North American Recent Soft-shelled Turtles (Family Trionychidae) Part 3

The six subspecies of _spinifer_ can be separated into two groups on the basis of the juvenal pattern. One group (subspecies _spinifer_, _hartwegi_ and _asper_) has a pattern of dark spots or ocelli of various sizes on the carapace, whereas the other group (subspecies _pallidus_, _guadalupensis_ and _emoryi_) has a pattern of small white dots or tubercles on the carapace. The two groups differ also in the manner in which the mottled and blotched pattern first appears on the carapace of females. Usually, contrasting lichenlike figures initially surround the dark spots or ocelli on the carapace in females of the _spinifer_ group (less evident in _pallidus_), whereas females of the _emoryi_ group usually lack a contrasting pattern early in ontogeny.

In general, the two groups differ in the degree of pigmentation. The _spinifer_ group has larger marks and more contrasting patterns on the head and limbs, and more extensive pigmentation on the ventral surface than members of the _emoryi_ group. _T. ater_ is more closely related to those subspecies of the _emoryi_ group but differs in having the ventral surface heavily speckled with black and an over-all blackish, dorsal coloration; the underlying pattern of _ater_ resembles that of _emoryi_.

Clinal Variation

Several characters are arranged in a geographical gradient or cline.

Some characters are relatively uniform and represent a terminus in the _spinifer_ group. Some characters change gradually and successively through the subspecies _pallidus_ and _guadalupensis_, and terminate in _emoryi_ and _T. ater_. Some characters of _ater_, in turn, show affinity with _T. muticus_ and _T. ferox_.

_Pattern on Snout_

The pattern (Fig. 5) on the snout usually consists of pale, dark-bordered stripes that form an acute angle in front of the eyes in _spinifer_, _hartwegi_ and _asper_, but the corresponding marks form a dark triangle the base line of which joins the anterior margins of the orbits in _emoryi_ and usually in _guadalupensis_. In _pallidus_, the geographic range of which is between _guadalupensis_ and _hartwegi_, there are different patterns that are in various degrees intermediate between those described immediately above for _hartwegi_ and _guadalupensis_.

_Pattern on Side of Head_

The change in pattern (Fig. 6) and its contrast with the ground color on the side of the head parallels the sequence of changes in pattern on the snout. The pattern on the side of head contrasts with the ground color and consists of dark markings below the eye and on the neck, an indication of a postl.a.b.i.al stripe, and a pale, dark-bordered postocular stripe that may be variously interrupted (_spinifer_ and _hartwegi_; _asper_ usually has uninterrupted postocular and postl.a.b.i.al stripes that unite on the side of the head). The pattern is contrasting but variable in _pallidus_. _T. s. emoryi_ and usually _guadalupensis_ have fewer dark markings, sometimes none, and an interrupted postocular pale stripe that produces a pale blotch just behind the eye.

[Ill.u.s.tration: FIG. 6. Pattern on side of head of some American species and subspecies of the genus _Trionyx_. Note the gradual reduction in contrast of pattern and interruption of the postocular stripe from that of _spinifer_ (b) to that of _emoryi_ (f).

a. _T. ferox_ (UMMZ 102276, 1/3) b. _T. spinifer spinifer_ (UMMZ 54401, 2/3) c. _T. spinifer asper_ (KU 50843, 2/3) d. _T. spinifer pallidus_ (KU 50830, 3/4) e. _T. spinifer guadalupensis_ (SM 659, 2/3) f. _T. spinifer emoryi_ (KU 2922, 3/4) g. _T. muticus muticus_ (KU 48228, 2/3) h. _T. muticus calvatus_ (KU 47117, 2/3) ]

[Ill.u.s.tration: FIG. 7. Pattern on the dorsal surface of the distal part of the right hind limb of some American species and subspecies of the genus _Trionyx_. Note the gradual reduction in contrast of pattern from that of _hartwegi_ (a) to that of _emoryi_ (d).

a. _T. spinifer hartwegi_ (KU 15932, 3/4) b. _T. spinifer pallidus_ (KU 40175, 2/3) c. _T. spinifer guadalupensis_ (TU 10165, 3/4) d. _T. spinifer emoryi_ (KU 3153, 5/6) e. _T. muticus muticus_ (KU 48228, 3/4) f. _T. ferox_ (UMMZ 102276, 1/2) ]

[Ill.u.s.tration: FIG. 8. Shape of tubercles on anterior edge of carapace in some American species and subspecies of the genus _Trionyx_ ( 1/2). Note the gradual reduction in size of tubercles from that of _hartwegi_ (b) to that of _muticus_ (h).

a. _T. ferox_ (UMMZ 90010) b. _T. spinifer hartwegi_ (KU 3346) c. _T. spinifer pallidus_ (TU 13213) d. _T. spinifer guadalupensis_ (TU 10160) e. _T. spinifer emoryi_ (KU 2906) f. _T. ater_ (KU 46906) g. _T. muticus muticus_ (KU 48229) h. _T. muticus muticus_ (KU 48232) ]

_Pattern on Dorsal Surface of Limbs_

A corresponding sequence of change occurs in the size of dark markings on the dorsal surface of the limbs (Fig. 7). The hind limb usually has larger markings than the forelimb. The change is gradual from larger and darker markings (contrasting pattern) in _hartwegi_, _spinifer_ and _asper_ to smaller and paler markings (non-contrasting pattern) in _emoryi_.

_Tuberculation_

There is also a cline in tuberculation (Fig. 8) that parallels geographically the sequence of changes in patterns mentioned immediately above. The size of the tubercles along the anterior edge of the carapace changes in both s.e.xes from those that are enlarged and equilateral or conical in shape in _spinifer_, _hartwegi_, _asper_ and _pallidus_ to those that are scarcely elevated in _guadalupensis_, _emoryi_ and _T. ater_. Indeed, in the three kinds mentioned last, the tubercles are absent in some specimens. There seems to be a corresponding reduction in the size and number of small, sharp-tipped tubercles that cover the carapace in adult males; the carapace of _T.

ater_ is mostly smooth and has only a few small, whitish tubercles.

[Ill.u.s.tration: FIG. 9. Anteroposterior position of plane of greatest width of carapace (CL/PCW) in some American species and subspecies of the genus _Trionyx_. Symbols as in Fig. 3.

The greatest width of carapace is midway between anterior and posterior ends in _ferox_, _spinifer_, _hartwegi_, _asper_ and _muticus_, and farther posterior in the other subspecies of _spinifer_.]

_Ratios_

The clinal tendencies in PL/HW (Fig. 3) and CL/CW (Fig. 4) that parallel those mentioned above for pattern and tuberculation have already been mentioned under the section "Ontogenetic Variation."

The ratio of CL/PCW (Fig. 9) was used in an effort to show further differences in the shape of the carapace, especially the plane on the carapace where the greatest width occurs. Figure 9 shows the greatest width to be approximately midway between the anterior and posterior ends in the subspecies _spinifer_, _hartwegi_ and _asper_, and in the species _ferox_ and _muticus_ (CL/PCW of 2.00). The greatest width of carapace is more posterior and at approximately the same plane in _pallidus_ and _guadalupensis_, and farther posterior in _emoryi_.

Calculated ratios for 12 specimens of _T. ater_ average 2.15, a value that suggests closer affinity with _pallidus_, _guadalupensis_ and _emoryi_ than to the other species and subspecies.

Comparison of the relative lengths of snout (HW/SL, Fig. 10) in different populations of _T. spinifer_ shows a character gradient. To facilitate a comparison utilizing large samples, the subspecies _spinifer_ was combined with _hartwegi_, and _pallidus_ with _guadalupensis_. The snout is longer in the subspecies _spinifer_ and _hartwegi_ than in _emoryi_; the length of the snout of _emoryi_ resembles that of _T. ferox_. The snout is proportionately the longest in _T. muticus_. The average ratio of HW/SL for 12 individuals of _T.

ater_ is 1.37, and is nearer that of _pallidus_, _guadalupensis_, _emoryi_ and _ferox_ than that of _muticus_ or the other subspecies of _T. spinifer_.

[Ill.u.s.tration: FIG. 10. Ratio of width of head to length of snout (HW/SL) in some American species and subspecies of the genus _Trionyx_. Symbols as in Fig. 3. Values for _spinifer_ are combined with those of _hartwegi_, and those of _pallidus_ with _guadalupensis_. The snout is proportionately the longest in _muticus_.]

Size of the ocelli increases from west to east in populations of _T.

spinifer_ in the upper Mississippi River and Great Lakes drainages.

The ratio of OD/PL (Fig. 11) varies considerably but gradually increases from Kansas northeastward to Michigan. The minimal diameter of any ocellus recorded was one millimeter; solid dots on the carapace (_hartwegi_) were also recorded as one millimeter. Larger ratios are usually derived from measurements of larger individuals. Seemingly, there should be a clinal tendency in ontogenetic variation paralleling the size of ocelli and dependent on it; ontogenetic variation should be least in western populations in which the size of ocelli does not change appreciably with increasing size, and should be greatest in eastern populations in which the ocelli on adult males are larger than those on the carapace of young turtles. It is difficult to demonstrate ontogenetic variation because specimens of corresponding size from the same general area may have ocelli of different sizes.

The gradient in size of ocelli is also indicated by specimens from other states. I have the subjective impression that there is least variation in specimens from Michigan (Great Lakes-St. Lawrence River drainage), but this is not clearly shown by Figure 11.

[Ill.u.s.tration: FIG. 11. Ratio of diameter of ocellus to length of plastron (OD/PL) in _T. spinifer_ from some states in the upper Mississippi River and Great Lakes drainages. Symbols as in Fig. 3. The size of the ocelli on the carapace gradually increases from Kansas northeastward to Michigan.]

Character a.n.a.lysis

_Snout_

The snout (Fig. 12) is tubate having terminal nostrils separated by a vertical septum. One of the princ.i.p.al characters distinguis.h.i.+ng _T.

ferox_ and _T. spinifer_ from _T. muticus_ is a lateral, whitish ridge projecting from each side of the nasal septum (hereafter referred to as septal ridges but often referred to in the literature as a papilla). The shape of the end of the snout is truncate in _T. ferox_ and _T. spinifer_, and the nostrils are larger than in _T. muticus_.

In _muticus_ the snout usually terminates somewhat obliquely, and the nostrils tend to be slightly inferior; also, the end of the snout is usually rounded and somewhat pointed, causing the nostrils to be visible in lateral view. Some _T. muticus_ do not differ markedly from _ferox_ or _spinifer_ in shape of the end of the snout. Stejneger (1944:14) mentioned indication of a septal ridge that did not reach the opening of the nostril in _muticus_. I have slit the outer edge of the nostril on several specimens of _muticus_, and have not noticed an indication of a septal ridge.

[Ill.u.s.tration: FIG. 12. Shape of snout in _T. spinifer_ (left, a-d, from KU 46907) and _T. muticus_ (right, e-h, from KU 48236).

Lateral views--a, e ( 1); anterior views--b, f ( 5); dorsal views--c, g ( 2.5); ventral views--d, h ( 2.5).]

_Tuberculation_

Tubercles or obtuse prominences occur on the anterior edge of the carapace (Fig. 8) or on the dorsal surface of the carapace. _Trionyx muticus_ lacks tubercles, although some individuals show shallow, widely s.p.a.ced wrinkles that suggest prominences on the anterior edge of the carapace. Both s.e.xes of _T. ferox_ have prominences, resembling flattened hemispheres, on the anterior edge of the carapace and in the nuchal region. Large females of _ferox_ have obtuse prominences in the center of the carapace posteriorly, some of which are often arranged in longitudinal rows. The surface of the carapace in both s.e.xes of _T.

ferox_ has small closely-set, blunt tubercles arranged in rows that resemble longitudinal ridges (most evident in juveniles).

Large females of _T. spinifer_ have obtuse prominences in the center of the carapace posteriorly, some of which in many specimens are arranged in longitudinal rows; I cannot discern any correlation of number or arrangement of prominences with size in _spinifer_ or _ferox_. The carapace in adult males of _spinifer_ bears small, sharp tubercles that make the surface feel like sandpaper. The tubercles on the anterior edge of the carapace in adults of both s.e.xes vary from round to equilateral and conical to low and flattened (see comments on tuberculation under subsection ent.i.tled "Geographic Variation"). Some large females of the same subspecies have tubercles on the anterior edge of the carapace that may be conical (higher than wide) or equilateral. The difference in shape of the tubercles seems not to be correlated with size because one _T. s. pallidus_, 30.5 centimeters (TU 13212) has prominent but blunted and equilateral tubercles, whereas, another female of _pallidus_, 20.8 centimeters (TU 13210), from the same locality has higher, conical tubercles. The blunted, equilateral tubercles may be the result of environmental wear, or the difference in shape of tubercles may be due to individual variation.

_Pattern on Carapace_

Two features of the pattern on the carapace are of taxonomic worth: 1) the width and distinctness of the pale rim at the periphery of the carapace (marginal rim), if present, and 2) the kind of pattern on the carapace (juvenal pattern). The marginal rim is absent in females of _T. ater_, and only faintly evident in males. The marginal rim is obscured or absent (adult males and females) and is not separated from the ground color of the carapace by a dark marginal line in hatchlings of _T. ferox_. The carapace of _T. muticus_ has a marginal rim that is usually separated from the ground color of the carapace by an ill-defined, dark marginal line; some individuals lack the marginal dark line. The subspecies of _T. spinifer_ have a well-defined, dark, marginal line that separates the marginal rim from the ground color of the carapace; _T. s. asper_ has more than one dark marginal line on the carapace. The marginal rim is ill-defined and blotched, or absent, in large females of all species of _Trionyx_.

The marginal rim is widest at the posterior end of the carapace and lacking in the nuchal area. The width of the pale marginal rim is very narrow, almost to the degree of being absent, in juveniles of _T.

ferox_. _T. s. emoryi_ has a pale, marginal rim that is four or five times wider posteriorly than it is laterally, whereas posteriorly the width of the rim in the other subspecies of _T. spinifer_ and in the species _T. muticus_ is only two or three times wider posteriorly than it is laterally.

The juvenal pattern commonly consists of whitish tubercles or dots (_T. s. emoryi_, _T. s. guadalupensis_, _T. s. pallidus_, _T. ater_), large black ocelli (_T. s. spinifer_), small black dots and ocelli (_T. s. hartwegi_, _T. s. asper_), large dusky spots or ocelli (_T. m.

calvatus_), or small dusky dots or short streaks and dashes (_T. m.

muticus_). Some hatchlings of _pallidus_ and _emoryi_ have a uniform pale brown or tan carapace; hatchlings of _T. ferox_ have a distinctive pattern (Pl. 31). Further comments and ill.u.s.trations pertaining to kind of pattern on the carapace are offered under the accounts of species and subspecies.

_Pattern on Dorsal Surface of Snout (Fig. 5)_