<Tr> <Td> Tahirids </Td> <Td> 821--873 </Td> </Tr> <Tr> <Td> Saffarids </Td> <Td> 863--900 </Td> </Tr> <Tr> <Td> Samanids </Td> <Td> 875--999 </Td> </Tr> <Tr> <Td> Ghaznavids </Td> <Td> 963--1187 </Td> </Tr>

What were key events of the umayyad dynasty