Field records.

A giant ocean-going lizard with powerful jaws, paddle-shaped limbs and a deep, two-lobed tail. Mosasaurs had flexible skulls, but did not simply dislocate their jaws like movie snakes.

Scale

Large individuals roughly 12–17 m

Fossil range

Europe, North America and other Late Cretaceous seas

A large mosasaur with an elongated, toothless tip to its snout. Fossils and stomach contents show an opportunistic predator of fish, sharks, seabirds and other marine reptiles.

Scale

About 12–14 m at the upper end

Fossil range

Western Interior Seaway of North America

A robust-skulled mosasaur with stout teeth suited to resisting heavy loads. Related Prognathodon species processed turtles, ammonites and other hard prey.

Scale

A large mosasaur; estimates vary

Fossil range

Known from Maastrichtian marine deposits

One of the largest described pliosaurs: a short-necked marine reptile propelled by four enormous flippers. Its high-Arctic fossil provenance is genuine.

Scale

Roughly 10–13 m; incomplete remains

Fossil range

Svalbard, Norway

A massive-headed pliosaur with conical teeth and a compact, powerful body. Historical reconstructions were sometimes assembled too long, so modern size estimates are more restrained.

Scale

Common estimates around 9–11 m

Fossil range

Australia; close relatives known elsewhere

A long-necked plesiosaur with a small head, fish-catching teeth and four flippers. Its neck was flexible, but not capable of the upright swan pose common in old artwork.

Scale

Roughly 10–14 m, over half of it neck

Fossil range

Western Interior Seaway of North America

A semi-aquatic marine reptile with a long body and interlocking teeth. Its limbs were less specialized than those of later plesiosaurs, supporting a life divided between water and shore.

Scale

Large species reached several meters

Fossil range

Europe and adjacent Triassic seas

An enormous megatooth shark known from teeth, vertebrae and feeding traces on whale bone. Its exact body shape remains debated because cartilage rarely fossilizes.

Scale

Most modern estimates place adults near 15–20 m

Fossil range

Warm and temperate seas worldwide

A shark-like chondrichthyan more closely related to modern ratfish, famous for a spiral whorl of teeth housed in its lower jaw. CT studies resolved a puzzle that lasted more than a century.

Scale

Possibly 5–8 m; estimates remain uncertain

Fossil range

Fossils from North America, Eurasia and Australia

A large lamniform shark known from exceptionally complete fossils and bite traces on marine reptiles. Its streamlined anatomy supports an active, fast-swimming lifestyle.

Scale

Approximately 6–7 m

Fossil range

Widespread, especially well known from North America

A freshwater xenacanth—an extinct shark lineage—with a long body, double-cusped teeth and a prominent spine behind the skull.

Scale

Up to roughly 3 m

Fossil range

Freshwater systems of Europe and North America

A shark-like chondrichthyan with curved tooth blades in the upper and lower jaws. Fossils preserve the teeth far better than the body, and its feeding mechanics remain actively debated.

Scale

Large species may have approached 6 m

Fossil range

Marine deposits in North America and Eurasia

An armored placoderm whose sharpened bony jaw plates acted as cutting edges. Recent body-proportion studies favor a shorter, deeper animal than the old 8–9 meter reconstructions.

Scale

Likely around 3.5–4.5 m for most large adults

Fossil range

North America, Europe and North Africa

A large predatory bony fish with an upturned jaw and long teeth. The celebrated “fish within a fish” fossil preserves a nearly intact Gillicus swallowed whole.

Scale

Usually 4–6 m

Fossil range

Cretaceous seas of North America and beyond

The largest known ray-finned fish was a suspension feeder, using delicate gill structures to strain small organisms from seawater.

Scale

Perhaps 9–16 m; estimates are difficult

Fossil range

Marine deposits in Europe and South America

A recurring sonar return whose movement, scale and depth profile resist comparison with known vessels or catalogued marine life.

Scale

Estimate withheld

Fossil range

High-latitude deep water