Rhyniophyta and the Origin of Vascular Plants: Fossil Evidence from Early Land Flora
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
- All the members included in this division are extinct.
- Only sporophytes of these fossils are known so far; no gametophyte has been discovered.
- Members have a sporophytic plant body differentiated into rhizomes (underground stems) and aerial branches.
- Aerial branches are more or less dichotomously branched.
- Roots are completely absent, but unicellular rhizoids are present in tufts.
- 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:
-
Cuticle Presence:
- It helps prevent desiccation (water loss).
-
Stomata:
- Openings for gas exchange between air and inner plant tissues.
- Can close to prevent excessive water loss.
-
Xylem Evidence:
- Some fossils show evidence of xylem.
-
Life Cycle Changes:
- Adaptations to external water are not required.
-
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)
1 Comments
Roll no1211
ReplyDelete5th semester