Aren't you begging the question here? I think you need to establish that your premise is correct in the first place; that the US congress was rowdier and that it did change. I am not an expert on the subject, but my impression is that it's always been a more subdued gathering than parliament.

In his book covering LBJ's Senate Years "Master of the Senate," Robert Caro covers a lot of American Legislative history.

I would not describe the Senate ever as "informal." It always has had a set of rules governing conduct to maintain a gentlemanly vibe. Like when directly addressing Chuck Schumer you wouldn't call him by name but you might say, "The Senior Senator from New York."

But the Senate did have a different vibe back in the 1800s when it was both smaller and more powerful. Back then the Senate had more power when it came to foreign policy and even domestic policy than it did after the presidencies of the Roosevelts who concentrated more power in the executive office.

So in a way, the smallness of the Senate, combined with it's power, led to way more heated exchanges.