According to Cappetta (1987:158), this genus is known from a distinctive and nearly complete rostra as well as fragmentary rostrum and isolated spines. Based on his description and accompanying illustrations, these spines should prove quite distinctive. The spines of Propristis schweinfurthi DAMES 1883, which can reach 3 cm, are flat and roughly as deep as long. The posterior edge is convex as is the upper anterior edge. However, there is a shoulder-like projection 50-60% down the anterior margin, and the basal portion of this edge is concave (where it comes in contact with the next spine). He lists Propristis as reported from the Middle-Upper Eocene and ?Miocene of Africa and the Upper Eocene of Georgia, USA.
The Folmer collection included a large number (18) of small (length < 11.0, depth < 5.0 mm) sawfish rostral spines, which at first glance, had appeared to be saw shark (Pristiophorus). When detailed attention was directed to them, it became apparent that they represented very small (neonate) sawfish spines. Compagno (pers. com. 1999) confirmed this speculation and, after viewing an image of the specimens (Fig. a-d), went on to suggest that fetal teeth may be present as well.
Small rostral spines with a convex posterior basal edge (?Anoxypristis) have not been illustrated (only 4 examples were present, and all lacked enameloid -- deemed to be in poor condition). On the other specimen's posterior edge, the "cusp" was convex and the base, straight (n=8) or concave (n=5). At the time of the writing (1999), the author considered this group to represent a single species (Pristis cf lathami). Those specimens with poorly developed bases probably represent fetal rostrals, and the others being derived from neonates.
In addition to rostral teeth, complete or fragmentary rostra are found. The calcified cartilage, alveoli and internal channels can be clearly seen in the below image of a Castle Hayne (Middle Eocene) specimen.
Nanjemoy Discussion (1999)
Having had the opportunity to review a good number (50+) of pristid rostral spines from Potapaco Bed B, I tender the above Anoxypristis identification with less than full confidence. There are undoubtedly two (or more) pristids in the fauna, and one is Pristis cf lathami. Another taxon is represented by teeth that bear the paleontological identification of Anoxypristis. Part of the difficulty is determining the key characteristics to employ. The "channeled" vs "convex" posterior edge seems simple enough, but the designs bearing convex posteriors when small, tend to become straight (and even weakly concave, Fig. c) as the tooth gets larger. Another specimen
(Fig. c) has all the characteristics of Pristis but the posterior face is flat and not concave.
Compagno (pers. com. 1999) noted that in extant sawfishes, the rostral "teeth" of Pristis tend to be much more deeply rooted than those of Anoxypristis. Applying this insight to the isolated fossil teeth requires a dependence on two unverified conclusions: 1) the portions of the spine imbedded in the socket have no enameloid coating and 2) once emerged, and prior to abrasive wear, the spines bear a thin enameloid covering. Only a few of the studied specimens were in a condition that would reflect these characteristics, and those thought to be Pristis had greater non-enameloid bases than those ascribed to Anoxypristis. Of greater diagnostic value was a coincidental observation; anterior and posterior basal spine edges are roughly parallel in Pristis but flare basally in the other design -- a characteristic that can be recognized in most specimens.
Although the depth to width ratio of the spine's base is relevant, the length has been deemed of little value. The continually growing rostral spines may be lost or broken during life which could allow a rostrum to bear teeth of significantly different sizes when regrowth occurs.
It is the author's current opinion that only a single species of Anoxypristis should be attributed to the Potapaco fauna -- differences are attributable to ontogenetic variations, functional wear and/or spine position. Included within this genus are those spines with a relatively sharp posterior edge and those with a nearly straight to gently concave edge. Anoxypristis spines tend to be more dorso-ventrally compressed (2.75:1) than those of Pristis. The growth bands are quite strong on these rostral spines and may appear ornamented when intersected by the apico-basal ridges.