Sprecher
Beschreibung
Nematoclasts are smooth-walled and banded organic tubes of nematophytes origin found as dispersed elements in Silurian non-marine deposits. Recent molecular compositional analyses from the Rhynie Chert by Loron et al. indicate Prototaxites taiti to be distinct from fungi, eroding support for fungal affinity of nematoclasts at large. We analyzed a set of Nematothallus lobata specimens of Wenlock (Silurian) age for their $^{13}$C content, which yielded a mean $\delta$$^{13}$C value of -27.92 ± 0.74 -- consistent with derivation from C3 (Calvin cycle) land plant photosynthesis. We speculate that N. lobata may have been the protonemal phase in an early thalloid plant life cycle, which formed resistant-walled caulonemal filaments. A survey of Silurian nematoclasts from the central Appalachians shows an evolutionary increase in wall complexity and variety. This provides a developmental (morphological) model for the origin of both smooth and banded nematoclasts. Protonemal thalli were directly exposed to UV-B during life, so it is possible that phenylpropanoid pathways in protonemal cells were under selective pressure to provide UV-B absorption, stimulating the primordial synthesis of lignin.
We hypothesize that the acquisition of the underlying developmental (genomic) basis for caulonemal walls was retained and later repurposed into walls of “proto-tracheary elements” derived from the primitive shoot apical meristem (SAM) in early embryophytes. The ability to grow an organic tube, deposit secondary wall thickenings, and synthesize lignin may have become part of the ancestral embryophyte toolkit during the early Silurian just prior to incorporation into the full genetic repertoire of the land plant embryo.