Rhodophyta Rhodomelaceae

Premise of research. Although largely neglected by the paleobotanical literature, the Early Devonian genus Sphondylophyton Schultes and Dorf is on record as the oldest sphenophyte. Given current understanding of the fossil record, a revised interpretation of the depositional environment at the fossil locality, and the discovery of new specimens, reconsideration of the taxonomic affinities of Sphondylophyton is necessary.
Methodology. All known Sphondylophyton specimens were examined for morphological comparison with plant and algal lineages exhibiting a similar whorled architecture.
Pivotal results. Sphondylophyton is characterized by sparsely branched, flexuous axes bearing whorled, simple, terete appendages with dimorphic apical morphology. Sphenophyte affinities are ruled out, as whorled taxis does not appear in vascular plants until the Late Devonian. Evidence for marine influences in the depositional environment of Sphondylophyton warrants consideration of algal affinities. Although dasycladalean and charophycean green algae share superficial morphological similarities with Sphondylophyton, they can be excluded upon detailed comparison (e.g., branching, lateral appendage dimensions and complexity).
The combination of characters of Sphondylophyton falls within the morphospace of the Rhodophyta. Among these, Sphondylophyton is most similar to rhodomelacean taxa in overall habit, appendage morphology, and thallus durability. Generic and specific emended amplified diagnoses are provided.
Conclusions. Sphondylophyton is not a sphenophyte as previously suggested; dasycladalean and charophycean
affinities are not supported. We demonstrate that Sphondylophyton is a red alga most comparable to Rhodomelaceae, although no taxonomic placement below the phylum rank is proposed; as such, it contributes to the limited fossil record of the Rhodophyta.