. Bulletins of American paleontology. 16 Bulletin 359 tionary research (Raup and Stanley, 1978, p. 55), yet it is absent from many contemporary paleontological studies. An understanding of ontogenetic variability is necessary for recognizing members of the same spe- cies at different ages and comparing morphological differences among individuals of similar ages. A com- prehensive understanding of Priinum ontogeny at the species level is particularly important because: (1) On- togenetic variation is important for species discrimi- nation. Species-diagnostic characters, such as callus morphology
. Bulletins of American paleontology. 16 Bulletin 359 tionary research (Raup and Stanley, 1978, p. 55), yet it is absent from many contemporary paleontological studies. An understanding of ontogenetic variability is necessary for recognizing members of the same spe- cies at different ages and comparing morphological differences among individuals of similar ages. A com- prehensive understanding of Priinum ontogeny at the species level is particularly important because: (1) On- togenetic variation is important for species discrimi- nation. Species-diagnostic characters, such as callus morphology, shape of the outer lip, presence of an ex- ternal varix, and the presence and morphology of den- ticulations, are variable through ontogeny; (2) Juvenile stratigraphic and geographic distributions are not com- pletely concordant with adult distributions, so assign- ing juveniles to species is necessary for accurately de- termining the temporal and geographic ranges of spe- cies; (3) Evolutionary change within populations, spe- cies, and clades is often generated by alterations in the timing of development (Williamson, 1987; McKinney and McNamara, 1991). Separation of marginellid specimens into juvenile and adult classes is possible because living and fossil species, including Prunum, develop unique morpho- logical features at adulthood, when growth in body size and shape ceases. In gastropods, determinate growth is recognized by the development of a lip var- ix, internal lip thickening, an ascending suture, and apertural callusing patterns (Vermeij and Signor, 1992). Prunum develops six morphological features at adulthood (Text-fig. 8): (1) An aperture margin callus; (2) Lip denticulations and/or crenulations; (3) Inner lip thickening; (4) A terminal inflection of the body whorl; (5) an external varix; and (6) A posterior lip callus. Importantly, size and shape are not characters used to identify adults and juveniles, thus it is possible to independently study size and shape
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