This was hands down my favorite piece in this series till date! I find sea creatures really easy to design simply because they all have an overall similar morphology, of course that brought new challenges like how to make everything similar yet also stand out. I think I've pulled it off, and decided to do some creative lighting!
The Thalattophidia, also called the sea serpents, are a clade of draconimorphs that belong to a group known as the Serpentiformes, a sister lineage to the Draconiformes (wyrms, wyverns, drakes & dragons). Fossil evidence suggests that the group was at one point quite diverse, with at least 5 distinct clades, of which the Thalattophidia are the sole survivors. Their evolutionary origins are as murky as the waters they inhabit; while it is known that they are parareptiles, some have hypothesized, based on anatomical similarities, that they are actually descended from the mesosaurs, a basal group of parareptiles that hold the distinction of being the first reptiles to become secondarily aquatic. If this is the case, it would make sea serpents the single oldest group of aquatic amniotes (all tetrapods excluding amphibians), with an evolutionary ancestry dating back nearly 290-300 million years, as well as making the draconimorpha an unnatural grouping, as the draconiformes are known to be descended from procolophonian parareptiles rather than mesosaurs. However, the current consensus, based on molecular studies and fossil evidence, indicates that the serpentiformes do share a procolophonian ancestry with the draconiformes, and as such Draconimorpha is indeed a valid clade, with the perceived anatomical similarities simply being a result of convergent evolution.
With just 11 known species, the Thalattophidia are a very poorly-studied group, despite their global distribution, which in turn makes classification very difficult. Furthermore, their generally benthic and/or abyssopelagic nature means that sightings and direct observations are rare and infrequent, with much of what is known about their biology coming from the occasional washed-up carcass. Thalattophidians get their name from their generally elongated, serpentine bauplan, but unlike snakes, whose necks & tails are much shorter than their mid-sections, sea serpents have uniformly elongated bodies. Their tails are laterally flattened, and are the primary locomotory organs, while their limbs are modified into paddle-like polydactyl flippers similar to those of the distantly-related plesiosaurs and ichthyosaurs.
Fossil evidence shows that the sea serpents first appeared roughly 150 mya during the late Jurassic, but it was only in the aftermath of the K-Pg extinction (and the resulting extinction of the mosasaurs & plesiosaurs), that they diversified, and only in the last 30 million years did most of them become deep-diving benthic animals, likely due to increased competition with cetaceans, pinnipeds and sea drakes, as evidenced by a scarce fossil record from that time period onwards. Nonetheless, a 150-million-year evolutionary history makes them among the oldest extant lineages of marine reptiles and marine amniotes in general (alongside sea turtles), a fact that becomes obvious from their adaptations to an aquatic life. Their secondary palate is completely closed, thus separating their nasal passage from their oral passage, allowing them to swallow food while underwater; their external ears have been lost but their lower jawbone is modified to conduct sounds to the inner ear, granting them excellent underwater hearing; lingual salt glands to excrete excess salt; extremely dense bones that make diving easier; and having among the highest erythrocyte counts of any vertebrate, as well a high concentration of myoglobin in their tissues for oxygen storage.
Most species have elongated but small lungs, which primarily aid in regulating buoyancy rather than storing oxygen. All known species are also able to respire through a modified, highly vascularized gular sac rich in blood-filled papillae, as well as through their cloacas in a manner similar to sea turtles. It is estimated that sea serpents can actually meet upto 50% of their oxygen requirements through this combination of gular and cloacal (non-pulmonary) respiration, allowing them to dive deep for prolonged periods of time.
Sea serpents are viviparous, a trait shared with lindwurms, although this appears to be a case of convergent evolution. They are also the only known draconimorphs to possess thecodont dentition (teeth placed in distinct sockets in the jaw), a trait that appears to be widespread among the serpentiformes but non-existent in the draconiformes, which possess either pleurodont or acrodont teeth.
In contrast to most extant marine reptiles, thalattophidians are able to venture into cold waters thanks to a network of countercurrent heat exchangers in their bodies along with a thick layer of blubbery skin, effectively making them endotherms. Their skin is covered in small, multi-keeled scales that channel water backwards, similar to shark skin, making swimming much more efficient and significantly reducing any turbulence caused by their movement.
Although the phylogenetic relationships within the Thalattophidia are yet to be fully resolved, the clade is generally classified into 2 suborders: the very basal Paludisauria and the more advanced Anguillisauria. The Paludisauria are represented by a single species, the knucker (Paludisaurus flaviceps) though fossil evidence suggests that the group was more diverse in the past. Found in wetlands, rivers and estuaries through much of Europe, parts of northwestern Asia and Atlantis right up to the Arctic circle, knuckers are among the few sea serpents capable of tolerating fresh, brackish and salt water, though on the few occasions when they do take to the sea, they rarely dive deeper than 30 m even though they can hold their breath for a maximum of 1.5 hours. Their limbs are somewhat intermediate between walking legs and flippers, with a high degree of flexibility and muscular strength that allows them to move about fairly comfortably on land, similar to seals and sea lions. Thalattophidian fossils dating back to the Cretaceous bear striking similarity to the modern knucker, indicating that this is the ancestral condition in the clade as a whole, with fully aquatic forms seemingly having appeared only in the Paleogene.
Growing to 4 - 5 m in length, knuckers are the smallest thalattophidians, but are nonetheless fierce predators feeding on everything from invertebrates to fish and other small to medium-sized vertebrates. Remarkably, knuckers appear to be immune to the deadly venom of lindwurms, with which they are often sympatric. Being similar in size but with a more powerful bite and sharp, slicing teeth, they are among the lindwurms only known predators, so effective that regions with high knucker populations have few lindwurms. In fact, knuckers will actually hibernate in lindwurm burrows during winters, often by killing the burrow builder first. Although viviparous just like their fully marine cousins, knuckers usually give birth on land as their offspring are born head-first rather than tail-first. This has been used to support a hypothesis that viviparity is an ancestral trait in Thalattophidia that appeared even before the group became aquatic, but since no fossils of pregnant terrestrial thalattophidians have been found this is difficult to prove. While mostly solitary, females will generally congregate in slow-moving water bodies to give birth, but disperse shortly afterwards and care for their 2-6 offspring for roughly 3 months.
The Anguillisauria, or advanced sea serpents comprise the remaining 10 known species. These are extremely well-adapted to their marine ecosystems, with modified skin receptors that allow them to detect vibrations in the water as well as shark-like electroreception. Remarkably, they are able to smell underwater thanks to a unique anatomical trait - their external nares (nostrils) cannot close fully unlike the majority of marine reptiles and mammals, and instead have muscular valves in their internal nares (choanae) that prevent water from entering the lungs. This trait allows them to smell underwater by channeling water through their nostrils into the olfactory passage. This group is further split into 2 distinct clades, the Squalosauridae and the Thalattophiidae.
The former is represented by a single species, the nakaïwha (Squalosaurus pectinognathus) or lizardshark. A seemingly young clade evolutionarily, just 2 other species are known from a very scant fossil record, and the sole surviving species appearing as recently as 4 mya in the Pliocene. Although they strongly resemble an older extinct clade of similar Serpentiformes, this resemblance is superficial, and appears to be a highly derived condition in Thalattophidia. Growing to a length of 6-7 m and weighing 350-450kg, the nakaïwha isn't a very large sea serpent, but greatly differs from other species in its morphology. In contrast to the generally anguilliform locomotion of other sea serpents, this species shows carangiform locomotion; its body is more compact and less elongated, its flippers are larger & more pointed, its tail fluke is larger and its spine is stiffer with a small dorsal fin, all of which make it a fast & powerful swimmer, able to achieve speeds of up to 45 kmph. A mesopelagic species, it spends most of its time in the open ocean rather than near the sea bed. Nakaïwha have rather large lungs in comparison to other sea serpents, and do seem to rely on them as an oxygen storage while swimming close to the surface in addition to their gular and cloacal respiration. Able to dive to depths of 300 m and hold their breath for 1 hour, these sea serpents actually seem to exhale as they dive, similar to many marine mammals, which minimizes the risk of decompression sickness. Highly active animals, they can actually maintain a near-constant body temperature of 28 - 30°C thanks to their thick blubber and an extensive layer of brown adipose tissue, making them homeotherms or endotherms.
Distributed though the Pacific, Indian and Southern oceans, these sea serpents feed on small fish, squid and notably krill, their numerous slender teeth functioning in a similar manner to the baleen of whales. It is thought that this particular diet is the primary reason for this species' success, even though it shares some niche space with sea drakes and dolphins. Indeed, the nakaïwha does oftentimes fall prey to sea drakes such as the makara, and macropredatory dolphins such as orcas and false killer whales. But while these predators are faster, most of them do not dive nearly as deep, and few can match its maneuverability. The only known social sea serpent, nakaïwha are known to travel in small pods of 5-10 individuals, often in close proximity to those of large whales perhaps for protection against predators. Females give birth to 2-3 offspring, which are looked after for 6-8 months, longer than any other thalattophidian species.
The remaining species are classified under the family Thalattophiidae. These sea serpents have rather small lungs for their size, which are very narrow and elongated and appear to be a result of their generally elongated, serpentine bauplan. Oxygen is stored mainly in body tissues and the blood, and anaerobic respiration appears to contribute significantly during prolonged dives. Their rib cages are collapsible like those of cetaceans, and immediately collapse at greater depths, emptying the lungs of air so as to lower the risk of decompression sickness as they dive. This also seems to restrict blood flow to the lungs, further reducing the chances of any remaining gases from dissolving into it. In order to compensate, they have an extremely large spleen as well as numerous blood-filled haemal nodes, similar to those of some mammals, which store large quantities of erythrocytes that may act as oxygen reserves. The lungs do however appear to play a significant role in maintaining buoyancy: as a sea serpent begins to ascend towards the surface, the rib cage expands, and the lungs begin to refill with air, reducing their overall density and thus reducing the energy required to resurface.
Previously, the group was divided into 2 subfamilies purely on the basis of their anatomy: the 'sea lizards' and the 'sea orms'. The sea lizards comprised those species with larger flippers and tail flukes for more efficient swimming and a greater reliance on their underwater vibration sense to detect moving prey, and which spent more time cruising in the water column, while the sea orms traditionally comprised those species with smaller flippers and narrower tails and a greater reliance on electroreception to detect hidden prey, and which spent most of their time close to the sea bed. Genetic studies performed on beached carcasses however have shown that this classification is inaccurate, with several species of 'sea orms' actually being more closely related to 'sea lizards' than to each other and vice versa. The same lines of evidence have also enabled a more accurate classification of the group into 2 subfamilies: the Onychopteryginae and the Thalattophiinae.
Members of the former are known as the coastal sea serpents, and comprise 2 species in a single genus - the Pacific coastal sea serpent (Onychopterygius longirostris) and the Atlantic coastal sea serpent (Onychopterygius melanurus). At 6 - 7.5 m in length and weighing 180-240 kg, the Pacific coastal sea serpent is slightly larger than its Atlantic cousin, and is distributed through much of the Pacific coast, from Asia and Australia to the Americas, and as far south as Aotearoa. The prominent claws on all 4 of its flippers are what give the genus its name. As their common name suggests, these sea serpents inhabit shallow coastal waters, rarely over 50 m deep, and have a very slender, streamlined build that allows them to snake their way through the coral reefs and kelp/seaweed forests that dominate their preferred habitats. Their clawed flippers allow them to "walk" on the seabed as well as grip onto surfaces. As with the knucker, they can also tolerate brackish and freshwater, and can crawl on land (though not nearly as well). The onychopterygine morphology appears to be the ancestral condition of thalattophiids and Anguillisaurians in general, as all pre-Oligocene Anguillisaurians bear a strong superficial resemblance. As with knuckers, their tails are paddle-shaped without any flukes, an adaptation for hunting close to the sea floor; uniquely however, the neural spines on their caudal vertebrae are jointed, and have muscle attachments that enable them to be bent downwards to make the tail smaller to fit through narrow spaces, or bent upwards to make the tail taller to provide additional thrust for swimming faster. Even so, this species isn't nearly as fast as some of its relatives, and relies more heavily on its size and being able to hide in enclosed spaces to escape a threat. Coastal sea serpents also rely on their streamlined shape to hunt - ambushing prey while concealed or actively searching for prey such as fish and invertebrates hiding in rocks, corals or in sand, and using their long, narrow snouts with curved upper jaws as tweezers to capture such hidden prey items. Because of this, their electroreceptors are only present at the very tip of the snout to increase the chances of accurately detecting & gripping something, in contrast to other sea serpents which have such receptors distributed more evenly over their heads.
These are also the only sea serpents known to be anadromous, travelling into freshwater in order to give birth, though sheltered lagoons and sea caves are oft-used alternatives. Although they generally inhabit shallow waters, they have been known to hold their breath for nearly 2 hours by relying heavily on anaerobic respiration, and can thus venture deep into sea caves and remain nearly undetected in rivers and lakes, breaking the water surface with just their nostrils at the tip of the snout to breathe. The sea caves and larger rivers of Aotearoa are a particularly favored breeding colony for this species. Females give birth to 4-6 offspring, which are cared for roughly 3 months.
The Thalattophiinae comprise the other 7 species, all of which inhabit deep waters and as such are ecologically not very well understood. Most of what is known of their phylogeny and biology comes from the occasional washed-up carcass and/or gut contents of predators/scavengers.
The Northern sea orm (Thalattophis platyurus) is a typical member of the - now invalid - sea orm clade (although the common name has stuck), this species has shorter, more paddle-shaped flippers and a narrower tail with no prominent fluke. Distributed throughout the North Atlantic and North Pacific, even into the Arctic circle, this is a large benthopelagic sea serpent measuring 16-18 m in length and weighing well over a ton. This species isn't built for speeding through the water, but rather for foraging close to the ocean floor at depths of 1000 m or more at times. Its bones show extreme pachyostosis, an adaptation for deep-diving, and its eyes, while still sensitive to light, are relatively small, indicating that they do not play a crucial role in hunting except for perhaps locating bioluminescent prey. Instead, it appears to rely more on its sense of smell and electroreception to locate prey. The 3 pairs of long, fleshy tendrils on its snout, known as barbels, are packed with nerve endings that help it detect vibrations and water turbulence, such as those made by swimming prey. Along with the related Southern sea orm (Thalattophis rudicollis), this species is notable among sea serpents for its crocodile-like osteodermal armor, most pronounced on its back but also on its undersides and flanks. This is believed to help protect it from attacks by predators such as titan drakes & some sea drakes, as well as giant cephalopods such as krakens and giant squid. It also has a somewhat crocodilian head, and a powerful bite force that aids in crushing the shells of the mollusks and giant crustaceans that dwell at such great depths. These sea serpents have been known to stay underwater for as long as 5 hours, preventing the accumulation of CO2 and lactic acid in their tissues by non-pulmonary respiration via their modified gular sac and cloaca. Their thick skin and underlying blubbery fat layer, together with temperature-independent swimming muscles and brown adipose tissue allow them to maintain a body temperature roughly 12°C higher than the surrounding water without the need for constant muscle activity, a necessity given their rather slow locomotion and the extreme cold of their feeding areas.
Although primarily deep-sea predators, sea orms will sometimes hunt near the surface, especially at night. The frequency of these surface hunts increases significantly during the breeding seasons of smaller pinnipeds and sea birds such as great auks, with the sea orm taking full advantage of the high population densities of these animals during these times, making it the best studied of the deep-diving sea serpents because of these frequent surface hunts. Surprisingly, the sea orm is rather ill-equipped as a marine mammal/bird predator. Compared to its regular prey, pinnipeds and auks are incredibly agile, and can easily escape from an attack by a much slower sea orm. For this reason, sea orms hunt at night, when their dark coloration makes them nearly invisible in deep water, and target the relatively naive juveniles, which aren't nearly as adept at avoiding predators as the adults. There haven't been very many documented attacks on cetaceans however, mostly because the smaller species are able to detect them via echolocation and larger ones are simply far too large and powerful. Little is known about the sea orm's breeding habits, but what is known is that they generally give birth far from open waters and close to continental shelves, where their main predator, the devil whale, rarely ventures.
Among the most common sea serpent species is the banded sea serpent (Neptunisaurus fasciatus), found in tropical and subtropical waters worldwide. Growing to a length of 10 - 12m and weighing upto 800 kg, this sea serpent is an active predator of fish and squid, with the enlarged teeth at the front of its jaws helping grip onto its prey. A related species, the white-bellied sea serpent (Neptunisaurus leucogaster) has a similar distribution, but has less prominent teeth & stronger jaws adapted for tackling more robust prey. A typical member of the - now invalid - sea lizard clade, banded sea serpents have longer fore flippers than hind flippers, a broad tail fluke, can swim surprisingly fast, and their flexibility allows them to outmaneuver their prey. Because of their slender build, they can silently approach their prey before striking, a strategy believed to be similar to that of the ancient long-necked plesiosaurs. Their large eyes indicate that they are highly visual predators, and using them in conjunction with acute hearing and a powerful sense of smell allows them to hunt in relatively dark waters. It hunts primarily at depths of 350-800m, and can hold its breath for 1.5 - 2 hours, though most dives only last for 20-30 minutes.
Each year during the breeding season, female banded sea serpents migrate to warm, shallow water coves to give birth. A seemingly r-selected species, a single female can give birth to 10 small offspring, which it will abandon after 45-50 days. The young will stay in the coves for several months more, before venturing out into deeper waters. Offspring survival rate is unclear, but given their small size and the number of predators in their ecosystem, it's believed to be quite small.
Sea serpents are prime examples of abyssal gigantism, with deeper diving species growing to larger sizes than those that prowl shallower waters, and sometimes, far larger. Growing to a length of 18-21 m and weighing 5.5 tons, the tiamat (Typhoneustes regius) or giant sea serpent is among the largest thalattophidians, and among the largest extant reptiles. While outwardly similar in appearance to the banded sea serpent, genetic samples from beached carcasses have shown that it is actually more closely related to the sea orm, though it occupies a different niche as a mesopelagic, bathypelagic predator, cruising through the open ocean generally far away from continental slopes and far above the abyssal plain. With a worldwide distribution, this species is among the largest extant macropredatory marine reptiles, able to tackle large prey such as giant cephalopods, large sharks and marine mammals from small seals to medium-sized baleen whales using its powerful jaws lined with curved, serrated teeth. It is deceptively fast for its size, able to reach speeds of upto 45 kmph, though only for extremely short bursts of a few seconds. This however is sufficient to ambush prey in the pitch-black depths of its hunting zone, with its dark coloration providing additional camouflage. Tiamat generally hunt at depths between 300 - 1500 m, with a few exceptional dives of 2000 m, and can remain underwater for an impressive 6 hours, though as with all marine reptiles and mammals, dive time decreases significantly with greater activity. They have been observed using their pale undersides to spook small fish and squid and corralling them into bait balls, before lunging forward to engulf several at a time, their wide jaws allowing them to suction-feed to an extent, but their teeth appear ill-suited for the task. Instead, they spend most of their time tracking large prey using their sense of smell as well as keen vision to spot bioluminescent prey. Once a suitable prey item has been found, the sea serpent will rush towards it, usually from the side and bite down on its locomotory organs or, in the case of large cephalopods, directly on the head, before pulling backwards or even performing a death roll akin to that of crocodiles to tear off large chunks of flesh. The prey usually dies swiftly of blood-loss and shock. Tiamats are also known to frequently hunt near the surface, particularly at night, following the vertical migration of squid. Despite their prowess, they aren't considered true apex predators. Smaller individuals are known to frequently fall prey to sharks such as great whites, larger sea drakes hunting in packs may even take on adults, and on one occasion, an individual was documented being killed by a pod of orcas, which took turns ramming the animal and tearing off its flippers, but its primary predator appears to be the devil whale. A confrontation has never been witnessed, but studies of whale gut contents has shown that this sea serpent is fair game for the giant physeteroid whale, which is thought to use its sonar to stun its prey before dispatching it.
Due to the tiamat's general rarity and the depths at which it usually hunts, little is known of its breeding habits. Females are known to grow larger than males, as evidenced by beached carcasses of adults, probably to make room for developing offspring, but this remains unclear.
The largest, as well as most mysterious, thalattophidian is undoubtedly the jörmungandr (Poseidonophis titanis), also called the greater sea orm on account of its similar appearance, though once more, this resemblance is merely superficial, and this species seems to be more closely related to the banded sea serpent. Growing to 22 m in length, with the largest known individual at 25 m, and weighing an impressive 7-8 tons, the Jörmungandr is one of the largest extant reptiles, with only the titan drake regularly exceeding it in mass at 9 tons. It is known to be among the deepest diving marine tetrapods, a deep-sea apex predator that has been recorded diving to depths of 2500 - 3000 m for a duration of 8 hours, with the longest recorded dive lasting a little over 11 hours, and seem to forage equally in benthic and pelagic zones. Due to the sheer inaccessibility of their foraging areas, there's no comprehensive information about their feeding ecology, barring some observation of hunting behavior and prey selection, and what is known of their biology comes solely from a small handful of carcasses. Their teeth & jaws are similar to those of the banded sea serpent, albeit on a much larger scale, indicating a somewhat similar feeding strategy, though given its size, it's likely to also hunt far larger prey such as sharks and cephalopods such as krakens, giant and colossal squid, with the numerous deep scars covering their bodies and the presence of cephalopod beaks in their guts attesting to this. The retinae of their large eyes are unique in having only rod cells, which function extremely well in low light, and no cones, making them completely colour blind, though this isn't an issue given the pitch darkness of their habitat. As with the sea orm they have 3 pairs of barbels on their heads, though these differ in their placement on the head, as well as possessing both mechanoreceptors (for sensing vibrations) as well as electroreceptors (for detecting bioelectric fields). Jörmungandr actually seem to have these 2 kinds of receptors distributed all over their bodies, but they're especially concentrated on these barbels, which can be upto 2 m long in this species, leading some to theorize that they may function similar to human fingers. Given their rarity, virtually nothing is known of their overall ecology and behaviour, though it is clear that the world they inhabit in the vast darkness of the abyss is one that's as alien as their appearance.
Those are very interesting indeed. I had ideas for manticores, sphinxes, unicorns and mermaids, and might just end up incorporating the rest of these too, if I can come up with a plausible evolutionary history for them