Is this rude email etiquette towards a Biology professor.?

So I had a small email conversation with a bio professor at my university and I was wondering if my last email was kind of rude. He ended his reply to my first email with his first name, so I didn't know whether or not to call him Dr. LastName again or by his first name. So I just didn't address him (I... show more So I had a small email conversation with a bio professor at my university and I was wondering if my last email was kind of rude. He ended his reply to my first email with his first name, so I didn't know whether or not to call him Dr. LastName again or by his first name. So I just didn't address him (I didn't say hi or anything) when I responded to his reply. Is this rude, weird, or just both?
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Also isn't it weird calling a PHD professor by his first name?
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