About the usage of 'reckon'
The passage below comes from Sapiens by Yuval Noah Harari.
http://erenow.com/common/sapiensbriefhistory/11.html Living around what is now Adelaide in southern Australia were several patrilineal clans that reckoned descent from the father’s side.I'd like to ask a question about the underlined 'reckoned'.
I don't understand why this world-famous author chose to use 'reckoned' instead of 'are reckoned' the way this word is usually used.
For the chance of there's something I miss, I checked out a few dictionaries, but I couldn't find none that suggests this aberrant usage.
I need your help.
Regards.