The Garbani Channel deposits, part of the Tullock Formation exposed in northeastern Montana, have yielded a large sample of vertebrates that probably lived during the Puercan 3 North American Land Mammal Age (NALMA). Four fossils in this sample — three isolated teeth and a medial phalanx — document the presence of a stylinodontid taeniodont, cf. Wortmania. Discovery of cf. Wortmania in the Tullock Formation extends the documented range of taeniodonts during Puercan 3 approximately 500 miles (800 km) northward from the San Juan Basin, New Mexico. Evaluation of the oldest records of taeniodonts, from the Lancian, Puercan, and Torrejonian NALMAs, highlights biases warranting future research. Recent phylogenetic analyses that resulted in numerous ghost lineages indicate that the available fossil record is far from complete. They open the possibility that the origin and initial radiation of taeniodonts occurred in areas yet to be sampled and their first occurrences might reflect immigration of invasive species. The available fossil record of taeniodonts is biased with significantly more abundant and complete specimens discovered in the San Juan Basin than at localities to the north. This bias is also apparent in the available samples of two other lineages of large Puercan mammals, the multituberculate Taeniolabis and the “triisodontid” Eoconodon. Where they occur, taeniodonts are relatively rare members of any local fauna. Is their rarity a product of an ecological bias or a reflection of decreasing population size related to increasing body size?