Human red blood cells found in Carboniferous fossil

A Carboniferous fossil found in Pennsylvania
shows round red blood cells over 10 micrometer in diamter, as well as
bone cells. They should be human red blood cells, because non-mammals
do not have "round", "nucleus-less" red blood vessels while no other
mammals have such large red blood cells.
These red blood cells are shown at:
http://www.wretch.cc/album/album.php?id=lin440315&book=3&page=1

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