Ventarura

Ventarura axes

Above: Polished slab of Windyfield chert showing aerial and rhizomal axes of Ventarura lyonii. Many of these axes are inverted and therefore not in growth position.

Introduction

Morphology

Relationships

Palaeoecology

 

Introduction

Ventarura is the most recently identified element of the Rhynie flora. The plant was formally described by Powell et al. (2000a) and assigned the name Ventarura lyonii. The float blocks from which the plant were described all originated from the Windyfield chert of the Rhynie hot spring complex. Many thin sections made of this material have revealed details of the subaerial and probable rhizomal axes, the pattern of branching, and the morphology of the sporangia of the plant.

A preliminary reconstruction of the sporophyte stage of the plant has recently been attempted (see below), though the gametophyte stage of the plant remains unknown. The overall morphology and palaeoecology of Ventarura is outlined below.

 

Morphology

Aerial Axes

The aerial axes of Ventarura (see inset right) have a maximum diameter of 7.2mm and exhibit primarily dichotomous branching. The angle of dichotomy usually being at 250. It is not known what height the plant attained, but it was in excess of 12cm. The axes are smooth though the epidermis exhibits unicellular peg-like projections. The details of the cuticle are not fully known and the morphology of the stomata has not been resolved. Occasionally, enigmatic elliptical features are seen on the surface of the cuticle. It is not known exactly what these are, but they may represent the sites of projections that have been lost.

The cortex of the aerial axes of Ventarura is quite distinctive. An inner and outer cortical layer is separated by a dark middle cortex zone of sclerenchymatous cells (see inset below left). The outer cortex comprises closely packed thin-walled cells. The inner cortex again comprises thin-walled cells but with a well-developed inter-cellular air space network.

 

Right: Longitudinal section of a fertile aerial axis with laterally attached sporangia (s) (see below) (scale bar = 2mm).

 

Fertile axis of Ventarura with lateral sporangia

The vascular strand of Ventarura comprises a generally poorly preserved outer zone of uniform width, tentatively interpreted as phloem tissue. The xylem strand is terete and exarch with annular and spiral thickenings (see inset below right).

Oblique cross section through an axis Xylem strand of Ventarura

Above: Slightly oblique section through an aerial axis of Ventarura showing outer cortex (o), middle cortex (s), inner cortex (i) and xylem strand (x). The cuticle of the axis is coated by a brown coloured microbial mucilage (m) (scale bar = 2mm).

Above: Close-up of xylem strand (x) of Ventarura. The dark patches are framboids of pyrite (scale bar = 100Ám).

 

Rhizomal Axes

Observed within many of the chert blocks containing the aerial axes of Ventarura are other smaller axes of a slightly differing but distinctive morphology that are believed to represent the rhizomes of the plant although there remains no unequivocal evidence for organic continuity between the two.

These axes branch repeatedly and tend to exhibit unicellular peg-like outgrowths from the epidermis (interpreted as representing rhizoids) and characteristically lack the sclerenchymatous middle cortex seen in the aerial axes.

 

Right: Straw of a Ventarura axis with microbial mucilage (m) cut by a later branching rhizomal axis (a) (scale bar = 1mm).

 

Straw and rhizomal axes of Ventarura

 

 

Sporangium

The sporangia of Ventarura are reniform (kidney-shaped) to pear shaped with dimensions up to a maximum 5.2mm by 2mm. They are attached laterally to the aerial axes with a vascularised sporangial stalk. Their spatial distribution is not fully known but at least appear to form in a vertical sequence along the fertile axes (see inset top right in section on 'aerial axes'). The sporangia are bi-valved with a well-developed marginal dehiscence mechanism (see inset right).

 

Right: Section through an empty sporangium of Ventarura showing the well-developed marginal dehiscence mechanism (d) (scale bar = 1mm).

 

Sporangium

 

Preliminary reconstruction

Preliminary reconstruction of Ventarura
Serial sections of a block of Windyfield chert have revealed more of the general anatomy of Ventarura than could be described by Powell et al. (2000a). This reconstruction (see inset left) was displayed at the conference on the Rhynie chert held in Aberdeen in September 2003.

In the sketch the individual sections of the plant that could be traced in the block are darkly shaded, and reveal a variety of branching styles are present.

Numbered features on the reconstruction are:

1. Branched sections of aerial axes traced within a chert block.

2. Sporangia in ?2 regular en echelon rows on axes of 2-3 mm diameter. Sporangia up to 4 mm wide.

3. Branched horizontal rhizomes attached to aerial axis.

4. Rhizomes penetrating bacterial meshworks and decayed aerial axes in water. Growth of the plant probably extended into floating bacterial mats, resulting in vertical penetration of rhizomes into water underneath the mat.

5. Portions of aerial axes were preserved in an inverted position after floating in pools adjacent to growing areas (see heading photograph).

We are still seeking the connections between the fertile axes seen in thin sections and the main aerial axes, also further information on rhizome branching, sporangial arrangement, and the terminations of aerial axes and rhizomes. The search continues prior to formal publication!!





Relationships

Ventarura exhibits a number of features that assist in classification; particularly the morphology of the xylem tissue, and the morphology and disposition of the sporangia. The xylem strand is characteristically exarch with thickenings. The sporangia are reniform to pear-shaped with a well-developed dehiscence mechanism, and have a lateral disposition on the aerial axes to which they are attached by a vascularised sporangial stalk. These features indicate Ventarura is undoubtedly a tracheophyte allied with the zosterophylls and as such is the second zosterophyll known from the Rhynie chert (see also Trichopherophyton). 

 

Palaeoecology

To date Ventarura has only been found in a number of chert float blocks from pods of the Windyfield chert. Although locally very well preserved, much of the vegetative and fertile material appears to be at least partially transported, and deposited in freshwater pools evidenced by many of the branched aerial axes being inverted (see heading photograph). The chert texture is typical of silicified subaqueous deposits, containing the crustacean Lepidocaris, microcoprolites and bacterial coatings on plant debris.

As such it remains difficult to sufficiently interpret the palaeoecology of this new plant but with the presence of apparently in situ rhizomatic axes in the chert blocks it seems likely that the plant probably grew at the edges of or at least within the vicinity of these pools and may have preferred sandy and organic-rich substrates.