In Genesis 38:1-10, the detailed account of Er and Onan's deaths explains why they are listed as deceased here.

In Genesis 46:12, the genealogy of Judah includes Er and Onan who died in Canaan—same record.

In 1 Chronicles 2:3-8, the Judahite genealogy includes Er and Onan, confirming their deaths.

In Genesis 38:3, Er is born—the same person whose death is noted in this census.

In Genesis 38:4, Onan is born—the same person whose death is noted in this census.

Genesis 38:7 explains Er's death was due to his wickedness, supplementing the bare record of his death in this census.

Genesis 38:10 explains Onan's death was due to his wicked act, giving reason for his absence from the tribe.