North American Recent Soft-shelled Turtles (Family Trionychidae) Part 5

_Opisthotic Wing_

This term refers to the laterally directed, posterior part of the opisthotic that is visible in occipital, lateral and ventral views. In ventral view the opisthotic wing is most easily seen and is wider in _muticus_ than in _spinifer_ or _ferox_. In _muticus_ the distal part is truncate, whereas in _ferox_ and _spinifer_, it is more tapered and gently rounded, although somewhat unevenly flared medially. Also there is more of a downward curvature (in ventral view) of the opisthotic wing in _muticus_ than in _ferox_ or _spinifer_; consequently the tip of the wing in _muticus_ is often just visible in dorsal view (on lateral side of squamosal), certainly in lateral view.

The distal part or tip of the opisthotic wing is not visible in dorsal view on skulls of _ferox_ or _spinifer_.

_Articular Surface of Quadrate_

The ventral surface of the quadrate that articulates with the mandible is composed of a lateral condyle and a medial articular surface. The condyle and medial articular surface are separated by a furrow. On skulls of _ferox_ and _spinifer_ the lateral condyle, which is not conspicuously tapered posteriorly, is slightly larger than the medial articular surface, and the furrow is shallow. On skulls of _muticus_, the lateral condyle is conspicuously tapered posteriorly, is slightly smaller than the medial articular surface, and the furrow is deep.

_Contact of Maxillaries Above Premaxillaries_

The contact of the maxillaries above the premaxillaries is of diagnostic value in distinguishing skulls of _ferox_ and _spinifer_ from those of _muticus_. I have seen no skulls of _muticus_ on which the maxillaries were in contact, and no skulls of _ferox_ on which the maxillaries were separated. Stejneger (1944:19), however, reported a skull of _muticus_ (USNM 102677) having the maxillaries in contact.

Maxillaries are in contact (sometimes just barely) in 65 of 74 skulls of _spinifer_ (88%); the premaxillaries are separated on nine skulls (12%).

_Carapace_

The dorsal surface of the bony carapace of American trionychids consists of a nuchal, seven or eight pairs of pleurals, and seven or eight, rarely nine, neurals (Fig. 16). The lateral parts of the nuchal overlie the second pair of ribs. The distal parts of the second through the ninth pair of ribs extend laterally beyond the lateral edges of the pleurals. There are no marginal ossifications. The posterior part of the bony carapace bears blunt, rounded or ovoid to linear, prominences mostly on the last pair of pleurals princ.i.p.ally on large females of _spinifer_ and _ferox_; I have seen only one adult male (stuffed, MCZ 46633) having a semblance of welts on the bony carapace. The nuchal, pleurals and neurals are sculptured.

As growth proceeds, the single, transversely-oriented, fontanelle of young turtles that separates the nuchal from the first neural and first pair of pleurals divides into two fontanelles that generally decrease in size and finally disappear. Occasionally only one (unilateral) large fontanelle is present (USNM 54734, _muticus_). The largest specimens noted that retain fontanelles are a _ferox_ (USNM 029474) having a plastron 24 centimeters long, and a _spinifer_ (USNM 54731) having a plastron 20 centimeters long. The fontanelles probably are present in some larger individuals.

[Ill.u.s.tration: FIG. 16. Carapace of _Trionyx spinifer_ (a), and sketches of posterior parts of carapaces (b-i) of three American species, showing number and variation in arrangement of neurals and pleurals (not to scale; seventh neural, n7, and pleural, p7).

a. KU 2226, Lewisville, Lafayette County, Arkansas ( 1/3); sculpturing incompletely shown. Labels: r, ribs; nu, nuchal; n, neurals 1-7; p, pleurals 1-7.

b. _ferox_, USNM 60496, Auburndale, Polk County, Florida.

c. _muticus_, KU 1964, Doniphan Lake, Doniphan County, Kansas.

d. _spinifer_, USNM 100380, Plaquemine, Iberville Parish, Louisiana.

e. _muticus_, TCWC 7260, Red River, 8 mi. NW Ringgold, Montague County, in Clay County, Texas.

f. _spinifer_, USNM 59266, Homer, Winona, Minnesota.

g. _muticus_, KU 2840, White River, DeValls Bluff, Prairie County, Arkansas.

h. _muticus_, USNM 115939, Mississippi.

i. _muticus_, USNM 54734, Mississippi River, Fairport, Muscatine County, Iowa.

Most variation concerns the number of neurals and pairs of pleurals, and their arrangement posteriorly (H. M. Smith, 1947:121, table; Stejneger, 1944:18). Table 5 shows the frequency of occurrence of the number of neurals, pairs of pleurals, and the separation or contact of the seventh pair of pleurals; figure 16 ill.u.s.trates some of the configurations of these plates posteriorly (e, g, and i not included in Table 5). The eighth pair of pleurals is reduced or absent (Loveridge and Williams, 1957:417). Eight neurals and eight pairs of pleurals occur in all three species. The seventh pleurals may contact each other in all three species, and their separation has been observed only in the species _spinifer_ and _muticus_. Seven neurals and contact of the seventh pair of pleurals, or eight neurals and separation of the seventh pair of pleurals from each other occurs with approximately equal frequency in the species _muticus_. _T. ferox_ and _spinifer_ most often have seven neurals, seven pairs of pleurals, and the seventh pair of pleurals in contact. Stejneger (_loc. cit._) mentioned a specimen in MCZ having nine neurals; I recorded nine neurals for USNM 54734 (Fig. 16i) for which Stejneger (_loc. cit._) recorded eight. AMNH 57384 (_ferox_) has a small eighth pleural on the left side only, and USNM 115939 (_muticus_) has an eighth pleural only on the right side (Fig. 16h). Anomalous conditions observed included: an accessory bone between the first and second pleurals on the right side that contacts the first and second neurals in USNM 54733, (_muticus_); only six neurals in USNM 95193 (_spinifer_); a small accessory bony element between the first and second neurals in AMNH 57383 (_ferox_); and, only six pleurals (second and third fused) on the right side in USNM 54734 (_muticus_).

TABLE 5. Frequency of Occurrence of Number of Neurals, Pairs of Pleurals, and Separation or Contact of the Seventh Pair of Pleurals Among Species of American Soft-sh.e.l.l Turtles

===================+=================+================================== Number | Contact (+) or | Species --------+----------+ separation (-) +---------+------------+----------- | Pairs of | of seventh pair | _ferox_ | _spinifer_ | _muticus_ Neurals | pleurals | of pleurals | (16) | (60) | (34) --------+----------+-----------------+---------+------------+----------- 7 | 7 | + | 9 (56%) | 50 (83%) | 13 (38%) 7 | 8 | + | 5 (31%) | 2 (3%) | 2 (6%) 8 | 7 | + | 2 (13%) | 3 (5%) | 3 (9%) 8 | 8 | + | | 4 (7%) | 2 (6%) 8 | 7 | - | | 1 (2%) | 14 (41%) --------+----------+-----------------+---------+------------+-----------

Ventrally, the bony carapace shows ten thoracic vertebrae, the second through the ninth having well-developed, depressed ribs that are fused (no sutures) to the pleurals. The ribs of the first thoracic vertebra are represented by bony struts that extend posterolaterally and contact the anterior borders of the second pair of ribs. The two ribs of the ninth pair are free for most of their length and often are broken; they are slightly shorter than the eighth pair of ribs. The ribs of the tenth thoracic vertebra may be well-developed (KU 2219, 2666, 50856, _spinifer_, and 16528, _ferox_), but are usually broken off and represented only by transverse processes.

Kyphosis

Kyphosis (angular curvature of the vertebral column) or the hump-backed condition in American softsh.e.l.l turtles has been summarized by Nixon and Smith (1949:28). Cahn (1937:185, pl. 25e) ill.u.s.trated the condition in an individual of _T. spinifer_, and H.

M. Smith (1947:119) mentioned kyphotic softsh.e.l.ls representing the species _spinifer_ (subspecies _hartwegi_ and _emoryi_) and _muticus_.

Neill (1951:10) mentioned two kyphotic _T. s. asper_ and Nixon and Smith (_loc. cit._) recorded the report of a kyphotic _T. ferox_. I have noted the condition in four _muticus_ (subspecies _muticus_, KU 1959-60, 23230; INHS 2148) and seven _spinifer_ (CNHM 22925; subspecies _hartwegi_, USNM 55689; subspecies _spinifer_, UMMZ 52948, 95615; subspecies _emoryi_, KU 2219, 33523, TU 16240). The smallest kyphotic specimen, a hatchling, TU 16240, has a plastral length of 3.5 centimeters. Kyphosis is to be expected in all kinds of softsh.e.l.ls as are other abnormalities, such as albinism (reported for _Lissemys_ by D'Abreu, 1928, and partial albinism noted in _T. cartilagineus_ by Mohr, 1929) or congenital absence of limbs (reported by Dutta, 1931, as occurring in the genera _Trionyx_ and _Lissemys_). The cause of kyphosis is not known. Smith (_op. cit._:120) suggested an abnormally early fusion of the costals (= pleurals) with the ribs, and a subsequent differential rate of growth between them and the vertebral column as a hypothesis; Williams (1957:236) proposed that late retraction of the yolk ma.s.s, or retraction of an excessively large yolk ma.s.s may cause kyphosis. The cause of kyphosis may be of genetic origin or due to some environmental damage to the vertebral column prior to the cessation of growth. The variation in rate of growth of the vertebral column may produce humps of different shapes and sizes.

Some of the specimens noted above (UMMZ 52948, 95615) have the carapace only slightly arched and are considered partly kyphotic.

There seem to be degrees of kyphosis, a fact that should be taken into account in considering the occurrence of variation in greatest depth of sh.e.l.l.

Plastron

The plastron is united to the carapace by ligamentous tissue and is somewhat flexible anteriorly and posteriorly. Anteriorly the plastron is somewhat hingelike and may contact the anteriormost edge of the carapace. The bony elements are reduced. There is usually a median vacuity, which is relatively smaller in larger specimens and may be divided into two vacuities (a posteromedial and an anteromedial) by the medial juxtaposition of the hyo-hypoplastra, especially in _muticus_. Williams and McDowell (1952) have recommended a change in nomenclature for some of the plastral bones on the basis of reinterpretation of their h.o.m.ologies. The nine plastral bones include: an anterior pair of preplastra (= epiplastra, _auct._); an unpaired, median bone, representing fused epiplastra (= entoplastron, _auct._), hereafter referred to as the epiplastron; a pair of hyoplastra; a pair of hypoplastra; and, posteriorly, a pair of xiphiplastra (Fig. 17).

Siebenrock's (1902) synopsis of living trionychids was based entirely on plastral characters. He distinguished between _muticus_ and _spinifer_ princ.i.p.ally by the shape of the epiplastron; _T. ferox_ was not considered different from _spinifer_. The median angle formed by the boomerang-shaped epiplastron is obtuse and somewhat greater than 90 degrees in _muticus_ (Fig. 17a); the angle of the epiplastron in _spinifer_ and _ferox_ is smaller than in _muticus_ and forms an approximate right angle (Fig. 17b). Williams and McDowell (_op.

cit._:277, Pl. 1, Fig. 3) presented an ill.u.s.tration of the anterior plastral elements of an adult _T. ferox_. Siebenrock provided ill.u.s.trations of the plastrons of _muticus_ (_op. cit._:823, Fig. 5) and _spinifer_ (_op. cit._:830, Fig. 10).

[Ill.u.s.tration: FIG. 17. Plastron of _Trionyx muticus_ (a) and _T.

spinifer_ (b); sculpturing of callosities incompletely shown. ep, epiplastron; hp, hyoplastron; hyp, hypoplastron; pp, preplastron; xp, xiphiplastron. a--KU 1868, White River, Devall's Bluff, Prairie County, Arkansas ( 2/3); b--KU 1869, same locality ( 2/3).]

Much importance has been credited to the fusion (no suture) or separation (suture present) of the hypoplastra and hyoplastra. The fusion of these bones distinguishes the genera _Lissemys_, _Cyclanorbis_ and _Cycloderma_ from _Trionyx_, _Pelochelys_, and _Chitra_ (Siebenrock, _op. cit._:815, 817; Loveridge and Williams, 1957:415). This character is also one of the criteria used by Hummel (1929: 768) in his erection of the two subfamilies Cyclanorbinae (= Lissemyinae) and Trionychinae.

In my examination of specimens this character, unfortunately, was not given full attention. I have noted the fusion of the hypoplastra and hyoplastra in KU 1878 (_muticus_, right side only), KU 2219 (kyphotic _spinifer_), KU 16528 (_ferox_) and KU 60121 (_ferox_). Dr. Ernest E.

Williams informs me in a letter of November 17, 1959, that of six specimens of _ferox_ in the MCZ, the hyoplastra are fused with the hypoplastra in three (54689-90, 54686). I suspect that these bones in the three American species of the genus _Trionyx_, especially in _ferox_, fuse more often than is supposed.

In _muticus_ the constricted part of the hyoplastron and hypoplastron is wider anteroposteriorly than in _spinifer_ or _ferox_ (Fig. 17).

The three American species have on the hyoplastra, hypoplastra, and xiphiplastra well-developed callosities, which enlarge with increasing size. The medial borders of the hyoplastral and hypoplastral callosities in larger specimens are rounded and closely approximated, often touching, as do the callosities of each xiphiplastron; seemingly, the callosities are relatively larger in _muticus_ than in _spinifer_ and _ferox_. I have seen one adult male _muticus_ (KU 41380) that lacked median fontanelles or vacuities owing to the contact of the plastral elements (as viewed through overlying skin, alcoholic specimen). The bony plastron (approximately 9 cm. in maximal length) of a small _muticus_ (KU 19460) resembles the plastron of larger individuals of _muticus_ in having well-developed hyoplastral and hypoplastral callosities that are closely approximated medially. Large individuals of _muticus_ usually have small, ovoid callosities on the preplastra, and a well-developed, angular callosity on the epiplastron (Fig. 17a).

Siebenrock (_op. cit._:823) suggests that the presence of callosities on the preplastra and epiplastron of _muticus_ is subject to individual variation. I can not substantiate or dispute the supposition of Baur (1888:1122), Siebenrock (1924:193) and Stejneger (1944:12, 19) that the callosities are larger in males of _muticus_ than in the females. Some individuals of _spinifer_ have seven plastral callosities (KU 2842) as does _muticus_, but the callosities on the preplastra and epiplastron are less frequent and less well-developed in large specimens of _spinifer_ than in _muticus_. The small epiplastral callosity in _spinifer_ is located at the medial angle and does not extend posterolaterally to cover the entire surface of the epiplastron as it may in _muticus_ (Fig. 17b). The epiplastron of a _spinifer_ (KU 2826) has a medial callosity and another on the right posterolateral projection; three separate callosities occur on the epiplastron of MCZ 46615. The last specimen mentioned, a large, stuffed female, possesses a round, intercalary bone that tends to occlude the posteromedial vacuity.

Seemingly, the callosity on the epiplastron appears prior to those on the preplastra; I have not seen any plastra having callosities on the preplastra and lacking a callosity on the epiplastron. I have not noted callosities on the preplastra or epiplastron of specimens of _ferox_.

The callosities on the plastral bones are sculptured; small, recently formed callosities on the preplastra and epiplastron lack sculpturing.

The pattern of sculpturing on the plastral bones as well as that of the carapace is generally of anastamosing ridges. I am unable to discern any differences in pattern of sculpturing between the three American species. Stejneger distinguished adult specimens of _ferox_ from the other American species by the coa.r.s.eness of the sculpture of the bony callosities (1944:24) and of the bony carapace (_op. cit._:32). The sculpturing on the plastral callosities and carapace seems to be correlated with size; larger specimens (_ferox_) have coa.r.s.er sculpturing than do smaller specimens (_muticus_). Stejneger also mentioned that the sculpturing on many specimens of _ferox_ is specialized into prominent, longitudinal welts (_loc. cit._); these welts occur also on the carapace of _spinifer_.

On the basis of the osteological characters examined by me, _T. muticus_ is distinguished from _spinifer_ and _ferox_ by a number of characters (plastron and especially skull) whereas the species _spinifer_ and _ferox_ are not easily distinguished from one another.

Composition of the Genus _Trionyx_ in North America

a.n.a.lysis of the characters previously mentioned and their geographic distribution permits the recognition of ten taxa, comprising four species and eight subspecies. Two subspecies, _T. spinifer_ pallidus and _T. s. guadalupensis_ are described as new. The four species and the included subspecies here recognized are:

_Trionyx ferox_ _Trionyx spinifer spinifer_ _hartwegi_ _asper_ _emoryi_ _guadalupensis_ _pallidus_ _Trionyx ater_ _Trionyx muticus muticus_ _calvatus_

The following key is designed to permit quick identification of living individuals; therefore, ratios and osteological characters are avoided as much as possible in favor of other characters that are the least variable and most "typical." Because there is considerable variation correlated with s.e.x and size, each taxon occurs in the key in more than one couplet. Large females having mottled and blotched patterns will be the most difficult to identify. The characters listed should be used in combination because one character alone may not be sufficient; it is advisable to read both choices of each couplet. The text, figures and ill.u.s.trations should be consulted for final identification.