marine reptiles – Nix Illustration

Xiphodracon

Xiphodracon goldencapensis was an ichthyosaur that lived in marine waters covering what is now the Jurassic Coast of the southern United Kingdom during the Early Jurassic, about 188 million years ago.

Around 3m long (~10′), it had fairly large eyes and a long narrow snout lined with small slender pointed teeth.

It was part of the leptonectid family, closely related to other long-snouted forms like Eurhinosaurus. Although currently only represented by a single fossil specimen, it’s actually the most complete ichthyosaur known from the Pliensbachian age of the Early Jurassic.

Preserved gut contents show that it primarily fed on fish, and also that its stomach was positioned on the left side of its body. The fossilized individual also suffered from multiple injuries during its life, including malformed teeth, a fractured clavicle, and avascular necrosis in its upper limb bones. It appears to have died after a bite to the skull from a predator – likely the larger ichthyosaur Temnodontosaurus – and additional bite marks on one hindlimb may be evidence of scavenger activity.

Megacephalosaurus

Megacephalosaurus eulerti was a one of the last of the pliosaurs – a group of short-necked big-headed plesiosaurs – living during the Late Cretaceous (~93 million years ago) in what is now the Midwestern United States, a region that at that time was covered by the Western Interior Seaway.

Although known only from fossil skulls and a few neck bones, based on the proportions of related pliosaurs it probably reached around 9m long (~30′) with its 1.75m (~5’9″) head alone making up 20-25% of that measurement.

Its elongated jaws were lined with pointed conical teeth, and pits in the bones of its snout may have housed a complex sensory system, possibly giving it the ability to detect the movements or even bioelectric fields of nearby prey.

Rhacheosaurus

Metriorhynchids were a group of fully marine crocodyliforms known from the mid-Jurassic to the early Cretaceous of Europe and the Americas. They were the most aquatic-adapted of all known archosaurs, with streamlined bodies, smooth scaleless skin, small front flippers, larger hind flippers, and shark-like tail flukes. They may also have been endothermic, and might even have given live birth at sea rather than laying eggs.

Rhacheosaurus gracilis here was a metriorhynchid that lived in warm shallow waters around what is now Germany during the late Jurassic, about 150 million years ago. Around 1.5m long (~5′), its long narrow snout lined with delicate pointed teeth suggests it fed on small soft-bodied prey, a niche partitioning specialization that allowed it to coexist with several other metriorhynchid species in the same habitat.

Unlike most other marine reptiles metriorhynchids didn’t have particularly retracted nostrils, which may have had a limiting effect on their efficiency as sustained swimmers since higher-set nostrils make it much easier to breathe without having to lift the whole head above the surface. The lack of such an adaptation in this group may be due to their ancestors having a single nasal opening formed entirely within the premaxilla bones at the tip of the snout, uniquely limiting how far it could easily shift backwards – other marine reptiles had nostrils bound by the edges of multiple different bones, giving them much more flexibility to move the openings around.

(By the early Cretaceous a close relative of Rhacheosaurus did actually evolve nostrils bound by both the premaxilla and the maxilla, and appeared to have started more significant retraction, but unfortunately this only happened shortly before the group’s extinction.)

Metriorhynchids also had well-developed salt glands in front of their eyes, but the large sinuses that accommodated these glands may have made their skulls ill-suited to deep diving, being more susceptible to serious damage from pressure changes and restricting their swimming to near-surface waters only.

Preserved skin impressions in some metriorhynchid fossils show several unusual “irregularities”, including curl shapes, small bumps, and cratering. It’s unknown what exactly caused these marks, but they may represent scarring from external parasites such as lampreys and barnacles.

Megapterygius

Most mosasaurs all had very similar body plans: they were streamlined scaly monitor-lizard-like marine reptiles with four rounded paddle-shaped flippers, and many of them also had large shark-like tail fins.

But Megapterygius wakayamaensis here seems to have been doing something a bit different.

Living towards the end of the Cretaceous, about 72 million years ago, in the waters covering what is now western Japan, this mosasaur was around the size of a modern orca, roughly 6m long (~20′).

Unlike other known mosasaurs its flippers were huge, bigger than its own head and distinctively wing-shaped, with the back pair being larger than the front. This is an arrangement oddly reminiscent of the unrelated plesiosaurs, and may suggest a convergent sort of highly maneuverable “underwater flight” swimming ability – but unlike plesiosaurs Megapterygius also still had a powerful fluked tail, so how exactly all of its fins worked together is still unknown.

It’s also the first mosasaur known to preserve potential evidence of a dorsal fin. Some of its back vertebrae show a change in orientation at the point where a fin base would be expected to be, closely resembling the vertebrae shape of cetaceans like the modern harbor porpoise.

Wapitisaurus

Back in the 1980s, a fossil of a partial reptile skull was discovered in British Columbia, Canada, dating to the Early Triassic about 250 million years ago. Its triangular skull shape, large eye sockets, and what seemed to be distinctive spiky frills on the back of its head initially caused it to be identified as a relative of the gliding weigeltisaurids.

But the aptly-named Wapitisaurus problematicus would have had to be a very unusual member of this group. With an estimated length of up to 2m (6’6″) it was much larger than any other known weigeltisaurid, it was the only one known from the Triassic side of the “Great Dying” mass extinction event, it was found in a completely different part of the world, and its teeth seemed more like those of marine reptiles like thalattosaurs.

In recent years new discoveries and re-analysis of weigeltisaurid fossil material have resulted in much better modern understanding of their skull structure – and with that came the realization that Wapitisaurus really didn’t seem to match with them after all.

So a new study has finally identified what this problematic reptile really was… and it turns out the teeth didn’t lie! It was a marine thalattosaur all along!

Wapitisaurus had rather large eyes compared to most other North American thalattosaurs, and although the front parts of its jaws are missing it probably had a long slightly hooked snout similar to its close relative Thalattosaurus. It’s also now one of the oldest known members of the thalattosaur lineage, showing that some of their specialized skull features like retracted nostrils had actually appeared very quickly during their evolutionary history.

…Oh, and those “spiky frills” on the back of Wapitisaurus’ skull? They were actually all teeth from both the upper jaw and the palate, on broken shards of bone that had been displaced to just the right spot to muddle up its identity for over three decades.

Hupehsuchus

Hupehsuchians were small marine reptiles closely related to ichthyosaurs, known only from the Early Triassic of southwestern China about 249-247 million years ago. They had toothless snouts, streamlined bodies, paddle-like limbs, and long flattened tails, along with a unique pattern of armor along their backs made up of overlapping layers of bony osteoderms.

Hupehsuchus nanchangensis was a mid-sized member of the group, about 1m long (3’3″). Newly-discovered fossils of its skull show that its long flattened snout had a distinctive gap between the bones (similar to the platypus-like snout seen in its relative Eretmorhipis) with an overall shape surprisingly convergent with that of modern baleen whales – suggesting that this hupehsuchian may have been a similar sort of filter-feeder.

A diagram comparing Hupehsuchus' skull to that of a modern baleen whale. — *Hupehsuchus skull compared to a modern minke whale*
From fig 2 & fig 3 of Fang et al (2023). First filter feeding in the Early Triassic: cranial morphological convergence between *Hupehsuchus* and baleen whales. *BMC Ecol Evo* 23, 36. https://doi.org/10.1186/s12862-023-02143-9

Grooves in the bones along the outer edges of its upper jaws may be evidence of filtering structures similar to baleen, although with no soft-tissue preservation we don’t know exactly what this would have looked like. Its slender flexible lower jaws probably also supported a large expandable throat pouch, allowing it to filter plankton out of larger volumes of water.

Crystal Palace Field Trip Part 2: Walking With Victorian Dinosaurs

[Previously: the Permian and Triassic]

The next part of the Crystal Palace Dinosaur trail depicts the Jurassic and Cretaceous periods. Most of the featured animals here are actually marine reptiles, but a few dinosaur species do make an appearance towards the end of this section.

A photograph of a Crystal Palace ichthyosaur statue, posed hauled out of the water like a seal or crocodile. It's partially obscured by plant growth, and is in a state of slight disrepair – moss and lichen patches cover its sides, and a plant is growing out of a crack on its back. A moorhen can be seen in the water swimming towards it.

Although there are supposed to be three Jurassic ichthyosaur statues here, only the big Temnodontosaurus platyodon could really be seen at the time of my visit. The two smaller Ichthyosaurus communis and Leptonectes tenuirostris were almost entirely hidden by the dense plant growth on the island.

Two photographs of the Crystal Palace ichthyosaurs. On the left the island is clear of foliage and all three can be seen; and on the right is the current overgrown state. — Ichthyosaurs when fully visible vs currently obscured
Left side image by Nick Richards (CC BY SA 2.0)

Two photographs of the large Crystal Palace ichthyosaur, showing closer views of the eye, flipper, and tail fin. Int he background a second ichthyosaur can be seen through the foliage. A moorhen is pecking around near the flipper. — *Head, flipper, and tail details of the Temnodontosaurus. A second ichthyosaur is just barely visible in the background.*

Ichthyosaurs were already known from some very complete and well-preserved fossils in the 1850s, so a lot of the anatomy here still holds up fairly well even 170 years later. They even have an attempt at a tail fin despite no impressions of such a structure having been discovered yet! Some details are still noticeably wrong compared to modern knowledge, though, such as the unusual amount of shrinkwrapping on the sclerotic rings of the eyes and the bones of the flippers.

An illustration comparing the Crystal Palace depiction of an ichthyosaur with a modern interpretation. The retro version has long toothy jaws, very large eyes, a seal-like body, four scaly-looking flippers, and a small eel-like fin on its tail. The modern version is a much more dolphin-like animal with smaller eyes, smooth triangular flippers, a dorsal fin, and a vertical crescent-shaped tail fin.

Serpentisuchops

While the most iconic types of plesiosaur were long-necked with small heads and short blunt snouts, some of these marine reptiles actually developed the opposite sort of arrangement, with groups like the polycotylids and the pliosaurs independently evolving short necks, larger heads, and long snouts.

…Except some of them didn’t keep it quite that simple.

Serpentisuchops pfisterae here lived during the late Cretaceous, about 70 million years ago, in the ancient Western Interior Seaway covering what is now Wyoming, USA. This 7m long (~23′) plesiosaur was a member of the polycotylid lineage, but along with a long slender snout it also had an unusually long neck.

Some earlier polycotylids like Thililua had fairly long necks, too, but all of Serpentisuchops’ closest relatives were short-necked species, so it seems to have actually re-evolved this condition rather than inheriting it from its ancestors. Since no other marine reptiles in its habitat had this particular body plan, it was probably occupying a very specific ecological niche – the presence of attachment points for powerful neck muscles suggest it was able to swing its head sideways to snap its jaws at prey at high speed, with its longer neck giving it more reach than other polycotylids.

Kyhytysuka

The first definite ichthyosaur fossil found in Colombia was a single well-preserved skull, found in Early Cretaceous deposits dating to between 130 and 112 million years ago.

Although first discovered in the 1970s, this marine reptile wasn’t described until the late 1990s, at the time being named as a species of Platypterygius. But since then more pieces of the skeleton have been recovered, and the Platypterygius genus has been found to be a wastebasket taxon in need of revision, so in 2021 the Colombian ichthyosaur got a more detailed redescription and its own distinct name: Kyhytysuka sachicarum.

Kyhytysuka was a mid-sized ichthyosaur, about 5.5m long (18′) – about the size of a small modern orca – with a large head and a long robust snout. Its teeth varied in size, shape, and spacing along its jaws, with several different regions that were specialized to catch, slice, and crush its prey.

It could also open its jaws very widely, possibly up to an angle of 75°, suggesting it was able to tackle particularly large prey such as other marine reptiles. Possible soft tissue preservation around its lower jaw might also be evidence of elastic connective tissue that would have allowed its throat to expand out while swallowing big prey items.

This makes Kyhytysuka the first known example of a Cretaceous-aged ichthyosaur with an apex predator lifestyle, convergently evolving a similar ecological role to some earlier Triassic and Jurassic species.

Umoonasaurus

Umoonasaurus demoscyllus was a small short-necked plesiosaur, about 2m long (6’6″), that lived in the polar shallow seas covering much of what is now Australia 115 million years ago during the Early Cretaceous.

Its known fossil remains include a specimen nicknamed “Eric”, one of the most complete opalized vertebrate skeletons ever found.

While most of its body was fairly generalized for a plesiosaur, its skull was unusually ornamented. A raised ridge along the middle of its snout shows evidence of supporting a larger keratinous crest, and smaller ridges over each of its eyes may have also had similar structures. These crests were fairly delicate so were probably mainly used for visual display, and might have been brightly colored.