Nematophytes

Above: Nematoplexus rhyniensis showing typical open meshwork of spirally coiled tubular cells (scale bar = 200µm) (Copyright owned by University Münster).

 

Introduction

Morphology

Palaeoecology

 

Introduction

Nematophytes or the Nematophytales are an extinct group of enigmatic plants known only from Devonian and Carboniferous sedimentary rocks. Their systematic position is unresolved, showing certain affinities with fungi, algae and also, tentatively, with vascular plants. Their gross morphology and habitat (particularly as to whether they were aquatic, semi-aquatic or fully terrestrial plants) are also not fully known, but some appear to have been cylindrical organisms up to 1 metre in length (see Stewart 1999). Nematophytes appear to generally comprise networks of intertwined spirally coiled tubular cells showing both smooth and spirally-thickened walls (the latter similar in appearance to tracheids in vascular plants), with branching localised in distinct 'knots'. The plants appear to have had a variably preserved cuticle-like layer on their outer surface. Certain Carboniferous nematophytes appear to have attained quite a large size, in life probably looking like prostrate logs.

At Rhynie, sandstone casts of probable nematophytes have been found in the past in the Quarry Hill Sandstone. Two incomplete nematophytes have been described from the Early Devonian Rhynie chert: Nematophyton taiti (Kidston & Lang 1921b) and Nematoplexus rhyniensis (Lyon 1962). Fragmentary nematophyte remains also occur in the Windyfield chert (Fayers & Trewin in press). For the purposes of this resource, the morphology of one of the Rhynie nematophytes, Nematoplexus rhyniensis is outlined below:

Morphology

Nematoplexus rhyniensis

Nematoplexus rhyniensis was first described by Lyon (1962), and is only known from incomplete, partially decayed, and generally fragmentary remains. As such, like other nematophytes, the gross morphology of the organism and its overall size is unknown. Conforming to the general internal nematophyte structure, Nematoplexus comprises an open meshwork or plexus of intertwined, spirally coiled tubular cells (see heading photograph). The tubes show no preferred orientation, but in the sample described by Lyon (1962) they appear more compacted towards the outer surface of the plexus where they abut onto a structureless clear zone which may represent a partially decayed natural surface layer or membrane. Branching of the tubes occurs in what appear to be dark, amorphous spots, but in better preserved samples these, in fact, comprise very tightly coiled knots of tubes showing repeated and closely spaced branching (see insets right and below right). These branch-knots occur in two discrete sizes, the smaller ranging from 45µm to 76µm in size and the larger knots between 99µm and 270µm.

Above: Taken from the holotype of Nematoplexus rhyniensis, this image shows a typical branch knot (k) with tubes with thickenings (t). Smooth tubes (s) are also shown (scale bar = 100µm) (Copyright owned by University Münster).

The tubes show two distinct morphologies. The most abundant form are smooth-walled, non-septate tubes ranging from 7µm to 10µm in diameter, typically appearing as lax intertwined coils (see inset below left). Branching of the smooth-walled tubes occurs within the smaller branch-knots. The second type are thin-walled tubes displaying conspicuous spiral thickenings within the wall layer. These range from 2µm up to 28µm in diameter and are again non-septate (see inset below right). The spirally thickened tubes are generally found as short isolated fragments or as irregular clusters within the smooth-walled tube plexus. Branching of these tubes occurs within the larger branch-knots. Occasionally tubes of both types may originate from a single, large branch-knot, though no organic connectivity between the two types have been observed.

Above: Another branch knot (k) of Nematoplexus with emerging tubes with spiral thickenings (t) (scale bar = 100µm) (Copyright owned by University Münster).

Above: A spirally coiled smooth-walled tube of Nematoplexus (scale bar = 50µm) (Copyright owned by University Münster).	Above: A thin-walled tube of Nematoplexus with spiral thickenings (t) (scale bar = 10µm) (Copyright owned by University Münster).

Palaeoecology

As stated in the introduction, the habitat of Nematoplexus and nematophytes in general remains unresolved.

A number of authorities believe that nematophytes were aquatic plants. In the Rhynie chert the two nematophytes Nematoplexus and Nematophyton are typically found in association with filamentous green algae (chlorophytes), charophytes, cyanobacteria, occasionally with the crustacean Lepidocaris and with coprolites in a 'clotted' chert matrix; indicative of silicification in a freshwater, aquatic environment. However, in these instances, it may be that the nematophyte remains had been transported, after death, into such aquatic settings or alternatively had been drowned in situ after a flooding event.

They may, however, have been terrestrial or at least semi-aquatic organisms, suggested by the resemblance of the spirally-thickened tubes seen in the Rhynie chert nematophytes to the thickened tracheids in the xylem of vascular plants. It may be that these organisms grew in shallow water with the spirally-thickened tubes differentiated in emergent fronds or 'leaves'. However, to date, no unequivocal evidence of such differentiation has been found in any fossil nematophytes.