The Ziegler Reservoir fossil site in Snowmass Village, CO presents a continuous late-Pleistocene fossil record spanning 130 – 45 ka between the Bull Lake and Pinedale Glaciations, and consists of fine-grained lake and marsh sediments interbedded with debris flows. Twelve distinct stratigraphic units are delineated. Tiger salamanders, Ambystoma tigrinum, the most widely distributed salamanders in North America, are present in most units, and act as a sensitive indicator of paleoenvironmental conditions.

Preliminary data represents ~80% of the total salamander elements collected from roughly half of the sediment volume. More than 23000 elements (NISP) have been identified, including one complete skull, one partial skeleton, more than 5700 non-atlas vertebrae, and 361 atlases (MNI). Vertebral morphology is used as proxy for individual developmental stage. With the exception of the basal silt unit (NISP = 31, MNI = 6) which predominantly contains terrestrial adults, neotenic adults represent the most abundant morphotype in all units, suggesting an early switch in the population’s life history strategy. Terrestrial juveniles are the least abundant morphotype in all units, suggesting conditions that favored a longer aquatic lifestyle, and/or selective removal by predation. The ratio of neotenic adults to terrestrial adults is >1 in all units and varies from 1:1 to 5:1. The relative abundance of neotenes is consistent with the presence of a permanent pond that allowed for indefinite postponement of facultative metamorphosis.

Both terrestrial and aquatic forms can be accounted for by predation by birds and/or mammals. The cannibalistic morphotype, characterized by distorted enlargement of the entire vertebra, was uncommon but present in most units containing salamander elements. Suitable pond conditions (i.e. food availability, low predation risk) discouraged metamorphosis, although a complete switch to a neotenic life strategy did not occur. The absence of complete neoteny in the population over ~85ky can be attributed to environmental stressors, and overpopulation, which led some salamanders to become cannibalistic and others to metamorphose.

Ongoing data collection and analysis seek to provide better resolution, especially for the youngest units, and a complete population history.