Certain facets of phyletic tooth development in some species of sharks seem to be mirrored in their ontogenic development. One such example was given in Hubbell (1996) with his observation that some juvenile teeth of Carcharodon carcharias are similar in appearance to those of the late Paleocene shark Palaeocarcharodon orientalis (p. 13). Ward and Bonavia (2001) outline phyletic changes, which are copied by ontogenic changes in Carcharocles teeth between the Eocene and Pliocene. This study attempts to identify a phyletic change in teeth of Hemipristis serra, which is almost certainly an ontogenic change as well. Thus, care must be taken to limit the use of juvenile specimens because they might express the earlier pattern.

Compagno (1988, pp. 269-270) has observed that in teeth of young Hemipristis elongata (the extant species, see Figure 4):

"...a few distal cusplets are present on upper laterals (five on fifth upper laterals of a 532 mm. specimen) but these become more numerous on adults and subadults (ten or more on fifth upper lateral) and turn into coarse serrations."

Assuming the teeth of all Hemipristis species behaved similarly, Compagno's (1988) description provides a means for distinguishing immature teeth. Typical features include small size, a few large distal "cusplets" (serrations), reduced or no mesial serrations, and a long unserrated tip.

Teeth of the earliest species, Hemipristis curvatus, might be considered as a template of the immature form. As can be seen in Figure 5, the H. curvatus tooth (label 5.1) greatly resembles Compagno's (1988) immature specimen in Figure 4. Examining the scans produced very few examples of what could be considered immature teeth from the other samples. Figure 5 illustrates examples of Santee, Belgrade, Pungo Reject, and Yorktown immature-form teeth. There are no good candidates from among the Pungo Float, Calvert, or Recent samples.

Among the four samples used for statistical modeling, teeth from Belgrade were the smallest and had the fewest serrations. However, very few of the teeth from the Belgrade sample resemble the juvenile tooth figured by Compagno (1988). Of the complete Belgrade collection of more than 200 teeth, most were too fragmentary to use in this study. Several of these fragmentary teeth were of the juvenile configuration pictured by Compagno (1988). Thus, there were immature sharks in the population, which provided the Belgrade sample.

It is possible the Belgrade teeth are all from subadult individuals. There is no clear way to determine the age of the individuals, which produced the sample teeth. If distal edge length is used as a proxy for age, histograms of the four float samples used in the modeling indicate the four populations have reasonably similar distributions (see Figure 6).

As a species Hemipristis serra extends from the late Oligocene to the early Pleistocene (Sánchez-Villagra et al. 2000); however, examples of teeth from late Oligocene – early Miocene deposits are rare in the Atlantic coastal plain. A database search of the collection in the Florida Museum of Natural History returned 140 specimens of Hemipristis teeth, and only one of these was identified as being from (late) Oligocene sediments. The Chandler Bridge Formation sediments in the coastal plain of South Carolina correlate well (in age) to the Belgrade Formation (Harris and Zullo 1991) and do produce Hemipristis teeth. M. Havenstein, a very experienced collector in the Charleston, South Carolina, area, indicated that he would consider "large" any Chandler Bridge Hemipristis tooth with mesial edge length in excess of 25 mm (Chandler, personal commun., 2005). By this measure there were at least four "large" teeth in the Belgrade sample. The complete absence of large teeth, the similarity in range of variation among samples, and comparison with the largest known teeth from the Belgrade Formation suggest strongly that the ontogenic distribution of the Belgrade sample is not size or age biased.