Evidence of the relative age of two genera of turtles, as suggested by their degree of differentiation into minor taxa, and the degree of difference between populations of two genera that inhabit adjacent drainage systems, may indicate the relative ages of particular river systems. For example, the slight resemblance of G. versa to kohni and the close resemblance of T. s. guadalupensis to pallidus in Texas may reflect the age of the genus Trionyx and the youth of the genus Graptemys. Remembering that the genus Graptemys is relatively recently evolved and assuming G. versa to be the most primitive and ancestral species of the genus (at least it is monotypic, the most aberrant species, and unlike any other species of the genus), it seems logical to suppose that the physiographic changes responsible for the Colorado-Brazos divide and the isolation of versa occurred early in the evolutionary history of the genus Graptemys. The degree of differentiation of Trionyx suggests that that genus is, comparatively, much older, and that the same physiographic changes responsible for the Colorado-Brazos divide and differentiation of the subspecies pallidus and guadalupensis occurred late in the evolutionary history of the genus Trionyx.
In general, patterns of distribution of turtle populations support physiographic evidence concerning changes in stream confluence and relative age of river systems.
In North America, soft-shelled turtles (genus Trionyx) occur in northern México, the eastern two-thirds of the United States, and extreme southeastern Canada. The genus fits the well-known Sino-American distributional pattern. In North America there are four species. Three (ferox, spinifer and muticus) are well-differentiated and one (ater) is not well-differentiated from spinifer. Characters of taxonomic worth are provided by the following: size; proportions of snout, head and shell; pattern on carapace, snout, side of head, and limbs; tuberculation; sizes of parts of skull; number of parts of carapaces; and, shape and number of some parts of plastra. Many features show geographical gradients or clines. T. ferox is the largest species and muticus is the smallest. Females of all species are larger than males. With increasing size of individual, the juvenal pattern is replaced by a mottled and blotched pattern in females of all species; adult males of spinifer retain a conspicuous juvenal pattern, whereas the juvenal pattern is sometimes obscured or lost on those of ferox and muticus. The elongation of the preanal region in all males, and the acquisition of a "sandpapery" carapace in males of spinifer occur at sexual maturity. There is a marked secondary sexual difference in coloration in a population of T. s. emoryi (side of head bright orange in males and yellow in females). The sex of many hatchlings of T. s. asper can be distinguished by the pattern on the carapace. Slight ontogenetic variation occurs in some proportional measurements. Large skulls of ferox and some asper (those in Atlantic Coast drainages) have expanded crushing surfaces on the jaws. Considering osteological characters, muticus is most distinct; there is less difference between ferox and spinifer than between those species and muticus.
T. ferox is monotypic, confined to the southeastern United States, and resembles Old World softshells more than it does any American species. The northern part of the geographic range of ferox overlaps that of T. s. asper; there, the two species are ecologically isolated. T. spinifer is polytypic, has the largest geographic range, and is composed of six subspecies, of which two are described as new (pallidus and guadalupensis). The subspecies are divisible into two groups. One, the spinifer group (spinifer, hartwegi and asper) is recognized by a juvenal pattern having black spots or ocelli; asper is the most distinctive and shows little evidence of intergradation in the lower Mississippi River drainage with the spinifer-hartwegi complex, which, northward, is differentiated into two subspecies in which there is an east-west cline in size of the ocelli on the carapace. The emoryi group (pallidus, guadalupensis, emoryi) is recognized by a pattern of white spots; emoryi is most distinctive. Each of several characters behaves as a cline if traced from east to west through the three subspecies. T. s. pallidus intergrades with the spinifer-hartwegi complex in the lower Mississippi River drainage. T. s. emoryi is the most variable subspecies; in its most notable population the males have orange coloration. T. s. emoryi has been introduced into the Colorado River drainage of Arizona. T. ater most closely resembles T. s. emoryi, but shows alliance with T. muticus and T. ferox. T. ater is confined to ponds of crystal-clear water in central Coahuila, México. T. muticus is completely sympatric with spinifer, and is composed of two subspecies (muticus and calvatus). T. m. calvatus shows no evidence of intergradation in the lower Mississippi River drainage with T. m. muticus, corresponding somewhat to the relationship of T. s. asper with the intergradient population of T. spinifer in the Mississippi River.
Softshells have pharyngeal respiration and probably are incapacitated by rotenone. T. ferox and the subspecies of spinifer occur in a wide variety of fresh-water habitats; muticus is more nearly restricted to running water (especially in the northern parts of its range) than spinifer, and may be less vagile than spinifer. T. ferox is more tolerant of marine and brackish waters than are muticus or spinifer. Small size and pallid coloration seem correlated with arid environments. The largest species (ferox) and the smallest population of spinifer (resembling muticus) both occur in the southernmost part of the range of the genus. Diurnal habits include basking on shores or débris in water, floating at the surface, procuring food, and burrowing in shallow and deep water (no observations for spinifer and muticus in deep water). Softshells are principally carnivorous; the food consists mostly of crawfish and insects; there is evidence of cannibalism involving predation on first- and second-year-old turtles. The capture of food is triggered primarily by movement of prey; sight seems to be more important than smell to Trionyx in capturing food. There is no indication of a food preference between species; enlarged crushing surfaces of jaws in some ferox and asper may be an adaptation for feeding on mollusks. Schools of fish are reported to follow softshells, and presumably acquire food that is dislodged by the grubbing and scurrying of the turtles on the bottom. Softshells are wary. They are good swimmers, and travel rapidly on land. The depressed body is an adaptation for burrowing and concealment. Permanent growths of algae do not occur on the dorsal surface of softshells. There is evidence of some nocturnal activity, and a general parallel in habits between trionychids and chelydrids. Softshells sometimes move overland; they move little in aquatic habitats. The normal annual period of activity of spinifer in latitudes 40° to 43° is approximately five months from April into September, depending on the weather; they hibernate under a shallow covering of mud in deep water. The southernmost populations may be active throughout the year.
Males of spinifer are sexually mature when the plastron is 9.0 to 10.0 centimeters in length (some when 8.0 long), whereas those of muticus are sexually mature at 8.0 to 9.0 centimeters. In the mentioned size range, the smaller adult males are probably in their fourth growing season, and the larger males in their fifth. Most females of spinifer are sexually mature at a plastral length of 18.0 to 20.0 centimeters and are probably in their ninth year; the smaller individuals probably are in their eighth. Females of muticus are sexually mature when the plastron is 14.0 to 16.0 centimeters long. Most of these are seven years old but some are only six years old. Some large females contain immature ovaries. The near-maximum length of carapace of spinifer is 18 inches, and such turtles are perhaps 60 years old; ferox perhaps attains a length of two feet.
T. ferox deposits eggs from late March to mid-July, whereas northern populations of spinifer and muticus usually deposit theirs from mid-June to mid-July. Sandy sites are preferred for nests, although movement to other sites occurs if the preferred sandy sites are submerged or otherwise rendered unusable. T. muticus limits its nest sites to the open areas of sand bars and does not lay inland where it must traverse vegetated areas, as does spinifer. Nests of ferox and spinifer seem to differ from those of muticus in being flask-shaped.
The seasonal reproductive potential is perhaps less in northern populations (averaging 20 eggs per clutch and only one clutch per season) than in southern populations (averaging about 10 eggs per clutch, but three clutches per season). Larger females deposit more eggs than smaller females. Eggs laid in northern latitudes are slightly smaller than those laid farther south. In any latitude the incubation period probably is at least 60 days. Hatchlings presumably leave nests at dusk, nighttime or dawn, and may winter over in eggs or nests.
Man is a great enemy of softshells. Predation on eggs probably accounts for most mortality. Physical conditions of the environment (overcrowding of nest sites, inadequate hibernation sites) and probably some kinds of parasitism contribute to mortality. Softshells are eaten locally and sometimes appear in the market of large cities, but over most of their range, there probably is no general demand and no special efforts are made to capture them. Fish, mostly minnows, comprise a small proportion of the diet. There is no evidence that softshells are active predators on any kind of fish, but their known food habits suggest that they compete with game fishes for food. Softshells are scavengers.