From Large to Small to Large: Phylogenetic Systematics of Rotulid “Sand Dollars” and their Tiny Relatives

Abstract

The Clypeasteroida (sand dollars and allies) comprises three clades: Clypeasterina, Laganina, and Scutellina (true sand dollars). The diverse Laganina contains taxa that vary greatly in size, from the large, sand dollar-like rotulids to miniaturized forms like Fibulariella. Fibulariella, though once included in the family Fibulariidae, is actually more closely related to rotulids than fibulariids. Previous analyses showed two separate derivations of tiny laganines lacking internal buttresses: 1) fibulariids, and 2) Fibulariella (Mooi et al. 2001). I tested this scenario by examining material and the literature to better circumscribe key taxa, especially Fibulariella and its Eocene relatives, Thagastea and Tarphypygus. A previously unrecognized group of tiny Fibulariella-like laganines (the “Unknowns”) in the Eocene neotropics was also included in the study. To reveal underlying processes and driving forces behind laganine evolution, a phylogenetic analyses of 59 characters was performed using PAUP, and taxa were mapped in time and space.