Ontogenetic morphometrics of some Late Cretaceous trochospiral planktonic foraminifera from the Austral Realm

Huber, Brian T. Ontogenetic Morphometries of Some Late Cretaceous Trochospiral Planktonic Foraminifera from the Austral Realm. Smithsonian Contributions to Paleobiology, number 77, 85 pages, 37 figures, 10 plates, 15 tables, 1994.—Biometric analysis of ontogenetic changes in test morphology is employed to determine the taxonomic status of several trochospiral planktonic foraminiferal species from southern high latitude Upper Cretaceous sediments. Ontogenetic morphometric data obtained from specimens of Hedbergella sliteri Huber, Archaeoglobigerina australis Huber (both micromorph and normal-sized populations), and Archaeoglobigerina mateola Huber are compared with topotype populations of Hedbergella holmdelensis Olsson, H. monmouthensis (Olsson), Costellagerina pilula (Belford), and Rugoglobigerina rugosa (Plummer). Southern South Atlantic specimens of Archaeoglobigerina bosquensis Pessagno and Archaeoglobigerina cretacea (d’Orbigny) are also analyzed for comparison. Numerous biometric data, measured from exterior observations of whole tests, contact microradiographs, and Scanning Electron Microscope (SEM) micrograph images of serially dissected foraminifera, are used to characterize developmental changes in morphology of the planktonic foraminiferal species. The most useful variables for discriminating taxonomic differences are discussed for each method. Results indicate that the ontogenetic morphometric approach to study of planktonic foraminifera can be effectively used to resolve problems in taxonomic classification, particularly for species that appear homeomorphic in exterior view. This approach was particularly useful for demonstrating that the ontogenetic morphologies of A. australis, C. pilula, and R. rugosa are very different and, therefore, previous assignment of A. australis morphotypes to various species of Rugoglobigerina and Costellagerina were incorrect. This study also demonstrates that the growth morphology of the new high latitude species H. sliteri significantly differs from H. holmdelensis and H. monmouthensis, thus confirming recognition of H. sliteri as a valid taxon. However, taxonomic uncertainty persists for some high latitude morphotypes that have external characteristics similar to R. rugosa (e.g., faint umbilical apertures, faint costellae that are meridionally aligned, presence of tegilla), but ontogenetic morphologies more similar to A. australis. Morphologic changes during ontogeny, including changes in (1) shell pore characteristics (pore diameter, pore density, and porosity), (2) rates of increase in cross-sectional chamber area, (3) apertural position, (4) chamber surface ornamentation, and (5) umbilical diameter, were used to recognize ontogenetic stages in the foraminiferal shells. These include the prolocular, juvenile, neanic, and adult stages. The growth patterns of H. holmdelensis, H. monmouthensis, H. sliteri, and C. pilula are very uniform and do not show discemable transitions from the juvenile to neanic and adult stages. All four ontogenetic stages were recognized in A. australis, A. bosquensis, A. mateola, A. cretacea, and R. rugosa, although the abruptness of the transitions and the chamber number where these transitions occur are variable within and between species. Recognition of these growth stages enables taxonomic identification of pre-adult morphologies that occur in smaller size fractions. This is particularly useful since these smaller forms have dominated in unstable environments such as the highly seasonal circum-antarctic oceans. Official publication date is handstamped in a limited number of initial copies and is recorded in the Institution’s annual report, Smithsonian Year. Series cover design: The trilobite Phacops rana Green. Library of Congress Cataloging-in-Publication Data Huber, Brian T. Ontogenetic morphometries of some Late Cretaceous trochospiral planktonic foraminifera from the austral realm / Brian T. Huber. p. cm. — (Smithsonian contributions to paleobiology ; no. 77) Includes bibliographical references. 1. Foraminifera, Fossil. 2. Paleontology—Cretaceous. 3. Foraminifera—Morphology. I. Title. II. Series. QE701.S56 no. 77 [QE772] 560 s-dc20 [563M2] 93-27089 ® The paper used in this publication meets the minimum requirements of the American National Standard for Permanence of Paper for Printed Library Materials Z39.48—1984.