Ontogeny of Upper Cretaceous (Turonian-Santonian) scaphitid ammonites from the Western Interior of North America : systematics, developmental patterns, and life history. Bulletin of the AMNH ; v. 185, article 2

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[New York] : American Museum of Natural History
"Scaphites are a group of late Cretaceous heteromorphic ammonites in which the final body chamber partially uncoils, thereby marking the attainment of the adult stage. This distinctive change in shape permits unequivocal separation of variation due to developmental stage from phenotypic variation among adults. In the Western Interior of North America, scaphites are represented by a wide diversity of endemic species. Many of these species are abundant and well preserved and, therefore, are especially suitable for a detailed investigation of ontogenetic development. I studied the ontogeny of several species of Turonian-Santonian scaphites in the genera Scaphites, Clioscaphites and Pteroscaphites, utilizing both whole fossils and polished sections. The study of their ontogeny bears on the questions of scaphite systematics and morphological development, and ammonite life history in general. The initial whorls of scaphites, as in other ammonites, consist of a bulbous protoconch and part of a planispiral whorl (referred to as the ammonitella). The ammonitella displays a uniform tuberculate micro-ornamentation extending 0.75 whorls to a depression (primary constriction) after which growth lines appear. The shell wall is prismatic in microstructure and nacre first appears at the constriction where it forms an internal pad (primary varix). These morphological observations support a scheme of direct development in which the constriction marks the aperture of the embryonic shell. Preserved ammonitellas of Scaphites ferronensis and Baculites cf. B. asper, B. codyensis suggest that hatching may have occurred after the development of the Proseptum. In scaphites, the proseptum displays a unique necklike attachment that appears as a superimposed saddle on the prosuture. The caecum and its prosiphonal attachment are similar among all the species studied. The diameter of the embryonic shell averages 700 [micrometers] and ranges from approximately 600 to 800 [micrometers]. The ammonitella angle averages 270, which is similar to the angular length of the juvenile body chamber. Unlike modern Nautilus, the embryonic shell is comparable in size to the young of many Recent dibranchiate cephalopods and may have followed a planktonic mode of life immediately after hatching. The juvenile shell conforms to a logarithmic spiral but exhibits a conspicuous change in morphology at approximately 3-4 mm diameter corresponding to two whorls from the primary constriction. The change involves modifications in the growth patterns of the umbilical diameter and spiral radius and coincides with the first appearance of macro-ornamentation. It also corresponds to a minimum in septal spacing and the attainment of a stable, ventral position of the siphuncle. These changes may indicate a transition from a passive planktonic to a more active mode of life. Similar morphometric changes occur in many other ammonites at this approximate size and whorl number and may represent a common developmental pattern. In micromorph scaphites of the genus Pteroscaphites, this whorl size coincides with the initiation of an accelerated maturity. As in modern Nautilus, the period of septal secretion in scaphites and other ammonites was probably dependent on the rate of apertural growth and buoyancy requirements rather than external astronomical rhythms. The period may also have displayed an increase over ontogeny, although the absolute rate of growth is unknown. Maturity is expressed by the development of an uncoiled body chamber, although the degree of uncoiling varies widely among species. Interspecific comparisons are therefore facilitated by examination of the more similarly shaped phragmocones. Within species, histograms of adult phragmocone diameter form unimodal distributions although a well-marked sexual dimorphism appears in many species. The ratio of maximum to minimum phragmocone diameter ranges from 1.7 in S. preventricosus to 4.6 in S. carlilensis. The diameter of the adult phragmocone and the number of postembryonic whorls exhibit a positive correlation within and among species. The adult size and the extent to which the mature body chamber uncoils also covary within and among species. Evolutionary changes in size, with concomitant changes in the timing of sexual maturation, may thus explain interspecific variation in the degree of mature uncoiling"--P. 119.
p. 118-241 : ill., map ; 26 cm.
Includes bibliographical references (p. 234-241).