- Main article: Triassic Divergence
Holocephali is the group including all modern chimaerae, both freshwater and marine. They inhabit a variety of niches and have a generally global distribution.
Natural History Edit
The holocephali originated sometime in the Devonian and had moderate success during the Paleozoic in a variety of forms. In the Permian and Triassic, most forms died out, but the chimaeriformes persisted. Though hit hard by the Triassic Mass Extinction, they did moderately well during the Mesozoic and persist to the present day.
During the extended Triassic, a Gondwanan lineage of holocephalan evolved a tolerance for fresh water. This lineage gave rise to all the living neochimaerae -- the freshwater chimaerae. The various groups continued to diversify throughout the Mesozoic, with the neochimaerae doing particularly well in the wet conditions of the Jurassic and early Cretaceous. The Cretaceous-Paleogene Mass Extinction mostly hit the marine chimaerae, though the freshwater forms in Laurasia died out. Since that time, evolution of the neochimaerae has been shaped by biogeography. Each southern continent seems to have an endemic lineage of chimaerae that evolved in isolation from the others. With Africa contacting Eurasia and South America colliding with North America, the neochimaerae have once again spread into the northern continents. However, these can be traced back to southern ancestors in South America or Africa.
The archaeochimaeria includes all living chimaerae that inhabit salt-water. Thus, it is likely that (since the other groups are derived freshwater animals) it represents a paraphyletic grade rather than a monophyletic lineage. Archaeochimaeria then represents the sister group to -- or a grade leading up to -- the neochimaeria. This group includes all of the various lineages that ancestrally inhabit freshwater (though there are a few brackish forms that may be found in marine environments). Within the neochimaeria, the australochimaeria (the chimaerae of Oceania and possibly one lineage in Madagascar) come out to the sister group of the rest. The neochimaerae (chimaerae of the Americas) are then the sister group to the fluvochimaeria and benthochimaeria (the two lineages of Old World chimaerae).
The archaeochimaeria represent a clade (or grade) of chimaerae that are ancestrally marine. They are not nearly as diverse as their freshwater counterparts, but they are more diverse than those of our timeline. All swim using their pectoral fins, and most have crushing tooth plates allowing them to feed on hard shellfish.
Members of the archaeochimaeria are found in marine habitats worldwide, from reefs to polar oceans and from estuaries to the abyss. Most, however, are limited to environments without many other fish species, such as polar oceans or the deep sea.
Parahydrolagus - "Rabbitfish" common below 200 m, usually in mid to high latitudes. They tend to be generalized predators of shellfish.
Parachimaera - "Ratfish" that tend to live in environments closer to shore than Parahydrolagus but are less common. They tend to be generalized predators of shellfish.
Archaeochimaera - Plesiomorphic chimaerae that tend to be found shallow habitats with muddy bottoms throughout the low and middle latitudes. Feeding on shellfish with a fairly generalized form, they seem to be morphologically similar to the ancestor of most living chimaerae.
Batochimaera - Chimaerae remarkably similar to the rays of our timeline and the freshwater Heliochimaera of this one, they tend to inhabit sandy flats in shallow environments worldwide.
Litochimaera - Similar to Batochimaera, though they tend to frequent nearshore habitats where waves and tides cause constant environmental disturbances.
Labridochimaera - An extremely diverse genus of reef-dwelling chimaerae with a wide range of diets and coloration.
Expleretochimaera - Chimaerae known from the extreme deep ocean, where they tend to live around hydrothermal vents. They are completely lacking in pigment and have exceptionally low metabolic rates.
Abyssochimaera - Slow moving scavengers and predators of the abyssal plane. They have chisel-shaped teeth and great bite force, allowing them to tackle any sort of food they come across.
The freshwater chimaerae of Oceania are found in a number of roles but are unusual in that there are relatively few piscivores in this lineage. The reason for this seems to be due to competition from limnarchians. Otherwise, they occupy a variety of detritivore and herbivore niches, as well as having many forms that typically eat shellfish.
The range of the australochimaeria is limited to fresh and brackish waterways in Oceania, primarily Australia, New Guinea, New Zealand, and Tasmania. Interestingly, one ambiguous genus in Madagascar tends to group with the Australochimaeria in cladistic analysis.
Australochimaera - The most common and widespread genus of chimaerae in Australasia, it is found across the mainland of Australia. They feed on detritus on river and lake beds and have unspecialized dentition.
?Madagascochimaera - A genus of chimaera endemic to the island of Madagascar. They are unspecialized bottom-feeders that seem to be the sister group to the chimaerae of Oceania.
Among the living chimaerae, the members of the neochimaeria are the most diverse. They and the xenacanths were the only fish groups to survive the Cretaceous-Paleogene Mass Extinction on that continent, allowing both groups to radiate quickly and widely. They occupy every ecological niche available to them, from midsize predators to tiny insectivores, and are represented in a variety of body forms.
Neochimaeria are the endemic holocephalans of the New World. They are found from Patagonia to Alaska and everywhere in between, though most common in South America.
Neochimaera - The "typical" New World chimaera, it contains a number of species of bottom-feeding detritivores. They are found across the waterways of South and Central America.
Islachimaera - Chimaerae isolated on the islands of the Caribbean during the Last Ice Age, they have speciated into a number of forms, especially on the larger islands.
Unusual among chimaerae, members of the fluvochimaerae are characterized by an enlargement of the caudal fin, allowing them to accelerate rapidly to capture prey or escape predators. Mostly, however, they swim using their pectoral fins. They evolved in the isolation of early-Cenozoic Africa to dominate the water column. Fluvochimaerae are represented by a variety of forms, some comparatively large piscivores, herbivores, top-feeding insectivores, and suspension-feeders. Size, too, ranges widely. Some of the smallest of the living chimaerae belong to this group, while it also includes some of the largest predatory forms.
Fluvochimaeria are found in open waterways throughout the Old World. Most common in Africa, they have representatives in Europe and Asia as well.
Fluvochimaera - Typical midwater chimaerae of Africa and South Asia, similar to our perch. They are opportunistic feeders.
Malagasiochimaera - The native fluvochimaerae of Madagascar. They are opportunistic but tend to spend more time on the bottom than their mainland relatives due to the lack of benthochimaerae in Madagascar.
Tanganyikachimaeridae - An extremely specious family of chimaera that have diversified from an ancestral stock in Lake Tanganyika.
Malwichimaeridae - The chimaerae of Lake Malawi, represented by about 800 species in several poorly-resolved genera.
Victoriachimaeridae - Lake Victoria chimaerae, less diverse than their cousins in Lake Tanganyika and Lake Malawi but still numbering in hundreds of species.
Likely the most derived of the living lineages of chimaerae, the benthochimaeria are adapted to life on the bottom of lakes and river systems. The pectoral fins have evolved into "wings" running most of the length of the body, giving them a decidedly ray-like appearance. Some genera feed by suction, some have crushing plates to feed on shellfish, and a few are herbivorous, feeding on algae and plantlife on river and lake beds. The gills are covered by a cartilaginous operculum and are moved towards the dorsal surface of the animal (and the external nares do the same), allowing it to respire while resting on the bottom.
Benthochimaeria range across the Old World, including Europe, Asia, and Africa.
Heliochimaera - Viewed from the ventral surface, this chimaera is almost perfectly circular. It inhabits shallow fresh and brackish waterways of South and Southeast Asia, where it spends most of its time buried in the sediment. It tends to feed primarily on detritus and slow-moving, easy-to-catch prey.