This fish was quite small, only getting to around 25–30 cm or 10-12 inches long. This is about as big as some of the smallest sharks around today, like the pygmy laternshark.^[1] Falcatus was a chondricthian known as a "cladodont-toothed stethacanthid holocephalan".^[2] The first material known from the genus were the prominent fin spines that curve anteriorly over the head of the animal. When first described in 1883 from the St. Louis Limestone, these remains were given the name Physonemus falcatus. However, in 1985, fossils of a new type of condrichthyan from Montana were described that displayed a high degree of sexual dimorphism. The same spines that were previously named P. falcatus were found on one of the morphs, identified as the male due to the presence of valvae.^[3]

Despite often being called a shark, Falcatus and its relatives were part of the order Symmoriiformes, which itself was part of the subclass Holocephali.^[3] This means that this fish was more closely related to Chimaeras than to true sharks.^[4] Other members of its family include Ozarcus from the Carboniferous of Arkansas,^[5] and Cretacladoides from the Cretaceous of Austria.^[6]

The bear gulch limestone is a fossil deposit from the Big Snowy Mountains of Montana. It is a smaller part of the larger St. louis limestone, which dates to the middle carboniferous. During the time, the area was a series of mudflats and lagoons with brackish and freshwater.^[7] Many theories have been put forth for the preservation. One is that the creatures sank to the bottom and died of asphyxiation in the oxygen poor waters, being preserved without scavenging took place.^[8] Another theory is that the bottom of the bay created mudslides because of heavy rainfall, which rapidly buried the creatures.^[9] However, because many of the fish fossils were found with distended gills, this would suggest death by asphyxiation.^[10] Falcatus lived alongside many strange creatures like the chondrichthyans Agassizodus, Listracanthus^[11] and Delphyodontos.^[12] It also lived alongside many ray-finned fish like Discoserra^[13] and Paratarrasius.^[14] Other fish included the rhabdodermatid Cardiosuctor,^[15] the rhizodont Strepsodus,^[16] and Hardistiella, one of the oldest known lamprey.^[17] The invertebrates of bear gulch were very diverse creatures, like the hoplocarids, which include modern day mantis shrimp,^[18] Anderella, which is the youngest known synziphosurine^[19] and more enigmatic creatures like Typhloesus,^[20] and the "Square objects" which might be sea salps or cnidarians.^[21] Other inverts include, mollusks like the nautiloid Tylonautilus,^[22] pterioid bivalves which have been found encrusting sargassum like brown algae^[23] as well as productid brachiopods,^[24] Paleolimulus,^[25] phyllocarids^[26] and echinoderms like Crinoids, echinoids, sea stars, brittle stars and a many armed starfish called Lepidasterella montanensis.^[27]

• David Norman. (2001): The Big Book Of Dinosaurs. Pg.84 and Pg.286, Walcome books.

• http://www.elasmo-research.org/education/evolution/golden_age.htm
• https://web.archive.org/web/20070927011823/http://comenius.susqu.edu/biol/202/Animals/DEUTEROSTOMES/CRANIATA/CHONDRICHTHYES.htm