Study site and fossil collection
The Greater Yellowstone Ecosystem (GYE) is often considered one of the last intact, temperate ecosystems in the world. This ecosystem contains all native mammals and few exotics, and is thought to be functioning in a relatively natural state . The GYE is located in northwestern Wyoming, and contains portions of southern Montana and eastern Idaho (center of park: 44° 36' 53.25"N Latitude, 110° 30' 03.93" W Longitude). The core of the GYE is Yellowstone National Park (YNP), which was established as the world's first national park in 1872. The preservation of this park means that we are able to extend current ecological conditions to the recent past.
The A. tigrinum fossils used in this analysis were excavated from Lamar Cave, a paleontological site in YNP. The details of the excavation and stratigraphy are described elsewhere . Isotopic analysis has shown the sampling radius of the cave to be within 8 km (with 95% confidence) . Within this radius there are at least 19 fishless, modern ponds of generally similar permanence that are potential habitat for A. tigrinum. The A. tigrinum samples are most likely from predation in these ponds and surrounding lands. The current study analyzes fossils obtained from 15 of the 16 stratigraphic levels from the excavation (level 11 did not contain any Ambystoma specimens). For the analyses the levels were pooled into five intervals, labeled A-E (youngest to oldest). This aggregation was based on 95% confidence limits around the radiocarbon dating of the intervals .
Easily identified A. tigrinum fossils include femora, humeri, vertebrae, and various skull bones. We used the fossil vertebrae because of their abundance (N = 2850) and because they record metamorphic state. All vertebrae were identified, but for the purposes of this study only the first cervical and sacral vertebrae were used. Because these particular vertebrae are unique to every skeleton, they are useful in determining the minimum number of individuals from a locality . The fossils were grouped within each stratigraphic layer into four morphologically distinct classes: Young Larval, Paedomorphic, Young Terrestrial, or Old Terrestrial. The developmental stage and age of each individual was determined from diagnostic characteristics of the neural arch and centrum [32,33]. Specifically, the Young Larval had an open (unfused) neural arch and open centrum with little or no ossification; the Young Terrestrial were characterized by an open neural arch and constricted, or partially fused, centrum with little ossification; the Paedomorphic were typified by a fused neural arch and an open centrum with some ossification present; the Old Terrestrial were described by a fused neural arch and a closed, or fused, centrum with visible ossification (Figure 1).
Abundance was determined by a standardized minimum number of individuals (MNI) . The MNI was taken as the larger of the two values for sacral or cervical vertebrae (axis), since the Ambystoma skeleton contains only one of each of these elements. The abundance levels were then standardized by dividing by the MNI of the wood rat, Neotoma cinerea. Unlike other common small mammals found at this site, wood rats show a constant relative abundance . This pattern is consistent with a broad habitat preference for this species, and is especially important because the wood rat is the main collection agent of the Lamar Cave fossils. Their relative evenness thus indicates taphonomic constancy of the cave , which is corroborated with isotopic analyses .
Because plasticity in growth rate cannot be directly measured in the fossil record, it is inferred from body size in different age classes. The centrum length and anterior width of each specimen were measured with electronic calipers. A body size index (BSI) was created for each specimen by dividing the centrum length by the anterior centrum width .
Percent paedomorphosis by time interval was determined by dividing the standardized MNI of paedomorphic vertebrae by the standardized MNI of all adult vertebrae, defined as Old Larval, Young terrestrial, and Old Terrestrial morphs. Thus, we calculate abundance, mean body size, and percent paedomorphosis, each as potentially independent responses of the salamander population to the abiotic environment around Lamar Cave.