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Description
Elasmosaurus platyurus

Plesiosaurs were one of the most diverse and interesting groups of tetrapods that ever took to the water. By the end of the Cretaceous, they took had dominated most of the oceans. With some reaching 40 feet in length and filling niches like the dolphin-like Polycotylids replacing the Ichthyosaurs at the dawn of the Cretaceous to the powerful Pliosaurs with massive jaws.

Among the most iconic was Elasmosaurus. Its name literally means "flat-plate lizard" about the animal's flat bones in the pelvic region. Existing around 80,000,000 years ago, the type genera for the Elasmosaurine plesiosaurs was over 30 feet long and 3 tons and weight and a long 20+ foot long neck. With a small head at the end of the neck, tipped with peg-like teeth. Swimming through the vast Western Interior Seaway that cut through North America during the Late Cretaceous period.

Described by Edward Drinker Cope in 1869 in a paper he published and a drawing of what he thought was the creature's skeleton. He thought its head was positioned at the tail, and the long 20+ft neck was an incomplete tail. However, his mentor Joseph Leidy pointed out the error, and he tried to buy back all the journals. However, he was unsuccessful. His old friend, and now rival, Othniel Charles Marsh, also pointed this out in a New York Herald article. He said that "I informed Professor Cope of it his wounded vanity received a shock from which it has never recovered, and he has since been my bitter enemy"[New York Herald (oceansofkansas.com) ].
This was the dawn of the Bone Wars, and soon, the two men would be locked in a scientific endeavor that would both expand the field of Paleontology and subsequently ruin their reputations.

Cope and other early paleontologists up to the 1950s used to think these animals rowed themselves through the water. Their flippers acting as oars. However, it's believed this is an inefficient method of locomotion through the water. Paleontologist Jane Robinson proposed they flew through the water using alternating strokes of their forelimbs and hindlimbs. Studies from the 80s and 2010 showed the model could have worked, especially if they varied the timing between strokes of their flippers. Elasmosaurus would likely have done this as well. Other theories suggest they flapped all 4 flippers at once, but paleontologists will never know how these magnificent animals swam without observing a real-life creature.

The soft tissue of Mesozoic Marine life, excluding the famous Ichthyosaurs, has only recently begun to see the light. Of course, tail-flukes in Mosasaurs have emerged, but Plesiosaurs have been more enigmatic. Discoveries have emerged, painting a more vivid picture of how these animals lived.
First and foremost, they likely had a tail-fluke. Studies from 2010-2013 indicate that Cryptoclidus, Pantosaurus, and Rhomaleosaurus had horizontally positioned flukes on their tails. Compared to Ichthyosaurs and Mosasaurs, it was not that big. And given the structure of their tail and their powerful flippers, it was not used in any form of locomotion. However, it probably decreased drag acting on their bodies when moving through the water. Its shape is unknown, but many paleoartists have restored it to look like some amphibian tails such as salamanders. Until a fossil mummy of a Plesiosaur is found, it's pure conjecture as to what these animals' flukes looked like.
Furthermore, the flippers probably had more flesh on them, making them far broader than the fossils show. A broader flipper would have given them more power in how they swam through the water.

Most Plesiosaurs have long necks, but Elasmosaurs, Elasmosaurus itself, and its kin included had the longest necks proportional to their bodies. A 34-foot specimen would have had a neck ~23 feet in length with an 11ft-long body and tail after.
Early Paleoart and skeletal restorations show it with a swan-like "S" curve held above the water. Cope theorized this too, thinking the neck would have been like a snake. However, from studying the vertebra, it is now accepted that for any plesiosaur, even the long-necked Elasmosaurs, that position would be impossible. Recent studies indicate their necks had a range of 75–177˚ of ventral movement, 87–155° of dorsal movement, and 94–176° of lateral movement. It would have been impossible for them to achieve the swan-shaped neck curve.
The use of the necks was probably for feeding purposes, but what the feeding methods looked like is a mystery. The Cretaceous seas were home to Pliosaurs, Mosasaurs, and sharks. Apex predators could have hunted Elasmosaurus. Even with their size, their necks would have been a soft target for predators.
While it's not known what function their neck was used for, there is a theory on how they could have hunted. They could have used their long necks for grazing off schools of fish in the Western Interior Seaway. Sneaking up from the deaths before ambushing the schools and plucking them using their sharp peg-like teeth to pierce the flesh and then swallow them down. Paleontologists also have found gastroliths in Plesiosaurs to help them process food. Meaning they didn't chew their food. They ate small fish and swallowed them down to have them digested in their gut. It can be assumed that Elasmosaurus did the same.

The Western Interior Sea Way was shallow, only around 2,500 feet deep at its deepest point. Fossils of Elasmosaurus have been found in the Pierre Shale Formation in Kansas, which would have been open water. So Elasmosaurus was probably an open-ocean pelagic animal, hunting the shoals of fish that congregated in open water. Elasmosaurus competed with creatures like Mosasaurs, smaller Plesiosaurs like Dolichorhynchops, large Pliosaurs like Kronosaurus, and sharks Cretoxyrhina. If Elasmosaurus did graze off schools of fish in the Seaway, they likely filled niches occupied by cetaceans of today. However, where cetaceans use bursts of speed to catch fish, Elasmosaurus used its neck to catch them.

When the Western Interior Seaway started to retreat some ~80-75,000,000 years ago, Elasmosaurus and its kin vanished from the fossil record in North America. Its kin would survive up to the KT-Event when an Asteroid struck the earth some 66,000,000 years ago.
___________________________________________________________________________________________
I haven't done a marine reptile so, I thought I'd do my favorite marine reptile.

This was a minor mod to Tyranachu's Elasmosaurus. I beefed up the neck a little to give it more of a modern restoration of how Paleontologists think Elasmosaurus looked. The original one looked fine, but I still wanted to adjust it a little to bring it more in line with other restorations.

Is this going to be in Past Meets Present? No. There's no way I can realistically rescue marine creatures. And considering we can't even keep Orcas in captivity, how the hell would we manage giant Elasmosaurs? I just threw this together in literally 5 minutes doing quick edits in Blender.

Elasmosaurus by Tyranachu, modified by me:
Elasmosaurus (Tyranachu) | ZT2 Download Library Wiki | Fandom

Description
Elasmosaurus platyurus

Plesiosaurs were one of the most diverse and interesting groups of tetrapods that ever took to the water. By the end of the Cretaceous, they took had dominated most of the oceans. With some reaching 40 feet in length and filling niches like the dolphin-like Polycotylids replacing the Ichthyosaurs at the dawn of the Cretaceous to the powerful Pliosaurs with massive jaws.

Among the most iconic was Elasmosaurus. Its name literally means "flat-plate lizard" about the animal's flat bones in the pelvic region. Existing around 80,000,000 years ago, the type genera for the Elasmosaurine plesiosaurs was over 30 feet long and 3 tons and weight and a long 20+ foot long neck. With a small head at the end of the neck, tipped with peg-like teeth. Swimming through the vast Western Interior Seaway that cut through North America during the Late Cretaceous period.

Described by Edward Drinker Cope in 1869 in a paper he published and a drawing of what he thought was the creature's skeleton. He thought its head was positioned at the tail, and the long 20+ft neck was an incomplete tail. However, his mentor Joseph Leidy pointed out the error, and he tried to buy back all the journals. However, he was unsuccessful. His old friend, and now rival, Othniel Charles Marsh, also pointed this out in a New York Herald article. He said that "I informed Professor Cope of it his wounded vanity received a shock from which it has never recovered, and he has since been my bitter enemy"[New York Herald (oceansofkansas.com) ].
This was the dawn of the Bone Wars, and soon, the two men would be locked in a scientific endeavor that would both expand the field of Paleontology and subsequently ruin their reputations.

Cope and other early paleontologists up to the 1950s used to think these animals rowed themselves through the water. Their flippers acting as oars. However, it's believed this is an inefficient method of locomotion through the water. Paleontologist Jane Robinson proposed they flew through the water using alternating strokes of their forelimbs and hindlimbs. Studies from the 80s and 2010 showed the model could have worked, especially if they varied the timing between strokes of their flippers. Elasmosaurus would likely have done this as well. Other theories suggest they flapped all 4 flippers at once, but paleontologists will never know how these magnificent animals swam without observing a real-life creature.

The soft tissue of Mesozoic Marine life, excluding the famous Ichthyosaurs, has only recently begun to see the light. Of course, tail-flukes in Mosasaurs have emerged, but Plesiosaurs have been more enigmatic. Discoveries have emerged, painting a more vivid picture of how these animals lived.
First and foremost, they likely had a tail-fluke. Studies from 2010-2013 indicate that Cryptoclidus, Pantosaurus, and Rhomaleosaurus had horizontally positioned flukes on their tails. Compared to Ichthyosaurs and Mosasaurs, it was not that big. And given the structure of their tail and their powerful flippers, it was not used in any form of locomotion. However, it probably decreased drag acting on their bodies when moving through the water. Its shape is unknown, but many paleoartists have restored it to look like some amphibian tails such as salamanders. Until a fossil mummy of a Plesiosaur is found, it's pure conjecture as to what these animals' flukes looked like.
Furthermore, the flippers probably had more flesh on them, making them far broader than the fossils show. A broader flipper would have given them more power in how they swam through the water.

Most Plesiosaurs have long necks, but Elasmosaurs, Elasmosaurus itself, and its kin included had the longest necks proportional to their bodies. A 34-foot specimen would have had a neck ~23 feet in length with an 11ft-long body and tail after.
Early Paleoart and skeletal restorations show it with a swan-like "S" curve held above the water. Cope theorized this too, thinking the neck would have been like a snake. However, from studying the vertebra, it is now accepted that for any plesiosaur, even the long-necked Elasmosaurs, that position would be impossible. Recent studies indicate their necks had a range of 75–177˚ of ventral movement, 87–155° of dorsal movement, and 94–176° of lateral movement. It would have been impossible for them to achieve the swan-shaped neck curve.
The use of the necks was probably for feeding purposes, but what the feeding methods looked like is a mystery. The Cretaceous seas were home to Pliosaurs, Mosasaurs, and sharks. Apex predators could have hunted Elasmosaurus. Even with their size, their necks would have been a soft target for predators.
While it's not known what function their neck was used for, there is a theory on how they could have hunted. They could have used their long necks for grazing off schools of fish in the Western Interior Seaway. Sneaking up from the deaths before ambushing the schools and plucking them using their sharp peg-like teeth to pierce the flesh and then swallow them down. Paleontologists also have found gastroliths in Plesiosaurs to help them process food. Meaning they didn't chew their food. They ate small fish and swallowed them down to have them digested in their gut. It can be assumed that Elasmosaurus did the same.

The Western Interior Sea Way was shallow, only around 2,500 feet deep at its deepest point. Fossils of Elasmosaurus have been found in the Pierre Shale Formation in Kansas, which would have been open water. So Elasmosaurus was probably an open-ocean pelagic animal, hunting the shoals of fish that congregated in open water. Elasmosaurus competed with creatures like Mosasaurs, smaller Plesiosaurs like Dolichorhynchops, large Pliosaurs like Kronosaurus, and sharks Cretoxyrhina. If Elasmosaurus did graze off schools of fish in the Seaway, they likely filled niches occupied by cetaceans of today. However, where cetaceans use bursts of speed to catch fish, Elasmosaurus used its neck to catch them.

When the Western Interior Seaway started to retreat some ~80-75,000,000 years ago, Elasmosaurus and its kin vanished from the fossil record in North America. Its kin would survive up to the KT-Event when an Asteroid struck the earth some 66,000,000 years ago.
___________________________________________________________________________________________
I haven't done a marine reptile so, I thought I'd do my favorite marine reptile.

This was a minor mod to Tyranachu's Elasmosaurus. I beefed up the neck a little to give it more of a modern restoration of how Paleontologists think Elasmosaurus looked. The original one looked fine, but I still wanted to adjust it a little to bring it more in line with other restorations.

Is this going to be in Past Meets Present? No. There's no way I can realistically rescue marine creatures. And considering we can't even keep Orcas in captivity, how the hell would we manage giant Elasmosaurs? I just threw this together in literally 5 minutes doing quick edits in Blender.

Elasmosaurus by Tyranachu, modified by me:
Elasmosaurus (Tyranachu) | ZT2 Download Library Wiki | Fandom