Leidybatis jugosus ( Leidy, 1876 )

Ebersole, Jun A., Cicimurri, David J. & Stringer, Gary L., 2019, Taxonomy and biostratigraphy of the elasmobranchs and bony fishes (Chondrichthyes and Osteichthyes) of the lower-to-middle Eocene (Ypresian to Bartonian) Claiborne Group in Alabama, USA, including an analysis of otoliths, European Journal of Taxonomy 585, pp. 1-274 : 140-144

Leidybatis jugosus ( Leidy, 1876)

Fig. 52

Myliobates jugosus Leidy, 1876, p. 86 .

Myliobates jugosus – Leidy 1877: 240 , pl. 31, figs 4–5.

Myliobatis cf. jugosus – Leriche 1905: 182 , pl. 52.

Myliobatis jugosus – Leriche 1942: 26 .

Leidybatis jugosus – Cappetta 1986: 189 , pl. 4c. — Van den Eeckhaut & De Schutter 2009: 22, figs 5–7. Leidybatis cf. jugosus – Clayton et al. 2013: 64 , pl. 4a–c.

Material examined

UNITED STATES OF AMERICA – Alabama • 71 isolated teeth; Claiborne Group ; ALMNH PV1992.28.21, ALMNH PV1992.28.6.3, MMNS VP-5643 (2 specimens), MMNS VP-8189 (6 specimens), MMNS VP-8224 (5 specimens), MSC 35005, MSC 37080, MSC 37081, MSC 37082 (4 specimens), MSC 37083, MSC 37084, MSC 37085, MSC 37086, MSC 37135, MSC 37135, MSC 37249 (2 specimens), MSC 37256, MSC 37257 (3 specimens), MSC 37282, MSC 37291, MSC 37321 (3 specimens), MSC 37681, MSC 38829 (3 specimens), MSC 38876 (2 specimens), MSC 38948, NJSM 24033 , SC 2012.47.16, SC 2012.47.17 (2 specimens), SC 2012.47.197 (3 specimens), SC 2012.47.198, SC 2012.47.199, SC 2012.47.200, SC 2012.47.201 (2 specimens), SC 2012.47.202, SC 2012.47.203 (2 specimens), SC 2012.47.204, WSU 18 , WSU 30 , WSU CC 513 (2 specimens), WSU CC 522 (2 specimens), WSU 5009 (3 specimens) .

Description

Median teeth very wide and narrow. All median teeth with distinctive swelling on the crown; swelling generally located medially but can be closer to the lateral margin (resulting in an asymmetrical labial/ lingual outline). Swelling is in contrast to remainder of crown, which is dorsoventrally very thin. Occlusal surface has thick enameloid covering that is heavily tuberculated on unworn parts of tooth; surface is punctate where enameloid is missing, due to in vivo use or taphonomic processes (in vivo use is indicated by flat wear facets that on the highly convex portion of the crown of some teeth). Labial crown face overhangs the root; lingual face with thick and rounded transverse ridge at the crown base. Labial and lingual crown faces possess coarse parallel vertical folds that anastomose and become finer apically. Distal ends of crown angular. Root at least as high as thinnest part of crown; is subdivided into thin lamellae by evenly spaced nutritive grooves. Lamellae do not extend lingually past the crown. Lateral teeth are six-sided, nearly symmetrical. Lateral teeth with same tuberculation and wrinkling as observed on median teeth, but root subdivided into only two or three lamellae.

Remarks

Ascribing the teeth described above to a particular species is complicated by the fact that fossil Myliobatidae are often based on a single median tooth, and our knowledge of how the tooth was incorporated into the upper or lower dentition is far from complete. Whole and even partial upper and lower dentitions are more informative, but as Hovestadt & Hovestadt-Euler (2013) have recently shown, there can be a great deal of intraspecific variation within the dentitions of extant species. Several fossil species based on isolated median teeth, including Myliobatis siculus Salinas, 1901 , M. pachyrhizodus Fowler, 1911 , M. tumidens Woodward, 1889 , and M. jugosus Leidy, 1876 , are very convex, but this phenomenon can largely be attributed to the teeth being from the upper dentition (i.e., see description for Aetomylaeus above). Leidy’s (1876) taxon, the jugosus morphology, has unusually convex median teeth like the ones from the Lisbon Formation we described above. Cappetta (1986) erected the genus Leidybatis to include the jugosus morphology, as well as his new species, L. granulosus . Mendiola (1999) named several species of Leidybatis , but Cappetta (2012) considered those species as a nomina dubia because they lacked stratigraphic context. Hovestadt & Hovestadt-Euler (2013) indicated that the various tooth morphologies attributed to Leidybatis fall within the range they observed in extant species of Aetomylaeus and Myliobatis , and they considered L. jugosus as a nomen dubium. However, Leidybatis possess labial and lingual ornamentation of parallel vertical folds, which contrasts with the pitting and granulation seen on these areas of Aetomylaeus teeth ( Leidy 1877; Cappetta 1986; Mendiola 1999; Cappetta 2012; Hovestadt & Hovestadt-Euler 2013). In addition, the labial root face is vertical on Leidybatis teeth and root lobes do not extend lingually past the crown base. This contrasts with the roots of Myliobatis and Aetomylaeus , which have oblique labial faces and the lobes very conspicuously extend past the lingual crown face.

The root characteristics of Leidybatis are similar to the condition seen on Rhinoptera teeth, and specimens illustrated by Cappetta (1986: fig. 3, 1–8) indicate that L. granulosus had lateral teeth of decreasing width like Rhinoptera . In support of this is a Leidybatis lateral tooth morphology that is comparable to those within more proximal lateral rows of a Rhinoptera dentition. SC2012.47.202 ( Fig. 52 SS–TT) is twice as wide as long and is similar to Rhinoptera lateral teeth in our Claiborne sample. We assign this tooth to Leidybatis based on the distinctly ornamented occlusal surface and lingual and labial crown ornamentation that is identical to that on the median teeth described above.

We herein support the validity of Leidybatis based on our samples of Myliobatinae and Rhinopterinae teeth from the Tallahatta and Lisbon formations. As mentioned above, the labial and lingual vertical wrinkling on Leidybatis teeth contrasts with the ornamentation we observed on Aetomylaeus teeth from both formations. If one considered the Leidybatis morphology aberrant, as do Hovestadt & Hovestadt- Euler (2013), the labial and lingual ornamentation of teeth should still serve as a generic identifier. As an example, we examined a partial Aetomylaeus median tooth (MSC 37079, Fig. 46 EE–II) that exhibits a convex medial portion but thinner lateral side, similar to Leidybatis . The vertical wrinkling seen on Pseudaetobatus (Tallahatta Formation) and Aetobatus (Lisbon Formation) is much weaker than that of Leidybatis , and the root lobes of the former two taxa conspicuously extend lingually past the crown. Lastly, a conspicuous feature of Leidybatis teeth is the thick enameloid covering on the crown, which is tuberculated on the occlusal surface. This contrasts with the smooth enameloid occurring on all other Myliobatinae and Rhinopterinae teeth within Claibornian strata.

Within the species of Leidybatis , the crowns of L. granulosus , L. rusticus , and L. zemensis appear to be more uniformly convex than L. jugosus . MSC 35005 ( Fig. 52 A–E) is virtually identical to the type specimen illustrated by Leidy (1877: pl. 31, figs 4–5) and Hovestadt & Hovestadt-Euler (2013: pl. 49, fig. 19), and it compares closely with a specimen from the middle Eocene of western Africa shown by Cappetta (1986: pl. 2, fig. 4). Leidy (1877) made no mention of the tuberculated ornamentation we observed on our Claiborne specimens, and he attributed the “dull” surface and thinness of the lateral parts of the crown to in vivo usage. The apparent lack of ornament on Leidy’s illustrated specimen could be the result of wear or poor representation by the illustrator, but our Claiborne specimens show that the thinness of the sides of the crown reflects the natural morphology of the tooth. Four specimens within our sample indicate a degree of heterodonty within the median row of Leidybatis dentitions. MSC 37082 ( Fig. 52 T–X) has a broad but very low convexity that is skewed to one side of the tooth. MSC 37086 ( Fig. 52 K–O) has a more convex swelling that is also offset to one side of the crown, and the shoulders are more elongated than on MSC 37082. MSC 37083 ( Fig. 52 NN–RR) is a smaller tooth with high, medially located swelling and short shoulders, whereas MSC 35005 (a much larger tooth, Fig. 52 A–E) has a very high medial swelling and elongated shoulders. Unfortunately, we lack dentitions and cannot accurately distinguish upper from lower teeth, but we believe the Lisbon sample represents intraspecific variation within a single species. There is currently no evidence to suggest that the two tooth fragments from the Tallahatta Formation are not conspecific with L. jugosus .

Stratigraphic and geographic range in Alabama

Two tooth fragments were collected from the lower Tallahatta Formation at site ADl-1. The remaining sample is from the contact of the Tallahatta and Lisbon formations at sites ACh-14 and ACov-11, and the basal Lisbon Formation at site ACov-11. Upper Ypresian to middle Lutetian, zones NP14 and NP15.

Figures

Fig. 52 (opposite page). Leidybatis jugosus (Leidy, 1876), teeth. A–E. MSC 35005, median tooth, Tallahatta/Lisbon formation contact zone. A. Profile view. B. Labial view. C. Lingual view. D. Oral view. E. Basal view. F–J. MSC 37080, lateral tooth, Tallahatta/Lisbon formation contact zone, courtesy of Bruce Relihan. F. Labial view. G. Profile view. H. Lingual view. I. Oral view. J. Basal view. K–O. MSC 37086, median tooth, Tallahatta/Lisbon formation contact zone, courtesy of James Lowery. K. Profile view. L. Labial view. M. Lingual view. N. Oral view. O. Basal view. P–S. MSC 37291, lateral tooth, basal Lisbon Formation. P. Profile view. Q. Lingual view. R. Oral view. S. Basal view. T–X. MSC 37082, median tooth, Tallahatta/Lisbon formation contact zone, courtesy of Bruce Relihan. T. Profile view. U. Labial view. V. Lingual view. W. Oral view. X. Basal view. Y–CC. MSC 37081, lateral tooth, Tallahatta/Lisbon formation contact zone, courtesy of Bruce Relihan. Y. Lingual view. Z. Profile view. AA. Labial view. BB. Oral view. CC. Basal view. DD–HH. SC2012.47.198, median tooth, basal Lisbon Formation. DD. Profile view. EE. Labial view. FF. Lingual view. GG. Oral view. HH. Basal view. II– MM. MSC 37135, lateral tooth, Tallahatta/Lisbon formation contact zone, courtesy of James Lowery. II. Labial view. JJ. Profile view. KK. Lingual view. LL. Oral view. MM. Basal view. NN–RR. MSC 37083, median tooth, Tallahatta/Lisbon formation contact zone, courtesy of Bruce Relihan. NN. Profile view. OO. Labial view. PP. Lingual view. QQ. Oral view. RR. Basal view. SS–TT. SC2012.47.202, lateral tooth, basal Lisbon Formation. SS. Oral view. TT. Basal view. Labial at top in oral and basal views. Scale bars: A–E, K–O, T–X, DD–HH, NN–RR =1 cm; F–J, P–S, Y–CC, II–MM, SS–TT=5 mm.

Fig. 46. Aetomylaeus sp., teeth. A–E. MSC 38836, lower median tooth, “upper” Lisbon Formation. A. Oral view. B. Basal view. C. Lingual view. D. Labial view. E. Profile view. F–J. MSC 199.2, median tooth, basal Gosport Sand. F. Oral view. G. Basal view. H. Lingual view. I. Labial view. J. Profile view. K–O. MSC 199.1, median tooth, basal Gosport Sand. K. Oral view. L. Basal view. M. Lingual view. N. Labial view. O. Profile view. P–T. MSC 38872, lower median tooth, Gosport Sand. P. Oral view. Q. Basal view. R. Lingual view. S. Labial view. T. Profile view. U–Y. MSC 37910, lateral tooth, Tallahatta/Lisbon formation contact zone, courtesy of James Lowery. U. Oral view. V. Basal view. W. Mesial view. X. Distal view. Y. Labial view. Z–DD. MSC 38851, lateral tooth, basal Gosport Sand. Z. Oral view. AA. Basal view. BB. Mesial view. CC. Distal view. DD. Labial view. EE–II. MSC 37079, upper median tooth, Tallahatta/Lisbon formation contact zone, courtesy of Bruce Relihan. EE. Oral view. FF. Basal view. GG. Lingual view. HH. Labial view. II. Profile view. Labial at the top in oral or basal views of the median teeth. Scale bars = 5 mm.