Equisetales

The Equisetales are an order of living and extinct spore-bearing plants that exhibit whorled growth in branching and leaf insertion. The modern forms, Equisetum spp., known as the horsetails or scouring rushes are herbaceous plants with whorled photosynthetic branches, and highly-reduced whorled leaves. Ancient members of this group, such as Calamites, grew to heights over 30 meters. They produced wood to reach these heights and displayed laminate photosynthetic leaves for photosynthesis (Annularia)

Ecology & Form

One living genus (Equisetum); ~60 fossil genera including Calamites
Almost all members, living or extinct, inhabit wetland environments
Modern forms are known to absorb macromolecules from soil

Stems

Whorled branches in the "horsetails"; the "scouring rushes" usually lack branches or have smaller side branches
Distinct nodes and internodes (bamboo-like appearance)
- Internodes are hollow; nodes are solid
- Living forms are herbaceous; extinct forms exhibited a vascular cambium with wood production
- Periderm has been observed in the fossil Arthropitys (Cichan and Taylor, 1983)
Pseudomonopodial growth
- Upright stem and side branches formed from the unequal division of an apical cell

Leaves

Whorled leaves at nodes
Highly reduced in living forms (e.g., Equisetum)
Laminate and photosynthetic in extinct forms

Roots

True roots are present
Roots may be the only organ that is not whorled in horsetails

Reproduction

Sporangia aggregated into a cone (=strobilus)
Sporangia are on recurved stalks, called sporangiophores, and inserted in a whorled fashion
Eusporangiate: Sporangia are larger with many spores
Spores exhibit arm-like elaters, which aid in wind dispersal
- Elaters react to humidity; opening in dry air and wrapping around the spore in humid air

Classification

└Equisetales

Geologic Range

- Mississippian - present

Above: close-up of E. fluviatile cone. Below the cone the reduced leaves are visible

Below: cross-section of Equisetum cone showing recurved sporangia

Diversity

The Equisetales has several families, including:

Archaeocalamitaceae †

Middle Devonian - Early Permian
Simplified cone structures and lack of peltate sporangiophores found in derived equisetophytes (e.g., Equisetaceae, Calamitaceae)

Archaeocalamites †

Štúr, 1875

A. prolxus

A. scrobiculatus

Paragiridia taioensis †

Boardman, Iannuzzi, & Dutra, 2016
Early Permian of Brazil

Protocalamostachys †

Bateman, 1991
Early Mississippian of the UK

P. arranensis

P. farringtonii

Peltotheca furcata †

Escapa & Cúneo, 2005
Early Permian of Argentina

Above: 1, reconstruction of Paragiridia taioensis; 2, specimen ULVG 5458, showing Paracalamites australis-like aerial axes with foliar axes that bear Phyllotheca-like leaves; 3, specimen ULVG 5300, showing the fertile parts of a reproductive axis, with four fertile internodes, with the sporangiophores occurring in the middle to upper portion of the internodes and formed of clusters of sporangiophores bifurcated one or more times and supporting only one terminal clavate sporangium per stem; 4, specimen ULVG 5472 ac A, showing the non-articulated roots bearing rootlets that arise irregularly on the entire surface. Scales: 10 mm (Boardman, Iannuzzi, & Dutra, 2016, Plate III)

Calamitaceae †

Calamocarpon insignia †

- Baxter 1963, 1964; Good 1975
- Pennsylvanian of the USA

Calamostachys †

- Ca. americana
  - Good 1975
  - Pennsylvanian of USA
- Ca. binneyana
  - Good 1975
  - Pennsylvanian of UK and USA
- Ca. casheana
  - Williamson and Scott 1894; Lacey 1941
  - Pennsylvanian of USA
- Ca. inversibractis
  - Good 1975
  - Pennsylvanian of USA

Cruciaetheca †

- Cúneo and Escapa 2006
- Early Permian of Argentina
- Cr. patagonica
- Cr. feruglioi

Mazostachys pendulata †

Kosanke 1955; Good 1975
Pennsylvanian of USA

Paracalamitina striata †

Naugolnykh 2002
Early- Middle Permian of Russia

Palaeostachya †

- Good 1975
- Pennsylvanian of USA
- P. andrewsii
- P. decacnema

Pendulostachys cingulariformis †

Good 1975
Pennsylvanian of USA

Weissistachys kentuckiensis †

Rothwell and Taylor 1971a, 1971b
Pennsylvanian of USA

Above: Reconstruction of Calamites from the Carboniferous

Above: Cast fossil of Calamites

Tchernovianceae †

Gondwanostachyaceae †

Equisetaceae

Equisetum

E. arvense
E. bogotense
E. clarnoi †
E. dimorphum †
E. diffusum
E. fluviatile
E. fluviatoides †
E. giganteum
E. haukeanum †
E. hyemale
E. laevigatum
E. laterale †
E. myriochaetum
E. palustre
E. pratense
E. ramosissimum
E. scirpoides
E. similkamense †
E. sylvaticum
E. telmateia
E. thermale †
E. vancouverense †
E. variegatum

Equisetites †

E. arenaceus †
E. lyellii †
E. bogotense †

Neocalamites †

Neocalamostachys arrondoi †

Nododendron †

Schizoneura †

Spaciinodum collinsonii †

Above: E. arvense in a woodland leaf litter

Above: Large stems of E. myriochaetum

Incertae sedis

Phyllotheca †

Above: Phyllotheca

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