A recent article published in BMC Ecology casts an interesting light on tiger salamander evolution:
Researchers analysed a late-Holocene fossil record to track morphological traits in the Tiger Salamander through the last 3,000 years. The team, led by Elizabeth Hadly from Stanford University, United States, analysed trends in the fossil record within the context of known climate change, to distinguish patterns of response correlating to specific climatic periods during this time.
The fossils were all collected from Lamar Cave in Yellowstone National Park in Wyoming, United States. The cave deposits were dated and divided into five time periods according to their estimated age. The researchers then grouped the fossils into four morphologically distinct groups: young larval, paedomorphic, young terrestrial or old terrestrial, and measured the body size index (BSI) of fossils in each group and time period.
The team found that paedomorphic individuals – sexually mature, yet still aquatic and retaining larval characteristics – were much smaller than terrestrial adult individuals, during the Medieval Warm Period (MWP). The authors claim that this is eveidence for a response to warm and dry climate conditions, which allowed a terrestrial ectotherm to flourish. They conclude that the fossil record of the Tiger Salamander reflects known climatic conditions during the MWP, a time period characterised by a warm and dry climate that occurred approximately 1150 to 650 years ago.
The entire paper can be found here