Giant ammonite

(Dog for size comparison)

Few of the ammonites occurring in the lower and middle part of the Jurassic period reached a size exceeding 23 cm (9 in) in diameter. Much larger forms are found in the later rocks of the upper part of the Jurassic and the lower part of the Cretaceous, such as Titanites from the Portland Stone of Jurassic of southern England, which is often 53 cm (2 ft) in diameter, and Parapuzosia seppenradensis of the Cretaceous period of Germany, which is one of the largest known ammonites, sometimes reaching 2 m (6.5 ft) in diameter. The largest documented North American ammonite is Parapuzosia bradyi from the Cretaceous, with specimens measuring 137 cm (4.5 ft) in diameter.

Via the Pics subreddit.