Retro vs Modern #06: Plesiosaurus dolichodeirus

Plesiosaurs were first recognized as a distinct group of fossil animals in the early 1820s, only a few years after ichthyosaurs. Initially they were perceived as being closer in form to reptiles in the “chain of being” than the more fish-like ichthyosaurs were, and so the group’s scientific name ended up reflecting that early interpretation – “plesiosaur” roughly translates to “near to reptiles”.

The first named species of plesiosaur was Plesiosaurus dolichodeirus, based on a near-complete skeleton discovered by Mary Anning that revealed the strange long-necked proportions of these animals for the first time.


1830s-1850s

Early reconstructions of plesiosaurs in the 1830s compared them to “a snake threaded through a turtle”, giving them highly sinuous necks and a turtle-like body. Much like ichthyosaurs they were assumed to be amphibious, using their flippers to crawl up onto the shore like a sea turtle.

The 1850s Crystal Palace plesiosaur statues show a variant of this design with smooth skin textures and fairly flexible reptilian bodies, with powerful shoulders and flipper postures that give them an overall almost seal-like appearance.


1860s-1990s

From the 1860s onwards a more upright S-shaped neck pose became the most common depiction of plesiosaurs. The writhing snake-like necks persisted in some reconstructions of the extremely long-necked elasmosaurids, but the overall design for these animals that caught hold for the next century was an egg-shaped body with oar-like flippers and a swan-like neck – a body plan that would end up so influential in pop culture that it was incorporated into modern lake monster folklore, with the Loch Ness Monster being the most famous example.

During this period plesiosaurs were often portrayed as floating or swimming at the water’s surface, rowing along with their flippers and using their long necks to snatch up prey. They were generally assumed to still haul out turtle-style to lay their eggs on the shore, although it wasn’t clear how the very largest species would have been able to support their own weight.


2020s

Since the 1990s a boom of new plesiosaur species and biomechanical studies have brought a lot of changes to our understanding of these marine reptiles.

Their necks are now considered to have been less flexible, capable only of more gentle curving, and were probably much thicker and more streamlined with the body than previously depicted. Rather than oar-like rowing all four of their flippers were probably used in more of an “underwater flying” vertical motion similar to modern sea turtles – which is pretty fitting, considering that their closest living relatives are now thought to actually be turtles.

They gave live birth and were probably warm-blooded, with a thick layer of insulating blubbery fat and a teardrop-shaped body outline. Their skin texture was smooth, but one exceptionally well-preserved specimen shows a covering of tiny thin millimeter-sized scales that wouldn’t have been visible in life except in extreme closeup.

We now know Plesiosaurus itself was a fairly small species, around 3.5m long (~11’6″), with a broad body and a short thick tail that probably had a rudder-like fin – usually assumed to be vertically-oriented, but possibly horizontal instead. It lived during the Early Jurassic, about 201-183 million years ago, in the shallow tropical sea that covered what is now southern England, and had a rather small head compared to other plesiosaurs, with its eyes facing upwards and to the sides.

It had sharp needle-like teeth that would have been used to catch soft-bodied aquatic prey like fish and cephalopods. It’s not known whether it had extensive fleshy lips, croc-like snaggletoothed jaws, or something in-between, so the facial soft tissue on this particular reconstruction is rather speculative.

Retro vs Modern #05: Ichthyosaurus communis

Fossilized ichthyosaur bones have been found for centuries, but were initially misidentified as being the remains of fish, dolphins, and crocodiles. More complete skeletons began to be discovered in the early 19th century – particularly by pioneering paleontologist Mary Anning – and Ichthyosaurus communis was one of the first species of these ancient “fish lizards” to be scientifically recognized.


1830s-1870s

Early reconstructions of ichthyosaurs in the 1830s depicted flippered crocodile-like animals with long straight eel-like tails and strangely shrinkwrapped features, showing the sclerotic rings of their eyes and the internal bones of their flippers as highly visible externally. They were also frequently portrayed as being amphibious, hauling themselves out of the water to bask.

By the late 1830s impressions of smooth scaleless skin had been found, and specimens with tail-tips that were always “broken” in the exact same place were interpreted as evidence of the presence of some sort of paddle-like tail fin. The 1850s Crystal Palace Ichthyosaurus statues show this slightly updated version, along with a low dorsal ridge on their backs reminiscent of a beluga whale.


2020s

From the 1880s onwards the discovery of exceptional ichthyosaur specimens preserving whole body outlines revealed a fully aquatic streamlined shape, a triangular dorsal fin, and a crescent-shaped vertical tail fluke. Numerous examples of fossilized pregnant females also showed that ichthyosaurs gave live birth rather than laying eggs.

This highly dolphin-like version of ichthyosaurs quickly caught on and became the standard depiction into the early 20th century, frequently showing them as highly active animals – swimming in groups, chasing fish and ammonites, and leaping dramatically out of the water like their modern cetacean counterparts. While we don’t actually know if they were social or acrobatic like dolphins, it was still a surprising and refreshing contrast to the increasingly lumpy and sluggish depictions of non-avian dinosaurs that were happening around the same time.

Actual further paleontological study on ichthyosaurs was scarce for decades, however, with a general attitude that the group was already scientifically “complete” and there wasn’t much new or interesting left to learn about them anymore. It wasn’t until the late 20th century that they began to have their own “ichthyosaur renaissance” alongside the dinosaurs, with a sharp rise in research in the last few decades bringing us a lot of new information about their diversity and biology.

Ichthyosaurus communis was just one of several species in the Ichthyosaurus genus, living during the Early Jurassic, about 196-183 million years ago, in the shallow tropical seas of what is now Europe. About 3.3m long (~11′), it was adapted for high-speed long-distance swimming like a modern tuna, and it probably had a large keeled peduncle on the sides of its tail.

Bone structure and isotope analysis show that ichthyosaurs were all warm-blooded. One exceptional specimen also preserves an insulating layer of cetacean-like blubber, along with some evidence of its coloration: overall darker on the top and lighter on the underside in a countershaded pattern.

(I’ve given this reconstruction some speculative disruptive camouflage, too.)

Some of the preserved pigmentation has enough microscopic detail to show what appear to be branched melanophore cells associated with the ability to change color – suggesting that ichthyosaurs may have been able to actively darken and lighten their coloration like some modern lizards.

Ophthalmothule

The cryptoclidids were fairly standard-looking plesiosaurs, with long necks and small heads – but those tiny skull bones were also rather fragile and so there’s very little good fossil material of their heads, making it difficult to figure out both their feeding ecology and their exact evolutionary relationships.

But a recently-discovered specimen from the Svalbard archipelago actually preserved a mostly-complete skeleton, including an unusually intact skull.

Given the name Ophthalmothule cryostea (meaning “frozen bones of the Northern eye”), this cryptoclidid lived about 145 million years ago, right at the boundary between the Jurassic and the Cretaceous.

It measured around 5m long (16’5″) and had proportionally huge eyes that faced upwards on its head – an adaptation for seeing in low-light underwater conditions, maximizing the amount of light reaching it from above.

Those big dark-adapted eyes suggest it may have been nocturnal, or spent a lot of time diving into very deep waters in search of food. Its skull had weak jaw muscles and delicate teeth, and its gut region contained a lot of fine gravelly sediment, so it probably mainly grubbed around for small soft-bodied prey on the sea floor.

At that point in time Svalbard would have been a little further south than it is today, at a subarctic latitude, but the area would have still experienced particularly long nights during the winter. So it’s possible Ophthalmothule also developed such big sensitive eyes to help it survive through those darker seasons.

Ichthyosaur Blubber

In early 2017 evidence of blubber was found in plesiosaurs, indicating that they were probably much more chubby than they’re usually reconstructed, and now in late 2018 it’s been found in an ichthyosaur, too!

Living during the Early Jurassic (~183-179 mya) in the shallow seas that covered most of Europe at the time, Stenopterygius was an average-sized ichthyosaur growing up to about 4m in length (13′). A fossil found in Germany has some incredibly good soft-tissue preservation, showing smooth flexible scaleless skin, a layer of insulating blubber very convergently similar to that found in cetaceans, and even evidence of countershaded coloration.

While the confirmation of blubber is amazing, and gives further evidence that ichthyosaurs were warm-blooded, the color preservation might actually be even more interesting. The skin pigmentation is preserved in enough fine detail for branched melanophores to be visible under a microscope – a type of cell associated with the ability to change color. So there’s a possibility that ichthyosaurs could actively darken or lighten their color patterns, for purposes such as better camouflage, UV protection, or temperature regulation.