Jaws of late Cretaceous placenticeratid ammonites : how preservation affects the interpretation of morphology

2006·4.7 MB·English

by  Landman, Neil H.

-» TAMERICAN MUSEUM
JSIovitates
PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3500, 48 pp., 34 figures January 12, 2006
Jaws of Late Cretaceous Placenticeratid
Ammonites: How Preservation Affects the
Interpretation of Morphology
NEIL H. LANDMAN,1 CAMERON J. TSUJITA,2 WILLIAM A. COBBAN,3
NEAL L. LARSON,4 KAZUSHIGE TANABE,5 AND ROBERTA L. FLEMMING2
ABSTRACT
We describe upper and lower jaws of Placenticeras Meek, 1876, from the Upper Cretaceous
(upper Campanian) Bearpaw Shale and Pierre Shale of the Western Interior of North America
and lower jaws of the related ammonite Metaplacenticeras Spath, 1926, from the Campanian
Yasukawa Formation of Hokkaido, Japan. One lower jaw is preserved inside the body chamber
of Placenticeras costatum Hyatt, 1903. The other jaws are isolated but are generally associated
with fragments of placenticeratid shells. The jaws from North America are attributed to Pla¬
centiceras meeki Bohm, 1898, and P. costatum, while those from Japan are attributed to
Metaplacenticeras subtilistriatum (Jimbo, 1894). All of the jaws are presumed to be from
adults.
The jaws of Placenticeras attain lengths of up to 95 mm. They are preserved as steinkems
with a thin film of black material, representing diagenetically altered chitin. X-ray diffraction
analysis of samples of this material indicates that it consists of magnesium-rich calcite, pyrite,
and amorphous material (organic compounds). The upper jaw is approximately the same length
as the lower jaw and is U-shaped, with narrow wings that converge anteriorly to a dome¬
shaped hood. The lower jaw is composed of two lamellae. The outer lamella is broad and
consists of two wings terminating in a bilobate posterior margin. The inner lamella is one-
1 Division of Paleontology (Invertebrates), American Museum of Natural History ([email protected]).
2 Department of Earth Sciences, Biological and Geological Sciences Building, University of Western Ontario,
London, Ontario, N6A 5B7, Canada ([email protected]; [email protected]).
3 Division of Paleontology (Invertebrates); Home address: 70 Estes St., Lakewood, CO 80226.
4 Black Hills Museum of Natural History, P.O. Box 614, Hill City, SD 57745 ([email protected]).
5 Department of Earth and Planetary Sciences, University of Tokyo, Hongo 7-3-1, Tokyo 113-003, Japan (tanabe@
eps.s.u-tokyo.ac.jp).
Copyright © American Museum of Natural History 2006 ISSN 0003-0082
2 AMERICAN MUSEUM NOVITATES NO. 3500
half the length of the outer lamella. The two lamellae are separated except in the apical region
and along the sides. The junction between the lamellae appears as a U or V-shaped outline
on the anterior portion on the ventral surface of the jaw. This junction is especially conspicuous
in specimens in which part of the inner lamella has eroded away.
In crushed specimens, the lower jaw is subquadrate in shape. In specimens that retain some
or all of their original curvature, the central portion is gently convex and the sides bend steeply
dorsally. The rostrum projects slightly anteriorly and dorsally and there is a thickened rim of
chitin along the anterior margin where the two lamellae are doubled over. A small indentation
appears at the apical end and, in most specimens, develops into a midline slit that extends
posteriorly 10-15 mm. However, as shown in well-preserved specimens and based on com¬
parisons with the jaws of closely related ammonites, this slit represents the remnants of a
narrow ridge on the ventral side of the inner lamella. This ridge is surrounded by an elongate
boss of thickened chitin, which corresponds to a depression on the dorsal side. The ventral
surface of the outer lamella bears a midline ridge with a central groove, which essentially
forms a continuation of the ridge on the inner lamella. The ventral surface of the outer lamella
is ornamented with thin, radial striations and irregular broad undulations paralleling the pos¬
terior margin. The posterior end is generally incomplete, probably as a result of predation or
postmortem degradation, and the lateral margins are commonly creased, indicating postmortem
plastic deformation.
The lower jaws of Metaplacenticeras subtilistriatum are much smaller than those of Pla-
centiceras but are otherwise similar in morphology. However, they retain pieces of a very
thin, fibrous outer layer comprising two plates. X-ray diffraction analysis of samples of this
layer indicates that it consists of calcite enriched in magnesium. Each plate covers the ventral
surface of one of the wings and terminates at the midline ridge. Based on the close affinity
of Metaplacenticeras and Placenticeras, and in comparison with published descriptions of
placenticeratid jaws from elsewhere, we hypothesize that similar plates covered the lower jaws
of all placenticeratids, although these plates have not been found in any Placenticeras material
from North America. The thin nature and fibrous micro structure of this layer would have made
it susceptible to mechanical breakage and chemical dissolution. Furthermore, jaws are internal
structures embedded in the buccal bulb. The micro-environment within this bulb may have
promoted dissolution of the outer calcitic layer of the lower jaw.
The presence of a pair of calcitic plates (aptychi) and a midline ridge with a central groove
on the outer lamella of the lower jaw are unique features of the lower jaws of the Aptychophora
Engeser and Keupp, 2002. Although differences in preservation obscure this similarity, the
lower jaws of placenticeratids conform to the description of aptychus-type jaws. However,
unlike the thick calcitic aptychi of other Ammonitina, the thin calcitic aptychi of placenticer¬
atids probably did not function as opercula and would have served simply to strengthen the
lower jaw. The jaws of placenticeratids were probably designed for biting and cutting food
rather than for passively collecting and straining plankton. Other data about the habitat and
mode of life of placenticeratids are consistent with this interpretation. These ammonites prob¬
ably inhabited surface waters and were capable of pursuing and attacking sluggish prey. An
ecological analog of placenticeratids may be the modern ocean sunfish Mola mola (Linnaeus,
1758), which inhabits surface waters and feeds on gelatinous zooplankton.
INTRODUCTION common ammonites. One might expect that
the jaws of these ammonites, the shells of
Jaws of Late Cretaceous placenticeratid which sometimes attain diameters of approx¬
ammonites have been reported from Slovenia imately 1 m, would have been sufficiently
(Summesberger et al., 1996, 1999) and India robust to be preserved.
(Gangopadhyay and Bardhan, 1998). How¬ We describe several jaws of Placenticeras
ever, no placenticeratid jaws have been re¬ Meek, 1876, from the Upper Cretaceous
ported from the North American Western In¬ (Campanian) Bearpaw Shale of Alberta
terior. This is somewhat surprising given the (commonly referred to as the Bearpaw For¬
abundance of large, well-preserved speci¬ mation by most Canadian workers) and the
mens of placenticeratids in this region. At Pierre Shale of Colorado and South Dakota
some localities, placenticeratids are the most (fig. 1). One specimen occurs inside the body
2006 LANDMAN ET AL.: JAWS OF LATE CRETACEOUS AMMONITES 3
120° 110° 100°
Fig. 1. Map of parts of the United States and Canada showing the localities of the jaws described
in the text.
chamber of Placenticeras costatum Hyatt, buted. All of the jaws are presumed to be
1903. The other specimens are isolated but from adults.
are generally associated with shards of pla- In describing, comparing, and interpreting
centiceratid shell. They belong to either Pla¬ ammonite jaws, much depends on the state
centiceras meeki Bohm, 1898, or P. costatum of preservation of the specimens. Sometimes
because these are the only two species of a calcitic layer is preserved; sometimes just
Placenticeras that occur in these strata. We the impression of it; or sometimes none of it
also describe lower jaws of the closely relat¬ at all. However, the absence of a calcitic lay¬
ed ammonite Metaplacenticeras Spath, 1926, er does not necessarily imply that it did not
from the Campanian Yasukawa Formation of exist originally. In some specimens, the en¬
Hokkaido, Japan. These jaws occur as iso¬ tire chitinous part of the jaw, now altered, is
lated elements but are closely associated with preserved. In other specimens, part of the
specimens of Metaplacenticeras subtilistria- chitinous layer has broken off. It is important
tum (Jimbo, 1894), to which they are attri¬ to examine as many specimens as possible,
4 AMERICAN MUSEUM NOVITATES NO. 3500
because each specimen offers different infor¬ seum (USNM), Washington, DC; and the
mation depending on its unique state of pres¬ University of Western Ontario (UWO), Lon¬
ervation, and all of this information contrib¬ don, Ontario, Canada.
utes to an overall picture. Analysis of a sin¬
gle specimen, if poorly preserved, can lead LIST OL LOCALITIES
to erroneous conclusions.
Numbers refer to figure 1.
1. U.S. Geological Survey loc. D1353.
MATERIAL Upper Campanian, Baculites cuneatus Zone,
Pierre Shale, NW !4 SW % sec. 17, T. 3N, R.
The lower jaw of Placenticeras costatum
80W, Grand County, Colorado, USA.
from the Pierre Shale of Colorado is pre¬
2a. Upper Campanian, Baculites compres-
served inside the body chamber. The rest of
sus Zone, Pierre Shale, sec. 6, T. 3 N, R. 14E,
the North American material consists of iso¬
Elk Creek, Meade County, South Dakota,
lated jaws generally associated with frag¬
USA.
ments of placenticeratid shells in calcite- or
2b. Upper Campanian, Baculites cuneatus
siderite-cemented concretions. It is possible
Zone, Pierre Shale, approximately 11 km
that some of these jaws occur inside the body
northwest of loc. 2a, Cheyenne River Drain¬
chamber, but this is impossible to demon¬
age, Meade County, South Dakota, USA.
strate.
3. Upper Campanian, Baculites cuneatus
The Pierre Shale of South Dakota also
Zone, Bearpaw Shale, Korite Mine, adjacent
yields jaws of scaphites, baculites, and co-
to St. Mary River, Welling, Alberta, Canada.
leoids in addition to those of Placenticeras.
4. Upper Campanian, Baculites reesidei
However, the lower jaws of these other ceph-
Zone, Bearpaw Shale, immediately below the
alopods differ from those of Placenticeras in
lowest sandy transitional beds of the Horse¬
either size, shape, or ornament. The lower
shoe Canyon Lormation, Travers Dam, Al¬
jaws of scaphites are usually smaller (15-40
berta, Canada.
mm long in adults) and nearly triangular in
5. Upper Campanian, Yasukawa Lorma-
shape (Meek, 1876; Landman and Waage,
tion, Rubeshube Creek, Enbetsu Town, north
1993). The outer lamella bears a prominent
Hokkaido, Japan.
flange with a central groove and a pair of
thick aptychi ornamented with lirae parallel¬
CONVENTIONS
ing the posterior margin. The lower jaws of
baculites vary in size, but are usually less The terminology used to describe ammo¬
than 40 mm long in adults (Klinger and Ken¬ nite jaws is modified from Kanie (1982),
nedy, 2001; Larson et al., in press). They are Tanabe (1983), Clarke, (1986), and Tanabe
elongate in shape with a pair of thick aptychi and Lukuda (1987) and illustrated in figure
ornamented with prominent rugae. The lower 2. Upper and lower jaws are described with
jaws of coleoids are sharply folded without respect to the anterior, posterior, ventral, and
a median ridge, and the apical end is strongly dorsal directions. (The ventral and dorsal sur¬
projected forward. In contrast, the lower jaws faces of the lower jaw are equivalent to the
of Placenticeras, as reported in this paper, outer and inner surfaces, respectively; con¬
are massive, up to 95 mm in length, and sub¬ versely, the ventral and dorsal surfaces of the
quadrate in shape. The outer lamella is char¬ upper jaw are equivalent to the inner and out¬
acterized by a midline ridge with a central er surfaces, respectively.) The left and right
groove and a pair of very thin aptychi. sides of the jaw refer to the jaw as it was
Specimens are reposited in the American oriented in life. However, because all of the
Museum of Natural History (AMNH), New lower jaws are preserved with the ventral
York; the Black Hills Museum of Natural side up, contrary to their position in life, the
History (BHMNH), Hill City, South Dakota; left and right sides of the jaws are opposite
the Royal Tyrrell Museum of Palaeontology to what we observe. The terms “inner and
(TMP), Drumheller, Alberta, Canada; the outer lamellae” (Tanabe and Lukuda, 1987)
University Museum of the University of To¬ are used instead of “inner and outer plates”
kyo (UMUT), Tokyo; the U.S. National Mu¬ (Dagys et al., 1989) to conform with the ter-
2006 LANDMAN ET AL.: JAWS OF LATE CRETACEOUS AMMONITES 5
Upper Jaw
Anterior Anterior
a
A
Posterior Posterior
Dorsal View Ventral View
Lower Jaw
Anterior Anterior
Slit
Fig. 2. Diagram of upper and lower jaws illustrating terminology and measurements. A. Dorsal view
of upper jaw. B. Ventral view of upper jaw. C. Ventral view of lower jaw. D. Dorsal view of lower
jaw. Abbreviation: a = apical angle.
6 AMERICAN MUSEUM NOVITATES NO. 3500
minology of modern cephalopod jaws. The from AMNH 47275, and three samples from
two symmetric halves of the lower jaw are BHMNH 5456). At the AMNH, we used a
referred to as wings, but this does not imply Rigaku DMAX/RAPID Micro X-ray diffrac¬
that they are separate structures. The apical tion system employing a 100 (Jim monocap¬
tip of the jaw is called the rostrum. The hood illary collimator and monochromatized Cu-
of the jaw is the median portion just posterior Ka radiation to produce diffractions on an
of the rostrum. image plate. Samples were oscillated about a
The terms “anaptychus” and “aptychus” vertical axis («) and rotated around a 4>-axis
are commonly used to describe ammonite inclined at 45° (fixed) from w to obtain suf¬
jaws (Lehmann, 1981; Engeser and Keupp, ficient diffractions on the cylindrial image
2002), but often in different ways. For ex¬ plate. Images were integrated into diffracto-
ample, the term “aptychus” sometimes re¬ grams of two-theta (20) versus intensity.
fers to the entire lower jaw; other times, it They were interpreted with Jade 7.0 (MDI)
only refers to the calcareous plates covering software, with phase identification based on
the ventral surface of the lower jaw. In the the ICDD PDF-2 diffraction database and/or
interests of consistency, we define an anap- synthetic diffraction patterns derived from
tychus-type jaw as a lower jaw in which the the Mineralogical Society of America
outer lamella lacks a median fold, with or (MSA) Crystal Structure Database (http:
without a calcareous deposit. An aptychus- www.minsocam.org/MSA/CrystaLDatabase.
type jaw is defined as a lower jaw in which html).
the outer lamella bears a median fold, as well At the University of Western Ontario, the
as a calcareous layer in the form of two mineralogical composition of the jaws was
plates—the aptychi. determined using a Bruker D8 Discover dif¬
In measuring jaws, it is important to re¬ fractometer. The diffractometer was operated
member that most of our specimens are in¬ with Cu-Ka radiation generated at 40 kV and
complete. The length (L) and width (W) of 40 mA. A Gobel mirror parallel optics sys¬
the jaws were measured following Kanie tem was employed to remove K[3 radiation
(1982) and Tanabe and Fukuda (1987). The and minimize the effect of sample displace¬
measurement of width depends on whether ment. A beam collimator snout was used to
the jaw is flattened (ironed out) or retains its reduce the Ka beam diameter to 500 |am.
original shape. The ratio of width to length Grains were mounted on double sided tape
of a single wing is a more reliable estimate on an XYZ stage, which was moved via re¬
of shape than the ratio of width to length of mote control to analyze selected points. An
the entire jaw because both wings are seldom optical microscope/video camera and laser
preserved intact. The measurement of the system permitted precise positioning of each
apical angle (also called the rostral angle) is sample. The 0-0 geometry of the diffractom¬
also dependent on the state of preservation eter allowed the sample to remain stationary
of the jaw. (flat), while the source and the detector
Specimens are photographed with and moved independently. The angle between the
without coating (magnesium oxide). The sample and the source was while the angle
photographs are accompanied by camera lu- between the sample and the detector was 02,
cida drawings highlighting certain features. where 0j + 02 = 20.
We also illustrate hypothetical cross sections The majority of samples were analyzed in
through various parts of the jaws, based on Coupled Scan Mode, where 0j = 02 and 0j
observations of whole specimens. + 02 = 20. This is a stationary mode, where
the sample, the beam, and the detector re¬
METHODOFOGY main stationary. This is useful for polycrys¬
talline samples (or nearly so). The sample
To determine the mineralogical composi¬ from BHMNH 5456 was analyzed in Omega
tion of the jaws, we analyzed samples from Scan Mode, where the sample remains sta¬
six specimens (one sample each from UMUT tionary but the X-ray optics (source = 0,;
MM28896, UMUT MM28897, UWO.KMC. detector = 02) are rotated through an omega
100126, and BHMNH 5454b, two samples angle of 30°. At the start of the omega scan,
2006 LANDMAN ET AL.: JAWS OF LATE CRETACEOUS AMMONITES 7
the source and detector were positioned at 6° on the lateral margins. There is a narrow slit
and 30° 0, respectively. Both the source and on the midline approximately 1.5 mm wide
detector were rotated clockwise, by an ome¬ at the apex. This slit extends posteriorly 9
ga angle of 30°, stopping at 36° and 3° 0, mm, narrowing down completely, although
respectively. At all points during rotation, 20 the point of the slit is covered. The edges of
remained constant at 0j + 02 = 36°. The sec¬ the slit are sharply defined. At the posterior
ond frame was collected with 01(start) = 27° end of the slit, the wings appear to be joined
and 02(start) = 45°, rotated through 23° to 01(stop) together, forming a ridge. The slit is bordered
= 50° and 02(stop) = 22°, yielding a constant on each side by narrow radial ridges, the out¬
20 = 72°. Omega scans were performed for er margins of which form radial depressions.
time intervals of 30 minutes per frame (at These flexures are covered in places by
500 pm beam diameter). patches of coarsely crystalline black materi¬
Diffracted X-rays were detected using a al, eliminating most of the relief. The black
General Area Diffraction Detection System material consists of finely fractured square
(GADDS) 2D detector positioned at a dis¬ and rectangular pieces, resembling a honey¬
tance of —15 cm from the sample and inter¬ comb, with tiny clear to yellowish crystals
secting a 35° segment of diffraction space, embedded in the fractures. The black mate¬
centered at 28.5° 20. Powder lines and/or dif¬ rial in this region represents the thickened
fraction spots resulting from each lattice portion of the inner lamella.
plane produced an arc of radius 20hk,. The An elongate patch of golden, finely crys¬
GADDS software integrated the intensity talline material approximately 8 mm long
along each arc, and corrected for the effect covers the midline starting at the end of the
of flat screen detection (unwarping) to create slit. (This material also occurs in patches on
a one-dimensional plot of intensity versus 20, adjacent parts of the jaw where it overlies the
similar to a conventional powder diffraction black material.) This elongate patch is sur¬
pattern. The resulting diffraction pattern was rounded by coarsely crystalline black mate¬
imported into the evaluation software EVA, rial that ends in a lobate outline, representing
where phases were identified, after baseline the border of the outer lamella. The rest of
subtraction, using the ICDD database. the jaw is partly covered by a thin film of
fine-grained black material. There is a low
DESCRIPTION OF JAWS median ridge between the wings but no gap.
Starting at the midpoint of the jaw, the two
Placenticeras meeki/Placenticeras costatum
wings are tom apart and become progres¬
Lower Jaw sively more widely separated. The margins
are ragged, probably resulting from postmor¬
Bearpaw Shale, Alberta
tem degradation coupled with sediment com¬
AMNH 47275 is preserved as a flattened paction.
steinkern with a him of black material (figs. Where the black material is missing ex¬
3, 4). It is embedded in a chunk of maroon- posing the steinkern on the anterior end of
colored concretion of calcite- and siderite-ce- the jaw, fine, nearly straight lirae are spaced
mented silty mudstone. No placenticeratid at intervals of approximately 0.75 mm. The
shell material is preserved in the concretion. lirae are oriented perpendicular to the mid¬
The jaw is subquadrate in overall view, line and slant backward at a slight angle to
measuring approximately 95 mm in length the anterior margin. The posterior end of the
and 65 mm in width (W/L = 0.68). The left jaw on the right side is covered by thin, ir¬
side of the anterior margin is missing, and regular ridges that are concave toward the
the jaw is compressed due to sediment com¬ posterior margin. There are also irregular
paction. However, the jaw is weakly convex longitudinal creases along the left lateral
in cross section, its lateral edges turning edge. The surface of the jaw shows bumps
obliquely dorsally for a few millimeters. and wrinkles suggesting postmortem plastic
The anterior margin is narrowly rounded, deformation.
forming an apical angle of approximately AMNH 47276 is preserved in a fragment
105°, increasing to a broader angle of 145° of a concretion composed of siderite-ce-
AMERICAN MUSEUM NOVITATES NO. 3500
Fig. 3. Lower jaw of Placenticeras meeki Bohm, 1898, or Placenticeras costatum Hyatt, 1903,
AMNH 47275, Baculites cuneatus Zone, Bearpaw Shale, Welling, Alberta, Canada, ventral view, an¬
terior on top. A. Overview showing the ragged posterior margin, XI. B. Close-up of the anterior end
showing the apical slit and U-shaped outline of the junction between the inner and outer lamellae
(arrow), X3.
2006 LANDMAN ET AL.: JAWS OF LATE CRETACEOUS AMMONITES
B 1 mm
Fig. 4. Drawings of AMNH 47275 (see fig. 3). A. Hypothetical cross section near the apical tip. B.
Close-up of the anterior end. The cross hatched area indicates coarsely crystalline black material; the
lined area indicates finely crystalline yellowish material.
10 AMERICAN MUSEUM NOVITATES NO. 3500
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Fig. 5. Lower jaw of Placenticeras meeki Bohm, 1898, or Placenticeras costatum Hyatt, 1903,
AMNH 47276, Baculites cuneatus Zone, Bearpaw Shale, Welling, Alberta, Canada, ventral view, an¬
terior end on top, showing the curved anterior margin, X1.
merited sandstone (fig. 5). It is associated along the midline in the anterior region, but
with an angular shard of iridescent shell that it is indistinct. It is bordered by two ridges
is probably a fragment of Placenticeras. that diverge posteriorly. The ridges develop
The jaw is preserved as a steinkem with into broad, raised areas, posterior of which
patches of black material. It is approximately the middle portion of the jaw is not pre¬
85 mm long and 95 mm wide, but it is in¬ served. The jaw ends in a ragged edge.
complete (W/L = 1.12). The right and left The material of the jaw consists of three
wings are each approximately 50 mm wide layers. A patch of golden crystalline material
at their widest points (W/L of each wing = makes up the outermost layer and occurs on
0.59). The jaw is convex in cross section, part of the anterior one-third of the jaw. This
with a gently arched median area and more material is underlain by a layer of shiny,
steeply sloping sides. A small piece of the crackled black material, which in turn is un¬
right anterior portion of the jaw is missing, derlain by a layer of more finely textured
and part of the posterior region is torn and black material—that is, the dorsal surface of
crushed against a gastropod shell. the inner or outer lamella, depending on the
The jaw is parabolic in outline. The ante¬ position.
rior margin is rounded, with an apical angle Distinct, longitudinal striations parallel the
of 145°. The apex is slightly projected, with midline and occur at intervals of approxi¬
a shallow indentation on each side. A slit is mately 0.6 mm on the right wing, especially
not present at the apex, although the speci¬ at its anterior end. They are visible on the
men is broken in this area. A groove occurs layer below the crackled material—that is, on