EARLY EVIDENCE FOR LAND INHABITATION

Ordovician & Early Silurian Terrestrial(?) Plants

Strother et al. 1996 (Geology, v. 24, p. 55-58) have studies the Middle Ordovician shales of Saudi Arabia.

• Nearshore marine setting from which palynomorphs (acritarchs and cryptospores) and phytoclasts recovered.
• Spore-like tetrads (fundamental character of embryophytes), dyads and cuticle-type fragments with cell clusters.

Dyads are known from Cooksonia and Rhynia-type sporangia (Fanning et al., 1991). They argue for a terrestrial origination for this assemblage; plants at a bryophyte-grade of development (permanently bound tetrads). No extant algae are known to produce either trilete spores or resistant-walled, tetrahedrally arranged spore tetrads. Earliest trilete spore in the Ashgillian (latest Ordovician). Taylor (1995) has shown that the ultrastructure of Dyadospora (Ashgillian Ohio) has a lamellated spore wall that is similar to modern hepatics.

Remains of what have been interpreted to be the earliest known vascular land plants appear in the mid-Silurian (Wenlockian). This evidence consists of spores, recovered from shallow marine environments, similar to those isolated from early land plants. Spore diversity by Late Silurian time is greater than that known from megafossil evidence. This may be due to spore release from vegetation living on better-drained soils, evidence for the parent plant not having been preserved.

Evidence for early land plants is first encountered in fluvial sediments (Massanutten Sandstone) of the Early Silurian (Llandoverian) of Virginia. The PASSAGE CREEK ASSEMBLAGE is comprised of:

1) Banded and smooth tubular elements.

• Elements with an annular or spiral banding embedded in a lighter wall. Widths range from 11-31 m; lengths up to 200 m. Clusters of tubes or parallel pairs are common.
• Smooth-walled elements are abundant, the ends of which appear to be jagged suggesting that these elements were longer. Mean width is 21 µm, lengths up to 45 µm.

2) Cuticular and membranaceous fragments (one cell and several cells thick).

• Cellular sheets contain upwards of 200 cells, the shape of which are equidimensional and often orbicular.

3) Alete and trilete spores and tetrads.

• Trilete spores (35-41 µm), alete spores (14-30 µm), and sphaeromorphs up to 41 µm in diameter.

4) Small septate hyphal filaments and filamentous mats.

No evidence of true vascular tissues has been recovered.

One of the earliest inhabitors of land may have been the lichen, an organism that is the symbiotic relationship between a green alga and a fungus. These organisms today colonize bare rock surfaces and are responsible for the breakdown of lithic materials via biochemical weathering. The earliest known lichen has recently been reported from the Early Devonian Rhynie Chert.

Evidence in Paleosols

Paleosols (ancient soils) are a record of land surfaces and the vegetation that shaped their character. Plants that colonized these soils can be interpreted from root traces and soil (paleosol) profile (once diagenetic effects have been accounted for). Retallack has proposed two terms:

• Polsterland - well-drained soils dominated by multicellular plants that lack roots and rhizomes;
• Brakeland - soils that support herbaceous plants with rhizomes, runners, or fibrous roots

Polsterlands are believed to exist in the Late Ordovician of Nova Scotia and Pennsylvania (which also include abundant burrows, but no root traces). More evidence exists for the identification of brakelands. Brakelands are found as early as Late Silurian and continue into the Carboniferous. Evidence of macroinvertebrate bioturbation exists in these oxidized calcareous soils.

Recent reports of early vascular plants include:

Pinnatiramosus from China (Cai et al. 1996) believed to be Silurian due to regional stratigraphic correlations. The "rooting" structures are 2 meters below the Silurian/Permian unconformity.

• Rooting structures are pinnate, cross-cut bedding and are found horizontal along bedding surfaces. Vascular cells are interpreted from casts of the internal anatomy.

Cai et al. argue that these are SILURIAN vascular plants and, hence, the level of evolution of plant clades is higher than previously thought. Approximately >40 MY of time between Silurian/Permian; similar to that of present/Eocene. Possible that Silurian sediments were not lithified and Permian plants rooted. Rooting structures, then, may represent Permian vegetation and not Silurian.

Cooksonia has been found in the Late Silurian of Bolivia (Morel et al. 1995) at 50^o - 60^o South Latitude indicating that it has spread to high latitudes soon after its appearance in the Wendlockian. It is the 4^th Silurian record from Gondwana: Pridoli of Libya & Pridoli of Bohemia.

Several environmental changes are noted to occur in the early Silurian (435 Ma)

• Autotrophs reproduced and increased in diversity
• Production of oxygen that exceeded consumption - formation of ozone layer providing protection
• Decrease in tectonic activity - new sizeable and stable near shore environments
• Local movement of the earth's crust causing cyclical emergence and subsidence in the littoral zone - littoral inhabitants exposed to longer periods of drying out
• Plants increasing rate of weathering of rocks and contributing to the formation of soils (e.g., fossil lichens have been reported from the Devonian-aged Rhynie chert - 400 million years old; Taylor et al. 1995)

© Copyright 1997 by Robert A. Gastaldo. All rights reserved. No part of these lecture notes may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission from the author.