North American Recent Soft-shelled Turtles (Family Trionychidae) Part 23

_Widened alveolar surfaces of jaws._--An ontogenetic variation affecting large skulls of _T. ferox_ and some individuals of _T.

spinifer asper_; presumably confined to females. Of especial interest is its presence in some populations of _asper_ that are not otherwise distinguishable (external characters) from the rest of the individuals comprising that subspecies.

_Sculpturing._--No differences in pattern (generally of anastomosing ridges) on carapace or plastron; fineness or coa.r.s.eness seemingly correlated with size; frequency and kind (k.n.o.blike or ridgelike) of bony prominences on carapace variable; bony prominences confined to species _spinifer_ and _ferox_, occurring princ.i.p.ally on large females.

_Fontanelles of carapace._--Closure more or less correlated with increasing size, although much variation noted between individuals of same size; small individuals have fontanelles confluent (medially), thus separating nuchal from contact with first neural and first pair of pleurals.

_Number and arrangement of neurals and pleurals._--Neurals number six to nine, usually seven or eight; pleurals number seven or eight pairs, and may or may not be in contact with each other posteriorly; eighth pair of pleurals when present reduced, never contacting seventh neural; arrangement posteriorly variable (see Fig. 16 and Tab. 5).

_Plastral callosities._--Increase in size with advancing age causing corresponding reduction in size of plastral vacuity; relatively best developed in _muticus_ (all elements touching medially on KU 41380 leaving no plastral vacuity); probably no callosities on preplastra or epiplastron of _ferox_; callosity on epiplastron of _spinifer_ not covering entire surface (as it may in _muticus_).

_Epiplastron._--Obtusely-angled (greater than 90 degrees) in _muticus_; acutely-angled (90 degrees or less) in _ferox_ and _spinifer_.

_Hyo-hypoplastral suture._--Usually present, but occasionally absent, in all species.

The fossil turtles of North America have been treated monographically by Hay (1908), who apportioned fossil trionychid remains into eight genera (three living) of two families. Recently, Romer (1956:514) relegated all trionychid fossils to the genus _Trionyx_. Characters, as gleaned from Hay's synopsis (_op. cit._:465-548, Pls. 85-113), that seem especially worthy of taxonomic consideration are: (1) The presence of a preneural, which is not known to occur in the living American species (seemingly the preneural is fused with the first neural and represents the elongate first neural in living species); (2) The large eighth pair of pleurals, especially when they contact the seventh neural; (3) The thickness of the costal plates, a condition probably correlated with the size of some fossils, which are larger than any living species (for example, Hay, _op. cit._:518, mentioned the greatest dimension of a nuchal bone as approximately 300 mm.).

The approximate extent of the known horizontal distribution of fossils is indicated in Figure 24. A comparison of known localities of fossils and the distribution of living softsh.e.l.ls (introduced population of _T.

s. emoryi_ in Colorado River drainage omitted) shows that the distribution was more widespread in former times. Localities of fossils are centered on the Atlantic Coast from New Jersey to North Carolina and in the Rocky Mountain-Great Plains region from Alberta and Saskatchewan to northwestern New Mexico; the oldest fossils, which occur in each region, are found in Upper Cretaceous deposits. Many fossils occur in marine and brackish water deposits. Most localities depicted on the map are mentioned by Hay (1908:36-37, 465-548). Other localities included on the map are in southern Alberta (Russell, 1929:164; 1930:27; Sternberg, 1926:104), southern Saskatchewan (Russell, 1934:109), northern South Dakota (Hay, 1910:324), central Utah (Gilmore, 1946), western Colorado (Schmidt, 1945), southwestern Kansas (Galbreath, 1948:284), southeastern Texas (Hay _in_ Stejneger, 1944:65), southern California (Brattstrom, 1958:5), and northeastern Coahuila, Mexico (Mullerried, 1943:623). Hay's record of the living _Platypeltis_ (= _Trionyx_) _ferox_ and other remains from the Peace Creek formation in Hillsborough County, Florida (_op. cit._:548), presumably is the same record mentioned by Pope (1949:305).

[Ill.u.s.tration: FIG. 24. Geographic distribution of Recent soft-sh.e.l.led turtles (bordered by heavy black line) and fossil trionychids (black circles) in North America. The introduced population of _T. s. emoryi_ in the southwestern United States is not shown.]

Ameghino (_in_ Hay, _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, Mexico, (material now lost), but their geographical provenance was unknown. The former extent of range southward is not known; it is improbable that trionychids occurred in South America (Simpson, 1943:423).

Phylogeny

The occurrence of _T. ferox_ in Florida and the suggestion of _ferox_-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 by _ater_, correspond to aquatic habitats supporting _ferox_ in Florida. The splitting of the geographic range into eastern and western parts possibly resulted from a southward s.h.i.+ft 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 between _ferox_ and the forms in southwestern Texas and northern Mexico, 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 softsh.e.l.ls 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; softsh.e.l.ls persisted at least until the Upper Eocene on the west coast (Brattstrom, 1958:5). The factors responsible for the disappearance of softsh.e.l.ls on the Atlantic Coast probably were related to the glacial advances in the Pleistocene; the most recent fossils known occur in Miocene deposits.

The relations.h.i.+ps 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.

Pliocene Pleistocene Recent ==========================================================================

+--F-----------------------------------F (_ferox_) | | +---------Mm (_muticus muticus_) | +--M------------------+ | | +---------Mc (_muticus calvatus_) | | FMSA-+ | +--Ss (_spinifer spinifer_) | | +------+ | | | +--Sh (_spinifer hartwegi_) | | +--Ssha-+ +--MSA-+ | | | | +---------Sa (_spinifer asper_) | | | +--S---+ +--Sp (_spinifer pallidus_) | | | +--Spg-+ | | | | +--Sg (_spinifer guadalupensis_) +--SA--+ +--Sepg-+ | | | +--Se-----Se (_spinifer emoryi_) | +--A---------------------A (_ater_)

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 the _ferox_, _muticus_ and _spinifer_ stocks.

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 the _ferox_ stock 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 Mexico and along the Gulf Coast at least as far as Alabama. Farther eastward, the _ferox_ stock 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 Mexico), the SA and _muticus_ stocks were separated.

The _muticus_ stock 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 by _ater_ and _T. s. emoryi_, the most variable subspecies; the distribution of the most distinctive population of _emoryi_ indicates a former isolated inland drainage. The multiple fragmentation of the SA stock presumably terminated by the end of the Pliocene. The progenitors of _T. ater_ probably closely resembled _ferox_. _Trionyx ater_ and _T. ferox_ resemble each other morphologically and in habitat. Therefore, the progenitors of _ater_ are considered to have undergone comparatively little differentiation.

The _spinifer_ stock, occurring princ.i.p.ally in the area included within the present drainage basin of the Rio Grande, extended its geographic range eastward and became sympatric with _muticus_ and _ferox_. 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 the _spinifer_ stock to unite, and permitted that stock to extend its range eastward. The concurrent continental glaciation permitted the _spinifer_ stock to extend its range eastward only in a belt approximately 300 miles wide along the Gulf Coast, and also displaced the ranges of _ferox_ and _muticus_ to southern lat.i.tudes. Perhaps _ferox_ was less tolerant of decreased temperatures or changes in habitat than was the _spinifer_ stock but, for some unknown reason, _ferox_ did not extend its range westward. Because _T. ater_ closely resembles _T. s. emoryi_, continued isolation of _ater_ since the beginning of the Pleistocene seems unlikely and _ater_ may have been reunited in subsequent pluvial periods with the _spinifer_ (_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 of _spinifer_ in the general region of western Louisiana, resulting in the differentiation of the _spinifer_ group of subspecies to the east and the _emoryi_ group of subspecies to the west, and the differentiation of _T. s. asper_ and _T. m. calvatus_, both having corresponding western limits of distribution (Mississippi River drainage), are a.s.sociated 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 by _T.

m. calvatus_ and _T. s. asper_. The _spinifer-hartwegi_ stock probably developed in southern Louisiana in a.s.sociation 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 const.i.tuted a barrier, of a sort, to free communication between the east and west. Westward the _emoryi_ group 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 of _muticus_. However, the juvenal pattern of the subspecies _muticus_ that inhabits the Gulf Coast streams is slightly different (having less short lines) from that of _muticus_ elsewhere.

The Rio Grande (inhabited by _emoryi_) presumably had its own exit to the Gulf whereas rivers westward to (and including) the Red River (inhabited by _pallidus-guadalupensis_ cline) probably were joined near their mouths forming a large drainage system. Hubbs (1957:93) pointed out that the Rio Grande-Nueces divide also limits a large number of species of fish. The differentiation of _pallidus_ and _guadalupensis_ is 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, the _spinifer-hartwegi_ stock 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 by _T. s. spinifer_) that resembles _T. 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 Rio Grande drainage the geographic distribution of the population of _emoryi_ having orange color in males is approximately the same as that of _Pseudemys scripta gaigeae_; the corresponding distributions suggest that a part of the Rio Grande drainage consisting of the Rio 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 Cienegas in central Coahuila, Mexico, is endemic (except _T. s. emoryi_). And the coincidence of the geographic ranges of _T.

muticus calvatus_ and _Graptemys pulchra_ in the southeast suggest a former a.s.sociation of the included (Pearl to Escambia) river systems.

The occurrence of _T. s. pallidus_ in the Red River drainage indicates that the Red River was formerly a.s.sociated with the Gulf Coast streams of eastern Texas and western Louisiana (inhabited by _pallidus_) 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. calvatus_ and _T. 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 of _Trionyx_ and _Graptemys_ in Texas suggests a faunal break between the drainage systems of the Brazos and Colorado rivers. _Graptemys versa_ occurs in the Colorado and Guadalupe-San Antonio drainages. To my knowledge _versa_ hitherto has not been recorded from the latter drainage system. I have seen one specimen of _Graptemys_ (custody of Gerald Raun, University of Texas) from the Guadalupe River drainage, which I judge to be representative of _versa_, and Olson (1959:48) has reported _Graptemys_ (probably _versa_) in the San Antonio River. The distribution of _G. versa_ parallels in a general way, the distribution of _T. s. guadalupensis_. _G. kohni_ and _T. s. pallidus_ occur in the Brazos River and eastward. Also, it is notable that the population of _T. m. muticus_ occurring 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, a.s.suming that each genus had had a similar 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 genus _Graptemys_, 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 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 a.s.suming _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.

SUMMARY

In North America, soft-sh.e.l.led turtles (genus _Trionyx_) occur in northern Mexico, 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 sh.e.l.l; 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 prea.n.a.l region in all males, and the acquisition of a "sandpapery" carapace in males of _spinifer_ occur at s.e.xual maturity. There is a marked secondary s.e.xual difference in coloration in a population of _T. s. emoryi_ (side of head bright orange in males and yellow in females). The s.e.x 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 crus.h.i.+ng 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 softsh.e.l.ls 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, Mexico. _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 relations.h.i.+p of _T. s. asper_ with the intergradient population of _T. spinifer_ in the Mississippi River.

Softsh.e.l.ls 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 sh.o.r.es or debris in water, floating at the surface, procuring food, and burrowing in shallow and deep water (no observations for _spinifer_ and _muticus_ in deep water). Softsh.e.l.ls are princ.i.p.ally 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 crus.h.i.+ng surfaces of jaws in some _ferox_ and _asper_ may be an adaptation for feeding on mollusks. Schools of fish are reported to follow softsh.e.l.ls, and presumably acquire food that is dislodged by the grubbing and scurrying of the turtles on the bottom. Softsh.e.l.ls 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 softsh.e.l.ls. There is evidence of some nocturnal activity, and a general parallel in habits between trionychids and chelydrids. Softsh.e.l.ls sometimes move overland; they move little in aquatic habitats. The normal annual period of activity of _spinifer_ in lat.i.tudes 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 s.e.xually mature when the plastron is 9.0 to 10.0 centimeters in length (some when 8.0 long), whereas those of _muticus_ are s.e.xually 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 s.e.xually 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 s.e.xually 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.