A Monographic Revision of Retrophyllum (Podocarpaceae)

Abstract

The living species of the genus Retrophyllum C.N. Page (Podocarpaceae) are revised. A key to the six species recognised is given. Retrophyllum filicifolium (N.E.Gray) R.R.Mill comb. nov. from New Guinea and the Moluccas is recognised as a species distinct from R. vitiense (Seemann) C.N. Page, which is restricted to material from Fiji and the Solomon Islands. Additional characters are given by which to separate Retrophyllum piresii (Silba) C.N. Page (Brazil) from R. rospigliosii (Pilg.) C.N. Page (Andes). The six species fall into three species-pairs that differ in reproductive characters: one pair in Papuasia and Melanesia, a second on New Caledonia and a third in South America. In the New Caledonian species-pair, the adult leaves are flattened in four ranks with only minimal heterofacial twisting, whereas in the species-pair inhabiting Fiji, New Guinea and neighbouring areas, as well as the pair inhabiting South America, the adult leaves are arranged in two ranks with significant heterofacial twisting. The names Podocarpus filicifolius N.E.Gray and Nageia minor Carrière, respectively the basionyms of Retrophyllum filicifolium and R. minus, are lectotypified, and the typification of Nageia minor comprehensively discussed in an appendix. Adult female epitypes are additionally designated for Podocarpus filicifolius, which was originally based on juvenile foliage of Retrophyllum mixed with a detached seed of Nageia wallichiana, and for Nageia minor, which Carrière described on the basis of sterile material. Two other appendices provide a list of accepted names and synonyms, and a list of exsiccatae. Illustrations and distribution maps are provided for each species.

The fossil flip‐leaves (Retrophyllum, Podocarpaceae) of southern South America

“…1–9; Table 1) is a small genus of tropical rainforest conifers with a disjunct Neotropical–West Pacific distribution and a scarce, Gondwanic fossil record. The six currently recognized living species (Table 1) include a pair of close relatives in each of three areas: Papuasia and Melanesia excluding New Caledonia, New Caledonia itself, and tropical South America (Mill, 2016). Per its name, Retrophyllum is readily identified from its oppositely twisted “flip‐leaved” foliage (e.g., Page, 1989; Figs.…”

“… Notes: Extant species data primarily from Mill (2016), combined with several additional sources (Gray and Buchholz, 1948; Gray, 1962; de Laubenfels, 1969, 1972; Silba, 1983; de Laubenfels, 1991; Farjon, 2010) and authors’ observations. All measurements in millimeters; extreme measurements omitted; leaf dimensions from free portion of blade only, usually recorded for fossils as the largest nearly complete leaf on a shoot.…”

“…The Retrophyllum species are physiologically constrained to require very wet conditions (Brodribb and Holbrook, 2006; Brodribb, 2011; Brodribb et al, 2014); they inhabit a range of humid environments, from lowland rainforest to cloud forest, and can play significant hydrological roles (Jaffré, 1995; Ataroff, 2002; Farjon, 2010; Mill, 2016). The genus includes the rheophytic, small‐statured, and endangered R. minus , from low elevations of New Caledonia (de Laubenfels, 1972; Jaffré, 1988; Jaffré et al, 1998, 2010), as well as the tall (to 50 m) Andean timber tree R. rospigliosii (Gray and Buchholz, 1948; de Laubenfels, 1982, 1991; Vicuña‐Miñano and Mostacero León, 2003; Veblen et al, 2005).…”

“…Accordingly, the leaves are heterofacially oriented, such that all leaves on the left side of the twig (when viewed from above toward the distal direction, as in Fig. 1) have the abaxial surface facing the sun, and all the opposing leaves display the adaxial surface (Florin, 1940b; Hill and Pole, 1992; Brodribb and Hill, 1999a; Eckenwalder, 2009; Farjon, 2010; Mill, 2016). In all other podocarps with twisted and flattened foliage, such as Afrocarpus (J. T. Buchholz & N. E. Gray) C. N. Page, opposing leaves twist in opposite directions in relation to the twig, and the same surface faces the sun on either side (e.g., Hill and Pole, 1992; Brodribb and Hill, 1999a).…”

“…The rows of stomata extend across the full leaf surface without concentration into stomatal bands and are separated by well‐marked longitudinal striations (Figs. 6, 9; Mill, 2016).…”