Access

You are not currently logged in.

Access your personal account or get JSTOR access through your library or other institution:

login

Log in to your personal account or through your institution.

If You Use a Screen Reader

This content is available through Read Online (Free) program, which relies on page scans. Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.

Holarctic Landmass Rearrangement, Cosmic Events, and Cenozoic Terrestrial Organisms

Malcolm C. McKenna
Annals of the Missouri Botanical Garden
Vol. 70, No. 3 (1983), pp. 459-489
DOI: 10.2307/2992083
Stable URL: http://www.jstor.org/stable/2992083
Page Count: 31
  • Read Online (Free)
  • Subscribe ($19.50)
  • Cite this Item
Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Holarctic Landmass Rearrangement, Cosmic Events, and Cenozoic Terrestrial Organisms
Preview not available

Abstract

Rearrangement of landmass configurations has profoundly affected the evolution of mammals and other organisms in the Northern Hemisphere. At the north end of the Atlantic Ocean, evidence is very strong for a now sundered landmass of Euramerica. This landmass existed toward the end of the Paleocene but began to separate into at least two landmasses in early Ypresian time at the beginning of the Eocene. Throughout most of the Eocene, Western Europe south of the Baltic Sea remained fragmented and separate from both Asia and the part of Europe and the attached Barents Shelf that lies north of the Baltic. Northern Scandinavia may have been essentially a peninsula of North America until the end of Eocene time. Thus at the end of the Paleocene and until some point very early in the Eocene, two land bridges may have simultaneously connected North America with separate parts of Europe across the north end of the Atlantic: one dominated by compression in the Greenland/Svalbard area plus any excess lava pile generated by early activity of the Yermak hotspot northwest of Spitsbergen, the other by the early activities of the Icelandic hotspot. Possibly a connection from western Asia and Europe to the northeastern end of North America existed at about 37 million years ago (Ma) through the combined activities of the Yermak hotspot, waning compression between Greenland and Svalbard, and the drying of Eurasian seaways. Throughout nearly all of the Cenozoic a land connection between Asia and Alaska existed at the Bering Strait, but the paleolatitude of the Bering Bridge was initially high and climate must have exerted a strong filtering action on mammals and other terrestrial organisms. It is possible, but only weakly supported on geological grounds, that a second northern Pacific bridge between Asia and North America existed via the Aleutians at the time the Kula Ridge was subducted. Other rearrangements that have strongly affected the composition of northern mammalian faunas and presumably would have affected many other organisms as well have been the reconnection of South America to North America in the Pliocene, the collision of India with Asia in the early Cenozoic (possibly earlier), and various Tethyan microplate transfers. Although paleobotanists and others have sometimes suggested that the warm polar climates of the early Cenozoic were the result of a lower mean spin axis obliquity of the Earth than the present 23.5⚬, no one has explained adequately how mean spin axis obliquity can be permanently changed significantly in the Cenozoic, where the necessary force would come from to shift it, how such a force would be coupled to the Earth, or how the resulting crustal heat would be dissipated. At present, therefore, known high latitude early Cenozoic environments of mammals and other organisms are not well understood, but a significant change of Earth's mean spin axis obliquity since the early Cenozoic is evidently out of the question. A catastrophic theory of Apollo-class asteroid impact at supposedly 35 Ma has recently been invoked to explain major Eocene/Oligocene faunal turn-over. However, major turn-over in both the marine and terrestrial realms occurred well before 35 Ma in both Europe and North America, as did the "North American tectite strewn field." Several such fields exist, however. Under a stable continent rationale, concepts of corridors, filters, and sweepstakes routes were developed by the 1950s by G. G. Simpson. Under a mobile continent rationale additional paleogeographic concepts have been added since then, such as Noah's arks and grounded Viking funeral ships for colliding or coalescing terranes bearing respectively either living or fossil contents. In addition, vicariance biogeography, with its emphasis on congruent cladograms of both the genealogies of the biota and of the geologic history of various areas, has been born and is undergoing growing pains. To these concepts may be added the notions of escalator counterflow and escalator hopscotch, whereby the existence of archaic elements of a biota can be reconciled with their presence on young mid-ocean-ridge islands like Iceland or midplate volcanic edifices such as those of the Hawaiian-Emperor chain. The retreat from, and return to, continental coasts by island arcs affected by subduction zone jumps provides a geological mechanism for biological voyages to nowhere and return, as would happen if the landmass of the volcanic arc separated from its parent continent, remained for a while in isolation, and then once more contacted the continent. Vicariant biotas produced by such means and nurtured in isolation would thus eventually be passively returned to contiguity with the parent continent. A certain amount of caution is called for before one should invoke far-traveled tectonic blocks, such as those plastered against the present shores of the North Pacific, as carriers of vicariant biota, now endemic to a terrane embedded in a foreign land. Before accepting such a block as a possible carrier of a terrestrial or even shallow water marine biota, it is important to determine if the block possesses continental or oceanic tectonic basement. Was the surface of the block always above sea level? Were its organisms transported only as fossils? Are the proposed vicariant sister groups at reasonable taxonomic levels in view of the time of tectonic transport of the block while in isolation? A parsimonious approach to all natural history, not merely biology, is necessary. Ad hoc hypotheses should be held to a minimum. The expanding Earth hypothesis is discounted as an explanation of discernible late Phanerozoic biogeographic pattern. Studies of mammalian diversity based on the fossil record have often been biased by tacit assumptions that biostratigraphic units have represented aliquot divisions of time. Calibration of mammalian biostratigraphy is under way, but until the subject is more advanced, published diversity/time relations should be taken with a grain of salt.

Page Thumbnails

  • Thumbnail: Page 
[459]
    [459]
  • Thumbnail: Page 
460
    460
  • Thumbnail: Page 
461
    461
  • Thumbnail: Page 
462
    462
  • Thumbnail: Page 
463
    463
  • Thumbnail: Page 
464
    464
  • Thumbnail: Page 
465
    465
  • Thumbnail: Page 
466
    466
  • Thumbnail: Page 
467
    467
  • Thumbnail: Page 
468
    468
  • Thumbnail: Page 
469
    469
  • Thumbnail: Page 
470
    470
  • Thumbnail: Page 
471
    471
  • Thumbnail: Page 
472
    472
  • Thumbnail: Page 
473
    473
  • Thumbnail: Page 
474
    474
  • Thumbnail: Page 
475
    475
  • Thumbnail: Page 
476
    476
  • Thumbnail: Page 
477
    477
  • Thumbnail: Page 
478
    478
  • Thumbnail: Page 
479
    479
  • Thumbnail: Page 
480
    480
  • Thumbnail: Page 
481
    481
  • Thumbnail: Page 
482
    482
  • Thumbnail: Page 
483
    483
  • Thumbnail: Page 
484
    484
  • Thumbnail: Page 
485
    485
  • Thumbnail: Page 
486
    486
  • Thumbnail: Page 
487
    487
  • Thumbnail: Page 
488
    488
  • Thumbnail: Page 
489
    489