Phylogenetic diversification of Equisetum (Equisetales) as inferred from Lower Cretaceous species of British Columbia, Canada

Abstract

Three types of anatomically preserved vegetative shoots with features that characterize crown group Equisetum have been discovered in Lower Cretaceous deposits (≈136 Ma) of British Columbia, Canada, suggesting the genus is much older than currently believed. Specimens include two types of aerial shoots described as E. haukeanum sp. nov. and E. vancouverense sp. nov. and one type of subterranean rhizome. Shoots are 1-2 mm in diameter, jointed, and in cross section have fluted stems with a hollow pith. Distinctive patterns of cortical sclerenchyma and different ridge morphologies characterize each shoot morphotype. Nodes display irregular branching, highly fused leaf sheaths, and a nodal diaphragm. The aerial stem morphospecies have vallecular canals on alternating radii with carinal canals of an equisetostele surrounded by only a few tracheids. No secondary tissues are produced. Bands of surficial stomata flank the furrows of one morphospecies. Rhizomes and aerial shoots are of a similar size, suggesting that the plants were equivalent in stature to the smallest living Equisetum species. These fossils augment our understanding of evolutionary transformations that led from Paleozoic Archaeocalamitaceae and Calamitaceae to crown group Equisetaceae, suggesting that the initial diversification of Equisetum began far earlier than suggested by molecular-clock-based estimates.

Equisetum thermale sp. nov. (Equisetales) from the Jurassic San Agustín hot spring deposit, Patagonia: Anatomy, paleoecology, and inferred paleoecophysiology

“…Equisetum thermale possesses all the general vegetative features that define the family Equisetaceae, including herbaceous, jointed, aerial, and subterranean stems, with numerous wiry roots, and very thin leaves with a single vein borne in whorls and joined in a sheath (e.g., Stanich et al, 2009). Growth in the plant was exclusively primary, and it has vascular anastomosis typical of the family (e.g., Stewart and Rothwell, 1993).…”

“…The strobili conform in organization to those of Equisetaceae in that they lack subtending bracts between sporangiophores. Features that differentiate the Equisetaceae from other sphenophytes are tabulated in Stanich et al (2009, Table 1).…”

“…The fossil record suggests that species morphologically (and to a degree anatomically) of modern aspect appeared in the Mesozoic, and it is likely that the extant clade originated from one of these Mesozoic lineages. In a recent paper documenting two such plants, Stanich et al (2009) provided evidence that all the characteristic synapomorphies of living Equisetum had evolved by the Lower Cretaceous (136 Ma), and they hypothesized that the extant crown group probably originated much earlier in the Mesozoic. The data substantiating their claim were based on evidence from at least two fossil species.…”

Reconstruction and Phylogenetic Significance of a NewEquisetumLinnaeus Species from the Lower Jurassic of Cerro Bayo (Chubut Province, Argentina)

“…Differences between the latter two genera have been extensively discussed (e.g., Harris, 1961;Gould, 1968;Stanich et al, 2009), and many of their representatives have been suggested to belong to the same evolutionary lineage leading to the extant horsetails (Boureau, 1964;Gould, 1968).…”

“…(Early Cretaceous) from the fossil record (Stanich et al, 2009) and possibly extending far back into the Jurassic, since nearly all synapomorphies that define the crown group Equisetum already occur in the anatomically preserved Equisetum thermale Channing, Zamuner, Edwards and Guido, 2011 from the Jurassic of Argentina (see Channing et al, 2011). Nonetheless, a phylogenetic analysis including both living and fossil representatives of the genus is required in order to confirm the position of fossil Equisetum species as part of the crown group, and therefore, the diversification minimum age for that group.…”