The tiny snails of the genus Carychium, which grow to a maximum height of 2.5 millimeters and a width of 1.5 millimeters, are common across the American continents from Canada to Panama in protected, moist habitats. "Unlike the recent species, however, fossils of the dwarf horn snails are rare east of the Mississippi River. In our latest research, we have now produced the first fossil record of the genus in the southeastern United States, as well as the first fossil record ever for the species Carychium floridanum," explains project leader Adrienne Jochum of the Senckenberg Research Institute, the Frankfurt Museum of Nature and the Bern Museum of Natural History. The study, which has now been published, is freely accessible on "ZooKeys".
The researchers had luck on their side: During construction work for a track bed of the Brightline railroad, which will connect Port Canaveral with Orlando International Airport, the civil engineers accidentally came across a one-meter-thick layer of fossilized non-marine snails between two marine shell beds. "This rock sandwich was formed during the Pleistocene. This geologic age is characterized by repeated glaciations, climate changes, and fluctuations in water level, and it greatly influenced and shaped the region around present-day Florida. The shell layer is sandwiched between rock layers from the Lower Pleistocene, 2.58 to 0.77 million years ago, and the Upper Pleistocene, 140,000 to 120,000 years ago, and contains 14 freshwater and 28 terrestrial snail species."
Among them is the snail species Carychium floridanum, whose current representative still lives in moist, forested and undisturbed habitats in central and northern Florida. The researchers newly described Carychium nashuaense, a species less than 1.6 millimeters in size that was previously unknown to science. "To dislodge the fossil miniature snails from the rock layers, we washed them through a graduated series of sieves. Subsequently, 32 Carychium shells were filtered out under a microscope from a mixture of other mollusks and rock debris. A high-resolution X-ray tomograph helped us examine the structure in the fragile fossil shells and compare them with 3D reconstructions of the inner shell of still-living thorn snail species from the southeastern U.S., Mexico, Central America and Jamaica," Jochum explains the methodology.
The snails were measured by the Malacology Department of the Natural History Museum in Bern and imaged with slice photography. The result brought out that the finds are two different species. While the expression of the inner shell structure of Carychium floridanum has hardly changed since the Pleistocene until today, the shell structure of Carychium nashuaense, which was first described, indicates a relationship with Central American land snails. "We suspect that the spread of the snails occurred via birds, mammals and reptiles. The small snails traveled in their guts, fur, or feathers to the wetlands where the alluvial sediments of the rock layer we studied originated. The subsequent mixing with conspecifics led to the emergence of new species," concludes Jochum.