Pteroplatytrygon violacea
 
This is a small (to 80 cm/32 inches disc width) pelagic ray of worldwide distribution in tropical and temperate waters. It feeds on squid, herring, mackerel, octopus, and crustaceans. The disc is trapezoidal in shape, purple in color and bears thorns on the back and tail.

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.

NOTE: The discussion of this particular dentition is based on teeth in-situ, it was not torn apart to properly measure and study the teeth. Various teeth in the process of being shed were removed and measured, but most measurements could only be made directly from the dentition using a millimeter-scaled ruler and a loupe. Although the authors feels these measurements are useable, they may not be as accurate as implied by their notation in tenths of a millimeter.

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 tooth 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).

Pteroplatytrygon (Dasyatis) violacea

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 When viewing the teeth as a dentition, the files appear to be segregated into two groups, medial and lateral. (Please note that there will be a further refinement when discussing the teeth themselves.)

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 count includes a single seed or germ tooth and a 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.

Tooth Characteristics

There appear to be several morphological constants in these teeth - many are characteristic to the family, others are less common.

• All non-pathological teeth have bilobate, lingually directed roots with a single central foramen. The teeth have not been stained or coated to enhance details, but lateral foramina, on the interior surface of the lobes, have not been observed. A second foramen in the nutrient groove has not been noted although at least one central pore appeared to be made up of two adjoining foramina.
• The lateral profile of most cuspidate teeth are of typical dasyatid design. Some cuspidate teeth have an unusual feature: they are cuspidate but the slope of the lingual face is labially oriented -- a cuspidate transverse crest.
• The labial face of all teeth have a consistent characteristic, there are narrow, closely-spaced, intersecting enameloid ridges running from the labial visor to the apex of the cusp. This creates a pitted appearance, which becomes slightly more elongated apically.
• In most teeth, there is a transverse depression on the labial face. In cuspidate teeth, it is often weak and located between the visor and cusp portions of the crown. In posterior and some lateral teeth, it is often very deep, helping to create the prominent transverse crest. Most teeth reflect longitudinal grooves on the upper lingual face in the area of the transverse ridge.
• These teeth often have a notched lingual rim which gives the appearance of a two uvula, one above each root lobe. This is likely a result of the dense packing of these teeth.

General Description

Male and female pelagic stingray teeth consist of a bilobate root and a cuspidate crown. The teeth are small, ranging in width (mesio-distal) from 2 to 4 mm and depth (labio-lingual) from 1 to 2-1/2 mm. A moderately deep and wide nutrient groove is present between the two, lingually directed, lobes. There is a single central pore which usually contains a single foramen. Lateral foramina, on the interior surface of the lobes, have not been observed. The root lobes do not extend beyond the lateral margins of the crown.

Anterior and lateral teeth of both sexes have cuspidate crowns, in contrast to the typical globular design associated with most stingrays. The medial teeth have strong, lingually directed cusps, which are similar to the "typical" high-cusped male stingray tooth -- such as those from Dasyatis americana HILDEBRAND & SCHROEDER, 1928, D. sayi (LESUER, 1817) and D. sabina (LESUER, 1824). Unlike the cited examples, the labial face does not have strong enameloid ridges or a medial depression running the length of the cusp. In lateral teeth, the cusp is weaker and more upright. In lieu of a cusp, the posterior teeth have a prominent, laterally elongated, transverse crest which adjoins a strong transverse depression on the labial face. On the lingual face, the transverse crest has longitudinal ridges. Medial and lateral teeth have a weak depression on the labial face which is located at the juncture of the crown base and the cusp.

The labial face of all teeth have a pitted appearance which results from the depressions created by numerous small meandering enameloid ridges. No teeth have a "cutting edge" as in many shark or skate teeth. Rather, the acute angle of the transverse edge of the cusp and transverse crest have a serrate edge created by the numerous enameloid ridges.

As a dasyatid, there is a unique feature represented in these teeth -- the development of a second cusp. An incipient reflection of this characteristic can be seen in many teeth, usually as a longitudinal depression in the lingual face of the crown. Eight percent of the teeth, originating in medial and lateral positions, have a second cusp. These bicusped teeth appear sporadically within the dentition, and are present in a file of teeth which are otherwise single-cusped. Unlike the mobulids, the other family developing additional cusps, this bicuspidate condition in the pelagic stingray is not associated with the mesio-distal elongation of the crown.

High-Cusped Teeth (Anterior)

Several distinct types of strongly cusped teeth can be found in the sample. Because observations by MBA revealed that the female stingrays had much larger teeth than the males, it seemed safe to differentiate these teeth by sex.

Female Anterior hgt = 3.0, wid = 3.7, dep = 2.6 mm.

Female Anterior hgt = 3.0, wid = 3.7, dep = 2.6 mm

Male anterior hgt = 2.6, wid = 2.3, dep = 1.9 mm

Male anterior. hgt = 1.9, wid = 2.1, dep = 1.6 mm

Low-Cusped Teeth (Lateral)

When viewed laterally, these teeth have a sharp apical surface, but there is no elongated cusp directed lingually. Viewed labially or cuspally, the crown has an angular (pointed) apex which provides a cusp-like appearance.

Female lateral hgt = 2.8, wid = 3.5, dep = 2.2 mm

Lateral hgt = 1.8, wid = 2.5, dep = 2.1 mm

Non-Cuspidate Teeth (Posterior)

These teeth when viewed laterally have a relatively sharp apical angle, but when viewed from a labial aspect, have little or no acute apical angle, but rather an elongated apical edge.

Posterior. hgt = 2.1, wid = 2.5, dep = 2.2 mm

Posterior. hgt = 1.5, wid = 2.1, dep = 1.6 mm

Bi-cuspidate Teeth

An unexpected morphological condition in this study was the presence of a second cusp in the teeth of this species.

Bi-cusped Tooth hgt = 2.1, wid = 3.2, dep = 2.0 mm