The family Heteromyidae was defined by Wood (1935:81) essentially as
follows: Cheek teeth brachydont to hypsodont and even rootless; usually
six cusps per molar, three on each loph; enamel rarely divided into two
plates, never reduced to one; skull light, thin and papery; mastoids
inflated, mastoidal breadth often greatest, never appreciably less than
zygomatic breadth; interorbital space wider than rostrum; palate nearly
horizontal and little if any below level of zygomata; nasals extended
beyond incisors; zygomata slender, with greatly reduced malar, almost,
or quite, abutting against tympanic; frontals and parietals broad, with
latter reaching, or nearly reaching, orbits; frontal trapezoidal;
parietal quadrate, to pentagonal or triangular; interparietal
primitively large, secondarily reduced; squamosal mostly or entirely
confined to orbit; tympanic vesicular and inflated, in some forms highly
inflated; mastoids inflated and bullous, reaching top of skull, and
forming part of occipital surface; occipitals contracted and limited in
area on occiput, but extending onto dorsum of skull; coronoid processes
small, inclined caudad and lying below level of condyle; jaw small and
weak with large, everted angle; tail as long as, or longer than, head
and body; claws of manus elongate, fossorial, but forelimb slender;
pelage usually coarse and frequently spinose; ears and eyes large; body
murine in form; locomotion in many forms saltatorial.
This characterization of the family includes all of the members of the
subfamilies Perognathinae, Heteromyinae and Dipodomyinae as well as the
genus _Microdipodops_ which I am disinclined to place with any of the
three subfamilies. Apparently it is more closely related to the
subfamily Perognathinae.
The subfamily Dipodomyinae, which contains the genera _Dipodomys_,
_Prodipodomys_ and _Cupidinimus_, might be characterized after Coues'
(1875) original description of the subfamily as follows: Cheek teeth
progressively hypsodont, in _Dipodomys_ becoming ever-growing; enamel
progressively interrupted, eventually reduced to anterior and posterior
plates; upper and lower third molars reduced in size; tooth pattern
rapidly destroyed, leaving only an enamel oval; upper incisors smooth
(some fossils) or grooved (living forms); progressive expansion of the
auditory bullae and increase in saltatorial ability; pterygoid fossa
double; calcaneal-navicular or even calcaneal-cuneiform articulation;
tail tufted.
Owing to the fact that so little paleontological material is known and
because even that is fragile and not easily accessible for study,
knowledge of the fossil representatives has been drawn primarily from
the literature, especially from Wood's (1935) account.
Heteromyids are known from the Chadron formation, of early Oligocene
age, in which a single tooth was found. In the Orellan stage of the
mid-Oligocene where the genus _Heliscomys_ occurs, it is notably
generalized, in comparison with other members of the family, but it may
not be ancestral at all. The lower premolar is tricuspidate and the
first and second molars are quadritubercular with a broad cingulum. The
teeth are bunodont and brachydont, with the cusps not uniting to form
lophs. Wood (1935:78) shows _Mookomys formicorum_ (from the Arikeean) as
the next heteromyid in the evolutionary sequence and postulates that
this species arose from _Heliscomys gregoryi_. _Mookomys_ is judged by
Wood to be the common ancestral form of the perognathines and the
dipodomyines.
_Cupidinimus_, the genus next in line, is characterized by smooth upper
incisors; lower molars with incipient H-pattern; cheek teeth
progressively hypsodont and lophate (but always rooted); and
calcaneal-navicular articulation.
The time range of this genus is from the late Miocene (Niobrara River,
Local Fauna) of Nebraska to the medial Pliocene, Thousand Creek
(Hemphillian) of Nevada.
Hibbard (1937:462) described _Dipodomys kansensis_ from the Ogallala
formation (Hemphillian age) of Kansas. He redescribed his species, and
made it the type of the new genus _Prodipodomys_ (Hibbard, 1939:458),
differentiating it from _Dipodomys_ on the basis of the three-rooted p4,
double-rooted m1 and m2 and the single rooted m3. It is shown to be
closely allied to _Dipodomys_ by the form and position of a large
foramen posterior and labial to m3, and by the development of the
masseteric ridge.
The next youngest heteromyid fossils which have been described are of
the genus _Prodipodomys_? from Arizona. Gidley (1922:123) described
_Dipodomys minor_ from the Benson (Blancan) which Gazin (1942:486)
refers to the genus _Prodipodomys_?. Wood (1935:156) described
_Dipodomys gidleyi_ from the Curtis (Pleistocene). Both of these species
are primitive as regards dentition; that is to say, the enamel ring of
the tooth is complete and lacks any sign of a break. The limb bones of
_D. gidleyi_ show lesser saltatorial ability, and therefore appear to be
more primitive, than those of any living _Dipodomys_.
Several heteromyids which have not been assigned to any genus are known.
Wilson (1939:36-37) recognized some from the Avawatz (Clarendonian) and
the Ricardo (Clarendonian). Another, possibly of the genus
_Diprionomys_?, from the Barstow (Barstovian) was described by Wood
(1935:197) as follows: "The general shape of the tooth as figured
strongly suggests either one of the most advanced species of _Dipodomys_
or else a Geomyid.... It is much more advanced than are any known
contemporary heteromyids, and compares fairly well with such late
Tertiary and Pleistocene geomyids as have been described. It certainly
is not referable to any known heteromyid genus other than _Dipodomys_,
and should probably be called a Geomyid." Wilson (_loc. cit._) refers to
these specimens as Dipodomyine (?) n. gen. and sp. If these specimens
referred to by Wood and Wilson are true heteromyids then a change in the
phylogenetic scheme proposed by Wood (1935) would be necessary. Wilson
(_loc. cit._) says, referring to the Avawatz specimen, "The cheek teeth
are very hypsodont but are apparently not persistent in growth,... Wide
enamel breaks are present in M/1 dividing the enamel into anterior and
posterior bands. The enamel of P/4 is complete in the present stage of
wear, but an examination of the tooth indicates that breaks would
develop with additional attrition at the buccal and lingual margins of
the metalophid, and at the buccal border of the protolophid. The incisor
is of the slender heteromyid type."
Wood (1935:118) in referring to the ancestry of _Cupidinimus_ with
regard to the grooving of the incisors says: "The philosophy of
evolution which would prohibit its derivation from _Mookomys_, because
of the grooved incisors in the latter genus, would require a separate
line leading back at least to the Lower Miocene."
In view of the above statements, it is conceivable that additional
material will be found carrying the dipodomyine line back into the
early Miocene. Perhaps the line involving _Mookomys_ and _Cupidinimus_
which was regarded by Wood as the line of descent, is merely an aberrant
side branch that parallels in its structures the main line of evolution
of the dipodomyines (Figure 1).
[Illustration: FIG. 1. Phylogeny of the Dipodomyines (modified after
Wood, 1935).]
As Wilson (1939:37) says: "Indeed it is hard to recognize such a form as
_Cupidinimus nebraskensis_ as directly ancestral to _Dipodomys_ in view
of the occurrence of the much more advanced Avawatz specimen in deposits
that are at most only slightly later than those in which the former is
found. The kangaroo rats were apparently much farther along in their
development by lower Pliocene time than heretofore supposed."
Wood (1935:78) suggested that _Dipodomys gidleyi_ gave rise to
_Dipodomys spectabilis_ and _Dipodomys ordii_, and _Dipodomys minor_
gave rise to _Dipodomys compactus_. However, my own study indicates that
_Dipodomys compactus_ is conspecific with _Dipodomys ordii_ and should
stand as _Dipodomys ordii compactus_. Consequently a different phyletic
arrangement than that proposed by Wood (_loc._ _cit._) is required.
Since _D. compactus_ is more closely allied to _Prodipodomys? minor_
than _D. ordii_ is to _D. gidleyi_, it is possible that _P.? minor_ gave
rise to _D. ordii_ and that _D. spectabilis_ is the end product of the
phyletic trend of _D. gidleyi_ (Figure 1).
The trend of phyletic development in the dipodomyines has been toward
the saltatorial habit. To acquire this habit from a scampering ancestor,
certain morphological modifications were necessary. Among these
modifications were a lengthening of the tail, a lengthening of the hind
legs, the development of a calcaneal-navicular-ectocuneiform contact
instead of a calcaneal-navicular contact for additional strength in
leaping, a shortening of the forelimb, an increase in size and inflation
of the mastoid and tympanic portions of the skull with a consequent
reduction in size of the interparietal region and the fusion of certain
of the cervical vertebrae. Late Miocene (_Cupidinimus_) and Pliocene
(Avawatz specimen and _Prodipodomys_) forms had acquired certain of
these morphological modifications that are present in the modern genus
_Dipodomys_.