Early Vendel Period baldric mounts from the Ottarshögen barrow, Vendel parish, Uppland.
Archaeological periods are defined by artefact types. For instance, the Early Neolithic of Sweden is defined by the appearance of Funnel Beaker pottery, thin-butted flint axes and pointed-butt axes (and a long list of other things). Before these types appear, the period has not begun. When they cease to be produced and are replaced by other types, we are no longer in the Early Neolithic. This means that archaeological chronology is largely structured around long lists of artefact types with as stringent definitions as possible, allowing us to place each closed find in its own pigeon hole. Such relative chronology is our backbone, and various techniques are used to fix the calendar dates of each pigeon hole in absolute chronology. This is tricky for many reasons, and absolute chronology is constantly refined (though the changes made to it become smaller and smaller over the decades as more and better data become available).
In the mid-to-late 1st Millennium, great grave barrows were built in much of agricultural southern Sweden. In his massive 1936 doctoral thesis, Sune Lindqvist pigeonholed the largest ones in the later Migration Period, whose end date he took to be about AD 600. The end of this period and the beginning of the Vendel Period marks the most dramatic change in material culture since the introduction of metalworking to Sweden over two millennia earlier. We now know that this cultural shift actually occurred about AD 540, and that it may have had to do with a volcanic eruption documented in dendrochronology and written sources that clouded the atmosphere for several years and caused crops to fail disastrously.
The great grave barrows of Old Uppsala and Vendel date from the late 6th century and the decades around AD 600. Lindqvist actually placed them quite well in the absolute chronology. But he also stated, erroneously, that they belonged to the Migration Period, whose absolute dates have since been modified, and this is what almost every Swedish archaeologist still remembers from his basic training. Having little reason to look into matters like these as they do their daily fieldwork, everyone therefore believes that the barrows date from about AD 500. Yet, looking at a helmet, a ceremonial staff and baldric mounts from these barrows, it is clear that they belong in the Vendel Period pigeon hole of the relative chronology.
Now, why does this matter? Most importantly, because we now know that the great barrows do not mark the end of a Migration Period elite: on the contrary, they are monuments to newly risen leaders in the radically changed political landscape of the Early Vendel Period.
Thanks to Svante Fischer for making me check what was actually found in the Ottarshögen barrow. Until I did, I still believed it to be a Migration Period burial.