New information about the elusive life history of early tetrapods - the earliest four-limbed vertebrates - is presented in a study published online in Nature this week. The authors re-examine in detail a group of tetrapod fossils (genus Acanthostega) from present-day Greenland and show that the specimens, which are about 365 million years old, were all juveniles at the time of death and had an aquatic lifestyle.
The transition from fish to tetrapods is one of the major events in the evolutionary history of vertebrates. However, many aspects of the life history and behaviour of the earliest tetrapods, such as Acanthostega, have long remained unknown, partly because early tetrapod fossils are rare and often fragmentary.
Sophie Sanchez and colleagues use a non-destructive technique - propagation phase contrast synchrotron microtomography - to visualize and describe four upper arm bones (humeri) of Acanthostega specimens collected from the Britta Dal Formation on Stensio Bjerg, East Greenland. This locality, which has previously yielded more than 200 skeletal elements, comprises at least 20 animals that died together, probably as a result of a drought following a flooding event. The authors use details of growth pattern in the bones (such as bone patterns that indicate arrested growth) to show that all individuals in the assemblage - even the largest - were juveniles at the time of death. Furthermore, the late onset of bone growth in the limbs of Acanthostega suggests that these juveniles were exclusively aquatic.
Based on these bone analyses, the authors suggest that Acanthostega had a long, aquatic juvenile stage and that, at least at times, the juveniles formed schools with few or no adults.