FROM CENTRAL MEXICO AND A POSSIBLE QUATERNARY PALEOCLIMATIC INDICATOR

Limnocythere bradburyi is a new species of nonmarine ostracode that is living in several lakes in the central Mexican Plateau. These lakes are shallow, turbid, and pan-shaped, having relatively unstable and fluid substrates. Water levels of these lakes are high in the summer and low or dry in the winter. These lakes usually contain fresh to slightly saline water during the rainy season (summer-fall) and slightly saline water during the dry season (winter-spring), and have a solute composition that is dominated by Na+, HCO3--CO32-, Cl- ions. The regional climate is characterized as humid temperate with mild equitable temperatures throughout the year. Winter temperatures are usually above 0?C, whereas summer temperatures are commonly below 30?C. The water temperature of the lakes containing L. bradburyi generally reflects atmospheric tem- peratures. The ostracode's life cycle coincides with the climatic wet cycle and is therefore completed during the warmest period of the year, which is in marked contrast to ostracodes living in lakes in the United States and Canada that usually begin their life cycle with the spring rain and snow melt in cold water and complete their life cycle in warm water. This contrasting climatic life-cycle pattern between central Mexico and the United States may be sufficient to explain why L. bradburyi occurs commonly in many Quaterary deposits in the southwester United States, but has not been found living in the United States.