MOOI, R.*; PIMENTEL, F.; Calif. Acad. of Sciences, San Francisco; Kalamazoo Coll., Kalamazoo: From large to small to large: Phylogenetic systematics of rotulid �sand dollars� and their tiny relatives

Recent phylogenies of the Clypeasteroida (sand dollars and allies) recognize 3 clades (Clypeasterina, Laganina, Scutellina). The Laganina contains the strangest of all clypeasteroids. Although many laganine taxa are of medium size (approximately 50 mm test length), the group contains the smallest of all echinoids (less than10 mm) as well as large forms (more than 90 mm). Phylogenetics indicate that a monophyletic Laganina includes the bizarre rotulids (�alien catcher�s mitts�). Fibulariella, miniaturized laganines once included in the Fibulariidae, is actually more closely related to rotulids than fibulariids. Previous analyses showed two separate derivations of tiny, almost spherical clypeasteroids lacking internal buttresses: 1) fibulariids, and 2) Fibulariella. Our present results undermine this interpretation through cladistic studies of plate architecture and other features of certain key taxa, especially Fibulariella and its Eocene relatives in Thagastea and Tarphypygus. Placement of all these taxa in time and space suggests that there are unexpected changes in diversity and morphology during the evolution of the group. Most significantly, there is a previously unrecognized clade of tiny Fibulariella-like laganines in the Eocene neotropics that seems to link the fibulariids with Fibulariella. Our phylogeny suggests that Fibulariella does not represent a second miniaturization event in the laganines. Fibulariella appears to have originated from an ancestor in common with fibulariids. The surprising result is that the rotulids derived their large, flattened 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.