Aetomylaeus, Garman, 1908

“ Aetomylaeus View in CoL ” sp.

Fig. 19M View Fig –DD

Material examined

UNITED STATES OF AMERICA – Mississippi • 49 isolated teeth; Catahoula Formation ; MMNS VP-12064 (3 teeth), MMNS VP-12065 (4 teeth), MMNS VP-12066 ( Fig. 19R–V View Fig ), SC 2013.28.383 ( Fig. 19W–Z View Fig ), SC 2013.28.384 ( Fig. 19 View Fig AA–DD), SC 2013.28.385 to 28.388 , SC 2013.28.389 (8 teeth), SC 2013.28.390 , SC 2013.28.391 (3 teeth), SC 2013.28.392 (3 teeth), SC 2013.28.393 (20 teeth) • 2 dentitions; Catahoula Formation; MMNS VP-12064.1 ( Fig.19M–N View Fig ), MMNS VP-12065.1 ( Fig. 19O–Q View Fig ).

Description

Teeth in the sample are generally mesio-distally much wider than long (labio-lingually). In occlusal view, the six-sided crown exhibits obtuse lateral angles that are located closer to the lingual margin, and the occlusal outline ranges from straight to slightly arcuate. Other teeth have a four-sided appearance in occlusal view due to the diminutive area of the labial and lingual faces. These teeth are much longer (labio-lingually) than wide (mesio-distally) and have an elongated diamond-shaped outline ( Fig. 19R View Fig ). In profile view, the labial and lingual faces of all teeth are lingually inclined ( Fig. 19U View Fig ). Well-preserved teeth show that the labial face bears a reticulated network of ridges that may transition to vertical ridges near the apex ( Fig. 19Y View Fig , CC). The lingual face is largely tuberculated but bears irregular vertical ridges near the apex ( Fig. 19X View Fig , BB). The labial crown foot overhangs the root, and the basal surface exhibits a longitudinal furrow. A very thin and sharp transverse ridge occurring at the lingual crown foot is inconspicuous. The crown also overhangs the root on the mesial and distal sides, but lingually the root extends well beyond the crown foot. The root is low and may have a straight or convex basal attachment surface, depending on tooth position. In basal view, the root is differentiated into numerous thin, closely spaced, parallel lamellae by nutritive grooves ( Fig. 19Z View Fig , DD).

Remarks

Monognathic, dignathic, and ontogenetic heterodonty in our sample was determined based on the morphological criteria described above for “ Myliobatis ” sp. Specimens MMNS VP-12064.1 ( Fig. 19M– N View Fig ) and 12065.1 ( Fig. 19O–Q View Fig ) show that the lower dentition was flat labio-lingually and mesio-distally. Symphyseal teeth of “ Aetomylaeus ” sp. can be separated from “ Myliobatis ” sp. and “ Rhinoptera ” sp. (see below) by the more uniformly reticulated ornamentation on the labial and lingual faces, lateral angles that are located closer to the lingual crown margin, the very thin and inconspicuous transverse ridge at the lingual crown foot, and a root that extends well beyond the lingual crown foot. Lateral teeth of “ Aetomylaeus ” sp. ( Fig. 19R–V View Fig ) also differ from those of “ Myliobatis ” sp. by their being labio-lingually longer than mesio-distally wide (whereas they are roughly equal in these dimensions in the latter taxon). Except for the ultimate lateral file, lateral teeth of “ Rhinoptera ” sp. are all six-sided. Those from medial lateral positions are roughly hexagonal, whereas the ultimate lateral tooth has a pentagonal occlusal outline. We note that specimens are often worn through in vivo use down to the crown foot. However, even highly worn specimens can be accurately identified based on the nature of the lingual transverse ridge and lingual elongation of the root.

It is difficult to determine the Oligocene geographic and stratigraphic distribution of “ Aetomylaeus ” because: 1) Oligocene fish faunas in the USA are relatively uncommon and 2) Paleogene Myliobatidae teeth generally appear to be (mis)identified as Myliobatis or Rhinoptera simply based on overall shape (see discussion in Ebersole et al. 2019). Cicimurri & Knight (2009) reported a tooth from the Chattian Chandler Bridge Formation of South Carolina (as Myliobatidae gen. indet.) that compares favorably to the Catahoula Formation “ Aetomylaeus ” sp. Ebersole et al. (2021) recently reported similar teeth derived from the Rupelian (NP23) Byram Formation of Alabama that they identified as “ Aetomylaeus ” sp. As with “ Myliobatis ” sp., the teeth in our sample likely represent an undescribed Oligocene member of the extant Aetomylaeus lineage and are therefore referred to herein as “ Aetomylaeus ” sp.