Miranthus, E. M. FRIIS, P. R. CRANE et K. R. PEDERSEN, 2021

Friis, Else Marie, Crane, Peter R. & Pedersen, Kaj Raunsgaard, 2021, Early Flowers Of Primuloid Ericales From The Late Cretaceous Of Portugal And Their Ecological And Phytogeographic Implications, Fossil Imprint 77 (2), pp. 214-230 : 225-226

Miranthus

Characters of the Miranthus flowers clearly place the fossil taxon in the primuloid clade, which is firmly nested among the Ericales (e.g., Anderberg et al. 2002, Schönenberger et al. 2005). In most classifications the primuloid clade comprises four groups recognised either as families of the order Primulales ( Maesaceae , Theophrastaceae , Primulaceae , Myrsinaceae ; Anderberg and Ståhl 1995, Anderberg et al. 1998 a, 2000 a, Källersjö et al. 2000, Schönenberger et al. 2005) or as subfamilies of the family Primulaceae (Maesoideae, Theophrastoideae , Primuloideae, Myrsinoideae; APGIII 2009). A fifth, monogeneric lineage, Samolaceae /Samoloideae, is now usually treated as a tribe, Samoleae, within the Theophrastaceae / Theophrastoideae group (e.g., Källersjö et al. 2000, Caris and Smets 2004). We here use the inclusive definition of Primulaceae s. l.

Extensive studies on extant primuloids by Anderberg and colleagues (e.g., Anderberg and Ståhl 1995, Anderberg et al. 1998 a, b, 2000a, b, 2007, Källersjö et al. 2000, Källersjö and Ståhl 2003, Wanntorp and Anderberg 2011), as well as by other researchers (e.g., Caris et al. 2000, Ma and Saunders 2003, Caris and Smets 2004, Morozowska et al. 2020), have been pivotal for our comparative studies and for recognizing the affinities of the fossil flowers. Relationships among the main primuloid lineages are well supported based on phylogenetic analyses of both morphological and molecular data. The monogeneric Maesoideae is resolved as the sister lineage to the remaining taxa, with Theophrastoideae resolved as sister to a well-supported Primuloideae- Myrsinoideae clade ( Källersjö et al. 2000, Bremer et al. 2002, Schönenberger et al. 2005). Within Theophrastoideae , the monogeneric Samoleae are sister to all other members of the subfamily.

Shared features for Miranthus and the Primulaceae include the pentamerous and isomerous organisation of the flowers, persistent calyx, haplostemonous androecium and the highly characteristic free central and dome-shaped placenta. Among Ericales , a free central placenta is known only for the Primulaceae and is a clear synapomorphy for the group ( Schönenberger et al. 2005).

Few floral features distinguish the various primuloid lineages and a separation of the lineages based on floral morphology is only poorly supported. As noted by Caris and Smets (2004) “it seems that no unambiguous morphological synapomorphies can be found that justify a position of Samolus in either Primulaceae or Theophrastaceae ”, but a position as sister to Theophrastoideae is well supported by molecular data ( Källersjö and Ståhl 2003). Nevertheless, the semi-inferior ovary of Miranthus flowers, and the presence of staminodes alternating with the corolla lobes, are important distinctive floral features. Furthermore, among extant primuloids, a semi-inferior ovary is known only for Maesa FORSSK. and Samolus L., and staminodes are present only in Maesa , the genera of Theophrastoideae (including Samolus ), and Soldanella L. of the Primuloideae ( Anderberg and Ståhl 1995, Anderberg et al. 1998a). Anectariferous zone on the upper part of the ovary, referred to as “nectarostomata on the flanks of the ovary” by Caris and Smets (2004), is a further feature shared with Maesa and Samolus that is not present in other primuloid taxa ( Caris et al. 2000, Caris and Smets 2004). Miranthus thus appears to be nested securely in the primuloid clade.

Maesa comprises about 150 species of tropical and subtropical vines, shrubs, and trees in the Old World ( Anderberg et al. 2000a), whereas species of Samolus are herbaceous and less diverse with about twelve species. Four or five species of Samolus occur in the southern United States, Cuba and Mexico, and one species ( Samolus valerandi L.) is cosmopolitan in distribution. The remaining species are restricted to the Southern Hemisphere, with three species in South America, four in Australia and Tasmania, and one in South Africa ( Ståhl 2004a, Wanntorp and Anderberg 2011). Maesa is distinguished from Miranthus and all other primuloid taxa by the presence of two bracteoles immediately under the calyx. Miranthus and other primuloids display the likely plesiomorphic condition in which such bracteoles are lacking ( Anderberg et al. 2000a). Miranthus is also more like Samolus in having a long and slender pedicel.

The close relationship of Miranthus and Samolus suggested by their morphological similarities is supported by the results of the phylogenetic analysis ( Text-fig. 9 ). The most parsimonious position of Miranthus is as sister to Samolus in the Theophrastoideae (131 steps). However, a sister group relationship to Theophrastoideae , or a position as sister to Clavija RUIZ et PAV. + Theophrasta L., is only one step longer (132 steps), while a sister group relationship to either Clavija or Theophrasta is less parsimonious (136 steps). A sister group relationship to Myrsinoideae is also only two steps less parsimonious than the Miranthus - Samolus relationship, as also is a sister group relationship to Theophrastoideae + Primuloideae + Myrsinoideae, to Primuloideae + Myrsinoideae, and to Maesa (133 steps). All other positions are less parsimonious (134 steps or more; Text-fig. 9 ).

Figures

Text-fig. 9. Phylogenetic tree indicating the number of required character state changes (steps) under parsimony for various positions of Miranthus gen. nov. in a molecular based backbone tree (see material and methods for additional details).