Antitropicality and Vicariance: A Reply to Briggs

Briggs (1987a, b) objects to my hypothesis (White, 1986), that many modern antitropical distributions have a common paleoclimatic origin, for several reasons. He suspects that the decade-old paleotemperature references I cite misinterpret Cenozoic oxygen isotope fluctuations because of faulty assumptions about the history of Antarctic glaciation. He implies that my hypothesis calls for Oligocene surface temperatures of 15?-17?C that are inconsistent with the existence of ancient coral reefs, and objects to my attribution of "all antitropical distributions to a single warming event that began in the Middle Miocene" (Briggs, 1987a:206) because antitropical taxa occur at several levels of the taxonomic hierarchy and must therefore have originated at different times. That marine oxygen isotope ratios are sensitive to both salinity and temperature effects has long been known. It is nonetheless true that the tropical paleotemperature curves I cite (Savin et al., 1975) are based on the assumption that prior to 15 mya, temperature was the primary driving force behind isotopic fluctuations because only after this time did Antarctic ice significantly alter oxygen isotope ratios. Briggs (1987a, b) claims that this assumption is invalid because substantial southern glaciation occurred as long ago as 25 mya in the "Middle Miocene" and cites van Andel (1985) in support. The relevant passage stating that a "true Antarctic icecap did not form until the Middle Miocene, about 25 million years ago" (van Andel, 1985:172) should actually read 15 mya (van Andel, pers. comm.). Matthews and Poore (1980) do not demonstrate stable Cenozoic tropical sea-surface temperatures of 27?-28?C as Briggs (1987a, b) implies, they only assume them as a way of estimating the maximum magnitude of hypothetical glacial events from oxygen isotope data. Their assumption can lead to non-sensical conclusions when applied to isotopic databases different from their own (Shackelton et al., 1984). There is no sedimentary evidence to support the idea that oxygen isotopes responded very much to advancing ice-sheets during the Paleogene (Murphy and Kennett, 1986). The correction factor for modern icevolumes is only ca. 3?C (Miller et al., 1987). The appropriate Oligocene correction would almost certainly be less. Finally, a variety of marine micropaleontologic and terrestrial palynologic and paleontologic data are consistent with the global Eocene / Oligocene cooling and the Middle Miocene low-latitude warming that I think shaped many modern antitropical distributions (White, 1983, 1986). The implication that my hypothesis calls for tropical temperatures between 150-170C (Briggs, 1987a:206) is false. I clearly stated that "In the middle Eocene, the earth cooled suddenly, with tropical marine surface temperatures plummeting almost 90C and remaining at or near 200C during the entire Oligocene and early Miocene" (White, 1986:182). I agree with Briggs that the coral fossil record is inconsistent with tropical surface temperatures less than 200C. I use this and other fossil evidence to conclude that global cooling in the Eocene/Oligocene "was not severe enough to cause wholesale extinction of tropical communities because, even during the coldest part of the Oligocene, low-latitude marine surface temperatures were near 20?C" (White, 1986:184). I do not neglect alternative disjunctive mechanisms for antitropical distributions. My abstract states that "hypotheses promoting long-distance dispersal, tropical submergence, competition and predation contribute to a fuller understanding of the