The most mysterious lineage of marine mammals, thylacophocids are a cryptic but incredibly diverse clade ranging from small, otter-like forms to whale sized marine predators. They are the most successful and disparate group of non-therian mammals, their ancient origins in part implicating for both their bizarre body plans and ecologies as well as cryptic habits, having been pushed to specialised niches by more recent placental swimmers.

Thylacophocids diverged from all other mammals in the early Jurassic, being slightly closer to therians than to monotremes. Genetic divergence rates within the group suggesting a Maastrichtian or Paleocene origin, which is supported by the appearance of Waitoreke-like forms in the early Paleocene of New Zealand and Antarctica. The group probably evolved in these regions, where ‘archaic’ mammals would continue to survive well after they became extinct in northern landmasses; in fact the Miocene Saint Bathans mammal fossils may very well represent small thylacophocids. Thylacophocids probably evolved in response to the extinction of large marine reptiles and the declining landmass of Zealandia, becoming coastal seal-like species.

Thylacophocid diversity remained mostly focused on the southern oceans, but by the Eocene they already developed fully aquatic forms, perhaps driven further by the aftermath of the PETM. It is possible that fully aquatic lifestyles developed independently twice or even more, but regardless these mammals enjoyed a brief global success, becoming important components of marine faunas around the globe.

However, the evolution of cetaceans and the climatic crises at the end of the Eocene began to thin out thylacophocid diversity, pushing fully marine forms to more specialised niches. This ecological displacement wasn’t immediate, and some may even argue that an outright decline didn’t happen per se, but regardless across the Cenozoic fully marine thylacophocids were pushed into more extreme ecological roles while whales underwent several adaptive radiations. ‘Seal-like’ forms nonetheless remained common in the southern oceans until the Miocene, when pinnipeds made their way down there. Unlike the gradual and possibly ambiguous displacement of fully marine thylacophocids by cetaceans, this event was almost immediate and obvious; by the Pliocene only a few freshwater species remained.

The exact number of extant thylacophocid species is unclear, with some arguing for a surprisingly high diversity comparable to cetaceans and pinnipeds, but it is clear that they are highly cryptic animals, being rather hard to observe and generally leading solitary lives.

Waitoreke (Zealandotherium mantelli)

The most basal living thylacophocid (though its ancestors were probably marine and recolonised freshwater biomes in the late Miocene), the Waitoreke makes its living in the lakes and rivers of New Zealand’s South Island. The smallest living thylacophocid at around 5 kg, it is an otter-like mammal with sprawling limbs, a spotted coat and a large broad tail. Much like other ancient non-therian mammals, it possesses venomous spurs in its hind limbs, the only thylacophocid to retain them; occurring in both sexes, these deliver a milder venom than that of the platypus, inducing severe pain that lasts for a few days in human beings. This renders them relatively unaffected by introduced predators aside from particularly stupid dogs.

The Waitoreke is a mostly nocturnal animal, resting during the day on the shore or in the shallows and foraging at night. It is possible that this behaviour was acquired after the Maori arrived to New Zealand and that it was originally cathemeral, much as the kiwi, as it has a rather bright coat and an excellent eyesight which can perceive ultraviolet light. The average sleeping time is 14 hours, though it might engage in hibernation that lasts for up to a week at a time during winter months.

Other than resting and molting it spends most of its time in the water, hunting fish, invertebrates and waterfowl, though it occasionally ventures into land to catch bird hatchlings, lizards, tuataras, insects, bats and introduced mammals such as rats. The Waitoreke swims mostly with its limbs, reserving the tail for quick bursts of speed when ambushing prey.

Adults are mostly territorial, leaving only their turf during the mating season which lasts from June to early August. Both juveniles and traveling adults prefer to wander through water, either taking advantage of local floods or venturing into new water bodies via the sea, but they are known to travel long distances on land if forced. Intersexual competition occurs in both males and females, resorting to the spurs to stab each other.

Like all non-placental mammals, Waitorekes give birth to undeveloped young, pregnancy lasting for about 28 days. Like in platypodes, females lack a pouch and instead use their broad tails as a pouch-like environment, pressing them against their stomach and shielding their young. Also like in monotremes there are no nipples, the young being rather active and walking around their mother’s abdomen in search of mammary glands to lick. The mother stays on shore for about five weeks, when the puggles grow large enough to stay unattended while she forages. They will remain with their mother for about an year of their lives, learning to hunt from her, before they go separate ways. Sexual maturity is reached at about 6 years of age.

Bunyip (Thylacophoca hirsuticus)

Occurring across most of Australia, Tasmania and southern New Guinea, the Bunyip is the last representative of a lineage of seal-like thylacophocids that was common across the southern oceans, disappearing due to competition from true seals. Its ancestors colonised the Australian freshwater habitats during the Miocene, more or less at a time when Australia was becoming drier. As such, while a mostly aquatic mammal that stays in water ways whenever possible, it is also adapted to travel long distances on land, and it bears unique camouflaging behaviours to protect itself.

About as large as a small seal, the Bunyip bears a thick shaggy coat, black or dark brown in colour. Like the Waitoreke, it has an excellent eyesight, and it has traded the typical mammalian scent communications for primarily visual ones, developing large, floppy ears for this express purpose. This lack of scent allows it to blend in while resting on land: by pressing itself against tree trunks or the ground, the shaggy coat breaks its bodily contours, allowing it to remain concealed from predators. As a downside, it has little insulating oil, so like a cormorant it spends a great deal of its time basking. Unlike the Waitoreke it does no possess venomous spurs; its main predators are saltwater crocodiles, dingoes and wedge-tailed eagles.

The Bunyip is cathemeral, bearing more nocturnal behaviours in warmer regions and more diurnal ones in colder areas. It uses its powerful paddle-like limbs to swim, chasing a variety of fish, amphibians, reptiles, water rats, crustaceans, aquatic birds and turtles, occasionally ambushing terrestrial prey as well. When swimming it presses its ears against its neck and head, allowing for a more hydrodynamic profile. The limbs are proportionally larger than those of the Waitoreke, allowing simultaneously for higher speed while swimming as well as more efficient terrestrial locomotion, a difference similar to that between seals and sea lions. Instances of Bunyips collaborating to trap fish have been recorded, and for this and its complex social behaviour it is considered to be among the most intelligent of thylacophocids.

The Bunyip is a rather nomadic animal, travelling across waterways but occasionally walking long distances on land. It is mostly solitary, but conversely it is among the most social of thylacophocids, gathering in small groups whenever there are enough resources to sustain several individuals. The breeding season is highly variable, but it usually occurs from July to September. Competition for mates is mostly non violent, animals displaying their large ears in a complex series of rituals.

Much as in the Waitoreke, the female Bunyip uses her tail as a pouch, staying on land for about five weeks. The young remain with her for up to two years, sometimes even longer if circumstances allow for it. Sexual maturity occurs at 4 years of age.

Dingonek (Dingonek odobenomimus)

Dingonekines are an unique lineage of walrus-like tusked thylacophocids. Although they likely evolved in a marine context — the earliest representative of this group, Eodingonek mangarsahoci, is known from Miocene marine deposits of Madagascar — they have colonised the water ways of Africa where they can only be found today, and are most well represented in Miocene and Pliocene fossil sites, where they co-exist with early hominids. They have likely been ingrained in human consciousness for the longest time, as several African cultures that do not even directly co-exist with the Dingonek have words for this animal and depict it in art. The sole living species occurs in the Great Lakes region; as populations in Lake Victoria and Lake Turkana have been likely seperated for thousands of years, it is possible that they represent different subspecies.

The Dingonek is a massive animal; reaching up to six meters and surpassing a ton in weight, it is way out of any predator’s league aside from the very largest Nile crocodiles. Unlike the Waitoreke and Bunyip — and indeed quite possibly all other thylacophocids — it relies on its massive, muscular manatee-like tail to propel itself, swimming slowly and deliberately but capable of reaching speeds of 30 kilometers per hour in short bursts. It is almost fully aquatic, only periodically dragging itself ashore with its tusks; combined with the warm climate of its habitat, this lead to a loss of its bodily hair, but some individuals still retain patches, which often form clumps that are mistaken for scales. It resorts on blubber to insulate itself, though naturally it has relatively little of it compared to the Arctic walruses.

Being almost fully aquatic, it displays a remarkable reproductive strategy. Like more derived thylacophocids, it has developed an actual pouch, opening close to the cloaca, though this presumably evolved independently from its relatives and in order to free its tail as a propulsive mechanism. Like in both the marsupial Yapok and more derived thylacophocids, this pouch has a sphincter and can be sealed off, allowing for the young to remain waterproof. It hasn’t yet developed into the complex mechanism of other thylacophocids, however, so the female periodically leaves the water to clean it and relax the sphincter. The single puggle is abruptly expelled from it at about four months of age, when it grows too large to be contained, and it remains with the mother for about seven years, when it reaches sexual maturity and leaves. Breeding happens every two years.

The Dingonek is mostly solitary, often defending patches of territory from competitors with its tusks. It is a generalistic carnivore, but it feeds primarily on molluscs and crustaceans it finds along the lake bottom, sucking them up with its lips. Besides dragging the animal unto land and being used in fights, the tusks are also used to feel along the bottom, trailing across the substrate as the Dingonek feeds. The Dingonek is cathemeral and is active across the day. It has a relatively slow metabolism, almost ectothermic, which allows it to save energy and consume less food.

Gambo (Gambonessa burnhami)

Occuring in the Congo River system, the Gambo is the last representative of a lineage of fully aquatic dolphin-like thylacophocids that prospered during the Eocene and disappeared with the appearance of basilosaurid-grade whales, and is essentially their answer to a river dolphin. It has lost most of its fur covering as well as its external ears and its limbs have become paddle-like, allowing it to swim in a rather peculiar way, flying like a penguin using its forelimbs but also undulating its body in a dolphin-like manner; its muscular tail lacks flukes, so it uses the pelvic flippers in their stead, a method of locomotion likely also practised by early whales.

Its teeth have become conical much like those of cetaceans, and while some of its extinct relatives might have had a decent eyesight the murky river water has ensured that the Gambo’s eyes are vestigial; instead, it relies on its long vibrissae as well as electroreceptors along its snout to feel its way. Its nostrils remain close to the snout’s tip, allowing the animal to surface only the tip of its jaws as a “snorkel” and otherwise remain fully submerged.

At 4.6 meters in length the Gambo is the apex predator of the Congo River. It feeds on everything from crustaceans to crocodiles to carrion, bearing a preference for large fish and aquatic birds. It has a dark brown upper side, allowing it to contrast against the murky waters and ambush its prey. It is cathemeral, hunting at all times of day. When it rests, it remains in a diagonal or vertical position, its nostrils surfacing; sleeping is rather brief, lasting for about 2 minutes at most, but performed in series for a period of up to seven hours. Compared to other thylacophocids, the Gambo has a relatively large brain, but there is little evidence of particularly intelligent behaviour in relation to its relatives. Rather, the large brain evolved as means to process the extremely well developed vibrissae-derived touch and electroreception.

The Gambo is a solitary, nomadic predator, travelling up and down river in responses to the seasons, taking advantage of floods to hunt in the flooded rainforest and making its way down to the estuaries during the dry season. Breeding occurs year round, and it usually takes place every four years. Gambos lodge each other to their partner’s flippers with their teeth, often leaving bite marks and occasionally even gouging limbs off. In most non-placental mammals the penis is bifurcated, having somewhere between two (marsupials, platypus, most thylacophocids) and eight (echidna, Waitoreke, Dingonek) heads. The Gambo is however unique in possessing an asymmetrical penis, with one right head and two left heads; the latter tend to be undeveloped, but become larger in response to the right head being damaged.

As a fully aquatic thylacophocid, the Gambo’s pouch is a perfect illustration of the complexity seen in the more derived species. Muscular and well vasculated, this organ essentially functions as an “external” womb, its opening close to the cloaca. After gestating for 28 days, the puggle passes immediately to the pouch only briefly contacting external water, finding it full of milk. During the first week or so of its life, the puggle has non-functional lungs, so it absorbs oxygen and nutrients from this specialised milk, before a combination of vascularisation-driven oxygen deposition and muscular contractions expel the milk and replace it with humid air. The pouch actually has higher oxygen levels and less carbon dioxide than atmospheric air, so the puggle survives on this sealed environment for a period of 3 months. During this time the female surfaces more often, breathing more frequently to supply the pouch with oxygen and expel carbon dioxide from her system, and is in general less active. After the puggle grows large enough, it is expelled from the pouch, which is emptied by surfacing the abdomen and compressed against the body until the next reproductive cycle. The young remains for a period of about five months before leaving. Sexual maturity is reached at around seven to eight years.

A species of bichir relative, the aptly named Congo Country Fish (Cunnopiscis congoensis) bears a commensal relationship with the Gambo, a relationship that may be rather old as these fish diverged from other bichir in the Eocene. These fish, usually free swimmers, are drawn to female Gambo prior to the puggles being born, likely detecting milk particles that may escape into the water. They gather around the pouch and enter it alongside the puggle. They quickly establish themselves within it, feeding on the milk and the puggles faeces and fending off parasites like worms and crustaceans. Males guard a patch usually near to one of the mammary glands, where females lay their eggs. When the pouch is emptied, the eggs and larvae exit it, but some of the adults stay, walking around with their fins and cleaning the puggle, sometimes opening wounds to drink its blood. They are inevitably crushed to death by the growing puggle, which may eat their corpses before exiting the pouch.

Long-Necked Seal (Megalotaria longicollis)

In spite of its name, the Long-Necked Seal isn’t a true pinniped, but a member of a clade of specialised thylacophocids which specialised to a similar style of swimming to true seals, undulating their bodies laterally and losing their tails altogether, resorting instead to their fused hindlimbs. Unlike true pinnipeds, it is fully aquatic, and it developed a specialised pouch similar to that of the Gambo independently, differing in that it is empty rather than temporarily filled with milk and juveniles need to crawl into it, doing so while the mother surfaces her abdomen. Consequently, they have somewhat leg-like forelimbs, and small individuals can still crawl ashore if they find it absolutely necessary; this is not possible for adult animals, which may reach spectacular lengths of 19 meters, making them among the largest non-cetacean mammals of all time.

Besides their massive size, Long-Necked Seals are incredibly bizarre mammals. They have unique elevated snorkel-like nostrils, the only thylacophocids to show substantial nasal retraction, which appear almost horn-like, as well as long necks that may account for a third of the animal’s length; these are used to allow it to breathe without having to surface very quickly, reducing the risk of DCS. It is mostly a bottom feeder, using its muscular lips to dislodge sea cucumbers, starfish, sponges, soft-bodied corals and other invertebrates, essentially grazing the depths. Larger individuals forage deep in the abyssal plains, though it is found just as often on shallower waters, even in freshwater bodies. Having a rather slow metabolism, these animals can hold their breath for up to an hour, though females with puggles usually don’t hold it for more than 12 minutes.

Long-Necked Seals are cosmopolitan, and it is likely that they may form a worldwide species complex. During the last glaciation, a number of populations have ended up in isolated lakes such as the Loch Ness; being only recently separated from their marine counterparts, they are not very genetically distinct, but already show signs of dwarfism, being considerably smaller and rarely exceeding 7 meters in length. Their slow metabolism allows them to survive in these relatively ecologically impoverished areas, feeding on the detrivores that composed most of the fauna.

Sexual maturity is reached at around 9 years of age, and animals usually breed in spaces of two to five years. At their large size only orcas and large thylacophocids pose a threat, with large sharks occasionally attacking juveniles; lake populations have almost no predators to fear.

Tizheruk (Tizheruk suchocephalus)

Inhabiting the waters of the Arctic and North Pacific, the Tizheruk is a seal-like predator that also resorts to the same locomotion style as the Long-Necked Seal. They are in fact close relatives, but in the Tizheruk the pouch opens in sea water, allowing the puggles to swim to it before it closes and ejects the salt water, quickly replacing it with milk. Not needing to climb, the young have no need for forelimbs, which are now vestigial, handless stubs. Tizheruks produce litters of 5 to 7 puggles, which initially co-exist within their mother’s pouch for a period of two weeks before it is emptied, prompting them to eat each other.

Tizheruks are mostly pelagic animals, occupying a macropredatory role away from the more coastal and benthic domains of the orcas, polar bears and sleeper sharks, though they may lurk near ice shelves during winter months. At lengths of 6 meters they are formidable predators of their own right, relying on their speed and crocodile-like dentition to tear chunks of flesh from larger animals such as cetaceans and other thylacophocids, though most prey items are still comparatively smaller fish. They rely mostly on their eyesight to hunt, having lost their vibrissae in order to avoid drag, but they also have electroreceptors along their upper lip.

They are the thylacophocids most hostile to humans, and apparently have learned that turning kayaks and other small vessels results in tasy, defenseless treats. This, as well as global warming and depleting fish stocks, have drastically reduced Tizheruk populations, rendering them the most visibly endangered thylacophocid.

Tizheruks are solitary through most of the year, but during their mating season lasting from April to May they gather in small groups. Males have inflatable nasal and horn-like sacs much like those of several seal species, usually concealed to avoid breaking their hydrodynamic profile but inflated when aroused. As water temperatures are rather too cold they display these structures by surfacing their heads and necks, bobbing them in mad frenzies and occasionally breaching, seemingly running on the ocean surface like grebes.

Cadborosaurus (Cadborosaurus willsi)

The closest relative of the Tizheruk, both lineages diverged around 12 million years ago. It has been speculated that they represent a clade of North Pacific thylacophocids, though this may not be the case as the Cadborosaurus once also occurred in the North Atlantic, Mediterranean and Black Seas until medieval times, having been extirpated from these regions due to hunting. Currently, its only known breeding spot is in the Cadboro Bay, though adults have been found as far away as Rapa Nui.

Like its relative, the Cadborosaurus is a pelagic predator, and has modified its locomotive style to a cetacean-like vertical undulation, the fused hindlimbs forming its equivalent to a tail fluke. Like the Gambo, Long-Necked Seal and Tizheruk it has lost its fur, bearing particularly visible skin fibers similar to those of sperm whales. With these adaptations, it has reach speeds comparable to those of the fastest dolphins, and indeed it occupies a similar pelagic predatory niche, hunting fish and squids. At the size of an orca, it can hypothetically target prey as large as a bottlenose dolphin, but it otherwise prefers relatively smaller targets. Its horse-like visage allows it to perform suction feeding, bobbing its head like a seahorse and sucking small prey with its muscular lips. It has no teeth, facilitating this process.

By contrast, juveniles are coastal animals which sometimes will even breach to exploit crustaceans and other beach morsels, but the basic technique is still the same. Breeding takes place every three years, adults gathering offshore and courting through bizarre echolocations, resonating through their deep muzzles. Like in the Long-Necked Seal puggles crawl to the pouch as their mother surfaces the abdomen, and as such these animals still retain claws on their flippers, which come in handy during mating. Juveniles acquire independence rather early on: after six months inside the pouch, they fend for themselves, staying in Cadboro Bay until their 8th birthday, where they leave for the open sea and might not return for another six years.

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