Monilophyta P. D. Cantino and M. J. Donoghue in P. o. Cantino et al. (2007): E13 [J. A. Doyle, P. D. Cantino, and M. J. Donoghue], converted clade name Registration Number: 67 considered to be their stem relatives, such as Sphenophyllales, Archaeocalamites, and Calamites Definition: The largest crown clade containing in the case of Equisetum; Psaronius in the case Pteridium aquilinum (Linnaeus) Kuhn 1879 of Marattiales; and Ankyropteris in the case of (originally Pteris aquilina) (Leptosporangiatae) Leptosporangiatae (see Doyle, 2013). Less can be and Equisetum hyemale Linnaeus 1753 but not said about other fossil members because most 0ryza sativa Linnaeus 1753 (Spermatophyta) of the analyses that inferred the existence of this or Huperzia lucidula (Michaux) Trevisan dade included only extant plants, and some de Saint-Leon 1875 (originally Lycopodium analyses that included both fossils and extant lucidulum) (Lycopodiophyta). This is a max- plants did not infer the existence of this dade. imum-crown-dade definition. Abbreviated Kenrick and Crane (1997: Table 7.1) included definition: max crown V (Pteridium aquilinum the fossil groups Cladoxyliidae, Zygopteridae (Linnaeus) Kuhn 1879 & Equisetum hyemale (which may include additional stem relatives Linnaeus 1753 ~ Oryza sativa Linnaeus 1753 of Leptosporangiatae: see Galtier, 2010), and V Huperzia lucidula (Michaux) Trevisan de Stauropteridae within Moniliformopses (a clade Saint-Leon 1875). that may be either equivalent to or slightly more inclusive than Monilophyta; see Comments). Etymology: From the Latin monile, mean- In contrast, Rothwell (1999) inferred that ing necklace, in reference to the "position and Cladoxyliidae and Zygopteridae, along with ontogeny of protoxylem in the lobed primary Equisetum, are more closely related to seed xylem of early fossil groups" (Kenrick and plants than to extant ferns (thus the clade Crane, 1997: 248), and the Greekphyton (plant). Monilophyta as defined here does not exist on that phylogeny) and stauropterids are even more Reference Phylogeny: The primary reference distant from extant ferns. Some of the analyses phylogeny is Knie et al. (2015: Fig. 4). See also of Rothwell and Nixon (2006) inferred the exis- Rothfels et al. (2015: Fig. 1, the clade labeled tence of Monilophyta (as defined here) and oth- "Polypodiopsida"), Pryer et al. (2001: Fig. 1, as ers did not, but in those trees in which there is "Moniliformopses"), Pryer et al. (2004: Fig. 3, as a clade that fits our definition of Monilophyta, "ferns (monilophytes)"), Qiu et al. (2007: Fig. all three of the above-mentioned extinct groups 1), and Wickett et al. (2014: Figs. 2, 3). included in Moniliformopses by Kenrick and Crane lie outside Monilophyta. Composition: Equisetum, Psilotophyta, Ophio- glossales, Marattiales, and Leptosporangi.atae, their Diagnostic Apomorphies: In the analysis of most recent common ancestor, and all of its Kenrick and Crane (1997), the main synapo- ocher descendants. In addition to fossil repre- morphy is mesarch protoxylem con.fined to the sentatives of the five listed crown subgroups, outer lobed ends of the xylem strand (Crane fossil members presumably include various taxa and Kenrick, 1997); typically the protoxylem is Monilophyta parenchymatous, so that the metaxylem forms a e.g., Bierhorst, 1971) Psilotophyta. However, in conspicuous "peripheral loop" around a spongy the context of other phylogenies (e.g., Wick- protoxylem area. This feature occurs in fossil ett et al., 2014; Knie et al., 2015; Rothfels et cladoxylopsids, zygopterids, and Ankyropteris, al., 2015), where Equisetum is sister to all other and it has been assumed that the spongy pro- monilophytes, names such as Filicophyta are not toxylem areas were modified into the proto- synonyms of Monilophyta but instead apply to a xylem canals of Equisetum and related fossil major subclade comprising the "ferns" and Psi- calamites; however, homologous structures have lotophyta. not been recognized in living ferns. Schneider et al. (2009), considering extant groups only, Comments: Kenrick and Crane (1997) first reported four unambiguous apomorphies: spo- proposed the existence of a clade that includes rangia arranged in a sorus, presence of a pseu- ferns and Equisetum (represented in the tree doendospore, centrifugal spore wall formation shown in their Figure 4.32 by the presumed (also reported by Schneider et al., 2002), and fossil ferns Pseudosporochnus, a "cladoxylop- plasmodial tapetum. However, it is not clear sid," and Rhacophyton, a "zygopterid," and the that the groups of sporangia in Equisetum, presumed fossil sphenopsid Ibyka) but excludes Psilotophyta, Ophioglossales, and many presumed Lycopodiophyta and seed plants. Ferns and fossil monilophytes are more comparable to the Equisetum do not form a monophyletic group in typical sori of leptosporangiate ferns than are several other morphological studies (Bremer et the groups of sporangia and microsynangia of al., 1987; Rothwell, 1999; Rothwell and Nixon, seed plant stem relatives ("progymnosperms," 2006), but the two groups (also including "seed ferns"). It is also not certain that the spore Psilotophyta or "whisk ferns") are strongly sup- characters of monilophytes are derived relative ported as a clade by molecular analyses (Nickrent to the pollen of living seed plants, which have et al., 2000; Pryer et al., 2001, 2004; Wikstrom more complex exine development, or to the and Pryer, 2005; Schuettpelz et al., 2006; Qiu more fern-like spores and "pre-pollen" of seed et al., 2007; Ruhfel et al., 2014; Wickett et al., plant stem relatives, in which exine develop- 2014; Knie et al., 2015; Rothfels et al., 2015) and ment is largely unknown. weakly supported by a morphological analysis of extant taxa by Schneider et al. (2009). Synonyms: Moniliformophyta Crane and A maximum-crown-clade definition usually Kenrick 1997 is an unambiguous heterodefini- has only one internal specifier, but a second tional synonym in the context of some phyloge- internal specifier is included here in order to netic hypotheses but not others (see Comments). disqualify the name under certain conditions. Moniliformopses sensu Lecointre and Guyader In the context of a phylogenetic hypothesis in (2006) is a possible synonym (see Comments). which extant ferns share more recent ancestry The names Filicophyta, Filicopsida, Polypodi- with seed plants than with Equisetum (Bremer ophyta, Pterophyta, and Pteropsida are partial et al., 1987: Fig. 1), or one in which Equisetum synonyms of Monilophyta in the context of some shares more recent ancestry with seed plants phylogenies (e.g., Pryer et al., 2001) in that they than with extant ferns (Rothwell, 1999: Fig. 2; have been commonly applied co the paraphy- Rothwell and Nixon, 2006: Fig. 3A), the name letic group ("ferns") originating from the same Monilophyta would not apply to any clade. ancestor as the clade Monilophyta but exclud- Abandonment of the name would be appro- ing Equisetum and usually (but not always; priate in such cases because it is universally 248 Monilophyta associated with the hypothesis that ferns (includ- i.e., the ophioglossoid ferns and whisk ferns incr PsilotophJ1ta) and horsetails form a clade (Psilotophyta) are part of Monilophyta but lie ex~lu sive of seed plants and lycopodiophytes. outside of Moniliformophyta. The max imum-crown-clade definition that we One might argue that a name based on Filico-, have adopted here and the slightly different def- Ptero-, or Polypodio- (see Synonyms) should have inirion of Camino et al. (2007) both capture been chosen for this clade given that these par- rhe essence of this hypothesis. The two defini- tial synonyms are widely applied to the plants tions differ in the specifiers chosen to represent that are commonly called ferns. However, the Equisetum, Spermatophyta, and Lycopodiophyta, name Monilophyta has already been phyloge- with each definition using species included in netically defined for the clade of concern here rhe respective primary reference phylogeny. (i.e., including Equisetum), unlike any of the When we phylogenetically defined the name alternatives, and it avoids the suggestion that Monilophyta as referring to this crown clade the common ancestor of Equisetum and ferns (Camino et al., 2007), we selected this name was fernlike in having compound leaves, when because it closely approximates the informal it more likely had branch systems with dichoto- name "monilophytes," which is often used for mous ultimate appendages, as in "cladoxylop- this clade (e.g., Judd et al., 2002, 2008; Simpson, sids" (which may include stem relatives of the 2006). The name Monilophyta was applied to whole clade and some of its subgroups). Names this clade in a field guide (Cobb et al., 2005) but such as Filicophyta and Polypodiophyta are bet- without a description or diagnosis, so it did not ter reserved for a clade that excludes Equisetum, qualify as a preexisting name. Another candi- such as "fern clade 3" of Rothwell (1999: Fig. 2), date name, Moniliformopses Kenrick and Crane or the clade including all monilophytes except (1997: Table 7.1), was apparently apomorphy- Equisetum in Knie et al. (2015) and Rothfels et based (1997: Table 7.2), and it is unclear whether al. (2015). Indeed, the name Polypodiophyta has subsequent uses of this name (e.g., Lecointre already been phylogenetically defined to apply and Guyader, 2006) refer to the crown or an in precisely that way (Camino et al., 2007: El4); apomorphy-based clade. A third candidate the three specifiers were species