Ptilodus was a relatively large multituberculate of 30 to 50 cm in length, which is about the same size as a squirrel. Its feet, legs and long tail suggest it was a good climber, so it very possibly lead a squirrel-like lifestyle.
The genus was named by the famous paleontologist Edward Drinker Cope in 1881. Cope also mistakenly assigned some material belonging to this genus to Chirox in 1884. Elements from Ectypodus (Jepsen, 1940) and Neoplagiaulax have also been reassigned to this genus.
There are seven species, and others have been proposed at one time or another. P. nellieae (Bell, 1941) is apparently mentioned in a manuscript, rather than a publication. P. sinclairi (Simpson, 1935) seems to have become Parectypodus sinclairi. In botany, Ptilodus pyramidatus is an extinct Australian plant.
Ptilodus gnomus is the smallest known member of the genus. Hence, the species epithet 'gnomus', which is New Latin for 'dwarf'. Remains have been discovered in Wyoming in the United States, and Alberta, Canada. They were recovered from the Cochrane 2 deposits of the Paskapoo Formation, which have been dated to the Tiffanian stage of the Paleocene.
Remains of this species include over 100 teeth, (upper and lower), and at least one fragment of jaw. The teeth range from 2 to 5 mm in length. The fourth lower premolar (p4) is about 51% shorter than the corresponding tooth in P. mediaevus; 28% less than P. kummae; 15% less than P. tsosiensis; and 5% smaller than P. fractus, which gives some idea of the relative sizes of the various animals. There are also differences in shape and the number of serrations. (Scott et al, 2002)
A couple of isolated teeth which had been previously identified as perhaps belonging to Ectypodus, have been now been placed within this taxon. All referred specimens are held in the collection of the University of Alberta.
The species was named by C. R. Scott, R. C. Fox, and G. P. Youzwyshyn in 2002. Material assigned by Jepsen in 1940, as amended by Gazin in 1956, to Ectypodus hazeni has also been assigned to this species.
Remains of Ptilodus mediaevus have been recovered in New Mexico, Utah, and Wyoming in the United States. They have been unearthed from the depsits of the San Juan Basin, which are dated to the Torrejonian stage of the Paleocene.
Cope named the species in 1881 (Cope, 1881). The species Ptilodus feronensis, or Ptilodus ferronensis, was named C. L. Gazin in 1941 (Gazin, 1941), but all material was reassigned to P. mediaevus by Rigby in 1980. In 1929, Granger and Simpson did the same thing to the species P. plicatus, which was originally named by J. W. Gidley in 1909. Chirox plicatus was named by Cope in 1884. He described it as a marsupial in 1884, but it is also part of this species.
Ptilodus montanus is a large species, about 650 g. A brain cast indicates a well developed sense of smell. Remains were recovered in Montana, in the rock of the Silberling Quarry, which is dated to the lower Tiffanian stage of the Paleocene.
The species was named by E. Douglass in 1908 (Douglass, 1908). The species P. admiralis, first described by Hay in 1930; and P. gracillis, first described by J. W. Gidley in 1909 (Gidley, 1909) are now considered part of this species.
Ptilodus tsosiensis has been discovered in New Mexico, the Puercan stage deposits of the Lower Paleocene San Juan Basin. The species was named by R. E. Sloan in 1981 (Sloan, 1981).
Remains of Ptilodus wyomingensis have been found in the Middle Paleocene deposits of the Rock Bench Quarry, in Wyoming and North Dakota in the United States. The species was named by G. L. Jepsen in 1940 (Jepsen, 1940).