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Research Article
First flower inclusion and fossil evidence of Cryptocarya (Laurales, Lauraceae) from Miocene amber of Zhangpu (China)
expand article infoSimon Beurel‡, Julien B. Bachelier§, Jérôme Munzinger|, Fuchen Shao§, Jörg U. Hammel, Gongle Shi#, Eva-Maria Sadowski‡
‡ Museum für Naturkunde, Berlin, Germany
§ Freie Universität Berlin, Berlin, Germany
| Université Montpellier, Montpellier, France
¶ Institute of Materials Physics, Geesthacht, Germany
# Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Palaeoenvironment, Chinese Academy of Sciences, Nanjing, China

Corresponding author: Simon Beurel ( simon.beurel@mfn.berlin )

Academic editor: Florian Witzmann
© 2024 Simon Beurel, Julien B. Bachelier, Jérôme Munzinger, Fuchen Shao, Jörg U. Hammel, Gongle Shi, Eva-Maria Sadowski.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation: Beurel S, Bachelier JB, Munzinger J, Shao F, Hammel JU, Shi G, Sadowski E-M (2024) First flower inclusion and fossil evidence of Cryptocarya (Laurales, Lauraceae) from Miocene amber of Zhangpu (China). Fossil Record 27(1): 1-11. https://doi.org/10.3897/fr.27.109621
ZooBank: urn:lsid:zoobank.org:pub:145575FB-0BE7-4916-88D0-2CA73D69B62C
Open Access

Abstract

Lauraceae have one of the oldest fossil records of angiosperms with the earliest known evidence from the mid-Cretaceous. However, most of these records are based on leaves, especially from the Cenozoic of Asia, which are often challenging to assign to extinct or extant genera or species. In contrast, fossils of reproductive organs are more informative, but remain scarce. We here described the first Cenozoic Lauraceae flower of Asia and confirmed the presence of Cryptocarya in the Miocene Zhangpu flora (Fujian Province, south-eastern China) based on an amber inclusion. We scanned the specimen using synchrotron radiation-based micro-computed tomography (SRμCT) and then compared the fossil with extant flowers of the genus. The present fossil flower is small, bisexual, and polysymmetric, with a whorled and trimerous perianth and androecium along with a hypanthium around the gynoecium. The perianth comprises six undifferentiated tepals, the androecium consists of nine stamens and three innermost staminodes, and the gynoecium of a single carpel with a superior, unilocular (and uniovulate) ovary. Our study also shows that the fossil shares an unusual position of the typical staminal glands and a short androecial tube on the rim of the hypanthium with at least one extant Australian species of Cryptocarya, which have not been reported before. Nowadays, Lauraceae are still present in tropical to subtropical regions, mostly in American and Asian rainforests. The discovery of many Lauraceae leaf fossils in Zhangpu, as well as the amber flower of this study, is consistent with the current reconstruction of the amber source environment as a megathermal seasonal rainforest during the Mid-Miocene.

Key Words

Angiosperm, Cenozoic, palaeobotany, synchrotron X-ray tomography, 3D reconstruction

Introduction

The Lauraceae are one of the most species rich families of Magnoliidae, with worldwide ecological and economic importance. Some taxa are key ecosystem components, especially in Asian and American tropical lowland and montane forests. Lauraceae also comprise crops and spices, such as avocado, cloves and cinnamon (Rohwer 1993). This family has a vast meso- to megafossil record, mostly consisting of leaves from all over the world (Friis et al. 2011). Lauraceous leaves are notoriously difficult to assign to species but also to generic level, as key features are widespread across extinct and extant taxa and geographical areas (Hill 1986; Christophel et al. 1996). In contrast, fossil fruits and flowers show more diagnostic features than leaves and are particularly valuable for generic or even species level identification (Rohwer 1993). Such reproductive structures are nevertheless rare, as they are not often preserved in the sedimentary fossil record (Friis et al. 2011).

The oldest currently known lauraceous fossil flower stems are from the Early Cretaceous Puddledock locality (von Balthazar et al. 2007). Additionally, fossils of lauraceous origin from North America (Drinnan et al. 1990; Friis et al. 2011), Europe (Kvaček 1992; Eklund and Kvaček 1998; Kvaček and Eklund 2003; Viehofen et al. 2008; Coiffard et al. 2009; Moreau et al. 2016) and South Asia (Poinar 2017) show that early members of the family were already widely distributed and diverse by the mid-Cretaceous. In Asia, the earliest Lauraceae fossil records date back to the Cenomanian with a flower inclusion of Cascolaurus burmitis Poinar in Burmese amber (Poinar 2017) and inflorescences and flowers of Mauldinia hirsuta Frumin, Eklund and Friis from Kazakhstan (Frumin et al. 2004). Additional Cretaceous fossils include flowers from the Coniacian of Japan (Takahashi et al. 1999, 2001, 2014). However, molecular clock study infers that diversification of modern Lauraceae in Asia is more recent, and probably took place during the early Cenozoic (Chanderbali et al. 2001). This is supported by the large fossil leaves record, mostly from China (e.g. Hu et al. 2007; Jacques et al. 2011; Sun et al. 2011; Shi et al. 2014c; Huang et al. 2016; Wang et al. 2019; Wang et al. 2021), but fossil flowers from this time period remain scarce. Therefore, the discovery of additional Cenozoic lauraceous reproductive organs is particularly interesting to gain more insight into the evolution and diversification of Lauraceae.

In this paper, we aim to describe a new fossil flower of Lauraceae from Chinese Miocene Zhangpu amber by using non-destructive X-ray imaging techniques. We noticed similarities, such as an the unusual position of the staminal glands and an androecial tube, with an extant member of Cryptocarya R. Br. from Australia, and discuss the significance of the oldest unambiguous fossil of the genus in the Miocene Zhangpu flora.

Material and methods

The amber specimen derives from the Zhangpu amber deposit of the Fotan Group, in Zhangpu County (Fujian province, SE China; see detailed map in Wang et al. 2021: fig. 1). The group is formed by sedimentary layers of conglomerates, sandstone and mudstone alternating with layers of basaltic rocks (Zheng and Wang 1994; Wang et al. 2021). Plant macro-remains are commonly found co-occurring with the amber in the same layers of blue-grey mudstone with lignite and diatomite (Wang et al. 2021). Based on the 40Ar/39Ar dating of the basaltic units associated the fossil layers, the age of the amber has been well constrained at 14.8 ± 0.6 Ma to 14.7 ± 0.4 Ma, corresponding to the Langhian stage of the Middle Miocene (Ho et al. 2003; Zheng et al. 2019; Wang et al. 2021). This is consistent with the previous palynological studies of the Fotan Group (Zheng and Wang 1994). The terpenoid composition of Zhangpu amber indicates that it derived from the tropical Dipterocarpaceae. Additionally, fossilized winged fruits with affinities to this angiosperm family were found in the same deposit (Shi and Li 2010; Shi et al. 2014a, b). The amber piece was deposited in the palaeobotanical collections of the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences in China (NIGPAS) under the collection number PB21525.

Separation of the fossil flower from other syninclusions in the same amber piece was achieved by using the sawing machine ALGE. Then, the piece containing the fossil was ground and polished with a LaboPol-25 machine (Struers, Sarasota, Florida, USA) using wet silicon polishing papers with successively finer grit sizes (250 to 4000 nm, Struers) to create even surfaces parallel to the inclusion (see Sadowski et al. 2021 for details).

The amber specimen was studied under a Carl Zeiss AxioScope A1 compound microscope (Carl Zeiss, Oberkochen, Germany), using incident and transmitted light simultaneously. Images were taken with a Canon EOS 80D digital camera (Canon, Tokyo, Japan) that was mounted on the microscope. To accommodate the three-dimensionality of the inclusion, 65 single photographs were taken and then digitally stacked to a composite image, by applying the HeliconFocus 7.7 software.

Imaging of the flower inclusion was performed using synchrotron-radiation based X-ray microtomography (SRμCT) at beamline P05 of the storage ring PETRA III (Deutsches Elektronen-Synchrotron, DESY, Hamburg, Germany) that is operated by Helmholtz-Zentrum Hereon (Haibel et al. 2010; Greving et al. 2014; Wilde et al. 2016). The amber piece was mounted on a sample-stub with beeswax and imaged using an attenuation contrast setup (Greving et al. 2014). The specimen was scanned using a commercial CMOS camera system with an effective pixel size of 0.45 µm, a sample to detector distance of 75 mm and a photon energy of 18 keV. A total of 3501 projections were recorded for the tomographic scan, at equal intervals between 0 and π. A transport of intensity phase retrieval approach and a filtered back projection was used to perform the tomographic reconstruction implemented with the Astra Toolbox (Palenstijn et al. 2011; van Aarle et al. 2015, 2016) and Matlab (Math-Works), integrated in a bespoke reconstruction pipeline (Moosmann et al. 2014). Raw projections were binned twice during processing, resulting in an effective voxel size of 0.91 µm for the reconstructed volume.

The size of the resulting tomographic images was decreased from 32-bit floating point data to 16-bit, by applying ImageJ 1.52 (Wayne Rasband, National Institutes of Health, Bethesda, Maryland, USA). The software Volume Graphics Studio Max, version 3.4 (Volume Graphics, Heidelberg, Germany) was used to complete the segmentation, 3D reconstructions, and length measurements.

We additionally examined flowers of some extant species of Cryptocarya from the Oceanic region, including one from Australia (originally C. hypotephra F. Muell., now C. vulgaris B. Hyland; MB.Pb.HB.00569 [Schrader Herbarium (BHUPM), Museum für Naturkunde Berlin (MfN)]), and four from New Caledonia (C. adpressa Munzinger & McPherson [Munzinger et al. 5832, P, Type specimen], C. aristata Kosterm. [Munzinger et al. 5866, P], C. cf. odorata Guillaumin [Munzinger (leg. Waikedre) 6716, MPU], and Vanuatu (C. wilsonii Guillaumin [Munzinger & Bruy 8232], NOU). Each flower sample was mounted on a specimen holder to facilitate microCT scanning with a Phoenix X-ray Nanotom at the MfN. The scans comprised 1440 to 2000 projections and were conducted using the following settings: voltage = 80 to 100 kV, current = 75 to 100 µA, exposure time = 0.75 to 1 s, average = 3 to 6, skip = 1 and isotropic resolution = 1.64 to 3.6 µm.

Data resources

Supplementary data including SRμCT videos, 3D models of the Cryptocarya flower inclusion and of extant flowers of the genus are available here: https://doi.org/10.7479/pzxg-2x16.

Results

Systematic palaeontology

Order Laurales Juss. ex Bercht. & J. Presl, 1820

Family Lauraceae Juss. 1789

Genus Cryptocarya R. Br., 1810

Cryptocarya sp.

Figs 1, 2

Specimen studied

PB21525. Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences in China (NIGPAS).

Locality

Zhangpu County, Fujian Province, China.

Horizon and age

Middle part of the Fotan Group, Langhian (ca. 14.7–14.8 Ma), Middle Miocene.

Description

The flower is 3.11 mm long and 1.66 mm wide (without the pedicel, not preserved) (Fig. 1A, B). It is perfect, polysymmetric, and comprises six tepals, nine stamens, and three staminodes, which are arranged in six trimerous and alternate whorls around the gynoecium (Fig. 1C–G). It is also perigynous with a hypanthium (1.71 mm long and 0.60 mm wide) that encloses the superior ovary and lowest third of the style entirely (Fig. 1D). The position of perianth organs forming a narrow opening and of the apical flaps still covering the pollen sacs suggest a late pre-anthetic stage or early (female) anthetic phase.

Figure 1. 

Cryptocarya fossil flower from Miocene Zhangpu amber, PB21525. A. Overview of the flower inclusion, taken under the light microscope; B–D. Virtual 3D reconstruction from segmented SRµCT data; B. Overall flower; C. Cross section; D. Longitudinal section; E–G SRµCT cross-sections, indicated in D and arranged from top to bottom; E. Bilocular anthers with two valves and laterally fused pollen sacs (septa indicated by white arrowheads); F. Style, staminodes, glands, and the filaments of the fertile stamens adnate to a tepal lobe; G. Short staminal tube resulting from the fusion of the third whorl of stamens with the whorl of staminodes. Perianth – grey; Stamens of the first whorl – red; Stamens of the second whorl – orange; Stamens of the third whorl – beige; Anther valves – white; Staminal glands – pink; Staminodes – yellow; Gynoecium – purple; Ovule – blue. Scale bars: 0.5 mm (A–D); 0.4 mm (E–F); 0.3 mm (G).

The tepal lobes (1.72–2.04 mm long and 0.66–1.05 mm wide) are free and arranged in an outer and inner whorl, and share a similar narrowly ovate to elliptic shape and an acute tip (Fig. 1A, B).

All stamens are shorter than the tepals in length and have basifixed anthers with unilocular thecae opening by two slightly lobed to flattened apically attached or hinged flaps (Figs 1C–E, 2A, C). Stamens of the first and second whorls have introrse, ovate and incurved anthers (ca. 0.57–0.68 mm long and 0.40–0.50 mm wide). Their filaments are adnate to a tepal lobe (Fig. 1F) and distinctly shorter than their anthers (ca. 0.25–0.48 mm long and 0.08–0.11 mm wide). Three pairs of large glands are inserted on the hypanthium rim, between the united bases of the tepals and filaments (Fig. 1D). These glands are formed by a subglobose distal part (ca. 0.34–0.56 mm long and 0.29–0.38 mm wide) and a long stalk (ca. 0.31–0.40 mm long and 0.09–0.12 mm wide; Figs 1D, F, 2B). Stamens of the third whorl have latero-extrorse, erect and narrow anthers (ca. 0.68–0.75 mm long and 0.37–0.39 mm wide). The filaments are just about as long as their anthers (ca. 0.64–0.74 mm long and 0.09–0.14 mm wide) and their bases form, together with those of the staminodes, a short staminal tube projecting beyond the rim of the hypanthium (ca. 0.13 mm long and 0.45 mm wide; Figs 1D, G, 2C). The staminodes have a sagittate (triangular-ovate and acuminate) sterile anther and a short and stout filament (ca. 0.60–0.65 mm long and 0.38–0.40 mm wide and 0.31–0.41 mm long and 0.15 mm wide respectively; Figs 1D, F–G, 2C).

Figure 2. 

Androecium of the Cryptocarya flower from Miocene Zhangpu amber, PB21525. A. Stamens of the first and second whorl; B. Staminal glands; C. Stamens of the third whorl fused with the staminodes at the base of the filaments, forming a short staminal tube. Stamen of the first whorl – red; Stamen of the second whorl – orange; Stamens of the third whorl – beige; Anther valves – white; Staminal glands – pink; Staminodes – yellow. Scale bars: 0.2 mm (A); 0.3 mm (B–C).

The gynoecium consists of a single carpel (ca. 2.03 mm long). The slender superior ovary (1.22 mm long and 0.18 mm wide) is entirely enclosed in the hypanthium. The style (ca. 0.81 mm long and 0.14 mm wide) ends in an inconspicuous stigma (Fig. 1D). The ovary has a single locule and an apical pendulous ovule (ca. 0.71 mm long and 0.16 mm wide).

The flower is densely pubescent with appressed simple acute trichomes covering all organs, except for the inner surface of the distal part of the hypanthium, the fertile and sterile anthers and the gynoecium (Fig. 1).

Syninclusions

The specimen contains inclusions of two Tetramorium and five Carebara ants (Formicidae), one Collembola, two Chironomidae, and wood fragments.

Discussion

Systematic affinity

Our non-invasive and non-destructive approach shows that our fossil flower exhibits essentially the same Bauplan as most extant Lauraceae, with two trimerous whorls of tepals, followed by three trimerous whorls of fertile stamens and an innermost trimerous whorl of staminodes, and a unicarpellate gynoecium. In addition, like in all Lauraceae, the anthers are unmistakingly valvate with apical flaps, and the single ovary is superior and unilocular with one apical ovule (Rohwer 1993; van der Werff 2001; Li et al. 2008a; Simpson 2010). The bithecal anthers also exhibit a bilocular structure with septal remains in some thecae. However, whether lateral fusion of the two pollen sacs happened before anthesis or earlier is unclear, but such anthers may have evolved multiple times in Lauraceae, with intermediate forms observed in several genera (Rohwer 1994). According to Rohwer (1993), bilocular anthers most likely originated in Lauraceae from the reduction of the upper or lower pollen sac(s) but may also have been formed by the lateral fusion of the pair of pollen sacs, which is a synapomorphy of the “larger part of the Cryptocarya group” that contains at least the genera Cryptocarya, Beilschmiedia Nees, Endiandra R. Br., Aspidostemon Rohwer & H.G. Richt., Potameia Thouars, Eusideroxylon Teijsm, & Binn., Potoxylon Kosterm., Dahlgrenodendron J.J.M. van der Merwe & A. E. van Wyk, Sinopora J. Li, N.H. Xia & H.W. Li, Triadodaphne Kosterm. and Yasunia van der Werff (Rohwer et al. 2014).

Within this group, the fossil flower exhibits a combination of distinctive features with members of the extant genus Cryptocarya, including six tepals, nine stamens that are all fertile and a deep hypanthium entirely encompassing the ovary (Fig. 3; van der Werff 1991, 2001; Rohwer 1993; Moraes 2007; Li et al. 2008b).

Figure 3. 

Extant Cryptocarya vulgaris flower from Australia, MB.Pb.HB.00569. A. Overview of a flower, taken under the light microscope; B–D. Virtual 3D reconstruction from segmented microCT data; B. Overall flower; C. Cross section; D. Longitudinal section; E–G. microCT cross-sections, indicated in D and arranged from top to bottom; E. Immature bilocular anthers of the androecium and the bilobed stigma; F. Style, staminodes, glands, and filaments of the fertile stamens; G. Staminal tube resulting from the fusion of the third whorl of stamens with the whorl of staminodes; note the glands inserted at the division point of the tepal lobes. Perianth – grey; Stamens of the first whorl – red; Stamens of the second whorl – orange; Stamens of the third whorl – beige; Staminal glands – pink; Staminodes – yellow; Gynoecium – purple; Ovule – blue. Scale bars: 0.4 mm (A–D); 0.3 mm (E–G).

Extant Cryptocarya flowers are small (about 3–5 mm, Fig. 3), actinomorphic, bisexual and perigynous. The perianth also consists of six tepals arranged in an outer and inner whorl usually of similar size and shape. The androecium comprises three whorls with three fertile stamens and an innermost whorl of staminodes. The first and second whorl of stamens have introrse anthers, while the ones of the third whorl can be extrorse, lateral or rarely introrse (Kostermans 1937; Moraes 2007). The stamen filaments are inserted on the rim of the hypanthium, each close or connected to the base of a tepal. In addition, stamens of the first/second or third whorl bear a pair of glands, distant or adnate to the base of their filament (Fig. 3G; Hyland 1989; Moraes 2007). A fourth whorl of conspicuous, sagittate staminodes also occurs in the flowers. The style is long and the stigma is small or inconspicuous (Kostermans 1937; Moraes 2007).

Position of glands and staminal tube in Cryptocarya

Androecial characters are highly variable among extant Lauraceae and may also vary within genera. In most cases, the pairs of staminal glands are connected to or born on the filament bases of the third whorl of stamens (Rohwer 1993). However, they can also occur on the outermost androecial whorl such as in Rhodostemonodaphne Rohwer and Kubitzki, Urbanodendron Mez, Chlorocardium Rohwer, Richter and van der Werff, and Brassiodendron C.K. Allen (Rohwer 1993). Also, in some extant representatives of Cryptocarya from Australia and South America, the paired staminal glands are associated with the two outer whorls of stamens or rather distant from the filaments (in C. brassii C.K. Allen, C. grandis B. Hyland, C. pleurosperma C.T. White and W.D. Francis, C. putida B. Hyland, C. vulgaris in Hyland 1989; C. moschata Nees and Martius, C. guianensis Meisner, C. riedeliana P.L.R. Moraes in Moraes 2007). As in some of these extant species, the staminal glands of our Cryptocarya fossil flower are free from the filament and inserted towards the distal end of the floral cup (Fig. 1D). However, it is often unclear in the literature and in our material which whorl the glands are associated with. For instance, we observed in the herbarium specimen C. vulgaris (MB.Pb.HB.00569) that they are inserted at the division point of the tepal lobes (Fig. 3G) whereas Hyland (1989) associates the glands with the first whorl of stamens (fig. 37G).

In the fossil flower from Zhangpu, as well as in the herbarium flowers of Cryptocarya vulgaris (MB.Pb.HB.00569), the filaments of the third stamen whorl are similarly fused with those of the staminodes whorl (Figs 1G, 2C, 3G), forming a short staminal tube extending beyond the rim of the hypanthium. Judging from flowering stage, the staminal tube in the fossil is less prominent than in the extant specimen. In addition, the tube observed in extant Cryptocarya flowers is more defined when the flower is less mature, suggesting that, depending on the developmental stage of the specimen, a staminal tube is only temporarily present in the early development of some Cryptocarya flowers. They can thus be superficially different in older stages and prevent a detailed comparison with classic descriptions of fully developed flowers in the literature. These morphological variations should be considered in future morphological studies of the genus.

The glands and the staminal tube are distinctive in the herbarium flower (MB.Pb.HB.00569) and in the fossil. However, such a staminal tube elaborated only by the third and fourth androecium whorls has, to our knowledge, never been pictured or mentioned before in Lauraceae. Similarly, pairs of staminal glands on different whorls were also only occasionally depicted (Hyland 1989, figs 13G, 21G, 31I, 32G, 37G; Moraes 2007, figs 33D, 52D). Therefore, future morphological and anatomical studies are needed to gain more insight into the variation of the gland position and staminal tube development in Cryptocarya. Additionally, our results show that X-ray based methods, like micro-CT scanning, is a valuable tool for analysing extant plant specimens, as they allow detailed observations of reproductive plant organs non-invasively, compared to standard methods like microtome sections, which are usually applied in botany.

Due to the morphological variations in Cryptocarya flowers and the lack of distinctive characters in the fossil, it does not seem reasonable to assign the flower inclusion to any extant species of Cryptocarya. Furthermore, additional plants organs, such as leaves or fruits would be necessary to clarify the affiliation of the fossil to the species level. For all these reasons, we refrain from describing the amber inclusion as a new fossil species.

Fossil records and challenges in assigning Cryptocarya and Lauraceae fossils

According to molecular data, Cryptocarya diverged around about 90 ± 20 million years ago and is probably of Gondwanan origin (Chanderbali 2001). Nevertheless, fossils with affinities to the Cryptocarya group, and more specifically to Cryptocarya, are only known since the Cenozoic. The oldest representatives are possibly fossil leaves from the Eocene of Australia (Hill 1986, 1988; Conran and Christophel 1998; Carpenter et al. 2004) and North America (MacGinitie 1941). In addition, numerous putative leaves and fruit fossils were discovered in the Miocene and Pleistocene of New Zealand (Holden 1982; Pole 1993, 2007, 2019; Bannister et al. 2012), the Miocene of New Caledonia (Garrouste et al. 2021), as well as in the Oligocene, Miocene and Pleistocene of China (Liu 1993; Shi et al. 2014c; Wang et al. 2019), while fewer were discovered in the New World (i.e. in the Paleogene of Patagonia; Carpenter et al. 2018). It is only recently that a fossil has been reliably assigned to the genus in Asia, based on Pleistocene wood (Huang et al. 2023).

However, most of those fossils are leaf impressions and compressions or cuticle fragments. Moreover, the fossil pollen record of Lauraceae is scarce, as their pollen has a low fossilisation potential (Herendeen et al. 1994; Friis et al. 2011) and thus the pollen record for Cryptocarya is inexistent. Although placement of fossil leaves in Lauraceae is unequivocal, the assignment to genera or even species is more complex with vegetative organs alone, as leaf morphology varies greatly. This is also the case in extant and fossil Cryptocarya (Hill 1986; Christophel et al. 1996; van der Werff 1991, 2001; Bannister et al. 2012). For instance, Christophel et al. (1996) observed that the presumed diagnostic cuticular “butterfly-shaped” ledges of Australian Cryptocarya also occur in other extant Lauraceae genera of South America.

Additionally, van der Werff (1991, 2001) noted that the majority of extant genera are defined by floral characters; however, extant flower and fruit samples are difficult to obtain or even unavailable for study (Rohwer 1993; Li et al. 2008a). This makes the identification of fossil Lauraceae even more challenging. Moreover, small delicate reproductive organs are often not preserved as fossils. It should be noted that the presence of Cryptocarya in the Miocene of China was previously suggested by fossil fruits found in Zhangpu county which showed similarities to extant Cryptocarya bhutanica Long (Wang et al. 2019), but could not be unambiguously assigned to the genus. Therefore, the fossil flower of this study confirms the occurrence of Cryptocarya in the Miocene of Zhangpu and gives new insight into its flower morphology in the geological past.

Extant biodiversity and palaeoecological implications

Lauraceae are pantropical with about 50 genera and probably 2500 to 3500 species that are mostly evergreen trees and shrubs (Rohwer 1993). While they are mainly distributed in tropical to subtropical regions of Central and South America as well as Southeast Asia, the family is particularly diverse and sometimes dominant in rainforests (Rohwer 1993; Li et al. 2008a; Simpson 2010). About 25 genera and 445 species occur in China and mainly inhabit the southern provinces (Li et al. 2008a, Tan et al. 2023). In the family, Cryptocarya is one of the most widespread genera throughout the tropics, with between 200 and 350 species growing in South America, South Africa, Madagascar, Asia, Australia and Oceania, and a centre of diversity in Malaysia (Hyland 1989; van der Werff 1992, 2001; Rohwer 1993; Moraes 2007; Li et al. 2008b; Rohwer et al. 2014). It has been noted that the disjunct distribution of some Cryptocarya species was probably caused by fruits dispersion of birds from Gondwana across the Pacific, but that the genus was nonetheless, explicitly monophyletic (Rohwer et al. 2014). Although Cryptocarya as a whole still requires a global extensive revision (Rohwer 1993; Rohwer et al. 2014), some studies have recently improved our knowledge of the taxonomy of this genus in South Asia (de Kok 2015, 2016; Nishida et al. 2016).

The Zhangpu amber deposit, where the fossil flower was collected, is a highly diverse megathermal rainforest biome. Amber inclusions of arthropods include more than 250 families, with at least 200 being insects. Bryophytes are represented by about seven liverworts genera, as well as five extant moss taxa (Wang et al. 2021). In contrast, seed plant inclusions from Zhangpu amber are rare and up to date; only three species were described (Canarium wangboi Beurel et al. and Canarium leenhoutsii Beurel et al. (Burseraceae); Beurel et al. 2023; Parrotia zhiyanii Wu et al. (Hamamelidaceae); Wu et al. 2023). Nevertheless, plant megafossils from the amber-bearing sedimentary rocks are extremely rich, including about 24 families. Most of them are megathermal pantropical plant families, such as Annonaceae, Melastomataceae and Moraceae. Among these taxa, Dipterocarpaceae, Leguminosae, Lauraceae and Clusiaceae are the most abundant and diverse ones, with only slight morphological variations compared to their modern members, indicating a possible morphological stasis in the plant community (Wang et al. 2019, 2022; Wang et al. 2021). Within Lauraceae, fossils leaves with probable affinities to Alseodaphne Nees, Cinnamomum Schaeff. and Lindera Thunb. were recently discovered (Wang et al. 2021, supplementary material, table S1). The palaeoclimate of Zhangpu, as well as the fossil plant taxa are similar to tropical rainforests of Southeast Asia today, suggesting that the Zhangpu amber forest was a megathermal seasonal rainforest during the Mid-Miocene Climatic Optimum (Jacques et al. 2015; Zheng et al. 2019; Wang et al. 2021). Nowadays, Cryptocarya is still an important element of evergreen broad-leaved forests in South China, where it is part of the canopy and subcanopy (Wang et al. 2003; Li et al. 2008b). For instance, C. chinensis Hemsl. can be found in lower subtropical monsoon evergreen broad-leaved forests (Wang et al. 2003). Thus, the presence of Cryptocarya in the Zhangpu amber forest is consistent with the current palaeoclimatic and palaeoenvironmental interpretations (Jacques et al. 2015; Wang et al. 2021).

Conclusion

In our study, we present the first Cenozoic Lauraceae flower of Asia, based on an amber inclusion from Zhangpu. The application of non-invasive X-ray computed tomography techniques facilitated a detailed morphological examination of the amber inclusion and an in-depth comparison of the fossil flower with extant Cryptocarya flowers, revealing their remarkable resemblances. The fossil and extant flowers both have a perianth with six tepals, an androecium comprising nine fertile stamens with bilocular anthers and three staminodes, and a gynoecium with a unicarpellate carpel forming a superior ovary with one apical ovule surrounded by a hypanthium. The distinct presence of a similar staminal tube and position of staminal gland pairs are reported for the first time in Cryptocarya, in both the examined extant C. vulgaris from Australia and our fossil. This highlights the need to re-evaluate the occurrence (and potential significance) of those characters in other species.

In addition, the amber fossil supports the morphological stasis of Zhangpu plant communities alongside insects in tropical rainforests of Asia since at least the Middle Miocene (Wang et al. 2019, 2022; Wang et al. 2021; Beurel et al. 2023). However, it should be noted that previous affinities of Lauraceae fossil leaves to Cryptocarya are still debatable, as the family exhibits a wide range of leaf characters and morphological convergence between genera (Christophel et al. 1996). Nevertheless, the inclusion of this study represents the first fossil record of a flower of Cryptocarya and confirms the presence of this genus in the Miocene Zhangpu flora, which was only previously assumed from putative fossil fruits (Wang et al. 2019).

Acknowledgements

We would like to thank Bo Wang (NIGPAS) and Vincent Perrichot (Univ. Rennes) for the fossil specimen acquisition. We recognise the provision of beamtime, related to the proposal BAG-20210019 at beamline P05 at PETRA III at DESY, a member of the Helmholtz Association (HGF). This research was supported in part through the Maxwell computational resources operated at Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany.

We are grateful to Kristin Mahlow (MfN) for the scanning of the extant Cryptocarya flowers and to David Bruy (NOU, IRD), Pete Lowry (Missouri Botanical Garden), Chrissen Gemmill (Univ. Waikato), Sven Buerki (Univ. Boise), Martin Callmander (Conservatoire et Jardin Botaniques de Genève), and Jean Waikedre (IRD, Nouméa) for help in the field. Cryptocarya wilsonii was collected thanks to the Plants mo Pipol blong Vanuatu project (National Science Foundation grant 1555657 to the New York Botanical Garden). Angelika Haufe (MfN) provided access to extant specimens of Australian Cryptocarya from the Schrader herbarium (MfN, Berlin). We also thank Raymond J. Carpenter (Univ. Adelaide) for literature discussion and Corentin Jouault (MNHN) for insect identification. Conservation authorities of the North and South Provinces of New Caledonia (DDEE and DDDT) provided JM with collecting permits. This study was supported by the German Research Foundation [DFG, grant no. 423862824]. We would like to thank Yong Yang and Maria von Balthazar for their comments, which have improved the manuscript.

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