Smithsonian Miscellaneous Collections

SMITHSONIAN MISCELLANEOUS COLLECTIONS VOLUME 152, NUMBER 1 Smithsonian Publication 4695 Cdatlesi Ji. anb iHarp Vanx tlHIIalcott iElesieartt) Jf unti CRETACEOUS THYASIRA FROM THE WESTERN INTERIOR OF NORTH AMERICA (With Five Plates) By ERLE G. KAUFFMAN U. S. NATIONAL MUSEUM SMITHSONIAN INSTITUTION THE SMITHSONIAN INSTITUTION PRESS CITY OF WASHINGTON JUNE 30, 1967 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOLUME 152, NUMBER 1 Smithsonian Publication 4695 Cfjarlej; M. anb ifttarp "âux Malcott iêjieatcl) jFunb CRETACEOUS THYASIRA FROM THE WESTERN INTERIOR OF NORTH AMERICA (With Five Plates) By ERLE G. KAUFFMAN U. S. NATIONAL MUSEUM SMITHSONIAN INSTITUTION THE SMITHSONIAN INSTITUTION PRESS CITY OF WASHINGTON JUNE 30, 1967 Library of Congress Catalog Card Number 67-60093 PORT CITY PRESS- INC. BALTIMORE, MD., U. S. A. CijarlcsJ B. anb iWarp Uaux Malcott lêjfearcf) jFunb CRETACEOUS THYASIRA FROM THE WESTERN INTERIOR OF NORTH AMERICA By ERLE G. KAUFFMAN U. S. National Museum Smithsonian Institu Hon (With Five Plates) ABSTRACT The unique lucinoid Thyasira is represented in the Western In- terior Cretaceous by 7 new species and 10 new subspecies distributed through 11 Campanian ammonite zones. Two species complexes are recognized, containing five evolving lineages with Atlantic Realm affinities. Early Campanian radiation of one stock occurs prior to introduction of Thyasira into the Interior with southern migration of arctic waters ; abrupt Late Campanian radiation of the second stock accompanies replacement of the initial complex. Southern migration of Thyasira proceeds through the Campanian ; it disappears from the Interior during the Late Campanian, having attained maxi- mum southern migration. The morphology, ecology, and anatomy of Thyasira are similar in Cretaceous and living species ; evolution has been conservative since the Cretaceous. The initial phylogenetic deri- vation of Thyasira is not documented in the fossil record but may lie in deep-water deposits. Primary evolutionary trends in Cretaceous lineages are: (1) reduction of convexity; (2) elongation of the height (burrowing) axis; (3) reduction of projecting flanks; and (4) size restriction, straightening, and posterior migration of the primary sulcus. These are adaptive trends for more rapid, efficient burrowing, or reflect anatomical modifications related to change in shell form. Living Thyasira are anatomically unique, and adapted to life in cool waters of low productivity, usually on the outer shelf and slope, commonly in dark mud, oxygen-poor, hydrogen sulfide- rich substrate supporting a restricted molluscan assemblage. Creta- SMITHSONIAN MISCELLANEOUS COLLECTIONS, VOL. 152, NO. 1 2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I52 ceous species show similar substrate and molluscan associations, adaptive features, and latitude distribution. They are excellent paleoecologic indicators of the restricted habitat preferred by con- temporary species. IXTRODUCTION The lucinoid genus Thyasira contains anatomically unique bivalves adapted to restricted cool water, infaunal habitats, including oxygen- poor and/or hydrogen sulfide-rich environments with a limited food supply and an otherwise impoverished molluscan assemblage. The shell is relatively simple—thin, fragile, edentulous or pseudo- dentate, normally with two posterior folds and sulci, and lacking in ornamentation other than growth lines. Generic and subgeneric classification is based entirely on the shell, and is a matter of con- troversy among malacologists. The anatomy and ecology of living Thyasira have been of considerable interest to biologists and are well documented. Conversely, the fossil Thyasira, and in particular the ancestral Mesozoic forms, are poorly known. In North America, the genus Thyasira has been recorded only from Cretaceous and younger rocks, and the fossil record is sparse. The Cretaceous Pacific Coast Thyasiridae are generally distinct from those in the Western Interior and Atlantic Provinces, a con- dition which generally has persisted to the present. The Cretaceous Indo-Pacific forms (group of "Thyasira" cretacea Whiteaves) have been generally treated taxonomically. but no species have been previ- ously descriljed from the Western Interior and only scattered reports of thyasirid genera have been published from this area. In recent years, numerous well-preserved specimens of Thyasira have been found in middle Lower through middle Upper Campanian rocks of the W'estern Interior United States and Canada (Pierre Shale and equivalents), distributed through nine Baculites zones (as established by \\\A. Cobban. 1958, p. 660; 1962. p. 704-706; 1964, fig. 2; personal communication), with a total range transgressing 11 ammonite zones, at 20 distinct stratigraphic levels. Many of these were discovered in conjunction with the U.S. Geological Survey's Pierre Shale Project. These collections, which contain good adult populations from several levels, form the basis for this study. The W'estern Interior Cretaceous Thyasira belong to 7 new species and 10 new subspecies which are distributed into 2 principal species complexes with predominantly Atlantic Realm affinities. Inasmuch as the shell is not complex and the primary radiation of Thyasira : NO. I NORTH AMERICAN CRETACEOUS THYASIRA—KAUFFMAN 3 apparently took place prior to the Upper Cretaceous, evolution within the genus has been demonstrably conservative from Cretaceous to Recent times. The differences between species and subspecies within any lineage are not dramatic, and can best be recognized through basic biometric analysis of ontogentic and adult variation suites of specimens. Each species group exhibits small-scale evolutionary change in the Cretaceous, primarily in the outline and mea- sured angles of the shell, in the development, position and curvature of the beaks, umbos, and sulci, in convexity, and in the development of the lunule and escutcheon. The expected variation within fossil species, and the taxonomic and evolutionary significance of differences shown by chronologically successive populations of Cretaceous Thyasira are in part defined here by studies in variation of the large, morphologically similar living species, Thyasira sarsi (Phillipi) {=C. insigm's Verrill and Bush; fide Ockelmann, 1961, p. 51). Based on radiometric dating the average evolutionary rate of Upper Cretaceous species of Thyasira is 2.3 million years, and of subspecies 0.86 million years (text fig. 1 ) . These are coincident or slightly longer periods than found for species of ammonites (primarily Baculites) during the same period, but are restricted enough to make Thyasira useful in dating and biostratigraphic correlation of Cretaceous units. Evolutionary changes in the morphology of Thya- sira are primarily concerned with better adaptation of the shell for rapid, efficient burrowing and anatomical modifications resulting from changes in shell morphology. The conservative evolution of the shell in Thyasira observed since the Cretaceous is also demonstrated by the anatomy of the animal. The many unusual anatomical features of living Thyasira are reflected wholly or in part by the interior morphology of the shell (muscle impressions, pallial line, etc.), forming a basis for the interpretation of paleoanatomy, and the study of functional morphology in the Cretaceous species. A thorough study of Recent species was neces- sary before paleontologic interpretation could be attempted. Among the outstanding anatomical modifications of Recent Thyasira are (a) loss of posterior siphons, and abnormal development and func- tion of posterior exhalent and inhalent apertures; (b) a highly modified foot approximately 10 times the length of the body; (c) an anterior inhalent tube of mucous cemented sediment formed by the foot; (d) a modified anterior adductor muscle; and (e) modifications of the stomach, and reduction of the palps and sorting mechanisms of the gills as an adaptation for feeding on larger 4 SMITHSONIAN MISCIiLLANEOUS COLLECTIONS VOL. 152 particles ; and many other diversions from the normal pattern of infaunal bivalve anatomy. The interior morphology of Cretaceous Thyasira strongly indicates that by the Late Cretaceous, the Thyasiridae had already developed most of the unusual anatomical features which characterize the modern species. Little basic change took place in the genus during the Tertiary. The early steps in the development of the Thyasiridae from primitive Lucinacea are not yet known and apparently took place in the Early Cretaceous or prior to the Cretaceous. The study of Cretaceous Thyasira is of additional significance because of its potential contribution to paleoecologic interpretations in \\'estern Interior strata. Inasmuch as structural analysis indicates that Cretaceous and Recent species were similarly adapted for infaunal living, and the anatomy and ecology of living forms is well known, it is logical to assume that rather precise paleoecologic interpretations can be based on the form and inferred anatomy, and faunal associates of the fossil species. Living species of Thyasira are widespread in both the Indo-Pacific and Atlantic Realms. Their distribution is predominantly controlled by substrate and water temperature ; most living Thyasira display a marked preference for dark organic clay mud and sandy clay substrate in cool waters. The bathymetric range of many species deepens toward the southern end of their geographic range in response to temperature control on distribution. Some close parallels can be drawn with the distribution

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