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 coarseness seemingly
correlated with size; frequency and kind (knoblike or ridgelike) of
bony prominences on carapace variable; bony prominences confined to
species _spinifer_ and _ferox_, occurring principally 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 softshells (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, México (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).
[Illustration: 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 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,
México, (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 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 between _ferox_ and 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.
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 México 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 México), 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 principally in the area included within
the present drainage basin of the Río 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 latitudes. 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 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 by _T.
m. calvatus_ and _T. s. asper_. The _spinifer-hartwegi_ stock 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 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 Río 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 Río 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 Río 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 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 (except
_T. s. emoryi_). And the coincidence of the geographic ranges of _T.
muticus calvatus_ and _Graptemys pulchra_ in the southeast suggest a
former association 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 associated 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, assuming 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 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.
SUMMARY
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_.