The Sawdoniales are an order or

plesion In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

of extinct

zosterophyll The zosterophylls are a group of extinct land plants that first appeared in the Silurian period. The taxon was first established by Banks in 1968 as the subdivision Zosterophyllophytina; they have since also been treated as the division Zosteroph ...

s. The zosterophylls were among the first

vascular plant Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They al ...

s in the fossil record, and share an ancestor with the living lycophytes. The group has been divided up in various ways. In their major

cladistic Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...

study of early

land plants The Embryophyta (), or land plants, are the most familiar group of green plants that comprise vegetation on Earth. Embryophytes () have a common ancestor with green algae, having emerged within the Phragmoplastophyta clade of green algae as sist ...

, Kenrick and Crane placed most of the zosterophylls in the Sawdoniales (which they treated as a

). Like other zosterophylls, members of the Sawdoniales bore lateral, reniform sporangia. They branched dichotomously, and grew at the ends by unrolling (

circinate vernation Vernation (from ''vernal'' meaning ''spring'', since that is when leaves spring forth in temperate regions) is the formation of new leaves or fronds. In plant anatomy, it is the arrangement of leaves in a bud. In pine species, new leaves are sho ...

). Some had smooth stems, others were covered in small spines; fungal bodies have been reported in some spines.

Taxonomy

In 1997, Kenrick and Crane placed most of the zosterophylls in the plesion Sawdoniales, characterizing the group as having "marked bilateral symmetry". Their summary cladogram did not resolve the taxa within the Sawdoniales, other than placing '' Zosterophyllum divaricatum'' within the zosterophylls but outside the Sawdoniales. Hao and Xue in 2013 criticized their approach, and placed many of the members of Kenrick and Crane's Sawdoniales in the order

Gosslingiales Gosslingiales is an order of extinct zosterophylls. The zosterophylls were among the first vascular plants in the fossil record, and share an ancestor with the living lycophytes. The group has been divided up in various ways. Hao and Xue in 20 ...

, characterized among other features by the absence of terminal sporangia (i.e. with only lateral sporangia), and hence indeterminate growth.

Families

In Kenrick and Crane's treatment, the Sawdoniales are divided into four families: * † Gosslingiaceae * † Sawdoniaceae * †

Barinophytaceae The barinophytes are a group of extinct vascular plants (tracheophytes). Their relationship with other vascular plants is unclear. They have been treated as the separate class Barinophytopsida, the order Barinophytales of uncertain class and as ...

* †

Hsuaceae ''Hsua'' is a genus of extinct vascular plants, known from the Devonian. The name of the genus honours the Chinese palaeobotanist, Jen Hsü. Features The main stems (axes) of ''Hsua robusta'' are about an inch thick, with circinate, pseudo-m ...

Hao and Xue place the first two families in their Gosslingiales, but exclude both the Barinophytaceae and the Hsuaceae from the zosterophylls.

References

Bibliography

* *

External links

* Silurian plants Devonian plants Prehistoric lycophytes Prehistoric plant orders Paleozoic life of New Brunswick Zosterophylls {{Devonian-plant-stub