Pollen of six extant Rehderodendron species were photographed under LM and SEM and their sizes measured Figures 2, 3, Table 2). Depending on the length of heating during the acetolysation process, the pollen changed shape from originally suboblate (unacetolysed state) to subspheroidal and then to more subprolate shapes (fully acetolysed state; Figure 2). Pollen measurements displayed considerable differences: LM photographs of pollen in glycerol with a micrometer scale result in larger sizes than the measurements of pollen photographed with SEM (more-or-less desiccated state of pollen after being washed in alcohol, partly desiccated and sputtered under Argon atmosphere; Figure 2, Table 2). The measurements of the width and height of the endopori was only possible under LM. However, all measured sizes fall into the size ranges of previously measured Rehderodendron pollen in [24 page 87: R. kwangtungense, R. kweichowense and R. indochinense = is actually R. macrocarpum according to the junior author] and [25 table 1: R. macrocarpum]. The same is valid for the fossil Rehderodendron pollen (see below descriptions).
Description of extant pollen
Ericales Dumortier
Styracaceae Dumortier
Rehderodendron Hu
R. kwangtungense Chun (Figures 2A, 2A1, 3A)
Tricolporate (rarely tetracolporate), spheroidal to subprolate pollen grains, triangular to circular in polar view and angular, elliptical to subcircular in equatorial view; measurements: LM polar axes ca. 32.4 to 38.7 µm, equatorial axes ca. 34.5 to 42.7 µm and under SEM polar axes ca. 27.8 to 29.0 µm, equatorial axes ca. 26.2 to 31.2 µm; endoporus shape is rectangular to quadrangular and measures ca. 7.2 to 11.8 µm in height and ca. 5.4 to 7.4 µm in width (Figure 1, 2). Tectum: tectate, perforate, shallowly fossulate, and faintly rugulate with rugulae bordered by faint fossulae or perforations (comparable to R. indochinense, but less pronounced), towards the colpi more micro-verrucate; the ectexine in the polar areas and mesocolpi is regularly ornamented with supratectal blunt micro-echini (or micro-gemmae), colpus membrane, when visible, is micro-verrucate; pollen wall ca. 1.2 to 1.3 µm thick with the sexine (ca. 0.7 to 0.8 µm) thicker than the nexine (0.4 to 0.5 µm), endexine columellar to granular (columellae height max. 0.3 µm).
R. kweichowense Hu (Figures 2 B, 2B1, 3B)
Tricolporate, suboblate, subspheroidal to prolate pollen grains, triangular to circular in polar view and angular, elliptical to subcircular in equatorial view (Figures 1, 2); measurements: LM polar axes ca. 27.3 to 34.9 µm, equatorial axes ca. 17.8 to 39.6 µm and under SEM polar axes ca. 25.0 to 27.2 µm, equatorial axes ca. 20.9 to 29.7 µm ; endoporus shape is rectangular to quadrangular and measures ca. 3.2 to 8.4 µm in height and ca. 2.7 to 9.8 µm in width. Tectum: tectate, micro-verrucate to occasionally micro-rugulate, perforate, whilst the diameter of the micro-verrucae is ca. 0.2 µm, several micro-verrucae and micro-rugulae can locally fuse to produce areolae of ca. 0.5 to 0.8 µm in diameter; the colpus membrane, when visible, is micro-verrucate; pollen wall thickness ca. 1.1 to 1.3 µm with the sexine thicker than nexine.
R. macrocarpum Hu (Figures 2C, 2C1, 3C)
Tricolporate, suboblate, subspheroidal pollen grains, triangular to circular in polar view and angular, elliptical to subcircular in equatorial view (Figures 1, 2); measurements LM polar axes ca. 28.2 to 34.2 µm, equatorial axes ca. 32 to 44.2 µm and under SEM polar axes ca. 24.3 to 26.3 µm, equatorial axes ca. 24.3 to 33.6 µm; endoporus shape is rectangular to quadrangular and measures ca. 4.9 to 11.8 µm in height and ca. 6.4 to 10.4 µm in width. Tectum: tectate, perforate fossulate, the fossulae border more-or-less elongated, occasionally curved, angular rugulae (or areolae) of ca. 0.3 to 0.8 µm width and ca. 1.2 to 2.8 µm length; the rugulae are flat and display regularly spaced rows composed of occasionally fused supratectal micro-echini (or micro-gemmae) arranged perpendicularly to the rugulae; colpus membrane, when visible is micro-verrucate; pollen wall thickness ranges between 1.4 and 1.7 µm with the sexine (ca. 1 to 1.2 µm, visible columellae max. 0.3 µm long) thicker than the nexine (ca. 0.3 to 0.5 µm).
R. microcarpum K.M Feng ex. T.L. Ming (Figures 2D, 2D1, 3D)
Tricolporate, suboblate to subprolate pollen grains, triangular to circular in polar view and angular, elliptical to subcircular in equatorial view (Figures 1, 2); measurements LM polar axes ca. 31.8 to 32.7 µm, equatorial axes ca. 37.3 to 42.4 µm and under SEM polar axes ca. 26.2 to 28.8 µm, equatorial axes ca. 26.6 to 31.2 µm; endoporus shape is rectangular to quadrangular and measures ca. 10.4 to 11.9 µm in height and ca. 5.9 to 9.1 µm in width. Tectum: tectate, perforate, fossulate, micro-verrucate to micro-rugulate, whereas the mesocolpium areas are more pronounced perforate and micro-verrucate and the colpus margins and the polar areas are more micro-rugulate and fossulate; micro-verrucae and micro-rugulae are regularly covered by supratectal micro-echini (or micro-gemmae) that can be occasionally arranged in rows and fuse (the micro-rugulae are considerably smaller than the micro-rugulae of R. macrocarpum); colpus membrane, when visible is loosely micro-verrucate; pollen wall thickness ranges between 1 and 1.3 µm with the sexine (ca. 0.7 to 0.9 µm, visible columellae max. 0.2 µm long) thicker than the nexine (ca. 0.3 to 0.4 µm).
R. indochinense H.L. Li (Figures 2E, 2E1, 3E)
Tricolporate, subspheroidal to suboblate or subprolate pollen grains, triangular to circular in polar view and angular, elliptical to subcircular in equatorial view (Figures 1, 2); measurements LM polar axes ca. 26.9 to 29.1 µm, equatorial axis ca. 30.1 to 38.2 µm and under SEM polar axes ca. 23.5 to 27.1 µm, equatorial axes ca. 23.1 to 37.5 µm; endoporus shape is rectangular to quadrangular of ca. 5.4 to 9.1 µm height and ca. 5.4 to 6.7 µm widths. Tectum: tectate, perforate, rugulate, shallow fossulate, whereas the rugulae are generally angular, irregularly shaped and bordered by shallow fossulae (grooves) and perforations; the rugulae are ca. 1 µm to 2 µm in length, generally less than 1 µm wide, regularly ornamented with supratectal blunt micro-echini (or micro-gemmae) and decrease considerably in size towards the colpus margins; the perforation are best seen in the polar areas and diminish towards the colpus margins; the colpus membrane is micro-verrucate; pollen wall thickness is ca. 1.4 to 1.5 µm with the sexine (ca. 0.8 to 1 µm, visible columellae max. 0.3 µm long) thicker than the nexine (ca. 0.4 µm).
R. truongsonense P.W. Fritsch, W.B. Liao & W.Y. Zhao (2F, 2F1, 3F)
Tricolporate, suboblate to spheroidal pollen grains, triangular to circular in polar view and angular, elliptical to subcircular in equatorial view (Figures 1, 2); measurements: LM polar axes ca. 30.5 to 32.7 µm, equatorial axes ca. 36.4 to 40.0 µm and under SEM polar axes ca. 26.1 to 28.2 µm, equatorial axes ca. 28.3 to 32.1 µm; endoporus shape is rectangular of ca. 6.3 to 12.7 µm in height and ca. 4.3 to 7.4 µm in width. Tectum: tectate, covered with regularly arranged micro-gemmae and fused micro-gemmae producing short rod-like structures, perforate to foveolate, the tectum becomes more pronounced micro-rugulate to micro-verrucate, fossulate towards the colpus margins; colpus membrane, when visible is loosely micro-verrucate; pollen wall thickness ranges between 1 and 1.4 µm with the sexine (ca. 0.8 to 0.9 µm, visible columellae max. 0.3 µm long) thicker than the nexine (ca. 0.5 µm).
Description of fossil pollen
Rehderodendron sp. 1 (Figures 3G, 4A)
Tricolporate, prolate pollen grain; polar axis 30.5 µm and equatorial axis 22.3 µm (SEM); endoporus is more-or-less quadrangular to rectangular ca. 7.2 µm in height and ca. 5.7 µm in width (compressed fossilized state). Tectum: rugulate, fossulate, perforate covered with supratectal micro-gemmae, the rugulae are more-or-less angular, occasionally curving ca. 0.6 to 1.7 µm long and ca. 0.4 to 0.8 µm wide; the micro-gemmae are regularly arranged in rows perpendicular to the rugulae lengths; wall thickness 1.2 to 1.4 µm with the sexine thicker than the nexine (Figures 2, 3).
Remarks: This pollen type originally has been wrongly affiliated with Ebenaceae (Diospyros in [22]) and comes from the Paleocene Eocene Thermal Maximum (PETM) section recovered in exploration drill cores of the London tube in Brixton (England). It resembles mostly R. kwantungense and R. macrocarpum.
Rehderodendron sp. 2 (Figures 3H, 4B)
Tricolporate, prolate pollen grain; polar axis 30.8 to 34 µm and equatorial axis 23.2 to 24.4 µm; endoporus is more-or-less quadrangular to rectangular ca. 3.8 to 4.9 µm in height and ca. 3.8 to 4.6 µm in width (compressed fossilized state). Tectum: irregularly shaped rugulate to micro-rugulate to irregularly shaped verrucate, fossulate, perforate, the rugulae are more-or-less angular, occasionally curving ca. (ca. 0.2 to 0.8 µm wide and 0.7 to 2.5 µm long) and ornamented with striae or linearly fused supratectal micro-gemmae arranged perpendicular to the length of the rugulae; wall thickness 1.2 to 1.3 µm with the sexine thicker than the nexine (Figures 2, 3).
Remarks: This pollen type has been reported from [23 fig. 3 G-I] from middle Miocene Schaßbach clay pit (Austria; Middle Miocene Thermal Maximum = MMTM). It resembles a mixture of R. kwantungense, R. microcarpum and R. indochinense.
Rehderodendron sp. 3 (Figures 3I, 4C)
Tricolporate, prolate pollen grain; polar axis 31. to 34.6 µm and equatorial axis 23.2 to 24.5 µm; endoporus is more-or-less quadrangular to rectangular ca. 4.8 to 6.7 µm in height and ca. 4.1 to 5.6 µm in width (compressed fossilized state). Tectum: rugulate, fossulate, perforate, the rugulae are more-or-less angular, occasionally curving ca. 0.8 to 1.8 µm long and ca. 0.3 to 0.8 µm wide and ornamented with striae arranged diagonally or perpendicular to the length of the rugulae; wall thickness ca. 1.1 to 1.3 µm with the sexine thicker than the nexine (Figures 2, 3).
Remarks: This pollen type has been reported from Hofmann and Sachse [31] middle Miocene (end of the MMTM) sand pit in Entrischenbrunn (Germany). It resembles more a mixture of R. macrocarpum, R. microcarpum and R. indochinense.
Summarizing results of extant pollen descriptions
When looking at the sexine sculpture and ornamentation of R. macrocarpum and R. microcarpum there are fluent transitions in the rugulae sizes (larger to smaller), however, R. marcrocarpum is more fossulate and less perforate whereas R. microcarpum displays more perforations (Figure 2). When comparing the sexine of R. microcarpum with R. kwangtungense and R. indochinense the rugulae sizes are also transitional (towards smaller and less pronounced rugulae and more obvious perforations), however R. kwangtungense, has more pronounced supratectal micro-gemmae (Figure 2). If not knowing which pollen belongs to which species, separation between these four taxa is a difficult task. On the other hand, R. truongsonense, which is conspicuously perforate with supratectal micro-gemmae, and R. kweichowense, which is micro-verrucate to areolate (Figure 2), can be easily differentiated from each other and the rest of the species, and the fossil pollen investigated show no reminiscence with these two taxa either.
Short description of extant fruits
We described the fruits of the extent species (Figure 5, Table 3). It can be be generally said, that all investigated species develop a thick spongy mesocarp and differ mostly in the rib number and complexity of the endocarp ray system.
The fruits of R. indochinense (Figure. 5A, B) have a characteristic long cylindrical shape with five obvious ribs, the fruit surface usually has large brown spots which is a unique feature in extant Rehderodendron species. The styles are persistent, and the stigma is beaked, and the endocarp rays are irregular.
R. kwangtungense (Figure 5C, D) generally is characterized by columnar fruits, that are conspicuously ribbed, the styles are not conspicuous and persistent. The endocarp ray system is complex and displays irregular rays.
The fruits of R. microcarpum (Figure 5E, F) are more narrow and smaller, with usually an ovoid, cylindrical to fusiform shape and an inconspicuously ribbed surface (five ribs usually visible). The styles are persistent (conical coracoid), with simple, thickened endocarp rays.
R. macrocarpum (Figure 5G, H) is characterized by its oblong to elliptic fruits that are conspicuously ribbed (8-12 ribs), its persistent styles are very short. The endocarp rays are thick and complex.
As mentioned above, the pollen morphologies of R. kweichowense and R. truongsonense are quite different to morphologies of the other four species investigated. This difference is also reflected in their fruits: The fruits of R. kweichowense (Figure 5I, J) are densely covered with stellate hairs. The fruits have also 10 to12 ribs and irregular endocarp rays. The fruits of R. truongsonense (Figure 5K, L) are short terete and inconspicuously ribbed, the endocarp is thickened and comprises a complex endocarp ray system.