Direct U-Th dating of vertebrate fossils with minimum sampling destruction and application to museum specimens

Although vertebrate fossils are commonly abundant in museum palaeontological collections, they are only rarely accompanied by contextual data (e.g., stratigraphic and taphonomic information) that allow them to be placed independently into reliable temporal frameworks critical for testing significant evolutionary and extinction hypotheses. Moreover, where critical samples do exist in such collections, sampling for direct geochronological analyses becomes a significant concern, especially where such sampling is destructive in nature. Here we apply a direct fossil dating, micro-drilling sampling approach that minimises damage to and destruction of precious museum specimens. We carried out a systematic U-Th dating study (n = 28 ages) of an isolated museum specimen of the extinct Palorchestes azael (megafaunal 'marsupial tapir') originally collected in 1977 from Tea Tree Cave, Chillagoe, northeastern Australia. We obtained 21 U-Th ages and constructed 230Th-age profiles across three teeth exposed in cross-section, using micro-drilling and thermal ionisation mass spectrometry. Individual sample masses were as little as 0.18 mg (U concentration 33-82 ppm), meaning that the sampling resulted in only minimal destruction of the specimen. The results show no evidence of U leaching, suggesting that the dates represent reliable minimum ages. For independent age control, we also dated calcite that had encrusted the sample (thus, providing a minimum age; n = 6) and an older calcite clast that had been reworked into the surrounding breccia at the time of burial (thus, providing a maximum age; n = 1). U-Th ages of the teeth are older than the calcite overgrowths and younger than the reworked calcite, consistent with their demonstrable relative age relationships. Collectively, the results unequivocally bracket the age of the fossil between 199.1 ± 8.9 ka and 137.4 ± 1.1 ka (2σ), adding another rare datum to inform the timing and geographic distribution of last occurrences of the species. The benefits of our dating approach of museum fossil specimens are threefold: 1) it is minimally destructive even compared with laser-ablation method; 2) the use of U vs. apparent age approach allows direct testing for potential U leaching as occasionally seen in fossil dating; and 3) the combination of fossil and associated speleothem dating provides the most robust means of securely bracketing the age of fossils that lack firm stratigraphic control.