In the 30 years he’s been working to identify, preserve and study fossils in our national parks, that number has gone up to 265 of the 414 units of the National Park System – in part, thanks to his work.

我尝试解释，不一定正确。看其他老师是否有不同观点。

如果将he's been working..., 和that number has gone up...看成并列句，则少了并列连词and. 

是否可以将he's been working...看作是省略了关系副词that（when不能省略）的定语从句，修饰30 years。这样的话，that number has gone up...就是主句了。

译文：在他一直对我们国家公园的化石进行鉴定、保护和研究的这三十年时间里，这一数字已经增加到414个国家公园单位中的265个，其中一部分要归功于他的贡献。