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Paleobotany |
Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6316 USA
Received for publication May 30, 2002. Accepted for publication September 24, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSDISCUSSIONLITERATURE CITED |
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Key Words: arborvitae • Arctic • climate • Coniferales • Cupressaceae • fossil • phylogeny • systematics • Thuja • Turonian
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSDISCUSSIONLITERATURE CITED |
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).
The occurrence of fertile and vegetative remains of Thuja in the fossil record indicates that the genus possessed a much wider distribution during the Mesozoic and Cenozoic, one that also extended into the polar regions of the Northern Hemisphere (Florin, 1963
; Schweitzer, 1974
; McIver and Basinger, 1989
; Bennike, 1990
). More importantly, it indicates that representatives of this genus probably grew under a more diverse range of climatic and environmental conditions throughout geologic time than is seen today.
The evolutionary history of the group is poorly known given that most of the Thuja fossils so far recovered consist predominantly of isolated non-diagnostic vegetative remains commonly assigned to Thuja, Thuites Newberry, Thujites Sternberg, or Cupressinocladus Seward (Miller, 1977
; McIver and Basinger, 1989
). Although Thuja-like seed cones are known from the fossil record (e.g., Biota borealis Heer [Heer, 1874
]), remains such as these must be reexamined prior to being considered in any systematic analysis of the genus. Consequently, the fossil record that includes unequivocal Thuja seed cones that are closely associated or attached to foliar remains is poor and is limited to five occurrences. They include T. ehrenswaerdi (Heer) Heer from the Paleocene of Greenland (Schweitzer, 1974
); T. polaris McIver et Basinger from the Paleocene of Ellesmere Island, Nunavut, Canada (McIver and Basinger, 1989
); T. nipponica Tanai et Onoe from the Miocene of Sikhote Alin, Russia, and the late Miocene of Akita Prefecture, Japan (Akhmetiev, 1973
; Huzioka and Uemura, 1973
); and T. occidentalis from the Plio-Pleistocene of Peary Land, North Greenland (Bennike, 1990
).
Evolutionary relationships between fossil and extant representatives of the conifers are commonly based on seed-cone morphology and anatomy. Whereas the addition of a new species is not likely to resolve species relationships among the extant representatives, aspects of the evolutionary history of the genus and the environment in which some of these representatives lived in the past become clearer. The Thuja described here from the North Slope of Alaska is the oldest known reproductive material for the genus and provides insight into the early evolutionary history of Thuja and the climatic conditions under which fossil Thuja grew, especially in the polar regions. Consideration of the climatic conditions under which extant species of Thuja grow today, as well as their physiological responses to known climatic conditions, indicates that the Cretaceous and early Tertiary representatives of the genus were probably well adapted to growing in the polar regions in the geologic past. In light of the physiological responses and requirements seen in extant species of Thuja together with the presence of Thuja, as well as other evergreen taxa in the Cretaceous and early Tertiary polar broad-leaved forests, it appears that the dark polar winters were probably cold to freezing, rather than being warm as previously suggested.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSDISCUSSIONLITERATURE CITED |
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).
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, Detterman, Bickel, and Gryc (1963)
, Chapman, Detterman, and Mangus (1964)
, and Smiley (1966
, 1967
, 1969a
, b
). On the basis of regional floras containing more than 380 species and faunas from 250 localities, Smiley established seven (1969a)
and later eight (1972a
, b
) biostratigraphic floral zones across approximately 600 km of the North Slope of Alaska (Fig. 2). The Thuja fossils from Kuk River fall into Floral Zone V, which is dominated by conifers and dicots, minor amounts of ferns, Ginkgo L., and the last occurrence of Podozamites Braun (Fig. 2; Smiley, 1972a
).
Specimen
The fossil is preserved as a compression specimen within a fine-grained mudstone matrix. The surface of the specimen was cleaned with 5% hydrochloric acid (HCl) and then rinsed with distilled water to increase the contrast between the fossil and matrix. The fossil illustrated and described in this paper is housed in the collections of the Department of Paleobiology, National Museum of Natural History (USNM), Smithsonian Institution, Washington, D.C., USA.
Taxonomy
Order Coniferales
Family Cupressaceae Neger—Genus Thuja L.
Thuja smileya LePage sp. nov. (Figs. 3–6).
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Holotype: Hic designatus
United States National Museum, Smithsonian Institution, catalog number USNM 519552, Figs. 3–6.
Collecting locality
Locality 41 in Lithologic Unit #6 on the Avalik River, a tributary at the head of the Kuk River, North Slope, Alaska.
Stratigraphy
Ketik flora, Floral Zone V (Smiley, 1972a
).
Age
Late Cretaceous (Turonian).
Etymology
This species is named in memory of Professor C. Jack Smiley.
Description of fossil and comparison with extant Thuja
The seed cones of T. smileya (Figs. 3–6) are comparable with those of all extant species of Thuja. Based on the size and shape alone, the Alaskan fossil most closely resembles the seed cones of T. koraiensis and T. occidentalis. However, the seed cones of T. smileya resemble those of young arborvitae seed cones that have not yet opened because none of the cone scales are reflexed, as is the case when extant seed cones reach maturity and shed their seed. If these seed cones are indeed immature, it is likely that they have not yet reached their full size. Given this scenario, these seed cones would then probably fall in the size range seen in those of T. standishii and T. plicata when mature. The seed cones of T. standishii and T. plicata range from 9 to 13 and from 12 to 18 mm in length, respectively, while those of T. occidentalis range from 8 to 12 mm and those of T. koraiensis range from 8 to 10 mm in length (McIver and Basinger, 1989
). The seed cones of T. sutchuenensis are excluded from this comparison because they are only 6 mm long and 4 mm wide; this is considerably smaller than the Alaskan fossils.
Although the morphology of the bract-scale complex has been used confidently for species level circumscription among other fossil and living conifers (see LePage [2001]
and references therein), diagnostic features of the bracts and ovuliferous scales that would enable species segregation are lacking in fossil and extant Thuja. Apart from the slight differences in size and shape and the presence of abaxial grooves on the cone scales in the living species, the cone scales of the extant species are difficult to distinguish from one another. In addition, all possess small umbos that can either be reflexed or non-reflexed (McIver and Basinger, 1989
). Comparison of the morphological features of the cone scales of T. smileya with the living representatives revealed no obvious differences.
The number of pairs of bract-scale complexes varies somewhat between species, but apart from T. sutchuenensis, which possesses only four pairs of cone scales, segregation of species based on the number of pairs of cone scales is difficult. The seed cones of T. occidentalis and T. standishii possess 4–6 pairs, T. koraiensis possesses 4–5 pairs, while T. plicata possesses 5–6 pairs (Dallimore and Jackson, 1948
; Silba, 1986
). Although the seed cones of Thuja smileya are closed, at least four pairs of cone scales are apparent (Figs. 4, 5, and 7). Given the arrangement of the cone scales preserved in the terminal cone, it appears that the rudimentary pair of basal cone scales that is present in all extant species of Thuja is poorly preserved and not clearly visible in the fossil (Figs. 3, 4, 6, and 7). In addition, if the most apical pair of cone scales visible in the fossil were the last pair of fertile cone scales (Figs. 4 and 5), one would expect to find another pair of rudimentary cone scales apically, as is the case in extant species. Therefore, although only four pairs of cone scales are visible, it seems likely that this species probably possessed five and possibly six pairs of cone scales.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSDISCUSSIONLITERATURE CITED |
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). As pointed out by these authors, slight differences such as seed-cone size and shape, umbo distinctiveness, and the presence of abaxial grooves on the cone scales would probably not be apparent in fossil seed cones unless the preservation was exquisite. With the exception of T. sutchuenensis, which only produces four pairs of cone scales compared with 4–6 pairs of cones scales seen in the other four living species, comparison between fossil and extant Thuja seed cones for determination of taxonomic affinity appears to be entirely subjective using seed cones alone.
Despite the difficulties associated with correctly identifying isolated fossil and living Thuja leaves from other Thuja-like leaves (e.g., Thuites, Chamaecyparis Spach, and Cupressinocladus), the importance of the leaves increases considerably for species-level identification when they are associated unquestionably with seed cones. For example, the seed cones of T. polaris were reported to most closely resemble those of extant T. occidentalis and T. plicata (McIver and Basinger, 1989
). However, comparison of the leaves of T. polaris with both extant species indicated that the leaves of T. occidentalis were considerably different than those of T. polaris, and therefore, T. polaris was deemed to be most similar to T. plicata (McIver and Basinger, 1989
). Similarly, Huzioka and Uemura (1973)
reported that their specimens of T. nipponica, which consisted of leaves and seed cones were the most similar to extant T. standishii based on Tanai and Onoe's (1961)
original description of the species that was based exclusively on leaves. Akhmetiev's (1973)
assignment of his specimens to T. nipponica was also based on the similarity of the foliage to that of T. standishii. Schweitzer (1974)
used features of the seed cones and foliage to assign his material to T. ehrenswaerdi. He noted that the foliage most closely resembled that of Chamaecyparis but that the cones most closely resembled those of extant T. sutchuenensis. Bennike (1990)
assigned fossil leaves, twigs, seeds, and cones to T. occidentalis, but provided no justification for such an assignment. Therefore, if any meaningful phylogenetic interpretation for Thuja is to be made, the comparisons must be based on foliar and seed-cone features when preserved together. The lack of identifiable foliage associated with the Alaskan cones precludes a meaningful systematic comparison with extant Thuja representatives. However, despite the lack of associated or attached foliar remains, the erection of a new name for the Alaskan cone is warranted given that it is the earliest known reproductive material for the genus and extends the fossil record for Thuja from the Paleocene to the Late Cretaceous, a span of about 35 million years.
The presence of Thuja in Alaska also illustrates a number of other points. First, it is not possible to infer phylogenetic affinity between the Alaskan fossil and living representatives of the genus based on seed-cone morphology alone. Comparison and phylogenetic interpretation must await the recovery of well-preserved foliar remains that are attached to seed cones from the site where the cones were discovered initially.
Second, there is a general tendency among the conifers towards reduction of the seed cones through the fusion or loss of parts (Miller, 1982
). Eckenwalder (1976)
proposed that within the Cupressaceae the generalized ancestor possessed woody, globular, many-scaled seed cones, with non-flattened branches. Although this hypothesis may still hold true, the Alaskan Thuja indicates that the reduction in the number of cone scales and the presence of thin, probably leathery cone scales had occurred by Turonian time and that such an hypothetical ancestor would have had to arise earlier, perhaps sometime during the Early Cretaceous. Detailed taxonomic reassessment of the Thuja and Thuja-like fossils reported in the literature, especially those from the Early Cretaceous, is warranted and may provide considerable insight into the early evolutionary history of the genus.
The presence of eight pairs of cone scales in T. polaris indicates that this species is distinct from all extant and extinct species of Thuja and probably represents a distinct lineage within the genus. More importantly, it suggests that the rates of evolutionary change among lineages differed, as exemplified by the "modern-looking" seed cones from Alaska and the "primitive-looking" Ellesmere Island Thuja. Although McIver and Basinger's (1989)
suggestion that T. polaris is the ancestral sister group to the T. koraiensis/T. standishii and T. plicata/T. occidentalis clades may be true, the relatively late appearance of "primitive-looking" seed cones such as T. polaris compared with the much older T. smileya suggests that T. polaris may not have been involved directly in the evolution of the modern lineages and may have been an evolutionary "dead end." The problem of determining which one of the extant species of Thuja is basal within the genus is further compounded by the lack of molecular analyses comprising all living species. Up until now, rbcL and matK sequence data have been analyzed for only T. plicata, T. occidentalis, and T. standishii (Gadek et al., 2000
).
Climate
Although the Cretaceous and Tertiary polar floras were dominated by deciduous taxa, evergreen representatives were indeed a part of these floras (Spicer and Parrish, 1986
, 1990
; Spicer, 1987
; McIver and Basinger, 1989
; Basinger, 1991
; LePage, 2001
, in press
). However, because the number of evergreen taxa relative to the deciduous elements is small, the evergreen representatives have been commonly ignored for interpreting paleoclimate. Despite being minor elements of these polar deciduous floras, the evergreen taxa, such as the Alaskan Thuja, can provide important constraints for interpreting past climate.
During the Cretaceous and Tertiary the evergreen constituents of the high-latitude forests such as Thuja from Alaska, Ellesmere Island, Greenland, and Spitzbergen grew well above the Arctic Circle and would have grown under the unique polar light regime. In other words, the trees would have experienced 3 mo of continuous light in summer and 3 mo of total darkness during the winter. If temperatures during the winter were at or above freezing, as has been suggested previously (Estes and Hutchison, 1980
; Basinger, Greenwood, and Sweda, 1994
; Liu and Basinger, 2000
), the evergreen trees would have then remained active metabolically in the dark. In a study designed to address the response of deciduous and evergreen trees to prolonged "winter" darkness, Read and Francis (1992)
were able to show that the damage to the evergreen plants was less severe when subjected to cooler, rather than warmer (4°C vs. 15°C) temperatures. They concluded that woody evergreen plants could have survived prolonged periods of darkness under cooler conditions. However, their study lasted only 10 wk or about half the length of time that trees growing in the polar regions would have experienced.
Although their plants were grown in a normal light regime (i.e., a diurnal light/dark cycle; Adelaide, Australia) prior to the dark treatment and would have had an adequate supply of starch stored in their tissues, the starch levels in both the deciduous and evergreen plants that were subjected to the 10 wk dark treatment were considerably lower than those growing under the normal light regime (Read and Francis, 1992
). They suggested that among the evergreen species examined the leaf respiration rate of plants growing in the dark was probably a significant factor for survival during extended periods of darkness. In other words, during periods of extended darkness the ability of plants to store carbohydrates appears to be crucial for survival. If the dark polar winters were warm, the higher rates of leaf respiration associated with the higher temperatures would have precluded a number of plant species from growing in the polar regions (Read and Francis, 1992
). Alternatively, leaf respiration rates decrease and fewer carbohydrates are required for survival during extended periods of darkness if winter temperatures are lowered.
Recent work with the deciduous Metasequoia glyptostroboides Hu et Cheng (Taxodiaceae) showed interesting results. Here plants were grown under "normal" (Orono, Maine, USA) and 24 h of daylight (i.e., supplemental light was added at night to the normal amount of daylight to mimic 24 h light conditions) for 3.5 mo. The results of this experiment revealed that the trees growing under continuous light had stored starch levels that were considerably lower than the plants growing under the diurnal light regime (R. Jagels, University of Maine, personal communication). Therefore, could fossil plants growing in a 24 h light regime produce and store enough starch in their tissues in the few short weeks between the light and dark seasons to remain metabolically active and survive an extended period of darkness if the polar winters were warm? It is reasonable to hypothesize that the plants growing above the Arctic Circle in the geologic past may have had an insufficient carbohydrate reserve to survive long periods of respiration in the dark at above-freezing temperatures. Alternatively, if the polar temperatures were below freezing during the winter months, the plants would have become dormant and could have survived the dark polar winters without depleting what little carbohydrates they had stored in their tissues.
Although ecophysiological data for Thuja growing under polar light conditions and different temperature regimes are lacking, consideration of the response of extant Thuja to the onset of winter and growth under different light levels provides insight into how this species may have dealt with winter in the geologic past. According to Sakai and Larcher (1987)
, in regions where climate is seasonal, plants acclimate to cold and undergo a transition from lower to higher levels of frost resistance in one of two ways. First, they can acquire a greater resistance to frost or tolerance to freezing temperatures by responding directly to the progressive drop in temperature. Alternatively, plants can adjust their metabolic and developmental activities to seasonal changes such as day length. Experiments on the seasonal patterns and environmental regulation of frost hardiness in Thuja plicata indicates that this species, as well as other members of the Cupressaceae, respond primarily to decreasing temperatures throughout their range and that their response to changes in photoperiod are weak (Silim and Lavender, 1994
). The weak response to photoperiod is perhaps not surprising given that an experiment with extant T. plicata seedlings along light gradients on three environmentally comparable sites revealed that the seedlings adapted well to the different light levels (Wang, Qian, and Klinka, 1994
). In fact, Drever and Lertzman (2001)
indicate that T. plicata radial and height growth responses were the greatest when the light levels ranged from 0 to 30% full sun light. When the light levels were >30% full sun light, growth responses were minimal. Therefore, if the fossil Thuja growing in the polar regions responded to temperature rather than light, it would then seem that they would have been well adapted to growing under low light conditions and began the process of winter hardening as temperature decreased.
Although there is no way to prove whether the fossils responded to changes in temperature or photoperiod, it is interesting to note that extant species of Thuja currently grow in regions where climate is cool and humid, rainfall is abundant during the summer, and winters are snowy and cool to cold (Sakai and Larcher, 1987
; Farjon, 1990
). Representatives of the genus grow where winter temperatures range from –12° to –50°C (Rushforth, 1987
), and all species have been shown experimentally to survive frost damage at temperatures as low as –25° to –38°C (Sakai, 1983
; Sakai and Larcher, 1987
). Despite the pronounced changes in photoperiod seen in the transition between summer and fall in the polar regions, the Cretaceous and early Tertiary Thuja probably began the process of winter hardening in response to decreasing temperatures, assuming that this strategy, as it is seen in the extant species, had evolved early in the evolutionary history of the genus.
The fossil record of Thuja (McIver and Basinger, 1989
) indicates that the genus was distributed widely throughout the northern hemisphere and probably grew under a wide range of climatic conditions. Interestingly enough, four of the six fossil species of Thuja discussed here grew in the polar regions of the northern hemisphere and temporally extend from the Late Cretaceous to the Plio-Pleistocene. Given that extant Thuja becomes more frost resistant by responding to dropping temperature and today grows in regions in which freezing temperatures are common, it may be reasonably assumed that the fossils probably possessed similar adaptations.
From the standpoint of high-latitude Cretaceous and early Tertiary paleoclimate, there is little doubt that the polar regions were much warmer than present. Nevertheless, there is a growing body of geologic and paleobotanical evidence that supports the idea that the polar winters were cold with freezing temperatures during the Cretaceous. However, similar data for the early Tertiary are more equivocal (Spicer and Parrish, 1986
, 1990
; Kemper, 1987
; Spicer, 1987
; Frakes and Francis, 1988
, 1990
; Gregory et al., 1989
; Spicer and Corfield, 1992
; Greenwood and Wing, 1995
; Fricke and O'Neil, 1999
; Tripati et al., 2001
).
Paleotemperature estimates for the early Tertiary Canadian Arctic based on the Nearest Living Relative approach indicate that the winters were cool with no severe frost (Basinger, Greenwood, and Sweda, 1994
). Temperature estimates based on this approach indicate that the mean annual and cold month mean temperatures during the middle Eocene at a site on Axel Heiberg Island were 12°–15°C and 0°–4°C, respectively. However, these estimates do not account for the physiological problems that the evergreen taxa (e.g., Taiwania Hayata, Chamaecyparis, Tsuga (Endlicher) Carrière, Pinus L.) would have had to endure during the dark and warm polar winters, nor do they take into account the presence of taxa such as Larix Miller and Picea A. Dietrich that today grow in some of the coldest parts of the world.
Foliar physiognomic data from two sites in the Canadian High Arctic provide another estimate of temperature that is more in keeping with the floristic composition of each site and the physiological requirements of evergreen taxa. Based on this method cold month mean and mean annual temperatures of –2.0°C and 8.2°C, respectively, are predicted for a late Paleocene locality on Ellesmere Island, while –0.8°C and 9.3°C, respectively, are predicted for the middle Eocene site on Axel Heiberg Island (Greenwood and Wing, 1995
). It is also important to point out that the physiognomic approach is based entirely on the angiosperms present in the flora. Floras dominated by conifers and cold-adapted conifers such as Larix and Picea, which are present in the late Paleocene and middle Eocene floras of the Canadian Arctic (LePage and Basinger, 1991
; LePage, 2001
; personal observation), are not included in physiognomic analyses. Consequently, these temperatures are probably best considered to be maximum estimates of the cold month mean and mean annual temperatures.
Consideration of the latitudinal temperature gradients derived for the continental interior of North America indicates a drop of 0.4°C/1° latitude during the Eocene (Greenwood and Wing, 1995
). Thus, using the mean cold month temperature and latitudinal data for the Eocene localities listed in Greenwood and Wing (1995)
, together with their predicted latitudinal gradient of 0.4°C/1° latitude, the cold month mean temperatures at 75–80°N would have ranged from 0.7° to –12.1°C. Comparison of the cold month mean temperature estimates based on foliar physiognomy indicates that the two early Tertiary Arctic sites fall within this latitudinally derived range of cold month mean temperatures.
Important to this discussion are the temperature estimates for the polar regions that are based on global climate model simulations. Although there has been considerable debate concerning the southern extent of the freeze line during the late Paleocene-early Eocene (Wing and Greenwood, 1993
; Sloan, 1994
; Greenwood and Wing, 1995
; Sloan and Morrill, 1998
), the global climate model simulations consistently predict freezing temperatures at the polar latitudes during the winter months (Sloan and Barron, 1990
; Sloan, 1994
; Sloan and Rea, 1995
; Sloan and Morrill, 1998
). However, the global climate model simulations consistently estimate early Tertiary mean annual temperature in the Arctic to be ca. –20°C, a 25°–30°C departure from the temperatures indicated by the fossil floras (Greenwood and Wing, 1995
; Sloan and Rea, 1995
; Sloan and Morrill, 1998
; Sloan and Pollard, 1998
; Sloan et al., 2001
). Although the fossil plant assemblages indicate that such temperatures are biologically not possible, a number of mechanisms have been proposed that could potentially raise the mean annual temperature and cold month mean temperature of the polar regions to levels that are consistent with the paleotemperature estimates derived from the fossil plants (Sloan et al., 1992
; Sloan and Pollard, 1998
; Sloan, Huber, and Ewing, 1999
; Sewall and Sloan, 2001
). Nevertheless, what is important to consider here is that during the winter (January) freezing temperatures, regardless of the conditions prescribed prior to running the simulations (e.g., elevated CO2, orbital forcing, and sea surface temperatures), are predicted at latitudes of at least 60°N and higher during the early Tertiary.
If the terrestrial polar winters during the Tertiary were warm (i.e., >0°–4°C), as has been previously suggested (Estes and Hutchison, 1980
; Basinger, Greenwood, and Sweda, 1994
; Liu and Basinger, 2000
), it appears unlikely that the evergreen taxa could have then survived given the basic physiological and metabolic requirements seen in the extant representatives. The Cretaceous and Tertiary high-latitude plant fossil record indicates that evergreen taxa (e.g., Picea, Pinus, Keteleeria Carrière, Tsuga, Glyptostrobus Endlicher, Taiwania, Thuja, Chamaecyparis, and Cathaya Chun et Kuang) were indeed a part of the predominantly deciduous circumpolar floras. Although Cathaya has been identified recently from the middle Eocene of Axel Heiberg Island (Liu and Basinger, 2000
), the absence of frost tolerance and ecological data for this species provides little additional information.
Frost resistance studies conducted on representatives of these genera (except Cathaya) indicate that they all are capable of surviving temperatures of at least –10°C (Sakai, 1971
; Sakai and Larcher, 1987
). Thus, without invoking significant genetic and/or physiological modifications to all of these genera, the most parsimonious explanation that would have allowed these evergreen representatives to survive the dark winter months would be to conclude that subfreezing temperatures did occur during the winter. The presence of Thuja, as well as other evergreen representatives in Late Cretaceous and Tertiary fossil floras from the high-latitude regions of the northern hemisphere, indicates that these plants were well adapted to growth in polar environments. The ability of the modern representatives to grow and reproduce successfully where winter temperatures are below freezing is consistent with the contention that the high-latitude polar winters during the Cretaceous and Tertiary were cold and that these forests were of a cool-temperate nature as has been previously suggested (e.g., Berry, 1930
; Chaney, 1940a
, b
). Moreover, it appears that the physiological adaptations possessed by the early ancestors of these taxa, such as T. smileya, have been retained by the living representatives and one day may allow them to return to the polar regions.
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FOOTNOTES |
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSDISCUSSIONLITERATURE CITED |
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Basinger J. F. 1991 The fossil forests of the Buchanan Lake Formation (early Tertiary), Axel Heiberg Island, Canadian High Arctic: preliminary floristics and paleoclimate. Geological Survey of Canada, Bulletin 403: 39-65
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Huzioka K. K. Uemura 1973 The late Miocene Miyata flora of Akita Prefecture, northeast Honshu, Japan. Bulletin of the National Science Museum, Tokyo 18: 661-738
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LePage B. A. In press A new species of Tsuga (Pinaceae) from the middle Eocene of Axel Heiberg Island, Canada and an assessment of the evolution and biogeographic history of the genus. Botanical Journal of the Linnean Society 141
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