Some range of error is almost invariably present in any attempt to determine the numerical age of paleontological specimens. For the purpose of linear regression a mean age for each sample will be assigned the individual teeth in that sample. Although perhaps somewhat contrived, this seems a better method than assigning a random age within the recognized limits of each sample.

Belgrade. Synthesizing early work, Harris and Zullo (1991) placed the Belgrade Formation as spanning the Oligocene-Miocene boundary. The age for the Belgrade float population to be used in the linear regression will be 23.8 Ma, the currently recognized age for this boundary (Berggren et al. 1995).

Pungo River. Riggs et al. (2000) divided the Pungo River Formation into four units, labeled A–D. The Pungo River float sample is from the uppermost Unit D whereas the Pungo River reject sample is from the ore matrix, Units B and C. To Unit B, Riggs et al. (2000) assigns an age range of 19.1–17.0 Ma whereas they estimate Units C and D range from 16.4 to 14.8 Ma. Because they did not resolve the geochronology in greater detail, a certain amount of arbitrariness must be used to set times for the linear regression. Choosing the midpoint of 16.4 and 14.8 gives 15.6 Ma as an approximate age for the C-D boundary. This averaging gives a range of 15.6–14.8 Ma for Unit D, and again choosing the midpoint gives a regression age of 15.2 Ma for the Pungo River float sample. The range of Units B and C (the ore body) of 19.1 Ma to 15.6 Ma gives an average regression age of 17.35 Ma for the Pungo River reject sample (Figure 3).

Yorktown. Snyder et al. (1983) identified the Yorktown Formation in the Lee Creek Mine as contained within planktonic foraminifera zones N19 through the middle of N20. However, collecting is concentrated in the deeper material, primarily N19. This zone yields an age range of approximately 5.0–4.0 Ma. Taking the midpoint yields a regression age of 4.5 Ma.

Calvert. The 32 teeth lent by the Calvert Marine Museum had collection information assigning them to precise Shattuck Zones. Shattuck Zones have been correlated with major biostratigrafic units in de Verteuil and Norris (1996) to give the range of ages for teeth in this sample as seen in Table 2 (some ranges are interpolated).

Santee. Eight teeth of Hemipristis curvatus, the Eocene species, are from the Santee Formation of South Carolina. Harris and Zullo (1991) place the middle of this Formation within Foraminifera Zone P12, to which Berggren et al. (1995) assigns an age range of 43.6–40.6 Ma. Averaging gives 42.1 Ma for the linear regression age.

Recent. Thirty teeth of Hemipristis elongata, an Indian Ocean and western Pacific Ocean resident and the only extant species of Hemipristis, were also measured. Their age is 0 Ma.