Mosasauria

Of the seas of the *New Dinosaur's* Holocene, some of the most largest and successful groups of marine reptiles thrive: Mosasaurs. Mosasaurs, large carnivorous squamates closely related to monitor lizards and snakes appeared first in the Cretaceous, evolving and diversifying into many forms. Initial forms were slender and sleek, but as time progressed on, the Cenozoic brought on some of the more derived mosasaurs, some even being more fish-like than forms before.

## History

The early mosasaurs, taxa like *Dallasaurus* are small, had distinguished fingers, and primitive-form tails. These unusual lizards were some of the early mosasaurs, of the family Mosasauridae. Mosasaurids became adapted for short bursts of speed, becoming ambush predators.

Many of the modern mosasaurs come from the superfamily selachesauroidea, but there are more primitive lineages just outside of the superfamily. As the world cooled during the Eocene, the mosasaurids became almost extinct, letting the selachesauroids take major niches. Selachesauroids have evolved adaptations to *pelagic*, active lifestyles, leaving the coasts and being open-ocean, or pelagic creatures.

Selachesauroids continued to diversify and evolved new body plans, like the weird bownoses, or the stocky, macropredatory Morcas, or even the unusual phinnol selachesaurs, or dolphin-like mosasaurs.

   In the Pleistocene, only a few groups in between selachesauroidea (and the selachesauroids themselves) and mosasauridae survived. The "normal" body plan for this group were a very strongly lobed, lunate tail fin, a dorsal fin and a sleek, streamlined build adapted for hunting. There were other groups, like the enigmatic anguillosaurs, or river mosasaurs, slender and flexible, the cetoselachids, filter-feeding species, the enigmatic gigantopods and the sleek pelagic selachesauroids.

Mosasaurs now take up most undersea niches, like specialised durophagous mosasaurs, piscivorous mosasaurs, generalists and macropredators. This extremely diverse group are essentially the "cetaceans" of their world.

## Anguillosauridae

Anguillosaurids are medium-sized, small mosasaurs that have anguilloform, or eel-like (hence the name) bodies. They range from sizes from a metre, to up to three metres. This group is extremely rare, only being found in several parts of Southeast Asia.

### Indonesian eelsaur (*Striatanectus minor*)

   The Indonesian eelsaur is the smallest known species of eelsaur, up to a metre in length. Typical in form of other anguillosaurids, just shrunk down, it has a pointed head, medium-length neck (which has distinct collar patterns on it, giving this species the very appropriate name, the *collared eelsaur*) and a long, anguilloform body with short front limbs and a hypocercal tail, like its relatives. It is very rare, thus not much is known about it.

### Australasian heesar (*Truculodentus australasianus*)

    A common species found around the region of Australasia, the Australasian heesar. This species of medium-sized anguillosaurid mosasaur is well known for its unusual claws on its fins, actually adapted scales that are strengthened by other scales, used to pull against interrupting flora that gets into its way. A large species, up to 2 metres in length.

## Praeteritovaranidae

Whereas the vast majority of mosasaur taxa have stayed fully marine or lacustrine, at some point during the Cenozoic a species evolved that defied that trend, seemingly undergoing a brief evolutionary reversal to terrestriality. While this did not last, the vestiges of this stage can be found in the only living taxon.

### Lesser Slizard (*Praeteritovaranus commensalis*)

The lesser slizard is one of the smallest mosasaurs, attaining a length of just 0.8 metres (2.8 ft), and weighing a mere 6 kg (15 lbs). An adept hunter of small fish, it diverged from other mosasaurs roughly fifty million years ago. Its body is eel-like and sinuous, its backbone more flexible. It occupies the niche that would otherwise be occupied by the moray eels of our reality.

Perhaps the most notable aspect of the lesser slizard’s biology, however, is its symbiosis. Its breeding season is synchronized with that of a stem-octopod, *Pachymuensterella polynesiene*. Whereas most octopods protect their young until their inevitable deaths, *P. polynesiense* has formed a symbiotic relationship with the intelligent lesser slizard, and the two taxa lay their eggs and rear their young, respectively, in the same general area. They take it in turns to protect each other’s young, an efficient strategy that allows *P. polynesiense* to maximize the amount of young produced.

In the absence of *Pachymuensterella*—or if the individuals concerned are unwilling to cooperate—a pregnant slizard will make her way to the deepest recesses of the reef. There, after staking out the area to find out the quickest way to the surface, she will give birth. Her pups are fully independent, though may stay with their mother for some time; unusually for varanoids, some mosasaur taxa tolerate the presence of their young unless the situation is truly dire.

The slizard’s morphology, notably the pronounced fingers and flexible torso, seem to indicate an evolutionary reversal—its ancestors briefly returned to the land. Roughly fifteen million years ago, it seems as if they simply had enough of this new environment, and the processes of evolution rapidly turned them into aquatic hunters once more. This is one of the more unusual occurrences to have taken place in this strange world.

## Gigantopodidae

In around the early days of exploration of this alternative planet, an unusual corpse washed onto the shore. It was odd, blue-grey with dark bands all over its body. This, looked like some ridiculous elasmosaur, but let me inform you, it was neither. Not a weird creature with an aberrant mutation, this was **darwin's sea serpent**, one of the oddest of all species of mosasaur.

### Darwin's sea serpent (*Caudopterus darwinus*)

The large specimen discovered on that dark night was quickly named 'darwin's sea serpent', as it shows some of the most unusual qualities of nature — evolution is blind, and will do *anything* to achieve a suitable niche. The darwin's sea serpent is a 15-metre long mosasaur with a slender build. The neck is relatively long, the head is slender with hundreds of tiny cone-shaped teeth in its maw. After the neck, two small flippers usually held flush against the body, and a slender body. The most unusual quality of this slender mosasaur is found at its pelvis: a pair of very large flippers. These flippers are used to gain extra thrust while catching its small cephalopod prey. The ischium of this creature is unique, actually creating a *keel* that holds wide muscles. From the front, it almost appears pot-bellied. The tail is heavily hypocercal, with a lower fluke much larger than the slender upper fluke.

The skeleton of this creature is unusual. The pelvis is loosely attached to a sort of sacrum, and instead of five fingers and toes, only digits I and II (on manus) have survived on the hand, and digits I and II on the back flippers are fused. The ribs are extremely small, and there is a surprising amount of haemal arches, This unusual morphology have baffled many, but are almost certainly perfectly adapted for their deep-water environment. They are teuthivores, eating squids, cuttlefish, belemnites and a wide variety of shell-less cephalopods.

Another notable trait is a small sac on the tail. Derived from the anus, this small sac has several gallons of a black liquid. While unknown what *exactly* this liquid is, it is most likely a defensive system, to essentially disappear in a cloud of faecal matter.

The species is found worldwide, in mesopelagic zones ranging from 400 to around 1,200 metres in depth. It hunts by slowly waiting in the open, with the bands and stripes on its body to conceal its form, and uses quick bolts of speed by using a combined effort of both its pelvic flippers and tail, to ambush unwary, foolish cephalopods. Most other aspects of its behaviour is unknown, such as reproduction, migrations and feeding patterns.

## Cetoselachidae

Mosasaurs had diversified to a great degree. Some of the most successful groups were the cetoselachids. Closest to the selachocetoideans, these mosasaurs have a general mix of basal and derived traits — acting as a sort of "transitional" clade between the more "basal" mosasaurians, and the "derived" selachocetoideans.

### White-spotted morwhale (*Albumaculatus angustucephalus*)

A common sight off beaches of North America, the white-spotted morwhale is a typical morwhale. The teeth are comb-like and thin, appearing as rake-like and clumped together tightly. This filter feeder uses the white spots on its chin, the white tips of its fin attract fish and cephalopods, appearing as a "pseudo-school" from afar, similar to the tactic that oceanic whitetip sharks employ in Our Earth.

### Chinwhale (*Acanthocephalus canorus*)

    A benthic species of cetoselachid, the abberant **chinwhale**, a sort of morwhale with a very large lower jaw. It feeds by dragging its enormous lower jaw on the sea bed, raking up small fish, worms and various planktonic benthic animals with relative ease. Adapting to such a niche has forced many adaptations of the chinwhale to appear. Whilst other morwhales have more rounded, shorter flippers, the chinwhale has longer, thinner flippers to push themselves across the sea floor. The prominent chin spikes are formed by fusing of cornified scales, clumping together messily.

With such chin spikes, from a first glance it appears to just be a head defence — but in fact, they are used for *battering* prey items. Occasionally, when benthic plankton is low, or when they need more nutrients, they make a remarkable dietary shift to eating small mosasaurs, hespies and other marine reptiles, using their powerful chins and spines to kill and batter prey to death.

Their melodic calls (hence *canorus* in the name — literally meaning "melodic") are used for stunning prey momentarily, or calling out for mates. Weak, old or young chinwhales (alongside other morwhales) are easy prey for morcas or macropredatory choridosteres. Chinwhales range in sizes from 6 metres to 15 metres in length, weighing up to over a ton.

### Zillwhale (*Angusticetus enigmaticus*)

A deepwater species, Zillwhales are a very enigmatic group. Being filter feeders, they average around sizes from 2 to 4 metres, a very small sizing for a clade known relatively well for their colossal sizes, the zillwhale is more of a small gnat compared to the rest. Zillwhales are supposedly very aggressive, as quoted by one of the explorers of Dinosaur-Earth, a Japanese person whose name was forgotten over the years.

## Pseudoselachidae

A small family, consisting of only 5 species, the pseudoselachids show a great degree of derived traits, such as adaptations to endothermy, by being mesotherms, lunate, lobed tails, carangiform and thunniform locomotion, a prominent dorsal fin and slender build. However, they still have several primitive traits, such as a still enlarged forked tongue (selachosaurids lost this trait, but they still have forked tongues, just reduced), a more flexible spine which isn't stiffened and a more reef-inclined behaviour.