Temporal range: 76 - 248 MyF
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The Halilycosauria are an order of very derived marine reptiles evolved from a family of theriosaurs known as venenosuchians. The order exists from roughly 76 MyF, up to the formation of Novopangaea, almost 250 MyF, and are the last of many lineages of large marine tetrapods.
Halilycosaurs are large animals, ranging in size from 3meters to over 30 in exceptional circumstances. Due to their ancestors retention and advancement of their venom delivery system, these reptiles are the largest venomous animals to have ever existed. The venom delivery system is still largely the same as their very distant ancestors of the modern day, the monitors, but the muscles powering the bite are much stronger than those of monitors, with the bone crushing force of almost 800 pounds per square inch. The act of closing the jaw actually forces venom through groves in the lower teeth. The venom is a viscous liquid similar to honey so as to stop it from disipating in the water.
The physical appearance of halilycosaurs is rather different to the marine reptiles of the Mesozoic, and of the Holocene. All halilycosaurs are largely similar in shape, a long neck, front fins, one or two dorsal fins, a ventral fin and the hypocercal caudal fin. All true halilycosaurs lack pelvic fins/hind limbs, and even the pelvic girdle is lost in the most derived genera. Like most marine reptiles, halilycosaurs are viviparous, and typically have litters of between 2 and 10 pups.
The Halilycosauria are descended from the family of theriosaurs known as the Venenosuchidae, superficially crocodile-like theriosaurs that are found worldwide in shallow seas and large river systems. One branch of the Venenosuchidae, the almost fully aquatic, and strongly venomous Piscisherpetoninae, started taking up niches of pinnipeds and even new mosasaurs, and in the process, they eventually lost visible hind limbs, and their necks lengthened to make up as much as half of their total length. When the new mosasaurs met their ultimate demise at the hands of the Rex Traps eruptions of 110 MyF, the then coastal halilycosaurs, among other survivors, rapidly evolved to fill the vacant niches. By 120 MyF, the halilycosaurs were dominating the sea.
Halilycosaurs are classified into two groups, much like whales are. The groups are distinguished by their skull and tooth structure.
The Cavodontia (also called the 'Halilycosauroidea') are the most diverse group of halilycosaurs, and all members have a very robust skull, rather different from the lightweight flexible skull of their ancestors, and much more like that of crocodiles. The venom gland resides inside an indentation in the lower jaw, and the act of biting compresses this gland and forces venom into the prey item. The venom is delivered through the teeth in the front third of the lower jaw - which form a single tooth whole similar to those possessed by certain Palaeozoic sharks.
Halilycosaurids are the namesakes of the order, and are small generalist predators that hunt in packs up to 30 strong. They are aided by their highly venomous bite, which is as deadly as a rattlesnake, and even small groups of the animals can overwhelm all but the largest of prey.
Mortanatatorids are the big game apex predators, equivalent to killer whales. The teeth are serrated shark-like blades.
Anguillosaurs are elongated slender halilycosaurs that have only one low dorsal fin and reduced fins.
The Tenodontia are composed of two families, the Thaumosauria and the rare deinovenenosaurs. Both families have highly kinetic skulls, much like modern snakes, and they have the ability to open their mouths very wide, up to 140 degrees.
The Thaumosauridae are the largest of all halilycosaurs, with the largest species being able to exceed 30 meters in length, and 60 tons in weight. Thaumosaurs lack venom as adults, but most pups are born with rudimentary glands. Pups leave their mother at a young age, and spend the first few years of their life as predators, similar in niche to modern reef sharks, before they lose their upper teeth and replace them with modified scales similar to baleen. At this stage, they largely stop producing venom, and become pelagic filter feeders.
Deinovenenosaurs are superficially very different from thaumosaurs, they have very strong venom, are long and serpentine, and have relatively small heads. Deinovenenosaurs are almost exclusively found in the waning pacific ocean, and due to this relative isolation, they have become the top predators of much of this domain.