Retrophyllum oxyphyllum (Freng. & Parodi) Wilf, 2020

Wilf, Peter, 2020, Eocene " Chusquea " fossil from Patagonia is a conifer, not a bamboo, PhytoKeys 139, pp. 77-89 : 77

Retrophyllum oxyphyllum (Freng. & Parodi) Wilf comb. nov. Figure 1A-C

Basionym.

Chusquea oxyphylla Freng. & Parodi, Notas del Museo de La Plata, Paleontología 6: 236 (1941: fig. 1).

Synonym.

Retrophyllum spiralifolium Wilf, American Journal of Botany 104: 1350 (2017).

Holotype.

Argentina. Chubut Province: Laguna del Hunco, Tufolitas Laguna del Hunco, Huitrera Formation, early Eocene. Museo de La Plata (MLP), MLP-4234. Collected by J. Frenguelli 1939 or 1940 ( Frenguelli and Parodi 1941: 236), precise collection location unknown. The holotype is the only specimen of the basionym.

Amended description.

The entire recent description of Retrophyllum spiralifolium Wilf, 2017 ( Wilf et al. 2017b: 1350-1352), verbatim, is here denoted as the amended formal description of Retrophyllum oxyphyllum comb. nov. but is not reproduced here due to its length. The holotype fully conforms to the described foliage, in particular the distichous foliage form, of R. spiralifolium . The new combination incorporates all associated material described, illustrated, and justified previously under R. spiralifolium ( Wilf et al. 2017b), including the distichous foliage form, helical foliage form, reduced foliage forms, and peduncle of pollen cones.

Diagnostic characters.

In the absence of a diagnosis of the basionym ( Frenguelli and Parodi 1941), a formal amended diagnosis cannot be provided. However, the characters listed in the specific diagnosis for Retrophyllum spiralifolium ( Wilf et al. 2017b: 1350) all now apply to Retrophyllum oxyphyllum comb. nov. That diagnosis ( Wilf et al. 2017b: 1350) is reproduced here for ease of use, with the characters preserved in the holotype (Fig. 1A-C ) indicated in bold font:

" Foliage with conspicuous central longitudinal band of thickened tissue and obscure midvein not separating rows of stomata. Lateral resin canals present. Principal leaves decurrent and extensively clasping twig, free portions either distichous and pectinate, with full heterofacial flattening, or spirally deployed with negligible to slight basal twisting, frequently broken off to leave spirally arranged stubs of clasping portions. Leaf apices acuminate to markedly acuminate. Terminal bud protected by reduced, modified leaves. Reduced foliage also including ovoid and narrow forms on separate shoot segments and narrow miniature leaves abruptly or gradually interspersed with principal leaves along shoots. Pollen cones pedicellate, long-cylindrical, in axils of narrow reduced leaves, distichously grouped on a common peduncle. "

Amended description of the holotype.

The holotype of Retrophyllum oxyphyllum comb. nov. (Fig. 1A-C ) is a leafy branch segment of axis length 6.4 cm with remains of ca. ten pairs of opposite, distichous (pectinate), decurrent and clasping, ovate-lanceolate, bifacially flattened leaves that are heterofacially twisted into a single plane at their departure from the twig. The clasping portions of the leaves entirely cloak the twig in an overlapping, zigzag pattern. It is not possible to determine whether the preserved view is abaxial or adaxial (see Wilf et al. 2017b). The bases of the leaves’ free portions are twisted counterclockwise if viewed laterally from leaf to twig, so that pairs of abaxial and adaxial leaf faces appear in the same plane on either side of the twig. Only ca. four leaves have their free portions well preserved; most leaves are broken off at or near twig departure, leaving behind their clasping leaf bases. Free leaf portion length is to 18.0 mm, width to 2.5 mm, apices acute but not completely preserved. Leaves have no venation visible but preserve a longitudinal, raised central band of thickened, coalified tissue whose width is ca. 25% of total leaf width; the central band presumably obscures the much smaller, true midvein running within. The remaining leaf surface has numerous parallel striations on both faces, continuous across the midvein, with slight relief but no evidence of vein tissue; there are no cross-lineations that could be interpreted as cross-veins.

Figures

Figure 1. Retrophyllum oxyphyllum (Freng. & Parodi) Wilf, comb. nov. from Laguna del Hunco. A-C Holotype, MLP- 4234, arrows in A indicate detail panels in B, C. D MPEF-Pb 8915 a (Museo Paleontologico Egidio Feruglio, Trelew, Argentina) from Laguna del Hunco quarry LH 6 of Wilf et al. (2003), part of an extensive suite of fossil Retrophyllum material here synonymized (Wilf et al. 2017 b; Wilf 2017). A General view of the holotype, with clasping, overlapping, zigzagging, decurrent, opposite leaf bases; heterofacially twisted, lanceolate free-leaf portions, many of them broken off at departure from the twig and leaving stubs; and thick central tissue band compressed to a coalified black stripe, most conspicuous in the basalmost preserved leaf. Leaves on the left side of the twig in this view are twisted " forward " and those on the right " backward, " i. e., counterclockwise in both cases when viewed from leaf to twig; original abaxial or adaxial orientation cannot be determined B detail of backward-twisted leaf (at right), with negative relief (from compression) of the raised central band visible, and broken leaf base (at left) C detail of forward-twisted leaf, thickened central band, and dense longitudinal striations across the leaf surface marking borders between former stomatal rows D terminus of a long penultimate branch (also Wilf et al. 2017 b: figs 10 - 12) with pairs of opposite, ultimate leafy branches, each similar to the holotype (A) with opposite leaves and linear-reduced leaves at the shoot bases (the holotype does not preserve the shoot base). Arrows indicate opposite branch scars on the exposed penultimate branch, completely unlike bamboo nodes. Scale bars: 2 cm (A, D); 2 mm (B, C).