Distribution
The geographic range of the family Trionychidae in North America is principally in the eastern two-thirds of the continent and contributes to the well-known floral and faunal resemblance of eastern North America to that of eastern Asia (Schmidt, 1946:149) because Trionyx ferox (see [Fig. 18]) resembles the species of the genus in Asia more closely than it does any North American species. The Recent distribution in America does not include the Neotropical region, whereas the geographical range in the Old World extends south of the equator ([Fig. 1]; Dunn, 1931:109, fig. 2; Gadow, 1909:333, fig. 72; Hay, 1908:35, fig. 16).
American softshells occur in all river systems in the United States and the two adjacent river systems on the east coast of México that drain into the Gulf of México. Softshells inhabit streams of the Great Plains and occur westward to the foothills of the Rocky Mountains in the western tributaries of the [579] Mississippi River. Only T. s. spinifer occurs in the southern part of the Great Lakes-St. Lawrence drainage. Softshells are absent from the Atlantic Coast drainage except the Hudson River and those rivers at least south of (and including) the Pee Dee River in South Carolina.
T. s. emoryi is not known to be indigenous west of the Río Grande drainage, and has probably been introduced across the Continental Divide via the Gila River in western New Mexico into the Colorado River drainage of Arizona (Miller, 1946:46); the species undoubtedly occurs in México on the Sonoran side of the Colorado River opposite Baja California (Bogert and Oliver, 1945:417).
In the summer of 1959, I trapped turtles and with a specimen in hand inquired about softshells occurring in the inland drainages of northern México. From two collecting stations on the Río Nazas in Durango, only specimens of Pseudemys and Kinosternon were obtained; local inhabitants had neither seen nor heard of softshells. Flooded conditions in August of 1959 permitted trapping in only one of the inland drainages of northwestern Chihuahua, the Río Santa María; only specimens of Kinosternon were obtained. Local residents near that river as well as those living near the Río Casa Grandes and Río del Carmen had not seen or heard of softshells. A person that I judge to be a competent observer reported seeing a softshell in June of 1958 in the Río Alamos (Arroyo Cuchujáqui) near Alamos, Sonora, in the Río del Fuerte drainage on the west coast of México. I was a member of a field party from the University of Kansas that visited that locality in late January of 1959; only specimens of Pseudemys and Kinosternon were collected. Possibly isolated populations occur in streams of the Pacific Coast drainage of northern México. If so, they may have entered Pacific Coast drainages by stream capture across the Continental Divide. Several species of fish that are characteristic of the Río Grande traversed the Sierra Madre Occidental at some former time (presumably via the Río Conchos and Río Papigochic) and occur in the Yáqui River drainage (Meek, 1904:xxxviii, xlvii; Miller, 1959:214-15, 217). Because of the probability that the Río Nazas at some former time flowed north into the Río Grande (Meek, op. cit.:xxxiv), it is notable that softshells are absent in the Río Nazas drainage; the Big Bend turtle, Pseudemys scripta gaigeae, occurs in both drainages.
Relationships
Characters of Trionyx ferox suggesting a closer resemblance to some Old World members of the family than to the other three American species are: large size; marked difference between juvenal and adult patterns on the carapace; the marginal ridge; and the longitudinal ridgelike prominences on the carapace, especially in juveniles. Other characters of ferox suggesting a corresponding, but less marked resemblance to Old World species of Trionyx are: the large size of the eighth pair of pleurals; the absence of callosities on the epiplastron and preplastra; frequent fusion of the hyoplastra and hypoplastra (more than in spinifer or muticus); and tolerance of marine waters (more than muticus or spinifer). Some fossils also suggest alliance with ferox and some Old World members of the genus in their large size, large eighth pair of pleurals, and occurrence in marine deposits; several Old World species have been reported at sea (Pelochelys, T. triunguis, T. sinensis). T. ferox is monotypic and has the most southeasterly displaced, geographic range.
Because ferox resembles softshells from the Old World more closely than it does any American species, ferox is assumed to be more closely related to Old World softshells than to any American species, and, because of resemblance to some fossils, ferox is assumed to resemble most closely the primitive, ancestral stock of softshells that occupied North America. T. spinifer, T. muticus and T. ater, which resemble each other more closely than any of them resembles T. ferox or any Old World species, are considered autochthonous in North America. T. spinifer and T. muticus are distinct, sympatric species. Burt (1935:321) suggested that the two species "may be variants of the same species." T. ater is weakly differentiated from T. spinifer emoryi. The species, ferox, spinifer and muticus are well-differentiated and were considered by Agassiz (1857), Gray (1869) and Baur (1893) as belonging to three different genera.
In the widely distributed T. spinifer, the subspecies spinifer, hartwegi and asper closely resemble one another; asper seems most distinct, whereas spinifer and hartwegi are terminal populations of an east-west cline in one character. The subspecies pallidus, guadalupensis and emoryi resemble one another more closely than any resembles any of the subspecies mentioned immediately above; T. s. pallidus, however, is annectent. T. s. pallidus and guadalupensis represent terminal populations of clines in several characters, some of which occur in emoryi, but that subspecies is more distinct from pallidus and guadalupensis than those subspecies are from each other. T. s. emoryi is the most variable subspecies. T. ater, known only from a restricted area in central Coahuila, is most closely related to T. s. emoryi, and possesses some characters judged to represent the attenuation of the geographic cline in pallidus, guadalupensis and emoryi mentioned above. Some characters of ater show alliance to the species muticus. Of T. muticus, whose geographic range is removed from that of ater, there are two subspecies. Four subspecies of spinifer (spinifer, hartwegi, pallidus and asper) intergrade in the Mississippi River drainage of Louisiana; few specimens, however, are typical of asper. The subspecies of muticus do not show definite evidence of intergradation. To facilitate quick reference, the occurrence of some characters that are shared by, or are approximated in, two or more forms are listed in [Table 10]. In addition to external characters, some ratios emphasize the clinal relationship between T. s. pallidus, guadalupensis, and emoryi mentioned above. Of especial interest is the frequent resemblance of those subspecies and T. ater to T. ferox (dorsal pattern on limbs of adults, reduction in anterior tuberculation, wide head, narrow carapace, and short snout), and the less marked resemblance of T. muticus to T. ferox; not shown in [Table 10] is the resemblance of ferox to T. muticus calvatus in having thick, black-bordered postocular stripes. Some populations of T. s. emoryi resemble T. muticus in the corresponding size at sexual maturity and in having well-developed plastral callosities. It is notable that the occurrence of ater, and to a lesser extent that of T. s. emoryi, which resembles ferox (and muticus), is in the southwestern United States and northern México.