Phylogenetic revision of the micro-echinoid genus, Echinocyamus

Sea urchins (Echinoidea) comprise a group exhibiting enormous diversity in shape and size. At the lowest end of the size scale is Echinocyamus, a poorly known genus of highly miniaturized relatives of the sand dollars sometimes called “sea peas.” There are sixteen known extant species of Echinocyamus and two species in the sister genus, Mortonia. Using elliptical Fourier analysis to compare these species, 13 of them with sufficiently large sample sizes (> ten specimens) were significantly distinct based on test shape alone. A morphological phylogeny indicates that two species, E. planissimus and E. platytatus, should be removed not only from the genus, but from the family to which they belonged, Fibulariidae. With the removal of these two species, Echinocyamus and Mortonia are monophyletic and sister to Fibularia. Mortonia was nested well within Echinocyamus and should be sunk into that genus.