Zosterophyllopsida

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Zosterophyllopsida
Temporal range: Ludlow to Devonian
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Zosterophyllum.JPG
Zosterophyllum species fossils
Scientific classification Red Pencil Icon.png
Kingdom: Plantae
Clade: Lycophytes
Clade:†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 or plesion 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]

The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago (Mya), to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era. As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by several million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when up to 60% of marine genera were wiped out.

Taxon Group of one or more populations of an organism or organisms which have distinguishing characteristics in common

In biology, a taxon is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and given a particular ranking, especially if and when it is accepted or becomes established. It is not uncommon, however, for taxonomists to remain at odds over what belongs to a taxon and the criteria used for inclusion. If a taxon is given a formal scientific name, its use is then governed by one of the nomenclature codes specifying which scientific name is correct for a particular grouping.

In biology, a phylum is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent. Depending on definitions, the animal kingdom Animalia or Metazoa contains approximately 35 phyla, the plant kingdom Plantae contains about 14, and the fungus kingdom Fungi contains about 8 phyla. Current research in phylogenetics is uncovering the relationships between phyla, which are contained in larger clades, like Ecdysozoa and Embryophyta.

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]

Enations are scaly leaflike structures, differing from leaves in their lack of vascular tissue. They are created by some leaf diseases. Also found on some early plants such as Rhynia, where they are hypothesized to aid in photosynthesis.

Vernation is the formation of new leaves or fronds. In plant anatomy, it is the arrangement of leaves in a bud.

In a vascular plant, the stele is the central part of the root or stem containing the tissues derived from the procambium. These include vascular tissue, in some cases ground tissue (pith) and a pericycle, which, if present, defines the outermost boundary of the stele. Outside the stele lies the endodermis, which is the innermost cell layer of the cortex.

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]

Flowering plant Class of flowering plants (in APG I-III)

The flowering plants, also known as angiosperms, Angiospermae or Magnoliophyta, are the most diverse group of land plants, with 64 orders, 416 families, approximately 13,000 known genera and 300,000 known species. Like gymnosperms, angiosperms are seed-producing plants. However, they are distinguished from gymnosperms by characteristics including flowers, endosperm within the seeds, and the production of fruits that contain the seeds. Etymologically, angiosperm means a plant that produces seeds within an enclosure; in other words, a fruiting plant. The term comes from the Greek words angeion and sperma ("seed").

David P. Penhallow American botanist

David Pearce Penhallow was a Canadian-American botanist, paleobotanist and educator.

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:

Bryophyte terrestrial plants that lack vascular tissue

Bryophytes are an informal group consisting of three divisions of non-vascular land plants (embryophytes): the liverworts, hornworts and mosses. They are characteristically limited in size and prefer moist habitats although they can survive in drier environments. The bryophytes consist of about 20,000 plant species. Bryophytes produce enclosed reproductive structures, but they do not produce flowers or seeds. They reproduce via spores. Bryophytes are usually considered to be a paraphyletic group and not a monophyletic group, although some studies have produced contrary results. Regardless of their status, the name is convenient and remains in use as an informal collective term. The term "bryophyte" comes from Greek βρύον, bryon "tree-moss, oyster-green" and φυτόν, phyton "plant".

In biological classification, class is a taxonomic rank, as well as a taxonomic unit, a taxon, in that rank. Other well-known ranks in descending order of size are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order.

Taxonomic rank Level in a taxonomic hierarchy

In biological classification, taxonomic rank is the relative level of a group of organisms in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, order, class, phylum, kingdom, domain, etc.

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 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

Related Research Articles

Lycopodiopsida class of plants

Lycopodiopsida is a class of herbaceous vascular plants known as lycopods, lycophytes or other terms including the component lyco-. Members of the class are called clubmosses, firmosses 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 formed huge forests that dominated the landscape and contributed to coal deposits.

Lycophyte phylum of 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 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 tree-like species, although extant (living) lycophytes are relatively small plants.

Drepanophycales order of plants

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

<i>Tortilicaulis</i>

Tortilicaulis is a moss-like plant known from fossils recovered from southern Britain, spanning the Silurian-Devonian boundary. Originally recovered from the Downtonian of the Welsh borderlands, Tortilicaulis has since been recovered in the famous Ludlow Lane locality.

<i>Zosterophyllum</i> genus of plants

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

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.

Trimerophytopsida

Trimerophytopsida is a class of early vascular plants from the Devonian, informally called trimerophytes. It contains genera such as Psilophyton. This group is probably paraphyletic, and is believed to be the ancestral group from which both the ferns and seed plants evolved. Different authors have treated the group at different taxonomic ranks using the names Trimerophyta, Trimerophytophyta, Trimerophytina, Trimerophytophytina and Trimerophytales.

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 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.

<i>Rhynia</i> extinct species of plant

Rhynia is a single-species genus of Devonian vascular plants. Rhynia gwynne-vaughanii was the sporophyte generation of a vascular, axial, free-sporing diplohaplontic embryophytic land plant of the Lower Devonian that had anatomical features more advanced than those of the bryophytes. Rhynia gwynne-vaughanii was a member of a sister group to all other eutracheophytes, including modern vascular plants.

<i>Pertica</i> genus of plants (fossil)

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.

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>Renalia</i> genus of 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.

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. Cladistic analysis suggests that it is a lycophyte, related to the zosterophylls.

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.

Dutoitia is a genus of Devonian rhyniophyte, named after the renowned South African geologist Alex du Toit. It is one of the earliest plants from Gondwana to colonize land. Its fossils were preserved in fine mudstones of the 400-million-year-old Bokkeveld and Witteberg Groups of South Africa. This erect, gracile plant is less than 10 cm high and very simple in structure. Its diminutive stems, which are devoid of leaflike appendages, branch in two and end in club- or cup-shaped sporangia, occasionally containing its reproductive spores. Stomata are present in the cuticle of their stems for gas exchange, and primitive cells inside the stems transported water from the roots to the aerial parts of the plant. Three species are recognized, D. pulchra Hoeg 1930, D. alfreda Plumstead 1967 and D. maraisia Plumstead 1967.

<i>Nothia</i>

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.

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

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.

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. 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 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. 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 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
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