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UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY
Volume 13, No. 10, pp. 429-611, pls. 31-54, 24 figs.
February 16, 1962
North American Recent Soft-shelled Turtles
(Family Trionychidae)
BY
ROBERT G. WEBB
UNIVERSITY OF KANSAS
LAWRENCE
1962
UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY
Editors: E. Raymond Hall, Chairman, Henry S. Fitch, Robert W. Wilson
Volume 13, No. 10, pp. 429-611, pls. 31-54, 24 figs.
Published February 16, 1962
UNIVERSITY OF KANSAS
Lawrence, Kansas
PRINTED IN
THE STATE PRINTING PLANT
TOPEKA, KANSAS
1962
[Illustration (Union Label)]
28-7818
North American Recent Soft-shelled Turtles
(Family Trionychidae)
BY
ROBERT G. WEBB
CONTENTS
PAGE
CONTENTS 431
INTRODUCTION 433
Collecting Methods 434
Materials and Procedure 437
Acknowledgments 439
TAXONOMY 439
Family Trionychidae Bell, 1828 439
Genus _Trionyx_ Geoffroy, 1809 443
Variation 445
Secondary Sexual Variation 446
Ontogenetic Variation 449
Geographic Variation 453
Character Analysis 460
Composition of the Genus _Trionyx_ in North America 476
Artificial Key to North American Species and Subspecies
of the Genus _Trionyx_ 476
Systematic Account of Species and Subspecies 479
_Trionyx ferox_ 479
_Trionyx spinifer_ 486
_Trionyx spinifer spinifer_ 489
_Trionyx spinifer hartwegi_ 497
_Trionyx spinifer asper_ 502
_Trionyx spinifer emoryi_ 510
_Trionyx spinifer guadalupensis_ 517
_Trionyx spinifer pallidus_ 522
_Trionyx ater_ 528
_Trionyx muticus_ 531
_Trionyx muticus muticus_ 534
_Trionyx muticus calvatus_ 539
NATURAL HISTORY 541
Habitat 541
Daily and Seasonal Activity 547
Diurnal Habits 547
Behavior Adaptations 549
Movement 552
Nocturnal Habits 553
Seasonal Occurrence 553
Food Habits 555
Reproduction 558
Size of Males at Sexual Maturity 558
Size of Females at Sexual Maturity 560
Sexual Activity 563
Deposition of Eggs 565
Reproductive Potential 568
Eggs 572
Incubation and Hatching 573
Age and Growth 574
Mortality 576
Parasites 576
Economic Importance 577
EVOLUTIONARY HISTORY 578
Distribution 578
Relationships 579
Fossils 582
Phylogeny 585
The Importance of the Study of Turtle Populations in
Relation to the History of River Systems 588
SUMMARY 590
LITERATURE CITED 594
INTRODUCTION
Is it true that the greater the degree of resemblance between two
populations the shorter the time the two have been spatially isolated?
Are aquatic environments more stable than terrestrial environments?
These questions occurred to me while I was collecting turtles from river
systems of the Gulf Coast. As a general rule, each kind of turtle seemed
to occur throughout one continuous river system or large tributary, and
with no barriers to dispersal therein and with the lapse of enough time
for a population to reach its limits of dispersal, the question arose,
"Where do subspecies and zones of intergradation occur?" It seemed
logical to think that each isolated and continuous aquatic environment
would not contain more than one subspecies of the same species. In
terrestrial environments subspecies and transitions between them were
recognizable. Terrestrial habitats were continuous for longer distances
than the isolated, aquatic habitats. But, different species of turtles
prefer different kinds of aquatic habitats. Also, barriers occur in
large drainage systems, such as the Mississippi, where, in general,
the western tributaries are sluggish, turbid and shallow, and the
eastern tributaries are fast-flowing, clear and deep. But in young,
relatively small, river systems that do not traverse radically different
physiographic regions, and that show no gross ecological differences,
habitats or microhabitats that do exist probably are only partial
barriers and seem not to prevent the dispersal of most kinds of aquatic
turtles. Consequently, it seemed that study of the degree of difference
between closely related populations of turtles that occurred in one
drainage system, or in adjacent drainage systems would indicate the
length of time, respectively, that the drainage system had been
continuous or the length of time that two or more systems had been
isolated from one another.
Rivers or series of river systems having endemic kinds of turtles or
having the most kinds of turtles that are different from those in
adjacent rivers may be the oldest geologically, or may have been
isolated the longest. Knowledge of the kinds of turtles and their
relationships and distribution could indicate chronological changes in
aquatic habitats. Of course, modifying factors such as differences
between populations of turtles in rates of evolutionary change, degrees
of vagility, rates of dispersal, and overland migrations need to be
taken into account.
My accumulation of data on soft-shelled turtles was begun in the early
nineteen-fifties. Although American softshells have been discussed in a
revisionary manner by Agassiz (1857), Siebenrock (1924), Stejneger
(1944) and Neill (1951), the relationships of all the component
populations have not hitherto been appreciated. The present account
attempts to combine in one publication what is known concerning the
taxonomy, geographic distribution, life history, and relationships of
the Recent American species and subspecies of the genus _Trionyx_.
Collecting Methods
Nocturnal collecting, by hand, from a boat that was nosed among brush
piles along the shore line of rivers (Chaney and Smith, 1950:323) in the
early 1950's on rivers of the Gulf Coast drainage east of Texas yielded
many turtles of the genus _Graptemys_ but few softshells. Chaney and
Smith (_loc. cit._) reported only one softshell among 336 turtles taken
in 21 collecting hours on July 5, 6 and 7 on the Sabine River; Cagle and
Chaney (1950:385), however, recorded 11.6 per cent softshells of 208
turtles (collecting time not stated) taken on the Caddo Lake Spillway in
Louisiana. Using hoop-nets is probably the most efficient method for
collecting softshells considering the time and effort involved, and is
the chief method I have used. Lagler (1943a:24) mentioned the use of
watermelon rind as an effective bait. Kenneth Shain (field notes)
trapped _T. spinifer emoryi_ in hoop-nets baited with bread. I have used
chopped fresh fish with most success; canned sardines have also been
satisfactory. These baits seem to be more successful for trapping
_spinifer_ than they are for _muticus_. Hoop-nets were used to trap
turtles in Lake Texoma, Oklahoma, from June 14 to July 2, 1954. The
number of traps (usually four, rarely five) and trapping success varied
with location. Of 156 turtles, 19 (12%) were _T. spinifer_ and one was
_T. muticus_.
Trotlines and set lines frequently catch softshells; sport fishermen
often complain of catching these turtles on hook and line. Live worms,
soft-bodied insects, small crawfish, minnows, small pieces of fish and
other kinds of meat are adequate bait. Capture depends on the skill of
attachment of the bait and the size of hook used. In my experience,
softshells (mostly _spinifer_) were taken on trotlines that were set in
lakes or the slower-moving parts of rivers a few inches below the
surface. I have records of only two _muticus_ taken on trotlines.
Goin (1948:304) stated that commercial fishermen catch softshells on
trotlines set for catfish on the bottom of river beds. Evermann and
Clark (1920:595) found softshells to be caught more often than any
other kind of turtle in traps, on set lines, and by anglers in Lake
Maxinkuckee, Indiana. Some residents of the South tell of so placing
baits that turtles are lured to tread water against an object set with
recurved hooks upon which the webbing of the forelimbs are impaled.
Individuals of _muticus_ and _spinifer_ frequently bury themselves in
sand in shallow water and can be collected by hand by noting swirls or
disturbances on the bottom caused by a turtle withdrawing its head
(Conant, 1951:156, 159). Professional turtle collectors take them by
"noodeling" (Conant, _op. cit._:160); Lagler (1943a:22) elaborated on
the method of "noodling." P. W. Smith (1947:39) remarked that 20 or more
softshells were taken "within a few hours by probing sand bars at the
water edge" near Charleston, Illinois. From a distance I observed an
individual of _T. s. asper_ bury itself in shallow water on the Escambia
River, Florida. Small individuals of _muticus_ have been taken by hand
along the shore of Lake Texoma. Along the Flint River near Bainbridge,
Georgia, two hatchlings that were buried in sand in shallow water
emerged at my approach and scurried a few inches, then buried themselves
again. Larger turtles seem to be more wary. One that was disturbed,
emerged from the sand and swam toward deep water.
In clear water, water-goggling may be effective in securing softshells.
Marchand (_in_ Carr, 1952:417-18) mentioned that softshells (_ferox_)
can be found buried in deep water with only the heads visible; the
turtles are not easily frightened under water and may be captured by
grasping their necks. A similar technique described by Allen and Neill
(1950:3) resulted in the capture of trionychid turtles. In clear water
of the White River, Arkansas, I collected a few softshells by hand as
they lay on the bottom.
In shallow-water areas of large rivers, lakes and tributaries, seining
often procures softshells. Methods used in fisheries investigations such
as the application of rotenone and electric shockers, and even
dynamiting, sometimes yield soft-shelled turtles. Carr (1952:419) wrote
that numbers of _ferox_ were incapacitated by rotenone in Florida lakes,
although no other species of turtle was affected. I captured a snapping
turtle (_Chelydra serpentina_) that was immobilized by the current from
an electric shocker in a small, alga-choked tributary of Cache Creek,
Comanche County, Oklahoma; presumably turtles must come in close contact
with the electrodes to be affected (see discussion by Gunning and Lewis,
1957:52).
The effectiveness of gill nets in trapping turtles is indicated by
information kindly supplied by Mr. Alfred Houser on gill-net operations
from July through December, 1952, under the direction of Mr. "Bud"
Oldham, a commercial fisherman. The 4-inch mesh nets were in Lake Texoma
at the mouth of Briar Creek, two miles south of Powell, Marshall County,
Oklahoma, in 25 to 30 feet of water. Eighty to 90 per cent of the
turtles secured were softshells; more were taken near shoreline than
away from shore even though the depth was about the same. An average of
only one turtle every four days was taken in July and August when the
turtles presumably are most active (Table 1). One gill-net day is
equivalent to one gill net, 200 yards long, operated for 24 hours.
TABLE 1. The Abundance of Turtles as Revealed by Gill-net
Operations in Lake Texoma, 1952.
===========+==========+===========+===============
| Gill-net | Number of | Gill-net days
MONTH | days | turtles | per turtle
-----------+----------+-----------+---------------
July | 835 | 213 | 3.9
August | 816 | 199 | 4.6
September | 743 | 42 | 17.7
October | 1661 | 82 | 20.3
November | 1322 | 48 | 27.5
December | 864 | 5 | 172.8
-----------+----------+-----------+---------------
Dr. Virgil Dowell, while making fishery studies two miles east of
Willis, Marshall County, Oklahoma, caught, on the average, 1.5 turtles
per day. Of 75 turtles collected from July 1 through October 18, 1953,
66 were _Trionyx_ (_spinifer_ and _muticus_), five were _Graptemys_ and
four were _Pseudemys scripta_. No more than two gill nets were used
simultaneously. The nets were moved from time to time and varied in
dimensions, but those used most of the time were 200 feet long and eight
feet deep with a 3-inch mesh.
The few captures by Houser probably resulted from long-continued
trapping in one place; the gill nets were not moved in the entire
six-month period or for some time previously. Breckenridge (1955:6)
commented on the sedentary nature of _spinifer_ (in Minnesota) and
quoted a professional turtle trapper as stating that "after a section of
a river has been trapped heavily for softshells, little success can be
expected in that area for as much as three or four years thereafter."
Both Houser's and Dowell's data indicate a higher percentage of
soft-shelled turtles collected than any other species. The number
caught probably depends, at least partly, on the food habits of the
species and is influenced by the enmeshed fish, which, serving as a food
source, attract the turtles.
Materials and Procedures
In the course of this study I examined 1849 soft-shelled turtles,
including some incomplete alcoholic or dried specimens, such as those
represented only by skulls or by other osteological material. Material
was examined from each of the collections named below (except KKA), and
these are mentioned in the text by the following abbreviations:
AMNH American Museum of Natural History
ANSP Academy of Natural Sciences, Philadelphia
BCB Bryce C. Brown, private collection, Baylor University
CM Carnegie Museum
CNHM Chicago Natural History Museum
INHS Illinois Natural History Survey, University of Illinois
KKA Kraig K. Adler, private collection, data in letter dated January 8, 1960
KU Museum of Natural History, The University of Kansas
LSU Louisiana State University
MCZ Museum of Comparative Zoology, Harvard College
MSU The Museum, Michigan State University
NHB Naturhistorisches Museum Basel, Switzerland
OU University of Oklahoma Museum, Division of Zoology
SM Strecker Museum, Baylor University
TCWC Texas Cooperative Wildlife Collection, Texas Agricultural and Mechanical College
TNHC Texas Natural History Collection, The University of Texas
TTC Texas Technological College
TU Tulane University
UA University of Alabama
UI Museum of Natural History, The University of Illinois
UMMZ Museum of Zoology, The University of Michigan
USNM United States National Museum
WEB William E. Brode, private collection, Mississippi Southern College
WTN Wilfred T. Neill, private collection
External measurements (listed under the section, "Variation") were
taken by the writer by means of a Vernier caliper or a steel tape.
Measurements of the skulls are in millimeters and tenths as taken by
the writer with dial calipers. Partial wrinkling of the carapace at
the edges of some specimens causes some error in measurements;
consequently, length of plastron is used as the measurement of
reference.
Scattergrams based on external measurements were constructed. Some
demonstrate considerable ontogenetic variation. An inspection of the
scattergrams indicated regressions essentially linear in nature, but
sometimes occasioned an arbitrary separation of samples into size
groups to show ontogenetic variation; no secondary sexual differences
could be discerned. Several ratios were developed from the
measurements. The data correspond to the regression model 1A in
"Statistical Methods" (Snedecor, 1956, sec. 6.13); consequently, the
sample ratios indicate the slope of regression and are useful in
comparisons. Sample-means and their estimated standard errors are
compared graphically to show general trends in proportional
characters. Comparisons of means and standard errors indicate
statistical significance between populations if the sample-means plus
or minus twice their standard errors do not overlap, but this method
of comparison is valid only when comparing two samples (Pimentel,
1959:100).
In the section on "Variation," general features applicable to all
kinds of soft-shelled turtles are discussed under the following
headings: secondary sexual, ontogenetic, and geographic; individual
variation is mentioned in accounts of species and subspecies. In the
section "Character Analysis" external and osteological characters
having taxonomic significance are discussed.
Vernacular names follow, as closely as possible, those recommended by
the Committee on Herpetological Common Names (1956). The synonymy of
each monotypic species or subspecies begins with the name as given in
the original description. The second entry is the name-combination
herein applied to the taxon. Other entries are first usages, in
chronological order, of other names (synonyms) that have been applied
to the taxon in question. Next, the type is briefly discussed followed
by the "Range" defined in general geographic terms, and, when
appropriate, in terms of river drainage systems. "Diagnosis" includes
a combination of characters that facilitates quick identification. In
polytypic species, the diagnosis of a subspecies is designed only to
distinguish it from other subspecies of that species. The comments
included under the subsection entitled "Description" pertain to
individuals from an area where the taxon is most clearly
differentiated. Because osteological characters are significant only
at the specific level, they appear under the accounts of each species
(excluding _ater_). Proportional characters as given in the
"Diagnosis" are only in general terms; more specific data are set
forth in the subsection, "Description" or in the various text figures,
mostly in the section on "Variation," page 445. Proportions pertaining
to the species _muticus_ were derived only from the nominal
subspecies, and appear under the account of the species. A subsection
"Variation" under the accounts of some subspecies includes information
concerning principally individual variation and coloration; because
color is not considered to be of major taxonomic importance, color
terms are used without reference to any standard color guide. The
subsection "Remarks" follows the section on "Comparisons," and may
include comments on nomenclature, intergradation and other information
related to the distribution or taxonomy of the subspecies.
The probable geographic range of each species and subspecies is shown
on one of the maps. Locality records of specimens that I have examined
are shown by solid circles. Additional records of occurrence
(published records or specimens otherwise not seen) are shown by
hollow circles. Localities only a short distance apart share the same
circle.
Under the subsection "Specimens examined," a number in parentheses
following a museum number indicates the number of specimens referable
to that museum number. All localities of specimens examined are
indicated on one of the maps. The list of specimens is arranged
alphabetically by states (Canadian provinces precede states of the
United States under the account of _T. spinifer spinifer_, and Mexican
states follow those of the United States under _T. s. emoryi_),
alphabetically by counties, and within a county alphabetically by
abbreviations of museums; then, museum catalogue numbers are arranged
consecutively. Records in the literature are not included if they
refer to the same locality from which at least one specimen has been
examined, or refer to a less restricted locality that includes the
area from which at least one specimen has been examined. Localities
within a county are arranged alphabetically by author; the appropriate
reference may follow several localities.
All generic, specific and subspecific names (but not all the different
kinds of name-combinations) that have been applied to American
soft-shelled turtles are listed in a subsection entitled "Synonymy"
under the heading "Genus Trionyx Geoffroy, 1809."
Acknowledgments
Completion of this study has been made possible only by the
co-operation of those persons in charge of the collections listed
above and I am grateful to them for the privilege of examining
specimens. Also I wish to thank Dr. E. Raymond Hall for the facilities
afforded by the Museum of Natural History at the University of Kansas,
as well as for editorial assistance in the preparation of the
manuscript, and especially Dr. Henry S. Fitch under whose guidance
this research was carried out.
In addition to various staff members, graduate students, and
individuals whose help is acknowledged at appropriate places in the
text, Dr. Rollin H. Baker, Dr. Fred R. Cagle, Mr. J. Keever Greer, Dr.
A. Byron Leonard, Dr. Carl D. Riggs, and Dr. Edward H. Taylor deserve
especial mention for aid extended in the course of this study. I am
indebted to Mr. J. C. Battersby, British Museum (Natural History),
London, for information concerning the type of _Trionyx ferox_, to Dr.
Jean Guibé, Museum d'Histoire Naturelle, Paris, for information
concerning the types of _Trionyx muticus_, _T. spinifer_ and _T.
carinatus_, and photographs of the types of _T. muticus_, _T.
spinifer_ and _T. ocellatus_, and to Dr. Lothar Forcart of the
Naturhistorisches Museum, Basel, Switzerland, for information
pertaining to a published record of _T. muticus_.
The maps and figures are the work of Miss Lucy Jean Remple and Mrs.
Lorna Cordonnier, University of Kansas. Dr. John M. Legler, University
of Utah, prepared most of the photographs on plates 1-20; photographs
as mentioned in the preceding paragraph were received from Dr. Guibé,
one was provided through the co-operation of Roger Conant and Isabelle
Hunt Conant, another was furnished by Mr. J. Keever Greer, and the
others were taken by me. Field work was financed in part by funds
provided by the Sigma Xi-RESA Research Fund.
TAXONOMY
Family Trionychidae Bell, 1828
Recent soft-shelled turtles comprise a well-defined assemblage of the
family Trionychidae. Although the scope of this study does not involve
an assay of the relationships of the soft-shelled turtles of the Old
World, a brief résumé that includes some of the salient characteristics
of the family is included.
_Diagnosis._--Articulation between last cervical and first dorsal
vertebrae by zygopophyses only; preplastra separated from hyoplastra
by /\-shaped epiplastron, entoplastron absent (Williams and McDowell,
1952:263-75); marginal bones absent or forming an incomplete series,
not connected with ribs that extend beyond pleural plates; claws on
only three inner digits; fourth digit having four or more phalanges;
plastron united to carapace by ligamentous tissue (Smith, 1931:147).
_General characters._--Size large, "... some attaining probably 5 feet
in length of carapace" (Boulenger, 1890:10); body depressed; carapace
and plastron lacking horny epidermal shields, covered instead with
soft skin; snout ending in fleshy, tubate proboscis; jaws concealed by
fleshy lips; tail short; digits well-webbed; cervical vertebrae
opisthocoelous (eighth having double articulation in front); neck
elongate, cervical region equaling or exceeding length of dorsal
vertebral column; head and neck completely retractile, bending by
means of sigmoid curve in vertical plane; ear hidden; skull elongate,
having three posterior projections (median one produced by
supraoccipital and two lateral projections formed chiefly by
squamosals); temporal region emarginate posteriorly, forming wide
shallow fossa; premaxillae fused; an intermaxillary foramen;
pterygoids separated by basisphenoid that contacts palatines; vomer,
if present, not separating palatines; pelvis not fused to carapace and
plastron; plastron reduced, a median vacuity usually present; plastral
bones developing sculpturing with increase in size, forming four to
seven so-called plastral callosities; carapace with or without
prenuchal bone; nuchal overlapping or overlapped by first pleural;
neurals in continuous series or interrupted by pleurals; bony plates
of carapace sculptured; mandible having well-developed coronoid bone;
cutaneous femoral valves that conceal hind limbs present or absent;
two or three pairs of scent glands; cloacal bursae absent (Smith and
James, 1958:89); forelimbs having antebrachial scalation; body of
hyoid apparatus formed of two or three pairs of bones; penis broad,
expanded and pentifid, sulcus spermaticus quadrifid having branches in
each of four lateral projections (Hoffman, 1890:298, pl. 47, fig. 2);
aquatic, principally in fresh water; mainly carnivorous; flesh of many
species eaten. (See Boulenger, 1889:237-41; Loveridge and Williams,
1957:412; Romer, 1956:513; Smith, _op. cit._:147-54).
_Recent distribution_ (Figure 1).--North America, from extreme
southeastern Canada and eastern United States west to Rocky Mountains
and south to northern México; introduced in southwestern United States
(Conant, 1958:69-73). Africa, from Egypt and Senegal south to Angola
and Zambesi River drainage (Loveridge and Williams, _op.
cit._:412-68); occurrence of _Trionyx triunguis_ in Syria (Boulenger,
_op. cit._:255) and coastal streams of Palestine (Schmidt and Inger,
1957:36) considered accidental by Flower (1933:753-54). Southwestern
Asia (Tigris and Euphrates River drainage) in eastern Turkey, Syria,
Iraq and northeastern Israel (Mertens and Wermuth, 1955:388).
Southeastern Asia, from Pakistan and India (Indus River drainage) and
Manchuria and adjacent Siberia (Amur River drainage) to Ceylon, Japan,
Formosa, Hainan, Luzon, Sumatra, Java, Borneo, Timor and southeastern
New Guinea (De Rooij, 1915:325-32; Okada, 1938:108; Pope, 1935:60-64;
Smith, 1931:158-79; Stejneger, 1907:514-532; Taylor, 1920:141).
_Trionyx cartilagineus_ is questionably recorded from the Moluccas (De
Rooij, _op. cit._:330). _T. sinensis_ has been introduced on Kauai
Island, Hawaiian Islands (Brock, 1947:142; Oliver and Shaw, 1953:83),
one of the Bonin Islands (Okada, 1930:187-94), and probably Timor (De
Rooij, _op. cit._:331). All insular records east of Borneo and Java
are probably the result of introductions, except perhaps those of
_Pelochelys_ on Luzon and New Guinea (Darlington, 1957:210).
[Illustration: FIG. 1. Geographic distribution of the family
Trionychidae.]
_Recent genera._--According to Mertens and Wermuth (1955:387-95), there
are 21 species belonging to six genera as follows:
_Chitra_ Gray, 1844 (1)
_Cyclanorbis_ Gray, 1854 (2)
_Cycloderma_ Peters, 1854 (2)
_Lissemys_ Smith, 1931 (1)
_Pelochelys_ Gray, 1864 (1)
_Trionyx_ Geoffroy, 1809 (14)
_Dogania_ is considered a synonym of _Trionyx_ (Loveridge and Williams,
_op. cit._:422).
_Geologic range._--Lower Cretaceous (possibly Upper Jurassic) to Recent
of Asia; Upper Cretaceous to Recent of North America; Paleocene (Upper
Jurassic, assuming _Trionyx primoevus_ is a trionychid) to Pleistocene
of Europe; Lower Miocene to Recent of Africa; Pleistocene to Recent in
East Indies (Loveridge and Williams, _op. cit._:412; Romer, 1945:594);
questionable trionychid fragments from Pleistocene of Australia
(Darlington, _loc. cit._). $/
_Remarks._--The genera _Lissemys_, _Cyclanorbis_ and _Cycloderma_ are
distinguished from _Pelochelys_, _Chitra_ and _Trionyx_ by several
characters (Loveridge and Williams, _op. cit._:414). The recognition of
two groups of genera caused Deraniyagala (1939:290) to erect two
families, Cyclanorbidae and Trionychidae. An appraisal of fossils
prompted Hummel (1929:768) to propose two corresponding subfamilies,
Cyclanorbinae and Trionychinae. Williams (1950:554) considered the two
groups as subfamilies (Lissemydinae and Trionychinae).
Baur (1887:97) regarded the Trionychidae as constituting a separate
suborder distinct from the rest of the living turtles. Later (1891),
however, he pointed out the resemblances of the Trionychidae and
Carettochelyidae (having one living genus in New Guinea), and the
cryptodiran affinities of _Carettochelys_. Bergounioux (1932:1408)
mentioned the close resemblance of the Carettochelyidae to _Trionyx_ but
considered the former as having pleurodiran affinities, a view adopted
by Deraniyagala (_loc. cit._). Most students now consider the two
families to be closely related, and conceive of both as members of the
suborder Cryptodira (Hummel, 1929:768; Williams, _loc. cit._; Mertens
and Wermuth, 1955).
The oldest trionychid fossil, _Trionyx primoevus_, is from marine
deposits of the Upper Jurassic (Kiméridgien) from "Cap de la Hève," and
its characters do not indicate the kind of cryptodiran ancestor from
which the family arose (Bergounioux, _op. cit._:1409; 1937:188). Lane
(1910:350) found that the entoplastron (= epiplastron) was paired in
embryos of _Trionyx_ and regarded that genus as the most primitive of
the order; he also mentioned Wiedersheim's report of rudiments of teeth
in embryos of _Trionyx_. Baur (1891:637-38) thought that the family
arose directly from the Amphichelydia, that the ancestors of the
Trionychidae closely resembled _Carettochelys_ in the structure of the
carapace and plastron, and that a progressive reduction in ossification
of those structures occurred. Nopcsa (1926:654) also wrote that the
family originated from ancestors having a well-developed plastron; he
maintained that the progressive reduction in ossification of the
plastron was a specialization for aquatic life, and that the more
primitive trionychids had the best developed bones and callosities.
Hummel (1929:772) also thought that there had been a progressive
reduction in ossification. Bergounioux (1932:1408; 1936:1088,
1952:2304), on the contrary, thought that there had been a progressive
increase in ossification of the marginal bones in both families as well
as of the plastron (1936:1088; 1937:190). Zangerl's study of the shell
elements of turtles (1939:393) indicated that _Trionyx_ was highly
specialized in having a well-developed epithecal armor (sculptured
callosities, neurals and costals), and that it occurred in most aquatic
turtles; the development in soft-shells suggested that members of the
family had maintained an aquatic mode of life over a long period of
geologic time, a view supported by Deraniyagala (1930:1066). Of interest
are Stunkard's remarks (1930:214-18) concerning several _Trionyx
spinifer_ that were obtained from a commercial supply house and found to
be infested with pronocephalid trematodes (_Opisthoporus_ [=
_Teloporia_] _aspidonectes_). The closest relatives of that trematode
(also recorded from _T. ferox_) live in marine turtles. Possibly, a
Mesozoic ancestor of marine and essentially fresh-water soft-shelled
turtles harboured ancestors of these trematodes, but possibly the
parasites may have transferred relatively recently to their present
hosts. Bergounioux (1937:190) judged the Trionychidae to be an ancient
group of marine origin. Hummel (1929:770) wrote that the Trionychidae
originated in east Asia (the region of most differentiation) in humid
climates.
Baur (1891:634, 637) pointed out that the dorsal aspect of the skull of
the closely related _Carettochelys_ resembles the skull of the
Dermatemydidae, Staurotypidae and Kinosternidae; the close relationship
of _Carettochelys_ and the Dermatemydidae is also mentioned by
Bergounioux (1952:2304) and Hummel (1929:769). Hummel (_op. cit._:771)
thought that the Carettochelyidae and "die Chelydroiden" had a common
ancestor, and that (_op. cit._:772) the origin of the Trionychidae was
older than those two groups. Dunn (1931:109) wrote that the
Kinosternidae, Carettochelyidae and Dermatemydidae represented the same
general ancestry. Williams (1950:552) has shown the resemblance of the
cervical articulations in members of the Chelydridae (including
Staurotypinae and Kinosterninae) and the Central American family
Dermatemydidae. The consensus of opinion, then, is that the families
Trionychidae, Carettochelyidae, Chelydridae and Dermatemydidae are
relatively closely related.
Genus =Trionyx= Geoffroy, 1809
_Testudo_ Linnaeus (in part), Syst. Nat., Ed. 10, 1:197, 1758; type,
_Testudo graeca_ Linnaeus by subsequent designation (Fitzinger,
1843:29).
_Trionyx_ Geoffroy, Ann. Mus. Hist. Nat. Paris, 14:1, August, 1809;
type, _Trionyx aegyptiacus_ (= _Testudo triunguis_ Forskål) by
original designation.
_Apalone_ Rafinesque, Atlan. Jour., Friend of Knowledge,
Philadelphia, 1 (No. 2, Art. 12):64, Summer, 1832; type, _Apalone
hudsonica_ (= _Trionyx spiniferus_ Lesueur) by monotypy.
_Mesodeca_ Rafinesque, Atlan. Jour., Friend of Knowledge,
Philadelphia, 1 (No. 2, Art. 12):64, Summer, 1832; type _Mesodeca
bartrami_ (= _Testudo ferox_ Schneider) by monotypy.
_Aspidonectes_ Wagler, Naturl. Syst. Amphib., p. 134, 1830; type,
_Aspidonectes aegyptiacus_ Wagler (= _Testudo triunguis_ Forskål)
by subsequent designation (Fitzinger, 1843:30).
_Amyda_ Fitzinger, Ann. Wiener Mus. Naturg., 1:110, 120, 127, 1835;
type, _Amyda subplana_ Fitzinger by subsequent designation
(Fitzinger 1843:30).
_Gymnopus_ Duméril and Bibron, Erpét. Gén., 2:472, 1835; new
(substitute) name for _Aspidonectes_ Wagler.
_Pelodiscus_ Fitzinger, Ann. Wiener Mus. Naturg., 1:110, 120, 127,
1835; type, _Pelodiscus sinensis_ Fitzinger by subsequent
designation (Fitzinger, 1843:30).
_Platypeltis_ Fitzinger, Ann. Wiener Mus. Naturg., 1:109, 120, 127,
1835; type, _Platypeltis ferox_ by subsequent designation
(Fitzinger, 1843:30).
_Potamochelys_ Fitzinger, Syst. Rept., p. 30, 1843; type,
_Aspidonectes javanicus_ Wagler (= _Testudo cartilaginea_
Boddaert) by original designation.
_Tyrse_ Gray, Cat. Tort. Croc. Amphis. Brit. Mus., p. 48, 1844;
type, _Tyrse nilotica_ Gray (= _Testudo triunguis_ Forskål) by
tautonomy (_Tyrse_, a name for the Nile River).
_Callinia_ Gray, Proc. Zool. Soc. London, p. 222, 1869; new
(substitute) name for _Aspidonectes_ of Agassiz (1857:403); type,
_Callinia spicifera_ (mispelling for _spinifera_) Gray by
subsequent designation (Stejneger, 1907:514).
_Euamyda_ Stejneger, Bull. Mus. Comp. Zool., 94:7, 9, 12, 1944; new
(substitute) name for _Amyda mutica_ of Agassiz (1857:399); type,
_Amyda mutica_ Agassiz by monotypy.
_Type Species._--_Trionyx aegyptiacus_ (= _Testudo triunguis_
Forskål).
_Diagnosis._--Cutaneous femoral valves absent; width of postorbital
arch of skull less than diameter of orbit; pterygoids usually not
contacting opisthotics; carapace lacking prenuchal bone and marginal
ossifications; nuchal bone lacking conspicuous ventral ridges;
posterior margin of nuchal overlying first pair of pleurals; lateral
parts of nuchal bone overlying second pair of ribs; neurals seven or
eight, rarely six or nine; pleurals seven or eight pairs, posterior
one or two pairs sometimes in contact medially; distinct suture
usually present between hyoplastra and hypoplastra; most laterad prong
of posteromedial process of hypoplastra inserted between bifid
anterolateral process of xiphiplastra.
_Synonomy._--Geoffroy published a synopsis of the species he
recognized (1809) prior to his formal description of the genus
_Trionyx_ (1809a). Schweigger, nevertheless, probably was the first
person to recognize the soft-shelled turtles as a distinct group, and
he proposed for it the name _Amyda_ in an unpublished manuscript that
he sent to Geoffroy. The latter author (1809a:15) relegated the name
_Amyda_ to the synonomy of _Trionyx javanicus_ by means of the
following entry: "_Amyda javanica._ Schweigger, dans un manuscript
communique a l'Institut." Stejneger (1944:7) maintained that this
publication of Schweigger's monotypic generic name clearly established
its availability for the species congeneric with _Amyda javanica_ (=
_Testudo cartilaginea_ Boddaert, 1770). Loveridge and Williams
(1957:422) contend that this mere mention of the name _Amyda_ neither
constitutes the proposal of a new name nor validates it, and that the
first valid usage of the name _Amyda_ is that of Fitzinger (1835:120),
who later (1843:30) designated the type species as _Amyda subplana_.
The name _Amyda_ cannot date from _Oken_ (1816:348) as Volume 3
[Zoologie] of his Lehrbuch der Naturgeschichte published in 1815-1816
has been placed on the Official Index of Rejected and Invalid Works in
Zoological Nomenclature with the Title No. 33; see Opinion 417
(Hemming, 1956).
There has been considerable debate as to whether Geoffroy did or did
not designate a type species of the genus _Trionyx_ (1809a). Although
not specifically designated as the type species, _Trionyx aegyptiacus_
(= _Testudo triunguis_ Forskål) is considered by Smith (1930:2),
Schmidt (1953:108, footnote), and Loveridge and Williams (1957:422) to
have been sufficiently indicated by Geoffroy as the type species. But
Stejneger (1944:6), H. M. Smith (1947:122), Conant and Goin
(1948:11), and Mertens and Wermuth (1955) maintained that Geoffroy did
not adequately designate a type species, and that Fitzinger (1843:30)
designated the type species as _Trionyx granosus_ (= _Lissemys
punctata_), a synonym of Geoffroy's species, _coromandelicus_.
If Fitzinger's designation of a type species is accepted, the name
_Trionyx_ is applicable to the forms herein referred to _Lissemys_,
and _Amyda_ to the American forms. If Geoffroy's designation is
accepted, the American forms are referable to _Trionyx_, and _Amyda_
is a synonym.
The preceding includes only those generic names (listed in
chronological order) that have been applied to Recent American
soft-shelled turtles. Generic synonyms of the genus _Trionyx_
applicable to Old World species are listed by Stejneger (1907:514),
Smith (1931:165), and Loveridge and Williams (1957:420-21).
_Trionyx_ is the most widespread genus of the family; most of the
species occur in southeastern Asia. All North American soft-shelled
turtles belong to this genus.
For quick reference, all the specific and subspecific names proposed
for soft-shelled turtles in North America are listed below in
alphabetical order (left hand column) with their nomenclatural status
as recognized in this paper. The synonyms are listed in the account of
the appropriate species or subspecies, and are discussed under the
subsection entitled "Remarks."
_agassizi_ _Trionyx spinifer asper_
_annulifer_ _Trionyx spinifer spinifer_
_argus_ _Trionyx spinifer spinifer_
_asper_ _Trionyx spinifer asper_
_ater_ _Trionyx ater_
_bartrami_ _Trionyx ferox_
_emoryi_ _Trionyx spinifer emoryi_
_calvatus_ _Trionyx muticus calvatus_
_ferox_ _Trionyx ferox_
_georgianus_ _Trionyx ferox_
_georgicus_ _Trionyx ferox_
_harlani_ _Trionyx ferox_
_hartwegi_ _Trionyx spinifer hartwegi_
_hudsonica_ _Trionyx spinifer spinifer_
_mollis_ _Trionyx ferox_
_microcephalus_ _Trionyx muticus muticus_
_muticus_ _Trionyx muticus muticus_
_nuchalis_ _Trionyx spinifer spinifer_
_ocellatus_ _Trionyx spinifer spinifer_
_olivaceus_ _Trionyx spinifer spinifer_
_spiniferus_ _Trionyx spinifer spinifer_
Variation
Aside from qualitative variations and comparisons of patterns of
pigmentation the following external measurements (to the nearest
millimeter) were used.
_Length of plastron_: Maximal straight-line measurement
(midventrally), from the anteriormost region of the ventral surface to
the posterior end of the plastron; this measurement includes an
anterior cartilaginous part.
_Length of carapace_: Maximal, straight-line measurement
(middorsally), from the nuchal region to the posteriormost region of
the free edge of the carapace.
_Width of carapace_: Maximal, straight-line measurement between the
lateral margins of the carapace.
_Plane of greatest width of carapace_: Maximal, straight-line
measurement from the posteriormost region of the free edge of the
carapace to a point on the middorsal line at the level or plane of the
greatest width of the carapace; this measurement and the last two, of
course, include the fringing cartilaginous parts of the dorsal bony
carapace.
_Width of head_: Maximal measurement between the lateral margins of
the head.
_Length of snout_: Measurement from tip of snout to interorbital
region of least breadth.
_Diameter of ocellus_: Maximal outside diameter of largest (not
conspicuously ovoid or oblong) ocellus on carapace.
The following ratios were developed from the measurements. Reference
to these ratios will be made by the abbreviations within the
parentheses: length of carapace/length of plastron (CL/PL); length of
carapace/width of carapace (CL/CW); length of carapace/plane of width
of carapace (CL/PCW); length of plastron/width of head (PL/HW); width
of head/length of snout (HW/SL); diameter of ocellus/length of
plastron (OD/PL).
Secondary Sexual Variation
_Size_
In many species of turtles, females are larger than males; the
difference in size between the sexes is probably most pronounced in
aquatic emydids. The ten largest individuals of each sex were selected
to indicate the relative difference in size between the sexes of the
three American species of _Trionyx_ (excluding _ater_, Table 2).
Female soft-shelled turtles attain a larger size than males. _T.
ferox_ is the largest species; _muticus_ is the smallest. The
approximate maximal size of each sex and the difference in size
between the sexes are more correctly expressed for _spinifer_ and
_muticus_ than for _ferox_, because fewer specimens of _ferox_ were
examined; presumably the approximate maximal size of males and females
of _ferox_ is larger than is indicated in Table 2.
TABLE 2. Secondary Sexual Difference in Maximal Size of North
American Species of the Genus Trionyx (excluding ater) Based
on the Ten Largest Specimens of Each Sex of Each Species.
The Extremes Precede the Mean (in parentheses).
=============+============================
SPECIES | Plastral length (cm.)
-------------+---------+------------------
_ferox_ | males | 17.0-26.0 (20.0)
| females | 23.3-34.0 (27.9)
| |
_spinifer_ | males | 13.8-16.0 (14.4)
| females | 26.0-31.0 (28.0)
| |
_muticus_ | males | 11.8-14.0 (12.3)
| females | 17.7-21.5 (18.9)
-------------+---------+------------------
_Pattern_
Secondary sexual differences in pattern are probably more pronounced
in soft-shelled turtles than in other species of turtles, except
perhaps for the well-known melanism and concomitant obliteration of
pattern acquired by some adult males of the _scripta_ section of the
genus _Pseudemys_.
The difference in pattern between the sexes of American species varies
with size of the individual and with the species and subspecies. The
juvenal pattern of some individuals of _T. spinifer asper_ differs
according to sex. In the other species and subspecies, there are no
secondary sexual differences in the juvenal pattern. That pattern in
females of all species and subspecies is partly or entirely obscured
by a mottled and blotched pattern as growth proceeds. This mottled and
blotched pattern is present on females not yet sexually mature, and is
of contrasting lichenlike figures, and in other individuals is less
contrasting and a more uniform coloration. The largest males of _T.
spinifer_ retain a conspicuous juvenal pattern; in those of _muticus_
the pattern may be well-defined or partly modified and obscured,
whereas in large males of _ferox_ the juvenal pattern is ill-defined
or absent. No male normally acquires a contrasting mottled and
blotched pattern on the carapace. The pattern on the carapace of many
large individuals of _ferox_ is not distinctive as to sex.
On the dorsal surface of the soft parts of the body there is a
contrasting pattern in adult males and hatchlings of some forms, but
in most large females the pattern is usually reduced to a near-uniform
coloration; the pattern on adult males of _ferox_ and _muticus_ is not
contrasting and resembles that on large females.
_Coloration_
Because most specimens examined were preserved, the detection of
secondary sexual differences in coloration was difficult. There is one
difference in coloration between the sexes in the subspecies _T. s.
emoryi_. Males from the Río Grande drainage, at least those from the
Big Bend region of Texas, and southwestward in the Río Conchos into
Chihuahua, México, are bright orange on the side of head (postlabial
and postocular pale areas); an orange tinge also occurs in pale
stripes on the snout, and pale orange blotches sometimes occur on the
dorsal surfaces of limbs, especially the hind limbs. The coloration of
these areas on females is pale yellow, lacking orange.
_Tuberculation_
In all subspecies of _spinifer_ the carapace of adult males is
"sandpapery" owing to abundant, small, spiny tubercles distributed
over its surface; all females lack spiny tubercles on the surface of
the carapace.
_Length of Tail_
Elongation of the preanal region of the tail resulting in the
extension of the cloacal opening beyond the posterior edge of the
carapace occurs in males of several kinds of turtles, including
_Trionyx_, at least in those from Louisiana, Texas, and Lake Texoma,
Oklahoma (Webb, 1956:121). Probably this elongation is characteristic
of males of all American softshells. Some females of _spinifer_ and
_muticus_ that exceed the maximum size attained by males have the tip
of the tail and cloacal opening extending a short distance beyond the
posterior edge of the carapace. Some large females of _ferox_ have
more elongate tails than those of _spinifer_ and _muticus_.
_Width of Alveolar Surfaces of Jaws_
Stejneger (1944:34-36, pl. 6) commented on a series of large skulls of
_ferox_ mostly from Kissimmee, Florida, some of which had
conspicuously expanded alveolar surfaces. He suggested that the
condition was confined to large males. A scattergram (Fig. 2) based on
measurements obtained from 45 skulls of _ferox_ shows widened alveolar
surfaces of the upper jaws on some of the larger skulls. Because the
maximal size of adult males is unknown and the difference in size
between the sexes of _ferox_ is slight, such large skulls might
represent either sex. The sex had been recorded for only three of the
45 skulls; none of the three exceeded 82 millimeters in basicranial
length or had widened alveolar surfaces. Some of the larger skulls of
approximately the same size differ markedly in width of the alveolar
surfaces; this difference suggests that both sexes are included and
that the sexes may be of approximately the same maximal size. On the
other hand, the variation observed in skulls is possibly confined to
one sex. To judge from what is known of the maximal sizes of the sexes
of _spinifer_ and _muticus_ (see Table 2), skulls of _ferox_ of more
than 85 millimeters in basicranial length probably are of females. The
largest alcoholic male (dissected) of _ferox_ that I examined had a
width of head of approximately 46.5 millimeters; that measurement
corresponds to a basicranial length of 70 to 75 millimeters. The
specimen of which measurements are depicted by the uppermost symbol in
the scattergram (represented by KU 16528) was recorded as a female.
Large females of _T. s. asper_ from rivers emptying into the Atlantic
Ocean have broadened alveolar surfaces.
[Illustration: FIG. 2. Basicranial length and greatest width of
alveolar surface of upper jaw on 45 skulls of _T. ferox_. Some
skulls (sex unknown) in which the basicranial length exceeds
85 mm. develop widened alveolar surfaces of the jaws.]
_Length of Claw_