# Paleohistology of the Cretaceous resin-producing conifer
Geinitzia reichenbachii using X-ray synchrotron
microtomography

Jean-david Moreau, Marc Philippe, Didier Nérandeau, Eric Dépré, Matthieu

Le Couls, Vincent Fernandez, Simon Beurel

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## To cite this version:

Jean-david Moreau, Marc Philippe, Didier Nérandeau, Eric Dépré, Matthieu Le Couls, et al.. Paleohistology of the Cretaceous resin-producing conifer Geinitzia reichenbachii using X-ray synchrotron microtomography. American Journal of Botany, 2021, 108 (9), pp.1745-1760. 10.1002/ajb2.1722 . insu-03340210

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# Paleohistology of the Cretaceous resin-producing conifer *Geinitzia reichenbachii* using X-ray synchrotron microtomography

Jean-David Moreau<fnref n="1" />

<figure><img src="image_3.png" /></figure>

Matthieu Le Couls<fnref n="3" />

Marc Philippe<fnref n="2" />
Vincent Fernandez<fnref n="5" />,<fnref n="6" />

Didier Néraveau<fnref n="3" />
Simon Beurel<fnref n="3" />,<fnref n="7" />

Eric Dépré<fnref n="4" />

<figure><img src="image_4.png" /></figure>

### Correspondence

Jean-David Moreau, Biogéosciences, UMR CNRS 6282, Université de Bourgogne Franche-Comté, 6 boulevard Gabriel, Dijon 21000, France.
Email: jean.david.moreau@gmail.com

### Abstract

**Premise:** The conifer *Geinitzia reichenbachii* was a common member of the Cretaceous Laurasian floras. However, the histology of *G. reichenbachii* leafy axes was never described in detail, and our knowledge of its paleoecology remains very limited. Using new and exquisitely preserved silicified material from the Upper Cretaceous of western France, we describe *G. reichenbachii* from the gross morphology to the cellular scale, then discuss paleoecological and taphonomical implications.

**Methods:** We examined specimens from two localities in western France (Claix and Moragne) using propagation phase-contrast X-ray synchrotron microtomography.

**Results:** The cuticle and the inner tissues of leafy axes are preserved in three dimensions. Epidermis, hypodermis, palisade parenchyma, spongy parenchyma, transfusion tracheids, and most of tissues of the vascular cylinder are clearly discernible. The numerous resin ducts are sometimes filled by persisting resin. Additionally, surfaces of some leaves preserved drops and flows of resin.

**Conclusions:** Depositional environmental context combined with histological features of *G. reichenbachii* suggest that this conifer was adapted to a range of marginal-littoral ecosystems including those open to the sea paleoenvironments and innermost ones influenced by strong continental inputs. *Geinitzia reichenbachii* was adapted to withstand intense sunlight, hot temperatures coupled with salty sea wind, and dry conditions. The frequent amber-bearing beds in the Cretaceous from western France with *Geinitzia* as a main component of the associated floras raises the possibility of a role for *Geinitzia* in the production of the “Charentese amber”.

### KEYWORDS

Coniferales, fossils, Geinitziaceae, leafy twigs, Mesozoic, paleobotany, paleoecophysiology, permineralization, silicification, X-ray synchrotron microtomography

The conifer *Geinitzia* Endl. is mainly known from Mesozoic deposits and more particularly Cretaceous ones (e.g., Raubeson and Gensel, 1991; Herman and Kvaček, 2010; Halamski, 2013). More rarely, the genus has also been tentatively reported from the Permian (Meyen, 1997; Gomankov, 2020). *Geinitzia* has a broad geographical distribution, including America, Asia, and Europe (e.g., Hollick and Jeffrey, 1909; Durante, 1983; Raubeson and Gensel, 1991; Kunzmann, 1999; Burger and Ward, 2016; Yang et al., 2019), but is most abundant in the Cretaceous paleobotanical sites of Europe (e.g., Stockmans, 1946; Němejc and Kvaček, 1975; Barale et al., 2002; Gomez et al.,

2002, 2015; Kunzmann et al., 2003; Sender et al., 2008; Bosma et al., 2009; Herman and Kvaček, 2010; Kunzmann, 2010; Halamski, 2013; Kvaček et al., 2015; Halamski and Kvaček, 2015, 2016; Halamski et al., 2018; Płachno et al., 2018). This conifer was clearly a common member of Laurasian floras during the Cretaceous. However, the histology of *Geinitzia* leafy axes was never described in detail, and our knowledge of its paleoecology remains very limited.

In western France, the genus *Geinitzia* was reported from numerous Albian (Lower Cretaceous) to Senonian (Coniacian-Maastrichtian; Upper Cretaceous) paleobotanical sites, particularly in the Charente, Charente-Maritime,

<footnote n="1">¹Biogéosciences, UMR CNRS 6282, Université de Bourgogne Franche-Comté, 6 boulevard Gabriel, Dijon 21000, France</footnote>

<footnote n="2">²Université de Lyon, Claude-Bernard Lyon-1, ENTPE, CNRS, UMR 5023 LEHNA, Villeurbanne 69622, France</footnote>

<footnote n="3">³CNRS UMR 6118 Géosciences, Université Rennes 1, Campus de Beaulieu, 263 avenue du Général Leclerc, Rennes, Cedex 35042, France</footnote>

<footnote n="4">⁴GIP-GEVES (Groupement d'Étude et de Contrôle des Variétés et Semences), Le Magneraud, Surgères F-17700, France</footnote>

<footnote n="5">⁵Imaging and Analysis Centre, Natural History Museum, Cromwell Road, London SW7 5BD, UK</footnote>

<footnote n="6">⁶European Synchrotron Radiation Facility, 7 avenue des Martyrs, Grenoble 38043, France</footnote>

<footnote n="7">⁷Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Invalidenstraße 43, Berlin 10115, Germany</footnote>
Dordogne, Maine-et-Loire, and Vienne departments (Gomez et al., 2004, 2019; Néraveau et al., 2005, 2020; Saint-Martin et al., 2013; Moreau et al., 2014b, 2016, 2017a, 2018, 2019). In most of them, plant remains are preserved as impressions or compressions in lignitic clay, with or without preserved cuticle (Gomez et al., 2004; Néraveau et al., 2005, 2020; Saint-Martin et al., 2013; Moreau et al., 2017a). Due to the nature of this preservation, tissues of *Geinitzia* from this area were never observed. Over the last few years, new prospecting in the Aquitaine Basin and the Seuil du Poitou led to the discovery of exquisitely preserved Cretaceous plants inside silica-rich nodules (Moreau et al., 2014a, 2016, 2018, 2019; Néraveau, 2015; Gomez et al., 2019). Silicified leafy axes ascribed to *Geinitzia* were reported in flints from Charente-Maritime (Moreau et al., 2014a, 2014b, 2019), Charente (Moreau et al., 2016), and Vienne (Moreau et al., 2018; Gomez et al., 2019). However, the inner structures of these specimens have never been investigated. Because of its utility in the analysis of paleobotanical material (e.g., Feist et al., 2005; Friis et al., 2007; Strullu-Derrien et al., 2014; Evreïnoff et al., 2017), we used a nondestructive imaging technique useful for observing inner and hidden structures: propagation phase contrast X-ray synchrotron microcomputed tomography (PPC-SRµCT). Here, given the preservation of this exceptional new material in hard flints, we discuss the taphonomic processes. Then based on the depositional context of plant-bearing beds and on the histological features of leafy axes of *Geinitzia reichenbachii* (Geinitz) Hollick et Jeffrey, we discuss the paleoecology of this common and widespread species.

## GEOLOGICAL SETTING

The plants studied here are preserved inside flints collected from two localities in the Aquitaine Basin (western France), of the Charentes region, at Claix in the Charente department, and at Moragne in Charente-Maritime (Figure 1). The Moragne area consists of a small hill of lower

Cenomanian sandstones and clays, which are overlain by Palaeogene alteritic deposits (Bourgueil et al., 1972). The fossiliferous flints studied here were all collected in the Eocene-Oligocene residual formation that constitutes the top of the hill and is mainly composed of gravel, sand, silt, and clay. In a recent study, Moreau et al. (2019) demonstrated the co-occurrence of marine organisms and terrestrial plants in the Upper Cretaceous flints from Moragne. The marine organisms consist of sponge spicules, echinoids (*Periaster undulatus* Agassiz et Desor), and rudists (*Sphaerulites foliaceus* Lamarck; hippuritids and radiolitids). Terrestrial plants are represented by foliar macroremains and woods. Foliar remains consist of fragmented pinnae of ferns and diverse structures of conifers including leafy axes (*G. reichenbachii*; Glenrosa carentonensis Moreau, Néraveau, Tafforeau et Dépré) and isolated ovulate scales or subtending bracts of cones. The paleontological assemblage shows that the Paleogene alterites from Moragne are condensed deposits resulting from the alteration of the Cenomanian to Turonian-Senonian regional series (Moreau et al., 2019).

At Claix, the Turonian limestone (Bourgueil et al., 1970) is overlain by Palaeogene alteritic deposits containing flints. The flints collected in this area are considered Turonian-Coniacian in age (Moreau et al., 2016). They are very rich in plant macroremains including leaves of angiosperms, leafy axes of conifers [*Brachyphyllum* Brongn., *Frenelopsis* (Schenk) emend. J. Watson, *Geinitzia reichenbachii*], and fern pinnae, which are too fragmentary to be identified. These foliar remains co-occur in flints with marine organisms such as spatangoid echinoids (*Periaster* d'Orbigny).

## MATERIALS AND METHODS

In both localities, *G. reichenbachii* is the main component of the paleobotanical assemblage. Specimens were found on broken surfaces of flint nodule fragments. They are embedded in a dense and hard silica-rich matrix. Specimens consist of isolated leafy axes without reproductive

<figure><img src="image_5.png" /><figcaption>**FIGURE 1** Geological map of Charente and Charente-Maritime (Charentes region) showing location of Claix and Moragne. Dark gray corresponds to the Albian-Cenomanian deposits</figcaption></figure>
structures. Plant remains are preserved in three dimensions. The collection includes 27 specimens and is housed at Géosciences Rennes, CNRS UMR 6118, Université Rennes 1 (Rennes, France).

To characterize the inner structures of *G. reichenbachii*, we examined four exquisitely preserved specimens (IGR-PAL-2684, IGR-PAL-2741-B, IGR-PAL-2784, IGR-PAL-2791) at beamline ID19, of the European Synchrotron Radiation Facility (ESRF, Grenoble, France) using PPC-SRµCT (Tafforeau et al., 2006). Because the plant-bearing nodules were large and dense, we followed a tomographic protocol combining high energy and high resolution (Moreau et al., 2015). We used an isotropic voxel size of 2.26 µm. A propagation distance of 0.8 m between the specimen and the detector was chosen to use the properties of the propagation phase contrast. Phase contrast imaging is more sensitive to density variation than traditional absorption-based µCT, resulting in a significant increase of contrast in the tomographic data. This approach enables reconstruction of virtual thin sections of the specimens and their inner microstructures, comparable to real thin sections that would be observed with classic light microscopy. We used an effective energy of 125 keV based on the white beam spectrum of a W150b wiggler set to a gap of 35 mm and filtered with 6 mm of copper. We used a PCO.edge 4.2 camera (PCO, Kelheim, Germany) mounted on lens-based optical system coupled with a 200-µm thick LuAG scintillator. To increase the reconstructed horizontal field of view, we used continuous rotation mode over 360° in half-acquisition mode (center of rotation shifted near the edge of the detector), 6000 projections, and an exposure time of 0.01 s per projection for the scans. The scans were reconstructed using a back-projection algorithm implemented in the PyHST2 software (High Speed Tomography in python version, ESRF; Mirone et al., 2014), coupled with a single-distance phase-retrieval process modified from the algorithm of Paganin et al. (2002), which incorporates an unsharp mask applied to the retrieved phase maps (Sanchez et al., 2012). Data were finally converted into a 16-bit tiff stack of slices corrected for residual ring artefacts (Lyckegaard et al., 2011) and cropped. The histological virtual sections and three-dimensional reconstructions were achieved with the software VG Studio Max 2.2 (Volume Graphics, Heidelberg, Germany). The resolution used in the current tomographic configurations and probable preservation bias, although limited, did not allow observation of all histological details such as tissues in the vascular bundle of leaves and the phloem and cambium of the stem.

Micrographs of the wood were taken using a Phenom XL Desktop scanning electron microscope (SEM) at Géosciences Rennes, University of Rennes 1. Microsamples of wood were removed from the surface of the specimen IGR-PAL-2801 using a needle. The sample was mounted on an aluminum stub using carbon-impregnated double-sided tape and sputter-coated with gold. Analyses were conducted using high-vacuum mode (1 Pa) at 10 kV.

# RESULTS

## Systematics

Order: Coniferales.

## Family

Geinitziaceae Kunzmann, 1999.

## Genus

*Geinitzia* Endlicher, 1847.

## Species

*Geinitzia reichenbachii* (Geinitz, 1842) Hollick et Jeffrey, 1909 (Figures 2–9).

## Material

Five specimens from the Turonian-Coniacian of Claix (IGR-PAL-2682 to IGR-PAL-2684, IGR-PAL-2740B, IGR-PAL-2741B) and 22 specimens from the Cenomanian-Senonian of Moragne (IGR-PAL-2783 to IGR-PAL-2794, IGR-PAL-2795A, IGR-PAL-2795B, IGR-PAL-2796 to IGR-PAL-2799A, IGR-PAL-2799B, IGR-PAL-2801 to IGR-PAL-2803). Collection of Géosciences Rennes, CNRS UMR 6118, Université Rennes 1 (Rennes, France).

## Description

Shoots are up to once branched. Branches arise at up to 45°. The longest specimen is 70 mm long (IGR-PAL-2783). Leafy shoots are up to 18 mm in diameter, straight to slightly curved (Figure 2). Leaves are helically arranged, elongated, and have a long free part that forms an angle of up to 70° with the main axis of the shoot (Figure 2). Leaves are rhomboidal to triangular in transverse section (Figures 3A, 3B, 4E–J), triangular in abaxial and adaxial views (Figure 4C, D), and falcate in sagittal section (Figures 2, 4A). They are up to 21 mm long and up to 2.5 mm wide and slightly keeled on the abaxial side (Figure 4G–J). The leaf margin is entire. The apex of leaves is acute and commonly slightly curved inward (Figures 2, 4A–D). Occasionally, the apex of leaves is slightly recurved outward.

In the stem, in transverse section, numerous resin ducts up to 1200 µm wide occur in the cortex around the vascular cylinder (Figures 3A–D, 5A). They are arranged parallel to the stem axis (Figure 3C–D). Up to 10 resin ducts occur at the base of a leaf (Figures 3A–B, 4E–F). Six unramified resin ducts occur in the free part of each leaf (Figure 4A–J). They
<figure><img src="image_6.png" /><figcaption>**FIGURE 2** Leafy twigs of *Geinitzia reichenbachii* embedded inside flint nodules, photographs (A, C, E, G, I, K, M, O) and interpretative sketches (B, D, F, H, J, L, N, P). (A, B) IGR-PAL-2769. (C, D) IGR-PAL-2798. (E, F) IGR-PAL-2799. (G, H) IGR-PAL-2789. (I, J) IGR-PAL-2791. (K, L) IGR-PAL-2784. (M, N) IGR-PAL-2741-B. (O, P) IGR-PAL-2785. All scale bars = 1 cm</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**FIGURE 3** PPC-SRµCT, virtual sections of *Geinitzia reichenbachii* leafy axis. (A, B) Transverse section of leafy axis and interpretative sketch. (C, D) Longitudinal section of leafy axis and interpretative sketch. Pi., pith; R.d., resin ducts; Xy.II, secondary xylem. Orange areas on sketches corresponds to part of resin ducts full of resin. IGR-PAL-2684. Voxel side = 2.26 µm. All scale bars = 1 mm</figcaption></figure>

converge toward the leaf apex and are located along and around the vascular bundle, on both the abaxial and adaxial sides. Although all resin ducts occur along the main length of the leaf, three resin ducts end just before the apex (Figure 4I, J). In cross section, the duct lumen is round to oval (Figure 4G–J). Lumina are surrounded by one layer of elongated, thick-walled sheath cells 40–190 µm long, 12–45 µm wide. In one of the specimens (IGR-PAL-2684), most ducts are filled with more or less transparent, orange-brown matter showing conchoidal fractures with a resinous bril-
liance (Figures 3A–D, 5A–C, 6). PPC-SRµCT virtual sec-
tions revealed that the density of this matter is much lower than that of the surrounding mineralization. We interpreted this matter as persisting resin. The preservation of resin ducts is greatly variable. Those filled with a silica matrix appear white to gray on PPC-SRµCT virtual sections, whereas those filled by resin (the least-dense material) ap-
pear black in the PPC-SRµCT virtual section (Figure 3).

Surfaces of some leaves preserved drops of resin and resin flows (Figure 5B–C). Locally, drops of resin are isolated in the silica-rich matrix around the leafy axis.

The cuticle is 5–10 µm thick (Figure 7A, B). Leaves are amphistomatic, and stomatal apparatuses form stomatal bands. Stomatal apparatuses are densely arranged and commonly oriented transversally or obliquely (sometimes longitudinally; Figure 7C–E). The stomatal apparatuses are monocyclic to amphicyclic (Figure 7D, E). They are 60–95 µm long and 40–55 µm wide. Guard cells are 60–95 µm long and 15–22 µm wide. Four to five (rarely six), bean-shaped sub-
sidiaries constitute an oval ring, 70–105 µm long and 55–70 µm wide (Figure 7D–E). The epidermis is 7–22 µm thick. Ordinary epidermal cells are square, rectangular, or polygonal, 20–95 µm long and 5–30 µm wide (Figure 7A–F). The anti-
clinal walls of ordinary epidermal cells are straight to slightly curved and up to 5 µm thick. A hypodermis mainly occurs on the adaxial surface; it is partially or totally absent on the
<figure><img src="image_8.png" /><figcaption>FIGURE 4 (See caption on next page)</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**FIGURE 5** PPC-SRµCT, virtual sections showing abundance of amber amongst *Geinitzia reichenbachii*. (A) Transverse section of leafy axis showing abundance of resin (dark area on tomographic reconstruction) preserved inside resin ducts. (B, C) Transverse sections of a leaf showing abundance of resin (dark area on tomographic reconstruction) preserved inside resin ducts as well as resin flows and resin drops preserved on the surface of leaves.
Abbreviations: R.d., resin ducts; Re.d., resin drops; Re.f., resin flows. IGR-PAL-2684. Voxel side = 2.26 µm. All scale bars = 500 µm</figcaption></figure>

abaxial surface (Figure 7F–H). The hypodermis consists of a single layer of elongated cells, up to 180 µm long, 7–30 µm wide, and 10–22 µm high (Figure 7F–H). The anticlinal walls of the hypodermal cells are straight. Palisade parenchyma usually consists of one layer (rarely two) of elongated cells (Figure 7F–I) on the adaxial and abaxial sides but is not developed along the entire leaf length (Figure 4A–D). Cells of the palisade parenchyma are 35–120 µm long and 10–70 µm wide (Figure 7F, H). Anticlinal walls are curved to straight and less than 3 µm thick. Spongy parenchyma is mainly present at the base of leaves on the abaxial and adaxial sides (Figures 4A–D,

7I). Spongy parenchyma is thinner toward the leaf apex. Spongy parenchyma consists of up to six layers of large and rounded cells (Figure 7J). Cells are separated by large intercellular spaces, forming lacunae (Figure 7K). Cells vary in size up to 95 µm in diameter. The anticlinal walls are curved. Individual or cluster of large, rounded sclerenchyma cells (probable stone cells) are dispersed between parenchyma cells (Figure 7L). They are mainly located at the base of leaves in the cortex of stem. Transfusion tissue is up to 200 µm thick, occurs laterally on both sides of the vascular bundle (Figure 4A–D, G–J), and consists of up to six layers of cells

<caption>**FIGURE 4** PPC-SRµCT, virtual sections of *Geinitzia reichenbachii* leaf (A, C, E, G, I) and interpretative sketches (B, D, F, H, J). (A, B) Sagittal section. (C, D) Tangential section. (E–J) Transverse section at the base of a leaf (E, F), in the free part of a leaf (G, H) and at the apex of a leaf (I, J). Abbreviations: Cu., cuticle; Ep., Epidermis; Hy., hypodermis; Pa.p., palisade parenchyma; R.d., resin ducts; Re.f. resin flows; Sp.p., spongy parenchyma; Tr.t., transfusion tracheids; Va.b., vascular bundle. IGR-PAL-2684. Voxel side = 2.26 µm. All scale bars = 500 µm</caption>
<figure><img src="image_10.png" /><figcaption>**FIGURE 6** *Geinitzia reichenbachii.* (A) Longitudinal section of a leafy axis found on broken surface of a flint fragment. (B) Interpretative sketch. (C) Detail of a resin duct (outlined by box in A) full of a more or less transparent, orange-brown matter showing conchoidal fractures with a resinous brilliance and looking like amber. Abbreviations: R.d., resin ducts; Re., resin. IGR-PAL-2684. Scale bars: A, B = 3 mm; C = 1 mm</figcaption></figure>

(Figure 7L). Transfusion cells are polygonal to ovoid, 10–70 µm in diameter, with straight to slightly convex walls. A single vascular bundle ends before the leaf apex (Figure 4A–D). This vascular bundle forms an angle up to 70° with the stem. The vascular bundle is 70–160 µm in diameter (Figure 4A–D, G–J).

In the stem, the vascular cylinder is continuous (Figure 8A). The secondary xylem cylinder is 240–480 µm wide (Figure 8A–C). Tracheids are 6–15 µm in diameter (Figure 8D). Radial tracheid rows number to ca. 55 tracheids. Growth rings cannot be clearly observed; however, in the thickest part of the woody cylinder, a darker crescent can be observed (Figure 8A), where tracheid walls seem somewhat thicker (image blur made accurate measurements impossible). In the opposite paler area (Figure 8A), a tangential row of radially flattened mimic a growth ring, but disappears laterally. Rays are abundant, heterogeneous, homocellular, mainly uniseriate, 1–11 cells totalling 22–430 µm high (Figure 8C, F–G). Each cell is polygonal, 18–38 µm long, 17–45 µm wide, and 22–53 µm deep (Figure 8C, F). Tangential pitting was not observed. Radial walls of tracheids have uniseriate pits that are 7–11 µm in diameter (Figures 8E, 9A) and arranged as either the araucarian type (Figure 8E) or the abietinean type (Figure 9A). Pit areolae, which are usually decisive to decipher whether pits are contiguous or distant, are not clearly seen in view 8E; however, the strict equidistance of pori is characteristic of araucarian type of radial pitting. Cross fields feature (1) 2–3

pits, arranged in a column (Figure 9B–C). These pits are 5–9 µm in diameter, contiguous, rounded, with a bordered pore (oculipores). Their preservation is not good enough to assign them either to the cupressoid, podocarpoid, or taxodoid type. Resin ducts are not present in the secondary xylem or in the axial parenchyma.

The pith forms a cylinder, which is star-shaped in transverse section (Figure 8A, H) and 350–730 µm wide. There are 5–6 exarch primary xylem strands. The pith cells are large, up to 220 µm in diameter, rounded to ellipsoidal, and display convex walls. They include large sclerenchyma cells and smaller medullar parenchyma cells (Figure 8H, I).

## Nomenclatural remarks

Based on Cretaceous conifers from the Czech Republic and Germany, Geinitz (1842) described *Araucarites reichenbachii* Geinitz, which was later transferred to *Geinitzia* by Endlicher (1847), together with another Cretaceous conifer described by Geinitz, *Sedites rabenhorstii* Geinitz. Endlicher merged both of Geinitz's species into one, which he named *Geinitzia cretacea* Endl. This name is superfluous (ICBN, art. 11.4; Turland et al., 2018). The combination *Geinitzia reichenbachii* was first published by Hollick and Jeffrey (1909), but this did not solve the nomenclatural problem, i.e., that
<figure><img src="image_11.png" /><figcaption>**FIGURE 7** PPC-SRµCT, virtual sections of a *Geinitzia reichenbachii* leaf. (A, B) Cuticle showing smooth external surface and imprints of epidermal cells in internal view. (C, D) Arrangement of stomatal apparatus (abaxial surface). (E) Interpretative sketch of D. (F) Uppermost abaxial surface with epidermis, hypodermis and palisade parenchyma in transverse section. (G) Hypodermis and palisade parenchyma in tangential section. (H) Tangential section of a leaf showing elongated palisade parenchyma cells in transverse section. (I) Palisade parenchyma in tangential section. (J) Tangential section of a leaf showing rounded spongy parenchyma cells and rounded sclerenchyma cells. (K) Detail of spongy parenchyma with lacunae in tangential section. (L) Transfusion tissues in sagittal section at the base of a leaf. Abbreviations: Ep., epidermis; Hy., hypodermis; La., lacunae; Pa.p., palisade parenchyma; R.d. resin ducts; Sc.c., sclerenchyma cells; Sp.p., spongy parenchyma; Tr.t., transfusion tissues; Va.b., vascular bundle. A, B, H, J-L: IGR-PAL-2684. C-G, I: IGR-PAL-2791. Voxel side = 2.26 µm. Scale bars: A, B, F, G = 100 µm; C-E, H-L = 200 µm</figcaption></figure>

*Geinitzia* is pre-dated by *Sedites*, as Endlicher included the only original species (thus the type) of *Sedites* Geinitz. Zijlstra et al. (2010) proposed to conserve *Geinitzia* against *Sedites*. This proposal was rejected because “members of the committee were not convinced that the taxonomic questions were sufficiently resolved” (Herendeen, 2011). Nevertheless, *Geinitzia* Endlicher is still used by several paleobotanists (e.g., Kvaček, 2013;

Kvaček et al., 2015; Halamski et al., 2018; Płachno et al., 2018; Yang et al., 2019). It is used here, together with the family name Geinitziaceae, as we are convinced that any reasonable nomenclatural solution cannot reject such a well-established name and noting that according to Kunzmann (2010), *Sedites rabenhorstii* and *G. reichenbachii* are completely different conifers belonging to different families.
<figure><img src="image_12.png" /><figcaption>**FIGURE 8** PPC-SRµCT, virtual sections of *Geinitzia reichenbachii* central cylinder. (A) General view of central cylinder in transverse section. (B, C) Longitudinal section of central cylinder. (D) Detail of wood with tracheids and rays in transverse section. (E) Tracheids and pits in radial section. (F) Tracheids and rays in radial section. (G) Mainly uniseriate rays in tangential section. (H) Pith in transverse section. (I) Pith in radial section. Abbreviations: Me.p., medullar parenchyma; Pi., pith; Ra., rays; Sc.c., sclerenchyma cells; Tr., tracheids; Xy.II, secondary xylem. A–D, F–I: IGR-PAL-2684. E: IGR-PAL-2791. Voxel side = 2.26 µm. All scale bars = 200 µm</figcaption></figure>

# DISCUSSION

## Comparison with other *Geinitzia* species

The species *G. reichenbachii* is clearly the most common species of the genus and one of the most widespread Cretaceous conifers in Europe and eastern North America. It has a broad stratigraphical and geographical distribution, being reported from the Hauterivian to the Maastrichian. In Europe, the species was reported from many Cretaceous

paleobotanical localities in the Czech Republic (Kunzmann, 2010; Halamski and Kvaček, 2016; Halamski et al., 2018), France (Gomez et al., 2004; Néraseau et al., 2005; Saint-Martin et al., 2013), Germany (Herman and Kvaček, 2010; Kunzmann, 2010), Hungary (Barale et al., 2002), Italy (Gomez et al., 2002), Poland (Halamski, 2013; Halamski and Kvaček, 2015; Kvaček et al., 2015; Płachno et al., 2018) Romania (Givulescu and Lăcătușu, 1978; Givulescu et al., 1980 in Herman and Kvaček, 2010), the Netherlands (Bosma et al., 2009) and Ukraine (Halamski, 2013).
<figure><img src="image_13.png" /><figcaption>**FIGURE 9** SEM, secondary xylem of *Geinitzia reichenbachii*. (A) Radial walls of tracheids showing uniseriate pits. (B, C) Cross-field pitting in radial view. IGR-PAL-2801. All scale bars = 20 µm</figcaption></figure>

*Geinitzia reichenbachii* was also reported from the Cretaceous of the eastern United States (Hollick and Jeffrey, 1909; Penny, 1947; Raubeson and Gensel, 1991). The gross morphology and cuticular characteristics of specimens from Claix and Moragne are similar to the description of *G. reichenbachii* proposed by Kunzmann (2010). However, characteristics of the resin ducts of *G. reichenbachii* from Charentes differ from descriptions given by Hollick and Jeffrey (1909) and Kunzmann (2010). From the study of exceptionally preserved *G. reichenbachii* specimens from the United States, Hollick and Jeffrey (1909) described three resin ducts in each leaf: a median large one and two smaller lateral ones. This is not consistent with the six resin ducts observed in the free part of leaves from the Charentes region. As we described here, some resin ducts are only present at the base of the leaf. Depending on the location of the leaf cross section, it is possible that Hollick and Jeffrey (1909) only observed some of the ducts, particularly in the distal free part of leaves. However, after carefully examining some figured thin sections of Hollick and Jeffrey (1909), we are not convinced that they identified all the ducts present, particularly the smaller ones, which may be poorly preserved (see plates 16 and 17 in Figure 4 of Hollick and Jeffrey, 1909). Kunzmann (2010) also observed lateral resin ducts in the cortex of the main axis as well as in leaves of *G. reichenbachii* from the late Turonian of the Czech Republic. He described a single central resin duct in each leaf. In contrast with the specimens from the United States described by Hollick and Jeffrey (1909) and the silicified specimens from Charentes (present study), the specimens described by Kunzmann (2010) are preserved as coaly compressions without inner structures and only observed in a longitudinal section. Such differences between these three occurrences could be tentatively explained by preservation bias.

Based on material from the Cretaceous of Germany, Heer (1871) erected *Geinitzia formosa* Heer. This species is known from seed cones, seeds, and leafy axes. *Geinitzia formosa* was also reported from the Campanian of Austria (Herman and Kvaček, 2010), the Santonian of Germany (Kunzmann, 1999; Kunzmann et al., 2003), and the Turonian-Santonian of the eastern United States (Penny, 1947). Although the gross morphology of leaves of *G. formosa* and those of *G. reichenbachii* are quite similar (Kunzmann, 1999), they differ in size, *G. formosa* showing smaller leaves (Kunzmann, 1999; Herman and Kvaček, 2010). The longest leaves of *G. formosa* reported by Kunzmann (1999) and Herman and Kvaček (2010) are 10 mm and 11 mm, respectively, in contrast with the 15–20-mm-long leaves of *G. reichenbachii* reported from the Campanian of Austria by Herman and Kvaček (2010) and with the leaves from Charentes that are up to 21 mm long. *Geinitzia formosa* also has generally more irregular or random orientation of stomata on the adaxial side, in contrast with the preferential transverse and oblique orientation of stomata of *G. reichenbachii* (Kunzmann, 2010).

Based on silicified seed cones from the Santonian of Belgium, Kunzmann et al. (2003) introduced a new species, *Geinitzia schlotheimii*. Kunzmann, Knoll and Gaipl. Kunzmann et al. (2003) described short leafy axes attached to some cones. Shoots show helically arranged, falcate, three- or four-sided leaves with decurrent base that are morphologically similar to *G. reichenbachii* leafy axes. An isolated ovuliferous cone attributed to *Geinitzia cf. schlotheimii* was reported from the Coniacian of Poland (Halamski and Kvaček, 2015; Kvaček et al., 2015). *Geinitzia formosa* differs from *G. schlotheimii* by the larger cones and the greater number of cone scales (Kunzmann et al., 2003).

*Geinitzia rigida* (J. Phillips) T.M. Harris was described from the Middle Jurassic of Yorkshire (Harris, 1979).
Sender et al. (2008) assigned some specimens from the Albian of Spain to *Geinitzia* sp. cf. *G. rigida*. *Geinitzia rigida* differs from *G. reichenbachii* in having smaller and shorter leaves, with rounded adaxial and abaxial surfaces, marginal hairs, and a slightly incurved apex (Harris, 1979).

Considering *Cryptomerites* Bunbury as a younger synonym of *Geinitzia*, Harris (1979) recombined *Cryptomerites divaricata* Bunbury under *Geinitzia* and introduced the new species *Geinitzia divaricata* (Bunbury) T.M. Harris. *Geinitzia divaricata* was reported from the Middle Jurassic of Yorkshire (Harris, 1979) and Sardinia (Scanu et al., 2015, 2016; Kustatscher et al., 2016) and differs from *G. reichenbachii* in showing well-spaced and never crowded leaves arising perpendicularly on larger twigs. Leaves of *G. reichenbachii* never lie in a horizontal plane.

We do not recommend assigning juvenile secondary xylem, as observed within our leafy shoots, to a wood genus (Philippe and Bamford, 2008). The type of pitting on the radial wall of tracheids can change greatly during development from juvenile to mature wood. Here, we note that the pits in the widest tracheids are arranged according to the abietinean type. Together with the oculipores, which seems to have rather broad pores, this finding would suggest affinities with *Taxodioxylon* Hartig or *Podocarpoxylon* Gothan. It is also possible that the juvenile wood observed here could be associated with a mature wood belonging to the *Agathoxylon* type. In the axis of the cone *Geinitzia gracillima*, Jeffrey (1911) observed some juvenile secondary xylem with a much more abietinean type of radial pitting.

## Preservation and taphonomy

Although *G. reichenbachii* is an extremely common conifer from the Late Cretaceous paleobotanical sites of Europe, fossils of this species are generally preserved as compressions, with or without cuticles (e.g., Barale et al., 2002; Kunzmann, 2010), or impressions (e.g., Halamski and Kvaček, 2015; Halamski et al., 2018). In most of these cases, they were flattened during the compaction of sediments, and only the gross morphology could be described. When cuticle is preserved, only the external periclinal walls and the anticlinal walls of the most external cells layer are preserved. In contrast, the permineralized specimens from the silica-rich nodules of Charentes are preserved in three dimensions. Cuticle as well as all tissues of leafy axes are preserved from the outermost to the innermost ones. This type of preservation can be termed permineralization according to the classification by Schopf (1975). Siliceous microcrystallization occurred over the outer cell wall and inside cells over the inner cell wall and/or within the cell walls themselves. Locally, microcrystals filled intercellular and intracellular spaces. Moreau et al. (2014a, 2017b, 2019) observed that the Cretaceous silicified floras of Charentes were commonly fossilized in a littoral context and that they co-occurred with abundant marine organisms such as

sponge spicules. These spicules are observed in the matrix of the plant-bearing flints and on the surface of plant remains. Sponges were probably a great source of silica and contributed to the mineralization of the plants (Moreau et al., 2014a, 2019). As demonstrated by in vitro experiments, plants (and particularly wood) are excellent “reception structures” for silica (Laroche et al., 1989). The strong affinity of some organic compounds (e.g., lignins) with the silica can result in rapid silicification of some tissues (Laroche et al., 1989). In this context, plant debris can serve as nuclei for the formation of flints. The exquisite preservation of *Geinitzia* tissues attests that an early silicification episode occurred during the sedimentation of the Cretaceous deposits or during the earliest phases of diagnosis (Moreau et al., 2014a, 2017b, 2019). However, as pointed by Moreau et al. (2014b, 2016, 2019) and Néraudeau (2015), plant-bearing flints from Charentes were collected in alterite deposits. These alterites result from an Eocene-Oligocene climate episode (Daniou, 1978, 1979; Néraudeau, 2011) with strong and long pedogenetic alteration locally, implicating a late silicification of Cretaceous carbonated beds.

From the preservation point of view, the specimens of *G. reichenbachii* from Charentes studied here are exceptional. However, over a century earlier, Hollick and Jeffrey (1909) described and figured exquisitely preserved *Geinitzia* leafy axes from the Cretaceous (Turonian?) of Staten Island (USA). The specimens they described are preserved as “charcoal or lignite, often impregnated or coated with pyrite”, preserving twigs in 3D with most of their inner tissues. Although they did not describe them in detail, Hollick and Jeffrey (1909) mentioned some inner structures and tissues of *G. reichenbachii* (e.g., resin ducts, vascular bundle, transfusion tissues, secondary xylem, pith).

Based on material from the Upper Cretaceous of New Castle County, Delaware (USA), Penny (1947) mentioned the presence of inner structure in twigs ascribed to *G. reichenbachii*, but he limited the description to tissues of the wood. To our knowledge, the specimens reported by Hollick and Jeffrey (1909) and Penny (1947) as well as those we report from Charentes are the only occurrences of *Geinitzia* in which most of the tissues inside leafy axes have been preserved.

Although the material preserved inside resin ducts strongly resembles amber (i.e., very low density, transparent, orange-brown color, conchoidal fractures, resinous brilliance), we cannot exclude the possibility that chemical changes of the original resin occurred during diagenesis. Preservation of amber inside silicified wood is extremely rare. Such an exquisite preservation of fossil resin inside tissues and on the surface of leaves is for the first time observed in the Cretaceous flints of western France. Because amber-bearing beds are abundant (Perrichot, 2005) and *Geinitzia* is one of the main components of the Cretaceous floras from this area (Moreau et al., 2017a), this raises the question of a possible role for *Geinitzia* in the production of
the “Charentese amber”. Although this study shows that *Geinitzia* produced large amounts of resin in and on its leaves, there is no direct evidence that it was the source of this amber. The “Charentese amber” was categorized as a class IB amber whose chemical composition indicates a conifer source in the families Araucariaceae or Cheir-olepidiaceae (Nohra et al., 2015). However, we note that recently, Seyfullah et al. (2020) reported class IB amber in the Erdtmanithecales, that correspond to an incompletely known order of gymnosperms. The amber from the Charentes is found associated with various types of conifer wood (e.g., *Agathoxylon* Hartig, *Brachyoxylon* Hollick et Jeffrey, *Podocarpoxylon*, *Gingkoxylon* Andr.; see Perrichot, 2005), by far most commonly with *Agathoxylon* (Moreau et al., 2017a). Again, it is possible that the juvenile wood observed here could be associated with a mature wood be-longing to the *Agathoxylon* type. Saint-Martin et al. (2013) recently discussed the plant remains associated with amber in Cretaceous of western France and hypothesized that *Geinitzia* was not the major amber source. Our findings challenge this hypothesis.

## Paleoecophysiological implications and paleoenvironments of *G. reichenbachii*

At Claix and Moragne, the abundance of sponge spicules and echinoids attests that *Geinitzia*-bearing flints were deposited in environments open to the sea and influenced by both continental and marine inputs. Similar to other Cretaceous conifers from Charente, Charente-Maritime, and Vienne (e.g., *Brachyphyllum*, *Frenelopsis*, *Glenrosa* Watson et Fisher, *Pagiophyllum* Heer emend. T.M. Harris; e.g., Gomez et al., 2004; Néraudeau et al., 2005; Moreau et al., 2015, 2020), *G. reichenbachii* was adapted to withstand intense sunlight, and coastal environments exposed to hot temperatures coupled with salty sea wind and dry conditions. Reduced leaves that are helically arranged lessened the impact of solar radiation (Thévenard et al., 2005). The well-developed hypodermis decreased the ultraviolet penetration in the leaves and allowed the thermal load to be reduced (Darling, 1989; Larcher, 2003). However, regionally *G. reichenbachii* was also reported in innermost, shallow and brackish environments (Néraudeau et al., 2020), suggesting that this taxon was adapted to a range of marginal-littoral paleoenvironments. The Aquitaine Basin is considered to be located at a latitude of 35°N during the Upper Cretaceous (Philip et al., 1993). Based on the δ¹⁸O and δ¹³O of oysters from this area, Videt (2004) demonstrated that the climate of the Upper Cretaceous was tropical and characterized by strongly contrasted seasons.

Although no clear growth rings were observed, the secondary xylem cylinder thickness and the observed laterally discontinuous row of radially flattened tracheids suggest that these twigs persisted for at least two growth seasons. Twigs with deciduous leaves are observed in extant conifers only among those that grow in environments where

water is not limiting (e.g., *Metasequoia* Hu et W. C. Cheng; Williams, 2005).

The darker crescent observed in the cross section of the woody cylinder is probably made of compression wood, although in this juvenile stage the compression wood does not display the typical rounded tracheids.

## CONCLUSIONS

Synchrotron microtomography reveals the exquisite preservation of silicified *Geinitzia* leafy axes from the Upper Cretaceous of Charente and Charente-Maritime. Although *Geinitzia* is a common member of the Cretaceous floras of western France, its histology is described for the first time. Leafy twigs have large resin ducts sometimes filled by persistent resin in the leaves as well as in the stem cortex. Additionally, surfaces of some leaves preserved drops and resin flows. Cuticle and most of tissues of the leaves are preserved, including epidermis, hypodermis, palisade parenchyma, spongy parenchyma, transfusion tissue, and vascular bundle. Tracheids and rays of the secondary xylem as well as the pith are well preserved. The leafy shoots of *G. reichenbachii* contained large amounts of resin suggesting a possible role as a source of the “Charentese amber”. Although *G. reichenbachii* probably lived in a range of marginal-littoral paleoenvironments, the palaeontological context of the plant bearing-flints, as well as the plant’s morphology and histology demonstrated that this conifer was adapted to withstand intense sunlight and coastal environments exposed to hot temperatures coupled with salty sea winds and aridity.

## ACKNOWLEDGMENTS

We thank the European Synchrotron Radiation Facility (ESRF) and particularly the ID19 beamline for the beam time and the material support. We thank the Associate Editor Paul Kenrick and Lutz Kunzmann and an anonymous reviewer for constructive and thoughtful reviews of the manuscript. We thank P. Tafforeau for useful help during tomographic experiments. We thank the society Terra Lacta, owner of the site of Claix, who gave permission for this study. We are grateful to Gea Zijlstra for her advice on *Geinitzia* nomenclature. This paper is a contribution to the research program SILVE (2012–2018) of the UMR CNRS 6118 Géosciences (Université Rennes I) and the project EC-412 financed in 2016 by the ESRF. J.-D.M. and M.L. received financial support from the CNRS UMR6118.

## AUTHOR CONTRIBUTIONS

Conceived and designed the research: J.-D.M. and D.N. performed experiments: V.F., J.-D.M., M.L.C. and S.B. analyzed data: J.-D.M., M.P., D.N., E.D., and S.B. J.-D.M. wrote the first version of the manuscript, that was later revised by M.P., D.N., E.D., M.L.C., V.F., and S.B.
DATA AVAILABILITY STATEMENT

All specimens are housed at Géosciences Rennes, CNRS UMR 6118, Université Rennes 1 (Rennes, France) under the numbers IGR-PAL-2682 to IGR-PAL-2684, IGR-PAL-2740B and IGR-PAL-2741B, IGR-PAL-2783 to IGR-PAL-2794, IGR-PAL-2795A, IGR-PAL-2795B, IGR-PAL-2796 to IGR-PAL-2799A, IGR-PAL-2799B, and IGR-PAL-2801 to IGR-PAL-2803. All the microtomographical data used for this study are available in the open access ESRF database: http://paleo.esrf.eu.

ORCID

Jean-David Moreau https://orcid.org/0000-0003-2978-5983

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How to cite this article: Moreau, J.-D., M. Philippe, D. Néraveau, E. Dépré, M. Le Couls, V. Fernandez, and S. Beurel. 2021. Paleohistology of the Cretaceous resin-producing conifer *Geinitzia reichenbachii* using X-ray synchrotron microtomography. *American Journal of Botany* 108(9): 1745–1760.

https://doi.org/10.1002/ajb2.1722