"Myliobatis" sulcidens DARTEVELLE & CASIER, 1943

When given close attention, teeth attributed to this species are quite distinct. The transverse depression of the occlusal face, when combined with an otherwise rhinopterid-like tooth, quickly alerts one to its presence in the fauna. They clearly represent another genus, the only true question being — what family?

It is unknown what a full mouthplate might look like, but it would appear to form a pavement of laterals and transversely expanded medials. The lingual face is covered with fine enameloid ridges suggesting that the teeth interlocked. A clear 'ledge' extends laterally across the base of the crown, above which, there is short depression of the lingual face. The labial face is similarly ornamented, roughly upright and with a slight forward projection at its base. The root lobes are short and do not extend beyond the lingual margin of the crown.

Cappetta (1987) includes this species in the Palaeocene and Lower Eocene of Africa. The author has found no reference of its presence in North America, but the illustrated specimen clearly suggests differently.

"Myliobatis" toliapicus AGASSIZ, 1843

The dentition has a laterally elongated medial tooth (6.5:1 ratio) and multiple lateral files. The example in figure shows two left hand laterals plus at least one (termino-lateral) missing file. Viewed occlusally, the lateral teeth are 6-sided and the lateral angle is centered. The teeth are weakly sutured, often imperceptibly on isolated (worn) teeth (see Fig ). The labial & lingual crown faces are vertical and bear baso-apical ridges. The lingual shelf is short but prominent.

Kemp et al (1990) include this species in the Eocene of England and they appear to be relatively common in Potapaco Bed B of the Nanjemoy.

"Myliobatis" latidens WOODWARD, 1888

The "M." latidens tooth design appears in many Eocene faunas of the Western Atlantic. The dentition has a laterally elongated medial tooth (6:1 ratio) and multiple lateral files. The example in Figure shows two lateral files, but it is uncertain if a termino-lateral is missing. Viewed occlusally, the lateral teeth are commonly 4-sided (Fig. & ), but may have more (Fig. ). It is arguable, that those "M." latidens-like dentitions which have lateral teeth with more than four sides should be deemed a separate species, but the evidence is currently inconclusive.

Continuing to view occlusally, the lateral angle is positioned somewhat lingually in lower teeth and more clearly so in the upper plate. The teeth are strongly sutured. The labial and lingual crown faces have a rhinopterid-like mortise & tendon-design wherein the lower labial face projects to join with a transverse depression in the lower lingual face. The labial & lingual crown faces bear baso-apical ridges. The lingual shelf is short and the roots extend well beyond the lingual face of the crown, a myliobatid not rhinopterid characteristic.

Kemp et al (1990) include this species in the Eocene of England. Kent (1999) includes this species in Potapaco Bed "B" of the Nanjemoy (Ypresian, VA) fauna. It is important to note, that Kent lists both "M." striatus BUCKLAND, 1837 and "M." latidens as present in these sediments. Based on the images included in Kent, both of these species are currently deemed to be "M." latidens by this author. These are the most common myliobatid teeth in Potapaco Bed "B".

"Myliobatis" sp

Teeth of this design are probably the most commom in the Piscataway Member of the Aquia Formation. They appear relatively similar to those of "M" latidens, however the dentitions reflect two large differences: 1) the lower medials are curved in this species and 2) the lateral angles are centered in latidens and positioned posteriorly in this species. These tooth plates (at least the lowers) have three lateral files (per side), see Fig. .

"Myliobatis" dixoni AGASSIZ, 1843

As can be seen by the below dentitions, the medial teeth are relatively deep (ratio 4:1) and the laterals (at least 2 files per side) labio-lingually elongated. The crown faces of the teeth bear baso-apical enameloid ridges. Viewed occlusally, the marginal angle of the medial's crown is anterior of center and inter-crown suturing is evident.

Teeth of this design are common in the Piscataway Member of the Aquia (Upper Palaeocene) but not particularly common (scarce when compared to "M." latidens) in Potapaco Bed B of the Nanjemoy (Lower Eocene). In these sediments, there appears to be a greater degree of variability in the depth:width ratio of the medial teeth then in the other myliobatids. It is possible another species is present, but it would appear unlikely.

Cappetta (1987) includes this species in the Lower Eocene of England and Palaeocene and Eocene of Africa. Ward & Wiest (1990) included this species in Brightseat (Danian) through Piney Point (Lutetian) sediments of Maryland and Virginia. Case (1994) in the Eocene of Mississippi and Kent (1999) in Potapaco Bed "B" of the Nanjemoy (Ypresian, VA).

Aetobatus irregularis (AGASSIZ, 1843)

Extant Aetobatus teeth are characterized by roots that extend well beyond the lingual margin of the crown (less so in the Paleogene species) and a straight lateral margin, resulting from the absence of lateral files. In the extant species, A. narinari there is strong dignathic heterodonty, the upper teeth are significantly broader and the lowers more acutely curved, giving the tooth plate a chevron-like appearance.

Cappetta (1987) noted two fossil species: A. irregularis from the Palaeocene of Africa, Eocene of Africa, Europe & Maryland, and A. arcuatus (AGASSIZ, 1843) from the Miocene of Europe & the Atlantic Coastal Plain of the US. Ward & Wiest (1990) included A. irregularis in Woodstock Mbr of Nanjemoy (Ypresian) and Piney Point (Lutetian) sediments of Maryland and Virginia. Kent (1999) includes this species in the Potapaco Bed B fauna, but his illustrated example has a marginal angle suggesting the presence of a lateral tooth file, therefore it would not appear to be A. irregularis.

Although a regular component of other Eocene faunas, the author has been unable to locate a specimen originating from Potapaco Bed B sediments. The earliest noted Nanjemoy specimens (uncommon) are from Potapaco Bed C.

"Myliobatis" nzadinensis DARTEVELLE & CASIER 1943

This species, originally described from the Palaeocene of Cabinda (Western Africa), was originally (1999-2000) thought by elasmo.com to be represented in the Paleogene of the Chesapeake Bay region by a somewhat similar tooth-design (see Fig and below). With better access to comparative material, it was decided in 2007 that the differences out-weighed the similarities and the association terminated; "M." nzadinensis appears to be much closer to the "M." dixoni-design..

Undescribed myliobatoid

Originally considered by this website to be somewhat similar to the "Myliobatis" nzadinensis tooth-design, there are a number of characteristics that strongly argue against this association:

Following are the comments specific to teeth recovered from the Nanjemoy sediments of Potapaco Bed B and written prior to reading Cappetta (1986).

Making identification difficult is the condition of complete specimens. To the author's knowledge, no complete (or partial) plates have been found; leaving only shed teeth to be evaluated. By virtue of its grinding nature and strong interlocking, the teeth of this species show extreme occlusal wear (including the labial face). Fortunately, the Folmer collection includes not only what appear to be lateral teeth but fragmentary medials as well. Using a complete specimen (Fig. a), a fragmentary medial (Fig. ) and an isolated lateral (Fig. ), a reconstruction was made of the general tooth-design (Fig. b).

The few specimens available for study have a relatively level lingual crown base and roots that are deeper medially. It is currently unknown if these represent upper teeth only, or if both dentitions have similar characteristics. The upper root bears low, broad and short meandering ridges on the lingual and lateral faces. The upper crown face is ornamented with small enameloid projections, undoubtedly associated with interlocking. [Cappetta's description notes that when viewed occlusally, the upper teeth are rectilinear and the lowers curved (convex labially). Viewed labially, the crown of the uppers is deeper medially and the root lobes of equal size. In the lowers, the occlusal surface is relatively flat and the lobes higher medially.]

The large isolated lateral tooth in Figure has dramatically deep and lingually directed roots. The occlusal profile, lateral face of the crown & more strikingly the roots, compare well with those of the medial teeth in figure and .

Nanjemoy Potapaco Bed B uncertain

In addition to the more common myliobatid teeth, teeth occasionally appear in the fauna that remain unidentified. A primary example would be the partial plate in Figure , outwardly similar to "Myliobatis" dixoni, however the upper marginal faces of the crown are nearly smooth (no ridges) and no suturing appears when viewed occlusally. It is evident that the dentition includes lateral files, but the possible count is unknown.

Another of the mystery myliobatid teeth is illustrated in Figure . Kent (1999) deemed this tooth-design to be Aetomylaeus. The dentition of the fossil species A. meridionalis includes no teeth of this type, but the genus includes three or four extant species, and the author is not familiar with most.

These teeth are often small with low width:depth ratios, and usually quite worn. When casually viewed the roots look pointed from an occlusal or lingual perspective. Actually, two similar designs are present in the fauna. The more common has smooth labial and lingual crown faces and a centered marginal angle. The other has small projections (interlocking) on the crown faces, and the marginal angle is off set.

Based on the normally small size, I originally suspected them to represent neonate/juvenile examples of "Myliobatis" dixoni. teeth. However this was proved incorrect when Mike McCloskey provided a 25 mm (3:1) and Daryl Serafin a 36 mm (4:1) specimen.

Acknowledgement

Although the Eocene teeth and mouthplates could be grouped by odontological design, placing them in described taxa required an indepth knowledge of these species. I need to thank David Ward for taking the time to confirm the identificaton of "Myliobatis" latidens and providing the proper identifications for the specimens presented above as "M." sulcidens, "M." toliapicus, "M." dixoni and cf "M." nzadinensis. Henri Cappetta was kind enough to provide additional details on Pseudaetobatus and respond to a cross-examination on various details.