SHELL SIZE SHIFTS OF A QUATERNARY HELICID FROM THE CANARY ISLANDS

Land snails are exposed to environmental (e.g., temperature and humidity) and ecological (e.g., intraspecific competition) factors that influence their final body size. We have characterized the relationships between various climatic and ecological factors, and the size for fossil specimens of the helicid Theba geminata from Fuerteventura Island, Canary Archipelago. Shell body size (n = 141) and number of specimens (n = 7032) were characterized over a stratigraphic sequence of six paleosols that spanned the time interval from ~48 to ~37 kyr. Stable carbon and oxygen isotopes of shells (n = 9) and sediment mineralogy (n = 5) of the deposits were determined to place the shells within a paleoclimate framework to determine potential correlations among the parameters investigated. Mean shell surface area fluctuated significantly from 100.4 ± 27.2 mm2 in the youngest paleosol to 165 ± 23.6 mm2 in the oldest horizon. Stable isotope compositions ranged from -3.9 to -6.5‰ V-PDB for carbon, and from 0.0 to +0.9‰ V-PDB for oxygen. The percentage of quartz (originating from Saharan dust storms) fluctuated from 0 to 50%. Finally, number of individuals per horizon shifted from an average of 881 at paleosol FBE-4 (~39 kyr BP) to 2,467 specimens at paleosol FBE-2 (~47 kyr BP). A negative correlation was observed between shell size and carbon isotope composition (r= -0.89; p=0.044), suggesting that larger shells occurred under wetter conditions (i.e., more negative d¹³C_shell values indicate higher abundance of C₃ plants and higher humidity). In addition, a positive correlation was observed between shell size and oxygen isotope composition (r= 0.88; p=0.047) which may imply that larger shells occurred under warmer environments (i.e., more positive d¹⁸O_shell values related primarily to higher d¹⁸O_water values). Alternatively, the source region for meteoric waters may have shifted to a lower latitude where the d¹⁸O value for water was higher. No correlation was observed between snail size and number of individuals collected in a horizon nor percentage of quartz. Consequently, climatic factors, such as humidity and temperature, may be responsible of the size of T. geminata shells but not intraspecific competition. These results suggest that land snail size may be used as a proxy to estimate paleoclimate in low latitude-oceanic islands.