The genus Tinodon was described by American paleontologist Othniel Charles Marsh in 1879, from remains found in the "Jurassic beds of the Rocky Mountains" (now known as the Morrison Formation).
[2][3] An additional species was described from teeth from England's Purbeck Group in 2000 by French paleontologist Denise Sigogneau-Russell and British geologist Paul Ensom.
[3] A tooth found by German paleontologist Georg Krusat in 1969 at the Lourinhã Formation in Portugal[6] has been tentatively assigned to Tinodon, but the identity of the specimen remains uncertain.
[8] Crompton and Jenkins also compared the teeth with the only known tooth (an upper molar) of Eurylambda, another genus of mammal from the Morrison Formation, and wrote that the molar outlines and corresponding wear patterns of both specimens made it "extremely probable" that the "Eurylambda" specimen simply represents an upper tooth of Tinodon.
[12] The family was later included in a 2010 study by Russian paleontologists Alexander Olegovich Averianov and Alexey Vladimirovich Lopatin that analyzed the relationships between various mammalian groups based on scientific consensus, which rejected the traditional idea that Mammalia could be divided into three subclasses (Prototheria, represented by living monotremes; Allotheria, containing the extinct multituberculates; and Theria).
[13] A similar 2014 consensus-based analysis by Chinese paleontologist Shundong Bi and colleagues contradicted the results of the 2010 study (without citing it directly) and reaffirmed the placement of Tinodon among crown mammals, specifically as a sister group to Allotheria.
However, the paper's authors noted that this was inconsistent with its usual taxonomic placement (citing Kielan-Jaworowska et al.) and that its position was unstable.
[3] Its Portuguese record is considered tentative, based on a single lower molar, with some of the specimen's features more closely resembling those of a spalacotheriid.
[7] Besides Tinodon, the Morrison Formation has yielded remains of over a hundred species of vertebrates, including dinosaurs (such as Stegosaurus, Allosaurus, and Apatosaurus), crocodiles, turtles, frogs, salamanders, and other mammals.
[16] The mammals of the Purbeck Group, while somewhat similar to those of the Morrison Formation and traditionally believed to have been Late Jurassic in age, have been more recently suggested to come from the Berriasian of the Early Cretaceous.
[17] The Purbeck Group has been known for over a century for its fossiliferous limestone, which is believed to have been deposited in shallow water lagoons of varying salinity, from hypersaline to brackish to fresh.
British paleobotanist and paleoclimatologist Jane Francis, whose doctoral dissertation was written on the fossil trees of the Purbeck Group, suggested that the shallow roots and sparse and waxy foliage of the trees were adapted for a climate with little rainfall and arid summers, similar to parts of modern-day North Africa.