Table 10. Frequency of Selected Characters Among Species and Subspecies of Trionyx in North America. Characters of muticus Refer to the Typical Subspecies; Horizontal Dashes Connecting X's Indicate that Computations for Those Subspecies Were Combined; Vertical Dashes Indicate that the Subspecies Is Intermediate Between the Adjacent Subspecies.
| Characters | Species and subspecies | ||||||||
| ferox | spinifer | hartwegi | asper | pallidus | guadalupensis | emoryi | ater | muticus | |
| Juvenal pattern: | |||||||||
| black spots, ocelli | X | X | X | ||||||
| white dots | X | X | X | X | |||||
| Pattern on snout: | |||||||||
| acute angle (reduced in muticus) | X | X | X | X | X | X | |||
| | | |||||||||
| triangular | X | X | X | X | |||||
| Pattern on side of head: | |||||||||
| contrasting marks | X | X | X | X | X | X | |||
| | | |||||||||
| non-contrasting marks (distinct stripe in muticus) | X | X | X | X | |||||
| Pattern on limbs of adults: | |||||||||
| contrasting | X | X | X | X | X | ||||
| | | |||||||||
| reduced or absent | X | X | X | X | X | ||||
| Tuberculation (anterior edge of carapace): | |||||||||
| conical, equilateral | X | X | X | X | |||||
| reduced or absent | X | X | X | X | X | ||||
| Head (PL/HW, [fig. 3]): | |||||||||
| wide | X | X | X | X | X | ||||
| narrow | X | X | X | X | |||||
| Carapace (CL/CW, [fig. 4]): | |||||||||
| wide | X | X | X | X | X | X | |||
| | | |||||||||
| narrow | X | X | X | X | |||||
| Level of Carapace Width (CL/PCW, [fig. 5]): | |||||||||
| middle of carapace | X | X | X | X | X | ||||
| farther posteriorly | X | X | X | X | |||||
| Snout (HW/SL, [fig. 6]): | |||||||||
| long | X —— | —— X | X | X —— | —— X | X | |||
| | | | | ||||||||
| short | X | X —— | —— X | X | X | ||||
Fossils
The known occurrence of fossil trionychids throughout the world indicates a former distribution more widespread than the family has today; the principal difference in the former and present distributions is the lack of living softshells in Europe.
I have not studied in detail the many fossil remains but such examination as I have made of them suggests that many of the characters used as a basis for distinguishing fossil forms in North America are subject to individual variation or are of no diagnostic value in the living species (Hummel, 1929:769). Knowledge of the variation in the living species of the Old World would aid in adequately appraising the North American fossils. Some osteological characters of the three living American species (excluding ater) together with data on variation within a given species are mentioned below. Some differences in skulls of the three species already were mentioned in the section "Osteological Characters." Because most fossil remains are those of the carapace and plastron, attention is here given to those structures.
Widened alveolar surfaces of jaws.—An ontogenetic variation affecting large skulls of T. ferox and some individuals of T. spinifer asper; presumably confined to females. Of especial interest is its presence in some populations of asper that are not otherwise distinguishable (external characters) from the rest of the individuals comprising that subspecies.
Sculpturing.—No differences in pattern (generally of anastomosing ridges) on carapace or plastron; fineness or 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.