Thirty-four species of Eocene pteropods (minute, shell-bearing, planktonic gastropods) are added to the 11 previously known from North America. They can, on occasion, be used effectively for global correlation of synchronous strata. As pteropods receive further attention, the number and accuracy of these correlations will increase.
Pteropods are one of the most abundant and ubiquitous members of the plankton community in modern oceans. They were just as diverse and abundant in Eocene seas. There are about 28 modern euthecosome species. We have identified 45 Eocene species in North America, 7 of which were already known in England and Europe; 27 are new. They were collected from outcrops in Texas and Alabama and from exploratory wells in Louisiana and the Nova Scotian shelf.
All euthecosomatous pteropods have aragonitic shells but there are at least 3 different kinds of microstructure: (1) most spirally coiled species (family Spiratellidae) have crossed-lamellar microstructure, (2) straight or bilaterally symmetrical shells (family Cavoliniidae and Creisidae) have a helical microstructure, and (3) the Eocene species, Plotophysops bearnensis Curry (family Spiratellidae), has both crossed-lamellar and helical microstructure.
Helical microstructure, first described in pteropods by Be, MacClintock, and Chew-Currie in the modern species, Cuvierina columnella Rang, is not known to exist in other molluscan groups. The helical rods are nested in such a manner as to give maximum strength to the thin fragile shell, a decided advantage for an organism with a planktonic life style.