Who is common ancestor of 7th Earl of Grantham (Robert Crawley) & his 3rd cousin once-removed Matthew Crawley (who also was his son-in-law)?

On Downton Abbey, Robert (the 7th Earl of Grantham) and Matthew are 3rd cousins once-removed. Who would their common Crawley ancestor? Incidentally, Matthew and his wife Mary (Robertโ€™s daughter) would be 4th cousins.
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