[求助]请教一道指代题

题目：A group of paleontologists recently announced that a site in Utah has yielded fossils of some of the biggest armored dinosaurs ever found, and that they were at least 25 million years older than any similar dinosaur type previously found in North America.

我的问题是答案中说划线部分的they指代不清。A group of paleontologists, a site都是单数吧，they要指代不是只能是fossils吗？怎么会指代不清呢？

还有就是recently announced that a site in Utah has，这里为什么用has呢？而不是had啊？

菜鸟一个，大虾多多指教。

they 还可以指代考古学家

先谢谢。但是they可以指代古生物学者吗？A group of paleontologists应该是整体吧。第二还有个has的问题。请大虾们继续帮忙。

你说的对 是我没看仔细