Zosterophyllopsida

The zosterophylls were 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 Zosterophyllophyta and the class Zosterophyllopsida. They were among the first vascular plants in the fossil record, and had a world-wide distribution. They were probably stem-group lycophytes, forming a sister group to the ancestors of the living lycophytes.[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]

Zosterophyllopsida
Temporal range: Ludlow to Devonian
Zosterophyllum
Zosterophyllum species fossils
Scientific classification
Kingdom: Plantae
Subdivision: Lycophytina
Class: Zosterophyllopsida

Morphology

Sawdonia ornata
Reconstruction of the zosterophyll Sawdonia ornata.

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 column 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. David P. 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, particularly 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]

Taxonomy and classification

At first most of the fossilized early land plants other than bryophytes were placed in the class Psilophyta, established in 1917 by Kidston and Lang.[9] As additional fossils were discovered and described, it became apparent that the Psilophyta were not a homogeneous group of plants, and in 1975 Banks developed his earlier proposal to split it into three groups, which he put at the rank of subdivision. One of these was the subdivision Zosterophyllophytina, named after the genus Zosterophyllum.[10][11] For Banks, zosterophyllophytes or zosterophylls comprised plants with lateral sporangia which released their spores by splitting distally (i.e. away from their attachment), and which had exarch strands of xylem.[12] Bank's classification produces the hierarchy:

Division Tracheata
  Subdivision †Zosterophyllophytina = zosterophyllophytes, zosterophylls
  Subdivision Lycophytina = lycopods
  + other subdivisions

Those who treat most of the extant groups of plants as divisions may raise both the zosterophylls and the Lycophytina sensu Banks to the rank of division:[13]

Division Zosterophyllophyta = zosterophylls, zosterophyllophytes
Division Lycophyta = lycophytes

In their cladistic study published in 1997,[14] Kenrick and Crane provided support for a clade uniting both the zosterophylls and the lycopsids, producing a classification which places the zosterophylls in a class Zosterophyllopsida of the subdivision Lycophytina:[15]

Division Tracheata
  Subdivision Lycophytina = lycophytes
    Class †Zosterophyllopsida = zosterophylls
    Class Lycopodiopsida = lycopsids

This approach has been widely used alongside previous systems. A consequence is that "lycophyte" and corresponding formal names such as "Lycophyta" and "Lycophytina" are used by different authors in at least two senses: either excluding zosterophylls in the sense of Banks or including them in the sense of Kenrick and Crane.

A further complication is that the cladograms of Kenrick and Crane show that the zosterophylls, broadly defined, are paraphyletic, but contain a 'core' clade of plants with marked bilateral symmetry and circinate tips. The class Zosterophyllopsida sensu Kenrick & Crane may be restricted to this core clade,[16] leaving many genera (e.g. Hicklingia, Nothia) with no systematic placement other than Lycophytina sensu Kenrick & Crane, but nevertheless still informally called "zosterophylls".

Under whatever name and rank, the zosterophylls have been divided into orders and families, e.g. the Zosterophyllales containing the Zosterophyllaceae and the Sawdoniales containing the Sawdoniaceae. Since the publication of cladograms showing that the group is paraphyletic[14][17] divisions of the class have been less used, being ignored, for example, in the 2009 paleobotany textbook by Taylor et al.[13]

Phylogeny and genera

In 2004, Crane et al. published a unified cladogram for the polysporangiophytes (plants with branched stems bearing sporangia), based on cladistic analyses of morphological features.[9] This suggests that the zosterophylls were a paraphyletic stem group, related to the ancestors of modern lycophytes.

lycophytes

Hicklingia

†basal groups (Adoketophyton, Discalis, Distichophytum (=Rebuchia), Gumuia, Huia, Zosterophyllum myretonianum, Z. lianoveranum, Z. fertile)

†'core' zosterophylls (Zosterophyllum divaricatum, Tarella, Oricilla, Gosslingia, Hsua, Thrinkophyton, Protobarinophyton, Barinophyton obscurum, B. citrulliforme, Sawdonia, Deheubarthia, Konioria, Anisophyton, Serrulacaulis, Crenaticaulis)

†basal groups (Nothia, Zosterophyllum deciduum)

lycopsids (extant and extinct members)

Genera which are included at or around the zosterophyll position in the cladogram or have otherwise been included in the group by at least one source, and hence may be considered zosterophylls in the broad sense, are listed below.

B = included by Banks in his 1975 description of Zosterophyllophytina.

Genera may not be assigned to this group by other authors; for example, Adoketophyton was regarded by Hao et al., who named the genus, as having evolved separately from the lycopsids, so that its taxonomic placement was uncertain.[18] Other genera, such as Jugumella, may be members of this group.[19]

See also

References

  1. ^ Gensel, P.G. (1992), "Phylogenetic relationships of the zosterophylls and lycopsids: evidence from morphology, paleoecology, and cladistic methods of inference", Annals of the Missouri Botanical Garden, 79 (3): 450–73, doi:10.2307/2399750, JSTOR 2399750
  2. ^ Kotyk, M.E.; Basinger, J.F.; Gensel, P.G. & de Freitas, T.A. (2002), "Morphologically complex plant macrofossils from the Late Silurian of Arctic Canada", American Journal of Botany, 89 (6): 1004–1013, doi:10.3732/ajb.89.6.1004, PMID 21665700
  3. ^ a b c Stewart, W.N. & Rothwell, G.W. (1993), Paleobotany and the evolution of plants (2nd ed.), Cambridge, UK: Cambridge University Press, ISBN 978-0-521-38294-6
  4. ^ Penhallow, D.P. (1892), "Additional notes on Devonian plants from Scotland", Canadian Record of Science, 5: 1–13
  5. ^ Zhu, W.-Q. & Kenrick, P. (1999), "A Zosterophyllum-like plant from the Lower Devonian of Yunnan Province, China", Review of Palaeobotany and Palynology, 105 (1–2): 111–118, doi:10.1016/S0034-6667(98)00070-0
  6. ^ Paton, J.A. & Pearce, J.V. (1957), "The occurrence, structure and functions of the stomata in British bryophytes", Transactions of the British Bryological Society, 3 (2): 228–259, doi:10.1179/006813857804829560
  7. ^ Edwards, D.; Edwards, D.S. & Rayner, R. (1982), "The cuticle of early vascular plants and its evolutionary significance", in Cutler, D.; Alvin, K.L. & Price, C.E. (eds.), The Plant Cuticle, London: Academic Press, ISBN 978-0-12-199920-9
  8. ^ Edwards, D.; Abbott, G.D. & Raven, J.A. (1996), "Cuticles of early land plants: a paleoecophysiological evaluation", in Kerstiens, G. (ed.), Plant Cuticles, an integrated functional approach, Oxford: BIOS Scientific, ISBN 978-1-85996-130-8
  9. ^ a b Crane, P.R.; Herendeen, P. & Friis, E.M. (2004), "Fossils and plant phylogeny", American Journal of Botany, 91 (10): 1683–99, doi:10.3732/ajb.91.10.1683, PMID 21652317
  10. ^ Banks, H.P. (1968), "The early history of land plants", in Drake, E.T. (ed.), Evolution and Environment: A Symposium Presented on the Occasion of the 100th Anniversary of the Foundation of Peabody Museum of Natural History at Yale University, New Haven, Conn.: Yale University Press, pp. 73–107, cited in Banks 1980
  11. ^ Banks, H.P. (1975), "Reclassification of Psilophyta", Taxon, 24 (4): 401–413, doi:10.2307/1219491, JSTOR 1219491
  12. ^ Banks, H.P. (1980), "The role of Psilophyton in the evolution of vascular plants", Review of Palaeobotany and Palynology, 29: 165–176, doi:10.1016/0034-6667(80)90056-1
  13. ^ a b Taylor, T.N.; Taylor, E.L. & Krings, M. (2009), Paleobotany, The Biology and Evolution of Fossil Plants (2nd ed.), Amsterdam; Boston: Academic Press, ISBN 978-0-12-373972-8, p. 1028
  14. ^ a b Kenrick, Paul & Crane, Peter R. (1997a), The Origin and Early Diversification of Land Plants: A Cladistic Study, Washington, D.C.: Smithsonian Institution Press, ISBN 978-1-56098-730-7
  15. ^ See, e.g., Berry, C.M. & Fairon-Demaret, M. (2001), "The Middle Devonian Flora Revisited", in Gensel, P.G. & Edwards, D. (eds.), Plants invade the Land : Evolutionary & Environmental Perspectives, New York: Columbia University Press, ISBN 978-0-231-11161-4
  16. ^ Zhu, W.-Q. & Kenrick, P. (1999), "A Zosterophyllum-like plant from the Lower Devonian of Yunnan Province, China", Review of Palaeobotany and Palynology, 105 (1–2): 111–118, doi:10.1016/S0034-6667(98)00070-0
  17. ^ Kenrick, P. & Crane, P.R. (1997b), "The origin and early evolution of plants on land", Nature, 389 (6646): 33–39, Bibcode:1997Natur.389...33K, doi:10.1038/37918
  18. ^ Hao, Shougang; Wang, Deming & Beck, Charles B. (2003), "Observations on anatomy of Adoketophyton subverticillatum from the Posongchong Formation (Pragian, Lower Devonian) of Yunnan, China", Review of Palaeobotany and Palynology, 127 (3–4): 175–186, doi:10.1016/S0034-6667(03)00119-2
  19. ^ Raymond, A.; Gensel, P. & Stein, W.E. (2006), "Phytogeography of Late Silurian macrofloras", Review of Palaeobotany and Palynology, 142 (3–4): 165–192, doi:10.1016/j.revpalbo.2006.02.005

External links

Anisophyton

Anisophyton was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Anisophyton in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids (living and extinct clubmosses and relatives).

Barinophyton

Barinophyton was a genus of Silu-Devonian land plant with branching axes.

A cladogram published in 2004 by Crane et al. places Barinophyton in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids (living and extinct clubmosses and relatives).

Crenaticaulis

Crenaticaulis was an early genus of slender, dichotomously branching, leafless land plants, known from the Devonian period and first described in 1969. They were probably allied to the zosterophylls, and are assigned to subdivision Zosterophyllophytina, or class Zosterophyllopsida. They bore branches and scalariform tracheids.

A cladogram published in 2004 by Crane et al. places Crenaticaulis in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids (living and extinct clubmosses and relatives).

Deheubarthia

Deheubarthia was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Deheubarthia in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids (living and extinct clubmosses and relatives).

Gosslingia

Gosslingia was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Gosslingia in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids (living and extinct clubmosses and relatives).

Konioria

Konioria was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Konioria in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids (living and extinct clubmosses and relatives).

Lycopodiophyta

The Division Lycopodiophyta (sometimes called lycophyta or lycopods) is a tracheophyte subgroup of the Kingdom Plantae. It is one of the oldest lineages of extant (living) vascular plants and contains extinct plants like Baragwanathia that have been dated from the Silurian (ca. 425 million years ago). Members of Lycopodiophyta were some of the dominating plant species of the Carboniferous period. These species reproduce by shedding spores and have macroscopic alternation of generations, although some are homosporous while others are heterosporous. Most members of Lycopodiophyta bear a protostele, and the sporophyte generation is dominant. They differ from all other vascular plants in having microphylls, leaves that have only a single vascular trace (vein) rather than the much more complex megaphylls found in ferns and seed plants.

Lycopodiopsida

Lycopodiopsida is a class of herbaceous vascular plants known as the clubmosses and firmosses. They have dichotomously branching stems bearing simple leaves called microphylls and reproduce by means of spores borne in sporangia at the bases of the leaves. Traditionally, the group also included the spikemosses (Selaginella and relatives) and the quillworts (Isoetes and relatives) but because these groups have leaves with ligules and reproduce using spores of two different sizes, both are now placed into another class, Isoetopsida that also includes the extinct Lepidodendrales. These groups, together with the horsetails are often referred to informally as fern allies.

The class Lycopodiopsida as interpreted here contains a single living order, the Lycopodiales, and a single extinct order, the Drepanophycales.

Nothia

Nothia was a genus of Early Devonian vascular plants whose fossils were found in the Rhynie chert in Scotland. It had branching horizontal underground stems (rhizomes) and leafless aerial stems (axes) bearing lateral and terminal spore-forming organs (sporangia). Its aerial stems were covered with small 'bumps' (emergences), each bearing a stoma. It is one of the best described early land plants. Its classification remains uncertain, although it has been treated as a zosterophyll. There is one species, N. aphylla.

Oricilla

Oricilla was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Oricilla in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids (living and extinct clubmosses and relatives).

Polysporangiophyte

Polysporangiophytes, also called polysporangiates or formally Polysporangiophyta, are plants in which the spore-bearing generation (sporophyte) has branching stems (axes) that terminate in sporangia. The name literally means many sporangia plant. The clade includes all land plants (embryophytes) except for the bryophytes (liverworts, mosses and hornworts) whose sporophytes are normally unbranched, even if a few exceptional cases occur. While the definition is independent of the presence of vascular tissue, all living polysporangiophytes also have vascular tissue, i.e., are vascular plants or tracheophytes. Fossil polysporangiophytes are known that have no vascular tissue, and so are not tracheophytes.

Protobarinophyton

Protobarinophyton was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Protobarinophyton in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids (living and extinct clubmosses and relatives).

Sawdoniales

The Sawdoniales were among the first vascular plants in the fossil record. They are zosterophylls, descending from the same ancestors that modern lycophytes evolved from. They bore lateral, reniform sporangia, 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.

Serrulacaulis

Serrulacaulis was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Serrulacaulis in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids (living and extinct clubmosses and relatives).

Streptophyta

Streptophyta, informally the streptophytes (from the Greek strepto, for twisted, i.e., the morphology of the sperm of some members), is a clade of plants. The composition of the clade varies considerably between authors, but the definition employed here includes land plants and all green algae except the Chlorophyta and possibly the more basal Mesostigmatophyceae, Chlorokybophyceae, and Spirotaenia.

Tarella

Tarella was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Tarella in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids (living and extinct clubmosses and relatives).

Thrinkophyton

Thrinkophyton was a genus of Silu-Devonian land plant with branching axes.A cladogram published in 2004 by Crane et al. places Thrinkophyton in the core of a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids (living and extinct clubmosses and relatives).

Thursophyton

Thursophyton ("plant from Thurso") is a form genus of extinct vascular plants known from anatomically preserved specimens originally described from the Givetian (Middle Devonian) of Scotland, and since reported from other parts of northern Europe. The taxonomic position of Thursophyton has been uncertain; most authors have considered it too imperfectly known to place, or have assigned it to the Lycopodiophyta. Unpublished research suggests similarities with spiny genera of the Zosterophyllopsida.

Zosterophyllum

Zosterophyllum was a genus of Silurian-Devonian vascular land plant with branching axes on which kidney-shaped sporangia were arranged in lateral positions.

Some species have been transferred to other genera:

Z. artesianum to Danziella artesiana

Z. contiguum to Demersatheca contigua

Z. subverticillatum to Adoketophyton subverticillatumA cladogram published in 2004 by Crane et al. places the species of Zosterophyllum in a paraphyletic stem group of broadly defined "zosterophylls", basal to the lycopsids (living and extinct clubmosses and relatives).

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