- Main article: Triassic Divergence
Limnarchia represents one lineage of truly ancient amphibians that live only in present-day Australasia.
Natural History
Pre-Divergence
Limnarchian "amphibians" first evolved in the coal swamps of the Carboniferous. Unlike other temnospondyls, they adapted to a more aquatic lifestyle. Though almost going extinct at the end of the Permian, they experienced another radiation during the Triassic before being largely outcompteted by archosaurs. In our timeline, they nearly went extinct at the end of the Triassic but managed to hold out until the Cretaceous in isolated areas of Gondwana.
Post-Divergence
With the phytosaurs more cold-intolerant than our crocodilians, limnarchian amphibians were able to diversify during the cooling of the Jurassic in the high latitudes of polar Gondwana. Towards the end of the comparatively dry Cretaceous, one family (Limnarchidae) seems to have evolved the ability to enter a dormant state similar to that of our lungfish. This lineage survived the Cretaceous-Paleogene Extinction (if only just) in Australia and diversified as the only large, semi-aquatic predators on that continent. Today, they may be found in any waterway large enough to support them during the rainy season and typically feed on large terrestrial animals such as cynognathians.
Phylogeny
Phylogeny of the living members of the limnarchia (limnarchidae)
Living limnarchians belong to the family limnarchidae, which is nested within the trematosauria. Interestingly, this is also the lineage containing the longest-lived limnarchians of our timeline. Presumably, the ancestral members of this family had the ability to enter a temporary dormant state to cope with seasonal changes. Within the limnarchidae, there are two subfamilies, one of which is likely paraphyletic. The archaelimnarchinae include those limnarchians which retain the ability to enter a seasonal dormant state, while the neolimnarchinae have lost this trait.
Archaelimnarchinae
Description
The archaelimnarchinae includes limnarchians which retain the ability to enter a dormant state in burrows for at least part of the year and may be paraphyletic. In addition to retaining this ability, these animals typically have better-developed limbs than members of the neolimnarchinae (to varying degrees), a greater reliance on lung-breathing than cutaneous respiration, and a paddle-like tail that does not increase in diameter towards the end. They are more limited in size than their relatives without a dormant state, however, meaning that the largest recorded members of this group are around 2 m (6.5 ft) in length.
Range
A greater capacity for terrestriality allowed members of the archaelimnarchinae to spread across the continent of Australia. They are found on the mainland in nearly every body of water capable of supporting them (including deep into the outback), on Tasmania and New Guinea, and on a few of the larger islands near Australia.
Genera
Limnarchosuchus - The larger members of archaelimnarchinae tend to belong to the genus Limnarchosuchus. They are ambush predators ranging across the entirety of Australia, New Guinea, and Tasmania, and they tend to feed on tetrapods. Their forelimbs are well-developed to allow for digging, but the hindlimbs are somewhat less robust. Any body of water more than a meter deep and a few meters across is likely to house at least one Limnarchosuchus, lying in wait for its prey.
Microlimnarchus - The smaller of the genera in the archaelimnarchinae, Microlimnarchus includes stocky limnarchians that tend to ambush smaller prey. They are the most terrestrially-adapted of the living limnarchians, and they can be found in and around bodies of water throughout the range of the living limnarchians. Forelimbs are more developed than hindlimbs, and eyes tend to be large.
Neolimnarchinae
Description
The neolimnarchinae is characterized by a loss of the ability to enter a state of dormancy. They evolved in response to the lack of large aquatic predators such as xenacanths and phytosaurs in Australia in the wake of the Cretaceous-Paleogene Mass Extinction and allows these animals to grow to much greater sizes. The largest members of this group are real monsters attaining lengths of up to 6 m (20 ft) that never leave the water. However, smaller members are also known that only reach 15 cm (6 in) in length at the maximum. Members of the neolimnarchinae are carnivorous or piscivorous ambush predators with comparatively poorly-formed limbs and spinal columns, a paddle-like or forked tail that increases in diameter towards the end, and typically wrinkled skin to allow for greater cutaneous respiration. There is a much greater degree of morphological and ecological diversity in this group than in the archaelimnarchinae, despite their reduced range.
Range
Most members of the neolimnarchinae are limited to the eastern and northern regions of mainland Australia, New Guinea, and Tasmania, where large bodies of standing water are present and common throughout the year. A few members of the group range into the interior of the continent, but they tend to be small piscivores.
Genera
Ichthyolimnarchus - Highly variable in size between species, it is the most fully aquatic of the known limnarchians. The hindlimbs are lost entirely, while the forelimbs are reduced to paddles. Because of its capacity for cutaneous respiration, Ichthyolimnarchus can remain underwater for extended periods of time and only needs to surface for a few seconds at a time in oxygenated water. The tail is prominently forked, and the body is more streamlined than most of its relatives, giving it a decidedly fishlike appearance. All members of this genus are piscivorous, and none is known to leave the water for any period of time. The range of Ichthyolimnarchus ranges across the continent and to New Guinea and Tasmania, though most species are found exclusively in the waterways in northeastern Australia and on the island of New Guinea.
Anguillimimus - Inhabitants of backwaters, swamps, and small ponds through most of the continent, Anguillimimus is a genus opportunistic predatory limnarchian that feeds on anything small enough to fit into its mouth. This includes the fish, tetrapods, and invertebrates with which it shares low-oxygen waters. The eyes of this genus are highly reduced, and the limbs are lost entirely. Cutaneous respiration is marginal in this genus, due to the oxygen-poor habitats it tends to inhabit. One interesting feature of this genus is its ability to stiffen its vertebral column using an intricate series of muscles and ligaments to switch between a low-energy eel-like swimming pattern and quick, stiff swimming pattern to pursue prey. Thus, the end of the tail is still enlarged in this genus, though much less than in some of its relatives. Though this genus remains in water for almost all its life, it has been observed slithering across land between waterways on damp nights. Anguillimimus is found in backwaters throughout the wetter parts of Australia, New Guinea, and Tasmania.
Limnarchus - The genus containing the largest living limnarchians, Limnarchus is an ambush predator that feeds almost exclusively on tetrapods. With extremely wrinkled skin, it can remain submerged for hours at a time without coming to the surface to breathe. However, it retains more well-developed limbs than other members of its sub-family due to its hunting strategy. It is known to lie in wait entirely still for the better part of a day before making an attack, when it lunges out of the water using its paddle-like tail. It then returns to the water to eat its prey, often not making another kill for weeks afterward. Only having to exert large amounts of energy once in several weeks, combined with the typically slow metabolic rate of the limnarchians, translates to enormous animals with an exceptionally low-energy lifestyle. Limnarchus is limited to large bodies of water in northern and eastern Australia.