A cyclic mode of shell growth and its implications in a Late Cretaceous heteromorph ammonite Polyptychoceras pseudogaultinum (Yokoyama)

18 Pages·1997·6.5 MB·English

by  Takashi Okamoto

©Paleontological Research, vol.1, no.1, pp.29-46, 15Figs., April 30, 1997
by the Palaeontological Society of Japan
3. A cyclic mode of shell growth and its implications in a Late
Cretaceous heteromorph ammonite Polyptychoceras
pseudogaultinum (Yokoyama)
TAKASHI OKAMOTO and MAKOTO SHIBATA
Department of Earth Sciences, Ehime University, Bunkyo-cho 2-5, Matsuyama, 790 Japan.
NS Environmental Science Consultancy Inc., Shinbashi 6-14-5, Tokyo, 105 Japan.
Received 18 May, 1996 Revised manuscript accepted 20 December 1996
:
Abstract Polyptychoceras, a Cretaceous heteromorph ammonite genus, is characterized by a trom-
bone-likeshell called a "hamitoid" shell. In ordertoclarifytheshell-forming mechanism,a largesample,
which consistsof morethan 320 specimens ofP. pseudogaultinum (Yokoyama) obtained from the Upper
Santonian of Hokkaido, was biometrically analyzed. Besides the shell coiling, cyclic changes of growth
pattern are recognized bythe analysesofthe shell ornamentation, the relativegrowth rateofshell height,
and the distancebetweensepta. Intermittentshellgrowth,which wasalsodeducedfromtheontogenetic
stagedistribution in the population samples, is probablythe causeofsuch peculiarshell coiling. Also,we
carried outsome computersimulationsto reconstruct hydrostaticallythe ontogenetic changeofthe living
attitudeofP.pseudogaultinum. It issuggested thatthe rateofabsoluteshellgrowth possiblydependson
the living attitude of this ammonite in the water column; the shell grows slowly when the shell aperture
faces upward, and grows rapidly when the aperture faces in other directions. It is likely that every
individual of this ammonite spent most of its life time with an upward-facing aperture.
Key words: Heteromorph ammonite, mode of growth, mode of life, Polyptychoceras pseudogaultinum
(Yokoyama), Upper Cretaceous
Introduction allied genera of the Diplomoceratidae achieved worldwide
distribution, because there are some reliable records of
Ammonites, a dominant cephalopod group during Jurassic occurrencefrom other provinces in the UpperSantonian and
and Cretaceous times, changed their shell form in various Lower Campanian, e.g. Vancouver Island in Canada
ways in order to adapt themselves to the environment. (Whiteaves, 1903) and Barra do Dande in Angola (Howarth,
Heteromorph ammonites were especially diversified and 1965). In the Early Santonian and earlier, however, the
showed various kinds ofshell shape in the Late Cretaceous, occurrences of such primitive genera as Scalarites and
the last phase of ammonites' prosperity. Polyptychoceras Rhyoptychoceras and early species of Polyptychoceras are
Yabe, 1927, belonging to the diplomoceratids, is one of the restricted to the Far East province. Early diversification of
most conspicuous heteromorph genera and is characterized the Diplomoceratidae is thus suggested from the fossil
by a trombone-like "hamitoid" shell ; several straight shafts records inthe FarEastprovince. Thisprovince is,therefore,
run in parallel, successive shafts being connected with a U- important in order to document the evolutionary process of
shaped semi-whorl. This genus was probably derived from the Diplomoceratidae.
Scalarites, presumably a founder of the Diplomoceratidae, The fossils of Polyptychoceras usually occur in fragments
and Eubostrychoceras, a nostoceratid, is probablytheances- in calcareous nodules of the Upper Yezo Group, Hokkaido,
tor of that family (Matsumoto, 1967). The two-dimensional Japan. Previous descriptive studies of Polyptychoceras
shell coiling as observed in the diplomoceratids generally have mostly been based on incomplete or fragmentary
originated from the three-dimensional shell coiling of nos- specimens (Yokoyama, 1890; Jimbo, 1894; Yabe, 1927;
toceratids in the Late Cretaceous (probably Turanian) Matsumoto, 1984). A peculiar life mode of Polyptychoceras
(Matsumoto, 1967 Ward, 1976). was imagined by Matsumoto and Nihongi (1979) they may
; ;
Several speciesofPolyptychoceras occur intheSantonian have lived in schools at the bottom and sometimes in dead
and Campanian of Hokkaido and Sakhalin (Matsumoto, shells of large ammonites. In their argument, however, it
1984; Verchangin et al., 1965). Polyptychoceras and some was not discussed why the peculiar shell form of Polypty-
30 Takashi Okamoto and Makoto Shibata
choceras was advantageous. structions of living attitude through growth in the water
A hamitoid shell morphology like that of Polyptychoceras column, and interpret the mode of life from the relationship
appeared at least three times in independent clades (i.e. between the growth and the reconstructed living attitude.
ptychoceratids, hamitids and diplomoceratids). It is, of Such an approach would make clear howthe hamitoid shell
course, regarded as morphological convergence probably morphologywasadaptivetoacertain modeoflifeintheLate
related to a similar mode of life. Cretaceous seas.
The purpose of this study is to clarifythe mode of growth
of Polyptychoceras by analyzing biometrically the Material and method
ontogenetic change of shell form. The material is Polypty-
choceras pseudogaultinum (Yokoyama) which occurs abun- The UpperYezoGroup inthereachesofKotanbetsu River
dantly in the Upper Yezo Group. We also attempt recon- contains numerousfossiliferouscalcareous nodules inwhich
Figure! Sampling points shown on geologic map of Kotanbetsu area, Tomamae province, Hokkaido. The
Upper Yezo Group in this area was divided into five members, Uf-Uk (Tanaka, 1963). All the localities of
Polyptychoceras pseudogaultinum (Yokoyama) are in Uk member, Upper Santonian. A useful population sample
was obtained from the inlier of this member at Onko-no-sawa.
Growth and form of Polyptychoceras 31
Polyptychoceras occurs abundantly (Figure1). In order to shown in the Appendix Table.
collect specimens of Polyptychoceras and to clarify their Perfectly preserved specimens give us important informa-
stratigraphie occurrence, we surveyed this area in the field tion concerning the ontogenetic change of shell growth and
seasons of 1992-1994. Layers of the Upper Yezo Group in shell ornamentation. Such specimens of Polyptychoceras
thisarea, generally have north-south strikesand aretilted to pseudogaultinum areextremelyrare,whilefragmentaryspec-
thewest,though somefoldingsarerecognized intheeastern imens occur very abundantly in this area. We had to
part. This group is unconformably covered by Tertiary analyze its ontogenetic change by connecting imperfect
sedimentary rocks in the western part of this area. The specimens. An idealized shell form of Polyptychoceras
Upper Yezo Group is mainly composed of dark gray sandy pseudogaultinum is reconstructed, and terms for every part
siltstones and contains many intercalations of tuff and are defined as shown in Figure2. An adult shell is com-
sandstone layers. Tanaka (1963) and Tanabe et al. (1977) posed of five shafts and fourturns connecting two adjacent
defined twelve members (Ua-UI) in this group, and Polypty- shafts.
choceras pseudogaultinum occurs particularly in Uk, one of In order to clarify the general shell morphology and its
the uppermost members. This ammonite is usually intraspecific variation, we mainly used a slide caliper and a
contained in acluster, and sometimesoccursgregariously in binocular. For observation of the septate phragmocone,
calcareous nodules. We treated a total of 323 specimens several specimens were cut along the median plane. The
of P. pseudogaultinum in this study. A large population linear distance between adjacent septa was measured bya
sample of this species was obtained from the Onko-no- profile projector (Nikon, V-12). Some specimens showing
sawa valley where the Uk member is locally exposed as an early shell morphology were observed and measured by
inlier. Several smallersampleswereobtainedfromthe main scanning electron microscope (SEM, Jeol JSM-5200). For
stream of Kotanbetsu River, tributaries of Kami-no-sawa, SEM observation, the polished surface cutalongthe median
and a western tributary of Horotate-zawa. All specimens planewasetched with 5 percentaceticacidforahalftotwo
are kept in the University Museum, University of Tokyo with minutes. For the computer simulation, programs written in
register numbers with the prefix UMUT. MM. We use N88-BASIC were run on a personal computer (NEC PC-
consecutivespecimen numberswith prefixH- orS- through- 9801RA) printing to an ink-jet printer (Canon, BJ-130J).
out this paper, and their correlation with register numbers is
5th shaft
3rd shaft
4th ttuurrnn L^^m
1st shaft
W"
1st turn
i
iV\ \. "~'ß
2nd shaft
3rd turn
4th shaft
_i_
IJHeight
mm
10
Breadth
Figure2. Schematic illustration of Polyptychoceras pseudogaultinum (Yokoyama).
32 Takashi Okamoto and Makoto Shibata
Bw*iS>-•'^yw»ft>
Rgure3. Early shell morphology ofPolyptychoceras spp. A. Younger shell ofPolyptychoceras sp. containing
an almost complete ammonitella, and following first shaft, S-199. B-E. Microstructure of embryonic shell of P.
pSsceauldeobgaaruslt:i5nummm(YfoorkoAy,am1a0)0,pmB:foSr-2B05a,ndC,C,D5:0S/-/2m04f,orED,: aountder20wa0llumoffaormmEo.nitella S-203 (stereographic pair).
Growth and form of Polyptychoceras 33
General morphology and remarks increases just after the nepionic constriction (Figure3).
Tuberculatemicroornamentisobservable insomeofthese
Protoconch and early shell morphology specimens. Homologous textures have been described by
Ammonitellasofthe Diplomoceratidaeare in general rarely Bändel et al. (1982) and Tanabe (1989) in several other
preserved. Hayakawa (1988) reported several ammonitellas superfamilies cf Ammonoidea. According to their argu-
of Polyptychoceras sp. from the Santonian of the Upper ments, these textures were secreted probably bythe interior
Yezo Group. We obtained some early shells of P. gland cells of the mantle margin and indicate that the
pseudogaultinum in which the ammonitella is followed by a ammonitella was enclosed by the reflected mantle lobe.
straight shaft (=first shaft). By SEM observation along the Namely, the ammonitella stage of these ammonites was
median plane, the main structural elements of the am- endocochliate. TheTurrrilitaceae, in whichPolyptychoceras
monitella such as the primary varix, protoconch, prosiphon, is included, possibly shared a similar mode of life until the
caecum and proseptum were recognized (Figure3). The nepionic constriction was formed.
microstructure of the embryonic shell of P. pseudogaultinum Doguzhaeva and Mutvei (1989,1993) suggested that
is somewhat similar to that of the Lytoceratina, though the Ptychoceras, an EarlyCretaceousammoniteshowingconver-
size of the ammonitella is considerably smaller. The am- gent shell form to Polyptychoceras, had an internal or semi-
monitella in this species is about 0.7mm in diameter, internal shell. The grounds of their argument are 1) the
whereas in the Lytoceratina it generally ranges between 1.1 mechanical truncation of the phragmocone terminus and 2)
mm to 1.4mm (Tanabe and Ohtsuka, 1985). It is character- the existence of an external coating layerwhich is secreted
ized by a relatively short, arcuate prosiphon and a siphun- probably on the shell surface by an epithelium. In Polypty-
cular tube rapidly shifting to the ventral margin. The ne- choceras pseudogaultinum, however, no evidence indicating
pionic constriction and accompanying primary varix, by an endocochliate shell has been obtained throughout the
which the ammonitella stage is terminated, appear at about post-embryonic growth stages.
320 degrees from the beginning of the first whorl, and is
followed by the first shaft. The shell height rapidly
LEGEND
T
-IT Flared rib
ir
Double collar rib
IN**"^^^>_^_^w'l Rhythmic phase
PHI
Wrinkly phase
4th shaft
Smooth ornamentation
S"122 10mm (W'A"""- -1 Turn of shell coiling
\
Double collar rib Flared rib Wrinkly phase Rhythmic phase
f
>.
ventral
f
iaäsääi&iiiii:
adorai
Figure4. Schematic diagram showing the cyclic change of shell surface ornamentation throughout growth.
Twophasesofornamentationappearingalternatelyarerecognizedafterthesecondturn. Thewrinklyphasewhich
is initiated from a flared rib appearing near the beginning of the new shaft, gradually changes into the rhythmic
phaseduringtheearlierhalfofshaft. Eachscalebar(openrectangle)ontherightofthe illustration indicates1 mm.
34 Takashi Okamoto and Makoto Shibata
On Poiyptychoceras subquadratum (Yokoyama) The shell growth ofP. pseudogaultinum isdivided intotwo
Polyptychoceras pseudogaultinum (Yokoyama) was estab- major stages by the first appearance of the flared rib which
lished on the basis of some young specimens showing the is observed just afterthe beginning ofthe third shaft (Figure
second tothe third shafts, thetransverse section ofwhich is 4). Intheearlierstage,several doublecollarribsperiodically
nearly circular. In the same paper, Yokoyama (1890) de- develop and weak striae are observed between them. In
scribed Polyptychoceras subquadratum (Yokoyama) from the the laterstage, a cyclic switch oftwo ornamentation phases
same localityoftheUrakawaarea,southern Hokkaido. The corresponds to the shell coiling. The first or wrinkly phase
latter species was established on the basis of a single of ornamentation is characterized by slightly notched ribs
fragmentary specimen showing a nearly adult growth stage occurring at irregular intervals and by numerous weak striae
having a subquadrate transverse section. In the samples between them. This phase usuallystartsfrom theflared rib
examined, the larger specimens, which are usually frag- and continues for about one-third of the shaft. The not-
ments, always show the characteristics of P. subquadratum ched ribs gradually change to the simple ribs. The second
(i.e. subquadrate cross section), whereas almost all the or rhythmic phase of ornamentation is characterized by the
younger specimens are clearly identified with P. simple ribs occurring at regular intervals. This phase con-
pseudogaultinum. Actually, these larger specimens may tinues to the end of the next turn and is terminated by the
look as if they belong to a different species from P. next flared rib.
pseudogaultinum, because the shell shape is considerably The obliquity of transverse ribs also changes cyclically
different, and because the sample shows bimodal size throughout growth. More or less prorsiradiate ribs are
distribution. However, a few well-preserved specimens observed through the later half ofthe second shaftand near
reveal that the difference between the two "species" is only theend ofeach shaft,whilerursiradiateorrectiradiatesimple
due to ontogenetic change within a single species (Figures ribs are formed during each turn and the early part of the
14,15). WethereforetreatP. subquadratum asasynonymof following shaft. The change of rib obliquity is, though,
P. pseudogaultinum, and all the samples examined are rather obscure in the fourth shaft.
regarded as belonging to a single species, P. pseudo-
gaultinum (syntypes: Ptychoceras pseudo-gaultinum Yo- Shell height and shaft length
koyama,1890,Palaeontographica, vol.36, p.181, pi.20,figs.1- The shell height at the beginning of every shaft and the
3; synonymy: Anisoceras subquadratum Yokoyama, 1890, shaft length were measured in all the specimens if these
Palaeontographica, vol.36, p.182, pi.20, fig.4). Though values were measurable. The mean values of shell height
these two names were established in the same paper, we from the second to fifth shafts are 1.58mm, 3.78mm, 8.41
designate P. pseudogaultinum as the name of this species, mm, and 16.50mm, respectively. The standard deviation of
because P. pseudogaultinum has more frequently appeared
in literature (e.g. Yabe, 1927; Shimizu, 1935; and Matsu-
moto, 1984). Moreover, the type specimens of Polypty- A
^_—
choceras haradanum (Yokoyama) and Polyptychoceras 5r i -
subundulatum (Yokoyama), both of which were established
also in the same paper (Yokoyama, 1890), are possibly 4 H+H21
includable in the intraspecific variation of P. pseudo- u 3 l-|H 27
gaultinum.
0)
J= 2
Ontogenetic change of surface ornamentation Î • i 1•0 1'5 20
Polyptychoceras shows a periodically changing pattern of S5 Shell Height
surface ornamentation. Sincethe ammonite shell isformed
by accretionary growth, the tempo and mode of growth at ^4 B
every growth stage must be recorded in the commarginal M
ornamentation on the shell surface.
3
Polyptychoceras pseudogaultinum has two kinds of char-
acteristic ribs (double collar ribs and single flared rib), which
occur periodically at definite stages of the shell growth
(Figure4). The double collar ribs appear at first near the
trisecting point of the second shaft at which the adjacent 50 100 150
mm
firstshaftterminates, and occurs periodically untiltheend of Shaft Length
the second shaft. The number and the position of the
double collar ribs are variable among the examined speci- Figures. Diagrammatic figures showing the average
mens. The flared rib appears just after the shell turning aShnedllinhteriagshpteciifsicmevaarsiuatrieodn oaft sthheellbheegiignhntinagndofsheavfetrylesnhgathf.t
pointofeverystage exceptfora pointjustafterthefirstturn. excluding the first shaft. Horizontal bar, solid rectangle and
While the flared rib is relatively clear in the third and fourth number on the right indicate presumable distribution range of
shafts, it is not very clear or sometimes unobservable in the 95% ofthe population (1.96 times of standard deviation), 95%
fifth shaft. The shell is thickened at these ribs, and the confidence range ofmean value(1.96times ofstandard error),
aperture at these stages probably become a little narrower. and the sample size, respectively.
Growth and form of Polyptychoceras 35
these measurements are relatively small except for the fifth Estimation of missing shaft length
shaft. The mean values of shaft length from the first shaft P. pseudogaultinum usually grows in parallel with the
to the fourth shaft are 17.86mm, 51.06mm, 61.50nrfm and previous shaft. When the shell passes the origin of the
97.87mm, respectively (Figure5). The first shaft length is previous shaft, the growth direction temporarily slightly shifts
indirectly estimated by the shape of the second shaft as to the dorsal side but soon recovers its original direction.
mentioned below, because in no specimen is the first shaft Therefore, the length of earlier shaft can often be estimated
perfect. Unlike the shell height, standard deviation of shaft by the shape of the later shaft, even if the earlier shaft is
length is generally large except for the first shaft. missing. The second shaft commonly bends slightly at
These results indicate that the variation of shell form is about 20mm after the first turning point. This probably
fairlywide in the growth stagesafterthesecond shaft. The means a release from the constraint of the first shaft.
shaft length does not increase proportionally either with the Actually, an ammonitella situated just near the bending part
progression of the shaft number or with the shell height. ofthe second shaft isobserved in well preserved specimens
The second shaft is relatively longerthan othershafts. This of a related species (Figure3A). As to P. pseudogaultinum,
may be a common characteristic feature in the genus a relatively long first shaft, which is merely 2.3mm shorter
Polyptychoceras. than the expected first shaft, is preserved in one of our
mm
GO 80-
• o 4th shaft
o 3rd shaft
A 2nd shaft
• 1st shaft
-,120
P-l^O 10 15 20
mm
Shell Height (H)
Figure6. Scatter diagrams showing the relationship of the shaft length versus the shell height measured at
the beginning of the shaft. Positive correlation between the shell height and the previous shaft length is
significantly recognized (B), while the correlation between the shell height and the current shaft length is rather
obscure (A). Regression line is also shown with the correlation coefficient(r), when the correlation is significant.
36 Takashi Okamoto and Makoto Shibata
specimens (S-194). The shell height atthe earliest point of Further biométrie analyses
this specimen is 0.32mm. Sincethe shell height measured
justafterthe nepionicconstriction inthisspecies isgenerally Relative growth between height and total shell length
about 0.31 to 0.35mm, it is reasonable to consider that the Relative shell growth pattern in normallycoiled ammonites
bending pointofthesecondshaftindicatesthe impressionof is usually shown by the shell diameter (or whorl height),
the protoconch. As a result, the first shaft length can be breadth and umbilicus measured in various stagesalong the
estimated as the distance between the end of the first turn transverse section. However, this method cannot be
and the bending point of the second shaft. The intra- applied to heteromorph ammonites at all. The ontogenetic
specific variation is very small about the first shaft length growth oftheshell heightshould be measured relativetothe
estimated in this way (Figure6B). total shell length. The total shell length is defined as the
The shell length would bethe mostadequateand feasible distance from a protoconch to a point of a certain growth
parameter to compare the same growth stages between stage represented bythetrackoftheapertural center. This
specimens. Since every specimen of P. pseudogaultinum definition is also applied to the phragmocone length (from
examined in this study is more or less incomplete, the protoconch to the last septum).
missing part of the shell must be reconstructed in order to The relative growth patterns between the shell heightand
analyze biométrie data statistically. The mean values can the shell length in four specimens are shown in Figure7.
hardly be employed for the estimation of the missing shaft Morphology of the earlier shell is observable in two of the
length because of the large intraspecific variation. So we four specimens (H-038 and H-078). Relatively advanced
examined the correlation between the shell height and the growth stages, from thethird turn totheearlypartofthefifth
shaft length (Figure6A). No obviouscorrelation is, however, shaft, are observed in the other two specimens (S-108 and
recognized between the shell height measured atthe begin- H-114). These growth patternsare combined bystandardiz-
ning point of each shaft and corresponding shaft length. ing the third turn. These four specimens are concordant
The length of the previous shaft is, however, positively with each other inthe growth pattern,showing cyclicoscilla-
correlated to the shell height (Figure6B). Therefore, the tion of whorl height. The height grows slowly at an almost
length of the previous shaft can be estimated by the shell constant rate until the earlier half of the second shaft, but
height and this relationship, even if the earlier shell is then the rate gradually decreases. Unlike the earlier stage,
missing. the height grows rapidly during the earlier halves ofthethird
20r
mm
w
•pH 10-
(V
100 200 300
Total Shell Length (L)
Figure7. Relativegrowthbetweenthetotalshell lengthandtheshellheight. Solidsymbolsindicatethedata
obtained atthe turning part of shell. Shell height rapidly increases during the earlier half of every shaft.
Growth and form of Polyptychoceras 37
and following shafts. Then the growth rate gradually growth. The P/T ratio was actually measured in twenty-
decreases during the later half oftheshafts, and thisfeature one specimens showing various growth stages (Figure8).
continues until the end ofthe following turn. Such periodic The P/T ratio is almost constant throughout various
changes of growth rate are observed until the fifth shaft. growth stages of the fourth shaft and ranges approximately
The shape of the transverse section of the shell changes from 0.57 to 0.61. Younger specimens, however, have
during ontogeny (Figure2). The section is almost circular relativelyshortphragmocones. Although relativelylittledata
until the middle part ofthethird shaft. Itgraduallybecomes was obtained for other stages, the ratios are similar to the
oval during the growth of the third shaft and finally subqua- valuesmeasured inthefourthshaft. Consequently,theratio
drate in the later stages. is roughly 0.6 and is almost invariable throughout growth
except for the youngest stage. The value is nearly equiva-
Relative length between phragmocone and total shell lent to or slightly smaller than the ratios of normally coiled
Every ammonite shell consists of a body chamber and a and other heteromorph ammonites (Okamoto, 1996).
septate phragmocone. The soft part filling in the body InPolyptychoceraspseudogaultinum, the expansion rateof
chamber advances adorally with growth, and a new septum the shell aperture is not constant, as shown by the cyclic
is formed behind the soft part. The life mode of an ammo- change of shell height during growth. If the species needs
nite would have been hydrostatically controlled by the total to maintain a total average density so as to achieve neutral
average densityofthewholeammonite body. Ifthe density buoyancy, for example, the P/T ratio may also change
was higher than that of sea water, the ammonite could not slightly with growth. So we classified the data into two
stay in the water column without active swimming, because groups by the growth stages; the open circles in Figure8
the buoyant force is not enough to cancel the gravity. indicate thatthe shell aperture is situated in the later half of
Conversely, if the density of an ammonite was positively the shaft whereas the solid ones indicate that the shell
buoyant, the ammonite would be forced to float on the aperture is in the other part. However, no significant differ-
surface. In order to maintain a mode of life suspended in ence was detected between the two groups.
the water column, like extant Nautilus, the total average
density of the ammonite had to be neutrally buoyant in sea Interval of septa throughout growth
water. The interval between septa becomes rapidly shortened in
The relative volume of phragmocone tothe whole ammo- manyammoniteswhen theyreachthefullymaturestage. If
nite body must be constant so as to maintain neutral buoy- a stagnation or break of shell growth has occurred during
ancy, if the ammonite keeps a constant density of phrag- ontogeny, similar shortening of septal distance may appear
mocone during growth. The relative volume of phrag- in the phragmocone. We measured the distance between
mocone is roughly expressed by the ratio between the two adjacent septa together with corresponding phrag-
phragmocone length and total shell length (P/T), if the mocone length.
expansion rate of the shell aperture is nearly constant with Because the specimens examined are more or less frag-
70
%
Average=58.0%
Oh
(»
on aso> 60- (
a u ()
o • __fJ_<f T _.
1 i •
> bae o <»
-(J
• 4th shaft
• growth stage
50
''
100 200 300 mm
Total Shell Length (T)
Figure8. Diagrammatic figure showing the length of phragmocone relativetothetotal shell length. Vertical
bars indicatethepresumablerangeoferrorowingtothe indirectestimationofmissingshell length. Solidandopen
circlesstandforspecimenswiththeshellaperturesituated intheearlierand laterhalvesofshaft, respectively. The
P/T ratio seems to be almost invariable throughout growth except for the two youngest specimens.
38 Takashi Okamoto and Makoto Shibata
Estimated Scale of Total Shell Length (T)
mm
100 200 300
1st s| 2nd shaft 1 3rd shaft 4th shaft 5th shaft
8.0
mm
CO o o
-£ 6.0 o
ft °o
CD
o
V ,
CD O oo
CD
Oo
£'4-0 Ov o
i
CD °1 ° A* §0^°v
CÜD v<yo
v
Ö
CO vvvv _wjcf • o S-083
2.0
GO o S-142
V S-110
AY
• S-114
100 200
mm
Phragmocone Length P)
(
Figure9. Diagrammatic figure showing the ontogenetic change of distance between septa. Solid symbols
indicate the values measured in the turning parts of shell. Estimated length of total shell corresponding to the
phragmoconelength isalsoenteredonthisdiagram. Thedistancebetweenseptarapidlyincreaseswhentheshell
aperture is in the later half of the shaft.
mentary, the developmental change of the septal distance or later half of every shaft.
cannot be measured in a single specimen. The septal
distance isactuallymeasured onthecrosssection cutalong Population analysis (Distribution of growth stage)
the median plane. Its ontogenetic change is synthetically Ifthefossilsare collected atrandom,thesample indicates
shown by connecting four specimens (S-083, S-142, S-110 thedistribution ofgrowth stagesinafossil population. Asto
and S-114) in Figure9. In this diagram, the lower horizontal normally coiled ammonites, the growth stage is usually
axis shows the phragmocone length, and the vertical axis shown by the total revolution angle until the shell aperture.
shows the distance between septa. The progressive In Polyptychoceras pseudogaultinum, however, the growth
stages, in which the distance increases more or less rapidly, stage has to be shown by the shaft number and relative
appears at 20-30mm, 60-80mm and 110-140mm in phrag- position of the shell aperture in the shaft.
mocone length. These stages are followed by some stag- A total of 71 specimens with complete apertures was
nant stages in which the septal distance is nearly constant. examined, and the frequency distribution is shown in Figure
Though this biométrie analysis shows that the progressive 10B. The intervals of horizontal axis corresponding toevery
and stagnant stages alternately appear throughout the shaft were standardized so as to increase proportionally to
ontogeny, the relation tothe position ofsepta isnotobvious. the corresponding shell height. Three peaks which appear
The cyclic change of the septal distance is well demon- in the earlier parts ofthethird tofifth shaftare recognized in
strated in relationtotheshell aperture. Ascaleoftotalshell thisdiagram, although thesamplesize isnotsufficientowing
length estimatedfromthephragmocone lengthand P/Tratio to the infrequent preservation of the shell aperture. This
is also shown in Figure9. The progressive stage of the method bears some difficulties in requiring randomness of
distance between septa corresponds well to the growth observed sample. For example, full-aperture specimens
stage where the shell aperture is located in the middle part having only one shaft (without turn and ammonitella) were