TABLE 4Proportionate Decrease of NasalsWidth of rostrumLength of nasalsLeast interorbital widthNasalsInterorbital × 100terrosus4.114.7513.591.6luteolus4.3513.912.9593.1evexus4.314.3513.7594.8montanus4.113.512.6593.8ordii3.513.313.097.7idoneus3.713.213.75103.5palmeri3.312.813.0101.1The first three columns represent the actual measurements of the various elements; the fourth column is the index established.Six different complexes (groups) of subspecies ofD. ordiihave probably arisen as a result of geographical separation.The Great Plains complex consisting ofD. o. richardsoni,D. o. oklahomae,D. o. evexus,D. o. terrosus,D. o. luteolus,D. o. priscusandD. o. mediusare, with the exception ofD. o. priscus, inhabitants of the high plains grassland habitat.D. o. priscusinhabits the red Desert of Wyoming.The Gulf Coast complex, comprisingD. o. sennettiandD. o. compactusare separable from all others by small auditory bullae and short tail.D. o. compactusprobably has differentiated fromD. o. sennettisince the cutting off, by wave action, from the mainland, of the islands on whichD. o. compactuslives.The Mexican complex consisting ofD. o. obscurus,D. o. fuscus,D. o. idoneusandD. o. palmerihave probably differentiated by natural selection acting on fortuitous variations, but I lack first hand knowledge of the region concerned.The southwestern complex consists ofD. o. chapmani,D. o. extractus,D. o. attenuatusandD. o. ordii.D. o. attenuatusandD. o. chapmaniare subspecifically distinct owing to geographic isolation, although both kinds show intergradation where their ranges approach that ofD. o. ordii.The western desert complex, composed ofD. o. monoensis,utahensis,cineraceus,columbianus,cinderensis,fetosus,celeripes,marshalli,inaquosus,pallidus,panguitchensisandfremontiwas isolated from the other complexes ofD. ordiiby the Quaternary uplift of the Wasatch Mountain mass, consisting of the Wasatch, Fish Lake and San Pitch mountains and the Wasatch, Aquarius, Paunsaugunt and Kaiparowits plateaus, and the concurrent reëstablishment of drainage systems. The drainages are those of the Colorado and Columbia rivers and that of the Snake River from Blackfoot, Idaho, to the junction with the Columbia.D. o. fremontihas been isolated on the upper reaches of the Fremont River which arises from the eastern side of the Wasatch Divide.D. o. panguitchensishas been isolated in Panguitch Valley as a result of the canyons formed by the Sevier River in Utah.D. o. cineraceus, although its subspecific and insular status are in doubt, appears to have been isolated on Dolphin Island, Great Salt Lake, Utah.The intermontane complex consisting ofD. o. montanus,longipes,cupidineus,nexilis,sanrafaelianduintensis, like the western desert complex, has become separated from the remainder of the subspecies of the speciesD. ordiiby Quaternary geologic events.D. o. cupidineushas been cut off by the gorges of the Colorado River to the south and the Virgin River to the north.D. o. sanrafaeliis separated fromD. o. uintensisby the Tavaputs Plateau and by the Roan and Book cliffs, and is separated from the range ofD. o. nexilisbythe Colorado River although there is intergradation betweenD. o. nexilisandD. o. sanrafaeli.D. o. longipeshas been separated from the rest of this intermontane complex by the San Juan and Colorado rivers, but to the east it intergrades freely with adjacent subspecies.D. o. montanushas been relatively isolated in the San Luis Valley of Colorado and New Mexico, but in the southern part of its range it does show intergradation with other subspecies.subspecies of Dipodomys ordii.Fig. 22.An arrangement, according to morphological indices, of the subspecies ofDipodomys ordii.The complexes mentioned above are represented graphically in Figure 22, in a way that expresses some of my ideas as to their genetic relationships.The indices used to determine the amount of specialization that each complex of subspecies has undergone are as follows:The Body index (head and body/length of tail × 100) is the expression of the elongation of the tail as an organ of balance while the length of the head and body remain relatively constant. As the tail elongates the index decreases and as the tail becomes shorter the index increases.The Pedal index (hind foot/head and body × 100) is the expression of the development of the hind foot as an element essential for the saltatorial habit. As the hind foot elongates the index will increase; elongation of the hind foot is interpreted as a specialization.The Cranial index (breadth across bullae/length of the skull × 100) reflects the degree of development of the tympanic or mastoid region, or both, and is thought to be an adaptation for more acute hearing and possibly for more delicate balance. Inflation of the tympanic bullae is thought to be a specialization. As the auditory bullae become more inflated, the index increases toward 100.The Bullar index (width of maxillary arches/breadth across bullae × 100) also expresses the degree of inflation of the auditory bullae. In a generalized mammal, at least in the heteromyids, the index would be 100, but as the auditory bullae become larger the index will decrease from 100.In attempting to arrange the subspecies ofD. ordiiaccording to degree of specialization, the geographic positions of the subspecies have been considered along with the information provided by the above-mentioned indices. These indices were used in the same way as were the indices for the species of the genus. InTables 5and6and in the accounts and maps the subspecies are arranged from the least specialized to the most specialized.TABLE 5Indices for the Subspecies ofDIPODOMYS ORDIIBodyPedalCranialBullarrichardsoni88.8534.3560.9588.25oklahomae86.7535.561.790.25compactus127.737.2559.7588.35sennetti94.2534.062.8585.95evexus80.135.760.592.9medius80.433.763.5585.9obscurus62.9586.4terrosus75.2535.0561.686.85fremonti80.5534.762.985.5uintensis77.235.362.386.0monoensis85.436.463.485.6ordii79.0537.662.7586.9luteolus75.037.0562.3586.3extractus83.6534.3564.384.25chapmani75.0536.3562.985.65montanus80.436.1564.2582.5cinderensis85.137.265.1584.75fetosus81.838.8563.9583.95utahensis80.236.9564.4584.35columbianus78.537.5564.2584.9idoneus72.336.664.285.0priscus74.939.4562.384.95celeripes91.8538.7565.084.25cineraceus75.539.163.984.8marshalli81.537.365.283.0inaquosus78.0537.964.2583.05attenuatus73.537.3564.083.4fuscus79.839.064.383.2longipes75.737.164.382.75pallidus76.940.7564.3584.65nexilis77.140.764.9578.45cupidineus73.1539.164.180.85palmeri72.2537.1565.180.45TABLE 6Numerals(derived fromTable 5)are Indicative of the Relative Degree of Specialization of the Subspecies ofDIPODOMYS ORDIIBodyPedalCranialBullarAveragerichardsoni43343.5oklahomae58525.0compactus119136.0sennetti2210106.0evexus158216.5medius12115119.75obscurus12710.0terrosus2564610.25fremonti115121410.5uintensis2077910.75monoensis612141311.25ordii17239513.5luteolus27158814.5extractus84252214,75chapmani2611111215.0montanus1310233019.0cinderensis718321919.0fetosus926172419.0utahensis1414282119.25columbianus1822211719.5idoneus3113201519.75priscus283061620.0celeripes325302320.25cineraceus2428161821.5marshalli1020332822.75inaquosus1924222723.0attenuatus2921182523.25fuscus1627242623.25longipes2316262923.5pallidus2232272025.25nexilis2131293326.0cupidineus3029193127.25palmeri3217313228.0
Proportionate Decrease of Nasals
The first three columns represent the actual measurements of the various elements; the fourth column is the index established.
Six different complexes (groups) of subspecies ofD. ordiihave probably arisen as a result of geographical separation.
The Great Plains complex consisting ofD. o. richardsoni,D. o. oklahomae,D. o. evexus,D. o. terrosus,D. o. luteolus,D. o. priscusandD. o. mediusare, with the exception ofD. o. priscus, inhabitants of the high plains grassland habitat.D. o. priscusinhabits the red Desert of Wyoming.
The Gulf Coast complex, comprisingD. o. sennettiandD. o. compactusare separable from all others by small auditory bullae and short tail.D. o. compactusprobably has differentiated fromD. o. sennettisince the cutting off, by wave action, from the mainland, of the islands on whichD. o. compactuslives.
The Mexican complex consisting ofD. o. obscurus,D. o. fuscus,D. o. idoneusandD. o. palmerihave probably differentiated by natural selection acting on fortuitous variations, but I lack first hand knowledge of the region concerned.
The southwestern complex consists ofD. o. chapmani,D. o. extractus,D. o. attenuatusandD. o. ordii.D. o. attenuatusandD. o. chapmaniare subspecifically distinct owing to geographic isolation, although both kinds show intergradation where their ranges approach that ofD. o. ordii.
The western desert complex, composed ofD. o. monoensis,utahensis,cineraceus,columbianus,cinderensis,fetosus,celeripes,marshalli,inaquosus,pallidus,panguitchensisandfremontiwas isolated from the other complexes ofD. ordiiby the Quaternary uplift of the Wasatch Mountain mass, consisting of the Wasatch, Fish Lake and San Pitch mountains and the Wasatch, Aquarius, Paunsaugunt and Kaiparowits plateaus, and the concurrent reëstablishment of drainage systems. The drainages are those of the Colorado and Columbia rivers and that of the Snake River from Blackfoot, Idaho, to the junction with the Columbia.D. o. fremontihas been isolated on the upper reaches of the Fremont River which arises from the eastern side of the Wasatch Divide.D. o. panguitchensishas been isolated in Panguitch Valley as a result of the canyons formed by the Sevier River in Utah.D. o. cineraceus, although its subspecific and insular status are in doubt, appears to have been isolated on Dolphin Island, Great Salt Lake, Utah.
The intermontane complex consisting ofD. o. montanus,longipes,cupidineus,nexilis,sanrafaelianduintensis, like the western desert complex, has become separated from the remainder of the subspecies of the speciesD. ordiiby Quaternary geologic events.D. o. cupidineushas been cut off by the gorges of the Colorado River to the south and the Virgin River to the north.D. o. sanrafaeliis separated fromD. o. uintensisby the Tavaputs Plateau and by the Roan and Book cliffs, and is separated from the range ofD. o. nexilisbythe Colorado River although there is intergradation betweenD. o. nexilisandD. o. sanrafaeli.D. o. longipeshas been separated from the rest of this intermontane complex by the San Juan and Colorado rivers, but to the east it intergrades freely with adjacent subspecies.D. o. montanushas been relatively isolated in the San Luis Valley of Colorado and New Mexico, but in the southern part of its range it does show intergradation with other subspecies.
subspecies of Dipodomys ordii.Fig. 22.An arrangement, according to morphological indices, of the subspecies ofDipodomys ordii.
The complexes mentioned above are represented graphically in Figure 22, in a way that expresses some of my ideas as to their genetic relationships.
The indices used to determine the amount of specialization that each complex of subspecies has undergone are as follows:
The Body index (head and body/length of tail × 100) is the expression of the elongation of the tail as an organ of balance while the length of the head and body remain relatively constant. As the tail elongates the index decreases and as the tail becomes shorter the index increases.
The Pedal index (hind foot/head and body × 100) is the expression of the development of the hind foot as an element essential for the saltatorial habit. As the hind foot elongates the index will increase; elongation of the hind foot is interpreted as a specialization.
The Cranial index (breadth across bullae/length of the skull × 100) reflects the degree of development of the tympanic or mastoid region, or both, and is thought to be an adaptation for more acute hearing and possibly for more delicate balance. Inflation of the tympanic bullae is thought to be a specialization. As the auditory bullae become more inflated, the index increases toward 100.
The Bullar index (width of maxillary arches/breadth across bullae × 100) also expresses the degree of inflation of the auditory bullae. In a generalized mammal, at least in the heteromyids, the index would be 100, but as the auditory bullae become larger the index will decrease from 100.
In attempting to arrange the subspecies ofD. ordiiaccording to degree of specialization, the geographic positions of the subspecies have been considered along with the information provided by the above-mentioned indices. These indices were used in the same way as were the indices for the species of the genus. InTables 5and6and in the accounts and maps the subspecies are arranged from the least specialized to the most specialized.
Indices for the Subspecies ofDIPODOMYS ORDII
Indices for the Subspecies ofDIPODOMYS ORDII
Numerals(derived fromTable 5)are Indicative of the Relative Degree of Specialization of the Subspecies ofDIPODOMYS ORDII