Orthacanthus donnelljohnsi, 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 : 650-656

Orthacanthus donnelljohnsi sp. nov.

Figs. 1–9 .

Etymology: In honor of the late Donnell F. Johns (1934–2002), who was professor of surgery, clinical professor of otolarynology and director of clinical research for the Department of Plastic Surgery at The University of Texas Southwestern Medical Center at Dallas. He was awarded the 2002 Frank R. Kleffner Clinical Career Award of the American Speech Language−Hearing Foundation, the most prestigious award in his profession, particularly for developing the pharyngeal flap procedure. The lives of hundreds of people were greatly improved by his direct intervention, particularly children.

Type material: Holotype: UAPL 23384, lateral tooth ( Fig. 1 ). Paratypes include 59 measured adult teeth comprising UAPL 5269 (one lateral), 23382 (42 laterals), 23383 (one lateral), 23386 (four posteriors), 23387 (one posterior), 23388 (one posterolateral), 23490 (tooth with convergent cusps), 23491 (one medial), 23492 (three posterolaterals), 23493 (three germinal laterals), and 23498 (one?medial); and 39 measured juvenile teeth comprising UAPL 23389 (33 teeth), 23390–23393 (four laterals), 23396 (one posterolateral), and 23497 (one?posterolateral). Other material includes UAPL 5270 (incomplete lateral tooth), 6335 (tooth fragments), 23385 (31 incomplete teeth, 30 laterals and one posterior), 23394 (six incomplete juvenile teeth, one is germinal), 23488 (three posteriors, two incomplete, one in matrix), 23489 (two incomplete germinal teeth), 23499 (juvenile?medial or?posterolateral), and 23500 (juvenile?germinal lateral).

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).

Diagnosis.—Teeth small to moderate size (<10 mm). Principal cusps compressed throughout with carinae lacking serrations; larger (major) cusp is posterior and more divergent than minor cusp. Single intermediate cusp present; none in posterior teeth. Labial margin of base usually thin. Lingual extension of basal tubercle usually extends to, and beyond center of base. Juvenile teeth with consistently thinner base, otherwise similar (but smaller) to adult teeth.

Description.—Based on adult teeth throughout the dental arcade; differences from lateral teeth noted below. Presumed juvenile nonsegregated teeth (see below) are compared in Table 1. Tooth base generally slightly wider (l−l) than long, probably equidimensional if the influence of the basal tubercle and lingual extension of the apical button is neglected ( Fig. 1C, D ). About 1/4 ( Table 1) have a thick base as seen in labial view ( Fig. 1B ), comparable to Orthacanthus texensis (Johnson 1999) , whereas about 1/2 have a thin base, comparable to O. platypternus (Johnson 1999) ; remaining teeth intermediate in thickness. Larger teeth tend to have a thicker base, but some are thin−based, and some small teeth have a thick base. Aboral surface flat or slightly concave in 3/4 of the teeth or distinctly concave ( Table 1). Four to six nutrient foramina occur on the aboral surface ( Fig. 1D ),> 6 in about 10% where a determination could be made; pattern random (Johnson 1999). Basal tubercle round in most teeth, or elongated (am−pl), with a convex surface or flat surface ( Table 1); nearly all with a lingual extension ( Fig. 1D ), which is short or reaches the center of the base in half the teeth, or extends beyond the center. Apical button ( Fig. 1A ) always isolated from cusps; shape is round, irregular, pear− or heart−shaped, nearly always has a lingual extension that is narrow to broad, reaching the lingual margin of the base ( Fig. 1A ). Oral surface usually with three or four nutrient foramina ( Fig. 1A , Table 1).

Principal cusps not equal. Major cusp largest by definition ( Fig. 1A ), always leans (or curves) posteriorly, as in Orthacanthus texensis (Johnson 1999: 231) , always bears carinae on both edges where a determination can be made ( Table 1). Minor cusp straight (near vertical) or leans slightly anteriorly ( Table 1). Both cusps usually 90–105° to the base (crown−base angle, Table 1), but not always equally. Major transverse axis in a plane passing through the cusp bases (Johnson 1999: fig. 1E) forms an angle <45° with the labial margin of the base between the cusps, usually <30° for both cusps, often much less for the minor cusp ( Table 1), 45° for major cusp in only one tooth.

Intermediate cusp less than half the length of the principal cusps ( Table 1), tends to be straight or lean slightly toward the posterior (major) principal cusp ( Table 1); all but one with “reversed compression”, in which the base is am−pl compressed, but the distal two−thirds is l−l compressed.

Principal cusps of medial teeth are equal in size and somewhat divergent ( Fig. 2 ). Both principal cusps in posterolateral teeth lean toward the posterior; intermediate cusp present ( Fig. 3 ); minor cusp may be proximally straight, with only the distal half leaning posteriorly. Principal cusps in posterior teeth lean posteriorly ( Fig. 4 ); the teeth are small (an exception is discussed below), lack an intermediate cusp, and sometimes lack a central foramen.

Measurements.—The adult teeth range in size from 0.75 mm (am−pl) × 1.16 mm (l−l) (a posterior tooth; the smallest lateral is 1.01 mm × 0.81 mm) to 8.06 mm × 7.76 mm ( Fig. 5 ); a second lateral is 7.71 mm × 8.70 mm; the former is about 9 mm high. Their mean dimensions ± one standard deviation are 3.67 ± 1.64 mm (am−pl) and 3.79 ± 1.71 mm (l−l) based on 60 measured teeth (holotype plus paratypes). A linear regression of l−l on am−pl with 95% confidence intervals yields a slope of 1.00 ± 0.08 and y−intercept of 0.11 ± 0.30 mm ( Fig. 6A ).

The presumed juvenile teeth range in size from 0.84 mm (am−pl) × 0.81 mm (l−l) to 2.21 mm × 1.68 mm. Another tooth is 2.05 mm × 2.01 mm; the l−l dimension is relatively large because of a prominent basal tubercle ( Fig. 6B ). The am−pl mean ± 1 s.d. is 1.32 ± 0.36 mm and the l−l mean ± 1 s.d. is 1.21 ± 0.31 mm based on 39 measured teeth. A linear regression of l−l on am−pl with 95% confidence intervals yields a slope of 0.80 ± 0.11 and y−intercept of 0.15 ± 0.16 mm ( Fig. 6B ).

Discussion

All available teeth with complete bases were initially divided into two categories. Teeth in the first category were assigned by Johnson and Thayer (1999) to Orthacanthus compressus , and the second category, consisting of small teeth with thin bases, was thought to represent a different species ( Xenacanthus cf. X. decheni ) or possibly O.? compressus medials or juvenile teeth. Detailed examination and description of each tooth revealed no significant differences in morphology, because many teeth in the first category also have thin bases, and a few are as small as those in the second category. Rodrigo Soler−Gijón (personal communication, May 1999) agreed that the second category may consist of juvenile teeth. Both categories contain medial and posterolateral as well as lateral teeth, but no juvenile posterior teeth have been identified. Segregation of juvenile teeth by position within the dental arcade is problematic, as described below. For purposes of discussion, and to facilitate future studies, the teeth remain segregated as adult and juvenile categories, although differentiation is sometimes subjective.

Adult teeth.—The diagnosis and most of the description are based on adult lateral teeth. Other teeth from the dental arcade, presumed to be adult, are less common. Whether the lingual extension of the apical button ( Fig. 1A ) is ever responsible for the protuberance on the base is uncertain, but generally it seems to be independent of the shape of the lingual margin. Attempts to observe carinae on the intermediate cusps of adult lateral teeth were largely unsuccessful because they were usually broken, covered by matrix, or possibly worn. Where they are reasonably complete, it was estimated that none exceeded half the length of the principal cusps ( Table 1).

Symphyseal teeth have not been recognized, nor were they by Johnson (1999). However, a single large tooth (UAPL 23490) has convergent principal cusps, not typical of Orthacanthus teeth, and the central foramen is offset beneath the primary principal cusp. All other features are normal in this tooth, including a complete intermediate cusp about half the length of the principal cusps, which suggest it is not deformed. And one of the posterior teeth (part of UALP 23386) with a broken cusp, discussed below, might actually have occurred near the symphysis.

Medial teeth are anterior to the laterals and typically occur in Orthacanthus dentitions (Johnson 1999). But only one Swisshelm medial tooth ( Fig. 2 ) is considered as adult, because it is at least 4mm high and has a moderately thick base. Other than the attitude of the principal cusps, no other morphological features are unusual, and it is included in Table 1 and the adult−tooth measurement database.

Posterolateral teeth are transitional between the lateral and posterior teeth. Johnson (1999: 233, 241) did not recognize them as a separate suite of teeth, but instead included them with the lateral teeth (but see Johnson 1999: figs. 5D, 7A–E, 18K–L). They are similar to lateral teeth, and the somewhat variable attitude of the minor cusp suggests a smooth transition between the two suites. Only four teeth (UAPL 23388 and 23492) from among those considered to be adult were recognized. All were measured and included in that database as their bases are not unique, although their labial margins range from “thin” to “thick”. UAPL 23388 ( Fig. 3 ) does not possess carinae on its cusps, but carinae do occur on two of the other teeth and the fourth has questionably worn highly compressed principal cusps.

Posterior teeth are the most unusual of those in the Orthacanthus donnelljohnsi sp. nov. dental arcade. There is no doubt these teeth belong to O. donnelljohnsi . The isolated apical button is not in contact with the lingual margin in UAPL 23387 ( Fig. 4 ), its cusp−base angle is about 120°, and its cusp−labial margin angle is about 45° (unusual for O. donnelljohnsi ; compare with Table 1); but its cusps possess carinae ( Fig. 4B ) and the base is normal, including a central foramen, so its identity is not questioned. Four additional teeth (UAPL 23386) are posteriors; a central foramen is present in one, absent in the second, very small in the third, and may be absent or very small in the fourth. A broken cusp in the second tooth may have been divergent from the preserved cusp, so it may not be a posterior. Also, its thick base is compressed more than usual (0.75 mm long, 1.16 mm wide), suggesting the possibility it is not a posterior, but perhaps occurred near the symphysis, although Fig. 6A suggests it is not significantly unusual. Hampe (2003: 206, fig. 10c) described a commissural, i.e., posterior, bicusped tooth of O. gibbosus and suggested it might have instead occupied a symphyseal position. Measurements of the five teeth were used in the database; they are the smallest teeth in Fig. 6A . None have a thin base. Three additional posteriors (UAPL 23488) are fragmentary or in matrix. Among the tooth fragments in UAPL 23385 is one that lacks a central foramen and intermediate cusp and has a minor cusp that leans toward the posterior; if complete, it would have been significantly larger than the five measured teeth.

Juvenile teeth.—Thirty−nine measured small teeth lacking a thick base ( Table 1; 20% have an intermediate thickness) may be teeth from juvenile sharks. On the basis of the orientation of the principal cusps, 13 may be medial teeth (six are questioned), 18 are laterals (five questioned), eight are posterolaterals (five questioned), and one is indeterminate. One germinal tooth was not measured. Although no posterior teeth are identified, and as an inordinate number of medial teeth are present compared to the teeth from adult sharks, it is clear that these teeth demonstrate a gradual change in cusp orientation in the dental arcade. Other than possessing slightly divergent cusps, the medials are similar to the laterals, as suggested by the number of teeth with questioned position in the arcade. The teeth illustrated in Fig. 7 are considered laterals, although one is questionable ( Fig. 7C ), as the distal half of the minor cusp and the intermediate cusp lean toward the posterior, and might be considered a posterolateral, but the principal cusps in general have an attitude more similar to typical laterals. Another tooth, interpreted as a posterolateral, has all three cusps leaning posteriorly, but even this is subjective, depending on the point of reference from which the tooth is viewed ( Fig. 8 ; compare the lingual−occlusal and labial views). This apparent dilemma arose following the drawing of the initial illustrations ( Fig. 8A–D ); additional illustrations ( Fig. 8E–H ) made independently nearly three years later confirmed that no error was involved (slightly differing orientations between similar views emphasize difficulties in accurately depicting characters, e.g., minor foramina, in very small teeth). Yet another tooth, questionably a posterolateral ( Fig. 9 ), is significantly different in cusp attitude and length:width (am−pl: l−l) ratio of the base. The proximal half of the minor cusp leans slightly anteriorly ( Fig. 9C ), but the distal half leans slightly posteriorly toward the major cusp; the intermediate cusp leans slightly posteriorly (barely discernable in Fig. 9A, C ). But more disconcerting is the length:width ratio of about 1.37, considerably greater than the tooth in Fig. 8 . As seen in Fig. 6B , UAPL 23497 ( Fig. 9 ) is not unique (note the four values below the lower end of the trend line; the ratio of the am−pl and l−l means in Fig. 6B is 1.09), but suggests that in reality base length:width ratio may be a factor in tooth placement within the dental arcade, not just cusp attitude. Even if UAPL 23497 were considered a lateral, it would be still distinctive (compare with Fig. 8 ). All other characters deem it to be Orthacanthus donnelljohnsi sp. nov. Some other aspect of heterodonty (dignathic, sexual) might be reflected. Adult vs. juvenile teeth.—Other than size differences, the ontogenetic differences in Orthacanthus donnelljohnsi sp. nov. teeth appear to be minor ( Table 1). From a practical standpoint, the juvenile teeth were difficult to identify until the identities of the other xenacanth taxa in the Swisshelm fauna were established, and because of their unexpected relatively large number. Their mean base length:width (am−pl: l−l) ratios of 0.97 (adult) and 1.09 (juvenile) and linear regression slopes (1.00 and 0.80) and y−intercepts (0.11 mm and 0.15 mm) presumably reflect ontogenetic change, but the differences are not great ( Fig. 6 ).

All of the teeth in Fig. 7 have a thin base, or at least slightly less thick than the adult laterals in Figs. 1 and 5 , which are considered to be thick. This comparison suggests the difference may not be significant, as all of these teeth (adult and juvenile) have a base thickness more comparable to Orthacanthus platypternus than to O. texensis (Johnson 1999, figs. 1A, C; 6, 11). Johnson (1999: 244–245) stated that of 73 O. compressus teeth, 16 had thick bases, of which nine had serrated principal cusps, and the remaining teeth were thin−based, of which two had serrated principal cusps. In that group of teeth, Johnson (1999: 245) stated that some thin−based and thick−based teeth were of similar size, thus precluding the possibility that the former were juvenile teeth. The mean base length:width ratio of those nonsegregated O. compressus teeth is 1.05 with a linear regression slope of 0.97 and y−intercept of 0.03 mm (Johnson 1999; Table 2), not significantly different from O. donnelljohnsi sp. nov. The O. donnelljohnsi juvenile and adult teeth also overlap in size ( Table 1 and Fig. 6 ), but the largest juvenile teeth are smaller than about 80% of the adult teeth (half of which are thin−based, Table 1). Furthermore, posteriors constitute most of the small adult teeth.

Remarks.—Assuming that tooth−base thickness is gradational and of unknown significance, then a lack of other distinguishing features would seem to preclude more than one Orthacanthus species present at the Swisshelm locality. Other factors such as sexual or dignathic dimorphism may be required to account for base thickness versus size during ontogeny.

Hampe (2003: 227) noted that probable juvenile teeth can display considerable intergeneric similarity. For example, Orthacanthus bohemicus juvenile teeth appear to be Xenacanthus −like (see earlier comment regarding Johnson and Thayer 1999; see also Soler−Gijón 2004 regarding juveniles of this species). And O. gibbosus juvenile teeth may possess both serrated and non−serrated cusps (a modification of Hampe 1988, that Orthacanthus juvenile teeth are serrated). He concluded that there is no unambiguous suite of characters that taxonomically segregate xenacanthid teeth. This observation appears to be confirmed by the above discussion, at least in part for Orthacanthus .

The lack of serrations in the Swisshelm Orthacanthus teeth strongly suggests that more than one species was present in Johnson’s (1999) study of O. compressus teeth, which also possess only a single intermediate cusp, except the posterior teeth (Johnson 1999: 248). However, he was not able to delineate more than one taxon (mainly because of the base thickness problem), and had difficulty in later distinguishing some of the O. compressus teeth from those of geologically younger O. texensis and O. platypternus teeth (Johnson 1999: 248; see also Hampe 2003: 210). But Hampe (2003: 205, 209, 227) observed that O. gibbosus juvenile teeth sometimes also lack serrations. As for O. donnelljohnsi sp. nov., there is no doubt that most of the xenacanth teeth in the Swisshelm fauna represent adult individuals.

In the presumed juvenile teeth, differences with adult teeth are probably largely insignificant ( Table 1). Some changes, such as increase in the number of aboral nutrient foramina, may be ontogenetic. The data suggest the same for tooth thickness, but exceptions may preclude this. It would seem reasonable that the change from thin−based to thick−based teeth was ontogenetic, because most of the observed teeth are laterals, which suggests position in the dental arcade is not responsible. But size discrepancies suggest the difference is not ontogenetic. And, as half the adult teeth are thin−based ( Table 1), the possibility of sexual dimorphism or dignathic heterodonty is significant. As with other characters, such as smooth carinae and only a single intermediate cusp, tooth thickness, which is often intermediate or gradational, is not here taxonomically discretionary, unlike the difference between Orthacanthus texensis and O. platypternus teeth.

Orthacanthus teeth from the Lower Permian of Texas (Johnson 1999) are not represented by any that could be morphologically regarded as juvenile, except by size, despite the large number available for study. This difference from the Pennsylvanian species (e.g., O. bohemicus , O. gibbosus , and O. donnelljohnsi sp. nov.) suggests a significant evolutionary change. Orthacanthus donnelljohnsi is unique among the Pennsylvanian species in lacking serrated cusps.

Germinal teeth.—Presumably unerupted teeth, but designated as germinal and generally similar to those from the Lower Permian ( Johnson 2005a, designated therein as underdeveloped), are present in the Swisshelm collection. Six teeth, including UAPL 23493 (three measured adult teeth), 23489 (two adult broken teeth), and one incomplete juvenile tooth included in UAPL 23394, are not fully developed, but not in similar ways. All of the teeth with complete bases are laterals; all have thin bases. The three measured teeth are included in Fig. 6A because of their size. An additional tooth is presumably a juvenile lateral (UAPL 23500) and might be considered as germinal; the principal cusps are compressed, but show no evidence of development of carinae, and the very short but “massive” intermediate cusp is barely developed. Its apical button is normal; the basal tubercle is largely indeterminate, as the aboral surface is missing (wear from transport?).

Germinal (underdeveloped) teeth are here recognized by their lack of cusp development ( Johnson 2005a). The principal cusps tend to be conical and may not be compressed (see? Orthacanthus sp. , UAPL 23400, below); the intermediate cusp may not be developed at all, or is merely a small conical point. Unlike many of the Lower Permian underdeveloped (germinal) teeth described by Johnson (2005a), none of the measured teeth have cusps with exposed pulp cavities, although one of the fragments does. One of the measured adult teeth has a relatively massive apical button, but in another it is completely absent, while in the third it is not fully developed and is comparable to the teeth described as “tooth embryos” by Hampe (1997).

Comparison with other species.—There are many species of Orthacanthus , but only those known to possess a distinct juvenile dentition need be considered. Orthacanthus compressus may indeed possess a juvenile dentition as commented on above [and a preliminary study ( Johnson 2007) of at least one locality in the Texas Permian, that is older than those used by Johnson (1999), tends to support this]. As stated above, only two other species, O. bohemicus and O. gibbosus , possess a juvenile dentition. However, their teeth possess serrated cusps, as does O. compressus . The only species that does not possess serrated cusps is O. platypternus (Johnson 1999) , but it lacks a distinct juvenile dentition. Orthacanthus donnelljohnsi sp. nov. is the only known species of Orthacanthus with a distinct juvenile dentition and whose teeth lack serrated cusps.

Stratigraphic and geographic range.—Lower Pennsylvanian, southeastern Arizona, USA.

Figures

Fig. 1. Diplodoselachid chondrichthyan Orthacanthus donnelljohnsi sp. nov., holotype, adult lateral tooth, UAPL 23384, Lower Pennsylvanian, Black Prince Limestone, Swisshelm Mountains, Arizona; lingual−occlusal (A), labial (B), anteromedial (C), and aboral (D) views. Compare with Hampe (2003: fig. 2).

Fig. 2. Diplodoselachid chondrichthyan Orthacanthus donnelljohnsi sp. nov., adult medial tooth, UAPL 23491, Lower Pennsylvanian, Black Prince Limestone, Swisshelm Mountains, Arizona; lingual−occlusal (A), aboral (B), labial (C), and anterior (D) views.

Fig. 3. Diplodoselachid chondrichthyan Orthacanthus donnelljohnsi sp. nov., adult posterolateral tooth (broken intermediate cusp), UAPL 23388, Lower Pennsylvanian, Black Prince Limestone, Swisshelm Mountains, Arizona; lingual (A), lingual−occlusal (B), labial (C), posterior (D), and aboral (E) views.

Fig. 4. Diplodoselachid chondrichthyan Orthacanthus donnelljohnsi sp. nov., adult posterior tooth, UAPL 23387, Lower Pennsylvanian, Black Prince Limestone, Swisshelm Mountains, Arizona; lingual−occlusal (A) anterior (B), and aboral (C) views.

Fig. 5. Diplodoselachid chondrichthyan Orthacanthus donnelljohnsi sp. nov., adult lateral tooth, UAPL 23383, Lower Pennsylvanian, Black Prince Limestone, Swisshelm Mountains, Arizona; lingual−occlusal (A), aboral (B), and labial (C) views.

Fig. 6. Scatter diagrams of Orthacanthus donnelljohnsi sp. nov. tooth base dimensions; adult teeth (A), and juvenile teeth (B).

Fig. 7. Diplodoselachid chondrichthyan Orthacanthus donnelljohnsi sp. nov., juvenile lateral teeth, Lower Pennsylvanian, Black Prince Limestone, Swisshelm Mountains, Arizona. A. UAPL 23390; lingual−occlusal (A1), labial (A2), aboral (A3), and anteromedial (A4) views. B. UAPL 23391; lingual−occlusal (B1) and aboral (B2) views. C. UAPL 23392; lingual−occlusal view (matrix prevented other views). D. UAPL 23393; lingual−occlusal (D1), (posterior margin of oral surface covered by matrix), labial (D2), anteromedial (D3), and aboral (D4) views.

Fig. 8. Diplodoselachid chondrichthyan Orthacanthus donnelljohnsi sp. nov., juvenile posterolateral tooth (distal 1/4 of major principal cusp is missing), UAPL 23396, Lower Pennsylvanian, Black Prince Limestone, Swisshelm Mountains, Arizona; lingual−occlusal (A, E), anterior (B, F), labial (C, G), and aboral (D, H) views; see text for explanation.

Fig. 9. Diplodoselachid chondrichthyan Orthacanthus donnelljohnsi sp. nov., juvenile?posterolateral tooth, UAPL 23497, Lower Pennsylvanian, Black Prince Limestone, Swisshelm Mountains, Arizona; lingual−occlusal (A), anterior (B), labial (C), and aboral (D) views.