Umbridae
Umbridae is a family of fish in the order Esociformes, which contains pike, pickerel, and mudminnows. The single living genus, "Umbra", occupies weed-choked freshwater habitats in eastern North America and eastern Europe. While the family traditionally contained the genera "Umbra", "Novumbra", and "Dallia", recent genetic and paleontological research have recovered this grouping as paraphyletic, with "Novumbra" and "Dallia" being moved to the family Esocidae. Distribution. Umbridae contains three extant species, all within the genus Umbra: "Umbra pygmaea", "Umbra limi", and "Umbra krameri". "U. pygmaea" can be found across the eastern United States and southeastern Canada. "U. limi" ranges  throughout the Great Lakes region and Mississippi River basin of North America. "U. krameri" can be found in the Danube and Dniester River basins of Europe. "Umbra "spp. are most commonly found in the Atlantic coast regions of North America, along the marshy, low-oxygen areas of the Mississippi River, and in similar environments in Europe. All fossil specimens have been recovered from Europe, occupying similar niches to extant species. Behavior. Like all esociformes, the extant species of Umbridae are sight-based ambush predators, with umbrids occupying the niche of a bottom-feeding generalist. Diet changes based on seasonal availability , however primarily consists of invertebrates in all extant species. Detritus is also a major stomach content throughout the year as reported in "U. pygmaea". During winter months, U. limi has been observed hunting fish. Cannibalism has been observed in at least "U. pygmaea". Umbrids, like "Dallia" and "Novumbra", have an extreme tolerance for hypoxic conditions. "Umbra" has been observed surviving under ice in particularly cold winters, and has the ability to survive desiccation by burrowing tail-first into mud in times of drought. "Umbra" can breathe atmospheric air using a modified gas bladder to absorb oxygen. Relationships. Umbrids have been  present in Europe with fossils of the primitive genus "Boltyshia" dating back to the Paleocene. Following "Boltyshia", fossils of "Palaeoesox", another primitive genus, entered the known fossil record in the Middle Eocene. In contrast to "Boltyshia", "Palaeoesox" had a much longer stratigraphical range, surviving until possibly the Late Miocene based on otoliths assigned to the genus, though at least the Middle Miocene based on the presence of body fossils. "Palaeoesox" overlaps its temporal range with "Umbra"; the two species coexisted within Europe for at least 10 million years, as fossils of the earliest members of the genus "Umbra" have been  recovered during the latest Oligocene. A single species has been assigned to the genus "Proumbra", which was recovered in fossil beds dating to the late Oligocene. Genetic work has reported that separation of the European and North American species in the genus "Umbra" occurred roughly at the end of the Late Cretaceous and earliest half of the Paleogene, at the date of 60.57 million years ago with the 95% highest probability density of 39.57-81.75. The cause of the split in the genus coincides with the separation of the European and North American continents. One of the basalmost genera, "Palaeoesox , preserves depressible teeth similar to those of Esox". This trait may have been present in the last common ancestor of Esocidae before the umbrid/esocid split, and is lost today in "Umbra", "Dallia", and "Novumbra". The following cladogram illustrates the relationship between umbrid genera.