Sawdoniales

The Sawdoniales are an order or plesion 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. In their major cladistic study of early land plants, Kenrick and Crane placed most of the zosterophylls in the Sawdoniales (which they treated as a plesion).[1]

Sawdoniales
Temporal range:
Sawdonia ornata with ground creeping roots (A); lateral reniform sporangia (B)
Sawdonia ornata
Scientific classification Edit this classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Lycophytes
Plesion: †Zosterophylls
Order: †Sawdoniales
Families

See text.

Like other zosterophylls, members of the Sawdoniales bore lateral, reniform sporangia. They branched dichotomously, and grew at the ends by unrolling (circinate vernation). Some had smooth stems, others were covered in small spines; fungal bodies have been reported in some spines.[2]

Taxonomy

In 1997, Kenrick and Crane placed most of the zosterophylls in the plesion Sawdoniales,[1] characterizing the group as having "marked bilateral symmetry".[3] Their summary cladogram did not resolve the taxa within the Sawdoniales, other than placing Zosterophyllum divaricatum within the zosterophylls but outside the Sawdoniales.[4] Hao and Xue in 2013 criticized their approach, and placed many of the members of Kenrick and Crane's Sawdoniales in the order Gosslingiales, characterized among other features by the absence of terminal sporangia (i.e. with only lateral sporangia), and hence indeterminate growth.[5]

Families

In Kenrick and Crane's treatment, the Sawdoniales are divided into four families:[6]

Hao and Xue place the first two families in their Gosslingiales, but exclude both the Barinophytaceae and the Hsuaceae from the zosterophylls.[7]

References
  1. Kenrick & Crane (1997), p. 12.
  2. Rayner, R.J. (1983). "New observations on Sawdonia ornata from Scotland". Transactions of the Royal Society of Edinburgh. 74 (2): 79–93. doi:10.1017/s026359330001018x.
  3. Kenrick & Crane (1997), p. 249.
  4. Kenrick & Crane (1997), p. 238.
  5. Hao & Xue (2013), pp. 52–54.
  6. Kenrick & Crane (1997), p. 239.
  7. Hao & Xue (2013), pp. 53, 55.

Bibliography


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