Kolponomos first surfaced in 1957, when a partial skull -- technically a rostrum, the front portion of the face -- was found at Slip Point near Clallam Bay, Washington, eroding from rocks of the Clallam Formation. Paleontologist Ruben A. Stirton of the University of California Museum of Paleontology in Berkeley described the new genus in 1960, naming it Kolponomos clallamensis after the formation where it was found. Stirton tentatively classified it as a relative of raccoons, but the fragmentary nature of the specimen left its true identity unresolved. A nearly complete cranium appeared at the same Slip Point location in 1988, offering far more anatomical detail but deepening rather than resolving the mystery. The animal's teeth and skull architecture did not match any known group of living carnivores in a straightforward way.

The second species, Kolponomos newportensis, owes its discovery to Douglas Emlong, an Oregon fossil collector with an extraordinary eye for marine mammal remains along the Pacific Coast. Near Newport, Oregon, Emlong found a concretion in two pieces -- the first in 1969, the second eight years later in 1977. Inside lay a nearly complete skull, jaw, and post-cranial bones, the most complete Kolponomos specimen ever found. The concretion had been cemented by tectonic forces into something approaching the hardness of industrial concrete. When R. Tedford, L. Barnes, and Clayton E. Ray formally described the new species in 1994, they noted the extraordinary difficulty of its preparation. The discovery proved decisive: K. newportensis showed clearly that Kolponomos was not related to raccoons at all but instead belonged to the evolutionary lineage that gave rise to pinnipeds -- seals, sea lions, and walruses.

Kolponomos lived during the early Miocene epoch, about 20 million years ago, when the coastline of what is now the Pacific Northwest looked nothing like it does today. The animal was bear-sized and powerfully built, with a feeding strategy unlike any living mammal. Research published in 2016 revealed that Kolponomos used its large, forward-projecting canine teeth to pry shellfish from rocks -- a stabbing, leveraging motion convergent with the bite mechanics of sabertooth cats. Once the shellfish was detached, the animal crushed the shells with its massive, flat-topped molars, a technique functionally similar to how sea otters use stones to crack open their prey. This combination of sabertooth-like acquisition and otter-like processing had never been documented in any other animal, living or extinct. Aspects of its jaw mechanics also resembled those of Siamogale melilutra, a giant otter from Miocene Asia, though Kolponomos was not an otter.

Before the discovery of K. newportensis, scientists debated where Kolponomos fit in the mammalian family tree. The raccoon hypothesis was the best guess available from the fragmentary K. clallamensis material. The complete Oregon specimen changed everything, placing Kolponomos as a stem-pinniped -- an early branch on the evolutionary tree that would eventually produce the seals, sea lions, and walruses of today's oceans. Both species are associated with the late Arikareean North American Land Mammal Age. They lived along a coastline rich in shellfish, where the transition from land-based predator to marine specialist was still underway. Kolponomos represents one of evolution's experiments in that transition: a large, powerful carnivore that had committed to the sea for its food but retained the body plan of something that still walked confidently on land.