Plesiosauroidea is an extinct clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 of carnivorous

Carnivore

A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...

 plesiosaur

Plesiosaur

Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods...

 marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 reptiles. Plesiosauroids, are known from the Jurassic

Jurassic

The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 and Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 Periods. After their discovery, some plesiosauroids were somewhat fancifully said to have resembled "a snake threaded through the shell of a turtle", although they had no shell.

Plesiosauroidea appeared at the Early Jurassic

Early Jurassic

The Early Jurassic epoch is the earliest of three epochs of the Jurassic period...

 Period (late Sinemurian

Sinemurian

In the geologic timescale, the Sinemurian is an age or stage in the Early or Lower Jurassic epoch or series. It spans the time between 196.5 ± 2 Ma and 189.6 ± 1.5 Ma...

 stage) and thrived until the K-T extinction

Cretaceous-Tertiary extinction event

The Cretaceous–Paleogene extinction event, formerly named and still commonly referred to as the Cretaceous-Tertiary extinction event, occurred approximately 65.5 million years ago at the end of the Maastrichtian age of the Cretaceous period. It was a large-scale mass extinction of animal and plant...

, at the end of the Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 Period.