Aetomylaeus sp., Garman, 1908

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 : 125-128

Aetomylaeus sp.

Figs 45–46

Myliobatis dixoni – Clayton et al. 2013 : fig. 4.d–f

Material examined

UNITED STATES OF AMERICA – Alabama • 392 isolated teeth; Claiborne Group ; ALMNH PV1992.28.21, ALMNH PV2000.1.45, ANSP 23425 , MMNS VP-8190, MSC 20988.5 , MSC 33247, MSC 33274, MSC 33280, MSC 33321, MSC 33324, MSC 33329, MSC 33333, MSC 33403, MSC 33421, MSC 33436, MSC 33445, MSC 33476, MSC 33494, MSC 33600, MSC 33653, MSC 33844.1 , MSC 33844.2 , MSC 33844.5 , MSC 33844.7 , MSC 33916, MSC 33931, MSC 33956, MSC 35748 (10 specimens), MSC 37079, MSC 37117 (4 specimens), MSC 37205, MSC 37246, MSC 37293, MSC 37302 (24 specimens), MSC 37403, MSC 37481, MSC 37528, MSC 37910, MSC 38402, MSC 38410, MSC 38778 (8 specimens), MSC 38780 (5 specimens), MSC 38781 (6 specimens), MSC 38782 (5 specimens), MSC 38783, MSC 38789 (8 specimens), MSC 38797 (9 specimens), MSC 38801 (2 specimens), MSC 38807 (5 specimens), MSC 38813 (2 specimens), MSC 38814 (2 specimens), MSC 38819 (4 specimens), MSC 38824 (5 specimens), MSC 38833 (7 specimens), MSC 38836 (3 specimens), MSC 38844 (101 specimens), MSC 38850 (3 specimens), MSC 38851, MSC 38853, MSC 38858 (12 specimens), MSC 38872 (3 specimens), MSC 38882 (35 specimens), MSC 38886, MSC 38942, MSC 38946, MSC 38963 (2 specimens), SC 2012.47.13, SC 2012.47.14 (4 specimens), SC 2012.47.15 (12 specimens), SC 2012.47.185, SC 2012.47.186 (50 specimens), SC 2012.47.187 (3 specimens), SC 2012.47.188, SC 2012.47.189, SC 2012.47.190, SC 2012.47.191, SC 2012.47.192, SC 2012.47.193, SC 2012.47.194, SC 2012.47.195, SC 2012.47.196, WSU 5006 , WSU 5007 , WSU 5011 , WSU 5021 , WSU 5027 .

Description

Median teeth are wider than long; those in upper dentition have a very convex occlusal surface and sinuous to arcuate (convex labially) occlusal outline. Labial and lingual crown foot straight. Teeth in lower dentition with flat occlusal surface; occlusal outline sinuous or arcuate. In labial and lingual views, crown is thickest medially; crown foot is convex and tapers toward the distal margins. Labial crown face of median teeth weakly concave; generally inclined lingually, but at times near vertical. Lingual face with reticulated ornamentation; forms network of fine pits across the surface. Less worn teeth show the ornamentation may have longitudinal component apically (that is obliterated as the crown is worn through in vivo use). Labial crown face weakly convex, usually lingually inclined, may be nearly vertical. Lingual face ornamented with interconnected beaded ridges. Median teeth with rounded, strongly obtuse lateral angles; intersection of labial and lingual parts of the angle located closer to the anterior or posterior crown margin, depending on tooth position. Labial crown foot overhangs the root; lingual face bears thin and sharp transverse lingual ridge at the boundary between the crown and root. Root is low; labial face strongly inclined basiolingually; lingual margin extends beyond the crown foot. Root of median teeth polyaulocorhize, subdivided into numerous thin lamellae.

Upper and lower lateral teeth longer than wide, six-sided, may have a diamond-shaped occlusal outline due to very narrow labial and lingual faces. Ornamentation on vertical faces the same as observed on median teeth. Root generally bisected by a single nutritive groove (occasionally two).

Remarks

This taxon is represented by several partial to nearly complete upper and lower dentitions. MSC 35808 is an upper dentition consisting of nine articulated median teeth, with the remnants of an additional (tenth) tooth preserved on the labial face of the anterior-most tooth. This specimen, as well as all the available upper dentitions, shows that the entire upper dentition was a convex plate. Median teeth in the labial portion of the dentition have a sinuous occlusal outline, whereas the outline becomes more arcuate towards the back of the mouth. Although teeth are thickest medially, the crown tapers laterally to a very short and thin shelf. Although no complete lateral teeth are preserved on specimen MSC 20988.5, a part of a lateral tooth row is preserved on one side. A second lateral row is indicated by distal articular surfaces on teeth in the first lateral row. MSC 33956 consists of two articulated median teeth, both of which have very convex occlusal surfaces, that we interpret as having formed part of an upper dentition.

ALMNH PV2000.1.45 ( Fig. 45 ) is a beautifully preserved lower dentition consisting of 16 articulated median teeth and 12–13 and 11–12 lateral teeth in two rows on each side (starting at the fourth median tooth). The dentition is virtually flat, except for the anterior-most six median teeth, which curve slightly basally. The anterior half of the dentition is heavily worn through in vivo use.

The crown ornamentation occurring on the labial and lingual faces of Aetomylaeus teeth, coupled with the sharply pointed labial crown foot that fit into a furrow just above the lingual transverse ridge, and overlapping root lobes, served to strongly articulate individual teeth. These features are likely what contributed to the relatively large sample of dentitions available for study. These same features can be used to easily separate isolated Aetomylaeus teeth from those of similar genera occurring within Claibornian strata. The combination of labial pitting and lingual tuberculation contrasts with the fine to coarse vertical wrinkling observed on teeth of Aetobatus , Pseudaetobatus , and Leidybatis , and the occlusal surface lacks the pustulose occlusal ornament seen on Leidybatis . Rhinoptera differs in having vertical and flat labial and lingual faces that bear vertical wrinkling, and roots are comparatively much lower and do not extend beyond the lingual crown foot. Aetomylaeus teeth are most similar to those of Myliobatis sp. 2 of the Lisbon Formation and Gosport Sand (see below), but median teeth of the former can be distinguished from the latter in having obtuse, rounded lateral angles, and lateral teeth are diamond-shaped. Additionally, the lingual transverse ridge of Aetobatus teeth is generally very thin and sharp, as opposed to thick and rounded as seen on teeth of the other taxa listed above.

The teeth we herein assign to Aetomylaeus are, we believe, morphologically identical to those previously assigned to Myliobatis dixoni . This conclusion was also reached by Cappetta (2012), who indicated that the species would be more appropriately placed within Aetomylaeus . Hovestadt & Hovestadt-Euler (2013) synonymized numerous fossil species of Myliobatis with Pteromylaeus , a genus placed in synonymy with Aetomylaeus based on phylogenetic analyses ( Naylor et al. 2012; White 2014). Unfortunately, Agassiz (1843) did not include descriptions of the labial or lingual crown ornamentation when naming the dixoni morphology, but Woodward (1888) later commented that the crowns exhibited granular and punctate ornamentation. Although he did not mention which sides of the crown bore these types of ornament, Woodward’s statement conforms with our observations and those of Cappetta (2012) and Hovestadt & Hovestadt-Euler (2013) regarding the labial and lingual ornamentation on Aetomylaeus teeth.

We could not directly compare our Claibornian Aetomylaeus with the dixoni type specimens, and we cannot therefore know if the labial/lingual crown ornamentation is similar enough to warrant placement of the Alabama taxon into this species. Unfortunately, comparisons of the Aetomylaeus teeth occurring within the three Claibornian formations was hampered by the preservation of the material and the limited sample size. The Aetomylaeus teeth within the available samples from the Tallahatta and Lisbon formations and Gosport Sand exhibit some degree of variability, with the same variation being observed among the different formations. We did not discern an appreciable difference in overall morphology or labial/lingual crown ornamentation between the various formations, and there is no indication of anything other than intraspecific variation and longevity of a single Claibornian species.

Stratigraphic and geographic range in Alabama

The specimens in our sample were collected from the Meridian Sand Member of the Tallahatta Formation and lower Tallahatta Formation at site ADl-1, the contact of the Tallahatta and Lisbon formations at sites ACon-6 and ACov-11, the basal Lisbon Formation at site ACov-11, the “upper” Lisbon Formation at site ACl-3, the basal Gosport Sand at site ACl-4, and the Gosport Sand at sites ACh-21 and ACl-15. Upper Ypresian to middle Bartonian, zones NP12 to NP17.

Figures

Fig. 45. Aetomylaeus sp., tooth plates. A–D. ALMNH PV2000.1.45, lower tooth plate, “upper” Lisbon Formation. A. Lateral view. B. Oral view. C. Basal view. D. Close–up of root lamilae. Labial at top in A–D. Scale bar = 2 cm.

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.