Lanthonia Coppard 2016

Lanthonia Coppard, 2016

Type species. Mellita longifissa Michelin, 1858.

Assigned species. L. longifissa (Michelin, 1858), L. grantii (Mortensen, 1948).

Distribution. Early Pliocene to Recent. Tropical and subtropical Pacific coast of the Americas (Mexico to Colombia).

Etymology. ‘ Lanth ’ meaning hidden and ‘ onia ’ a diminutive ending (gender feminine).

Diagnosis. Members of Lanthonia possess diagnostic features of the family Mellitidae, but are distinguished from members of Mellita by the presence of very narrow ambulacral regions between the food grooves and the ambulacral lunules on the oral surface, and by the structure of the bidentate pedicellariae which have simple valves (with no projecting stereom within the blades of the valves) and small peripheral teeth present along the edge of the blade.

Description. Members of the genus Lanthonia have a test outline that is pentagonal to subpentagonal, with four ambulacral lunules (no lunule in the anterior ambulacrum) and one interambulacral posterior lunule. The lunules are smooth (non-sinuous) and on the oral surface have shallow pressure drainage channels. The apical disc is monobasal with four gonopores. The apex of the test ranges from the centre to anterior of centre, with the greatest width of the test anterior of centre. Food grooves branch distally and are positioned close to the ambulacral lunules, with only a narrow ambulacral region between the food grooves and the lunules. The aboral club-spines are expanded distally, but not greatly expanded as in the type species of Mellita (see Fig. 4). The interambulacral lunule-margin spines are highly curved, rounded and expanded distally into a paddle. The biphyllous pedicellariae have open blades. The bidentate pedicellariae are very similar in both L. longifissa and L. grantii and have simple, moderately broad, rounded blades with small peripheral teeth along the edge of the blades that gradually increase in size distally. Such teeth are typically smaller in L. grantii (Fig. 4 Fi) than in L. longifissa (Fig. 4 Fi).

Habitat and distribution. Based on molecular data Coppard et al. (2013) proposed that M. notabilis and L. longifissa are sympatric from Northern Mexico to Panama in the eastern Pacific, but that L. grantii occurred in the Gulf of California and along the adjacent Pacific coast of the Baja California Peninsula. These three species therefore have potentially overlapping distributions in the Gulf of California and along the Pacific coast of Mexico. In this investigation all species of Mellita and Lanthonia were found to be restricted to sand that was terrigenous in origin. Sympatric M. notabilis and L. longifissa were both found intertidally and subtidally in the littoral zone, where they were observed feeding in the surface layer of sand. In a few localities (e.g. Punta Cham, Panama and Malcomb, Mexico) they were found on the same beach in mixed populations.