Fig. 24.Geographic distribution of Recent soft-shelled turtles (bordered by heavy black line) and fossil trionychids (black circles) in North America. The introduced population ofT. s. emoryiin the southwestern United States is not shown.
Fig. 24.Geographic distribution of Recent soft-shelled turtles (bordered by heavy black line) and fossil trionychids (black circles) in North America. The introduced population ofT. s. emoryiin the southwestern United States is not shown.
Ameghino (inHay,op. cit.:35) recorded specimens of a trionychid from the Cretaceous of Patagonia, a record that, at present, cannot be accepted (Simpson, 1943:423). Mullerried (loc. cit.) also mentioned some trionychid remains that were housed in Tuxtla Gutierrez, Chiapas, México, (material now lost), but their geographical provenance was unknown. The former extent of rangesouthward is not known; it is improbable that trionychids occurred in South America (Simpson, 1943:423).
Phylogeny
The occurrence ofT. feroxin Florida and the suggestion offerox-like characters in turtles from southwestern Texas and northern Mexico presents a distributional pattern that resembles the disjunct ranges of many other pairs of closely related taxa. The clear-water ponds in central Coahuila, which are inhabited byater, correspond to aquatic habitats supportingferoxin Florida. The splitting of the geographic range into eastern and western parts possibly resulted from a southward shift of colder climates in glacial stages of the Pleistocene, or from the development of an intervening arid region in the late Miocene and Pliocene (see discussions in Martin and Harrell, 1957, and Blair, 1959). An initial separation of range by an arid environment in the Pliocene may have been terminated by the colder climates in the Pleistocene.
The degree of morphological difference betweenferoxand the forms in southwestern Texas and northern México, suggests that the time of separation antedated the Pleistocene.
Trionychid turtles may have traversed the Bering land bridge between Asia and North America in late Mesozoic times for they occur as fossils on the Atlantic Coast and in the Rocky Mountain-Great Plains region in Upper Cretaceous deposits. Shallow, inland seas may have afforded no barrier to the dispersal of softshells which presumably were tolerant of saline waters. The orogeny and volcanic action with subsequent erosion and sedimentation of the Rocky Mountain system, which was later accompanied by drier climates, tended to obliterate suitable habitats in the western United States; softshells persisted at least until the Upper Eocene on the west coast (Brattstrom, 1958:5). The factors responsible for the disappearance of softshells on the Atlantic Coast probably were related to the glacial advances in the Pleistocene; the most recent fossils known occur in Miocene deposits.
The relationships of the living species and subspecies were probably correlated with geologic change in aquatic environments and drainage patterns. These changes probably included stream capture, flooding, drought, uplifting and planation. A hypothetical, evolutionary history is presented in the phylogenetic diagram where letter symbols represent species and subspecies, and grouped symbols (referred to in subsequent paragraphs) represent ancestral stocks.
Click hereto see text version of above chart.
Click hereto see text version of above chart.
An arid environment in the central and southern United States and northern Mexico may have increased in area especially southward from Miocene times into the Pliocene (Dorf, 1959:189, 191). The combination of physiographic changes and aridity, which modified the mesic, essentially continuous, aquatic habitats, may have isolated and aided in the differentiation of theferox,muticusandspiniferstocks. Encroachment of the Eocene seas, the maximal extent of which corresponded to the Gulf Coastal Plain and included a northerly extension as far as Cairo in southern Illinois (Mississippi embayment), possibly was an initial barrier isolating theferoxstock of the east.
In the late Miocene or early Pliocene, the MSA (muticus-spinifer-ater) stock presumably occupied a large region of the central United States, which extended southward into northern México and along the Gulf Coast at least as far as Alabama. Farther eastward, theferoxstock was isolated in more mesic, probably swampy, marshy habitats.
Later, in the southwestern part of the range of the MSA stock (southern Texas and northern México), the SA andmuticusstocks were separated. Themuticusstock occurred to the northeastward, and presumably no farther south than the area included within the present drainage basin of the Colorado River. Southward, the SA stock was isolated into several populations that are today represented byaterandT. s. emoryi, the most variable subspecies; the distribution of the most distinctive population ofemoryiindicatesa former isolated inland drainage. The multiple fragmentation of the SA stock presumably terminated by the end of the Pliocene. The progenitors ofT. aterprobably closely resembledferox.Trionyx aterandT. feroxresemble each other morphologically and in habitat. Therefore, the progenitors ofaterare considered to have undergone comparatively little differentiation.
Thespiniferstock, occurring principally in the area included within the present drainage basin of the Río Grande, extended its geographic range eastward and became sympatric withmuticusandferox. An expansion of range necessarily demands more mesic conditions; these were perhaps afforded by the pluvials (wet, rainy ages) that were coincident with the glacial periods in the Pleistocene (Antevs, 1948:168). The pluvials permitted the isolated populations of thespiniferstock to unite, and permitted that stock to extend its range eastward. The concurrent continental glaciation permitted thespiniferstock to extend its range eastward only in a belt approximately 300 miles wide along the Gulf Coast, and also displaced the ranges offeroxandmuticusto southern latitudes. Perhapsferoxwas less tolerant of decreased temperatures or changes in habitat than was thespiniferstock but, for some unknown reason,feroxdid not extend its range westward. BecauseT. aterclosely resemblesT. s. emoryi, continued isolation ofatersince the beginning of the Pleistocene seems unlikely andatermay have been reunited in subsequent pluvial periods with thespinifer(emoryi) stock. A climatic fluctuation between relatively wet and dry periods is corroborated by studies of soil profiles in Trans-Pecos Texas (Bryan and Albritton, 1943).
The separation of the range ofspiniferin the general region of western Louisiana, resulting in the differentiation of thespinifergroup of subspecies to the east and theemoryigroup of subspecies to the west, and the differentiation ofT. s. asperandT. m. calvatus, both having corresponding western limits of distribution (Mississippi River drainage), are associated with the activities of the Mississippi River and its flood-plain. The combined effects of the pluvials and interpluvials seem responsible for changes in the lower Mississippi Valley. Great volumes of summer melt-water in the glacial stages greatly increased the breadth of the channel of the lower Mississippi River (corresponding to the northern extent of the Mississippi embayment; Hobbs, 1950), and this, coupled with the encroachment of Pleistocene seas (especially in the Mississippi embayment) in the interglacial periods, perhaps separated populations eastward represented today byT. m. calvatusandT. s. asper. Thespinifer-hartwegistock probably developed in southern Louisiana in association with the meandering of the Mississippi River and its tributaries, and its broad alluvial plain. The biota of that plain differed from that adjacent to the east or west (see discussion in Viosca, 1944) and constituted a barrier, of a sort, to free communication between the east and west. Westward theemoryigroup of subspecies differentiated, its eastern limit probably being the Red River, which followed its own course to the Gulf along the lowlands on the west side of the Mississippi Valley and did not empty directly into the Mississippi until Recent times (Holland, 1944:20). There was not an equally-marked, corresponding separation of the range ofmuticus. However, the juvenal pattern of the subspeciesmuticusthat inhabits the Gulf Coast streams is slightly different (having less short lines) from that ofmuticuselsewhere.
The Río Grande (inhabited byemoryi) presumably had its own exit to the Gulf whereas rivers westward to (and including) the Red River (inhabited bypallidus-guadalupensiscline) probably were joined near their mouths forming a large drainage system. Hubbs (1957:93) pointed out that the Río Grande-Nueces divide also limits a large number of species of fish. The differentiation ofpallidusandguadalupensisis possibly due to a difference in the salt content of waters that drain the Edward's Plateau (see page 547), or to isolation of those subspecies in separate drainage systems that had their own exits to the Gulf.
In the lower Mississippi drainage, thespinifer-hartwegistock extended its range northward following the retreat of the last glacial stage, and differentiated into those two subspecies in the upper Mississippi drainage and Great Lakes-St. Lawrence drainage system.
I have seen one specimen (UMMZ 59198) from the eastern part of the Tennessee drainage (inhabited byT. s. spinifer) that resemblesT. s. asper(occupying the Gulf Coast drainages of the southeast). This resemblance tends to support the thesis of a former confluence of the Coosa (Alabama River system) and Tennessee drainages as believed by some malacologists to explain resemblances in molluscan fauna and as corroborated by physiographical evidence (see discussion in van der Schalie, 1945).
The Importance of the Study of Turtle Populations in Relation to the History of River Systems
In the Río Grande drainage the geographic distribution of the population ofemoryihaving orange color in males is approximately the same as that ofPseudemys scripta gaigeae; the correspondingdistributions suggest that a part of the Río Grande drainage consisting of the Río Conchos in Chihuahua and the Big Bend region of Texas was isolated in former times. Accordingly, the known aquatic chelonian fauna in the basin of Cuatro Ciénegas in central Coahuila, México, is endemic (exceptT. s. emoryi). And the coincidence of the geographic ranges ofT. muticus calvatusandGraptemys pulchrain the southeast suggest a former association of the included (Pearl to Escambia) river systems. The occurrence ofT. s. pallidusin the Red River drainage indicates that the Red River was formerly associated with the Gulf Coast streams of eastern Texas and western Louisiana (inhabited bypallidus) and not with the Mississippi River drainage. The lower Mississippi River valley forms a prominent barrier to the eastern and western dispersal of many kinds of species and subspecies of turtles.T. m. calvatusandT. s. asper, which occur in rivers of the Gulf Coast drainage east of the Mississippi, are well-differentiated subspecies showing little or no evidence of intergradation with their relatives in the Mississippi River. The large faunal break provided by the Mississippi River would seem to indicate greater age for that river than for other rivers of the Gulf Coast drainage.
A comparison of the distributions ofTrionyxandGraptemysin Texas suggests a faunal break between the drainage systems of the Brazos and Colorado rivers.Graptemys versaoccurs in the Colorado and Guadalupe-San Antonio drainages. To my knowledgeversahitherto has not been recorded from the latter drainage system. I have seen one specimen ofGraptemys(custody of Gerald Raun, University of Texas) from the Guadalupe River drainage, which I judge to be representative ofversa, and Olson (1959:48) has reportedGraptemys(probablyversa) in the San Antonio River. The distribution ofG. versaparallels in a general way, the distribution ofT. s. guadalupensis.G. kohniandT. s. pallidusoccur in the Brazos River and eastward. Also, it is notable that the population ofT. m. muticusoccurring in the Colorado River drainage differs slightly (more black pigmentation) from the same subspecies in the adjacent Brazos River system.
There is much difference in the patterns of distribution and degree of differentiation of different genera of aquatic turtles in the eastern United States. Tinkle (1958:41-43, Figs. 49-55) concluded that a general resemblance in the patterns of distribution of the different genera of turtles was evidence that the rates of evolution were essentially the same, assuming that each genus had had asimilar time interval for differentiation (op. cit.:42). If this is true, corresponding patterns of distribution might indicate the same relative age of the population of turtles concerned. Generally, the genera of turtles that on morphological grounds are considered the oldest and most primitive (Macroclemys,Chelydra) show less differentiation into species and subspecies than those considered younger and more recently evolved (Graptemys,Pseudemys). In the genusGraptemys, much differentiation occurs in the geologically, recently formed, Gulf Coast drainage systems of the southeastern United States. It would seem then, that faster rates of differentiation denote more recent genera, whereas older genera are endowed with a "genetic senility" and are less subject to change.
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 ofG. versatokohniand the close resemblance ofT. s. guadalupensistopallidusin Texas may reflect the age of the genusTrionyxand the youth of the genusGraptemys. Remembering that the genusGraptemysis relatively recently evolved and assumingG. versato 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 ofversaoccurred early in the evolutionary history of the genusGraptemys. The degree of differentiation ofTrionyxsuggests that that genus is, comparatively, much older, and that the same physiographic changes responsible for the Colorado-Brazos divide and differentiation of the subspeciespallidusandguadalupensisoccurred late in the evolutionary history of the genusTrionyx.
In general, patterns of distribution of turtle populations support physiographic evidence concerning changes in stream confluence and relative age of river systems.
SUMMARY
In North America, soft-shelled turtles (genusTrionyx) 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,spiniferandmuticus) are well-differentiatedand one (ater) is not well-differentiated fromspinifer. 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. feroxis the largest species andmuticusis 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 ofspiniferretain a conspicuous juvenal pattern, whereas the juvenal pattern is sometimes obscured or lost on those offeroxandmuticus. The elongation of the preanal region in all males, and the acquisition of a "sandpapery" carapace in males ofspiniferoccur at sexual maturity. There is a marked secondary sexual difference in coloration in a population ofT. s. emoryi(side of head bright orange in males and yellow in females). The sex of many hatchlings ofT. s. aspercan be distinguished by the pattern on the carapace. Slight ontogenetic variation occurs in some proportional measurements. Large skulls offeroxand someasper(those in Atlantic Coast drainages) have expanded crushing surfaces on the jaws. Considering osteological characters,muticusis most distinct; there is less difference betweenferoxandspiniferthan between those species andmuticus.
T. feroxis 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 offeroxoverlaps that ofT. s. asper; there, the two species are ecologically isolated.T. spiniferis polytypic, has the largest geographic range, and is composed of six subspecies, of which two are described as new (pallidusandguadalupensis). The subspecies are divisible into two groups. One, thespinifergroup (spinifer,hartwegiandasper) is recognized by a juvenal pattern having black spots or ocelli;asperis the most distinctive and shows little evidence of intergradation in the lower Mississippi River drainage with thespinifer-hartwegicomplex, which, northward, is differentiated into two subspecies in which there is an east-west cline in size of the ocelli on the carapace. Theemoryigroup (pallidus,guadalupensis,emoryi) is recognized by a pattern of white spots;emoryiis most distinctive. Each of several characters behaves as a cline if traced from east to west through the three subspecies.T. s. pallidusintergrades withthespinifer-hartwegicomplex in the lower Mississippi River drainage.T. s. emoryiis the most variable subspecies; in its most notable population the males have orange coloration.T. s. emoryihas been introduced into the Colorado River drainage of Arizona.T. atermost closely resemblesT. s. emoryi, but shows alliance withT. muticusandT. ferox.T. ateris confined to ponds of crystal-clear water in central Coahuila, México.T. muticusis completely sympatric withspinifer, and is composed of two subspecies (muticusandcalvatus).T. m. calvatusshows no evidence of intergradation in the lower Mississippi River drainage withT. m. muticus, corresponding somewhat to the relationship ofT. s. asperwith the intergradient population ofT. spiniferin the Mississippi River.
Softshells have pharyngeal respiration and probably are incapacitated by rotenone.T. feroxand the subspecies ofspiniferoccur in a wide variety of fresh-water habitats;muticusis more nearly restricted to running water (especially in the northern parts of its range) thanspinifer, and may be less vagile thanspinifer.T. feroxis more tolerant of marine and brackish waters than aremuticusorspinifer. Small size and pallid coloration seem correlated with arid environments. The largest species (ferox) and the smallest population ofspinifer(resemblingmuticus) 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 forspiniferandmuticusin 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 toTrionyxin capturing food. There is no indication of a food preference between species; enlarged crushing surfaces of jaws in someferoxandaspermay 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 ofactivity ofspiniferin 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 ofspiniferare sexually mature when the plastron is 9.0 to 10.0 centimeters in length (some when 8.0 long), whereas those ofmuticusare 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 ofspiniferare 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 ofmuticusare 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 ofspiniferis 18 inches, and such turtles are perhaps 60 years old;feroxperhaps attains a length of two feet.
T. feroxdeposits eggs from late March to mid-July, whereas northern populations ofspiniferandmuticususually 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. muticuslimits its nest sites to the open areas of sand bars and does not lay inland where it must traverse vegetated areas, as doesspinifer. Nests offeroxandspiniferseem to differ from those ofmuticusin 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 generaldemand 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.
Fossil material was not studied in detail. The fossil softshells indicate a more widespread, former distribution. Some osteological characters and their variation in the living species are mentioned as an aid to future workers concerned with an assay of fossil remains. Fossils occur in marine, brackish and fresh-water deposits, and many are much larger than the living species; the oldest American fossils are of Upper Cretaceous age.
The interrelationships of the living species and subspecies suggest that the speciesspinifer,ater, andmuticusare derivatives of aferox-like ancestor, and that they differentiated in North America; most differentiation occurs in southwestern Texas and northern México where characters of some populations indicate alliance withferox. It is hypothesized that aridity in the late Tertiary effected specific differentiation by the modification and isolation of aquatic habitats. Pluvial periods in the Pleistocene provided for confluence of aquatic habitats and expansion of geographic ranges, and coupled with physiographic changes, conceivably caused or enhanced some of the subspecific variation.
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