Thalassaurs are one of the most derived lizard lineages, tracing their origins to a population that took to the seas around the Gecko Isles over five million years ago. The family slowly began spreading farther and farther across the Medithalassic, reaching the coasts of Abeli and Loxodia before the end of the Ice Age. Today, they can be found throughout the entire ocean, with their diversity being highest around the Aglirian shores, Ailuropia's Bayoulands, and Abelox. In total, there are about thirty species occupying as many unique niches, from small grazers to aquatic apex predators.

The thalassaurs can be divided into three subfamilies. The most basal, Gekkosuchinae, are semi-aquatic ambush predators. The Late Muricene gheckial was an early representative of this clade, though that species went extinct with the loss of the seabasket forests. Like all Gekkosuchines, it retained claws on its feet and was still relatively nimble on land, and it could not make oceanic journeys longer than about 20 kilometers at a time. Consequently, unlike its more specialized relatives, this subfamily is restricted to Aglirium and its immediate vicinity. Geckodiles (Gekkosuchus) are a group that live only in freshwater environments, lacking the salt-excreting glands located below the tongues of other thalassaurs. The three species of geckodile are the largest members of their subfamily, with some reaching 125 kilograms and just shy of four meters in length. They have powerful jaws with a hooked tip that helps them hold onto struggling prey, which can vary from fish to birds to Abelithere rats. Hatchlings will even eat aquatic insects and isopods, while they must be wary of their own adult relatives, which are avid cannibals. The biggest geckodiles will sometimes even kill adults of smaller members of their genus, so only the very largest are ever truly safe.

The estuarine Gekkosuchines all belong to the genus Oragekko, a much larger taxon containing at least ten members. Many of their ranges overlap, so these species tend to be heavily niche-specialized. One of the most common Oragekko varieties is the Tideprowler (O. terrestris), which has the best walking abilities of any thalassaur. When the tide comes in, it bides its time swimming around near the shoreline or, if it's feeling cold, basking higher on the beach. It catches little food, as it's a slower swimmer than its relatives, and it lacks the long, snapping jaws necessary to snatch darting sea creatures. However, it is an effective low-tide scavenger, having edged out vampire geckos from that role, and can patrol several kilometers of exposed sand before the water rises once more. Fish stranded on dry land are its main targets, though it's also comfortable plucking them from tide pools in the parts of its range where rocky coasts predominate.

Another Oragekko species found around the full circumference of Aglirium is the Trenchtrawling Thalassaur (O. sabulonubis), a diving specialist that can hold its breath for thirty minutes or more. It scours the seabed, dragging its long foreclaws through the sand while propelling itself with its powerful tail and hindlimbs. This lets it uncover buried food, including benthic fish, Caenarthrids, and carrion that other scavengers failed to find. Small items can be eaten underwater, while larger finds are usually carried to the surface for further processing. Trenchtrawlers are also avid diggers on land, and when it's time to lay eggs the females will travel inland in search of more stable soil. There they create a tunnel up to three meters long, deposit their clutch in its depths, then disguise the entrance with sticks and leaf litter. The young are born with fully-developed claws and can easily climb out before making the dangerous journey to the water's edge. Over three-quarters die from exhaustion or predation before they ever make it to the sea, but once below the waves, the weapons on their arms keep them relatively safe.

The grazing thalassaurs are a group that only barely made it into the Arthrocene. They belong to the subfamily Placidosaurinae, which contains only a single genus and species, Placidosaurus pelagicus. This is the descendant of the siren gecko, the only member of the clade that survived into the Arthrocene. It has a broad mouth containing roughly 50 semicircular teeth. The tooth margins have a wave-like pattern that helps them scrape algae like rockslime from hard surfaces, but also works equally well against semi-aquatic vascular plants. Dwarf Kegweeds (Thalassodolius medithalassicus), for example, can be found around much of the Aglirian coast and make up over a third of this species' diet. Tideslopes are only a minor habitat in the Medithalassic, as the enclosed ocean only sees 3-6 meters of difference between high and low tide, but its small patches of kegweed forest are just as productive here as those in Post-Abeli, allowing placidosaurs to reach a mass of more than 150 kilograms. 

The final and most speciose subfamily contains the pursuit-predatory thalassaurs, also known as Euthalassaurinae. These "true thalassaurs" are fully dedicated to a marine lifestyle, lacking claws on their front or back legs. To reproduce, they must drag themselves onto the shore and dig a shallow hole that provides little safety for their developing young. In a single season, females of most species each lay about 150 eggs across a dozen or more clutches, of which only one or two are likely to survive their first year of life. Newly hatched euthalassaurs, like all members of their family, receive no care. Instead, hatchlings of this subfamily form mixed-species foraging groups. Since they all start out as small, unspecialized omnivores, they can easily commingle at this age, schooling together for safety until they grow into their adult niches. 

One such niche, occupied by the meter-long Spearflipper (Hastopteryx), is that of the speedy small-game hunter. The one-dozen species in this genus can be found everywhere in the Medithalassic, feeding on small forage fish, pelagarthrids, and tailtubes. Open-Sea Spearflippers (H. brevipteryx) are the most widespread, spending their time far from shore in pursuit of shoaling prey. Glerrings are a prime target, gathering in massive numbers every night, but their bright, chaotic displays make them far from an easy catch. Instead of using keen senses to zero in on their prey, the open-sea spearflipper uses its powerful, arrow-shaped tail to accelerate rapidly, impaling the fish on its thin, forward-projecting teeth. Dozens of glerrings can be caught in one strike, after which the spearflipper flicks its head from side to side, dislodging its food. To prevent other species from stealing fish caught in this manner, open-sea spearflippers are more gregarious than any other euthalassaur. Pods comprise up to ten same-aged individuals that met as hatchlings, originating from a dozen or more mothers' clutches that hatched around the same time on a particular beach. Other members of the multi-species juvenile group gradually leave to live solitary or semi-social lives, but the open-sea spearflipper cohort remains together. The few that survive to adulthood hunt cooperatively for their entire lives, helping their podmates retrieve their prey so it isn't lost.

Shortfin Spearflippers (H. forticauda) are the fastest swimmers in their family, reaching speeds of up to 35 km/h. This is possible because of this species' long, torpedo-like body, with short fins and a strong, lobe-shaped tail. This physique has its drawbacks; namely, the shortfin spearflipper has a very difficult time crawling onto land to lay its eggs. Females are so slow that they often fall victim to terrestrial predators, so most choose to nest on small offshore islands where seacocks are the only threat. To avoid falling off the cliffs of these islets, H. forticauda's eggs have thin, pliable skin that lets their mother wedge them into crevices, where they remain stable until hatching. The baby geckos, upon hatching, throw themselves into the sea, for climbing down carefully would only put them at risk of being picked off.

The Spinner Spearflipper (H. triangularis), the smallest member of its genus, inhabits more coastal environments, mostly the shallows of the northern half of the Medithalassic. Its armspan is roughly the same as its body length, with winglike front flippers that grant it maneuverability greater than any other thalassaur. It eats fast-moving, mid-sized fish like jaguarfish and female brook stoutlings. Though often slower than its prey, it compensates with agility and gregarity. Pods form opportunistically in prey-dense areas, with individuals cooperating to encircle shoals of fish. They use their long arms to make themselves look larger and herd the fish into an ever-shrinking sphere. Once they've packed their food into a sufficiently dense area, all 5-20 pod members begin their final attack, snapping at one another and fighting over their meal. Injuries are common, as spearflipper social bonds are weak and there may not be enough fish to go around. The frenzy lasts until none remain, either because they've all been eaten or they've all escaped the distracted lizards.

The largest of all thalassaurs are two forms that inhabit the whole of the Medithalassic, filling the role of oceanic top predators. Though one reaches four meters long and the other less than half that, they are, genetically, one and the same species. Male Dimorphogekko pentapinna, or Diamondtail Thalassaurs, can tip the scales at 175 kilos, while their female counterparts are hardly larger than a spearflipper. This is caused by the need to leave the water when laying eggs, which puts an upper limit on females' body mass. Their mates need never exit the sea as adults, and therefore male diamondtails have no such size restrictions. 

Both sexes have a large, arrow-shaped tail fluke and four broad flippers (though the latter are proportionally bigger and stronger in females), a hydrodynamic body, and a pointed snout with several of its conical, backward-facing teeth exposed. This species has an adaptable diet, feeding mainly on fish and tailtube worms when they're plentiful. When an opportunity presents itself, seawis and other thalassaurs are also potential victims. Compared to other cold-blooded reptiles, the diamondtail has surprising endurance. When cruising, it swims at around 5 kilometers per hour, propelled by slow undulations of its tail. When pursuing prey, it can also use tail-based propulsion to "sprint" at up to 25 km/h, though it cannot maintain this speed for more than a minute or so. During longer chases, it switches to using its flippers, powering itself at about 12 km/h. At any given moment, only one pair of limbs is active, with the other being used as stabilizers. When fatigue begins to set in, the lizard starts using the other pair, alternating every minute or two. This allows it to swim relatively quickly for up to a quarter-hour at a stretch, long enough to slowly catch up to prey that is often faster than itself in burst swimming. The only ways to escape a determined diamondtail are to exit the water or descend deeper than it can, a task often impossible in the offshore shallows that characterize much of the Medithalassic.