Evolutionary trends and the origin of the mammalian lower jaw

Paleobiology, Fall 2003 by Sidor, Christian A

Quantitative Data.-Two areas and four linear measurements constitute the quantitative data set (Fig. 5). The area of the dentary and postdentary bones was calculated by digitizing their respective outlines in NIH Image. Synapsid mandibles were oriented norma lateralis, and then the following measurements were taken parallel to the long axis of the jaw: (1) jaw length, measured from the anterior-most point on the dentary to the midpoint of the craniomandibular joint; (2) dorsal length of the dentary, measured from the anterior tip of the dentary to the sutural contact between the dentary and surangular along the dorsal margin of the mandible; (3) ventral length of the dentary, measured from the anterior tip of the dentary to the sutural contact between the dentary and angular along the ventral margin of the mandible; and (4) perpendicular to the previous measurements, height of the coronoid region, equal to the distance from the mandibular joint to the dorsalmost point on the lower jaw. Measurements under 200 mm were taken with digital calipers and recorded to the nearest one-tenth millimeter. Measurements over 200 mm were taken with a measuring tape and recorded to the nearest half-millimeter. Raw data are presented in Appendix 5.

From the six original variables, four ratios were calculated: (A) the relative position of the dentary/surangular suture, calculated as the ratio of measurement (2) to measurement (1), (B) the relative position of the dentary/angular suture, calculated as the ratio of measurement (3) to measurement (1); (C) the relative height of the coronoid process, calculated as the ratio of measurement (4) to measurement (1); and (D) the relative area of the dentary, calculated as the area of the dentary divided by the total area of the lower jaw. Measurements were originally collected from 764 mandibles (Sidor 2000: App. 6.1). Of these, 322 lower jaws preserved at least two measurements and were used to calculate mean values for each taxon for each of the four dentary ratios (Appendix 5). For the purpose of summarizing changes in all four ratios, each taxon's dentary index (DI) was calculated as the average of the four ratios when each was standardized to have a mean of zero and unit variance. Appendix 4 contains the four original ratios and the summary DI, in addition to each taxon's first and last appearances (in ARs), CR, and PD. Because of fossil incompleteness, not every taxon has a complete set of measurements and so could not be included in all analyses.

Measurement-based Analyses

Stratigraphic Results.-Figure 6A summarizes the results of the measurement-based analyses. The dentary index (DI) is plotted on the abscissa, with increasingly positive values indicating an overall larger contribution of the dentary to the composition of the lower jaw. Although only the DI is depicted, the four individual dentary ratios show similar patterns (Table 1). The ordinate represents the stratigraphic distribution of each taxon in ARs. Significant, positive correlation between the two axes corresponds to a preferential positioning of dentary relative sizes through time.