Harries, P.J., Kauffman, E.G., Crampton, J.S.
(Redacteurs), Bengtson, P., Cech, S., Crame, J.A.,
Dhondt, A.V., Ernst, G., Hilbrecht, H., Lopez, Mortimore,
G.R., Tröger, K.-A., Walaszcyk, I. & Wood, C.J.
(1996): Mitteilungen aus dem Geologisch -
Paläontologischen Museum der Universität
Hamburg, 77: 641-671.
The bivalve family Inoceramidae first evolved in the
Permian and became extinct at the close of the Mesozoic.
Inoceramids became dominant elements of many level-bottom
communities, and they achieved global dispersion during
the Jurassic and Cretaceous, especially during intervals
of restricted benthic oxygen and black-shale deposition.
Many groups appear to have been specifically adapted, in
terms of anatomy and morphology (Kauffman and Harries,
1992), and possibly through chemosymbioses (Kauffman,
1988b; MacLeod and Hoppe, 1992), to chemically
deleterious benthic environments. They are also found,
however, in a wide range of different facies
types/environments (from basinal black shales to
nearshore sands), which suggests that they had relatively
wide ecological tolerances at the genus and species
level. The majority of inoceramid species had
intercontinental or cosmopolitan distribution, mirroring
the widespread nature of their preferred habitats, their
broad adaptive ranges, and probably long-lived
planktotrophic larvae; large larval shells are known from
a few species. Despite their broad distribution, the
Inoceramidae appear to have evolved very rapidly, with
species ranges commonly averaging 0.2-0.5 Ma. This
greatly enhances their use in biostratigraphy, and
contrasts to the "normal" evolutionary rates of bivalves
(2 Ma; see Stanley, 1979) and to the evolutionary
hypothesis that cosmopolitan taxa should have slow
evolutionary rates because of the wide dispersion of
their component populations.
Despite the fact that the Inoceramidae have been
studied intensively for over a century, there are still a
wide range of problems remaining to be investigated. The
Lower Turonian, although only a brief portion of
inoceramid history, represents an ideal interval to study
a number of these problems which can, at least in
concept, be applied to many different time periods as
well. These include: 1) the effects of a mass extinction
(Cenomanian-Turonian) on inoceramids; 2) a switch in
generic dominance from Late Cenomanian Inoceramus s.s. to
Early Turonian Mytiloides within the same basic facies;
3) the unusually rapid evolutionary rates of
Cenomanian-Turonian inoceramids at a time when species
were achieving their widest dispersal (a seeming
contradiction); 4) the need for generic revision of the
Inoceramidae in this interval; 5) problems with
species-level taxonomy related to the seeming morphologic
plasticity of the forms; and 6) the extremely similar
forms of Early Turonian Mytiloides spp. to those which
evolved in the Late Turonian to basal to middle
Coniacian, possibly representing iterative evolution.
This overview paper will focus on the morphological
terminology required to describe most inoceramids, the
techniques of biometric analyses, concentrating on shape
analysis, that can be employed in inoceramid studies to
resolve problems of population variation in evolutionary
and taxonomic studies of the Inoceramidae, generic
diagnoses for all known Lower Turonian inoceramid genera,
and attempts to construct a Lower Turonian inoceramid
This report focuses on many, but not all, of the
topics discussed at the Hamburg workshop. Whereas most of
the taxonomic, morphologic, morphometric, ecologic, and
broad biostratigraphic aspects of this report represents
a consensus of the participants and co-authors,
differences of opinion naturally arose during our
discussions and subsequent manuscript reviews. Points of
major debate and differing opinions are noted throughout
the manuscript, which we intend as a
"state-of-the-science, 1993" statement designed to
stimulate further research and discussion. Harries,
Kauffman, and Crampton are solely responsible for
designing the format and compiling the initial draft of
the manuscript, as agreed upon by the meeting
participants. The present paper, however, incorporates
many observations, changes, and deletions suggested by
the workshop participants during the meeting and
manuscript review process. The Redacteurs are grateful
for the constructive suggestions of our colleagues.
MORPHOLOGIC FEATURES OF INOCERAMID SHELLS
Throughout the history of inoceramid research, there
have been a wide variety of different morphologic
features and various morphometric parameters employed to
make species determinations. A comprehensive,
nomenclaturally consistent system for defining
morphologic features of the inoceramid shell is essential
to morphometric, functional morphologic, and evolutionary
studies, as well as anatomical reconstructions, within
the family. External shell morphology, upon which the
great majority of taxa are based, has been well-defined.
However, it is complicated by iterative or parallel
evolution among only distantly related groups of
inoceramids and even their ancestors. A knowledge of
internal shell features have become critical for
taxonomic (especially generic) determination, although
for many species musculature and ligamenture are only
poorly known or not yet documented. Utilization of both
internal and external shell features in the Inoceramidae
will help to maintain a degree of constancy in
descriptive format and systematic concepts.
Figure 1 ,
Figure 2 , and
Figure 3 , compiled by
P. J. Harries and E. G. Kauffman, illustrate the key
morphologic features used today in systematic description
of most inoceramids. A glossary of these terms is
presented in Appendix A. The simplest, most descriptive
and most commonly used English language terminology is
utilized in this glossary, with equivalent German
language terms, where relevant, listed in italics within
parentheses. But the reader should be aware that this
morphologic review draws heavily from earlier work by
Heinz (1932), Seitz (1934), Dobrov and Pavlova (1959),
Tröger (1967), Kauffman and Powell (1977), Efremova
(1978), Keller (1982), and Rasemann (1986) and is a
compilation of terminology for internal and external
shell features, as well as for shell ultrastructure, used
by a variety of previous authors. Some new terminology
has been added by the authors.
BIOMETRIC ANALYSIS OF INOCERAMID BIVALVES
Inoceramid taxonomy is hampered by two general
problems. First, for genetic and taphonomic reasons, the
inoceramid taxonomist typically has relatively few
characters to utilize in taxonomic differentiation, and
existing classifications are based almost exclusively on
shape and external shell features. Secondly, inoceramids
commonly display considerable intraspecific (phenotypic,
ecophenotypic, ontogenetic) morphological variation, and
interspecific morphological convergence in their
evolution. For these reasons, an increasing number of
workers have attempted to use quantitative biometric
methods as objective aids in the classification of the
group. Two general classes of methods are discussed
below: 1) those based on the uni- and bivariate analysis
of distance and angle measurements and 2) multivariate
Univariate and bivariate methods
Distance and angle measurements have been used
extensively to describe inoceramid shape and ornament in
order to quantify intra- and interspecific variation. A
great number of measurements have been used, a reflection
of the diversity of inoceramid morphologies which
probably precludes definition of a universally applicable
set of parameters. Many of the morphological elements
measured have been reviewed by Sornay (1966), Efremova
(1978), Rasemann (1986), and Aliev et al. (1988).
shows a number of measurements
which have been used to describe Turonian Mytiloides and
Distance and angle measurements have typically been
examined and compared using standard univariate
statistics and bivariate plots (e.g., see Jones 1988 for
a review of some useful uni- and bivariate statistics).
By taking measurements from different growth stages of
single individuals some studies have examined patterns of
relative growth, or changes of shape occurring throughout
the ontogeny of an individual. Bivariate data should be
examined and summarized using the line of reduced major
axis (RMA). This line describes the relationship between
two correlated variables, but unlike ordinary linear
regression, assumes independence of the variables.
Whereas the slopes of two RMA's can be statistically
compared, there is at present no rigorous method for
comparing the positions of two lines (e.g., Jones
Examples of the use of uni- and bivariate methods to
characterize species, distinguish between species, and
describe intraspecific and ontogenetic variation include
Seitz (1934), Tröger (1986), and Noda (1988). In a
few cases, such data have also contributed to studies of
evolutionary patterns (e.g., Tanabe, 1973; Noda,
With the advent of inexpensive personal computers,
alternative methods for the objective quantitative
description of morphology have become readily available.
In particular, outline-shape analysis is well suited to
the study of inoceramids, which generally lack sufficient
unique, biologically homologous reference points for
landmark analysis (see Temple, 1992 and references
therein). Shape analysis provides an objective and
conceptually parsimonious complement to qualitative
visual processes (Scott, 1980); the eye is particularly
adept at identifying differences within small samples,
whereas shape analysis can be used to estimate degrees of
similarity within large samples.
The different methods of shape analysis fall into
three categories: eigenshape analysis, the fitting of
polynomial curves, and a family of methods based on
Fourier decomposition (e. g., see several papers in Rohlf
and Bookstein, 1990). Discussion of their merits has been
covered extensively elsewhere (e.g., see Foster and
Kaesler 1988, and references therein). Elliptic Fourier
analysis (EFA), however, has been favored in several
recent studies (e g., Rohlf and Archie, 1984, Ferson et
al., 1985, White et al., 1988, Temple, 1992). Ferson et
al. (1985) provide a brief introduction to the theory of
EFA, and a more detailed account of the methodology and
associated problems as applied to inoceramids will be
presented in Crampton (in prep.). Unlike many other
methods, EFA can describe complex shapes, does not
require explicit definition of a biologically homologous
or mathematically determined centroid, does not require
points on the outline to be equally spaced, and can
include simple normalizations for size, position,
orientation, and starting position of the trace. A
further property of Fourier methods is the ability to
invert the transformation and reconstruct an outline from
a set of Fourier coefficients. Hence, for example, an
"average" shape can be reconstructed from the mean
coefficients of a large number of outlines (e.g., Ferson
et al., 1985). Elliptic Fourier analysis has been used to
demonstrate an association between genotype and
morphology within two putative mussel species (Ferson et
al., 1985) and to examine patterns of anagenesis and
cladogenesis in Albian inoceramids from England
(Crampton, 1992, and unpublished data).
The sequence of steps Crampton's (1992) study are
shown in Figure 5
and discussed briefly below.
Digitized outlines are generated by manual tracing using
either a video camera linked to image-analysis software
or a digitizing tablet and photographs or camera lucida
drawings (Fig. 5B). Fossil material is generally
unsuitable for automated outline capture because of
adhering matrix, preservational imperfections, and the
desire to trace growth lines other than for the
last-preserved growth stage. It is desirable to digitize
outlines in a standard fashion (Fig. 5A, C), using the
hingeline for orientation, beginning the trace at the
umbo, and tracing in a standard direction (e.g.,
counterclockwise). Standard treatment obviates the need
for normalizations during computation of elliptic Fourier
(EF) coefficients, normalizations which result in some
information loss. Crampton (1992, and unpublished data)
has chosen to mirror right valve outlines prior to EFA to
remove the effects of primitive bilateral symmetry about
the plane of commissure (Fig. 5C). This step permits
meaningful comparison of left and right valve shapes,
which may be quite different depending on the degree of
inequivalvedness. If perfectly equivalved, then a left
valve and its mirrored right valve will plot at the same
point in multivariate space; separation will increase
with increasing inequivalvedness.
The software needed to perform EFA is available with
Rohlf and Bookstein (1990) and is written in Fortran for
IBM-compatible personal computers. Required input for
each outline is a string of xy-coordinates preceded by a
sample number and the number of outline coordinates. EFA
describes outlines in terms of harmonically-related
ellipses, and each ellipse is, in turn, described by four
coefficients. Because of the basically elliptical shape
of many inoceramids, relatively few harmonics are
required to describe their outlines and most of the same
information of interest (i.e., the variance) resides in
the first three or four harmonics (i.e., first 12 - 16
coefficients, Crampton unpublished data). The number of
harmonics required to accurately describe an outline can
be estimated in two ways. One can calculate the average
discrepancy between the original outline and the inverse
Fourier reconstruction based on n harmonics. The Fourier
series is truncated at the value of n corresponding to a
negligible discrepancy (e.g., smaller than the resolution
of hand digitization). Alternatively, one can sum the
variance for successive harmonics and compare this sum to
the total variance of the Fourier series based upon the
maximum possible number of harmonics (equal to half the
number of points on the digitized outline). The variance,
or power, of each harmonic is equal to half the sum of
the squares of the Fourier coefficients. The Fourier
series is truncated at the value of n at which, say 99%,
of the variance is retained. In shape analysis, the
effects of specimen size (which profoundly influences
harmonic amplitudes) can be removed during computation of
EF coefficients. This normalization utilizes parameters
of the first harmonic (i.e., best fitting) ellipse and is
probably appropriate in most studies. Information about
relative size, however, can be reincorporated into a
study during statistical analysis and is essential for an
understanding of shape changes through ontogeny.
Elliptic Fourier coefficients for each outline are
then treated as variables in a multivariate statistical
analysis. The number of outlines should greatly exceed
the number of variables per outline. The statistical
methods used will vary depending upon the nature of the
data and the aims of the study. If there is some a priori
knowledge of structure in the data, for example if they
can be grouped into stratigraphically or geographically
separated populations, then discriminant function or
canonical variates analyses might be appropriate.
Alternatively, if there is no such knowledge, then
cluster or principal components analyses might be used.
Where it is used, principal components analysis should
probably be based on unstandardized data (i.e., the
variance-covariance matrix), rather than standardized
data (i.e., the correlation matrix), as is more usual.
Although there are theoretical arguments both for and
against standardization, it has the disadvantage of
giving undue weight (and implied genotypic significance)
to high-frequency data and apparently reduces the quality
of the analysis (Rohlf and Archie, 1984, Crampton
In summary, biometric methods are becoming
increasingly important in the study of inoceramid
taxonomy and paleobiology. In particular, outline shape
is a fundamental aspect of morphology which is suited to
biometric description, using either linear and angular
measurements, or Fourier shape analysis. Using such
methods, it is possible both to quantify and to
objectively compare ontogenetic and intra- and
interpopulation variations in morphology. This
information is crucial to the interpretation of
apparently complex patterns of morphological change
through space and time.
LOWER TURONIAN GENERA OF INOCERAMIDAE
The genera and subgenera of Inoceramidae are
critically in need of revision. The proliferation of
generic names by Heinz (1932), some of them nomina nuda
or nomina dubia (Cox, 1969), created chaos in inoceramid
taxonomy. The names not only were improperly formulated,
lacking generic diagnoses and, in some cases, proper
designation of type species, but they also were based
solely upon subtle differences in external shell form and
sculpture. The nature of phenotypic and ecophenotypic
population variation, parallel evolution, and homeomorphy
in external shell features was not considered by Heinz
(1932) or the majority of early inoceramid workers
(Kauffman and Powell, 1977). Most inoceramid specialists
now recognize that homeomorphy in shell shape and
external morphology is a common phenomenon, not only
within the Inoceramidae but also between this family and
related Permian inoceramid ancestors such as Atomodesma,
Kolymia, Intomodesma, and Aphanaia (all Ambonychiidae;
Kauffman and Runnegar, 1975). Parallel and convergent
evolution in shell form compounds the difficulties of
establishing a comprehensive taxonomy for the group.
Cox (1969), therefore, took a conservative view of
generic and subgeneric classification of the Inoceramidae
in the Treatise on Invertebrate Paleontology. He placed
the great majority of Heinz's (1932) genera into synonymy
(mainly with Inoceramus s.s.) and utilized as subgenera
(formally authored by Cox) only the new names of Heinz
for which at least a skeletal description of generic
characteristics had been presented or those which had
been subsequently used in published literature (e.g.,
Cataceramus, Cremnoceramus and Spyridoceramus). The
majority of inoceramid workers have followed Cox's lead.
Annie Dhondt (personal communication, Dec. 6, 1993),
however, has pointed out that, according to the ICZN
rules of zoological nomenclature, all of Heinz's (1932)
new genera to which he had assigned a valid, previously
described and illustrated species as the genotype, are
still valid genera, whether or not they were ever
formally described. This may invalidate many of Cox's
(1969) assignments of Heinz's (1932) genera to "nomina
nuda.". Unfortunately, Cox died before these mistakes
could be rectified in the Treatise on Invertebrate
Paleontology (1969). In future revisions of inoceramid
taxonomy, retention of Heinz's (1932) generic names must
be carefully considered on a case-by-case basis.
A few names used by Cox (1969) as subgenera have
subsequently been formally elevated to generic status
(e.g., Mytiloides; Kauffman and Powell, 1977) or used as
genera in non-systematic faunal analyses (e.g., see
Kauffman, 1975, 1976a,b,c; Kauffman et al., 1976, 1993 in
press). Further, several of Heinz's (1932) names
considered by Cox (1969) as nomina nuda, were
subsequently formally described as subgenera (e.g.,
Platyceramus, Endocostea, Magadiceramus, Cordiceramus) by
Seitz (1961, 1965, 1967). But our modern knowledge of the
Inoceramidae, based on a broader suite of external and
internal shell features, suggests that the elevation of
several subgeneric names to generic status, and the
formulation of additional genera and subgenera within the
family is warranted.
Kauffman (1994, in manuscript) is currently revising
the genera and subgenera of the Inoceramidae utilizing,
for the first time, a combination of external shell form
and ornament (including ontogenetic changes), with
internal morphological characteristics such as the shape,
kinds and position of muscle insertion areas, the nature
and position of the pallial line relative to the
commissure, characteristics of the ligament and
ligamental plate, the umbonal septum, pseudodentition,
and the nature of the byssal slit, where present. Data on
these characters are commonly difficult to obtain,
especially for the weakly impressed muscles. Internal
shell characteristics appear to be genetically
conservative and clearly divide the Inoceramidae into
natural morphologic groups. A combination of internal and
external shell features best characterize natural generic
and subgeneric divisions of the Inoceramidae, and may
eventually allow phylogenetic details to be worked out in
great detail. In many cases, generic and subgeneric
divisions of the Inoceramidae based on internal shell
characters separate taxa which were formally grouped
under a single genus or subgenus based solely on external
shell features, thus defining parallel evolutionary
At present, four inoceramid genera are commonly
recognized in the Early Turonian of the Europe and the
Americas (others may exist among rare and geographically
isolated groups). These are: Inoceramus s.s. J. Sowerby;
Mytiloides Brongniart; Sergipia Maury; and Cladoceramus
Heinz (1932) (?=Rhyssomytiloides Hessel, 1988; see
below). A diagnosis and description of each of these
genera is presented below, reflecting the concepts
generally agreed upon in the 1992 Hamburg meeting
(dissenting views are cited for each genus, where
relevant). The genus Cremnoceramus was also discussed at
this workshop, and its possible relationship to
geniculate specimens of Late Turonian Mytiloides?
incertus (Jimbo) (= M. fiegei fiegei (Tröger) ). But
inasmuch as no Lower Turonian Cremnoceramus are yet
recorded, redescription of this genus is not included
herein. (This section was initially prepared by E. G.
Kauffman with descriptions taken from Kauffman and
Powell, 1977, as modified herein; Kauffman, 1994, generic
revision in manuscript; Kauffman and Villamil, 1994, in
manuscript for generic treatment of Sergipia ; the
current version incorporates all comments and revisions
subsequently offered by workshop participants).
GENUS INOCERAMUS J. Sowerby, 1814
TYPE SPECIES: Inoceramus cuvieri Sowerby (Cox, 1969,
p. N315 by subsequent designation).
DIAGNOSIS: Adult shells attaining >1 m height.
Inequivalve, left valve largest, most inflated;
prosocline, outline erect-ovate, subquadrate, or
subtrapezoidal; projecting beaks, umbones prosogyrous to
orthogyrous. Adult ornament of concentric rugae with
intercalated growth lines or lamellae; juvenile ornament
distinct. Well-defined posterior auricle, auricular
sulcus; post umbonal sulci, folds common. Flattened
anterior face with sinuous, slightly gaping byssal slit.
Moderately thick prismatonacreous shell; calcite prisms
elongated, needle-like. Ligamental plate thick,
elongated, bearing numerous close-set, subrectangular,
vertically elongated resilifers; umbonal septum and
subumbonal cavity prominent, subtriangular. Musculature
weakly impressed; pallial line thin, continuous or
anteriorly pitted, at distal edge of small radial muscle
tracks; posterior adductor boat-shaped, submarginal;
large pedal-byssal retractor muscle insertion area below
umbonal septum; 1-3 small, ovate, dorsoanterior
pedal-byssal protractor insertion areas situated just
outside of pallial line.
GENERIC DESCRIPTION: Small to large size (>1 m)
adult shells; slightly to strongly inequivalve with left
valve largest and most inflated. Shells prosocline; most
commonly with ovate, subquadrate, or subtrapezoidal
outline; beaks, umbones inflated, prosogyrous to
orthogyrous, normally projecting above hinge line and
anteriorly situated. Ornamentation characteristically of
weakly to strongly developed, unequally to subequally
spaced, concentric rugae with numerous, closely spaced,
evenly to subevenly developed growth lamellae or raised
growth lines between rugae. Many species with fine,
discrete, juvenile ornamentation on umbo, consisting of
closely and evenly to subevenly spaced raised concentric
growth lines without rugae. Posterior auricle
well-defined, triangular, separated from disc by
auricular sulcus; a post-umbonal sulcus occurs in some
species, as do very small anterior auricles ("ears").
Byssal slit sinuous, slightly gaping in many species,
bounded by a flattened anterior face or a pseudolunule
with low marginal ridges. Small, irregular, sinuous
costellae may occur on anterior face or in pseudolunule
at near-right angles to the growth lines. Shell
moderately to very thick, especially prismatic layer,
with greatest thickness along the broad posterior
ligamental plate; resilifers numerous, closely spaced,
narrow and vertically oriented, shallow to moderately
excavated, separated by narrow, rounded to flattened
ridges. Umbonal septum subtriangular, moderate to large
size, with prominent subumbonal cavity. Some species
exhibit a poorly defined pseudocardinal "tooth" (an
inward extension of a small anterior auricle or the
thickened dorsoanterior shell margin), with or without a
shallow socket on the opposite valve at the anterior end
of the umbonal septum. Muscle attachment areas weakly
impressed. Pallial line thin, continuous or mostly so,
situated just inside the commissure; some species have a
pitted pallial line segment dorsoanteriorly; mantle
attachment/retractor muscles, which form pallial line,
leave radial tracks (shallow faint grooves) on shell
interior except in the posterior adductor track. Some
accessory mantle attachment/retractor muscles occur
outside pallial line on inner surface of posterior
auricle. Monomyarian; posterior adductor muscle insertion
area faint, situated near posteroventral margin,
typically elongated and boat-shaped. A large, well
impressed, pedal-byssal retractor muscle attachment area
occurs on the underside of the umbonal septum in many
species (unknown in others); one to three small, ovate to
vertically elongate-ovate pedal-byssal protractor muscle
insertion areas occur dorsoanteriorly, just outside of
the pallial line, in numerous species. Gill suspender and
other muscles unknown to date. Prodissoconch known on a
few species, large and inflated, indicating long-lived
DISCUSSION: Kauffman and Powell (1977) utilized
Inoceramus as a subgenus within the genus Inoceramus J.
Sowerby to identify a lineage of Cenomanian-Turonian taxa
that was characterized by erect, flattened to moderately
inflated forms with weakly to moderately developed rugae,
subequal growth lines or lamellae, and which lacked
strong posterior and anterior sulci and folds. Thus
defined, Inoceramus (Inoceramus) includes Cenomanian
Euramerican species such as I. (I.) pictus Sowerby and
subspecies, I. (I.) prefragilis Stephenson, I. (I.)
virgatus Schlüter and subspecies, I. (I.) scalprum
Woods, some illustrated versions of I. tenuis Mantell
(although the type of I. tenuis may belong to
Birostrina), and closely related species and subspecies
of these taxa. Inoceramus (Inoceramus) also includes
Lower and Middle Turonian I. (I.) cuvieri Sowerby (the
type species of Inoceramus), I. (I.) apicalis Woods, I.
(I.) inaequivalvis Schlüter, I. (I.) saxonicus
Petrascheck, I. (I.) tuberosus Keller, and closely
related species and subspecies. Kauffman and Powell
(1977, p. 71) suggested that the Cenomanian lineage of
Inoceramus arvanus Stephenson - I. rutherfordi Warren,
characterized by strong posterior folds and sulci, and
less commonly, anterior sulci on the shell exterior,
should be placed in a different subgenus (unnamed) of
Inoceramus. We further suggest that the strongly sulcate,
coarsely rugate, posteriorly auriculate species plexus
centered around Inoceramus lamarcki might be treated
similarly. Inoceramus s.s. is not known below the
Cenomanian and ranges at least into the Middle Coniacian,
but forms provisionally assigned to Inoceramus s.s. are
reported in rocks as young as Late Campanian and Early
Maastrichtian (Kauffman, et al., 1993, in press). Many
pre-Cenomanian and post-Coniacian species assigned to
Inoceramus can now be placed in other genera; some are
homeomorphic on Inoceramus s.s. and can be differentiated
mainly on interior shell features, musculature, and
ligamenture (Kauffman, 1994, in manuscript).
GENUS MYTILOIDES BRONGNIART, 1822
TYPE SPECIES: Ostracites labiatus Schlotheim (by
monotypy) = Inoceramus (Mytiloides) labiatus (Schlotheim)
fide Cox, 1969, p. N320; = Mytiloides labiatus
(Schlotheim) fide Kauffman and Powell, 1977, p.
DIAGNOSIS: Moderate size, length <50 cm; slightly
to moderately inequivalve; left valve largest, more
inflated. Shape subcircular to characteristically
labiatoid; prosocline. Beaks, umbones inflated,
slightly-moderately projecting, prosogyrous to nearly
orthogyrous; ventral umbo commonly geniculated.
Characteristic ornament of weakly to strongly developed
rugae intercalated with equally to unequally developed
growth lines and lamellae. Juvenile ornamentation
distinct. Posterior auricle flat, triangular, separated
from disc by moderately excavated to broad, shallow
auricular sulcus. Anterior face rounded to subtruncated;
byssal slit slightly sinuous, poorly defined to absent.
Ligamental plate posterior, short to moderately long,
thin, delicate, bearing small, moderately spaced,
slightly excavated, subtriangular to ovate resilifers
separated by low, flat to rounded platforms. Umbonal
septum small or absent. Muscles very weakly impressed;
pallial line continuous, submarginal, without radial
muscle tracks; posterior adductor insertion area
submarginal, posteroventral, narrow, elongated; no
pedal-byssal muscle insertions known. Shell
prismatonacreous, very thin, with short, blocky, calcite
GENERIC DESCRIPTION: Shell of moderate size, rarely
exceeding 30 cm in length, ranging to 50 cm; subequivalve
to moderately inequivalve with left valve slightly larger
and more inflated; shell slightly to moderately biconvex.
Valves moderately to highly prosocline; beaks, umbo
anteriorly situated except in M. latus group (beaks just
anterior to midline), strongly prosogyrous to nearly
orthogyrous, slightly to strongly projecting above the
hinge line. Low geniculation occurs at ventral edge of
umbo, between juvenile and adult portions of the disc, on
many species. Shell outline ovate, subovate,
elongate-ovate, or tongue-shaped (labiatoid) on various
species. External ornamentation characterized by equally
to unequally spaced, weakly to strongly developed
concentric rugae with regularly (typical) to irregularly
developed raised growth lines and/or flat lamellae
between rugae over most or all of the valve surface. Many
species lack growth lines in the juvenile and/or early
adult growth stages; rare species lack them altogether,
or lack rugae over most of shell. Juvenile ornamentation
commonly distinct from adult, consisting of closely
spaced, subequally developed, fine to coarse, raised
growth lines without rugae. Rare species (e.g.,
undescribed smooth form) have small anterior projection
in front of beak ("ear" or anterior auricle). Posterior
auricle small to moderate size, flattened, subtriangular,
in some cases flared posteriorly forming an acute
auricular angle; posterior auricular sulcus poorly
defined as a broad shallow depression between auricle and
disc, accentuated in some species where umbonal area is
highly inflated. Anterior face normally curved, convex,
with a poorly defined, slightly sinuous byssal slit or
none at all; a few species have a flattened to slightly
concave anterior face and a more prominent, slightly
gaping, sinuous byssal slit. Shallow anterior radial
sulcus occurs on rare species (e.g., M. submytiloides).
Hinge line short to moderate length, extending posterior
to beak, umbo; ligamental plate very thin, delicate,
bearing small, subtriangular to ovate, slightly excavated
resilifers separated by broad low ridges or narrow
flattened platforms. Umbonal septum very small or absent;
subumbonal cavity not significantly excavated.
Musculature very weakly impressed. Pallial line
continuous (entire), situated just inside the commissure,
normally lacking radial traces of mantle attachment
muscles. Narrow, laterally elongated, posterior adductor
insertion area situated posteroventrally near to the
commissure. No traces of pedal-byssal or gill suspender
muscles observed. Shell very thin (typically ranging from
0.1-0.3 mm), especially the calcite prismatic layer,
relative to other Inoceramidae; prisms small and
DISCUSSION: Brongniart (1822) originally described
Mytiloides as a genus, but without a comprehensive
diagnosis. Until recently, most authors have placed the
name in synonymy with Inoceramus (e.g,. Seitz, 1934,
1965). Cox (1969) utilized Mytiloides as a subgenus of
Inoceramus. Kauffman and Powell (1977) re-described
Mytiloides Brongniart as a full genus, noting that it
differed from typical Inoceramus in its strongly
prosocline form, low shell inflation, subequivalve shells
lacking strong radial sulci and folds, very thin nacreous
and prismatic shell layers (especially in the hinge and
umbonal areas), and weak byssal slit, or none at all.
Internally, Mytiloides is further distinguished from
Inoceramus by the lack of preserved pedal-byssal muscle
insertion areas; by its thin continuous pallial line
without impressed, radial, mantle retractor muscle
tracks; by an elongated, subcrescentic, posterior
adductor insertion area and a thin, narrow, ligamental
plate with small, subtriangular, weakly impressed
resilifers; and by its small umbonal septum, seemingly
without a deeply excavated subumbonal cavity. Species of
Mytiloides can be divided into at least two major
morphologic groups of potential subgeneric rank: (a)
Mytiloides s.s. consisting of mytiloid-shaped, strongly
prosocline taxa (e.g., M. mytiloides, M. labiatus, M.
striatoconcentricus lineages), including the oldest (Late
Cenomanian) Mytiloides known (M. n. sp.; smooth form, M.
submytiloides, M. hattini); and (b) rounded to ovate,
suberect Mytiloides such as M. "latus" (sensu Woods,
1912, Fig. 41; other illustrated types of Woods are
probably Inoceramus waltersdorfensis), and possibly M.
incertus (Jimbo) (Note that some participants at the
Workshop would place this species in Cremnoceramus
because it has an adult geniculation; we have retained it
here in Mytiloides because adult geniculation is also
common in more inflated members of this genus, and M.
incertus is otherwise similar to more rounded Mytiloides
of the Early Turonian). This rounded clade of Mytiloides
will receive a new subgeneric name in Kauffman's
forthcoming inoceramid revision (in manuscript, 1994).
Mytiloides s.s. originates in the Late Cenomanian and
ranges through the Turonian, Coniacian, and possibly
Early Santonian, although its record is not continuous.
Older Mytiloides-like species reflect homeomorphy and
belong to different genera, as indicated by interior
shell features, musculature, and ligamenture (Kauffman,
1994, in manuscript).
GENUS SERGIPIA MAURY, 1925
TYPE SPECIES: Inoceramus (Sergipia) posidonomyaformis
Maury, 1925; = Sergipia posidonomyaformis (Maury) fide
DIAGNOSIS: Moderately small adult shells, length
<15 cm, slightly inflated, subequivalve, left valve
slightly larger than right. Shape subrounded to ovate;
length > height, rounded growth-line trace; slightly
prosocline. Beaks subcentral, very slightly projecting,
slightly prosogyrous to orthogyrous. Anterior, posterior
auricles subtriangular, poorly defined from disc by broad
concave trough. Surface ornament of subequally spaced low
rugae and/or faint to strongly raised, close-set, growth
lines and/or lamellae. Ligamental plate very thin,
delicate, extending on both sides of beak, bearing
moderately to widely spaced, weakly excavated,
subtriangular to ovate resilifers. No umbonal septum.
Short, prominent internal rib separates posterior auricle
from disc. Musculature very weakly impressed; poorly
known; pallial line complete, very thin,with faint radial
muscle tracks; posterior adductor insertion area small,
ovate, posteroventral near commissure; no pedal-byssal
muscles or byssal slit known. Shell prismatonacreous,
exceptionally thin, with short blocky calcite
GENERIC DESCRIPTION: Shell attaining moderate size;
length rarely exceeding 10 cm; adult shells averaging 4-5
cm in length. Shell slightly inflated to flattened, with
greatest inflation dorsocentrally at base of umbo.
Subequivalve, with left valve very slightly larger than
right valve. Shape subrounded, ovate, rarely
elongate-ovate with length greater than height; anterior,
ventral, and posterior margins with rounded trace.
Slightly prosocline; beak and umbo slightly prosogyrous
to orthogyrous, beak subcentral to slightly anterior of
midline, only slightly projecting above long dorsal
margin, which extends both anterior and posterior to
beaks. Anterior and posterior auricles triangular to
subtriangular, with angular to somewhat rounded
dorsolateral margins; auricles either merging
continuously with disc or separated from it by very
shallow broad auricular troughs. Surface sculpture
consisting of subequal, closely spaced, small rounded
rugae or large raised growth lines on umbo; rugae become
separated by a few, equally to unequally distributed,
fine raised growth lines on the adult disc; growth lines
more prominent than rugae on some species. Internally,
ligamental plate very thin, rounded, extending both
anterior and posterior to beak (anterior ligament plate
shorter), bearing small, moderately to widely spaced,
subtriangular, weakly to very weakly excavated resilifers
on both sides of beak. Surface sculpture also moderately
defined on interior of very thin prismatonacreous shell
characterized by short blocky prisms. A prominent,
narrow, rounded internal rib extends from the posterior
umbo to or near the commissure at the junction between
the posterior auricle and the disc. No umbonal septum
known. Musculature very weakly impressed and poorly
known. Posterior adductor insertion area small, ovate to
subrounded, located near the ventroposterior commissure.
Posterior adductor muscle track defined by small, weak
lateral grooves on shell interior. Faint, incomplete
traces of very small radial grooves suggest mantle
attachment muscle tracks; pallial line entire where
observed, but incompletely known. No pedal-byssal or gill
suspender muscles observed; byssal slit, if present, not
differentiated on observed specimens; species may not be
byssate as adults.
DISCUSSION: The extension of the resilifer-bearing
ligamental plate anterior to the beaks places the
assignment of Sergipia to the Inoceramidae (Cox, 1969) in
question. The same might be said for Inoceramya Ulrich,
which is inferred to have had similar distribution of
resilifers (Cox, 1969, p. N317, fig. C47-5b). An
alternative placement might be in the Posidoniidae, which
are externally very similar in form and general
ornamental characteristics, and which have the ligamental
plate extended anterior to the beaks in most species. But
the presence of multivincular resilifers along the
anterior and posterior hinge plate of Sergipia, features
not yet known from the Posidoniidae, supports placement
within the Inoceramidae at this time. This taxonomic
assignment is also supported by the external similarity
in form and ornamentation of Sergipia to the inoceramid
genus Steinmannia, which also has the ligamental plate
extended anterior to the beaks, but has resilifers
restricted to its posterior segment (Cox, 1969, fig.
C49-3b), and to the Mytiloides "latus " (sensu Woods,
1912, Fig. 41 only) species group. In fact, some authors
(e.g., Hessel, 1988) have assigned species that
apparently belong to the M. "latus " lineage, and which
lack anterior extension of the ligamental plate, to
Sergipia (e.g., S. hartti Hessel), and vice versa, based
solely on the similarity of external shell form and
ornamentation. We suggest that Sergipia may have been
derived from rounded Lower Turonian M. "latus" (sensu
Woods, 1912, Fig. 41) by changes in the ligamental plate,
normally a conservative character within inoceramid
genera and subgenera. Kauffman and Villamil (1994, in
manuscript) are currently undertaking a taxonomic
revision and documenting the early evolution of Sergipia
from the American species.
GENUS CLADOCERAMUS Heinz, 1932
TYPE SPECIES: Inoceramus undulatoplicatus var.
michaeli Heinz, 1928, p. 76 (=Inoceramus digitatus
Schlüter (non Sowerby) 1877, p. 267, pl. 36; fide
Seitz, 1961, p. 95).
DIAGNOSIS: Small (Turonian) to very large (Santonian;
>1 m axial length) adult shells; subequivalve to
slightly inequivalve; left valve slightly larger. Outline
ovate to elongate-ovate to subtrapezoidal; moderately
prosocline. Beaks, umbones prosogyrous, anterior,
slightly projecting; umbo non-geniculate. Anterior face
rounded; no byssal slit observed. Posterior auricle
relatively small, subtriangular; auricular sulcus
slightly to moderately concave. Juvenile umbonal ornament
of fine to coarse raised growth lines or small rugae;
adult ornament characteristically of weakly to strongly
developed, divaricate radial plicae or folds, most
strongly developed but fewer in number on the posterior
flank of the disc, but in some cases only developed
anteriorly. Relatively weaker concentric ornament of
coarse raised growth lines and small rugae. Shell
prismatonacreous, moderately thin. Ligamental plate
relatively thin for shell size, bearing numerous, closely
spaced, small, slightly-moderately excavated resilifers.
Muscles weakly impressed. Posterior adductor insertion
area large, canoe-shaped, close to posteroventral margin;
pallial line thin, continuous, incompletely known; no
pedal-byssal muscle insertion areas known. Umbonal septum
GENERIC DESCRIPTION: Adult shell small (Early
Turonian) to very large size (>1 m in Early
Santonian); subequivalve to slightly inequivalve with
left valve slightly larger and more convex dorsally;
shell slightly to moderately biconvex, moderately
prosocline. Shell outline ovate, subovate, elongate-ovate
to subtrapezoidal. Beaks, umbones moderately prosogyrous
to suberect, situated at or near anterior end of a short
to moderately long hinge line; beaks slightly projecting
dorsally above the hinge axis. Anterior margin straight
to slightly rounded (convex outward), bending slightly to
moderately inward to commissure, but without strong
anterior truncation or development of a pseudolunule. No
byssal slit observed, but anterior face poorly known.
Posterior auricle small relative to shell size,
subtriangular, weakly to moderately defined, separated
from disc by slightly to moderately concave auricular
sulcus on dorsoposterior flank of umbo, or the flank of
the first divaricating fold on the disc. Lateral and
ventral margins of adult shells slightly to moderately
curved, with undulating commissure at intersection of
radial folds or plicae. Surface of juvenile shell on umbo
characterized by a predominance of concentric
ornamentation (fine to coarse raised growth lines or
small rugae; in some cases nearly smooth or with weakly
developed radial costae); adult disc characterized by
small to large divaricate radial plicae or folds which
extend ventrally and curve laterally from a median line
or discontinuous ridge approximating the growth axis of
the shell; posterior plicae or folds commonly fewer and
larger than those anteriorly; individual plicae and folds
may bifurcate distally. Some species have plicae, folds
weakly developed on one flank (usually the anterior) of
the shell. Coarse, moderately to broadly spaced, raised,
subequal to unequal concentric growth lines and/or small
rugae cross folds, plicae; raised growth lines commonly
become denser and/or coarser and more evenly spaced on
umbo, with or without radial elements. Shell moderately
thin, becoming thicker near ligamental plate; plate
relatively thin for size of shell, bearing small closely
spaced, slightly to moderately excavated resilifers.
Posterior adductor insertion area large, canoe-shaped,
situated moderately close to the posteroventral
commissural margin. Some species have small umbonal
septum. Pallial line partially known, thin and entire. No
other internal features are well known at present.
DISCUSSION: Among the genera of Inoceramidae
discussed at the workshop, Cladoceramus was the focus of
greatest debate. Heinz (1932) proposed the new genus
Cladoceramus and designated Inoceramus michaeli (= I.
undulatoplicatus var. michaeli Heinz) as the type
species. Heinz (1932) synonymized "I. digitatus"
Schluter, 1877 (non I. digitatus Sowerby, 1829) with I.
undulatoplicatus michaeli Heinz (1928) in designating the
genotype species. Heinz (1932, p. 25) never described or
illustrated his new genus, as noted by Cox (1969; p.
N320), who stated that Cladoceramus was a nomen nudum and
synonymized this genus with Sphenoceramus J. Böhm
1915 (type species Inoceramus cardissoides Goldfuss,
1836; subsequent designation by Vialov, 1960) in the
Treatise on Invertebrate Paleontology. However, Cox
(1969) did not take in to account the ICZN ruling that a
genus does not have to be described to be valid if the
genotype species is validly described at the time the new
genus is proposed (Annie Dhondt, personal communication,
1993) and further overlooked the work of Seitz (1961,
1965), who noted many morphological distinctions between
Cladoceramus, as represented by Inoceramus
undulatoplicatus and related species, and Sphenoceramus,
as represented by I. cardissoides. Unfortunately, Cox
died before these oversights could be corrected in his
1969 Treatise article. Seitz (1961) further validated the
use of this name by providing a diagnosis of Cladoceramus
(as a subgenus of Inoceramus), and illustrated typical
species, i.e., I. (C.) undulatoplicatus Roemer, and I.
(C.) japonicus Nagao and Matsumoto (1961, 1965). Seitz
noted that: (a) Heinz's genus Cladoceramus should have
the rank of subgenus; (b) that Woods (1912) had clearly
separated I. digitatus Schlüter (a Cladoceramus)
from I. digitatus Sowerby (a Sphenoceramus?); (c) that I.
digitatus Schlüter, originally described as a
subspecies, was elevated to species rank (Heinz, 1932);
(d) that Cladoceramus was most closely related to the
subgenus Platyceramus, some species of which had weakly
developed flared plicae and folds (e.g., I. (P.)
cycloides wegneri; I. (P.) rhomboides heinei).
Cladoceramus was separated from Platyceramus by placing
all species with divaricating radial plicae or folds,
which were stronger than the concentric ornament, into
Cladoceramus (the concept basically used today); and (e)
that Cladoceramus was not closely related to, nor
evolutionarily transitional with, the genus Sphenoceramus
as represented by S. pachti, S. cardissoides, S.
steenstrupi and S. schmidti. Seitz (1965) subsequently
defined and illustrated the subgenus Sphenoceramus in
depth; Cox (1969) re-elevated Sphenoceramus to generic
rank. In his 1961 paper, Seitz correctly attributed the
subgenus Cladoceramus to Heinz (1932), even though he
proposed this name without diagnosis. Kauffman (1975,
1991) and Kauffman et al. (1993, in press) has
subsequently used the name Cladoceramus as a full genus,
but without formal justification; this is presented
Subsequent work on inoceramid genera confirms Seitz's
observations that Cladoceramus and Sphenoceramus are
morphologically quite distinct, and only distantly
related. Sphenoceramus is acutely triangular in shape,
with a strongly projecting beak and umbo, a short hinge
line and very small posterior auricle. Various species
either lack radial ornamentation or have radial ribs
weakly to moderately developed, but never bifurcating,
divaricating, or stronger than the concentric
ornamentation on the disc. Further, Sphenoceramus has a
very well defined posterior umbonal folds and sulci
(especially the deep posterior auricular sulcus) and may
have weakly defined anterior sulci on the disc.
Sphenoceramus has a diagnostic concentric ornamentation
of very large, asymmetrical, subevenly developed, angular
to subangular, concentric rugae between which are found a
few coarse raised growth lines which become more
regularly developed dorsally, and weaker ventrally.
Internal characteristics also seem to be very different
between these genera, although they are not yet fully
known. Sphenoceramus has a much thicker ligamental plate,
relatively larger resilifers, and a much larger, more
quadrate to ovate posterior adductor muscle insertion
area than Cladoceramus. Mantle retractor muscle tracks
are common and may be well-defined on the interior shell
surface. Sphenoceramus also has a truncated anterior
face, in some cases a lanceolate pseudolunule, and a long
sinuous byssal slit not found on Cladoceramus. In
Kauffman's revision (1994, in manuscript), Cladoceramus
and Sphenoceramus are regarded as discrete genera of the
Inoceramidae based on a survey of both internal and
external shell features. Both differ significantly from
Inoceramus in shape, size, ornamentation, development of
their folds and sulci, musculature, and the possession of
some prevalent form of strong radial surface sculpture.
Cladoceramus is typical of the Early Santonian of Europe
and North America (Seitz, 1961; Kauffman, 1975; Kauffman
et al., in press), but derived species range throughout
It now appears, however, that Cladoceramus had its
origins in the Early Turonian of Brazil, in the ammonite
zone characterized by Mammites, Kamerunoceras,
Neoptychites and Watinoceras spp. (ammonite zone 2 of
Bengtson, 1983). Hessel (1988) described five species of
a new genus, Rhyssomytiloides, in this middle Early
Turonian ammonite zone from the Sergipe Basin, Brazil: R.
mauryae, R. bengtsoni, R. alatus, R. beurleni, and R.
retirensis. These species have external characteristics
identical to those of Cladoceramus, including: (a) coarse
divaricating radial folds or plicae on the adult portion
of the shell, which are stronger than the concentric
ornamentation; (b) strongly developed, raised, subequal
growth lines or small rugae on the umbo and early adult
portions of the disc; (c) an undulating commissure where
the folds intersect it; (d) a moderately prosocline shell
with a slightly curved growth axis; (e) a small,
subacute, subanteriorly situated beak which barely
projects above the hinge line; and (f) a somewhat flared,
slightly undulating, dorsoposterior auricle which is
poorly differentiated from the disc by a broad sulcus. In
fact, Hessel's species are very similar to the earliest
growth stages of large I. (Cladoceramus) undulatoplicatus
undulatoplicatus, and I. (C.) undulatoplicatus michaeli
illustrated by Seitz (1961, pl. 5,6), suggesting that
evolution of the group between the Early Turonian and
Early Santonian mainly involved great expansion and
modification of the adult growth stage. Inasmuch as
interior shell characters of the species placed by Hessel
(1988) in Rhyssomytiloides are still poorly known (a
large posterior adductor muscle insertion area, and small
resilifers along the relatively thin ligamental plate
near the beak), parallel evolution of discrete inoceramid
genera to produce this shell form cannot be demonstrated.
Thus, at this time Rhyssomytiloides Hessel is best placed
in synonymy with Cladoceramus Heinz (1932). Whereas
Hessel (1988) discussed differences between
Rhyssomytiloides and Sphenoceramus Böhm, to which
she had originally assigned R. alatus and R. mauryae, no
generic level comparisons were made to Cladoceramus.
Hessel did note, (1988, p. 28), however, very close
similarities between R. mauryae and two common
Cladoceramus species, "Inoceramus" undulatoplicatus
Roemer, and "I. (Platyceramus)" japonicus japonicus. She
distinguished R. mauryae from these species primarily on
shell size, thickness, inclination, modest differences in
inflation, and varying development of the concentric
ornamentation. None of these are deemed generic-level
distinctions. The validity of Rhyssomytiloides as a
discrete inoceramid genus will depend upon the results of
detailed study of the internal shell features and their
comparison to those of Cladoceramus.
At the Hamburg inoceramid workshop, debate regarding
Cladoceramus as a valid genus in the Early Turonian
focused on two things: (1) whether or not Cladoceramus
should be placed in synonymy with the genus Platyceramus
, and (2) whether or not Rhyssomytiloides should be
allowed to stand as a genus, rather than being
synonymized with Cladoceramus.
Specifically, Matsumoto, Noda, and Kozai (1982), Noda
(1983), and Lopez (1986, 1992, personal communication,
1993), among others, have recommended placing
Cladoceramus in synonymy with Platyceramus, which has
page priority over Cladoceramus , and have noted
transitional forms between them, even within single
populations (e.g., Platyceramus higoensis; Noda, 1983).
Collectively, these authors have pointed out similarities
in shape, convexity, and concentric ornamentation between
Cladoceramus and Platyceramus. The only difference
between them, as currently defined, is that radial and
divaricating ornamentation is stronger than the
concentric ornamentation in species assigned to
Cladoceramus, whereas some Platyceramus species have
poorly developed radial ornamentation that is more weakly
defined than the concentric ornamentation, and which
occurs more irregularly in populations. Such transitional
forms are not yet known, however, in the genotype species
of Platyceramus. Annie Dhondt also now favors this view
(personal communication, Dec. 5, 1993), in contrast to
her generic usage of Cladoceramus in Dhondt and Dieni
(1990). Seitz (1961), in contrast, favored retaining
Cladoceramus as a subgenus for those inoceramids with
divaricating radial ornamentation that is more prominent
than the concentric ornamentation, although he clearly
noted difficulties in assigning a genus to transitional
species. Kauffman (1975, 1991) and Kauffman et al. (1993,
in press) have used Cladoceramus as a full genus, and
placed within it all large Santonian inoceramids in this
plexus with radial or divaricating ornamentation of any
sort within species populations, including some taxa
formally placed within Platyceramus. Both views have
validity and need to be further explored until
ontogenetic development, internal shell characteristics,
and ornamental development of the shells of Platyceramus
and Cladoceramus spp. can be critically compared; this is
beyond the scope of the present paper and the principal
authors choose to retain Cladoceramus as a valid genus
until this careful systematic work is completed.
Finally, three workshop participants (Tröger,
Lopez, Hilbrecht) expressed the view that
Rhyssomytiloides Hessel should not be placed into
synonymy with Cladoceramus until interior shell
structures can be compared to prove the linkage. This
view, however, promotes the continued use of a newly
proposed genus that cannot be differentiated in any
substantial way from a previously described genus with
priority. Rhyssomytiloides, except for its relatively
small size, appears to have identical ontogenetic
development, shell shape and ornamentation to species now
placed within Cladoceramus (e.g. C. undulatoplicatus),
and because maximum adult shell size of a population from
a single region is rarely even regarded as a
species-level characteristic within clades because of
potential ecological controls, there is no justification
for retaining Rhyssomytiloides as a distinct genus. There
are no known characters to separate it from Cladoceramus
spp. at present (or from Platyceramus, if Cladoceramus is
considered a synonym of this genus). The fact that a time
gap exists between the Turonian and Santonian occurrences
of Cladoceramus is not justification for generic
separation. Many inoceramid lineages show these gaps
(e.g., Early and uppermost Turonian occurrences of
Mytiloides, and Permian and Jurassic occurrences of the
Family Inoceramidae, without intermediate
LOWER TURONIAN INOCERAMID BIOSTRATIGRAPHY
The Inoceramidae are exceptionally good
biostratigraphic tools among Bivalvia because of an
unusual combination of traits (Kauffman, 1975). The great
majority of known inoceramid species have
intercontinental to cosmopolitan distribution in normal
marine, temperate zone facies; they are much less common,
but still widespread, in the Tethyan Realm. When compared
to ammonite and plankton zonal boundaries or to
widespread event surfaces/intervals, the Inoceramidae
appear to have had rapid, widespread dispersal
mechanisms; their range boundaries commonly approximate
(but do not equal) chronostratigraphic surfaces.
Dispersal was apparently by very long-lived
planktotrophic larvae, as is also suggested by large
inflated prodissoconchs known from a few species. Species
durations of biostratigraphically useful Inoceramidae are
remarkably short for such a cosmopolitan group, ranging
from 0.12 - 0.5 Ma per range zone in middle and Upper
Cretaceous strata of the Western Interior Basin of North
America (Kauffman, 1975; Kauffman et al., 1993, in
press), where they can be compared to new, closely
spaced,single crystal 40Ar-39Ar radiometric ages
(Obradovich, 1993, in press). They evolved at rates
comparable to those of ammonites and much faster than
those of marine plankton in the Cretaceous. Normally,
broad biogeographic dispersal of large populations slows
evolutionary rates in marine organisms (e.g., Jablonski,
1986), so that the Inoceramidae are unusual in this
respect. Finally, the Inoceramidae are the numerically
dominant macrofossil in most Cretaceous fine-grained
facies, especially those representing oxygen-restricted
benthic environments (e.g., Sageman, 1989, Kauffman and
Sageman, 1990), and are an important component of
communities in coarser-grained nearshore facies as well.
Their resistant, organically bound prismatic calcite
shell layer preserves in most marine facies, including
those where ammonites may be partially or wholly
dissolved during early diagenesis. These characters
enhance the biostratigraphic utility of the Inoceramidae
in Jurassic and Cretaceous sequences worldwide.
Several biozonal schemes have been proposed for Lower
Turonian Inoceramidae. Initially, authors in both Europe
and America utilized a single zone, that of "Inoceramus
labiatus" (s.l.) for this substage (e.g., Cobban and
Reeside, 1952). Kauffman (1975, 1976 a,b,c), Kauffman et
al. (1976, 1993, in press), Seitz (1934), Tröger
(1981), and Walaszczyk (1992), among others, have
proposed a more refined lineage zonation based on rapidly
evolving Lower Turonian species within the genus
Mytiloides. Initially, following the systematic concepts
of Seitz (1934), this zonation consisted of (in ascending
order) the zones of M. submytiloides, M. opalensis, M.
mytiloides and M. labiatus, and assumed only slight
stratigraphic overlap between these species range zones.
Subsequent high-resolution stratigraphic collection of
the Early Turonian in Europe and America, however,
revealed additional species and subspecies of Mytiloides,
and rare Inoceramus (s.s.) in this interval, as well as
more extensive stratigraphic overlap between some of the
zonal species. Elder (1991) described M. hattini as a
basal Turonian stratigraphic index; Kennedy et al. (1987)
correctly pointed out that the type of Inoceramus
opalensis Böse was distinct from Seitz's (1934)
concept and was probably Middle to Late Turonian or even
Coniacian in age, within the Mytiloides hercynicus and/or
Inoceramus waltersdorfensis lineages. Subsequently, Early
Turonian forms originally assigned to M. opalensis
(Böse) by Seitz (1934) have been reassigned to M.
kossmati Heinz (1933) by Walaszczyk (1992; confirmed in
this workshop), who also synonymized M. goppelnensis
Badillet and Sornay with M. kossmati. These observations,
in turn, paved the way for simple assemblage zonation,
and greater biostratigraphic refinement for the Early
Turonian Mytiloides. The attempt of Kennedy and Cobban
(1991) to simplify Early Turonian inoceramid zonation by
establishing two zones (Mytiloides columbianus and M.
mytiloides) for the section at Pueblo, Colorado, is
herein rejected. Their species concepts are far broader
than those utilized by specialists in the field (e.g.,
the Hamburg working group), and they have made serious
taxonomic errors in the manner in which these names are
Whereas the Hamburg working group on Lower Turonian
Inoceramidae discussed stratigraphic ranges for most
latest Cenomanian and Lower to Middle Turonian species,
no compilation of inoceramid biostratigraphy was
attempted at the meeting, but it was noted that somewhat
different zonal systems were utilized in Eurasia and
North America, that some taxonomic changes were needed
and species range zones modified, and that additions
could be made to both systems. Subsequently, some of the
workshop participants offered preliminary drafts of new
or modified zonal schemes for the latest Cenomanian -
Middle Turonian interval, and these are included herein
as preliminary zonal hypotheses for future discussion and
modification. Hilbrecht (November, 1993, personal
communication) further suggested a simplified global
zonal scheme for the Early and early Middle Turonian,
utilizing inoceramids, of Mytiloides hattini (base), M.
mytiloides, M. labiatus, and M. hercynicus.
represents the most recent
compilation of Lower Turonian inoceramid species and
subspecies ranges for the North American Gulf Coast and
Western Interior Cretaceous seaways, reflecting the
composite opinions of the WIK Chronology working group of
the Global Sedimentary Geology Program (GSGP: CRER).
High-resolution stratigraphic data from North America has
been primarily compiled by Kauffman (1975, 1977),
Kauffman et al. (1976, 1993, in press), Elder (1987,
1989), and Harries (1990, 1993). These ranges have been
established utilizing HIRES sampling techniques
(Kauffman, 1988a; Kauffman et al., 1991), and involved
continuous bulk sampling of 10-20 cm intervals across the
Cenomanian-Turonian boundary at nearly 100 sections from
Texas to southern Canada. Integration of range zone data
was by linear and graphic correlation utilizing 112
event-chronostratigraphic marker horizons/beds
(bentonites, Milankovitch climate cycle deposits, etc.).
The distribution of composite inoceramid taxon range
zones for North America is shown in Figure 6, in addition
to the probable stratigraphic position of Brazilian
Cladoceramus (= Rhyssomytiloides) from the Sergipe Basin.
Note that a number of chronologically (evolutionarily)
transitional forms between described species are
recognized and plotted to give our colleagues a better
view of the extent of each species plexus; some of these
may receive new subspecies names during planned taxonomic
revision. From this diagram, a series of assemblage
biozones can be constructed, utilizing Inoceramidae, and
tied to the standard ammonite zonation of Cobban (1985;
1993 in press) and Kennedy and Cobban (1991) for
Euramerica. Figure 7
shows simple composite range
zones (represented by the individual boxes) constructed
from the stratigraphic ranges of the most abundant and
widespread Lower Turonian inoceramids in America. These
composite zones can be used for inter-regional
correlation. Through the combination and overlap of these
composite range zones, the Lower Turonian can be
subdivided into eight different assemblage zones in this
presents the latest inoceramid
biozonation for the Western European Early to Middle
Turonian by Walaszczyk (1992), as endorsed by Tröger
(personal communication, Dec., 1993), and with the
addition of Mytiloides hattini Elder and M. wiedmanni
Lopez in the basalmost Turonian, as suggested by
Hilbrecht (personal communication, November, 1993) and
Lopez (personal communication, December, 1993),
respectively. In his communication, Lopez further
suggested inclusion of subspecies originally described
under M. goppelnensis, and now valid subspecies of M.
kossmati (see Walaszczyk, 1992, and previous discussion).
But because the stratigraphic ranges of these subspecies
are not yet well known, they have not been included in
this preliminary European biozonation (Fig. 8).
At the broad scale of zonation, the North American
and European biozones compare favorably among
Inoceramidae. Both define basal Turonian zones
characterized by typical Mytiloides hattini (regionally
co-occurring with M. wiedmanni or M. submytiloides), a
second biozone with M. kossmati (with or without M.
hattini), a third concurrent range zone of M. kossmati
with M. mytiloides, a fourth concurrent range zone of M.
kossmati, M. mytiloides, and M. labiatus, a fifth biozone
dominated by M. labiatus, and an late Early /early Middle
Turonian biozone characterized by M. labiatus with M.
subhercynicus, overlain by an M. hercynicus zone. Exact
ammonite boundaries are more difficult to define in
European successions due to the relative paucity of
ammonites, and further refinement is required to
pin-point the stage boundaries. Finer scale divisions of
the North American sequence may reflect higher intensity
of collecting, at closer stratigraphic levels, inherent
in high-resolution stratigraphy, as well as the
relatively less condensed nature of many North American
Lower Turonian sequences.
The Lower Turonian Inoceramidae underwent a major
evolutionary radiation following the Cenomanian-Turonian
boundary mass extinction, primarily among members of the
genus Mytiloides. Rapid evolutionary rates coupled with
rapid intercontinental to cosmopolitan dispersal of many
species present a perplexing evolutionary problem, but
produces one of the most refined inter-regional
biostratigraphic zonations of the Cretaceous. As with any
major radiation, taxonomic problems abound, and
phylogenetic relationships are sometimes difficult to
define. Even among experienced workers in the field who
attended the Hamburg workshop, no consensus could be
reached on the phylogenetic relationships of Lower
Turonian Inoceramidae. But the meeting did produce
important advances in our understanding of this unique
group of bivalves. Generic, and to a large degree,
species concepts were stabilized, morphologic and
morphometric parameters defined for future systematic
work, the ecology and life habit of these Inoceramidae
broadly discussed, and preliminary biostratigraphic
revisions made for North America and Western Europe.
Perhaps most valuable to all of us was the open forum for
discussion and the identification of areas of controversy
which will stimulate further work, and guide future
ACKNOWLEDGEMENTS: We would like to thank Prof.
Christian Spaeth (Hamburg) for allowing us to include our
workshop as part of the 4th International Cretaceous
Symposium (Sept. 26-Oct.4, 1992), and we would also like
to thank Heinz Hilbrecht (ETH, Switzerland) and Peter
Harries (Tampa, FL, USA) for the exceptional job they did
in organizing the workshop and making it a reality. The
input of all of our colleagues at the workshop and the
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