peck, Jr., Bull. Geol. Soc. Am. 66, 977 (1955). of emergence. When the Bolivar existed probably throughout the Terti- 13. A. A. Olsson, Bull. Am. Paleontol. 19, 68 Trough was not a barrier, other bar- ary, to the west and north of the isth- (1932); ibid., p. 18. 14. B. F. Uhl, cited in W. E. Nygren, Bull. Am. riers, possibly including climatic ones, mian region, but the tectonically active Assoc. Petrol. Geologists 34, 2005 (1950). apparently were operative during most isthmus probably was broken up into 15. W. E. Nygren, ibid., p. 2006. 16. H. J. Harrington, ibid. 46, 1804 (1962). of Tertiary time. The idea of climatic an archipelago during most of Terti- 17. R. A. Stirton, Bull. Geol. Soc. Am. 64, 612 barriers is appealing because Dicera- ary time. Between the islands ran the (1953). 18. D. E. Savage, Pacific Discovery 5, 18 (1952). therium and Brachycrus, of the Cu- Strait of Panama; from time to time 19. The geology is generalized from the follow- caracha fauna, certainly were not tropi- parts of the isthmian area were con- ing: T. E. White, Bull. Museum Comp. Zool. 92, 1, Fig. 9 (1942); G. W. Stose (compiler), cal forms, occurring as they did as fai nected to the stable land to the west, "Geologic Map of the United States" (U.S. north as Massachusetts and allowing eastward migration of land Geological Survey, Washington, D.C., 1932); Montana, S. H. S. Nejorada, "Carta Geologica de la respectively. Darlington (5, p. 593), animals. The mammals of North Amer- Republica Mexicana" (Comit6 de la Carta Geologica de Mexico, 1960); R. J. Roberts on the basis of the present distribu- ican affinity in the Cucaracha Forma- and E. M. Irving, U.S. Geol. Surv. Bull. tion of birds and the past history of tion were found only a few kilometers 1034, PI. I (1957); G. W. Stose, "Geologic map of South America" (Geological Society mammals, concludes that no important from the western end of the San Blas of America, New York, 1950). tropical area was attached to North Area, a stable land mass in eastern 20. S. J. Olsen, J. Paleontol. 38, 600 (1964); H. E. Wood, 2nd, Bull. Museum Comp. Zool. America during the Tertiary. The com- Panama that was separated from South 130, 378 (1964). position of the Cucaracha fauna sup- America by the Bolivar Trough during 21. R. A. Stirton, Am. J. Sci. 252, 634 (1954). 22. A. T. Hopwood, Quart. J. Geol. Soc. Lon- ports this conclusion. The northern most of the interval between Oligo- don 84, 573 (1928); R. van Frank, Am. Mu- of the realm of Mio- cene and Pliocene time The seum Novitates 1850, 22 (1957). edge neo-tropical (16). 23. S. Schaub, Abhandl. Schweiz. Palaeontol. cene time may have been, as Nygren Strait of Panama was a less stable bar- Ges. 55, 1 (1935). implies, in the northern part of rier than the Bolivar this be- 24. R. L. Collins, Johns Hopkins Univ. Studies (15) Trough; Geol. 11, 235 (1934). the Bolivar geosyncline. ing so, it is likely that the San Blas 25. H. G. Stehlin, Eclogae Geol. Helv. 21, 227 The contrast between Central Amer- (1928). Area was inhabited by land animals 26. C. Jacobs, H. Birgl, D. L. Conley, Am. ican and South American Miocene of North American rather than South Assoc. Petrol. Geologists Mem. 2 (1963), mammal faunas is pp. 62-72. particularly sharp American affinity. Thus, the disap- 27. We are grateful to Robert J. Fleming, Jr., when one compares the mammals from pearance of the Bolivar seaway in Governor of the Canal Zone, for encourage- ment and logistic support of extensive field the Cucaracha locality (Fig. 1, N5) Pliocene time would have allowed, work by Stewart and of fossil collecting by and those of the La Venta fauna of probably for the first time, mingling of Whitmore. We thank Childs Frick and Bryan Patterson for permission to study collections Colombia (Fig. 1, S 1), a varied as- the North and South American mam- under their care at the Frick Laboratory of semblage of late Miocene mammals, mal faunas. the American Museum of Natural History and at the Museum of Comparative Zoology composed completely of South Ameri- at Harvard College, respectively, and Mr. can forms The other South Frick and Clayton E. Ray, then of the Florida (17, 18). References and Notes State Museum, for the loan of specimens. American localities plotted in Fig. 1 We are grateful to C. B. Schultz, H. E. 1. G. G. Simpson, Am. Scientist 38, 363 (1950). Wood, 2nd, L. G. Tanner, and Beryl Taylor represent occurrences of only a few 2. -, ibid., p. 3; R. A. Stirton, Bull. Geol. for discussions of the fossil mammal fauna mammal bones, most of them of un- Soc. Am. 61, 1541 (1950); B. Patterson, Brev. and, especially, to W. P. Woodring and C. Museum Comp. Zool. Harvard 94, 11 (1958). E. Ray for review of the manuscript. Publi- certain age, and further emphasize the 3. J. J. Lloyd, Am. Assoc. Petrol. Geologists cation of this article is authorized by the paucity of Tertiary land mammals in Mem. 2 (1963), pp. 88-100. Director of the U.S. Geological Survey, 4. W. P. Woodring, U.S. Geol. Surv. Profess. Washington, D.C., and the Governor of the the circum-Caribbean area. Paper 306-A (1957). Panama Canal Zone.

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