From large to small to large: Phylogenetic systematics of rotulid "sand dollars" and their tiny relatives
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Recent phylogenies of the Clypeasteroida (sand dollars and allies) recognize three clades: Clypeasterina, Laganina, and Scutellina. 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 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 ale 2001). I tested this scenario by examining available material and the literature to better circumscribe key taxa, especially Fibulariella and its Eocene relatives, Thagastea and Tarphypygus. A phylogenetic analyses of 59 characters was performed using PAUP. Placement of taxa in time and space suggests that there were unexpected changes in diversity and morphology during the evolution of the group. Most significantly, there is a previously unrecognized group of tiny Fibulariella-like laganines in the Eocene neotropics that seems to link the fibulariids with Fibulariella. The phylogeny suggests that Fibulariella does not represent a second miniaturization event in the laganines, but originated from an ancestor in common with fibulariids. The surprising result is that the rotulids derived their large, sand dollar-like morphology in parallel with true sand dollars. Odd features of rotulids can be traced to a "heterochronic bottleneck" that occurred when miniaturized ancestors adapted to coasts along West Africa that lacked sand dollars. These findings shed new light on the paleobiogeography of marine taxa and provide significant data towards our understanding of the evolutionary origins of bizarre new morphologies.