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Zosterophyllaceae
Fossil range: Late Silurian (late Ludlow, 420Ma) - Devonian
Scientific classification
Kingdom: Plantae
Division: Lycopodiophyta
Class: Zosterophyllopsida †
Order: Zosterophyllales †
Cleal & B.A. Thomas, 1994
Family: Zosterophyllaceae †
Genera

The Zosterophyllaceae or Zosterophylls (class Zosterophyllopsida) were probably stem-group lycophytes, forming a sister group to the ancestors of the lycophytes. They were among the first vascular plants in the fossil record, and had a world-wide distribution.[1] By the late Silurian (late Ludlovian, about 420 million years ago) a diverse assemblage of species existed, examples of which have been found fossilised in what is now Bathurst Island in Arctic Canada.[2]

The stems of Zosterophylls were either smooth or covered with small spines known as enations, branched dichotomously, and grew at the ends by unrolling, a process known as circinate vernation. The stems had a central vascular stele in which the protoxylem was exarch, and the metaxylem developed centripetally. The sporangia were kidney-shaped (reniform), with conspicuous lateral dehiscence and were borne laterally in a fertile zone towards the tips of the branches.[3]

The Zosterophylls were named after the aquatic flowering plant Zostera from a mistaken belief that the two groups were related. Penhallow's generic description of the type genus Zosterophyllum refers to "Aquatic plants with creeping stems, from which arise narrow dichotomous branches and narrow linear leaves of the aspect of Zostera."[4] Zosterophyllum rhenanum was reconstructed as aquatic, the lack of stomata on the lower axes giving support to this interpretation.[3] However, current opinion is that the Zosterophylls were terrestrial plants, and Penhallow's "linear leaves" are interpreted as the aerial stems of the plant that had become flattened during fossilization.[5]

Stomata were present, particlularly on the upper axes. Their absence on the lower portions of the axes suggests that this part of the plants may have been submerged, and that the plants dwelt in boggy ground or even shallow water.[3] In many fossils these appear to consist of a slit-like opening in the middle of a single elongated guard cell, leading to comparison with the stomata of some mosses.[6] However, this is now thought to result from the loss of the wall separating paired guard cells during fossilisation.[7] [8]

See also

References

  1. ^ PG Gensel (1992) Phylogenetic Relationships of the Zosterophylls and Lycopsids: Evidence from Morphology, Paleoecology, and Cladistic Methods of Inference. Annals of the Missouri Botanical Garden 79, 450-473
  2. ^ ME Kotyk, JF Basinger, PG Gensel & TA de Freitas (2002) Morphologically complex plant macrofossils from the Late Silurian of Arctic Canada. American Journal of Botany, 89, 1004-1013
  3. ^ a b c WN Stewart & GW Rothwell (1993) Palaeobotany and the evolution of plants. 2nd edition. Cambridge University Press.
  4. ^ DP Penhallow (1892) Additional notes on Devonian plants from Scotland. Can. Rec. Sci. 5, 1–13
  5. ^ W-Q Zhu & P Kenrick (1999) A Zosterophyllum-like plant from the Lower Devonian of Yunnan Province, China. Review of Palaeobotany and Palynology 105, 111–118
  6. ^ JA Paton and JV Pearce (1957) The occurrence, structure and functions of the stomata in British bryophytes. Transactions of the British Bryological Society 3, 228-259
  7. ^ D Edwards, DS Edwards & R Rayner (1982) The cuticle of early vascular plants and its evolutionary significance In: The plant cuticle. D Cutler et al., Academic Press
  8. ^ D Edwards, GD Abbott & JA Raven (1996) Cuticles of early land plants: a paleoecophysiological evaluation. In G Kerstiens, ed., Plant Cuticles, an integrated functional approach. Bios Scientific Publishers

External links

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