Beginning spring of 1995 until present, unusual developmental malformations have been observed on many species of vertebrate and documented on individuals of four species of wild ruminates. The most frequently observed of a variety of skeletal anomalies is brachygnathia superior, also called mandibular prognathia or more commonly, underbite, caused by underdevelopment in length and width of the premaxillary bone forward of the premolars. Facial anatomy was examined for bone and tooth malformations on 724 white-tailed deer (Odocoileus virginianus) that were accident-killed or died of natural causes between January 1996 and December 2009 in northern Ravalli County in the Bitterroot Valley of west-central, Montana, USA. Smaller samples (145 total) of hunter-killed elk (Cervis canadensis), mule deer (O. hemionus) and pronghorn antelope (Antilocapra americana) from throughout Montana were examined for facial malformations in 2005-09. Incidences of underbite increased from none observed prior to spring of 1995 to more than 35% in 8 of 9 years since 2000 and over 70% in 2008 and 2009. This abnormality appeared abruptly, vastly exceeding expected and previously documented percentages in wild ruminates, and is a characteristic symptom of congenital hypothyroidism. Our data strongly indicates widespread interference with maternal and fetal thyroid hormone function has been occurring in wild ruminants in Montana since autumn 1994