Skip Navigation

Late Paleozoic Pelecypods: Pectinacea

Prev Page--Contents || Next Page--Shell Morphology


The aviculopectens and related pectinoids constitute a characteristic element of the Medial and Late Paleozoic faunas. (The term, pectinoid, is used in a nontaxonomic sense in reference to the Pecten-like shape of these shells; the very similar term, pectinid, denotes an actual member of the Pecten group, rather than a shell that merely resembles representatives of this group.) These shells form a distinctive group of genera which are superficially alike in the Pecten-like shape, but are remarkably diversified in ornamentation and hinge characters. Although their ancestry is uncertain, the Paleozoic pectinoids undoubtedly originated in some of the Late Ordovician or Early Silurian pterioids (Pteria-like shells) undergoing a marked florescence in the Devonian period. By analogy with comparable modern forms, it is probable that these ancient pectinoids were the most. active and highly specialized of the Paleozoic pelecypods. Some of them probably were able to swim about on one side by clapping the valves together after the habit of modern pectens; others (Pseudomonotis) were sessile, like the modern Hinnites and various Spondylidae. Probably all of the ancient forms, like the modern ones, employed a byssal attachment at some period in life, as suggested by the invariable possession of a byssal notch below the anterior auricle of the right valve.

Ever since McCoy, in 1851, enumerated in Aviculopecten, the characters by which he sought to distinguish Paleozoic pectinoids from the later ones, the classification of the early forms has been the subject of much discussion and controversy. The chaotic state of the classification is most apparent if reference is made to the widely varying ideas of the principal students of the group, Meek (1874), Waagen (1881), Hall (1884), DeKoninck (1885), Jackson (1890), Frech (1891), Etheridge (1892), Hind (1903), Girty (1903, 1904a, 1904b, 1904c, 1909), Etheridge and Dun (1906), Licharew (1927), and others. A surprisingly small part of the literature written on generic classification has been based on observations of specimens. It has been customary for each student to discuss the published accounts of various genera and thereby to deduce something of the generic characters. Commonly the author's diagnosis and opinions regarding his genus have been quoted time and again, and accepted as authentic. Very few of the Paleozoic genera, as determined by the type species, were accurately portrayed in the original descriptions, and few of the original accounts are sufficiently complete in essential particulars to be wholly satisfactory.

An inherent difficulty in the study of the Paleozoic pectinoids has retarded progress in the understanding of these shells. The extreme rarity of the specimens exhibiting critical characters of hinge and musculature has prevented a clear understanding of the fundamental nature of the majority of described forms. Only after special and tedious preparation of specimens by technique not used by all paleontologists can most of the characters of the older pectinoid shells be observed. It is remarkable, in consideration of the handicap under which investigators have worked, that there have been any large contributions to the classification of this group.

The name Aviculopecten, based upon supposed hinge characters, has been employed hundreds of times since McCoy's diagnosis, but the published accounts and illustrations of the hinge of the Paleozoic pectinoids are indeed few in number. It is small wonder that there is such lack of agreement concerning the identity and significance of characters described in various forms of pectinoids.

In my studies of Paleozoic pectinoids I have found that the characters of the ligament area in various unlike stocks are conservative and persistent, and therefore available for the classification of larger groups. The ligament area is the shell surface to which the ligament is attached. Four kinds of ligament areas are distinguishable, and are comparable respectively to those of modern Pectinidae, Pteriidae, Pernidae, and Arcidae. An examination of the ligaments in modern representatives of these groups shows that the ligament is a complicated structure. The conservatism of a particular type of ligament in various modern and fossil pelecypod tribes suggests that ligament structures afford an extremely important, but sorely neglected, line of supplementary evidence in determining relationships between various groups of these mollusks.

It has seemed advisable, after an analysis of the data at hand, to regard the general features of hinge, ligament, and musculature as family characters in the Late Paleozoic pectinoids. Type of ornamentation, general form, and the shape of the resilifers have proved to be useful generic characters. More trivial, yet more or less persistent details of form and ornamentation are useful specific characters.

I am not able to demonstrate the close affinities implied by the term variety between two closely similar forms, and consequently do not feel the need for three names for a form when two indicate the probable relationships just as well. The terms genus and species, as employed for extinct groups, can never mean just what they do in living forms. A paleontological species, as employed here, is the lowest practicable division in a hierarchy of biologic divisions. Of necessity, a fossil species must be described more or less objectively. If a suite of specimens has certain distinctive characters, a paleontologist is justified in describing these characters. He lacks, however, many of the criteria for evaluating them according to the concepts of zoology. Observed variations in fossil shells may equally well suggest ecads, mutants, or species in the biologic sense. The advisability of applying a name to a particular form should be determined by utility not only in stratigraphy, but also in tracing lines of descent. It is ordinarily unwise to describe a pelecypod species from a single specimen, but where there are several specimens in a collection from a single locality, it is possible to learn something of the variability of the species.

Summary of Results

The original plan for this volume included a restudy of the known Paleozoic pectinoid genera, based on genotype species. Unfortunately, it has not been possible to secure specimens in every case. Although I have had access to some of the finest collections in the country, the rarity of well-preserved specimens of pectinoids, particularly those from the Middle Paleozoic rocks, has been a handicap. Several genera have been based upon rare and poorly known foreign species and it is difficult to obtain good specimens for study. A serious attempt has been made to establish the generic status of the Pennsylvanian and Permian species. Naturally, this program necessitated work on a few type species from rocks of various geologic systems.

Available type specimens were restudied in an effort to establish the recognizable, valid species described from the North American Pennsylvanian and Permian rocks, and to discover synonyms. The difficulties in this program were many, some insurmountable. A large number of original type specimens were lost or destroyed. In such instances I have made every attempt to recognize the species, either through topotype specimens or from the original descriptions and figures. Many species cannot be authoritatively established. There is no merit whatsoever in paleontology in using a specific name that is not established by authoritative specimens.

At the beginning of this investigation, 11 generic names were in general use by American writers for Paleozoic genera of pectinoids. New and old Paleozoic genera, considered in the following pages, are 28 in number; of these, 19 are treated in greater or less detail. There are 21 securely established genera, and 5 are still poorly known. Two were found to be synonyms of previously established genera. Eight genera are defined here for the first time. It was discovered that 75 specific names had been introduced for American Pennsylvanian and Permian forms. Of these, I was able to secure the type specimens or topotype specimens for 53 species. Forty can now be securely established on the basis of the types, but 8 must be suppressed as synonyms or homonyms. Twenty are species inquirenda, and most of these can never be recognized with certainty. Four names are retained for distinctive species in the absence of type or topotype specimens. Thirty species are described as new, making a total of 74 species recognized as valid.

Two new families and one new subfamily are erected for the reception of distinctive divisions of the Paleozoic Pecten-like shells, and new information has required that one previously defined family and one subfamily be emended.

During this investigation it was found that four characters can be employed consistently in the classification of Paleozoic pectinoids: (1) Musculature. Two types of musculature are distinguished—one like that of modern Pectinidae (and Amussiidae), the other embodying features of both Pectinidae and Pteriidae. (2) Hinge. Four types of hinge, found also in other, unrelated, pelecypod stocks, occur in the Paleozoic Pectinacea. One group is characterized by a ligament like that of modern Arca, another like modern Pteria, a third like Perna, and a fourth like Pecten. In each of the four groups of Paleozoic shells there are forms unlike in shape, ornamentation, and shell structure. (3) Ornamentation. The two valves in most pectinoid shells are ornamented differently. The types and combinations of shell ornamentation, where both valves are considered, are useful in delimiting compact groups. (4) Shell microstructure. Each valve is characterized by having three unlike layers, a film like outer calcite layer, a massive inner aragonite layer, within which is a small area of fibrous aragonite at the muscle impressions. The outer layer is generally prismatic in the right valve and structureless in the left, and there are three characteristic shapes of prisms. The outer layer in some shells is alike in both valves, having radially crossed-lamellar structure. The massive inner layer is commonly concentrically crossed-lamellar, and in some instances nacreous.

A restudy of type species has shown that there is much misunderstanding regarding some of the significant characters of the older pectinoids. Emphasis is placed on the importance of hinge characters by some authors, on general form by others. The existing classifications are highly artificial and inconsistent. An examination of large collections shows that general configuration is the chief character by which the Paleozoic Pecten-like forms can be classed together under Pectinacea rather than Pteriacea.

Prevalent ideas regarding Paleozoic pectinoids are based partly on misconceptions regarding hinge characters, and there has been general lack of agreement regarding features which should be considered most important. The kind of hinge originally described for Aviculopecten is fictitious, according to my study of original specimens. The four different kinds of hinge structures among pectinoids serve for recognition of four families: Pterinopectinidae, Aviculopectinidae, Euchondriidae, and Amussiidae.

The shell microstructure is not yet known for many of the genera of the Pterinopectinidae, but in most of the Paleozoic representatives of the last three families, the outer shell layer of the right valve is composed of calcite prisms of a shape characteristic for each family. Within the families there are many types and combinations of ornamentation which serve for a natural classification into genera.

The Aviculopectinidae are divided into subfamilies, which are based on form and shell structure.


Probably never before has there been assembled for study such fine collections of various kinds of Upper Paleozoic pelecypods as those now available to me. The great collections of the Kansas and Nebraska Geological Surveys, with exact stratigraphic data, represent almost every fossiliferous Pennsylvanian and Permian horizon in the northern Mid-Continent region. These collections supply unusually complete data in various pelecypod stocks that may be used to aid in distinguishing slight but useful variations, and also for determining the more significant products of progressive change through a thick succession of rocks. The extensive collections at Peabody Museum, Yale University, including the Braun-Schuchert collection, contain many hundreds of choice Pennsylvanian specimens. The Yale material supplements admirably the collections from Oklahoma and north-central Texas in the Kansas and Nebraska Geological Survey collections. A small collection of good specimens was lent from the geological museum at the University of Oklahoma. Three other small collections of Oklahoma and Texas specimens were received from Richard Hollingsworth, Ralph King, and from the University of Iowa. An exceedingly fine assemblage that was presented to me by Dr. J. Brookes Knight includes all of the pelecypods in his unique collection from the St. Louis outlier region. A special feature of this collection is the large number of juvenile specimens, particularly in the Nuculacea, including many nearly complete ontogenetic series. Some unusually fine specimens described in this volume were lent by the U. S. National Museum at Washington. A number of Permian forms from the Glass Mountains of West Texas, property of the Texas Bureau of Economic Geology, were accessible to me at Peabody Museum. Many of the Pennsylvanian specimens were collected personally by me.

Although the present volume is devoted only to the Pecten-like shells, further studies are contemplated, based on these collections.

Preparation of Specimens

Specimens of Paleozoic pelecypods wholly free from rock matrix are seldom discovered. Unlike brachiopods, pelecypod shells do not withstand very well vicissitudes of exposure to weathering, nor are they commonly silicified, so that free specimens, exhibiting all of the shell features, are uncommon, except perhaps, in the case of nuculoids in soft shale.

I was introduced to an important technique for the preparation of fossil specimens by Drs. J. Brookes Knight and G. Arthur Cooper. Very surprising results can be secured by working with very sharp, fine tools in conjunction with a binocular microscope. The most important requisite in the work is the need for infinite care and patience. The equipment employed in this preparation consists of a small pin vise, such as can be procured from hardware supply companies, and a number of small sewing needles, fine phonograph needles, and two or three jeweler's broaches. Three kinds of points generally are sufficient for all purposes. The original point of the needles is suitable for a few uses, but is used least of all. The sewing and phonograph needles can be cut off obliquely at the tip for a kind of gouging tool to be used in excavating rounded spaces between external costae. The most useful tool of all is an oblique chisel edge. Such an edge can be obtained by grinding down the point by hand on a fine carborundum block; or, better, it can be shaped on a motor-driven carborundum wheel. It is highly important that the chisel edge of the needle be sharpened frequently.

In dealing with ordinary limestone, it was found that much of the matrix can be cut away with a corborundum dental drill. Then the matrix is shaved down with the small chisel edge, this work being always done with a binocular microscope at low or intermediate magnification. Generally it is necessary to keep the specimens wet with water, glycerin, or xylol, in order to distinguish between shell and matrix. When the matrix has been cut down to a paper-thin film, it can be flaked off some specimens without scratching the shell.

Potassium hydroxide can be used in a few cases for the removal of shaly matrix. It should be used only as a last resort, however, because it is commonly destructive to the surface of the shells.

Shell microstructure generally can be observed without use of thin sections if the specimen is examined under some liquid, such as xylol or Canada balsam. Thin sections are usually necessary, however, in photographing shell structure, because of the high magnifications required.

Modeling-clay or dental-wax impressions facilitate measuring the umbonal angle and shell convexity, and also aid in preparing diagrams of surface profiles.


Lithologic associations—Some types of the ancient pectinoids thrived apparently in only a restricted range of environment. Some hint of these ecologic restrictions are given by the kind of rock in which the pectinoid shells are found, and an attempt was made in this investigation to note and record the general rock types associated with different kinds of shells. For example, specimens of Dunbarella and Euchondria, according to my experience, are found in black carbonaceous shales or the calcareous equivalent, or argillaceous shales. I have never seen a representative of one of these genera in argillaceous limestone, oolite, or pure limestone. The new genera, Pterinopectinella, Annuliconcha and Girtypecten were found invariably in relatively pure limestones, presumably deposited in an environment of clear water. Fasciculiconcha, Clavicosta, Pseudomonotis, Streblochondria, and Aviculopecten appear more commonly in argillaceous limestones, but all of them are also found locally in limestone oolites, and sandy or shaly beds.

The genus Limipecten is not uncommon in shaly beds of the Pennsylvanian rocks in Texas and Oklahoma, but appears to be exceedingly rare in equivalent rocks in the northern Mid-Continent region.

Stratigraphic distribution—Pectinoids from various separate provinces in North America have been included in this study so that a single chart showing the stratigraphic range of various species is not feasible. In order to indicate the general sequence of stratigraphic divisions in the main provinces, the following correlation tables were compiled from various sources representing the most up-to-date and best-established correlations obtainable at the present time.

Table I—Principal Divisions of the American Marine Permian Rocks.

Trans-Pecos Texas North Texas Kansas, Nebraska, Oklahoma Cordilleran
Double Mt. Quartermaster
Day Creek
Absent Post-
Delaware Mt. Word Dog Creek
Bone Springs Leonard Kaibab
Upper Harpersville
Upper Sumner Big Blue series
Hueco Wolfcamp Lower Sumner
Council Grove
Bird Springs Upper Uralian
Carboniferous Rocks

Table II—Principal Divisions of the American Upper Carboniferous (Pennsylvanian) Rocks.

West Texas North-central Texas Oklahoma,
Kansas, Nebraska, Missouri Illinois Pennsylvania,
Ohio, W. Virginia
Gaptank Cisco Lower Harpersville
Virgil Wabaunsee
Absent Monongahela Lower


Brush Creek
Canyon Upper Caddo Creek Merom

La Salle

Shoal Creek

Lower Caddo Creek
Nellie Bly

Missouri Pedee
Kansas City
Palo Pinto
Strawn Upper Mineral Wells
Lower Pennsylvanian Lower Mineral Wells Holdenville
Des Moines Marmaton Gimlet
St. David
Pope Creek
Allegheny Moscovian
Garner Cherokee
Upper Haymond Millsap Lake Absent Kanawha Pottsville
Lower Haymond
Bend Smithwick
Marble Falls
New River
Mississippian Rocks


This study was undertaken and largely accomplished with the aid of a Sterling Research Fellowship at Peabody Museum, Yale University. The fine collections of Late Paleozoic pelecypods possessed by the respective State Geological Surveys of Nebraska and Kansas, and Yale University were placed at my disposal. This project would never have been undertaken without access to these collections.

A large number of individuals have spared no effort to help me secure type specimens and other material for study. Especially important were the services of Dr. J. Brookes Knight, of Princeton University, who was instrumental in kindling my interest in Paleozoic pelecypods, expressing confidence in my work by entrusting to me all of his unique collection of pelecypods from the St. Louis Pennsylvanian outlier, and most important, supplying a constant inspiration, both through personal contact and by example of achievement. Dr. J. Marvin Weller, of the Illinois Geological Survey, helped me borrow the type specimens of species described by Meek and Worthen.

Doctors Carl O. Dunbar, G. E. Condra, and R. C. Moore made the project possible by placing the splendid collections of Peabody Museum, Yale University, the Nebraska Geological Survey, and the Kansas Geological Survey at my disposal. In addition to furnishing constructive counsel as teacher and friend, Doctor Dunbar was instrumental in obtaining for me a Sterling Research Fellowship at Yale University, without which I could not have undertaken this study.

The generous assistance given by the following, and all other persons, in helping me obtain types and instructive specimens, or helping me in other ways is deeply appreciated: Dr. W. T. Wells. Australian Museum; Dr. Ray S. Bassler, U. S. National Museum; Dr. G. Arthur Cooper, U. S. National Museum; Dr. Leslie Bairstow, British Museum; Dr. W. D. Lang, British Museum; Dr. E. B. Branson, University of Missouri; Dr. J. Ernest Carman, Ohio State University; Mr. M. S. Chappars, Cincinnati University; Dr. N. M. Fenneman, Cincinnati University; Dr. Lewis M. Cline, Iowa State Agricultural College; Dr. A. K. Miller, Iowa State University; Dr. Carey Croneis, Chicago University; Dr. C. E. Decker, University of Oklahoma; Dr. J. J. Galloway, University of Indiana; Dr. Paul S. Galtsoff, U. S. Bureau of Fisheries; Dr. George H. Girty, U. S. Geological Survey; Dr. James S. Williams, U. S. Geological Survey; Mr. George D. Harris, University of Texas; Mr. Richard Hollingsworth, Shell Petroleum Company, Tulsa, Okla.; Dr. Marshall Kay, Columbia University; Dr. Edward M. Kindle, Victoria Memorial Museum, Ottawa, Canada; Mr. Ralph H. King, Wichita Falls, Texas; Dr. Harold Wanless, University of Illinois; Dr. B. K. Licharew, United Geol. and Prospecting Service, Leningrad; Mr. F. Steams MacNeil, U. S. Geological Survey; Dr. W. C. Morse, Mississippi Geological Survey; Dr. Henry A Pilsbry, Philadelphia Academy of Science; Prof. F. B. Plummer, Texas Bureau of Economic Geology; Dr. E. H. Sellards, Texas Bureau of Economic Geology; Dr. Percy Raymond, Harvard University; Dr. F. R. Cowper Reed, Sedgwick Museum, Cambridge University, England; Dr. O. M. B. Bulman, Sedgwick Museum, Cambridge' University, England; Dr. A. G. Brighton, Sedgwick Museum, Cambridge University, England; Dr. W. E. Schevill, Harvard University; Dr. F. M. Carpenter, Harvard University; Dr. Courtney Werner, Washington University, St. Louis; Mr. Dana Wells, West Virginia University; Prof. M. K. Elias, Kansas Geological Survey; Mr. Joe L. Borden, Pure Oil Company, Tulsa, Okla.

Finally, part of the burden of proofreading the manuscript has been shared by Mrs. Norman D. Newell and Miss Jewell Kirby. Dr. R. C. Moore has given many invaluable suggestions that have materially improved the general presentation of my results and has done the final editing.

Prev Page--Contents || Next Page--Shell Morphology

Kansas Geological Survey, Geology
Placed on web Dec. 1, 2017; originally published 1937.
Comments to
The URL for this page is