Zosterophyll

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Zosterophyll
Temporal range: Ludlow to Devonian
Zosterophyllum fossils.jpg
Zosterophyllum species fossils; left: with coiled (circinate) branch tips, right: with sporangium
Zosterophyllum sp. - MUSE cropped.jpg
Zosterophyllum Life restoration from MUSE
Scientific classification Red Pencil Icon.png
Kingdom: Plantae
Clade: Streptophyta
Clade: Embryophytes
Clade: Polysporangiophytes
Clade: Tracheophytes
Clade: Lycophytes
Plesion:† Zosterophylls
Order

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 Zosterophyllophyta or Zosterophyta and the class or plesion Zosterophyllopsida or Zosteropsida. 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]

Contents

Morphology

Reconstruction of the zosterophyll Sawdonia ornata Sawdonia ornata.gif
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.[ citation needed ] 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

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.

† 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)

zosterophylls
in the broadest sense

Genera

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 broadest sense, are listed below. [1] [14] [9] [18] [19] "B" indicates genera included by Banks in his 1975 description of Zosterophyllophytina. [10]

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. [20] Barinophytes, like Barinophyton, have been considered to be possible lycopsids, [21] or to fall between the lycopsids and the euphyllophytes. [19]

See also

Related Research Articles

<span class="mw-page-title-main">Lycopodiopsida</span> Class of vascular plants

Lycopodiopsida is a class of vascular plants known as lycopods, lycophytes or other terms including the component lyco-. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching stems bearing simple leaves called microphylls and reproduce by means of spores borne in sporangia on the sides of the stems at the bases of the leaves. Although living species are small, during the Carboniferous, extinct tree-like forms (Lepidodendrales) formed huge forests that dominated the landscape and contributed to coal deposits.

<span class="mw-page-title-main">Lycophyte</span> Broadly circumscribed group of spore bearing plants

The lycophytes, when broadly circumscribed, are a vascular plant (tracheophyte) subgroup of the kingdom Plantae. They are sometimes placed in a division Lycopodiophyta or Lycophyta or in a subdivision Lycopodiophytina. They are one of the oldest lineages of extant (living) vascular plants; the group contains extinct plants that have been dated from the Silurian. Lycophytes were some of the dominating plant species of the Carboniferous period, and included the tree-like arboresencent lycophytes, some of which grew over 40 metres (130 ft) in height, although extant lycophytes are relatively small plants.

<span class="mw-page-title-main">Drepanophycales</span> Extinct order of spore-bearing plants

Drepanophycales is an order of extinct lycopsid plants of Late Silurian to Late Devonian age, found in North America, China, Russia, Europe, and Australia. Sometimes known as the Asteroxylales or Baragwanathiales.

<i>Zosterophyllum</i> Extinct genus of spore-bearing plants

Zosterophyllum was a genus of Silurian-Devonian vascular land plants with naked branching axes on which usually kidney-shaped sporangia were arranged in lateral positions. It is the type genus for the group known as zosterophylls, thought to be part of the lineage from which modern lycophytes evolved. More than 20 species have been described.

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.

<i>Pertica</i> Extinct genus of plants

Pertica is a genus of extinct vascular plants of the Early to Middle Devonian. It has been placed in the "trimerophytes", a strongly paraphyletic group of early members of the lineage leading to modern ferns and seed plants.

Krithodeophyton is a genus of lower Devonian plant with branching axes. It is considered to be a barinophyte.

Psilophytopsida is a now obsolete class containing one order, Psilophytales, which was previously used to classify a number of extinct plants which are now placed elsewhere. The class was established in 1917, under the name Psilophyta, with only three genera for a group of fossil plants from the Upper Silurian and Devonian periods which lack true roots and leaves, but have a vascular system within a branching cylindrical stem. The living Psilotaceae, the whisk-ferns, were sometimes added to the class, which was then usually called Psilopsida. This classification is no longer in use.

<i>Yunia</i> Extinct genus of spore-bearing plants

Yunia is a genus of extinct vascular plants from the Early Devonian. It was first described from the Posongchong Formation of Yunnan, China. The leafless plant consisted of spiny stems, some 2 to 5 cm wide, which branched dichotomously at wide angles in a cruciate arrangement. Each stem contained vascular tissue with one or two strands of protoxylem. The spore-forming organs (sporangia) were elongated and borne on short stalks. The spores had a relatively smooth sculptural pattern and were trilete.

<i>Renalia</i> Extinct genus of vascular plants

Renalia is a genus of extinct vascular plants from the Early Devonian. It was first described in 1976 from compressed fossils in the Battery Point Formation. It is difficult to reconstruct the original form of the complete plant, but it appears to have consisted of leafless branching stems whose side branches had sporangia at their tips. It is regarded as an early relative of the lycophytes.

Huia is a genus of extinct vascular plants of the Early Devonian. The genus was first described in 1985 based on fossil specimens from the Posongchong Formation, Wenshan district, Yunnan, China.

<i>Adoketophyton</i> Extinct genus of spore-bearing plants

Adoketophyton is a genus of extinct vascular plants of the Early Devonian. The plant was first described in 1977 based on fossil specimens from the Posongchong Formation, Wenshan district, Yunnan, China. These were originally named Zosterophyllum subverticillatum; later the species was transferred to a new genus as Adoketophyton subverticillatum. One cladistic analysis suggested that it is a lycophyte, related to the zosterophylls. Other researchers regard its placement within the vascular plants as uncertain.

Discalis is a genus of extinct vascular plants of the Early Devonian. The name is derived from the Greek δίσκος, referring to the disc-shaped sporangia. The genus was first described by Hao in 1989 based on fossil specimens from the Posongchong Formation, Wenshan district, Yunnan, China.

Distichophytum is a genus of extinct vascular plants of the Late Silurian (Ludfordian) to Early Devonian (Emsian), around 426 to 393 million years ago. The genus has a tangled taxonomic history, also being known as Bucheria and Rebuchia.

<i>Nothia aphylla</i> Extinct species of spore-bearing plant

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, Nothia aphylla.

<i>Barinophyton</i> Extinct genus of plants

Barinophyton was a genus of early land plant with branching axes. It is placed in a group of early vascular plants (tracheophytes), the barinophytes, a group that has been given various ranks and scientific names. Known fossils are of Devonian to Carboniferous age.

Protobarinophyton was a genus of Silu-Devonian land plant with branching axes. It is placed in a group of early vascular plants (tracheophytes), the barinophytes, a group that has been given various ranks and scientific names.

Wenshania is a genus of extinct vascular plants found in the Posongchong Formation, Yunnan, China, which is of Early Devonian age. Plants consisted of leafless stems with simple dichotomous branching, and bore spore-forming organs or sporangia all around the sides of stems. Wenshania is part of the broadly defined group of zosterophylls.

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 2013 used the presence or absence of terminal sporangia as a major dividing feature. The order Zosterophyllales was used for species with terminal as well as lateral sporangia, which were considered to have determinate growth, with their sporangia generally being arranged in spikes. The paraphyletic order Gosslingiales was used for species without terminal sporangia, which were considered to have indeterminate growth, with fertile branches generally circinate. Species assignable to the Gosslingiales made up about 9% of all confirmed species in the Early Devonian flora.

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 a family or clade Barinophytaceae within the zosterophylls. They have also been considered to be possible lycopodiopsids.

References

  1. 1 2 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. 1 2 3 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. 1 2 3 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. 1 2 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. 1 2 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. 1 2 3 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, S2CID   3866183
  18. ↑ 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
  19. 1 2 Hao, Shougang & Xue, Jinzhuang (2013), The early Devonian Posongchong flora of Yunnan: a contribution to an understanding of the evolution and early diversification of vascular plants, Beijing: Science Press, p. 329, ISBN   978-7-03-036616-0 , retrieved 2019-10-25
  20. ↑ 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
  21. ↑ Taylor, T.N.; Taylor, E.L. & Krings, M. (2009). Paleobotany, The Biology and Evolution of Fossil Plants (2nd ed.). Amsterdam; Boston: Academic Press. pp. 325–326. ISBN   978-0-12-373972-8.
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