While conservationists are hard at work to keep biodiversity alive and well, it's no secret that many species have gone extinct.

While there are some well-known extinct animals, like various dinosaurs and the woolly mammoth, there are others that fewer people would know by name, like the archelon and the Castoroides.

Photo: YouTube/WTD

While long-extinct species may seem hard to imagine, many were similar to the modern species we see today - only different in size.

The data-comparison YouTube channel WTD created a 3D animation video comparing the size of existing animals and their extinct ancestors and it's fascinating to see.

Photo: YouTube/WTD

In the video, you can see that in many cases, the extinct animals were much larger than our modern extant animals. Just take the extinct Castoroides and the extant beaver, for example!

There are some exceptions though, including the Livyatan melvillei which is believed to have been just half the size of a blue whale we'd see today.

Photo: YouTube/WTD

While you can look at the numbers and read the sizes of the various species, seeing them laid out in a side-by-side 3D animation really makes it easy to conceptualize and understand.

The animals were also compared to a human, so you can get an even better idea of their size!

If you'd like to see it for yourself, be sure to check out the video below: