Main Article: Triassic Divergence
Xenacanthida are the freshwater "sharks" in the world of Triassic Divergence, found in the Americas and Africa.
Evolving in the coal swamps of the Carboniferous, the xenacanths were, for a time, the largest predators in the world's freshwater environments. They diversified during the Permian but almost died out during the mass extinction that marked the end of the period. During the drying of the Triassic, they declined even further due to a decrease in their characteristic environments. The Triassic-Jurassic extinction event of our timeline marked their total extinction.
In the colder, wetter conditions of the Extended Triassic, Jurassic, and Cretaceous, the xenacanths underwent a surprising radiation (especially in the southern continents). Some forms attained impressive sizes, including the giant Megaxenacanthus which attained sizes of over 5 m (16 ft) in length.
In the Cretaceous-Paleogene Extinction, much of the group's diversity was lost. The xenacanths of the northern continents entirely died out, as did the largest lineages in the south. The Australian xenacanths became extinct during the drying at the end of the Oligocene, and the Antarctic xenacanths presumably became extinct at some point before the continent became glaciated. The surviving xenacanths can be traced back to one ancestral lineage that survived the K-Pg Extinction in South America and one that did so in Africa.
The living xenacanths can be grouped broadly into two major clades based on their continent of origin. The Afroxenacanthidae is the family containing all the xenacanths known from Africa (though their prehistoric range extended beyond that continent), and the Neoxenacanthidae and Alloxenacanthidae represent the lineage that evolved from an ancestral lineage of xenacanth in South America. These xenacanths are known from across the Americas.
The afroxenacanthids tend to have a fairly typical "xenacanth" body form. They have long eel-like bodies, secondary dorsal fins that wrap entirely around the tail, low primary dorsal fins, semi-lobate pectoral and pelvic fins, and short snouts. The teeth of all afroxenacanthids show the typical arrangement of interlocking points, and many feed on tough shellfish. Species range in size from approximately 0.5 m (1.6 ft) in the smallest species to 2 m (6.6 ft) in the largest.
Afroxenacanthids are found today in all of Africa's major river systems (though most common in the Congo) and in the Rift Lakes. Within the afroxenacanthids, most of the phylogenetic grouping is based on the geography of local watersheds. The largest species are found in the channels of the Nile and Congo rivers, where they are known to feed on some terrestrial animals.
The alloxenacanthids deviate significantly from the traditional "xenacanth" form. They are characterized by hyper-lobate fins, which they use for most of their locomotion. The tail is shorter and stockier than other xenacanths, and the caudal fin has a much greater average radius. The primary dorsal fin is almost absent in most species, and the body is torpedo-shaped (or even almost discus in some species). A wide variety of diets are known within this general form. Most species are piscivores of some form, but some are specialized in eating tetrapods or very large fish. Some tend to eat shellfish, and related species are specialized in cracking seeds of the neomeiales that dominate their habitat. One species even eats wood of fallen trees, having almost rodent-like teeth and colonies of bacteria in its stomach capable of breaking down cellulose. By species number, they actually represent the largest group of living xenacanths, despite their limited range.
The alloxenacanthids are known only from the tropical rivers and lakes of northern South America. Almost all inhabit the Amazon watershed, though a few species are known from the Orinoco and Magdalena Rivers.
Neoxenacanthidss tend towards the more typical end of the xenacanth body form spectrum. Most retain a long eel-like form, a merged secondary dorsal, caudal, and anal fin, and spines on their backs. Most neoxenacanthids, however, are dorsoventrally compressed and have pectoral and pelvic fins that tend to be more lobate than those of the afroxenacanthids. These are adaptations to living in shallow water to feed on small to midsize terrestrial animals. Most adopt this lifestyle, though a few are piscivorous.
Neoxenacanthids are found across the tropical and temperate freshwater bodies of the Americas. They tend to live in more oxygenated areas, as they lack any system to breathe air. Within these constraints, however, they are very widespread. Neoxenacanthids are recorded from the Pampas in South America to the Missouri River watershed in North America.