Ptilodus is a mammal genus from the Paleocene of North America. It was a member of the extinct order of Multituberculata.
For those of a technical inclination, it's within the Suborder of Cimolodonta, family Ptilodontidae.
Genus: Ptilodus Cope ED, 1881
Aka: Chirox Cope ED, 1884; cf. Ectypodus (Jepsen, 1940); Neoplagiaulax (partly)
Remarks: Ptilodus was a relatively large multituberculate of 30 to 50cm 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.
A number of other species 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 GG, 1935, seems to have become Parectypodus sinclairi. In botonay, Ptilodus pyramidatus is an extinct Australian plant.
Species: Ptilodus fractus Dorr JA, 1952
Aka: Neoplagiaulax fractus Dorr, 1952
Place: Dell Creek, Wyoming
Age: Tiffanian, Paleocene
Remarks: Body weight of about 85g.
I've also seen the citation as (Dorr, 1952) Krause, 1987a, and I'm not sure which is correct! The formulation I'm following is in Scott et al, 2002 (p.694).
Reference: Dorr (1952), Early Cenozoic stratigraphy and vertebrate paleontology of the Hoback Basin, Wyoming. Bulletin of the Geological Society of America 63, p.59-94.
Species: Ptilodus gnomus Scott CR, Fox RC & Youzwyshyn GP, 2002
Aka: cf. Ectypodus hazeni (Jepsen, 1940), Gazin 1956a
Place: Cochrane 2, Paskapoo Formation, Alberta & Wyoming
Country: Canada & USA
Age: earliest Tiffanian, Paleocene
Remarks: The following is based on my reading of Scott et al 2002.
Remains of this species include over 100 teeth, (upper and lower), and at least one fragment of jaw. The dentition range between lengths of less than two to greater than five mm. The fourth lower premolar, p4, is about 51% shorter than the corresponding chopper in P. mediaevus; 28% less than P. kummae ; 15% below P. tsosiensis; and 5% smaller than for P. fractus, which gives some idea of the relative sizes of the various critters. There are also differences in shape and the number of serrations.
"Although more similar in size to homologous teeth referable to P. tsosiensis and P. fractus, the upper and lower ultimate premolars of this new species most closely resemble those of P. mediaevus in overall morphology", (Scott et al 2002, p.694). 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 authors mention there are three species of this genus known from the Alberta location, two of which have yet to be described. This will be attended to by Krause DW, (p.691). The species name is New Latin for 'dwarf', seeing as it's the smallest known member of the genus.
Reference: Scott, Fox & Youzwyshyn (2002), New earliest Tiffanian (late Paleocene) mammals from Cochrane 2, southwestern Alberta, Canada. Acta Palaeontologica Polonica 47 (4), p.691-704.
Species: Ptilodus kummae Krause DW, 1977
Place: Roche Percée, Saskatchewan
Age: lower Tiffanian, Paleocene
Remarks: Estimated weight, 120g. A further Alberta University holotype.
Reference: Krause (1977), Paleocene multituberculates (Mammalia) of the Roche Percee Local Fauna, Ravenscrag Formation, Saskatchewan, Canada. Palaeontographica Abt. A 186, p.1-36.
Species: Ptilodus mediaevus Cope ED, 1881
Aka: Chirox plicatus Cope, 1884; Ptilodus feronensis; Ptilodus ferronensis Gazin CL, 1941; Ptilodus plicatus Gidley, 1909
Place: San Juan Basin, New Mexico & Utah & Wyoming
Age: Torrejonian, Paleocene
Remarks: In a paper in 1887, Cope described Chirox as a marsupial. It wasn’t. P. plicatus was synonymized with P. mediaevus by Granger & Simpson, 1929. Ths same fate befell P. ferronensis according to Rigby, 1980.
References: Cope (1881), Eocene Plagiaulacidae. Amer. Naturalist 15, p.921-922.
Gazin (1941), The mammalian faunas of the Paleocene of central Utah, with notes on the geology. Proc. US Nat. Mus. 91, p.1-53, 29 figs., 3 pls.
Species: Ptilodus montanus Douglass E, 1908
Aka: P. admiralis Hay, 1930; P. gracillis Gidley JW, 1909
Place: Silberling Quarry, Montana
Age: lower Tiffanian, Paleocene
Remarks: This was a large species of something like 650g. A brain cast indicates a well developed sense of smell.
References: Douglass (1908), Vertebrate fossils from the Fort Union beds. Ann. Carnegie Museum, V, p.11-26, pls. i, ii.
Gidley (1909), Notes on the fossil mammalian genus Ptilodon, with descriptions of new species. Proc. US Nat. Museum XXXVI, p.611-626, 9 figs., pl.LXX.
Species: Ptilodus tsosiensis Sloan RE, 1981
Place: San Juan Basin, New Mexico
Age: Puercan, Lower Paleocene
Reference: Sloan (1981), Systematics of Paleocene multituberculates from the San Juan Basin, New Mexico, p.127-160, in Lucas et al (eds), Advances in San Juan Basin paleontology. University of New Mexico Press, Alberquerque.
Species: Ptilodus wyomingensis Jepsen GL, 1940
Place: Rock Bench Quarry, Wyoming & North Dakota
Age: Middle Paleocene
Reference: Jepsen (1940), Paleocene faunas of the Polecat Bench formation, Park County, Wyoming. Pro. Amer. Philos. Soc, 83, p.217-340, 21 figs., 5pls.
Page references: Kielan-Jaworowska Z & Hurum JH (2001), Phylogeny and Systematics of multituberculate mammals. Paleontology 44, p.389-429.
Scott CR, Fox RC & Youzwyshyn GP (2002), New earliest Tiffanian (late Paleocene) mammals from Cochrane 2, southwestern Alberta, Canada. Acta Palaeontologica Polonica 47 (4), p.691-704.
(This information has been derived from  MESOZOIC MAMMALS; Ptilodontoidea, an internet directory. As that's my webpage, there are no issues of copyright. Trevor Dykes)