U-PB AGE OF THE OLDEST AIR-BREATHING LAND ANIMAL

The oldest air-breathing land animal found to date, the millipede Pneumodesmus newmani, was found in the Cowie harbour fish bed at Cowie, Aberdeenshire, Scotland. Despite it’s importance for concepts of land colonization, its age is debatable as the fossil occurs in a tectonically isolated, non-marine succession that is difficult to correlate with the standard marine-based time scale. At present it is assigned a late Wenlock to early Ludlow age based on terrestrial palynomorphs. However, there are no precise radiometric dates from the succession. In this study, we dated two volcaniclastic beds below and above the fish bed. Zircon grains were extracted from the samples by heavy mineral separation and dated using the Laser Ablation-Inductively Coupled Plasma-Mass Spectrometer (LA-ICP-MS) at the University of Texas at Austin. The most precise U-Pb zircon ages from the lowermost bed are 414.3 +/- 7.1 Ma and 418.7 +/- 4.4; while the uppermost bed gave ages of 413.8+/- 6.3 Ma and 410.4 +/- 7.2 Ma. Using the youngest ages, the fish bed is between ca. 414-410 Ma in age. The current age for the Silurian-Devonian boundary is 419.2 +/-3.2 Ma. Therefore, the Cowie fish bed and its air-breathing millipede are lowermost Devonian in age, contrary to the previously accepted Silurian age. This has important implications for the evolutionary timetable of land colonization, as the Cowie age range overlaps a U/Pb zircon age of 411.5 ± 1.3 Ma for andesite just below the nearby Rhynie Chert with its much more advanced and better preserved terrestrial biota.