The earliest dinosaurs appeared about 245 million years ago during the Triassic Period, when most of the Earth's landmasses were still joined together as the supercontinent Pangaea.

恐龍最早大約出現在 2.45 億年前的三疊紀時期，當時地球上大部分陸地仍連接在一起，也就是超大陸「盤古大陸」。

Over millions of years, Pangaea split apart, causing the dinosaurs to become separated for the very first time.

數百萬年來，盤古大陸分裂，導致恐龍第一次分開。

This led the reptiles to adapt to their specific habitats and diversify, giving rise to many new dinosaur species.

這使得這些爬行動物為適應棲息地產生不同變化，從而產生許多新恐龍物種。

According to some estimates, more than 1,000 species of dinosaurs have roamed the Earth.

據估計，超過 1,000 種恐龍曾生存在地球上。

All dinosaurs descended from reptiles called "Archosaurs".

所有的恐龍都源自「主龍類」爬行動物。

From there, dinosaurs branched out into two major groups, Saurischia and Ornithischia, based on the shape and orientation of their pelvis.

以此為基礎，恐龍又根據骨盆形狀和方向分為兩大類，蜥臀目和鳥臀目。

In Saurischian dinosaurs, such as Tyrannosaurus Rex and Brachiosaurus, the pelvis' pubis bone faces forward and down.

蜥臀目恐龍，如霸王龍和腕龍，牠們骨盆恥骨就是朝前、朝下。

In Ornithischian dinosaurs, such as Stegosaurus and Hadrosaurs, the pubis bone faces backward and down.

而鳥臀目恐龍，如劍龍和鴨嘴龍，牠們的恥骨就是朝後、朝下。

Across both groups, dinosaurs varied greatly in size.

不論種類，恐龍大小差異十分顯著。

Some were small, like Compsognathus, which was barely larger than a chicken, whereas others were gigantic, like Dreadnoughtus, which was 85 feet long and weighed 65 tons, making it the largest land animal to have ever lived.

有些很小，例如沒有比雞大多少的細頸龍，有些則很大，如長 85 英尺，重 65 噸的無畏龍，牠是有史以來最大的陸地動物。

Dinosaur diets varied as well.

恐龍飲食也很多種多樣。

Herbivores, like Hadrosaurs, evolved to have specialized teeth for grinding tough plant material.

草食動物，如鴨嘴龍，牙齒進化成專門磨碎堅硬植物的樣子。

Carnivores made up roughly 40% of dinosaur species.

食肉恐龍約佔整體恐龍物種的 40%。

Some predators, like the raptor Deinonychus, even hunted in packs.

有些掠食者，如猛禽恐爪龍，甚至成群結隊地捕獵。

Social behaviors were also found in other dinosaurs.

在其他恐龍身上也可以發現社交行為。

Footprints and track ways indicate that some of the ancient reptiles traveled together.

一些足跡及線索顯示這些遠古爬行動物會結伴旅行。

Evidence also shows that herds may have made annual visits to certain sites to lay their eggs.

證據還表明，恐龍可能每年都會到固定地點下蛋。

But by around 66 million years ago, most dinosaurs died out.

但到了約 6600 萬年前，大多數恐龍都滅絕了。

And the reason why is still a mystery.

而箇中原由至今仍是個謎。

The most well-known explanation is an asteroid strike, but a multitude of factors may have contributed to this extinction.

最著名的解釋是小行星撞擊，但有許多因素都可能導致恐龍滅絕。

In fact, the dinosaur population was already in severe decline by the time the asteroid struck.

事實上，小行星撞擊時，恐龍數量早已嚴重下降。

However, several dinosaur species survived, and some of which evolved to become today's birds.

然而，一些恐龍倖存了下來，演化成了今日的鳥類。

The world has never again seen land creatures as great as the dinosaurs, but through their descendants and fossils left behind, their legacy lives on.

世界上再沒有像恐龍這樣宏偉的陸地生物，但透過留下後裔及化石，牠們得以長存。