It seems that a post-coital female ostracod became trapped in tree resin about 100 million years ago, and was recently discovered by scientists. The sperm within her body is likely twice as old as the next oldest known animal sperm specimen, but it is also nearly five times the size of the male ostracod that produced it! according to a co-author of the study:

"This is equivalent to about 7.30 metres in a 1.70-metre human, so it requires a lot of energy to produce them"

While the article suggests that the next oldest sperm sample is a mere 17 million years, an article in The Independent highlights a 50 million year old find.

Related:

• World’s oldest sperm found perfectly preserved after 100m years – and it’s gigantic
• Scientists find world’s oldest sperm in Myanmar amber
• Meet the world’s oldest baby – made with 23-year-old sperm
• World’s oldest sperm is discovered in Antarctica