Elizabeth J. Hermsen

Fossils are the physical records of the history of morphological character evolution on Earth and can provide valuable information concerning the sequence and timing of origination of derived characters. Knowledge of the timing of origination of synapomorphies makes it possible to estimate when unobserved character changes occurred in the geological past. Here we present a method for estimating the temporal interval during which synapomorphies evolved. The method requires either direct inclusion of fossil taxa (with or without extant taxa) in cladistic analyses based on morphological or combined data, or indirectly using the "molecular scaffold approach." Second, characters of interest are mapped on a most parsimonious tree and "minimum age node mapping" is used to place minimum ages on the nodes of the tree. Finally, characters of interest are evaluated for younger and/or older temporal constraints on the time of their origination; application of the older bound assumes ancestry of fossil terminals included in the tree. A key is provided herein describing the method. Among other applications, this approach has the potential to provide a powerful test of purported evolutionary cause-effect relationships. For example, the method has the ability to discover that derived characters of suggested adaptational significance may considerably pre-date the cause(s) that are hypothesized to have favored their establishment.

Cataphylls associated with the Middle Triassic stem genus Antarcticycas are described, and their impact on understanding cycad evolution is discussed. The cataphylls of Antarcticycas are triangular in outline and flattened adaxially with lateral flanges. The outer surfaces are covered with a ramentum of filamentous hairs, the epidermis is a single cell layer thick, and the ground tissue is parenchymatous with mucilage canals and sclereids. Vascular bundles form a distinct inverted omega‐shaped pattern characteristic of the Cycadales observed in petioles of extant species. The structures in Antarcticycas are interpreted as cataphylls based on overall morphology, presence of straight vascular strands in the cortex of the associated stem, and lack of fascicular cambia in the vascular bundles. Because much of the overall diversity of Cycadales is represented by fossils, integrating fossil taxa into explicit phylogenetic hypotheses is important for understanding cycad evolution. Therefore, character and minimum age mapping were performed on a phylogeny of extant and fossil taxa including Antarcticycas. The results suggest that major extant lineages of Cycadales had diverged by the Permian to Triassic and that certain synapomorphies for Cycadales had evolved by the Permian. Evidence of insect feeding on Antarcticycas suggests that associations between cycads and insects are ancient.