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Description
Archelon ischyros

In the Late Cretaceous, around 76,000,000 years ago, North America had an ocean dividing it right in the middle. Known as the Western Interior Seaway (WIS), it was home to many species of plesiosaurs, sharks, mosasaurs and turtles. Sea turtles made it to the ocean around 120-130,000,000 years ago. By this time, a myriad of turtles had appeared in the ocean. However, one of the largest was Archelon ischyros.

Described in 1895, the original specimen was found in South Dakota near the Pierre Shale. The find consisted of a relatively complete skeleton and was described by Yale paleontologist George Weber Wieland. He named the animal Archelon ischyros. Its generic name derives from the Greek words "Arche" meaning ancient/first and "chelone" meaning turtle. The specific name, "ischyros" means "mighty" or "powerful".

Superficially, it resembled modern sea turtles, but on a much bigger scale. The largest extant sea turtle is the leatherback turtle, clocking in at an average size of 1500lbs and being around 7ft long, it's one of the largest reptiles alive today. Archelon however would have dwarfed even larger leatherback specimens being over double the size. The average specimens would have clocked in at around 2.5 tons in weight and gotten to 15ft in length.
Compared to other sea turtles, Archelon had a narrower head and a more pronounced beak than other sea turtles. The size and shape of its beak suggest it was used for crushing shellfish like the various ammonites and nautiloids in the seaway. As well as the various jellyfish in the seaway like modern-day sea turtles prey upon. The top of their shell is believed to be covered in toughened skin similar to the extant leatherback and not a hardened carapace like most modern-day sea turtles. Furthermore, biomechanical studies indicate that this turtle was not built for speed, understandable given its size. However, paleontologists have suggested this animal had a similar build compared to the predatory Loggerhead Turtle. An animal capable of efficient open-ocean travel. Like many sea turtles, it would have needed to return to land to lay eggs. It can be assumed that a mother Archelon returned to the shores of Laramidia (western USA) and/or Appalachia (eastern USA) and dug out a pit to deposit her eggs, bury them in the sand and then she'd return to the oceans.
Archelon and its kin, the Protostegid sea turtles were once thought to be related to leatherback sea turtles. Most of the species had similar morphology compared to the leatherback. However, recent cladistic studies have not supported this theory. It's now believed that Archelon and Protostegids, in general, were believed to be the most basal of all sea turtles and were among the first to ever enter the oceans around 125,000,000 years ago. Fossils of Protostegids have been found across the world from the Americas, Australia and Europe. Archelon was the largest of all of them. Swimming in the WIS from as far south as the Gulf of Mexico and as far North into Canada.

The habitat was reminiscent of the tropical seas and oceans we see today, however, the deepest parts of this seaway was shallow. Its deepest parts would have been between 400-600ft deep. Archelon would have swum these waters consuming the shellfish and jellyfish that it came across. However, even for its size, it was not invulnerable. In fact, the holotype specimen found in South Dakota was partially missing a flipper (right-lower flipper). Analysis of the bone is indicative of it happening earlier in its life. There were a variety of candidates that could have inflicted this injury. Xiphactinus, a massive bony fish with menacing jaws would have made short work of the flipper when the individual was a juvenile. Mosasaurs of varying sizes swam the seaway, from 12-40ft species, these giant sea-going reptiles were the apex predators of the sea and easily could have ripped off a flipper.

Archelon and Protostegids' cause of extinction is a mystery. They've vanished from the fossil record a million year before the KT-Event. However, Archelon vanished around 74,210,000 years ago. The WIS started to recede at this time, predation on their eggs from dinosaurs (avian or non-avian) could have diminished their population as well, finally, the climate began to cool and made their habitat dwindle.
In any respect, these giant turtles died out, Protostegids would persist, but none would ever reach sizes as large as Archelon.
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Another icon from the WIS.

As I've said before with most aquatic animals, no, this won't be in Past Meets Present. Just had the itch to write about a prehistoric sea turtle and so, here it is.

The original model is from Tyranachu and no changes were made.

Also, I should add, this creature was featured in WWSM, but the idea that a Mosasaur of any size could bite it in half is ludicrous. It could bite off a flipper but that's about it.

Original model and skin by Tyranachu:
Archelon (Tyranachu) | ZT2 Download Library Wiki | Fandom

Description
Archelon ischyros

In the Late Cretaceous, around 76,000,000 years ago, North America had an ocean dividing it right in the middle. Known as the Western Interior Seaway (WIS), it was home to many species of plesiosaurs, sharks, mosasaurs and turtles. Sea turtles made it to the ocean around 120-130,000,000 years ago. By this time, a myriad of turtles had appeared in the ocean. However, one of the largest was Archelon ischyros.

Described in 1895, the original specimen was found in South Dakota near the Pierre Shale. The find consisted of a relatively complete skeleton and was described by Yale paleontologist George Weber Wieland. He named the animal Archelon ischyros. Its generic name derives from the Greek words "Arche" meaning ancient/first and "chelone" meaning turtle. The specific name, "ischyros" means "mighty" or "powerful".

Superficially, it resembled modern sea turtles, but on a much bigger scale. The largest extant sea turtle is the leatherback turtle, clocking in at an average size of 1500lbs and being around 7ft long, it's one of the largest reptiles alive today. Archelon however would have dwarfed even larger leatherback specimens being over double the size. The average specimens would have clocked in at around 2.5 tons in weight and gotten to 15ft in length.
Compared to other sea turtles, Archelon had a narrower head and a more pronounced beak than other sea turtles. The size and shape of its beak suggest it was used for crushing shellfish like the various ammonites and nautiloids in the seaway. As well as the various jellyfish in the seaway like modern-day sea turtles prey upon. The top of their shell is believed to be covered in toughened skin similar to the extant leatherback and not a hardened carapace like most modern-day sea turtles. Furthermore, biomechanical studies indicate that this turtle was not built for speed, understandable given its size. However, paleontologists have suggested this animal had a similar build compared to the predatory Loggerhead Turtle. An animal capable of efficient open-ocean travel. Like many sea turtles, it would have needed to return to land to lay eggs. It can be assumed that a mother Archelon returned to the shores of Laramidia (western USA) and/or Appalachia (eastern USA) and dug out a pit to deposit her eggs, bury them in the sand and then she'd return to the oceans.
Archelon and its kin, the Protostegid sea turtles were once thought to be related to leatherback sea turtles. Most of the species had similar morphology compared to the leatherback. However, recent cladistic studies have not supported this theory. It's now believed that Archelon and Protostegids, in general, were believed to be the most basal of all sea turtles and were among the first to ever enter the oceans around 125,000,000 years ago. Fossils of Protostegids have been found across the world from the Americas, Australia and Europe. Archelon was the largest of all of them. Swimming in the WIS from as far south as the Gulf of Mexico and as far North into Canada.

The habitat was reminiscent of the tropical seas and oceans we see today, however, the deepest parts of this seaway was shallow. Its deepest parts would have been between 400-600ft deep. Archelon would have swum these waters consuming the shellfish and jellyfish that it came across. However, even for its size, it was not invulnerable. In fact, the holotype specimen found in South Dakota was partially missing a flipper (right-lower flipper). Analysis of the bone is indicative of it happening earlier in its life. There were a variety of candidates that could have inflicted this injury. Xiphactinus, a massive bony fish with menacing jaws would have made short work of the flipper when the individual was a juvenile. Mosasaurs of varying sizes swam the seaway, from 12-40ft species, these giant sea-going reptiles were the apex predators of the sea and easily could have ripped off a flipper.

Archelon and Protostegids' cause of extinction is a mystery. They've vanished from the fossil record a million year before the KT-Event. However, Archelon vanished around 74,210,000 years ago. The WIS started to recede at this time, predation on their eggs from dinosaurs (avian or non-avian) could have diminished their population as well, finally, the climate began to cool and made their habitat dwindle.
In any respect, these giant turtles died out, Protostegids would persist, but none would ever reach sizes as large as Archelon.
___________________________________________________________________________________________
Another icon from the WIS.

As I've said before with most aquatic animals, no, this won't be in Past Meets Present. Just had the itch to write about a prehistoric sea turtle and so, here it is.

The original model is from Tyranachu and no changes were made.

Also, I should add, this creature was featured in WWSM, but the idea that a Mosasaur of any size could bite it in half is ludicrous. It could bite off a flipper but that's about it.

Original model and skin by Tyranachu:
Archelon (Tyranachu) | ZT2 Download Library Wiki | Fandom