Article: Spongiophyton from the late Lower Devonian of New Brunswick and Quebec, Canada

Compressed cuticles of the dorsiventral, presumed parenchymatous land plant Spongiophyton are described from several late Lower Devonian (Emsian) localities in northern New Brunswick and Gaspe, Quebec. They are assigned to Spongiophyton minutissimum Krausel. Thalli branch dichotomously several times in more than one plane and also produce short vertical branches. No reproductive structures are present. The cuticle is thicker dorsally, often being up to 250 micro m thick as opposed to 30-60 micro m for the ventral cuticle. Small circular to elliptical pores occur on the dorsal surface or along thallus margins. The many specimens obtained provide considerable information on variation in vegetative morphology and suggest a growth habit similar to some extant thallose liverworts. This new information expands the concept of S. minutissimum and supports the genus as a taxon quite distinct from Nematothallus, Prototaxites, or other plants with a putative filamentous organization. Absence of reproductive structures precludes improved understanding of Spongiophyton's relationship to algae or vascular plants; the presence of a resistant cuticle argues against these plants representing algae. They may instead represent a transitional grade between algae and higher plants. The new specimens also demonstrate that these cuticles are susceptible to some differential breakdown as a result of oxidation or heat. Degradation features include superficial cracks or pockmarks, a spongy appearance, or a pseudocellular pattern. It is suggested that possibly some features of other enigmatic Devonian plants (e.g. Orestovia, Rhytidophyton, etc.) may have resulted from degradation of their very thick cuticles.