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Calicioid lichens from European Tertiary amber

     Department of Ecology and Systematics, P.O. Box 65, FIN-00014 University of Helsinki, Finland

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS DISCUSSION LITERATURE CITED

 

Two species of calicioid lichens (Ascomycota) are reported from Baltic amber dating back 55–35 million years ago. The fossils are very similar to some modern Calicium and Chaenotheca species, but because ascus characteristics and photobiont identities cannot be determined they are not assigned to any extant species. Calicioid lichens seem to show a conservative maintenance of morphological adaptations to successful ecological niches, as do several other groups of ascomycetes. The fossils demonstrate that distinguishing features in the morphology of both genera have remained unchanged for at least tens of millions of years. The palaeohabitat of the fossil lichens, viz. that of conifer trunks in a humid, mixed forest, is consistent with the habitat preferences of many modern Calicium and Chaenotheca species.

Key words: Baltic amber, Calicium, Chaenotheca, fossil fungi, lichen

	INTRODUCTION

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Calicioid fungi are a heterogeneous grouping of ascomycetes, with small, usually well-stalked apothecia. Both lichenized and nonlichenized species are involved, and the latter include both parasitic and saprophytic forms (Tibell 1984, 1999a, Wedin and Tibell 1997, Wedin et al 1998, 2000, Tibell and Wedin 2000). Mature ascospores of calicioid lichens tend to accumulate on the upper surfaces of the apothecia, often forming extensive mazaedia.

Here two specimens of calicioid lichens, representing a Calicium Pers. (Caliciaceae) and a Chaenotheca Th. Fr. (Coniocybaceae), are described and illustrated from Baltic amber dating back 55–35 million years ago. Amber is fossilized resin and composed of nonvolatile terpenoid materials, which have been oxidized and polymerized to a point where they can withstand chemical and microbiological attack (Poinar 1992).

	MATERIAL AND METHODS

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Fossilized lichens are contained in two pieces of Baltic amber. Both specimens (V. Arnold 1285 & 1294) originate from deposits in northern Europe and belong to the private collection of Dr. Volker Arnold, Germany. Their exact age is unknown, but the age of Baltic amber generally varies from Late Paleocene to Late Eocene epochs (ca 55–35 MaBP). The forests that produced the resin were in the region of present Fennoscandia. The resin-producing tree usually is referred to as Pinus succinifera (Göppert) Conwentz. Over millions of years, large amounts of solidified conifer resin were transported by rivers to Baltic deposits south of the amber forests (Poinar 1992, Ganzelewski 1997).

The amber specimens have been polished to facilitate screening for inclusions. In this study no further destructive sampling was performed. All measurements and photographs were taken from the intact specimen under transmitted and/or incident light. Optical distortions were neutralized by coating the specimen in vegetable oil. The deep fields of focus in Figs. 1–2 were achieved by combining several optical sections into composite pictures. During this process several stellate hairs and other obscuring debris were removed from Fig. 1. This was done to facilitate a clear view of the fungal ascoma.

View larger version (94K): [in this window] [in a new window]   FIGS. 1–2. Chaenotheca in Baltic amber (V. Arnold, Nr. 1285). 1. Mature ascoma. 2. Cross section of crustose thallus on bark substrate. Scale bars = 200 µm

	RESULTS

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Calicium specimen (V. Arnold, Nr. 1294), Baltic amber (Fig. 5). The specimen consists of a single detached ascoma and numerous spores embedded in solid amber. The ascoma is well stalked, 1.2 µm high, without obvious pruina. Stalk 110–190 µm diam, smooth. Capitulum broadly obconical, 260 µm wide and 120 µm high. Excipulum well developed and continuous with the stalk tissue. Mazaedium well developed, compact. Spores 1-septate, ellipsoidal, 10–15 x µm 5–6 µm diam, apparently with a rough surface ornamentation. Dozens of detached spores occur in a narrow belt extending across the amber specimen, but they are embedded too deeply to be studied or photographed in high magnification. Preserved with detritus and stellate hairs (from flowering plant).

View larger version (175K): [in this window] [in a new window]   FIG. 5. Calicium in Baltic amber (V. Arnold, Nr. 1294). The hair-like projection at the stalk base is a fragment of a plant trichome. Scale bar = 250 µm

	DISCUSSION

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The fossil Chaenotheca is characterized by its whitish gray, well-developed thallus, relatively stout and short-stalked apothecia, pale stalk, well-developed excipulum, and spherical, relatively large spores. A similar combination of features is found in several modern species, including Chaenotheca cinerea (Pers.) Tibell. However, because many Chaenotheca species are morphologically plastic and distinguishing among them often requires examination of ascus and photobiont characteristics, the fossil cannot safely be assigned to any extant species. The fossilized Calicium ascoma basically is identical to those of several extant taxa, including Calicium viride Pers. However, due to the fragmentary nature of the fossil, it cannot be assigned to any modern species.

Some Chaenotheca species on modern conifers, such as C. ferruginea (Turner ex Sm.) Mig. and C. trichialis (Ach.) Th. Fr., sometimes grow on dry conifer resin. In addition, Calicium viride and Cyphelium inquinans (Sm.) Trevis can grow on old resin. An ability to tolerate dry resin would have made these lichens candidates for preservation in amber. The fossilized Calicium obviously was not entrapped in place but either transported by viscous resin or deposited in stationary exudate, after first becoming detached from a bark or lignum surface. The abrupt cut-off scar suggests that the ascoma was dry when it broke from its original substrate, possibly under the foot of a bird or climbing mammal.

Lichens are unlikely candidates for fossilization and thus very few well-preserved lichen fossils have been found. However, the oldest fossils of both ascomycetes and lichens are known from the early Devonian Period, more than 400 MaBP (Taylor et al 1997, 1999). The first lichen symbioses might have evolved even earlier (Selosse 2002). Recent molecular clock estimates have indicated that the major lineages of Fungi already had evolved 1000 MaBP (Heckman et al 2001). Amber fossils have proven that several extant lichen genera, and possibly even species, already were present in the Tertiary Period. Mägdefran (1957) described a fertile alectorioid lichen from Baltic amber. Two fossil species of Parmelia s. lat. have been described from Dominican amber (Poinar et al 2000), and specimens of Anzia recently were found in European amber (Rikkinen and Poinar 2002). In addition, a 12–24 million-year-old impression of a foliose lichen belonging to Lobariaceae was reported by Peterson (2000). The thallus morphologies of these fossils are very similar to those of extant forms.

The close relationship between a resinicolous Chaenothecopsis species from European Eocene amber and modern eastern Asiatic taxa suggests that the current distribution of the species is strongly relictual (Rikkinen and Poinar 2000). Another species of resinicolous calicioids, Mycocalicium sequoiae Bonar, grows only on exudates of Sequoiadendron giganteum (Lindl.) Buchh. and Sequoia sempervirens (Lamb.) Endl. in western North America (Bonar 1971, Tibell and Titov 1995). This relationship probably dates to the Mesozoic Era, when both paleoendemic hosts were widely distributed. Proposed explanations of the disjunctive distributions of some calicioid lichens among Mediterranean Europe/Macaronesia and SW North America, such as those of Thelomma californicum (Tuck.) Tibell and T. mammosum (Hepp) Massal, as Tethyan relicts would imply slow evolution (Jörgensen 1983, Tibell 1994). Printzen and Lumbsch (2000) used molecular evidence to show that diversification within Biatora (Lecanorales) started in the Late Cretaceous and took place during periods of climate cooling, when many new forest vegetation types evolved and spread in the Northern Hemisphere. Printzen and Ekman (2002) concluded that the strongly disjunctive distribution of Cavernularia hultenii Degel. (Lecanorales) among NW North America, Newfoundland and NW Europe is due to fragmentation of a formerly continuous range.

Many modern species of Calicium and Chaenotheca have wide distributions, ranging from Europe to East Asia and further into the boreal and temperate forests of North America. In subtropical and tropical areas, boreal and temperate species generally occur at higher altitudes and occupy zones corresponding to their latitudinal distribution patterns (Tibell 1975, 1980, 1994). Generally, there is little morphological differentiation among populations in the Calicium and Chaenotheca species, with antitropical distributions in both hemispheres (Tibell 1987, 1994, 1999b). This might indicate continuous gene exchange between the populations and/or that morphological differentiation has been very slow (Tibell 1994). However, recent molecular studies have shown that there is wide sequence variation in some morphologically uniform Chaenotheca species and that such differences can increase with the distance between populations. Thus, some Chaenotheca species as conceived from morphological evidence might contain several cryptic taxa (Tibell 2002, Tibell and Beck 2001).

The palaeoenvironment of the fossil calicioids, namely that of tree trunks in a humid forest, is consistent with the habitat preferences of many modern Calicium and Chaenotheca species. Both amber fragments contain stellate trichomes similar to those on bud scales of extant oaks. Such hairs, which commonly are found in Baltic amber, indicate that hardwoods grew in the vicinity of the calicioids. Extant calicioids occur in a variety of habitat types, ranging from bark and lignum to thin hardwood twigs and rocks. High species diversity often is found on the basal trunks of damaged, old trees, especially in humid, semi-open mixed forests. Many species favor aged forests with long habitat continuity, these being rich in suitable substrates and providing a continuously high atmospheric humidity. On the other hand, many modern species are widely distributed and calicioid communities tend to be uniform over large areas. For example, extant lichen communities with many crustose calicioids are quite similar in species composition over the whole circumpolar belt of boreal coniferous forests and adjoining areas of mixed coniferous-deciduous forest (Tibell 1975, 1992, 1999a).

View larger version (114K): [in this window] [in a new window]   FIGS. 3–4. Chaenotheca in Baltic amber (V. Arnold, Nr. 1285). 3. Stalk. 4. Detached ascospores on thallus. Scale bars = 20 µm

	ACKNOWLEDGMENTS

	FOOTNOTES

 
¹ Corresponding author. E-mail: jouko.rikkinen{at}helsinki.fi

Accepted for publication April 24, 2003.

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