Ever since the Triassic, when the first ichthyosaurs and sauropterygians set foot in the water, the seas have been the domain of the tetraopds. These early marine reptiles greately diversified and with few interruptions they persisted until the end of the cretaceous, when they were themselves the victim of a mass extinction. After this event, the mammals took over this role, and the whales, seals, sirenians and otters, amongst several extinct groups, have largely dominated the seas since then.
The reptiles and birds still, however, exist in the seas in relative abundance, the birds as a variety of flightless and volant seabirds, and the reptiles as sea turtles, two species of crocodiles and a few species of lizard. Pressures from humans, and the climate change caused by us, will likely take their toll on the larger marine mammals, and while they will probably survive, they probably won't reach their former diversity. This gives several animals an opportunity to evolve into their niches. This modern day extinction event divides the Cenozoic in two, with the K-Pg boundry to the end of the Holocene being known as the 'Early Cenozoic', or more informally as 'the age of mammals', and the 'Late Cenozoic'. The late cenozoic was still largely dominated by mammals, but many areas saw the evolution of several advanced sauropsids, such as a terrestrial caiman in South America, giant macropredatory sea snakes in the Indopacific regions, and the first flying reptiles since the Mesozoic, the Vivamids, small flying geckos common in the Old World, and ancestors of the larger Novozoic pterogeckos.
These surprising competators were Rhomaeleovaranids, or 'Romomonitors', a group that are the top terrestrial predators of the early Novozoic, equivalent to sebecid crocodiles in niche and in some ways appearance. One of the significant differences between the monitor lizards, which went extinct in the C-N extinction, and their Romomonitor descendants, is the evolution of a sophisticated venom delivery system, the Rhomaeleovaranids have hollow lower teeth, and strong muscles to pump venom into their prey. These lizards are every bit as lethal as the venomous snakes they coexist with, and can inject far more venom per bite. This advantage lead them to spread out and evolve into many forms, and a new age of reptiles had begun. Although they are not alone, and they coexist and compete with large iguanas in the Americas, and Uromastyx lizards in Africa, and with a group of flying gecko descendants known as 'pterogeckos', the dominating land reptiles are Romomonitors, and they ranged from small arboreal species, to lumbering herbivores, to ferocious carnivores, with birds and mammals being pushed into relatively minor niches, as mostly small generalists. One group also headed for water, and evolved into the Venenosuchines, which evolved into the niche of crocodiles on the northern supercontinent of Borealia, and pinnipeds in coastal settings around the world. These marine venenosuchians then evolved into the completely aquatic Halilycosauria, which soon outcompeted the giant sharks and drove them into mesopredator niches.
The Halilycosauria as a group lasted the longest out of all aquatic tetrapods, with the last genus, the scissor-toothed Aquatyrannid known as Acerpleurodon, living in the early stages of the formation of Novopangaea, over 200 million years in the future, and almost 170 million years after its earliest ancestor, Piscisherpeton. The group finally went extinct when the oceans again stagnated due to the landmasses clumping together once more. Though not a mass extinction, it was enough to cause an ocean turnover, leaving the land the only place where tertapods had some dominance. The global ocean surrounding Novopangaea would be populated by derived cephalopods and a miriad of sharks and some bony fish, but marine tetrapods comparable to today were history, never again would titans like the Halilycosaurs exist, this was the death of a dynasty.
The venenosuchians, generally called 'venenodiles', are the direct ancestors of the halilycosaurs, and are common semiaquatic predators, similar to crocodiles and pinnipeds of the Holocene, and champsosaurs of the earliest Cenozoic. Venenosuchians evolved in the early Novozoic, out of the ancestral Romomonitor stock, and have since adapted to a semiaquatic lifestyle. Most venenodiles have a finned tail, and flipper-like legs, though their need to move on land requires them to have mobile joints, and the venenodiles still have a knee and elbow joint in their limbs. Venenosuchians started off as semiaquatic ambush predators, and were restricted to rivers and lakes, but they quickly made the transition to the ocean and also started adopting pinniped niches. These marine species eventually developed longer flippers and a more powerful fluked tail. The first true halilycosaur, known, rather anomylously, as Piscisherpeton, evolved roughly 60 million years in the future, and quickly diversified into the variety found today.
Halilycosaurs have a skeleton that is similar to their ultimate ancestors, the monitor lizards, but with several derived features. The bones of the legs have shortened, and the toe and finger bones (phalanges) have simultaneously elongated, giving the flippers a structure uncannily similar to that of plesiosaurs. The skull in particular is very different from that of monitors, with the back of the skull being fused completely to the braincase, and the lower jaw is also completely fused to give the animal a powerful bite. The top teeth are more robust than modern varanids, and are similar in structure to crocodile teeth, though with small serrations on the edges. The lower teeth are weaker, though hollow to deliver venom. Like crocodiles and sharks, halilycosaurs shed teeth throughout their lives, and their teeth are common components of most sediments dated after 60 MyF.Like most fully marine reptiles, halilycosaurs are viviparous, and usually have up to three pups.
The large filter feeding Thaumobalaenids have longer but much weaker teeth, which are always tightly packed to help strain small fish and krill from the water. These genera also lack venom altogether, though they still show a more than usual resistance to the venom of other, more dangerous, halilycosaurs.
In life, all Halilycosaurs look like long necked pliosaurs such as the early Jurassic Rhomaleosaurus, including filter feeders, but have the long tail and to some extent the swimming style of a mosasaur, and the colour of an orca or great white shark. Unlike pliosaurs, or plesiosaurs in general, halilycosaur teeth are not visible when the mouth is closed, and as such the head resembles the head of a mosasaur, but with a more rounded profile, and with a prominent blowhole.
One distinctive feature of almost all halilycosaurs is their almost uniform enormity, with all species able to achieve 4 metres in length as adults. The cause is unknown, but may be related to their extreme longevity, most large species can live in excess of 70 years, and their reptilian ability to grow during adulthood.
There are at least five families of halilycosaurs, with possibly more waiting to be discovered. They are classified into two groups, much as whales are.
This group contains most macropredatory halilycosaurs. Cavodontiform means 'cavern tooth form', refering to their hollow lower teeth, used to deliver venom. This tooth design is also found in Deinovenenosaurid tenudontiformes.
The type family of the group, the Halilycosaurids have been termed the 'wolves of the sea', hence the name - Halilycosaur means 'salt/ocean wolf reptile - and these animals certainly live up to their name. They travel in groups up to 30 strong, and overwelm any animal that is neither fast or large enough to get away, and are aided by their highly venomous bite. Halilycosaurids are found in all oceans, and are one of two groups of halilycosaurs to act co-operatively, with the other being the more pelagic. Although they are vicious predators, Halilycosaurids are all rather small, averaging around 4 metres, and none are larger than 6 metres long.
Mortanatators are much larger relatives of the Halilycosaurids, but unlike them they don't hunt co-operatively. They are also much larger, with all species being over 9 metres long, and the biggest species may be up to almost 20 metres long. Like all predatory halilycosaurs, Mortanatators have venom, though compared to their smaller relatives it isn't as strong, and they rely more on their bite itself to do the damage. Like their smaller relatives, Mortanatators are found throughout the worlds oceans. 'Mortanatator' means 'death swimmer' in Latin, and this is because members of this group are known to attack other animals out of individual rage, and then discarde the carcasses without eating them.
The anguillosaurs are slender halilycosaurs that dwell in large rivers, lakes and coastal shallow waters. Unlike other halilycosaurs, anguillosaurs have long necks and tails, and small flippers, meaning they primarily swim using an eel-like horizontal ungulation of the whole body, hence then name - Anguillosaur means 'eel-lizard'. In the ocean, anguillosaurs are a common prey of their agressive relatives, and as a result they have an extrordinary tolerance to the venom of most other halilycosaurs, though this is rather ineffective against an attack by a Mortanatatorid. Anguillosaurs are themselves predators of fish and small animals, and often have a small head to avoid disturbing the prey before striking. The largest species may be up to 9 metres long, but most species average about 4 metres long. Anguillosaurs are the dominant predators in a large lake formed in the Bengal basin after the collision of east africa with South East Asia, where other halilycosaurs are absent.
The Aquatyrannids are a small family of halilycosaurs that are almost completely pelagic, and show some resemblance to small pliosaurs and Polycotylids. This group also have a much more forked caudal fin, which enables for rapid swimming. Like the Halilycosaurids, the Aquatyrannids hunt in groups, though they are more losely co-ordinated, and tend to go after smaller prey. Most species average about 6 mtres long, with no know species exceding 11 metres.
This group is named after the closely spaced, long thin teeth used to filter feed small animals from the water, hence the name, Tenudontiform means 'net tooth form'. These features are largely only evolved in the derived Thaumobalaenids, and the more primative Deinovenenosaurs physically have more in common with the Cavodontiformes.
The Thaumobalaenidae are, quite literally, the largest animals ever, with some leviathanic genera exceding 40 metres long, dwarfing even the mightiest of whales. Thaumobalaenids, like all halilycosaurs, have a long neck, powerful flippers, and a powerful tail fin, giving them considerable speed and manouverability. The real difference is the design of the skull, which is much more lightly built and flexible than that of the smaller predatory halilycosaurs they coexist with, which enables them to open their mouths very wide to hold vast amounts of water, which is then strained through the thin, needle-like teeth, which act the wame way baleen does in whales. The gape of the largest species can excede 4 metres wide, and is large enough to swallow a car. Adult Thaumobalaenids lack venom, as they have no need for it, though they still possess the gland, and the pups still have it to protect themselves from the miriad of predators they live with.
The Deinovenenosaurids are similar to the Mortanatators in niche and size, with all known species exceding 8 metres, but unlike the Mortanatators, the skull of Deinovenenosaurids is not massively reinforced, meaning that the genera of this family have proportionally weaker bites. They compensate this by having very powerful venom for such a large animal, quite in contrast to their behemoth nonvenomous relatives. Deinovenenosaurs are also more restricted in range, they only inhabit the Pacific (which is much smaller 100 Million years in the future than it is currently), and have developed an amount of speciality in that time. The similarity in skull design between the Deinovenenosaurs and the Thaumobalaenidae suggest that they are closely related, despite their significant differences.