EXTANT BATOIDS

Pelagic Stingray Dentition

Jim Bourdon, Scott Greenwald and Henry Mollet
Illustrated by Jim Bourdon, Copyright © 1997

Objectives

One of the original parameters of the pelagic stingray study was that it should have as little impact on the captive rays as possible. It was evident early that teeth could be obtained in quantity, however educated quesses as to the sex and tooth position, acceptable in paleontology, was not deemed an appropriate method for an extant species. An early priority was to obtain a male and female dentition. They would provide the necessary benchmark to:

A Starting Point

Nishida and Nakaya (1990) provided an illustration and tooth counts for this species. The cuspidate teeth appear to form a clutching/grasping dentition. The male dentition depicted high-cusped teeth in the medial positions gradually diminishing distally. As illustrated, the lower teeth appear to have a broader cusp than the lower. The female dentition is unusually cuspidate, compared with other stingrays, which is likely an adaptation to its pelagic way of life and feeding habits. The Nishida and Nakaya (1990) illustration shows broad medium-cusped teeth in the medial files growing less cuspidate distally.

Although not determinable from the illustrations, Nishida and Nakaya provide tooth (file) counts for these dentitions. Based on the examination of eight specimens (6 M, 2 F), they provide the range of 31-39 upper and 33-37 lower.

A Studied Dentition

The illustrated dentition is from a 47 inch (total length) ray caught by fishermen in deep waters off Santa Rosa Island, California on October 5th, 1996. Although not positively identified at the time of capture, the ray was described as having a dark purple back with some light spots and a light purple belly. They went on to note that it had rounded tips, a recurved back, a long tail and a Bat ray-looking head. The overall description, time and location of capture, long thin stinging barb (also retained) and the teeth themselves leave little doubt that it was a pelagic stingray.

DISCLAIMER. This section contains preliminary observations of this removed jaw set. There was no desire at this point to "tear apart" the dentition to properly measure and study the teeth from the dentition. Various teeth in the process of being shed were removed and measured. Most measurements could only be made directly from the dentition using a millimeter-scaled ruler and a loop. Although the authors feels these measurements are useable, they may not be as accurate as implied by their notation in tenths of a millimeter.
Dasyatis violacea
probably female, overall width 77 mm

The Jaws

In this monognathic heterodont dentition, all teeth could be said to be cuspidate. The higher-cuspsed teeth are in the medial position, growing lower distally. Although upper and lower teeth are very similar, they appear to be distinguishable in lateral profile.

As illustrated, the jaws are 77 mm wide and 58 mm high, the internal width (narrowest) is 48 mm. The upper dental band is 51 mm wide and the lower, 44 mm. The teeth in this dentition compare favorably with a female specimen at the Monterey Bay Aquarium.

Tooth design

The dental bands from this species are unlike those of most stingrays. Not only do both sexes have cuspidate teeth, but there are no crushing teeth in the lateral files.

The medial teeth have a traditional dasyatid (male) cuspidate (grasping) crown which is derived from an elongated cusp, rising from the lower crown and directed lingually. Viewed labially, these teeth have a distinctly pointed crown.

The lateral teeth, however, do not have the globular crown of a crushing dentition as is present in most stingrays. The labio-lingually compressed upper regions of the crown has produced a high, usually (meso-distally) elongated, transverse crest -- a cutting dentition. The crest is enhanced by enameloid ridges which would produce a serrate-like cutting edge. Certain anterior lateral teeth may be cuspidate when viewed labially.

Tooth Files

Dental Count Upper 17-1-17 (34 files); Lower 19-S-19 (39 files).
D. (Pteroplatytrygon) violacea, Upper dentition
Files: right 7 -- left 9, Series: 2 - 5, illustrated width 26 mm, labial to top

Cusps. For upper teeth, files #1 - 3/4 are high-cusped, files #4/5 - 7 low-cusped. In the lower teeth, files #S - 1 are high-cusped, 2-7 are low-cusped. The remaining files for each dental band, have a high transverse ridge which begin with a cusp-like apical angle in position #8 and gradually decrease in height distally.

File Groups For the purposes of this report, these files have been segregated into two groups, medial and lateral.

Medial teeth [files S(1) - 7]. In this dentition, these are the cusp bearing crowns and are of similar size (2.9 - 3.0 mm in width and about 2 mm in depth) although the symphyseal and file #1 are slightly narrower (laterally compressed).

Lateral teeth. In the upper dental band, U8 - 11 are the broadest teeth of the dentition, measuring to 3.3 mm in width. Beginning with U12 (2.9 mm) the teeth diminish in width to U17 (2.0 mm). In the lower band, L8 - L9 are 2.6 - 2.8 mm in width, U10 (2.4 mm) and continuing to narrow to U19 (1.8/2.0 mm).

Although few teeth could be measured properly, several removed teeth show a significant labio-lingual compression of the lateral teeth distally. Selected width x depth measurements include: UL5: 3.0 x 1.8, UR10: 3.3 x 1.9, UR15: 2.4 x 1.5 and LL5: 3.0 x 2.1, LR10: 2.4 x 1.8 and LR15: 2.2 x 1.3 mm.

Tooth Series

The series tooth count includes a single seed or germ tooth and to-be-shed tooth. Upper file #1 = 9, file #9 = 8, file #17 = 7 teeth and Lower file #1 = 9, file #9 = 9, file # 19 = 7 teeth.

Functional Teeth. In this dentition, the "first" (labial most) upper series is in the process of being shed, and the second appears functional. In the lower teeth, the second series appears worn and the third appears to be the primary functional series.

Tooth Wear: The series being shed showed no evidence of the cusps which were visible on the functional and replacement teeth. [A fossil tooth would likely not reveal the cuspidate design of these teeth.]

Bicuspidate Several teeth show evidence of a bicuspidate condition. In this dentition they were relegated to files 4 - 8. This condition was noted in replacement series (as high has series 7) suggesting this is not a wear condition but an odontological characteristic of the species.

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Page revised May 26, 1997