Rhyniophyta: Unveiling Earth's Ancient Vascular Plants

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Rhyniophyta (early vascular plants)

Discovery and Early History:

• Vascular plants first appeared in the Silurian period of the Paleozoic era.
• Fossils of early vascular plants were discovered in the early Devonian period of the Paleozoic era.
• The first discovered fossil, Psilophyton, was found in 1858 in the lower Devonian rocks of the Gaspe Sandstones, Canada, by Sir J.W. Dawson.
• The recognition and importance of fossil plants increased with the investigation of the Rhynie chert bed in Aberdeenshire, Scotland.
• A wonderful plant fossil, "Rhynia," was discovered in the Rhynie chert bed.

Taxonomic Classification

• Early vascular plants belong to the division Psilophyta.
• The division is divided into two classes:
  • Psilophytopsida contains extinct members (Cooksonia, Rhynia, Zosterophylum, Horneophyton, etc.).
  • Psilotopsida includes living members (Psilotum and Tmesipteris).
• In modern classification systems, extinct early vascular plants are placed in a separate division named Rhyniophyta.

Rhyniophyta (early vascular plants)

Discovery and Early History:

• Vascular plants first appeared in the Silurian period of the Paleozoic era.
• Fossils of early vascular plants were discovered in the early Devonian period of the Paleozoic era.
• The first discovered fossil, Psilophyton, was found in 1858 in the lower Devonian rocks of the Gaspe Sandstones, Canada, by Sir J.W. Dawson.
• The recognition and importance of fossil plants increased with the investigation of the Rhynie chert bed in Aberdeenshire, Scotland.
• A wonderful plant fossil, "Rhynia," was discovered in the Rhynie chert bed.

Taxonomic Classification:

• Early vascular plants belong to the division Psilophyta.
• The division is divided into two classes:
  • Psilophytopsida: contains extinct members (Cooksonia, Rhynia, Zosterophylum, Horneophyton, etc.).
  • Psilotopsida: includes living members (Psilotum and Tmesipteris).
• In modern classification systems, extinct early vascular plants are placed in a separate division named Rhyniophyta.

Characteristics of Rhyniophyta

1. All the members included in this division are extinct.
2. Only sporophytes of these fossils are known so far; no gametophyte has been discovered.
3. Members have a sporophytic plant body differentiated into rhizomes (underground stems) and aerial branches.
4. Aerial branches are more or less dichotomously branched.
5. Roots are completely absent, but unicellular rhizoids are present in tufts.
6. Terminal or lateral sporangia are present, producing homospores.

Diversity of Vascular Plants Classification: This class is divided into a single order, Psilophytales/Rhyniales, further segmented into 9 families encompassing 20 genera. Notably, five families have been extensively studied and firmly established.

1. Rhyniaceae

• Members: Rhynia, Cooksonia, and Horneophyton
• Characteristics:
  • Rootless and leafless sporophytes.
  • Dichotomously branched aerial shoots.
  • Terminal sporangia.

2. Zosterophyllaceae

• Members: Zosterophylum, Pectinophyton
• Characteristics:
  • Rootless and leafless sporophytes.
  • Profusely branched rhizome.
  • Sporangia is produced on the apices of lateral branches.

3. Psilophytaceae

• Members: Psilophyton, Trimerophyton
• Characteristics:
  • Rootless and leafless sporophytes.
  • Aerial branches with spinous outgrowth.
  • Terminal sporangia on small branches.

4. Asteroxylaceae

• Member: Asteroxylon
• Characteristics:
  • Branched rhizome, with some branches functioning as roots.
  • Aerial stems are covered with simple and oval leaves.
  • Star-shaped wood.

5. Pseudosporochnaceae

• Member: Unspecified
• Characteristics:
  • Plant body similar to algae but with vascular tissues in the stem.
  • Thick and bulbous stem, dichotomously branched above.
  • Oval-shaped sporangia on the tips of branches.

Common Characteristics of Early Plant Fossils:

1. Cuticle Presence:

   • It helps prevent desiccation (water loss).

2. Stomata:

   • Openings for gas exchange between air and inner plant tissues.
   • Can close to prevent excessive water loss.

3. Xylem Evidence:

   • Some fossils show evidence of xylem.

4. Life Cycle Changes:

   • Adaptations to external water are not required.

5. Symbiotic Relationships:

   • Developed in the earliest plants.

Cooksonia: The Evolution of the First Land Plants

Introduction:

• The evolution of the first land plants marked a significant event in Earth's history.
• A notable Silurian land plant, sized up to a couple of centimeters, is named Cooksonia.

Discovery and Initial Studies

• Found by W.H. Lang in the lower Devonian of Wales.
• The first species was published in 1937 by W.H. Lang: C. pertoni and C. hemisphaerica.
• These specimens were a few centimeters high, dichotomously branched, and bore more or less globose sporangia at the ends of the branches.
• The genus name Cooksonia was given in honor of the Australian paleobotanist Isabel Cookson.

Global Presence

Reports of Cooksonia species from various locations worldwide, including Wales, Scotland, England, Bohemia, Kazakhstan, Siberia, the state of New York, Canada, China, Bolivia, and Brazil.

• Well-known species include C. pertoni, C. hemisphaerica, C. cambrensis, and C. caledonica, described based on British fossils.
• Recently described species C. paranensis from Brazil by P. Gerrienne et al. (2001)

Cooksonia Morphology and Characteristics:

• Morphological features of Cooksonia are similar to those of Rhynia and Psilotum (a living member of Psilotopsida).
• Anatomical studies of stem and sporangium are poorly studied.
• Cooksonia sporangia lacks special adaptations for dehiscence at maturity; it is likely to tear open at the upper side.
• Characteristics include:
  • The plant body is divided into an underground prostrate rhizome and an upright aerial part.
  • Stems are naked, straight, and dichotomously branched.
  • Sporangium is terminal.

Challenges in Finding Cooksonia Fossils:

• Difficulties in finding Cooksonia fossils arise due to:
  • Limited occurrences (marine, delta of river deposits from the Late Silurian and the earliest Devonian).
  • The small size of the plant.

Unique Feature:

• The sporangium has a one-cell-thick opercle.

Important species with unique features

Cooksonia pertoni:

• The sporangia of this plant are much broader than high, often flat and plate-shaped.
• The subtending stalk strongly widens, making the bearing part nearly as wide as the sporangium.
• The sporangium is not sunken into the axis.

Cooksonia hemisphaerica:

• Sporangia are semiglobose, globose, or elliptical.
• The stalk widens below the sporangium, with the sporangium at most about three times as wide as the top of the stalk.
• Sporangia close to the bifurcation are mostly round, while sporangia at longer stalks are more irregularly shaped.

Cooksonia cambrensis:

• The plant bears globose or elliptical sporangia, sitting on stalks that are not or hardly thickening below the sporangium.
• The axis of C. cambrensis is relatively thinner immediately below the sporangium, distinguishing it from C. hemisphaerica.

Aberlemnia (Cooksonia) caledonica:

• Sporangia are variably shaped but mostly wider than high or kidney-shaped.
• In some cases, a coaly border can be seen on the upper side.
• The stalk somewhat widens below the sporangium.

C. paranensis:

• Described by P. Gerrienne et al. (2001) based on hundreds of specimens from the Lower Devonian of Brazil.
• The plant has a plate-shaped or bowl-shaped sporangium, more or less sunken within the axis.
• The sporangium and stalk together are trumpet-shaped, and the sporangium does not project like that of C. pertoni.

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