Arrigo et al. American Journal of Botany 100(8): 1672-1682. 2013. Data Supplement S2-Page

A number of heterosporous fossils from Carboniferous with dimorphic, decussate leaves are assignable to the stem group of the rhizophoric clade: Selaginella suissei from the Duckmantian (Thomas, 1997), S. gutbieri from the Westphalian, and S. primaeva from the middle Bolsovian. The rhizophoric clade is recognized by two synapomorphies: decussate sporophylls and rhizophores. Besides these two characters, microphyll dimorphism is a feature that occurs only in this clade, but it is unclear whether uniform or dimorphic leaves are the plesiomorphic condition. In the absence of rhizophores in selaginellalean fossils and scarce reports on sporophyll arrangements, dimorphic leaves were chosen as the diagnostic feature of this node. The minimum age assigned to the rhizophoric clade and, hence, crown group Selaginellaceae, was therefore set to 310 Ma, based on the first occurrence of S. suissei.

Megaspore fossils of the genus Erlansonisporites have been reported from many localities and range from Triassic to Cretaceous in age (Takahashi et al., 2001). They are easily distinguished by an ordered, grid-like exospore wall structure, a feature seen only in extant species in clade A in the Selaginella phylogeny. Erlansonisporites scanicus has, in addition, a proximal pole (Takahashi et al., 2001) most closely resembling megaspores of the extant S. gracillima, S. pygmaea, S. lyallii, and S. polymorpha, where the laesurae become highly convoluted and, together with interradial sculptural elements, form a complex mass at the pole (Korall and Taylor, 2006). The combination of cross-section structure and proximal pole sculpture is found only in a small clade of three species in the extant phylogeny, including S. lyallii, S. polymorpha, and S. moratii (Korall and Taylor, 2006). Erlansonisporites scanicus thus gives a minimum age constraint for the stem group of these three species of 85 Ma.

The prior distributions for fossil-constrained ages are always one of the main problems when setting up a BEAST analysis. With a clear synapomorphy-based approach (see fossil constraints above), we can tell what the minimum age for a node is, but assigning a distribution of possible ages is less trivial. The exponential prior distribution of ages implicitly suggests that it is most likely for the estimated ages to be close to the age of the fossil constraint, which may lead to an underestimate of the true age. The prior is, however, less prone to bias (e.g., than the widely used lognormal prior). For the root node (the split between Isoetes and Selaginella), we used a uniform prior of 370–420 Ma. For the internal fossil constraints an exponential prior was used, using the minimum age as offset and 10 as mean.

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