The amber specimen originated from the amber mines in the Cordillera of the Dominican Republic. The estimated age of Dominican amber is 15–20 million years (Early Miocene to early Middle Miocene) according to Iturralde-Vinent and MacPhee (1996, 2019). We exclude the possibility that the fossil is preserved in Quaternary resin (copal) because we observed that the plant tissue is partly replaced by minerals (probably pyrite).

The amber piece containing the fossil was manually ground and polished using a series of wet silicon carbide papers with grits ranging from FEPA P 600 (25.8 μm grain size) to 4000 (5 μm grain size, Struers company) to produce smooth surfaces. Thin layers of amber (less than 1 mm thick) were gradually ground away from the specimen to achieve a minimum distance between the inclusion and the amber surface (Sadowski et al. 2021). The amber piece was embedded in high-grade epoxy (Epo-Tek 301–2, Epoxy Technology) to stabilize it and to obtain a larger block that was easier to handle without risk of breakage.

For photography, the amber piece was placed on glass microscope slides with small pieces of modeling tack so that the upper polished surface was oriented horizontally. In order to reduce light scattering from fine surface scratches and improve optical resolution, a drop of water was placed on the upper surface of the amber and covered with a coverslip, as suggested by Sadowski et al. (2021). The prepared specimen was examined under a Carl Zeiss Stemi 508 dissecting microscope and a Carl Zeiss AxioScope A1 compound microscope, both of them equipped with Canon 5D digital cameras. The HeliconFocus software package (version 6.3.3 Pro) was used to stack images from eight to 34 individual focal planes (Sadowski et al. 2021).

The fossil was described following the definitions and terminology by Schmidt et al. (2022). The fossil was compared with published morphological descriptions of extant Selaginella species from the West Indies (Alston 1952; Proctor 1985, 1989; Caluff and Shelton 2003; Shelton and Caluff 2003; Caluff and Shelton 2009; Caluff and Shelton 2014; Valdespino et al. 2014; and Valdespino 2019) and with herbarium specimens from the B, BSC, GOET, HAC, and HAJB herbaria.

Class LYCOPODIOPSIDA Bartl., 1830

Order SELAGINELLALES Prantl, 1874

Family SELAGINELLACEAE Willk., 1854

 Selaginella jorelisiae A. G. López-García, A. R. Schmidt & L. Regalado, sp. nov.

Holotype.

Amber World Museum (Museo Mundo de Ámbar), Santo Domingo, Dominican Republic, collection number AWMJC010004 [Figs 1, 2] (only specimen available). The amber specimen is being arranged for integration into the amber collection of the Museo Nacional de Historia Natural “Prof. Eugenio de Jesús Marcano,” Santo Domingo, Dominican Republic.

Etymology.

The specific epithet honors Ms. Jorelis Caridad, Amber World Museum (Dominican Republic), for supporting our research by making the amber inclusion available for study.

Stratigraphic and geographic range.

Lower to early Middle Miocene amber-bearing sediments of the Dominican Republic, approximately 15 to 20 Myr old (Iturralde-Vinent and MacPhee 1996, 2019).

Diagnosis.

Dorsal trophophylls ovate, symmetric, base rounded (Figs 1B, C, 2A), slightly carinate, apex aristate (Fig. 1B, C) and minutely serrate, margins with scattered teeth or cilia. Ventral trophophylls elliptic (Figs 1D, 2B), base slightly asymmetric, rounded on both sides, with the basiscopic side more developed, apex apiculate (Figs 1D, 2B), margins with the acroscopic proximal third ciliate (Figs 1E, 2B), the basiscopic proximal third nearly entire or minutely serrate, and the medial and distal thirds serrate (Figs 1F, 2B).

Figure 1. 

Holotype of Selaginella jorelisiae A. G. López-García, A. R. Schmidt & L. Regalado, sp. nov. (AWMJC010004). A. Overview of shoot portion; B. Dorsal view of the trophophylls; C. Aristate dorsal trophophylls (B, C: white arrowheads indicate the aristate apex); D. Ventral trophophyll apiculate at apex; E. Proximal part of a ventral trophophyll with cilia at the acroscopic margin (cilia encircled); F. Medial part of a ventral trophophyll with serrate margins (black arrowheads indicate the teeth). Scale bars: 1 mm (A); 500 µm (B); 400 µm (C); 200 µm (D–F).

Figure 2. 

Reconstruction of trophophylls of Selaginella jorelisiae A. G. López-García, A. R. Schmidt & L. Regalado, sp. nov. (AWMJC010004). A. Dorsal trophophyll; B. Ventral trophophyll.

Description.

Rhizophores not preserved. Main stem once branched, branches of 12 and 8.8 mm length. Trophophylls dimorphic (Figs 1, 2). Axillary trophophylls not preserved. Dorsal trophophylls symmetric, distant, ascending, 0.80–0.98 × 0.2–0.32 mm, ovate (Figs 1B, C, 2A), base rounded, slightly carinate, apex aristate and minutely serrate (arista 0.27–0.35 mm long, approximately 1/3 of the dorsal trophophyll length, Fig. 1B, C), margins with scattered teeth or cilia (Fig. 2A). Ventral trophophylls close to imbricate, ascending, 1.20–1.45 × 0.45–0.62 mm, elliptic, base slightly asymmetric, rounded on both sides (Figs 1D, 2B), with the basiscopic side more developed, apex apiculate (Fig. 1D), margins of the acroscopic proximal third ciliate (Fig. 1E), the basiscopic proximal third nearly entire or minutely serrate, and the medial and distal thirds serrate (Fig. 1F), cilia 60–100 µm in length, teeth 15–40 µm. Epidermis of trophophylls not preserved. Strobili not preserved.

As modern Selaginella species show a clear pattern of endemism by biogeographical region (Schmidt et al. 2020), our morphological comparison with extant species focuses on Antillean species.

The characters present in Selaginella jorelisiae, i.e., dorsal trophophylls with scattered cilia and aristate apex and ventral trophophylls with cilia on the acroscopic proximal side and teeth all along the rest of the margin (Figs 1, 2), is a combination that can be found in several extant Greater Antillean species with either monomorphic or dimorphic sporophylls (Table 1). However, the fossil species can be distinguished from all of these extant taxa based on the presence of several characters, such as the shape and symmetry of the trophophylls, the shape of the apex, the distribution of the margin indentations, and the presence/absence of auricles at the outer basal margin of the dorsal trophophylls.

Among the species with monomorphic sporophylls, Selaginella heterodonta differs from the fossil by its broadly ovate dorsal trophophylls, with cilia present all over the margin and the length of the arista < 1/3 as long as the leaf (Fig. 3A, B). In the case of S. armata, the dorsal trophophylls are quite similar to those of S. jorelisiae, but the extant species can be differentiated by its broadly ovate and asymmetrical ventral trophophylls with abundant cilia on the acroscopic side (Fig. 3C, D). Selaginella confusa also differs from the fossil by having broadly ovate ventral trophophylls, with inequilateral bases and acute (rather than apiculate) apices (Fig. 3E, F), which are elliptic and nearly symmetric, whereas S. jorelisiae possessed apiculate apices. One of the extant species that closely resembles S. jorelisiae in the general morphology of its trophophylls is S. phiara. It can be distinguished from the fossil only by the asymmetry of the ventral trophophylls and the presence of conspicuous cilia in the medial and proximal third of the acroscopic margin (Fig. 3G, H).

Figure 3. 

Ventral (upper row) and dorsal (lower row) trophophylls of Greater Antillean species with monomorphic sporophylls. A, B. Selaginella heterodonta (MGC 7036, BSC); C, D. Selaginella armata (MGC 4375, BSC); E, F. Selaginella confusa (MGC 4451, BSC); G, H. Selaginella phiara (HFC 32879, HAJB). Number series MGC: Ferns and Lycophytes of the Fern Garden, Santiago de Cuba, HFC: Herbarium Flora of Cuba (Regalado et al. 2008). Scale bars: 500 µm (E); 250 µm (C); 200 µm (A, B, D, F, G, H).

Among the extant species with dimorphic sporophylls that share a similar trophophyll morphology with Selaginella jorelisiae is S. tenella, which differs from the fossil by having the ventral ones narrowly elliptic with a rounded apex (Fig. 4A, B). Similarly, S. cordifolia can be distinguished from the fossil by its dorsal trophophylls possessing an arista that occupies at least half the length of the leaf, and its broadly ovate ventral trophophylls are conspicuously asymmetric at the base (Fig. 4C, D). Selaginella consimilis differs from the fossil in the trophophyll morphology with the dorsal trophophylls narrowly elliptic and the ventral ones having cilia in both the acroscopic and basiscopic margins (Fig. 4E, F). Despite the similarities in the overall morphology, S. urquiolae is also clearly distinct from S. jorelisiae. Its dorsal and ventral trophophylls are asymmetric at the base, with the dorsal ones possessing an inner side exauriculate (Fig. 4G, H; Caluff and Shelton 2009). Selaginella flabellum is distinguished from the fossil by its dorsal trophophylls with inequilateral bases and an auricle on the outer side of the base (Fig. 4I, J), which is not present in the fossil morphology. In addition, the ventral trophophylls of S. flabellum are broadly ovate with conspicuous cilia in both the acroscopic and basiscopic proximal margins. These cilia are restricted to the acroscopic proximal margin in the fossil (Fig. 2B). In contrast to all character combinations observed in the extant dimorphic Greater Antillean species, the fossil is characterized by symmetric, ovate dorsal trophophylls, with an arista occupying one third of the leaf length, and elliptic, nearly symmetric ventral trophophylls that are apiculate at the apex and have only a few cilia in their acroscopic proximal margin (Figs 1, 2).

Figure 4. 

Ventral (upper row) and dorsal (lower row) trophophylls of Greater Antillean species with dimorphic sporophylls. A, B. Selaginella tenella (Serguera 011, BSC); C, D. Selaginella cordifolia (MGC 4848, BSC); E, F. Selaginella consimilis (Wright 3907, HAC); G, H. Selaginella urquiolae (MGC 4390, BSC); I, J. Selaginella flabellum (MGC 4716, BSC). Number series MGC: Ferns and Lycophytes of the Fern Garden, Santiago de Cuba. Scale bars: 400 µm (A, C, G, H); 300 µm (I); 250 µm (B, D); 200 µm (E, F); 150 µm (J).

To the best of our knowledge, the Miocene fossil record of Selaginellaceae comprises exclusively microfossils (Knobloch 1986; Negru and Dorofeev 1986; Graham 1987, 1988, 1989, 1999; Mai and Walther 1991; Mai 2000, 2004; Grímsson et al. 2011; Henniger et L. 2011; Bouchal 2019; Tcheumeleu et al. 2020; He and Wang 2021). We could not find any macrofossil record of Miocene Selaginellaceae. Although strobili and spores are not preserved, the newly discovered fossil Selaginella provides further insights into the evolutionary history of the genus.

In a phylogenetic context, the anisophyllous vegetative shoots observed in Selaginella jorelisiae place it within the so-called “rhizophoric clade” (Weststrand and Korall 2016b), excluding a possible assignment of the fossil to the “selaginella clade” (subg. Selaginella, Weststrand and Korall 2016b), which have isophyllous shoots. Within the “rhizophoric clade” (Weststrand and Korall 2016b), assignment of the fossil to subg. Rupestrae can also be ruled out because representatives of this subgenus possess helically arranged vegetative leaves. However, an assignment to one of the remaining subgenera (Ericetorum, Exaltatae, Gymnogynum, Lepidophyllae, or Stachygynandrum) is impossible due to the incompleteness of the fossil.

Dominican amber is a well-known source of plant inclusions and is particularly famous for its numerous liverwort and moss fossils (Frahm 2001; Frahm and Newton 2005; Kaasalainen et al. 2016; Heinrichs et al. 2018), indicating the presence of diverse epiphytic communities in the Caribbean during the Miocene, and suggesting that the Dominican amber forest was a moist tropical forest. This is also supported by the presence of diverse ferns (Gómez 1982; Lóriga et al. 2014; Schneider et al. 2015; Sundue and Poinar 2016; Regalado et al. 2024), fungi, and lichens (Rikkinen and Poinar 2001, 2008; Kaasalainen et al. 2016), which were considered representatives of these epiphytic communities.

Iturralde-Vinent (2001) proposed that resin production by the amber source species Hymenaea protera G.O. Poinar and subsequent deposition most likely took place under warm/humid conditions during the Middle Miocene Climate Optimum, which occurred at about 16 Ma (Kaandorp et al. 2005). This assumption was based on the apparent correlation between the climate and the hypersecretion of resin observed for several extant Hymenaea species (Iturralde and MacPhee 2019), as trees growing in tropical moist forests and along streams apparently synthesize the greatest quantity of resin (Langenheim et al. 1973; Langenheim 1995). This hypothesis also supports previous hypotheses that the source of Dominican amber was located in a tropical moist forest biome (Rikkinen and Poinar 2008; Penney 2010) but does not exclude the possibility that the amber derived from gallery or riparian forests.

Although the most common life forms of Selaginella species are the terrestrial and the epilithic ones (Tryon and Tryon 1982; Jermy 1990), some species are epiphytes, e.g., S. extensa Underw. (Tryon and Tryon 1982; Mickel and Smith 2004), S. erythropus (Mart.) Spring (Fraile et al. 1995), S. arsenei Weath and S. flagellata Spring (Mickel and Smith 2004), S. bryophila M.Kessler & A.R.Sm. (Kessler et al. 2006), S. involvens (Sw.) Spring (Zhang et al. 2013), S. repanda (Desv. & Poir.) Spring (Setyawan et al. 2015), S. saltuicola (Valdespino et al. 2015), S. squamulosa Valdespino (Valdespino 2017), and S. microphylla (Kunth) Spring (Krzyzanski et al. 2021). Particularly among the Greater Antillean species, S. cordifolia and S. tenella have been found growing on mossy tree trunks in Puerto Rico and Jamaica, respectively (Proctor 1985, 1989), and S. epipubens Caluff & Shelton occasionally grows epiphytically on the trunk base of the tree fern Cyathea myosuroides (Liebm.) Domin in Cuba (Caluff and Shelton 2009).

In the context of the Dominican amber forest, Selaginella jorelisiae was probably a component of the herbaceous layer, growing either on overhanging cliffs, moist shaded ledges, boulders, or soil banks over humus and leaf litter or on mossy rocks, probably mixed with bryophytes, as is the most common microhabitat for most of the extant Greater Antillean spike moss species (Proctor 1985, 1989; Caluff and Sheton 2003, 2009). However, we cannot exclude the occurrence of Selaginella jorelisiae as part of the epiphytic community of the Dominican amber forest.