Subspeciation in the Kangaroo Rat,Dipodomys ordiiByHENRY W. SETZERCONTENTSPAGEIntroduction477Methods and Acknowledgments478Paleontology480Phylogeny of the Species of the Genus484Dispersal of the Several Species498Subspeciation499Accounts of Subspecies511Dipodomys ordii511Dipodomys ordii richardsoni511Dipodomys ordii oklahomae514Dipodomys ordii compactus515Dipodomys ordii sennetti517Dipodomys ordii evexus518Dipodomys ordii medius519Dipodomys ordii obscurus521Dipodomys ordii terrosus523Dipodomys ordii fremonti524Dipodomys ordii uintensis525Dipodomys ordii sanrafaeli526Dipodomys ordii panguitchensis527Dipodomys ordii monoensis528Dipodomys ordii ordii530Dipodomys ordii luteolus533Dipodomys ordii extractus534Dipodomys ordii chapmani536Dipodomys ordii montanus538Dipodomys ordii cinderensis540Dipodomys ordii fetosus541Dipodomys ordii utahensis543Dipodomys ordii columbianus544Dipodomys ordii idoneus546Dipodomys ordii priscus547Dipodomys ordii celeripes549Dipodomys ordii cineraceus550Dipodomys ordii marshalli551Dipodomys ordii inaquosus552Dipodomys ordii attenuatus553Dipodomys ordii fuscus555Dipodomys ordii longipess556Dipodomys ordii pallidus558Dipodomys ordii nexilis559Dipodomys ordii cupidineus561Dipodomys ordii palmeri562Conclusions563Tables of Measurements565Literature Cited571INTRODUCTIONThe geographic range of the kangaroo rats, genusDipodomys, extends from southern Canada south to the southern limits of the Mexican Tableland and from the Pacific Coast east to the eastern limits of the Great Plains in Kansas, Oklahoma and Nebraska. These animals are usually restricted to sandy soils in semiarid regions. The speciesDipodomys ordii, with which this account is primarily concerned, is, to the best of my knowledge, almost exclusively confined to sandy areas.Since 1841, when Gray gave the generic nameDipodomysto the kangaroo rats, basing the name on the four-toed speciesDipodomys phillipsi, several other generic names have been applied. Fitzinger, in 1867, used the namePerodipusfor those animals with five toes on the hind foot, designatingDipodomys agilisas the type of his genus. In 1890, Merriam proposed the generic nameDipodopswithDipodomys agilisas the type, apparently being unaware of Fitzinger's name,Perodipus. Trouessart, in 1897, through what was an apparentlapsus calami, applied the generic nameCricetodipusPeale to all of the species of the then known generaPerodipusandDipodomys, but Trouessart later, 1904 or 1905, in his Supplementum, corrected thislapsusand used the namesDipodomysandPerodipus.Grinnell(1919:203) showed that some of the four-toedDipodomyshad five toes on one hind foot and four on the other and thatPerodipusmust fall as a synonym of the earlier generic nameDipodomyswhich was to be applied to all of the kangaroo rats.Dipodomys ordiiwas named by Woodhouse in 1853, from specimens from El Paso, Texas, but between that time and 1919 the nameordiiwas used in combination with all of the generic names mentioned above (see synonymies under the accounts of the subspecies).The nearest approach to a revision of the genus wasGrinnell's(1922) "A Geographical Study of the Kangaroo Rats of California." Since that time,Hall and Dale(1939) revised theD. micropsgroup andDurrant and Setzer(1945) reported upon the kangaroo rats of Utah. The present paper is a review of the speciesDipodomys ordii. Some of the objectives in this review have been to learn: (1) What kinds of kangaroo rats are subspecies of the speciesDipodomys ordii; (2) the limits of geographic range of this full species; (3) the extremes of color, and of size and shape of the skullin this one species; (4) the significance of different sizes, shapes and colors; (5) the reasons for the existence, or formation, of selected subspecies; and (6) the relationships ofDipodomys ordiito other species in the genus.METHODS AND ACKNOWLEDGMENTSAvailable specimens were arranged according to geographic origin. These were segregated as to sex and then under each sex by age. Individual variation was next measured in each of several samples in which individuals were of like geographic origin, sex, age and season. Finally, comparable materials were arranged geographically for detection of variations of systematic worth. Following preliminary studies of material thus arranged, additional specimens were collected from critical areas.When fully adult animals (see next paragraph) were segregated as to sex, and then measured, the degree of secondary sexual variation was found to be less than the degree of individual variation; therefore in the tables of indices, no distinction as to sex has been made.The only external measurements of the animals used were those recorded by the collectors on the labels attached to the skins. These measurements were total length, length of tail and length of hind foot. Measurements of the ear have not been used since they were not in all instances recorded by collectors and since measurements of dry ears proved to be unsatisfactory. Only measurements of fully adult specimens have been used. The term fully adult is applied only to those specimens in which the auditory bulla is shiny and translucent, the permanent P4 is fully erupted and worn, and the tail is fully striped and penicillate. No one of these characters alone was accepted as proof of adulthood but only the three in combination.The following measurements of the skull have been used in the tables:Greatest length.—From the most anterior tip of the nasals to the most posterior projection of the auditory bullae.Greatest breadth across bullae.—From the most lateral projection of the auditory bulla on one side to the corresponding position on the other bulla.Breadth across maxillary arches.—Greatest breadth across arches in a plane perpendicular to the long axis of the skull.Width of rostrum.—Width of the premaxillae and the nasals taken immediately anterior to the upper incisors (not greatest width of nasals which is attained farther anteriorly).Length of nasals.—Maximum length of a nasal bone.Least interorbital breadth.—Least width between the orbits immediately posterior to the lacrimal processes.Basilar length.—From the anterior margin of the foramen magnum to the posterior border of the alveolus of one of the upper incisors.Capitalized color terms are fromRidgway, "Color Standards and Color Nomenclature," Washington, D. C., 1912. Color determinations were made by comparing a masked area of pure color on the side of the animal with a masked rectangle of named color onRidgway'splates in natural light always from the same angle.Abbreviations used for specimens examined from the various collections are as follows:AMNH—American Museum of Natural History.BYU—Brigham Young University.CNHM—Chicago Natural History Museum.CM—Carnegie Museum.CMNH—Colorado Museum of Natural History.DJC—Dixie Junior College.DRD—Donald R. Dickey Collection.KU—Museum of Natural History, University of Kansas.LACM—Los Angeles County Museum.MHS—Collection of Myron H. Swenk.MVZ—Museum of Vertebrate Zoology, University of California.OU—Museum of Zoology, University of Oklahoma.RH—Collection of Ross Hardy.UM—Museum of Zoology, University of Michigan.UN—Museum of Natural History, University of Nebraska.USAC—Utah State Agricultural College.USBS—United States Biological Surveys Collection.USNM—United States National Museum.UU—Museum of Zoology, University of Utah.TCWC—Texas Coöperative Wildlife Collection.This study is based on 3,732 specimens which were assembled at the Museum of Natural History, University of Kansas, or studied at other institutions. For the loan of this material and for the opportunity afforded for its study, I am extremely grateful to the authorities of each of these institutions and to the owners of the private collections.Acknowledgement is made to the Office of Research and Inventions of the United States Navy for assistance with the field work which permitted the acquisition of essential specimens from several of the critical geographic areas while the author was research assistant on a larger over-all project (N6 ori-164-T02) of which the determination of the geographic range of this rodent species, a potential host of tularemia, was one facet. Tularemia was not detected in this genus.I extend my thanks also to Professor Stephen D.Durrant, of the University of Utah, for helpful corrections in the preparation of the manuscript; to Mrs. Virginia Cassell Unruh for the preparation of the drawings; to Professor E. RaymondHall, of the University of Kansas, for guidance in the study and critical assistance with the manuscript; to Professors H. H. Lane and Worthie H. Horr for valued suggestions; to Mr. J. R. Alcorn for providing specimens for dissection when he was working under the University of Kansas endowment fund; and to the other friends and associates who have given of their time and criticism.PALEONTOLOGYThe family Heteromyidae was defined byWood(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 genusMicrodipodopswhich 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 generaDipodomys,ProdipodomysandCupidinimus, might be characterizedafterCoues'(1875) original description of the subfamily as follows: Cheek teeth progressively hypsodont, inDipodomysbecoming 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 fromWood'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 genusHeliscomysoccurs, 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) showsMookomys formicorum(from the Arikeean) as the next heteromyid in the evolutionary sequence and postulates that this species arose fromHeliscomys gregoryi.Mookomysis judged byWoodto 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) describedDipodomys kansensisfrom the Ogallala formation (Hemphillian age) of Kansas. He redescribed his species, and made it the type of the new genusProdipodomys(Hibbard, 1939:458), differentiating it fromDipodomyson the basis of the three-rooted p4, double-rooted m1 and m2 and the single rooted m3. It is shown to be closely allied toDipodomysby 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 genusProdipodomys? from Arizona.Gidley(1922:123) describedDipodomys minorfrom the Benson (Blancan) whichGazin(1942:486) refers to the genusProdipodomys?.Wood(1935:156) describedDipodomys gidleyifrom 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 ofD. gidleyishow lesser saltatorial ability, and therefore appear to be more primitive, than those of any livingDipodomys.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 genusDiprionomys?, from the Barstow (Barstovian) was described byWood(1935:197) as follows: "The general shape of the tooth as figured strongly suggests either one of the most advanced species ofDipodomysor 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 thanDipodomys, 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 byWoodandWilsonare true heteromyids then a change in the phylogenetic scheme proposed byWood(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 ofCupidinimuswith regard to the grooving of the incisors says: "The philosophy of evolution which would prohibit its derivation fromMookomys, 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 theearly Miocene. Perhaps the line involvingMookomysandCupidinimuswhich was regarded byWoodas 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).Phylogeny of the Dipodomyines.Fig. 1.Phylogeny of the Dipodomyines (modified afterWood, 1935).AsWilson(1939:37) says: "Indeed it is hard to recognize such a form asCupidinimus nebraskensisas directly ancestral toDipodomysin 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 thatDipodomys gidleyigave rise toDipodomys spectabilisandDipodomys ordii, andDipodomys minorgave rise toDipodomys compactus. However, my own study indicates thatDipodomys compactusis conspecific withDipodomys ordiiand should stand asDipodomys ordii compactus. Consequently a different phyletic arrangement than that proposed byWood(loc.cit.) is required. SinceD. compactusis more closely allied toProdipodomys? minorthanD. ordiiis toD. gidleyi, it is possible thatP.? minorgave rise toD. ordiiand thatD. spectabilisis the end product of the phyletic trend ofD. 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 andProdipodomys) forms had acquired certain of these morphological modifications that are present in the modern genusDipodomys.PHYLOGENY OF THE SPECIES OF THE GENUSRepresentatives of nine species ofDipodomyswere dissected in an attempt to determine the degree of specialization and the relative systematic position of each species.The myology was found to agree in detail as to origin, insertion and innervation with that ofDipodomys spectabilisas reported byHowell(1932). The only variation noted in the muscular system was the size of the individual muscles in those animals of widely divergent body size.Dipodomys ordiiis the most generalized andDipodomys desertiis the most specialized of the kangaroo rats (seeTable 1), as judged by the osteology. Information gained by the study of the viscera of the various species supports this judgment. The visceral mass is relatively loose inD. ordii, but is markedly compact inD. deserti. This compactness appears to be brought about by the foreshortening of the mesenteries which support the entire gut and by the closer apposition of the large intestine to the caecum; both the intestine and caecum occupy a ventral position in the abdominal cavity. InDipodomys ordiithe entire visceral mass is loosely interconnected and the caecum is relatively small as compared to the tightly compact viscera and the large caecum inDipodomys deserti. Another striking feature is the size, proportion and position of the liver. In all animals dissected, even inD. deserti, the right lobe of the liverdescends and forms a capsule around the anterior end of the right kidney. In the Ord kangaroo rat, the bulk of the liver lies on the right side of the body cavity. That is to say, there is a greater bulk of the liver on the right side and it is situated more dorsad than in any of the other species examined. In the most specialized condition, as inDipodomys deserti, the bulk of the liver is almost equal on the right and left sides, and instead of having the greater bulk situated dorsally as inD. ordiiit is cup-shaped, with the dorsal and ventral parts of approximately equal size and situated on almost the same transverse plane. The entire mass of the liver is concave posteriorly.TABLE 1Skeletal Indices of DipodomysHumeroradialIntermembralCruralTibioradialFemorotarsal-metatarsalHumero-Cranialordii144.557.2127.7560.5588.463.4microps138.556.17132.357.2790.9560.8panamintinus146.155.3132.057.590.560.8agilis147.056.05133.6557.2594.5562.65heermanni142.954.2135.955.3592.260.93ingens142.954.1130.656.289.6566.2spectabilis140.953.05133.954.295.664.6phillipsii163.455.05137.8558.97101.564.5merriami160.7553.85137.557.3699.7563.9nitratoides155.054.1137.457.098.2565.5deserti149.553.4139.454.996.667.6The right kidney is variable in position in reference to the left. In all species the right kidney lies anterior to the left but in some,D. desertiandD. ingens, it is markedly anterior.InDipodomys agilis,D. merriamiandD. desertithere are small to large patches of lymphoid tissue on the caecum. These patches were not noted in any of the other species examined and I do not know their function. In the three above mentioned species, however,the large intestine is shorter in proportion to the small intestine than in any other species exceptD. heermanni(seeTable 2) and with the exception ofD. heermanni,D. venustusandD. ordiithe actual measurements are less.Inasmuch as little is known of the food habits of the various species of kangaroo rats, any ascription of adaptive significance to the varying proportions of the digestive system would be only speculative.Midgley(1938) describes the visceral anatomy ofD. ordiiandD. microps. Except for the differences here noted the description of the viscera as given byMidgley(loc. cit.) applies to the rest of the species studied.
Subspeciation in the Kangaroo Rat,
Dipodomys ordii
By
HENRY W. SETZER
The geographic range of the kangaroo rats, genusDipodomys, extends from southern Canada south to the southern limits of the Mexican Tableland and from the Pacific Coast east to the eastern limits of the Great Plains in Kansas, Oklahoma and Nebraska. These animals are usually restricted to sandy soils in semiarid regions. The speciesDipodomys ordii, with which this account is primarily concerned, is, to the best of my knowledge, almost exclusively confined to sandy areas.
Since 1841, when Gray gave the generic nameDipodomysto the kangaroo rats, basing the name on the four-toed speciesDipodomys phillipsi, several other generic names have been applied. Fitzinger, in 1867, used the namePerodipusfor those animals with five toes on the hind foot, designatingDipodomys agilisas the type of his genus. In 1890, Merriam proposed the generic nameDipodopswithDipodomys agilisas the type, apparently being unaware of Fitzinger's name,Perodipus. Trouessart, in 1897, through what was an apparentlapsus calami, applied the generic nameCricetodipusPeale to all of the species of the then known generaPerodipusandDipodomys, but Trouessart later, 1904 or 1905, in his Supplementum, corrected thislapsusand used the namesDipodomysandPerodipus.Grinnell(1919:203) showed that some of the four-toedDipodomyshad five toes on one hind foot and four on the other and thatPerodipusmust fall as a synonym of the earlier generic nameDipodomyswhich was to be applied to all of the kangaroo rats.
Dipodomys ordiiwas named by Woodhouse in 1853, from specimens from El Paso, Texas, but between that time and 1919 the nameordiiwas used in combination with all of the generic names mentioned above (see synonymies under the accounts of the subspecies).
The nearest approach to a revision of the genus wasGrinnell's(1922) "A Geographical Study of the Kangaroo Rats of California." Since that time,Hall and Dale(1939) revised theD. micropsgroup andDurrant and Setzer(1945) reported upon the kangaroo rats of Utah. The present paper is a review of the speciesDipodomys ordii. Some of the objectives in this review have been to learn: (1) What kinds of kangaroo rats are subspecies of the speciesDipodomys ordii; (2) the limits of geographic range of this full species; (3) the extremes of color, and of size and shape of the skullin this one species; (4) the significance of different sizes, shapes and colors; (5) the reasons for the existence, or formation, of selected subspecies; and (6) the relationships ofDipodomys ordiito other species in the genus.
Available specimens were arranged according to geographic origin. These were segregated as to sex and then under each sex by age. Individual variation was next measured in each of several samples in which individuals were of like geographic origin, sex, age and season. Finally, comparable materials were arranged geographically for detection of variations of systematic worth. Following preliminary studies of material thus arranged, additional specimens were collected from critical areas.
When fully adult animals (see next paragraph) were segregated as to sex, and then measured, the degree of secondary sexual variation was found to be less than the degree of individual variation; therefore in the tables of indices, no distinction as to sex has been made.
The only external measurements of the animals used were those recorded by the collectors on the labels attached to the skins. These measurements were total length, length of tail and length of hind foot. Measurements of the ear have not been used since they were not in all instances recorded by collectors and since measurements of dry ears proved to be unsatisfactory. Only measurements of fully adult specimens have been used. The term fully adult is applied only to those specimens in which the auditory bulla is shiny and translucent, the permanent P4 is fully erupted and worn, and the tail is fully striped and penicillate. No one of these characters alone was accepted as proof of adulthood but only the three in combination.
The following measurements of the skull have been used in the tables:
Greatest length.—From the most anterior tip of the nasals to the most posterior projection of the auditory bullae.
Greatest breadth across bullae.—From the most lateral projection of the auditory bulla on one side to the corresponding position on the other bulla.
Breadth across maxillary arches.—Greatest breadth across arches in a plane perpendicular to the long axis of the skull.
Width of rostrum.—Width of the premaxillae and the nasals taken immediately anterior to the upper incisors (not greatest width of nasals which is attained farther anteriorly).
Length of nasals.—Maximum length of a nasal bone.
Least interorbital breadth.—Least width between the orbits immediately posterior to the lacrimal processes.
Basilar length.—From the anterior margin of the foramen magnum to the posterior border of the alveolus of one of the upper incisors.
Capitalized color terms are fromRidgway, "Color Standards and Color Nomenclature," Washington, D. C., 1912. Color determinations were made by comparing a masked area of pure color on the side of the animal with a masked rectangle of named color onRidgway'splates in natural light always from the same angle.
Abbreviations used for specimens examined from the various collections are as follows:
AMNH—American Museum of Natural History.BYU—Brigham Young University.CNHM—Chicago Natural History Museum.CM—Carnegie Museum.CMNH—Colorado Museum of Natural History.DJC—Dixie Junior College.DRD—Donald R. Dickey Collection.KU—Museum of Natural History, University of Kansas.LACM—Los Angeles County Museum.MHS—Collection of Myron H. Swenk.MVZ—Museum of Vertebrate Zoology, University of California.OU—Museum of Zoology, University of Oklahoma.RH—Collection of Ross Hardy.UM—Museum of Zoology, University of Michigan.UN—Museum of Natural History, University of Nebraska.USAC—Utah State Agricultural College.USBS—United States Biological Surveys Collection.USNM—United States National Museum.UU—Museum of Zoology, University of Utah.TCWC—Texas Coöperative Wildlife Collection.
AMNH—American Museum of Natural History.BYU—Brigham Young University.CNHM—Chicago Natural History Museum.CM—Carnegie Museum.CMNH—Colorado Museum of Natural History.DJC—Dixie Junior College.DRD—Donald R. Dickey Collection.KU—Museum of Natural History, University of Kansas.LACM—Los Angeles County Museum.MHS—Collection of Myron H. Swenk.MVZ—Museum of Vertebrate Zoology, University of California.OU—Museum of Zoology, University of Oklahoma.RH—Collection of Ross Hardy.UM—Museum of Zoology, University of Michigan.UN—Museum of Natural History, University of Nebraska.USAC—Utah State Agricultural College.USBS—United States Biological Surveys Collection.USNM—United States National Museum.UU—Museum of Zoology, University of Utah.TCWC—Texas Coöperative Wildlife Collection.
This study is based on 3,732 specimens which were assembled at the Museum of Natural History, University of Kansas, or studied at other institutions. For the loan of this material and for the opportunity afforded for its study, I am extremely grateful to the authorities of each of these institutions and to the owners of the private collections.
Acknowledgement is made to the Office of Research and Inventions of the United States Navy for assistance with the field work which permitted the acquisition of essential specimens from several of the critical geographic areas while the author was research assistant on a larger over-all project (N6 ori-164-T02) of which the determination of the geographic range of this rodent species, a potential host of tularemia, was one facet. Tularemia was not detected in this genus.
I extend my thanks also to Professor Stephen D.Durrant, of the University of Utah, for helpful corrections in the preparation of the manuscript; to Mrs. Virginia Cassell Unruh for the preparation of the drawings; to Professor E. RaymondHall, of the University of Kansas, for guidance in the study and critical assistance with the manuscript; to Professors H. H. Lane and Worthie H. Horr for valued suggestions; to Mr. J. R. Alcorn for providing specimens for dissection when he was working under the University of Kansas endowment fund; and to the other friends and associates who have given of their time and criticism.
The family Heteromyidae was defined byWood(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 genusMicrodipodopswhich 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 generaDipodomys,ProdipodomysandCupidinimus, might be characterizedafterCoues'(1875) original description of the subfamily as follows: Cheek teeth progressively hypsodont, inDipodomysbecoming 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 fromWood'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 genusHeliscomysoccurs, 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) showsMookomys formicorum(from the Arikeean) as the next heteromyid in the evolutionary sequence and postulates that this species arose fromHeliscomys gregoryi.Mookomysis judged byWoodto 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) describedDipodomys kansensisfrom the Ogallala formation (Hemphillian age) of Kansas. He redescribed his species, and made it the type of the new genusProdipodomys(Hibbard, 1939:458), differentiating it fromDipodomyson the basis of the three-rooted p4, double-rooted m1 and m2 and the single rooted m3. It is shown to be closely allied toDipodomysby 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 genusProdipodomys? from Arizona.Gidley(1922:123) describedDipodomys minorfrom the Benson (Blancan) whichGazin(1942:486) refers to the genusProdipodomys?.Wood(1935:156) describedDipodomys gidleyifrom 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 ofD. gidleyishow lesser saltatorial ability, and therefore appear to be more primitive, than those of any livingDipodomys.
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 genusDiprionomys?, from the Barstow (Barstovian) was described byWood(1935:197) as follows: "The general shape of the tooth as figured strongly suggests either one of the most advanced species ofDipodomysor 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 thanDipodomys, 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 byWoodandWilsonare true heteromyids then a change in the phylogenetic scheme proposed byWood(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 ofCupidinimuswith regard to the grooving of the incisors says: "The philosophy of evolution which would prohibit its derivation fromMookomys, 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 theearly Miocene. Perhaps the line involvingMookomysandCupidinimuswhich was regarded byWoodas 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).
Phylogeny of the Dipodomyines.Fig. 1.Phylogeny of the Dipodomyines (modified afterWood, 1935).
AsWilson(1939:37) says: "Indeed it is hard to recognize such a form asCupidinimus nebraskensisas directly ancestral toDipodomysin 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 thatDipodomys gidleyigave rise toDipodomys spectabilisandDipodomys ordii, andDipodomys minorgave rise toDipodomys compactus. However, my own study indicates thatDipodomys compactusis conspecific withDipodomys ordiiand should stand asDipodomys ordii compactus. Consequently a different phyletic arrangement than that proposed byWood(loc.cit.) is required. SinceD. compactusis more closely allied toProdipodomys? minorthanD. ordiiis toD. gidleyi, it is possible thatP.? minorgave rise toD. ordiiand thatD. spectabilisis the end product of the phyletic trend ofD. 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 andProdipodomys) forms had acquired certain of these morphological modifications that are present in the modern genusDipodomys.
Representatives of nine species ofDipodomyswere dissected in an attempt to determine the degree of specialization and the relative systematic position of each species.
The myology was found to agree in detail as to origin, insertion and innervation with that ofDipodomys spectabilisas reported byHowell(1932). The only variation noted in the muscular system was the size of the individual muscles in those animals of widely divergent body size.
Dipodomys ordiiis the most generalized andDipodomys desertiis the most specialized of the kangaroo rats (seeTable 1), as judged by the osteology. Information gained by the study of the viscera of the various species supports this judgment. The visceral mass is relatively loose inD. ordii, but is markedly compact inD. deserti. This compactness appears to be brought about by the foreshortening of the mesenteries which support the entire gut and by the closer apposition of the large intestine to the caecum; both the intestine and caecum occupy a ventral position in the abdominal cavity. InDipodomys ordiithe entire visceral mass is loosely interconnected and the caecum is relatively small as compared to the tightly compact viscera and the large caecum inDipodomys deserti. Another striking feature is the size, proportion and position of the liver. In all animals dissected, even inD. deserti, the right lobe of the liverdescends and forms a capsule around the anterior end of the right kidney. In the Ord kangaroo rat, the bulk of the liver lies on the right side of the body cavity. That is to say, there is a greater bulk of the liver on the right side and it is situated more dorsad than in any of the other species examined. In the most specialized condition, as inDipodomys deserti, the bulk of the liver is almost equal on the right and left sides, and instead of having the greater bulk situated dorsally as inD. ordiiit is cup-shaped, with the dorsal and ventral parts of approximately equal size and situated on almost the same transverse plane. The entire mass of the liver is concave posteriorly.
Skeletal Indices of Dipodomys
The right kidney is variable in position in reference to the left. In all species the right kidney lies anterior to the left but in some,D. desertiandD. ingens, it is markedly anterior.
InDipodomys agilis,D. merriamiandD. desertithere are small to large patches of lymphoid tissue on the caecum. These patches were not noted in any of the other species examined and I do not know their function. In the three above mentioned species, however,the large intestine is shorter in proportion to the small intestine than in any other species exceptD. heermanni(seeTable 2) and with the exception ofD. heermanni,D. venustusandD. ordiithe actual measurements are less.
Inasmuch as little is known of the food habits of the various species of kangaroo rats, any ascription of adaptive significance to the varying proportions of the digestive system would be only speculative.
Midgley(1938) describes the visceral anatomy ofD. ordiiandD. microps. Except for the differences here noted the description of the viscera as given byMidgley(loc. cit.) applies to the rest of the species studied.