Review of Palaeobotany and Palynology 210 (2014) 113–118

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Research paper

An in situ preserved moss community in Eocene Baltic amber

Jochen Heinrichs a,⁎, Lars Hedenäs b, Alfons Schäfer-Verwimp c, Kathrin Feldberg a,Alexander R. Schmidt d

a Ludwig-Maximilians-Universität München, Department für Biologie, Systematische Botanik und Mykologie, Menzinger Str. 67, 80638 München, Germanyb Department of Botany, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Swedenc Mittlere Letten 11, 88634 Herdwangen-Schönach, Germanyd Georg-August-Universität Göttingen, Courant Research Centre Geobiology, Goldschmidtstraße 3, 37077 Göttingen, Germany

⁎ Corresponding author. Tel.: +49 89 17861 302.E-mail address: jheinrichs@lmu.de (J. Heinrichs).

http://dx.doi.org/10.1016/j.revpalbo.2014.08.0050034-6667/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 April 2014Received in revised form 6 August 2014Accepted 10 August 2014Available online 17 August 2014

Keywords:BryophytaClassificationFossilHypnalesPyrrhobryumRhizomnium dentatum

We describe a bryophyte community in a piece of Eocene Baltic amber housed in the American Museum ofNatural History in New York. Five gametophytic plant fragments with zig-zagged stems, rounded-obtuse,bordered leaves with shortly and narrowly decurrent bases, non-mamillose, rhomboidal or elongate-hexagonal cells and a single costa are described as Rhizomnium dentatum, sp. nov., differing from extant speciesby some leaveswith shortly-dentatemargins. Syninclusions are Pyrrhobryum sp. and a pleurocarpousmoss of theorder Hypnales. The ecology of the extant representatives of these taxa and the presence of a chilopod suggestthat the preserved bryophyte community grew in short distance to the amber-procucing tree or on its rootplate, rather than epiphytically.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

One of the best-documented Eocene terrestrial biocoenoses derivesfrom the “Baltic amber forest” (Weitschat and Wichard, 2002), awarm-temperate mixed forest area dominated by oaks and variousconifers (Goeppert and Berendt, 1845; Conwentz, 1890; Arnold, 1998;Standke, 2008; Wolfe et al., 2009). These conifers produced largeamounts of resin that over time transformed into the largest amberdeposit worldwide (Weitschat and Wichard, 2010). Baltic amber isfamous for its organismic inclusions, and represents a productive sourceof bryophyte fossils (Grolle and Meister, 2004; Frahm, 2010). To date,Baltic amber has yielded 64 species of mosses (Frahm, 2010). Amongthese were three representatives of Mniaceae, an early diverginglineage of acrocarpous mosses (Cox et al., 2010) being characterizedby the presence of buds in the axil of every leaf, bordered leaves witha simple costa, rounded-quadrate to hexagonal lamina cells, and thefrequent presence of polysety, with each spore capsule on a long seta(Koponen, 1968a). All inclusions of Mniaceae described thus far fromBaltic amber have been assigned to the genus Trachycystis (Frahm,1994, 2010), which represents the only extant genus of Mniaceae withmamillose leaf cells.

Recently, we discovered a piece of Baltic amber in the entomologicalamber collection of the American Museum of Natural History (NewYork) that includes gametophytic fragments of three mosses, as wellas a chilopod (=centipede). Among the moss fragments were fiveMniaceae-like shoots with sub-complanate, non-mammillose leaf cells.The texture of the leaf surface argues against affinities of the fossil toTrachycystis, and hence represents a taxon not recorded previously forBaltic amber. In this study the fossils are assigned to the extant genusRhizomnium and the name Rhizomnium dentatum is proposed for thespecies. In addition, we identify the syninclusions and discuss the possi-ble microenvironment of the moss community. We trust that the de-scription of this and further in-situ-preserved bryophyte communitieswill eventually contribute to our understanding of the Baltic amber forestecosystem.

2. Material and methods

A single block of Eocene Baltic amber (42 × 29 × 6 mm) containingseveral unidentified bryophyte inclusions and a chilopod occurs inthe collection of the American Museum of Natural History [speci-men AMNH Ba-0200 (Plate I)]. The piece of amber was fullyembedded in a high-grade epoxy (Buehler Epoxicure) under vacu-um [see Nascimbene and Silverstein (2001) for protocols]. The ageof Baltic amber ranges between 35 and 48 Ma (Standke, 1998).

Inclusionswere analyzed with a Carl Zeiss Stemi 2000 incident-lightmicroscope and a Carl Zeiss AxioScope A1 compound microscope, each

Fig. 1. Reconstruction of the gametophyte of Rhizomnium dentatum showing a sector of asterile shoot (A) and the leaf cell pattern in surface view (B).

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equipped with a Canon 60D digital camera. In some instances, incidentand transmitted light were used simultaneously. The illustrationsprovided in Plates I to II are digitally stacked photomicrographiccomposites of up to 110 individual focal planes obtained by using thesoftware package HeliconFocus 5.0 developed to enhance illustrationsof three-dimensional structures.

Plate I. Baltic amber piece AMNH-Ba-0200 with inclusions of bryophytes and a chilopod (all sc

Fig. 1 View from above, holotype of Rhizomnium dentatum sp. nov. next to arrowheFig. 2 View from below.

3. Systematic palaeobotany

The botanical inclusions consisted of 14–15 sterile fragments ofmossesthat represent three distinguished morphotypes. One morphotype isdescribed as a new species, the others are regarded as syninclusions.

3.1. Mosses identified to genus level

Phylum BryophytaSubphylum BryophytinaClass Bryopsida

3.1.1. Family MniaceaeGenus Rhizomnium (Mitt. ex Broth.) T.J. Kop.Species Rhizomnium dentatum Heinrichs, Hedenäs, Schäf.-Verw.,

Feldberg & A.R. Schmidt, sp. nov.Holotype: AMNH-Ba-0200 p.p. [1 shoot, Plate I, 1 (plant next to

arrowhead), Plate II, 3 (overview), 4, 5 (details)]Paratypes: AMNH-Ba-0200 p.p. (4 shoots, Plate II, 1, 2, 6, 7).Repository: All material is deposited in the American Museum of

Natural History (AMNH).Etymology: The specific epithet refers to the dentate leaf margins.Specific diagnosis: Stem zig-zagged; leaves with short apiculus and

uni- to bistratose border of elongate cells; leafmarginwith a few looselyarranged, short teeth or entire; costa simple, usually ending well belowleaf apex.

Description: Stems up to ca 7 mm long, unbranched, 120–210 μmbroad in central sectors of the gametophyte, near top of shoots only70 μm, weakly zig-zagged with one leaf inserted at each angle. Neithermicro- nor macronemata observed. Mature leaves rounded-oblong,suborbicular, or obovate, very narrowly decurrent at base, 2.20–2.80 mm long and 1.40–1.92 mm wide, apex broadly rounded andoften with a short, rounded apiculus. Margin entire or with a fewteeth; teeth coarse in distal portion but increasingly inconspicuoustowards the leaf base, 1–2 rows of marginal cells longly linear andforming a distinct border. Costa single and long, mostly ending clearlybelow the leaf apex, rarely extending almost to the apex, ca. 50 μmwide in lower third of leaf, composed of 2–3 rows of elongate cells.Median leaf lamina cells rhomboidal or elongate-hexagonal,24–35(45) × 15–24 μm, tending to form rows diverging forwards-outwards from the costa.

ale bars = 5 mm).

ad.

Plate II. Rhizominium dentatum sp. nov., a moss from the Eocene of the Baltic region of Europe (all scale bars = 500 μm).

Fig. 1 Paratype; view from above of the sterile gametophyte. Note that the whole gametophyte is heavily pyritisized and that the structures on the leaf laminas do notrepresent cell walls.

Fig. 2 Paratype; sterile gametophyte in lateral-ventral view.Fig. 3 Holotype; sterile gametophyte, view from above.Figs. 4 & 5 Leaf of holotype with dentate margin, ventral view.Fig. 6 Leaf of paratype, dorso-lateral view.Fig. 7 Leaf of paratype, dorsal view.

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3.1.2. Family RhizogoniaceaeGenus PyrrhobryumSpecimen AMNH-Ba-0200 p.p. At least six, possibly seven shoots of a

moss with linear leaves [Plate III, 1–4].Description: Stems simple, up to ca. 12 mm long but lowermost

portions not preserved, diameter ca. 0.2–0.3 mm. Leaves laxly–spirallyarranged, linear, in lower ca. half parallel-sided, above graduallynarrowing to a relatively broadly acuminate apex. Longest leaves2.80–4.40 mm long, 0.28–0.44 mm wide in basal portion. Costacomposed of elongate-rectangular cells, single, ending in apex or shortlyexcurrent, with single prominent teeth at least in upper half. Medianleaf lamina cells varying around quadrate (transversely rectangular toshortly rectangular), thin-walled or slightly incrassate, 12–15 μmwide, in upper leaf slightly narrower, with many cells only 10 μmwide. Marginal leaf cells similar in length or slightly longer than cellsfurther in, apparently bistratose (muchmore opaque than other laminacells), forming a distinct border. Upper and middle leaf margin withgeminate teeth pointing in forward direction.

The inclusions correspondwell with the plants thatwere recognizedby Frahm (2010: 39) as Pyrrhobryum sp. and putatively assigned to theextant species Pyrrhobryum spiniforme (Hedw.) Mitt. by Frahm andGröhn (2013). We abstain from adopting the above species identifica-tion because numerous taxonomically relevant characters are notpreserved in the fossils. Furthermore, the fossils differ in size fromextant plants of Pyrrhobryum spiniforme (Frahm, 2010).

3.2. Pleurocarpous mosses not identified to genus level

Specimen AMNH-Ba-0200 p.p. Three gametophyte fragments of apleurocarpous moss [Plate III, 5, 6].

Phylum BryophytaSubphylum BryophytinaClass BryopsidaOrder HypnalesDescription: Sterile unbranched shoot fragments. Leaves spirally

arranged, mostly patent to almost spreading, sometimes from patentbase directed forwards, slightly homomallous, ovate or narrowlyovate, the distal 2/3 gradually narrowing to relatively short acuminateapex (apical cell sometimes acute), with subterminal lateral teeth,size, except for the most apical leaves, 0.90–1.60 × 0.24–0.50 mm,concave. Costa single and long, ending well below leaf apex, ca. 20 μmwide near base. Margin coarsely denticulate near apex, becomingmore finely denticulate towards base, entire or almost entire nearbase, teeth single, margin not bordered. Leaf lamina cells linear ca.24–30 × 6 μm, slightly incrassate. Cells above shorter, ca. 14–18 μmlong. Differentiated alar groups not visible on shoots (at least not largeand well differentiated), leaves not decurrent.

The morphological features that are available do not allowassignment of these inclusions to any family or genus of mosses.However, the prosenchymatous leaf cells suggest affinities to thepleurocarpous mosses of the order Hypnales.

4. Discussion

Some 150 extinct or extant moss taxa are known from the Neogeneor Paleogene of Europe (Frahm, 2000) including several taxa of theMniaceae genera Trachycystis and Mnium. The Mniaceae fossilsdescribed in this study have non-mammillose leaf cells and thus donot belong to the genus Trachycystis. Moreover, they are distinguishedfrom the extinct Mnium rottense which has been described from an

Plate III. Syninclusions of Rhizomnium dentatum sp. nov. in amber piece AMNH-Ba-0200.

Fig. 1 Pyrrhobryum sp.; upper part of sterile gametophyte in lateral view (scaleFig. 2 Pyrrhobryum sp.; upper part of leaf (scale bar = 100 μm).Figs. 3 & 4 Pyrrhobryum sp.; central portions of leaves (scale bars = 50 μm).Figs. 5 & 6 Pleurocarpous moss; upper parts of shoots in lateral view (scale bars =

Oligocene Lagerstätte near Rott in Rhineland Palatinate, Germany(Weyland, 1937). This sediment-preserved fossil species is character-ized by acute leaves with a costa that extends to the leaf apex ratherthan suborbiculate leaves with a costa ending well below the apex.Thus, the new fossils from Baltic amber cannot be attributed to any ofthe Mniaceae taxa described previously from the Oligocene or Eocene,but rather represent a new taxon.

The current Mniaceae taxonomy is largely based on the work ofKoponen (1968a) who subdivided the genus Mnium into the generaCyrtomnium,Mnium s.str., Plagiomnium, Pseudobryum, and Rhizomnium,and recent molecular phylogenetic studies which demonstrated thatthe Mniaceae also include several additional genera such as Pohlia andMielichhoferia (Goffinet et al., 2001; Cox et al., 2010; Guerra et al.,2011). Characters of the sporophyte, which are required to separatesome of the above genera are not preserved in the fossils. However,the zig-zagged stem, the rounded-obtuse leaves with shortly andnarrowly decurrent bases and the rather short, single costa are typicalfeatures of Rhizomnium, to which the amber inclusions are assigned(Fig. 1). They differ from extant species mainly by the presence ofsome clearly toothed leaves. Nowadays, Rhizomnium is a common andwidespread genus in the Northern Hemisphere and includes some 13species (e.g., Koponen, 1968b, 1973, 1980; Noguchi, 1989; KoponenandCzernyadjeva, 2006).Rhizomnium typically grows in shady environ-ments, on rocks, soil and rotten wood. Possibly Rhizomnium dentatumgrew in a similar environment. The syninclusion Pyrrhobryum sp. pointsto a forest floor community and also the chilopod syninclusion suggestsa close proximity to leaf litter and soil habitats (Weitschat andWichard,2010), possibly at a short distance from the amber-procucing tree or toan occurrence on its root plate. This type of habitat is also typical ofmany hypnalean mosses. The lowermost parts of the bryophyte inclu-sions are lacking, providing complementary evidence of a terrestrialcommunity, in which only the upper portions of the plants at somepoint became enshrined by a resin flow. Although many amberinclusions of bryophytes seem to represent epiphytic taxa (Grolle andMeister, 2004), several examples of putatively terrestrial forms havealso been detected in amber (e.g., Sphagnum sp., Frahm, 2009). Thepiece of amber considered in this study appears to come from such ahabitat. Frahm and Newton (2005) identified a fragment of a mosswith 7 mm long, decurrent, oblong leaves in Miocene Dominicanamber as Plagiomnium cf. rhynchophorum (Hook.) T. Kop. This plantpossibly grew in a similar microhabitat as our community.

Themoss community described here indicates that themoss flora ofthe Baltic amber forest is not yet completely known, despite detailedand comprehensive studies by Frahm (2010 and references citedtherein) which led to the discovery of some 64 species. Frahm (2004,2010) postulated that many inclusions of bryophytes in Baltic amberbelong to extant species. This is somewhat unlikely with regard to theminimum age of the amber of 35 Ma (Standke, 1998) and frequentevidence of morphologically cryptic speciation in bryophytes(Hedenäs and Eldenäs, 2007; Kreier et al., 2010; Carter, 2012), however,little is known to date on the actual ages of bryophyte species (Huttunenet al., 2008; Heinrichs et al., 2009; Sun et al., 2014). It cannot be ruledout at present that a few isolated species lineages of bryophytes maybe Paleogene in origin, but this hypothesis still awaits testing throughdivergence time analyses based on DNA sequence variation with thefossil record integrated as age constraints. A reciprocal illuminationapproach comparing the age estimates obtained by analyses withdifferent fossil assignments may allow to balance between alternativetaxonomic conclusions, and assess the likelihood of at least certainextant species dating back to the Eocene (Feldberg et al., 2013;

bar = 500 μm).

100 μm).

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Heinrichs et al., 2013). Only a limited number of Mniaceae have beenincluded inmolecular phylogenetic analyses, preventing a reconstructionof their diversity in time and space. A detailed study of the Mniaceaegenus Plagiomnium based on chloroplast and nuclear DNA (Harris,2008) suggests topological incongruence and hybridization eventsincluding allopolyploidy (Wyatt et al., 1988). Similar observations areavailable for Rhizomnium (Wyatt et al., 1993; Jankowiak et al., 2005). Inthe light of these results and the morphological differences between thestudied Mniaceae fossils and extant taxa, we decided to describe thefossils as an extinct species, rather than attributing them to an extantbinomial. We hope that at some point in near future Rhizomniumdentatumwill be used for age calibration in the study of the historical bio-geography of Mniaceae.

Acknowledgments

We are indebted to David A. Grimaldi and Paul C. Nascimbene(American Museum of Natural History, New York) for supporting thestudy of bryophytes from the amber collection of the AmericanMuseumof Natural History. This is publication number 134 from the CourantResearch Centre Geobiology that is funded by the German ExcellenceInitiative.

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