Triodus elpia, Johnson & Thayer, 2009

Johnson, Gary D. & Thayer, David W., 2009, Early Pennsylvanian xenacanth chondrichthyans from the Swisshelm Mountains, Arizona, USA, Acta Palaeontologica Polonica 54 (4), pp. 649-668 : 658-662

Triodus elpia sp. nov.

Figs. 12–17 .

Etymology: After the acronym, LPIA, late Paleozoic ice age, utilized by Stanley and Powell (2003), and others ( Montańez et al. 2007, for example). Despite the Swisshelm locality being equatorial, this ice age influence may have been much closer at hand later in the Pennsylvanian ( Soreghan et al. 2008). Perhaps the data from xenacanths and other vertebrates influenced by changing marine environments will be sufficient enough in the future to be added to the invertebrate database.

Type material: Holotype: UAPL 23397 , lateral tooth ( Figs. 12 , 14 ) . Paratypes include 29 measured teeth comprising UAPL 23395 (21 laterals), plus three additional laterals ( UAPL 23398 , 23505 , 23506 ), UAPL 23501 (one posterolateral), UAPL 23503 (one “anteromedial”), UAPL 23504 (one posterior), and UAPL 23502 (one?posterolateral) .

Type locality: UAPL locality 7205, Swisshelm Mountains , southeastern Arizona, USA .

Type horizon: Upper Black Prince Limestone, Lower Pennsylvanian (upper Bashkirian), equivalent to the Westphalian A and B boundary ( Thayer 1985).

Referred material.—Includes nine incomplete teeth plus tooth fragments and isolated cusps (all in UAPL 23399) which provide no additional descriptive information and exhibit no anomalies.

Diagnosis.—Teeth with principal cusps moderately labio−lingually compressed; cristae present on lingual and labial sides, often with one that is carina−like; minor cusp leans posteriorly, major cusp straight. Crown−base angle 90–105°, sometimes greater; angle between minor cusp base transverse axis and labial side of base variable, averaging about 30°, about 15° for major cusp. Base asymmetrical with an anterolingual shelf, sometimes reduced, absent in some nonlaterals; central foramen present. Basal tubercle with concave surface; lingual extension absent. Apical button isolated from cusps and usually from base margin; lingual extension reduced or absent. Maximum dimension <2 mm. Heterodont dentition probable.

Description.—Based on 25 (= n) mostly complete lateral teeth (others discussed below); n <25 (<100%) noted for many features. Labial side of base ( Fig. 12C ) thin (84%). Anterolingual shelf ( Fig. 12 ) always present, aborally flexed; in oral view, 44% on left side, 56% right side; may be subdued; base nearly always asymmetrical. Aboral nutrient foramina range from two to five (88%), but up to eight. Basal tubercle nearly always concave, rarely flat; shape equally round, semicircular, or anteroposteriorly oval; lingual extension absent (80%) or defined principally by foramina. Aboral side of base concave (92%) or flat. Apical button isolated from cusps (92%) and margin of base (80%); shape irregularly round or pear−shaped, but generally oval or rectangular with one long side parallel to the posterolateral base margin; lingual extension present (20%), abbreviated and usually defined only by foramina (48%), or absent. Central (medial) foramen present (76%), questionably absent (8%) or indeterminate (matrix). Two to four oral nutrient foramina most common (88%), otherwise five or six, with one indeterminate.

Principal cusps unequal in size (breadth, not length; see Fig. 12 ), except in one tooth (n = 20); minor cusp posterior (one questionable), longer than major cusp (n = 7; all others indeterminate). Base of both minor and major cusps compressed in all teeth, more or less labio−lingually, increasing distally. Cristae ( Fig. 12 ) generally straight, converging at the tips, may proximally bifurcate, restricted to the distal half (n = 11), sometimes extending onto the proximal half (n = 6), especially where adjacent to the carina−like cristae; one to four on labial side, one to three on lingual side of minor cusp, and most often three to five on labial side, two to five on lingual side of major cusp. Carina−like cristae usually present on both cusps (minor, n = 14 with 3 questionable; major, n = 18, with 2 questionable), but often indeterminate, presumably because of wear or poor preservation. Minor cusp leans in posterior direction (n = 20 with 2 questionable); major cusp straight (n = 16 with 1 questionable), or leans posteriorly (n = 3) or anteriorly (n = 1). Crown−base angle (angle between the cusps and oral side of the base) 90° to 105° (n = 15),>105° to 120° (n = 6); angle between transverse axis of minor cusp base and base labial margin 15° to 30° (n = 13), 30° to 45° (n = 8), and major cusp 0° to 15° (n = 19),>15° to 45° (n = 5).

Intermediate cusp always present, but nearly always broken at or near its base (n = 23), leaving only two teeth where it is more than half complete. Base antero−posteriorly compressed (n = 15) or round to labio−lingually compressed (n = 7); cusp straight (n = 2), cristae may be absent (n = 2).

Measurements.—Twenty−nine teeth with complete bases were measured ( Fig. 13 ). All are included in a single database. The teeth range in size from 0.60 mm (l−l) × 0.57 mm (am−pl) to 1.47 mm × 1.14 mm (holotype); both are laterals. The height of the holotype is 1.4 mm. Their mean dimensions ± one standard deviation (n = 29) are 0.95 ± 0.20 mm (l−l) and 0.83 ± 0.15 mm (am−pl). A linear regression of am−pl on l−l with 95% confidence intervals yields a slope of 0.53 ±0.20 and y−intercept of 0.33 ± 0.20 mm ( Fig. 13 ). The labio−lingual measurements were considered to be more reliable and therefore the independent variable, the reverse of Orthacanthus donnelljohnsi sp. nov. measurements. The anteromedial−posterolateral measurements were sometimes rather subjective because of asymmetry ( Fig. 12A ). The labio−lingual measurements were taken from the lingual tip of the anterolingual shelf ( Fig. 12E ) to the opposite margin of the basal tubercle in the more asymmetrical teeth so as to emphasize the l−l> am−pl ratio. This ratio is reversed in five teeth ( Fig. 13 ).

Discussion

Remarks.—The holotype ( Figs. 12 , 14 ) is the only essentially complete tooth available and coincidently the largest of all the teeth assigned to this species, and one of the 20% to possess an apical button with a lingual extension ( Figs. 12B , 14B ). The am−pl measurements are not as precise as those normally acquired for other species (this report; Johnson 1999, 2003). Estimates based on Figs. 12E (1.59 mm) and 14E (1.57 mm) exceed the actual measurement (1.47 mm). This is probably caused by the highly flexed anterolingual shelf ( Fig. 12 ) in the holotype and the unusual asymmetry exhibited by most of the lateral teeth. The anterolingual shelf is sometimes subdued or it is mostly on the anterior margin, but is distinctly aborally flexed, similar to the anterior end of the base in Orthacanthus platypternus teeth (Johnson 1999). Figures 12 (which is more schematic) and 14 illustrate the subjective appearances of the cristae, some of which tend to be emphasized by differing angles of view and light sources.

Figure 15 illustrates a lateral tooth with reversed asymmetry compared to the holotype ( Fig. 12 ). Of the 25 measured laterals, the anterolingual shelf is on the left side (occlusal view) in 11 teeth and right side in 14. This difference would probably diminish in larger samples. Another chondrichthyan that possesses an asymmetric tooth base is Thrinacodus (a Devonian phoebodontiform, Ginter et al. 2002), although its crown also displays asymmetry. Ginter et al. (2002: 201–203, fig. 14) suggested a possible arrangement of the teeth in a Th. tranquillus dentition, which may be applicable to the Triodus elpia sp. nov. dentition. The dental asymmetry displayed in these two species may have a bearing on the relationship between the phoebodontiforms and xenacanths (see Bransonella comments below). Although other Paleozoic sharks, such as Denaea wangi ( Wang et al. 2004) , have asymmetrical teeth (mainly the crown), it is the similarity of the tooth bases in T. elpia and Thrinacodus that appears to be significant. The lateral tooth in Fig. 15 also illustrates the problem in determining the major and minor principal cusps in teeth with incomplete cusps, although it is nearly always less ambiguous than in this example.

As noted above, five of the measured 29 tooth bases ( Fig. 13 ) have reversed l−l and am−pl dimensions. Two are included in UAPL 23395 with 0.74 mm × 0.84 mm and 0.72 mm × 0.79 mm dimensions, and two additional laterals measure 0.70 mm × 0.87 mm (UAPL 23505) and 0.87 mm × 1.01 mm (UAPL 23506). The fifth tooth may be a posterolateral (UAPL 23502, described below) with 0.72 mm × 0.88 mm dimensions. These differences, all within one standard deviation, are probably insignificant ( Fig. 13 ).

Evidence of heterodonty.—Five of the measured teeth are not laterals. One ( Fig. 16 ) is considered an “anteromedial” tooth, and is closer to being a true medial than any other tooth in the available sample. Its base is 0.85 (l−l) × 0.72 (am−pl) mm, nearly symmetrical, without an anterolingual shelf. The apical button is in contact with the central foramen and minor (posterior) cusp ( Fig. 16A ).

Two teeth are interpreted as posterolaterals. The first (UAPL 23501) has complete principal cusps; the minor cusp is longest, curves posteriorly with a conical distal half. The major cusp is straight, but leans posteriorly. The intermediate cusp is broken and partly obscured by matrix. In all other aspects, it is similar to the lateral teeth. The second posterolateral (UAPL 23502) may be questionable only because the distal half of the major cusp is missing; the preserved portion is straight and appears to have leaned posteriorly. The minor cusp leans posteriorly. Both cusps are labio−lingually compressed as preserved. This tooth, also with a broken intermediate cusp, is otherwise similar to the laterals, except there is no anterolingual shelf, but the base is extended more anteriorly than usual (reversed l−l and am−pl dimensions, see above).

The last nonlateral tooth is interpreted to be a posterior tooth ( Fig. 17 ). The principal cusps, as preserved, are nearly equal in size. The presumably major (anterior) cusp is slightly labio−lingually compressed ( Fig. 17C ), but the minor cusp, as preserved, is nearly round to slightly antero−posteriorly compressed. The intermediate cusp appears to be absent. The tooth is similar to laterals in other aspects, but with a reduced anterolingual shelf; the apical button is isolated from the principal cusps (not evident in Fig. 17A ), and there is a prominent central foramen (compare Fig. 17A with Fig. 12A, B ). The presence of an anterolingual shelf, which is absent in one of the posterolaterals, as interpreted, suggests a more complex heterodonty.

Comparison with other species.— Triodus elpia sp. nov. is different from all other described species of Triodus , as its lateral teeth possess a somewhat to highly asymmetrical base with an anterolingual shelf, and all of the teeth, where a determination can be made (matrix interference), possess a central foramen, although it is sometimes very small. Hampe (1989, 2003) has provided the most comprehensive reviews of most of the other species. Triodus sessilis ( Hampe 1989) teeth are comparable in size to those of T. elpia , but they lack an asymmetrical base, the crown−base angle is always 90°, and represent a homodont dentition. Triodus lauterensis teeth ( Hampe 1989) are also small and have a comparable crown−base angle, and have a variably asymmetrical base suggesting heterodonty; but, the asymmetry is quite unlike the T. elpia teeth with their anterolingual shelf, and Hampe (1989) did not mention the presence of a central foramen. Triodus palatinus teeth ( Hampe 1989) represent a heterodont dentition, are slightly larger than T. elpia teeth, and have a comparable crown−base angle; but, although the bases are sometimes asymmetrical, their asymmetry is quite unlike that of T. elpia teeth, and Hampe (1989) did not mention the presence of a central foramen. Triodus obscurus teeth ( Hampe 1989) are of similar crown−base angle and size to T. elpia , and questionably represent a heterodont dentition, but they lack lingual cristae and show little base asymmetry. Triodus kraetschmeri teeth ( Hampe 1989) are smaller than T. elpia teeth, and although Hampe (1989) described them as representing a homodont dentition, distinctive posterior teeth lacking an intermediate cusp are present; their crown−base angle is constantly 100°, and the intermediate cusp is positioned labially relative to the principal cusps which are rounded and not compressed; the tooth bases show little asymmetry, and Hampe (1989) does not mention the presence of a central foramen.

Hampe (1993) provided a summary description of the Triodus species he described earlier ( Hampe 1989). He did not mention the presence or absence of a central foramen in any of them. However, in his summary description of Orthacanthus ( Hampe 1993) , he did mention its presence (median aperture). Therefore, a central foramen is very likely absent in T. sessilis , T. kraetschmeri , T. palatinus , T. obscurus , and T. lauterensis . However, T. sessilis does possess a central foramen (Oliver Hampe, personal communication, October 2007). The age of these species collectively range from Gzhelian to perhaps as late as Kungurian ( Hampe 1989; Menning et al. 2006), so all are younger than T. elpia sp. nov.

Triodus serratus teeth (Westphalian A−C, Hampe 2003; or Bashkirian−Moscovian, in part) are generally significantly larger than T. elpia sp. nov., have a distinctive aboral depression on the oval base, and lack a central foramen (median foramen of Hampe 2003). Its dentition is largely homodont, with some teeth showing some asymmetry in the base. Other than having a similar crown−base angle, T. serratus is quite unlike T. elpia .

Teeth assigned by Soler−Gijón and Hampe (1998) to Triodus ? frossardi (Asselian; type specimen is a spine; species not questioned by Hampe and Ivanov 2007b) are similar in size to T. elpia sp. nov. teeth; the crown−base angle is smaller in the former. The tooth base in T.? frossardi is asymmetrical ( Soler−Gijón and Hampe 1998: fig. 4D, E), but is unlike that in T. elpia , in lacking an anterolingual shelf. Curiously, the lingual extension of the apical button curves toward one side of the base and not to the tip of the base (their fig. 4E), similar to a tooth of T. serratus illustrated by Hampe (2003: fig. 20c) and the holotype of T. elpia ( Fig. 12 ). Soler−Gijón and Hampe (1998) did not state whether T.? frossardi teeth possess a central foramen. They did, however, provide a summary (their table 2) of tooth characteristics of most Triodus species. Parenthetically, they suggested ( Soler−Gijón and Hampe 1998: 342, 345) that Triodus should occur in the Lower Permian of Texas, based on neurocrania; there is no evidence of the occurrence of Triodus teeth in the Texas Permian, based on extensive collections (Johnson 1999, 2003).

Schindler and Hampe (1996) assigned three teeth from the Gzhelian [lowermost Permian of Central Europe ( Menning et al. 2006: fig. 4), but now uppermost Carboniferous in standard usage] to Triodus sp. ZÖ. They are similar in size to T. elpia sp. nov., and they possess a central foramen. However, the tooth base is quite symmetrical. Schindler and Hampe (1996) also provided a summary description of the species mentioned above, and also of T. carinatus teeth (also Asselian), but there is no mention of the presence of a central foramen or an asymmetrical base with an anterolingual shelf in the latter.

Hampe and Ivanov (2007b) assigned three very small teeth from Pennsylvanian (Moscovian) marine sediments of the Northern Caucasus to a new species, Triodus teberdaensis . They possess a central (median) foramen and prominent aboral and lingual foramina in the base, which is fairly symmetrical. The cusps are rather round in cross−section and possess four or five straight cristae, some of which may be carina−like (lateral cutting edges, Hampe and Ivanov 2007b: 182). Hampe and Ivanov (2007b) successfully delineated their new species from all other previously described species mentioned above, and confirmed the absence of a central foramen in all but two species ( T. teberdaensis and Triodus sp. ZÖ; plus T. sessilis as noted above).

The teeth of Hagenoselache sippeli , based on a nearly complete articulated (and only) xenacanth specimen (Hampe and Heidtke 1997) from the Namurian B (lower Bashkirian, Menning et al. 2006), possess a central foramen and show evidence of variable symmetry in their lingually extended base. Although the principal cusps possess cristae, the overall morphology of H. sippeli teeth (Hampe and Heidtke 1997: fig. 4) is quite unlike those of Triodus elpia sp. nov.

Therefore, the only Triodus species to possess a central foramen is T. sessilis from the Asselian, Triodus sp. ZÖ from the Gzhelian, and T. teberdaensis from the Moscovian, besides T. elpia sp. nov. from the upper Bashkirian. Triodus serratus is the only species, for which teeth are known ( Hampe 2003), that is of similar age to T. elpia , but they differ in this fundamental morphologic feature. And, while some T. serratus teeth have an asymmetrical base, only T. elpia lateral teeth are generally asymmetrical with an anterolingual shelf.

The number of cristae and their patterns demonstrate enough variability within species of Triodus to be of questionable significance (e.g., “ Bohemiacanthus ” carinatus in Schneider and Zajíc 1994: fig. 21), except the lack of lingual cristae in T. obscurus and that, in general, they are straight in this genus. However, the efforts of Soler−Gijón and Hampe (1998: table 2) and Hampe (2003: 223–225) are useful in delineating species, despite the variability of the cristae in each one.

To summarize, Triodus elpia sp. nov. is similar in one major aspect only to T. lauterensis , T. palatinus , and possibly T. obscurus and T.? frossardi, in possessing a heterodont dentition; but those species lack a central foramen. Triodus teberdaensis , T. sessilis , and Triodus sp. ZÖ possess a central foramen, but otherwise are unlike T. elpia . Some of the T.? frossardi teeth are more similar to T. elpia teeth than any other species, except for their lack of an anterolingual shelf (and central foramen). Triodus serratus and Hagenoselache sippeli (with a central foramen), the only species of comparable age to T. elpia , are quite different, as noted above. The combination of a central foramen and an anterolingual shelf on an asymmetrical base distinguish the teeth of T. elpia from all other species. Hampe and Ivanov (2007b) provided a phylogenetic analysis of the Triodus species, based on 13 tooth morphology characters. Unfortunately, the absence or presence of a central (median) foramen is not among them. This might help resolve Hampe and Ivanov’s (2007b: 185) comment that Triodus may not be monophyletic; but Schneider’s (1996) Bohemiacanthus is not the solution.

Age, distribution and habitat.— Triodus occurrences in the Pennsylvanian and Permian are limited to Europe and North America ( Hampe 1989, 2003), and South America ( Johnson et al. 2002). If the Triassic species questionably assigned to this genus (reviewed by Hampe 2003: 225) are included, then its ultimate distribution would be significantly greater ( India, Australia, as well as European and North American Upper Triassic). It should be noted that the South American occurrence (Upper Permian) is represented by teeth similar to those from the Upper Triassic.

Occurrences of Triodus are typically in nonmarine facies. However, Hampe and Ivanov (2007b) stated that Triodus teberdaensis was very likely a marine xenacanth, as the teeth and associated fossils were recovered from a marine carbonate facies (plant remains at the locality were found in clastic facies; Alexander Ivanov, personal communication, December 2008). It is possible that T. elpia sp. nov. was also a marine xenacanth, but because of the associated lepospondyl amphibian remains ( Thayer 1985), its habitat remains uncertain.

Figures

Fig. 12. Xenacanthid chondrichthyan Triodus elpia sp. nov., lateral tooth, holotype, UAPL 23397, Lower Pennsylvanian, Black Prince Limestone, Swisshelm Mountains, Arizona; occlusal (A), lingual−occlusal (B), labial (C), anteromedial (D), and aboral (E) views; note the lingual extension on the apical button in B.

Fig. 13. Scatter diagram of Triodus elpia sp. nov. tooth base dimensions.

Fig. 14. Xenacanthid chondrichthyan Triodus elpia sp. nov., lateral tooth, holotype, UAPL 23397, Lower Pennsylvanian, Black Prince Limestone, Swisshelm Mountains, Arizona; compare with Fig. 12 and see text for explanation. Occlusal (A), lingual−occlusal (B), labial (C), anteromedial (D), and aboral (E) views.

Fig. 15. Xenacanthid chondrichthyan Triodus elpia sp. nov., lateral tooth, UAPL 23398, Lower Pennsylvanian, Black Prince Limestone, Swisshelm Mountains, Arizona; lingual−occlusal (A) and aboral (B) views. Left cusp in A is presumably the major (anterior) cusp; compare with Fig. 12.

Fig. 16. Xenacanthid chondrichthyan Triodus elpia sp. nov., “anteromedial” tooth, UAPL 23503, Lower Pennsylvanian, Black Prince Limestone, Swisshelm Mountains, Arizona; lingual−occlusal (A), labial (B), anteromedial (C), and aboral (D) views. B–D drawn following reattachment of major cusp to the base after A was drawn.

Fig. 17. Xenacanthid chondrichthyan Triodus elpia sp. nov., posterior tooth, UAPL 23504, Lower Pennsylvanian, Black Prince Limestone, Swisshelm Mountains, Arizona; lingual−occlusal (A), labial (B),?anterior (C), and aboral (D) views. Cristae on the broken (?minor) cusp extend proximally to the base; there is the slightest hint that the central crista shown in C extends to the base, but is not shown. The right half of the aboral surface (D), lingual of the basal tubercle, is largely covered by matrix; the small foramen shown is probably valid. There is no evidence of an intermediate cusp.