Carboniferous Through Jurassic Conifers
The earliest remains attributable to conifers occur in the late Carboniferous (310 Ma); conifers diversified and were floristically important by the end of the Paleozoic era. Although resin has not been found in contact with these remains, there is evidence of secretory structures. Several genera of late Car-boniferous and Permian conifers in Europe are characterized by resin canals (Vogellehner 1965). Of further possible evolutionary importance is that spe-cies of late Carboniferous conifers (e.g., Europoroxylon) had only vertical canals, like medullosan seed ferns, whereas a Triassic conifer had developed the more complex system of both horizontal and vertical canals, like modern conifers. Although the amount of exudate from endogenous canals or cysts
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following injury in Paleozoic plants appears small compared to later plants, these older plants apparently had considerable internal storage of resin. Thus abundant microscopic pieces (resinite) led some early coal scientists such as D. White (1914) to suggest that tropical Carboniferous coal floras may have been as “richly productive” in resins as the later (particularly Cretaceous) coal floras. An unanswered question is whether these resins would fit the def-inition used in Plant Resins. Scattered evidence suggests answers of both yes and no.
Although resin canals occur in Permian and Triassic coniferous woods, perhaps only small amounts of resin were produced during cooler and drier climates during these periods. Questionable Permian evidence for amber comes from a limestone in the western piedmont of the Ural Mountains, which contains microscopic quantities of a chemically undetermined resin.
On the other hand, small pieces of positively identified Triassic amber have been found in strata in Austria (Vávra 1984). Well-preserved microorgan-isms, which may have formed a community in the resin-producing tree, have been described from the late Triassic (200–230 Ma) from Bavaria, Germany.
Unfortunately, neither IR nor NMR proved diagnostically useful regarding the botanical origin of this amber (Poinar et al. 1993).
More information is available on late Triassic amber from the Petrified Forest National Park, Arizona, although its plant origin is unclear (Litwin and Ash 1991). Detrital amber there occurs in carbonaceous coals that may represent slack water deposits. Preliminary IR data indicated that some of the amber may have been produced by an araucarian-type tree, which tradi-tionally has been considered to be the source of most of the large accumula-tion of petrified wood (Araucarioxylon). Frustratingly, none of this analyzed amber occurred in direct association with Araucarioxylon. Although evi-dence of amber has generally not been reported in the four densest accumu-lations (“forests”) of petrified wood within the park, there are fragments of wood with amber-like material on it. Turkel (1968) discussed resin secretory structures in Araucarioxylon, which he assumed to be araucarian. On the other hand, the wood anatomy of A. arizonicum suggests only equivocal rela-tionships to Araucariaceae, clouding the issue (Ash 1987). In sum, a definite botanical source for this North American Triassic amber remains unresolved, and it is quite possible that both the resin and the resin-producing plant are not referable to any extant taxon.
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There is little documented evidence of Jurassic amber although it has been reported anecdotally from Bornholm Island and elsewhere in Denmark.
Zherikhin and Eskov (1999) provided evidence of amber enclosed in a coni-fer cone from Jurassic coals in the Republic of Georgia. Moreover, Stockey (1978) showed resin canals in silicified cone scales of Araucaria mirabilis from the Cerro Cuadrado Petrified Forest of Patagonia, Argentina, consid-ered to be late Jurassic in age (Stockey 1989). It is interesting that secretory structures, and sometimes small amounts of resin, are reported in cones from Jurassic deposits, a possible early evolutionary indication that resin protected reproductive structures chemically. There has been no evidence of copious production of resin from tree trunks, which occurred later, in the Cretaceous and Cenozoic.
Cretaceous Conifers
Although some modern conifer families had made their appearance by the late Permian and Triassic, all families were present by the early Cretaceous.
Thus amber from several modern families could be anticipated during this period. A major climatic warming occurred toward the end of the early Cre-taceous, apparently correlated with copious resin production in some areas, as indicated by amber deposits in Austria and the Levantine belt (mountainous areas of Israel, Lebanon, and Jordan). These Middle Eastern ambers constitute the oldest extensive fossiliferous Cretaceous deposits, approximately 125–130 million years old. The numerous organic inclusions are being studied. IR sug-gests a probable araucarian source for this amber (Appendix 4; Vávra 1984, Nissenbaum and Horowitz 1992). Another early Cretaceous deposit rich in inclusions is in Álava, Basque Country, Spain, for which an araucarian source has also been suggested based on chemistry and pollen (Alonso et al. 2000).
Studies of the botanical origin of Lebanese amber, however, have impli-cated an extinct conifer, Protopodocarpoxylon, in the family Cheirolepida-ceae (Azar 2000). This determination was based on leaves impregnated with amber, a female cone associated with the leaves, and wood containing pieces of amber. Leaves of this type occur in the majority of Lebanese amber beds (Azar 2000). Cheirolepidaceae were a large family of Mesozoic conifers, rep-resenting a great many different types of plants that inhabited a variety of eco-logical niches (Watson 1988). The affinities of Cheirolepidaceae remain spec-ulative, with earlier ideas placing this diverse group within the Taxodiaceae,
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Cupressaceae s.s., or Araucariaceae based principally on foliage. Taylor and Taylor (1993) indicated that until more information is obtained about the organization and diversity of the ovulate cones, Cheirolepidaceae will con-tinue to stand as a dominant group of Mesozoic conifers whose progenitors and descendants remain equivocal. Interestingly, no cheirolepidaceous wood contains resin canals, indicating that the Lebanese amber found in the Pro-topodocarpoxylon wood must have been formed by resin cells characteristic of the Cupressaceae s.l. or was induced by injury.
By the late Cretaceous (95 Ma) there are numerous amber fossils, particu-larly in North America (Figure 4-2). For example, along the Atlantic Coastal Plain of the United States (from Massachusetts to Georgia) there are 24 locali-ties at which amber has been found. Plant sources have been suggested for a number of these (Langenheim and Beck 1968; Grimaldi et al. 1989, 2000).
Large deposits on Staten Island, New York, were discovered in open pits mined for clay to manufacture bricks. The amber was so plentiful that work-ers could pile it in barrels and burn it during winter to keep warm. Amber occurs in similar abandoned sandy clay pits in central New Jersey, where the most abundant amber in North American deposits has been found. There is an exceptionally diverse assemblage of organismal inclusions in the New Jersey amber; in fact, it is probably “one of the most significant Cretaceous deposits in the world” (Grimaldi et al. 2000). The diversity of organisms in the amber may in part be attributed to their having occurred in subtropical or warm temperate forests. The amber was deposited in a network of deltaic, slack water streams where peat accumulated in shallow anaerobic basins.
Amber occurs with lignitic remains in these Atlantic Coastal Plain depos-its. Grimaldi et al. (2000) reviewed the tortuous and controversial history of the botanical source of these lignites based on anatomy of the wood and asso-ciated foliage. They reevaluated the source of the amber in the wood and the foliage as well as the chemistry of the amber. The wood, foliage, and amber remains had been considered to come from three conifer families: Araucaria-ceae, Cupressaceae s.l., and Pinaceae. However, previous IR and Py-GC sug-gested an araucarian source for much of the amber in New Jersey as well as generally along the Atlantic Coastal Plain. Grimaldi and coworkers meticu-lously collected amber from the commonly occurring wood of Pityoxylon and confirmed by Py-GC-MS that Pityoxylon is the source of most of the New Jersey amber. Pityoxylon has been considered a noncommittal
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ous genus, related at various times to the Araucariaceae, Cupressaceae s.l., and Pinaceae because of characteristics shared by these families. Stewart (1983), however, suggested that Pityoxylon represents the earliest wood remains of Pinaceae in Cretaceous deposits. The chemistry supported a pina-ceous origin for the resin with a predominance of retene, a transformation product of abietic acid (Figures 1-4 and 4-1), which in large quantities is diag-nostic of resin from Pinaceae.
Amber was also retrieved from cone scales of Dammara, which are juniper-like, or cupressaceous. Early workers (e.g., Hollick and Jeffrey 1909) and Penny (1947), however, maintained an araucarian affinity beyond ques-tion. Miller (1977) indicated that Dammara and Protodammara are taxo-diaceous (Cupressaceae s.l.) rather than araucarian and represent a Cun-ninghamia lineage. On the other hand, the picture is not clear-cut in that the small cone scales of Cunninghamia may be a derived condition. In fact, Miller (1999) suggested that the taxodiaceous line that includes Cunninghamia is evolutionarily grouped with Araucariaceae. Thus the taxodiaceous cone scale-bract complex may still link Cunninghamia with Araucariaceae; alternatively, these features may have evolved in parallel. DTMS of amber within the cone scales of Dammara indicates a large amount of abietane diterpenoid skeleton, again suggestive of Pinaceae. Disseminated amber similar to that found in the cone scales further indicates that it originated from the same kind of amber tree, at least in the northern range of the Atlantic Coastal Plain. Thus this evidence points away from a direct and primary araucarian origin for these deposits but does not clarify the taxodiaceous–pinaceous dilemma, again demonstrating problems in sorting out coniferous identities during the Cre-taceous. The study by Grimaldi et al. (2000) shows that straightforward con-clusions are difficult to come by, in some measure probably because of lack of understanding of the complexities of conifer evolution during the late Creta-ceous. These problems also are evident in other Cretaceous studies but they have not been evaluated as carefully as those along the Atlantic Coastal Plain.
Amber from the Hanna Basin in southern central Wyoming occurs in extraordinarily thick late Cretaceous and Paleocene sediments of unequivo-cal stratigraphic superposition. The 20-million-year period between the high-est and lowhigh-est occurrences of amber is unprecedented in amber deposits (Grimaldi et al. 2001). The amber is preserved as small droplets in three geo-logic formations spanning the entire time interval. Although the amber is
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chemically and physically mature, probably because of age and depth of bur-ial, only minor diagenetic changes were detected in the samples. Distinctive conifer cone scales occurred in carbonaceous to lignitic strata, representing fluvial episodes bounded by incursions of epicontinental seas. The Hanna Basin cone scales are similar to those of the Atlantic Coastal Plain for which Grimaldi et al. (2000) suggested a taxodiaceous origin. The chemistry of the amber in the scales from the Atlantic Coastal Plain indicates a pinaceous ori-gin for the resin; the Hanna Basin resin from similar cone scales is sufficiently similar chemically to be considered from the same source. Thus, for both the Atlantic Coast and Wyoming ambers, the chemical evidence does not cor-roborate a taxodiaceous (Cupressaceae s.l.) origin. What is interesting, how-ever, is the presence of similar cone scales with amber preserved in the secre-tory tissue, occurring across a large area of North America through 20–30 million years.
Amber of unknown Cretaceous age from Cedar Lake, Canada, was sug-gested to be of araucarian origin by comparison of IR and FTIR of the amber with resin from present-day Agathis australis as well as New Zealand amber from that source. Py-MS further supports this conclusion (Poinar and Haver-kamp 1985). However, there was no plant fossil evidence to provide corrob-oration. Although Araucariaceae are often implicated as the source of Creta-ceous amber, Lambert et al. (1996) have generalized to a greater extent based only on NMR of the molecular composition. They concluded that their spec-tra of amber from Alaska, Canada, France, Greenland, Kansas, Mississippi, New Jersey, Switzerland, and the Middle East “suggests a common paleo-botanical source or family of sources related to Agathis (Araucariaceae) that had a broad geographical distribution during the Triassic, Cretaceous and Early Tertiary times.” This, indeed, is a daring conclusion without support-ing botanical evidence.
On the other hand, abundant pollen and associated foliage (including Sequoia, Sequoiadendron, and perhaps Taxodium) from late Cretaceous local-ities such as the Alaska Arctic Coastal Plain suggest a taxodiaceous source for some of the amber there (R. Langenheim et al. 1960, Smiley 1966). The paleo-botanists in this study concluded that the amber was probably derived from taxodiaceous trees growing close to lakes or coastal swamps. Although IR did not clearly indicate taxodiaceous resin and tended to support an araucarian one, J. Langenheim and Beck (1968) concluded that the amber might be from
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more than one botanical source. There is considerable variation in IR of mod-ern taxodiaceous resins, with Sequoia and Taxodium distinct from Meta-sequoia and Sequoiadendron (J. Langenheim 1969). Using Py-MS, however, Poinar and Haverkamp (1985) supported a close relationship of this Alaskan amber to Agathis-type resin. Poinar (1992a) also noted that Podozamites occurs in the amber-bearing beds along with the taxodiaceous remains. Podo-zamites is very similar to present-day Agathis, and Stewart (1983) indicated that it could belong to the Araucariaceae. Thus definite conclusions regard-ing the botanical source of the Alaskan amber remain tenuous.
Other late Cretaceous amber from Tishomingo County, Mississippi, con-sists of small pieces of amber embedded in fossil wood identified as Cupres-saceae s.l. and Pinaceae, thus suggesting both as resin sources. Megafossil remains and IR support the taxodiaceous Metasequoia as the only source of mid-Eocene amber from the state of Washington and neighboring British Columbia (Mustoe 1985). Furthermore, amber has been found in situ in large secretory cysts in a Metasequoia log (Plate 24) in Eocene deposits from Axel Heiberg Island, Canada. Py-GC-MS shows that this amber has a Class Ib polymer (Appendix 3) similar to that of Agathis. These results point out the possibility of confusing taxodiaceous and araucarian sources through the occurrence of communic acid-based polymers (K. Anderson and LePage 1995). Amber from the Fruitland formation (75 Ma) in the San Juan Basin, New Mexico, again provides direct botanical evidence because amber was embedded in a tree trunk identified as taxodiaceous (probably Metasequoia).
Several points stand out about Cretaceous conifer amber. First is the com-plex evolution of modern conifers, characterized by morphological charac-teristics and, possibly, resin chemistry shared by members of different fami-lies. However, Graham (1999) emphasized that it is not only difficult to relate Cretaceous plants to modern taxa but also to reconstruct the fossil North American plant communities because modern analogs do not exist. Macro-fossil and pollen evidence for taxa of Araucariaceae, Cupressaceae s.l., and Pinaceae indicate that the trees grew in mixed conifer forests. Graham further pointed out the importance of a diverse conifer element within Cupressaceae s.l., particularly the deciduous taxodiaceous taxa such as Metasequoia and Parataxodium (extinct) and perhaps those similar to the evergreen Callitris (now primarily Australian). Moreover, a Metasequoia-Taxodium-Sequoia-Sequoiadendron complex (Chapter 2) was abundant in swampy wetland
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habitats. These habitats would provide excellent opportunities for deposi-tion and preservadeposi-tion of resin. In fact, these are the kinds of sites where masses of resin are collected for human use today, for example, various kinds of copals (Chapter 9).
Late Cretaceous amber generally occurs in relatively small pieces and has matured through time. Significantly, the presence of Class Ib polymers (pre-dominance of communic acid or communol polymers) in amber characterizes either an araucarian or taxodiaceous resin and may account for some of the confusion regarding plant source when botanical evidence is lacking. Pina-ceous resin has also been suggested by the presence of retene, an abietic acid derivative in some ambers. Thus the three families are likely to have produced resin preserved in some sites, but apparently with a predominance of resin from one source at other sites (Appendix 4).
The Tertiary Baltic Amber Mystery
The botanical source of the extensive deposits of early Tertiary (Eo-Oligo-cene) amber from the Baltic coast and other areas of Europe has long been intriguing. It is important to note that the commonly used designation, Baltic amber, may be misunderstood as implying that the amber only occurs in and around the Baltic Sea. Although the largest deposits of this amber occur in the Baltic area, it was apparently eroded from marine sediments near sea level, carried ashore during storms, and subsequently carried by water and glaciers to secondary deposits across much of northern and eastern Europe (Bachofen-Echt 1949). Thus Baltic amber constitutes the largest and most widespread deposition of amber in the world. The enormous deposits of amber have been a part of human lore since historical records began, and its origin is part of mythology (Chapter 6). It has also been the amber most intensively studied by scientists since the 19th century.
Mineralogically named succinite, 90% of Baltic amber appears to be from one plant source, although some rarer types of European amber, such as gles-site, may be from another source. The botanical origin of succinite remains controversial because it has not been possible to resolve the conflict between paleobotanical evidence (from enclosed plant remains) and resin chemistry.
Over the years, different coniferous sources have been suggested. Although the amber contains remains of numerous genera of Cupressaceae s.l., mem-bers of Pinaceae have classically been considered to be the source of succinite.
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Preconceived notions that the amber-producing tree was a pine came from several sources. The idea, however, was formalized by Goeppert (1836) when he designated the amber tree as Pinites succinifer, from amber associ-ated with wood. Goeppert used the ending -ites to confer an affinity, but not an identity, of the fossil wood with that of living species of Pinus. Although Conwentz (1890) recognized that the anatomy of the wood was unlike that of any living pine, he included several species of pine and perhaps spruce under a single name, Pinus succinifera. His use of Pinus is probably the source of the considerable subsequent confusion that modern species of Pinus were the source of succinite. Schubert (1961) found droplets of amber in the resin canals in woody tissue characteristic of extinct pines but unlike that of mod-ern pines, providing the most convincing evidence for a pinaceous source.
Larsson (1978) summarized by concluding that an identification as Pinus stretches the evidence, and Pinites (pine-like fossil wood) is all that can be supported paleobotanically. Nonetheless, some investigators continued to seek a chemical relationship of Baltic succinite with that of modern pine resins (e.g., Rottländer 1970, Mosini and Sampri 1985).
Several lines of chemical evidence strongly negate a relationship to species in the modern genus Pinus, as ably summarized by Beck (1999). First, the pri-mary diterpenes, with abietane and pimarane skeletons, in the resin of present-day pines lack the structural prerequisite for polymerization, that is, a
Several lines of chemical evidence strongly negate a relationship to species in the modern genus Pinus, as ably summarized by Beck (1999). First, the pri-mary diterpenes, with abietane and pimarane skeletons, in the resin of present-day pines lack the structural prerequisite for polymerization, that is, a