teeth, but either smooth or grooved on upper teeth depending on taxon;
cheek teeth hypsodont, becoming progressively higher crowned in modern
groups, rooted in primitive groups (late? Oligocene to middle
Pliocene), rootless and ever-growing in modern groups (late Pliocene
to Recent); upper and lower premolars persistently bicolumnar; upper
and lower molars bicolumnar only in primitive groups (late? Oligocene
and early Miocene), becoming progressively monocolumnar in advanced
groups (early Pliocene to Recent), primitive bicolumnar pattern being
retained on occlusal surface only in early stages of ontogeny and in
third molar throughout life; enamel pattern of occlusal surface of
cheek teeth based on sextituberculate prototype (see Wood and Wilson,
1936:388-391), having cusps arranged in two transverse rows of three
cusps each, excepting three anterior cusps of premolars that are
arranged in trefoil, especially on p4 (sometimes only one or two,
rather than three, cusps develop in a particular set, especially in
p4), conules absent; protostyle and endostyle in upper teeth and
protostylid and hypostylid in lower teeth formed from cingulum; cusps
of each row uniting with wear into transverse enamel lophs (or
lophids), each tooth having two lophs, one on anterior column,
protoloph and protolophid, and one on posterior column, hypoloph and
hypolophid, that unite with additional wear forming continuous enamel
band; enamel lacking on sides of each column in advanced lineages,
thereby restricting enamel to anterior and posterior walls; with
extreme reduction, posterior plates of upper teeth and, more commonly,
anterior plates of lower molars, missing. Dental formula: 1/1, 0/0,
1/1, 3/3.
Key to the Subfamilies of Geomyidae
A Angular process of ramus mostly below alveolar level of
mandibular tooth-row; pattern of premolar like that of molars,
consisting of two subequal crests united at one or both margins
of tooth; molars persistently bicolumnar; molariform teeth
always rooted. Subfamily Entoptychinae p. 513
A´ Angular process of ramus mostly above level of mandibular
tooth-row; pattern of permolar unlike that of molars,
consisting of two prisms differing in size and united at their
mid-points but never at either margin; molars progressively
monocolumnar, except for early Miocene forms; molariform teeth
rooted only in primitive genera (late? Oligocene to middle
Pliocene), and rootless and ever-growing in later genera (late
Pliocene to Recent). Subfamily Geomyinae p. 514
Subfamily ENTOPTYCHINAE Miller and Gidley, 1918
Anterior face of upper incisor usually smooth, sometimes bearing faint
groove in center or near medial margin of tooth, at least in
_Gregorymys_; cheek teeth hypsodont, medium to high crowned, and
rooted in all but _Entoptychus_ (has rootless, ever-growing teeth);
cheek teeth identical in form, premolars resembling molars and lower
cheek teeth mirror images of upper teeth; crowns biprismatic, having
two columns joined at edge of protomeres (for description of term, see
discussion of primitive morphotype on page 537) and with persistent
lateral fissure between them; lateral re-entrant fold deep,
penetrating at least half width of crown, from external side in upper
teeth and internal side in lower teeth (in specialized genus
_Entoptychus_ lophs, upon additional wear, join also at edge of
parameres, thus uniting columns at both ends and thereby enclosing
interior part of lateral fissure as a transverse fossette in center of
tooth); enamel investment of prisms usually complete, including
inflection bordering re-entrant folds, occlusal pattern becoming
interrupted with wear only in _Entoptychus_, where enamel disappears
first from sides of crowns (following union of anterior and posterior
columns at both sides) and later, in final stages of attrition, from
anterior wall of lower molars and posterior wall of upper molars.
Maxillary bone without pronounced vertical depth in part supporting
cheek teeth, its inferior border only slightly lower than inferior
border of premaxillary and alveolar lips of molariform teeth
consequently approximately level with, or slightly below, alveolar lip
of upper incisor; squamosal without lateral expansion, therefore,
meatal tube of auditory bulla separated from zygomatic process of
squamosal by deep, well-developed postglenoid notch; angular part of
mandible below alveolar level of mandibular cheek teeth; angular
process only slightly reflected laterally; coronoid process low, tip
only slightly above condyle.
For information concerning the structure and relationships of the
known genera, and for accounts of species, see Wood (1936). A list
of the named genera in order of specialization is as follows:
*_Pleurolicus_ Cope, 1878. Proc. Amer. Phil. Soc., 18:66.
*_Gregorymys_ Wood, 1936. Amer. Mus. Novit., 866:9.
*_Grangerimus_ Wood, 1936. Amer. Mus. Novit., 866:13.
*_Entoptychus_ Cope, 1878. Proc. Amer. Phil. Soc., 18:64.
Five new species have been described since Wood's (1936) revision.
They are: _Pleurolicus clasoni_ MacDonald (1963:180); _Gregorymys
kayi_ Wood (1950:335); _Gregorymys montanensis_ Hibbard and Keenmon
(1950:198); _Grangerimus dakotensis_ MacDonald (1963:182);
_Grangerimus sellardsi_ Hibbard and Wilson (1950:623).
Subfamily GEOMYINAE Baird, 1858
Anterior face of upper incisor primitively smooth, grooves
consistently developed only in one modern lineage (Geomyini); cheek
teeth hypsodont, primitively rooted and having crown of medium height
(late Oligocene to middle Pliocene), being higher crowned, rootless
and ever-growing in modern lineages (late Pliocene to Recent);
primitively crowns of cheek teeth biprismatic, having two columns
joined at mid-points by narrow isthmus and entire crown sheathed in
continuous band of enamel; premolars retaining primitive biprismatic
form, anterior and posterior columns never uniting at edge of
protomeres or parameres, and with both lateral re-entrant folds
persistent throughout life; primitive biprismatic pattern becoming
decidedly modified in molars (except in M3), having two prisms
progressively uniting into one column by reduction and loss of lateral
inflections, primitive biprismatic patterns being retained only in
early stages of ontogeny; third upper molars retaining, at least
partially, primitive bicolumnar pattern (except in Thomomyini), with
relatively broad isthmus and horizontally shallow re-entrant folds,
lingual fold sometimes wanting; enamel pattern becoming discontinuous
(late Pliocene to Recent) owing to loss of enamel from sides of each
column; remaining enamel restricted to anterior and posterior plates,
or cutting blades, and enamel bordering lateral inflections in
premolars (considering both sides together, these plates constitute
essentially two transverse cutting blades); enamel pattern of M3
varying, depending on taxon; with specialization, anterior plates of
lower molars and posterior plates of upper premolar and molars may be
reduced or lost; except in primitive species (early Miocene), no
enamel fossettes retained in adult dentitions.
Maxillary bone having pronounced vertical depth in part supporting
cheek teeth, inferior border arching downward well below inferior
border of premaxillary; consequently, alveolar lips of molariform
teeth decidedly below level of alveolar lip of upper incisor;
squamosal with marked lateral expansion at expense of postglenoid
notch; notch compressed and reduced between meatal tube of auditory
bulla and zygomatic process of squamosal; angular part of mandible
mostly above alveolar level of mandibular cheek teeth; angular process
reflected laterally at right angles to axis of ramus and developed
into heavy knoblike projection; coronoid process well developed, tip
decidedly higher than condyle; fossorial specializations remarkably
well developed in advanced lineages, degree of specialization of
primitive Miocene species unknown but probably only semi-fossorial as
in Entoptychinae.
Key to the Tribes of the Geomyinae
A Enamel investment complete and uninterrupted, even in final
(adult) stages of wear; cheek teeth rooted, with crowns of medium
height; third lower molar biprismatic, the two columns separated
by inner and outer re-entrant folds as in lower premolar.
Tribe Dikkomyini p. 515
A´ Enamel investment incomplete and discontinuous, reduced, at least
in final (adult) stages of wear, to interrupted enamel plates;
cheek teeth rootless and ever-growing (except in extinct genus
_Pliogeomys_), crowns of maximum height; third lower molar
monoprismatic, without trace of inner and outer re-entrant folds
as in first and second lower molars.
B Upper incisors smooth, occasionally with a fine indistinct
groove near inner margin of tooth; form of third upper molar
same as M1 and M2, monoprismatic, anteroposteriorly compressed,
and having transverse enamel plates on both anterior and
posterior faces, and without suggestion of either labial or
lingual re-entrant folds; basitemporal fossa absent (except
for a shallow depression in one Recent species, _T. townsendii_);
forefoot small and narrow with claws not elongated for digging.
Tribe Thomomyini p. 518
B´ Upper incisors grooved, bearing either one or two sulci; form of
third upper molar distinctly different from M1 and M2, fully or
partially biprismatic (with a few exceptions discussed beyond),
without marked anteroposterior compression (either subtriangular,
elongated, suborbicular or quadriform in cross-section, but not
elliptical as in M1 and M2), and having typical transverse
anterior plate and two lateral plates (varying in their
development, depending on taxa), but no posterior plate, and with
lateral re-entrant folds usually developed, especially labial
inflection (although sometimes minute in a few species, as
described beyond); basitemporal fossa well-developed, although
occasionally shallow or absent (primitive species of _Zygogeomys_);
forefoot large and broad, with elongated claws for digging.
Tribe Geomyini p. 521
Tribe DIKKOMYINI, new tribe
_Genotype._--_Dikkomys_ Wood, 1936.
_Chronologic and geographic range._--Early to Middle Pliocene (early
Arikareean to mid-Hemphillian) in western United States. Known from
Miocene fossil sites in Montana, South Dakota, and Nebraska and
Pliocene sites in South Dakota, Oregon, Nevada, and southern
California. For precise localities see accounts of _Dikkomys_ and
_Pliosaccomys_ beyond.
_Diagnosis._--Small Geomyinae; lacking specializations of more
advanced tribes; upper incisors smooth, at least in _Pliosaccomys_;
molariform teeth always rooted and having crowns of medium height;
enamel investment of cheek teeth complete and uninterrupted in all
stages of wear; crowns of molars primitively biprismatic, having two
columns united at mid-points, thus forming narrow isthmus separating
lateral re-entrant folds as in premolars, and, with wear, also uniting
secondarily at protomeres (with exception of third lower molars),
consequently, isolating remnant of that inflection as shallow fossette
(columns uniting first at protomeres in _Pliosaccomys_); anterior and
posterior columns of first and second molars, both above and below,
becoming progressively united into one column in advanced Dikkomyini
(early and middle Pliocene), but m3 (M3 unknown) retaining primitive
biprismatic pattern, with columns joined at centers but never at
protomeres (for details of dentition see generic accounts); mandible
stout, its angle mostly above mandibular tooth-row; masseteric ridge
low; basitemporal fossa barely discernable in some fragments of
_Pliosaccomys_; postcranial skeleton unknown.
Key to the Genera of the Tribe Dikkomyini
A Molars biprismatic throughout life; anterior and posterior
lophs of first and second molars in pre-final stages of wear
uniting first at their mid-points and later at edge of
protomeres; anterior lophid of lower premolar having distinct
anteroexternal inflection. Genus _Dikkomys_ p. 516
A' First and second molars becoming monoprismatic in final
(adult?) stages of wear, biprismatic only in pre-final stages
of wear; third molars persistently biprismatic; anterior and
posterior lophs of first and second molars uniting first at
edge of protomeres; anterior lophid of lower premolar lacking
anteroexternal inflection. Genus _Pliosaccomys_ p. 517
Genus =Dikkomys= Wood
1936. _Dikkomys_ Wood, Amer. Mus. Novit., 866:26, July 2.
_Type._--_Dikkomys matthewi_ Wood, 1936, from Lower Harrison deposits
near Agate, Sioux County, Nebraska.
_Chronologic range._--Early Miocene, from early Arikareean (Lower
Harrison local fauna of Nebraska) to middle Miocene, late
Hemingfordian (Upper Rosebud local fauna, South Dakota, and the Deep
River Formation, Montana). According to MacDonald (1963:149-150), the
Upper Rosebud is middle Miocene rather than early Miocene.
_Description._--Size small, about as in small kinds of _Thomomys_;
known only from fragmentary mandible, including molariform dentition
in place, and isolated cheek teeth, including M1 (see Wood, 1936:26-28
and fig. 32; Galbreath, 1948:316-317 and fig. 1; and Black, 1961:13-14
and fig. 58); upper incisors unknown; cheek teeth hyposodont,
persistently rooted, and having crowns of medium height compared with
Recent geomyids; enamel investment complete and uninterrupted in all
molariform teeth in all stages of wear; P4 unknown, but probably
formed like p4; p4 persistently biprismatic, two crowns joined at
mid-points by relatively narrow isthmus separating lateral re-entrant
folds; anterior lophid of p4 having distinct anteroexternal
inflection; molars also biprismatic throughout life; two lophids of
lower molars first uniting at mid-points as in p4, and, with
additional wear, m1 and m2 secondarily uniting at edge of protomeres
and forming isolated enamel fossette between point of connection
(detailed description of stages of wear discussed in account of
phylogeny of subfamily); m3 permanently joined at mid-point only,
without lateral union at edge of protomeres; upper molars, judging by
M1 (M2 and M3 unknown), having same pattern as lower molars, but first
union of lophs decidedly on lingual side of center, consequently,
lingual re-entrant fold small; M1 probably developing U-pattern in
advanced stages of wear by union of protomeres, with minute lingual
fossette developing in transition as lophs secondarily become united
at lingual edge of columns; mandible stout and geomyidlike; masseteric
ridge weakly developed; basitemporal fossa absent.
Evidently, _Dikkomys matthewi_ is more primitive than _Dikkomys
woodi_. The modified H-pattern in m1 and m2, with the metalophid and
hypolophid joined at both their mid-points and also at their
protomeres (by union of the protostylid and hypostylid in the lower
dentition), is persistent throughout life. Therefore, the enclosed
enamel fossette is not eradicated with wear. In m1 and m2 of _Dikkomys
woodi_, the fossette is shallower, and, at least in advanced stages of
wear, it would disappear, therefore, forming a U-pattern on the
occlusal surface, as in M1 and M2, but lateral inflection horizontally
shallow rather than deep as in entoptychines.
Specimen (No. P 26284 FMNH) reported as _Dikkomys matthewi_ by
Galbreath (1948:316) is referable to the recently described species
_Dikkomys woodi_ Black, 1961.
_Specimens examined._--One, no. P 26284, Field Mus. Nat. Hist., from
upper Rosebud, Shannon Co., South Dakota.
_Referred species._--two:
_Dikkomys matthewi_ Wood, 1936. Amer. Mus. Novit., 866:26, July.
Type from early Arikareean Lower Harrison deposits (early Miocene)
near Agate, Sioux County, Nebraska.
_Dikkomys woodi_ Black, 1961. Postilla, Yale Peabody Museum, 48:13,
January 16. Type from Deep River Formation, late Hemingfordian
(middle Miocene), Meagher County, Montana; also known from Upper
Rosebud deposits (middle Miocene) near Wounded Knee, Shannon
County, South Dakota.
Genus =Pliosaccomys= Wilson
1936. _Pliosaccomys_ Wilson, Carnegie Inst. Washington Publ.,
473:20, May 21.
_Type._--_Pliosaccomys dubius_ Wilson, 1936, from Smiths Valley local
fauna in Lyon County, Nevada.
_Chronologic range._--Early Pliocene, late Clarendonian (Wolf Creek
local fauna, South Dakota, and Nettle Springs local fauna, California)
to Middle Pliocene, middle part of Hemphillian (Smiths Valley local
fauna, Nevada, and McKay Reservoir and Otis Basin local faunas,
Oregon).
_Description._--Size small (alveolar length of mandibular tooth-row
measuring 6.0 in holotype), about as in _Thomomys monticola_; upper
incisor relatively broad and flat, having anterior face smooth,
without trace of grooving; crowns of cheek teeth of medium height and
rooted; enamel investment continuous and uninterrupted in all stages
of wear; premolars permanently, biprismatic; P4 having anterior prism
subtriangular and decidedly smaller that sub-crescentic posterior
prism, and joined near centers by narrow, obliquely oriented isthmus;
p4 having anterior prism subovate, posterior prism strongly compressed
anteroposteriorly, and joined at mid-points by relatively broad and
straight isthmus; first and second molars, both above and below,
monoprismatic in final (?adult) stage of wear, derived ontogenetically
from primitive bilophate pattern by coalescence of two columns into
one; M1 and M2 mirror images of m1 and m2 in pre-final stages of wear,
two columns first uniting at edge of protomeres forming U-pattern, and
primitive H-pattern never developing in either series (for detailed
description of stages of wear, see account of phylogeny, p. 546); m3
(M3 unknown, but probably with same form as in Geomyini, see p. 552)
persistently biprismatic, two columns joined by relatively broad
isthmus at centers, consequently, forming H-pattern of primitive
ancestors; rostrum heavy and broad as in modern geomyids; palate
narrow and strongly ribbed; mandible stout; masseteric ridge and fossa
well developed; basitemporal fossa absent.
_Specimens examined._--Six, nos. 1796 (holotype)--1799, 1804 and 1806
(CIT) now in the Los Angeles County Museum, all from Smiths Valley
local fauna, Middle Pliocene, Nevada.
_Referred species._--two:
*_Pliosaccomys dubius_ Wilson, 1936. Carnegie Inst. Washington Publ.,
743:20, May 21. Known from early and middle Pliocene faunas
including Wolf Creek local fauna (late Clarendonian), Shannon
County, South Dakota; McKay Reservoir local fauna and Otis Basin
local fauna (Hemphillian), Oregon; type from Smiths Valley local
fauna (probably middle Hemphillian), Lyon County, Nevada.
*_Pliosaccomys wilsoni_ James, 1963. Univ. California Publ. Geol. Sci.,
45:101, June 26. Type from Nettle Springs local fauna of late
Clarendonian (early Pliocene), Ventura County, California.
Tribe THOMOMYINI, new tribe
_Type._--_Thomomys_ Wied-Neuwied, 1839.
_Chronologic and geographic range._--Known from late Pliocene (early
Blancan) to Recent. Known primarily from western North America from
southern Canada south to Central México in Pliocene, Pleistocene and
Recent and in middle and late Pleistocene of Maryland and Florida.
_Diagnosis._--Size small to medium (basilar length exclusive of _T.
bulbivorus_, measuring from approximately 24 to 45, including both
males and females); upper incisors without grooving, excepting fine,
indistinct sulcus rarely near inner margin (grooving more common in
_T. monticola_ than in other Recent species); crowns of cheek teeth
high, rooted and ever-growing; all molars, including M3, monoprismatic
and anteroposteriorly compressed, sometimes (especially in subadults)
having slight inflection on labial side in upper teeth and lingual
side in lower teeth; molars bicolumnar in pre-final stages of wear
(seen in juvenal teeth only), patterns of wear in both upper and lower
molars resembling those of _Pliosaccomys_, except that crowns of m3
and M3 unite into single column in final stages of wear; enamel
pattern interrupted in all cheek teeth, loss occurring only at sides
of each column; transverse enamel blade completely covering posterior
face of both P4 and p4; all upper and lower molars with two transverse
enamel blades, one on anterior surface and one on posterior surface,
of each tooth, including M3; small third plate sometimes persistent on
broad side of tooth, labial side in upper molars and lingual side in
lower molars (_T. bulbivorus_); skull generalized, neither unusually
narrow and deep or broad and flat; usually without marked cresting or
rugosity; masseteric ridge well developed and massive; basitemporal
fossa absent, sometimes shallow depression forming in _T. townsendii_;
pelage soft, never harsh or hispid, covering body with thick coat of
hair; forefoot exceptionally small for fossorial mammal, claws not
especially long; body form remarkably fossorial.
The tribe Thomomyini is monotypic, including only the genus
_Thomomys_.
Genus =Thomomys= Wied-Neuwied
1839. _Thomomys_ Wied-Neuwied, Nova Acta Phys. Med. Acad. Caesar.
Leop.-Carol., 19(1):377.
1836. _Oryctomys_ Eydoux and Gervais (in part), Mag. de Zool., 6:20,
pl. 21. Type: _Oryctomys_ (_Saccophorus_) _bottae_, from
coast of California, probably near Monterey.
1903. _Megascapheus_ Elliot, Field Columb. Mus., Publ. 76, Zool.
Ser., 3(11):190, July 25. Type: _Diplostoma bulbivorum_
Richardson, from Columbia River, probably near Portland, Ore.
1933. _Pleisothomomys_ Gidley and Gazin, Jour. Mamm. 14:354. Type:
_Pleisothomomys potomacensis_ Gidley and Gazin, from
Pleistocene, Cumberland Cave local fauna, Allegany County,
Maryland.
_Chronologic range._--Known from late Pliocene to Recent.
_Description._--Same as that given for the tribe Thomomyini above.
_Discussion._--Features characterizing _Thomomys_ and the tribe
Thomomyini are more advanced than those characterizing the tribe
Dikkomyini. Also, the Thomomyini retain more of the primitive
features of the Geomyinae than do the more specialized tribe Geomyini.
Specializations are few, but include the third molar being a single
column both above and below, enamel plates, and a masseteric ridge.
Key to the Subgenera of _Thomomys_
A Molars sub-crescent or ovate in cross-section, not
becoming abruptly narrower at one end of tooth.
Subgenus _Pleisothomomys_ p. 519
A´ Molars pear-shaped, not sub-crescent or ovate, in
cross-section, crown becoming abruptly narrow at one
end of tooth. Subgenus _Thomomys_ p. 520
Subgenus =Pleisothomomys= Gidley and Gazin
1933. _Pleisothomomys_ Gidley and Gazin, Jour. Mamm., 14:354,
November 13.
_Type._--_Pleisothomomys potomacensis_ Gidley and Gazin, 1933.
_Chronologic range._--Late Pliocene (Hagerman local fauna, Idaho) to
late Pleistocene. The latest records are from the fauna of Saber-tooth
Cave, Florida, a late Pleistocene assemblage that probably was
deposited in the Sangamon. The middle and late Pleistocene records are
from the eastern United States, suggesting that the subgenus
_Pleisothomomys_ was restricted to that region while the subgenus
_Thomomys_ occupied the western United States and parts of Canada and
México as it does today.
_Description and Comparison._--Separated from subgenus _Thomomys_ only
on basis of sub-crescentic shaped molars (only jaw fragments and
isolated teeth known), seemingly a primitive feature of the genus.
This dental structure continued into the late Pleistocene; none of the
Recent species expresses this feature of the molars, although the
molars of _Thomomys vetus_ of the late Pleistocene (Wisconsin
deposits), referred to the subgenus _Thomomys_ on the basis of
its alleged relationship to _Thomomys townsendii_ (see Davis,
1937:156-158), are less distinctly pear-shaped, and are more
sub-crescentic, than in any other known species of the subgenus
_Thomomys_. _Pleisothomomys_ Gidley and Gazin (_loc. cit._) was
proposed as a genus but is here considered as of no more than
subgeneric worth, and is recognized because of the apparent constancy
of the sub-crescentic molars in the earlier members of the genus and
in those populations of _Thomomys_ occurring in Pleistocene times in
the eastern United States.
_Referred species._--Three (all extinct):
*_Thomomys gidleyi_ Wilson, 1933. Carnegie Inst. Washington Publ.
440:122, December. Type from Hagerman beds, late Pliocene,
Idaho.
*_Thomomys potomacensis_ Gidley and Gazin, 1933. Jour. Mamm.,
14:354, November 13. Type from Cumberland Cave, middle and late
Pleistocene, Maryland.
*_Thomomys orientalis_ Simpson, 1928. Amer. Mus. Novit., 328:6,
October 26. Type from Saber-tooth Cave, late Pleistocene,
Florida.
Subgenus =Thomomys= Wied-Neuwied
1839. _Thomomys_ Wied-Neuwied, Nova Acta Phys.-Med. Acad. Caesar.
Leop. Carol., 19(1):377.
1903. _Megascapheus_ Elliot, Field Columb. Mus., Publ. 76, Zool.
Ser., 3 (11):190, July 25. Type: _Diplostoma bulbivorum_
Richardson, from Columbia River, probably near Portland, Oregon.
_Type._--_Thomomys rufescens_ Wied-Neuwied, 1839.