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More than 500 specimens of embryonic shells of orthocerid nautiloids from the Imo Formation were investigated. Although the material is recrystallized, the external and internal features of the early growth stages are exceptionally well preserved. The material comprises eight species of Pseudorthoceratidae: Pseudorthoceras knoxense (McChesney, 1860); Ristedtoceras teriliratum n. sp.; Mooreoceras imoense n. sp.; Mooreoceras striatulum n. sp.; Reticycloceras peytonense Gordon, 1965; Dolorthoceras tenuifilosum Gordon, 1965; Mitorthoceras girtyi Gordon, 1965; and Euloxoceras angustinus Gordon, 1965. A new genus, Ristedtoceras, is erected and the genus Pseudorthoceras Girty, 1911 is emended. The analysis indicates that the morphologic diversity of the early growth stages of the shells of these species are much more diverse than expected. The different species vary strongly in the embryonic shell size, cicatrix position and shape, numbers of septa in the embryonic shell at the time of hatching, embryonic shell ornamentation, and the outline of the first segment of the siphuncle and its position in cross section. This study shows that the shape and position of the cicatrix is a morphologic feature that has been under utilized in previous investigations. The high morphologic variance of the embryonic shells in these Imo orthocerids requires a revision of our understanding of the Pseudorthoceratidae. In addition, the implication of this analysis strongly supports using the morphology of the embryonic shell, and especially the cicatrix, in all future orthocerid systematic and phylogenetic analyses because it is proving to be an important set of characters in detecting homeomorphic evolutionary relationships that are not discernable in mature specimens.
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560
J. Paleont., 78(3), 2004, pp. 560–573
Copyright q2004, The Paleontological Society
0022-3360/04/0078-560$03.00
LOWER CARBONIFEROUS (CHESTERIAN) EMBRYONIC
ORTHOCERATID NAUTILOIDS
B. KRO
¨GER
AND
R. H. MAPES
Geologisch—Pala¨ontologisches Institut und Museum der Universita¨t Hamburg, Bundesstr. 55, D–20146 Hamburg, Germany, and Department of
Geologic Sciences, Ohio University, Athens 45701
A
BSTRACT
—More than 500 specimens of embryonic shells of orthocerid nautiloids from the Imo Formation were investigated. Although
the material is recrystallized, the external and internal features of the early growth stages are exceptionally well preserved. The material
comprises eight species of Pseudorthoceratidae: Pseudorthoceras knoxense (McChesney, 1860); Ristedtoceras teriliratum n. sp.; Moor-
eoceras imoense n. sp.; Mooreoceras striatulum n. sp.; Reticycloceras peytonense Gordon, 1965; Dolorthoceras tenuifilosum Gordon,
1965; Mitorthoceras girtyi Gordon, 1965; and Euloxoceras angustinus Gordon, 1965. A new genus, Ristedtoceras, is erected and the
genus Pseudorthoceras Girty, 1911 is emended.
The analysis indicates that the morphologic diversity of the early growth stages of the shells of these species are much more diverse
than expected. The different species vary strongly in the embryonic shell size, cicatrix position and shape, numbers of septa in the
embryonic shell at the time of hatching, embryonic shell ornamentation, and the outline of the first segment of the siphuncle and its
position in cross section. This study shows that the shape and position of the cicatrix is a morphologic feature that has been under
utilized in previous investigations. The high morphologic variance of the embryonic shells in these Imo orthocerids requires a revision
of our understanding of the Pseudorthoceratidae. In addition, the implication of this analysis strongly supports using the morphology
of the embryonic shell, and especially the cicatrix, in all future orthocerid systematic and phylogenetic analyses because it is proving
to be an important set of characters in detecting homeomorphic evolutionary relationships that are not discernable in mature specimens.
INTRODUCTION
L
ITTLE IS
known about the early juvenile stages of the shells
of orthocerids. In the last 50 years the work of Ristedt (1968)
stands as the most significant fundamental work on the topic,
showing that the Orthocerida are basically represented by two
types of early shell shape: early stages with an egg- to spherical-
shaped initial chamber and early stages without a constriction
between the initial chamber and the rest of the conch. He con-
cluded that except for longitudinal sculpture, the external orna-
mentation of the shell, formerly generally used for all systematic
levels of the Orthocerida, had only a taxonomic significance be-
low the subfamily level. He also showed that the shape of the
early shell is crucial for understanding the phylogenetic and sys-
tematic relationships of the Orthocerida. Despite the fundamental
discoveries of Ristedt, there has been little overall investigation
of early shell shape of the Orthocerida. Since the 1960s, the sum-
mary provided by Teichert (1964) and the reports by Ristedt
(1971), Serpagli and Gnoli (1977), and Blind (1987, 1988), along
with earlier studies of Schindewolf (1933, 1944), Shimansky
(1948, 1968), and Balashov and Zhuravleva (1962) provide little
insight into the diversity of embryonic shells of orthocerid spe-
cies. In general, when the juvenile and embryonic stages of or-
thocerid genera and species are studied, the more mature growth
stages are unknown. Also, virtually nothing is known about the
relation between macroscopic species diversity and diversity of
juvenile shell parts of orthocerids in a given fauna. This study of
the Imo Formation orthocerids is the first attempt to evaluate em-
bryonic and early juvenile diversity with more mature stages of
orthocerids using a relatively large (n520001) collection of
specimens. Additionally, some of the problems inherent in sys-
tematic and taxonomic evaluation of the Imo orthocerids are ad-
dressed and some considerations about the wider scale problems
of the orthocerid phylogeny and morphology are discussed.
BACKGROUND INFORMATION AND MATERIAL
The Imo Formation (Searcy County, Arkansas, USA) has been
the object of observation and systematic collection on almost a
yearly basis since the mid-1960s by RHM. The intensive surface
collection effort and the more than 5 metric tons of shale that was
disaggregated, washed through sieve screens, and picked under a
dissecting microscope, created the large collection of embryonic,
juvenile, and mature orthocone shells available for this study. The
preservation of the nautiloid macro fauna of the Imo Formation
is exceptionally good, including color patterns (Mapes and Davis,
1996, Table 3). The more mature stages of growth of most of the
Imo orthocerid taxa were described by Gordon (1965) and his
report provides an excellent platform for further work on the early
ontogenetic stages of the described orthocerids. With this large
collection of embryonic to mature growth stage orthocerids, it is
possible to examine Gordon’s systematic identifications and com-
pare the diversity of the heretofore undescribed early growth stag-
es with the diversity described by Gordon (1965) of the late
growth stages. This procedure, which delivers specific informa-
tion on the taxonomic status of the Imo orthocerids, gives im-
portant information of the phylogeny of the pseudorthocerids in
general.
The Upper Mississippian Imo Formation is defined by fora-
minifers and ammonoids of late Chesterian, Elviran Stage and is
equivalent to the Arnsbergian Stage (Namurian A) of Europe
(Saunders, 1973; Brenckle, 1977; Saunders et al., 1977). The Imo
Formation consists of gray, calcareous shales, siltstones, sand-
stones, and concretionary dark gray to black shales. Most of the
cephalopods occur in the shales.
The shelly material of the Imo Formation is excellently pre-
served. The overall invertebrate fauna is diverse, and because of
the good preservation, some of the fauna has been documented
in a multitude of investigations [a partial list includes cephalopods
(Gordon, 1965; Mapes, 1979; Saunders, 1973; Saunders et al.,
1977); gastropods (Jeffery et al., 1994); bivalves (Hoare et al.,
1989); ostracodes (Sohn, 1977; Hoare and Mapes, 2000); cono-
donts (Mapes and Rexroad, 1986), and rostroconchs (Hoare et al.,
1982)].
More than 500 protoconchs and more than 1500 juvenile and
mature specimens of Imo orthocerids are available for study. The
main collecting sites are the Peyton Creek roadcut and nearby
quarry in Van Buren County, Arkansas (seeSaunders, 1973;Man-
ger, 1977, p. 11 for additional details), and a natural outcrop in
the bed of the Middle Fork of the Little Red River, Van Buren
County, Arkansas, near the abandoned community of Elba (Saun-
ders, 1973; Mapes and Rexroad, 1986, for additional informa-
tion). The juvenile and mature orthocerid specimens are usually
561KRO
¨GER AND MAPESLOWER CARBONIFEROUS (CHESTERIAN) EMBRYONIC ORTHOCERATID NAUTILOIDS
T
ABLE
1—Comparison of the embryonic shell dimensions of Pseudorthoceratidae of the Imo Formation, ammonoids (*Landman and Tanabe, 1996) and
Nautilida (
1
Landman, 1988). The comparision shows nearly similar embryonic shell dimensions in ammonoids and pseudorthocerids. Volumesare calculated,
assuming a cone-shaped embryonic shell.
Taxon Diameter at nepionic
constriction in mm Length of embryonic
shell in mm
Approximated volume
of the embryonic shell
in mm
3
Dolorthoceras tenuifilosum
Reticycloceras peytonense
Mooreoceras imoense n. sp.
Mooreoceras striatulum n. sp.
Pseudorthoceras knoxense
Euloxoceras angustinus
Mitorthoceras girtyi
2
3
5
5
1
1
1
13
12
9
9
3
3
1
14
28
236
236
0.8
0.8
0.3
Ammonoidea diverse species*
Nautilida diverse species
1
1–2.5
10–25
0.5–10
10
3
–2 310
3
T
ABLE
2—Comparison of some characters of the embryonic shell of the Pseudorthoceratidae of the Imo Formation. A comparison of the embryonic shell
characters is consistent with the species level taxonomy but in conflict with valid higher systematics of the Imo nautiloids (see text for discussion).
Taxon Ornamentation Position of
cicatrix Shape of first
siphuncular segment
Dolorthoceras tenuifilosum
Reticycloceras peytonense
Ristedtoceras teriliratum n. sp.
Mooreoceras imoense n. sp.
Mooreoceras striatulum n. sp.
Pseudorthoceras knoxense
Euloxoceras angustinus
Mitorthoceras girtyi
longitudinal lirate
reticulate
smooth
faint growth lines
faint growth lines
longitudinal lirate
longitudinal lirate
smooth
central
dorsal
dorsal
dorsal
dorsal
dorsal
dorsal
ventral
tubular
tubular
tubular
globular
globular
globular
globular
globular
recovered loose on the surface of the greyish-black shales, al-
though some specimens are partly encased in ironstone concre-
tions that commonly occur in the shale. The tests of the shells are
recrystallized, but the original ornamentation is preserved in very
good detail. Recrystallized inner structures of the connecting rings
and outline of the septa are visible if the material is sectioned.
Because there are considerable differences in the terminology that
has been used to describe the embryonic morphology of cepha-
lopods, a brief set of terms and their definitions, as used in this
report, are provided below.
Cicatrix.—A lentoid or longitudinal structure or plate at the
apex of the shell; without growth lines; with a longitudinal, cen-
tral depression, notch or ‘‘scar’’ (sensu Erben and Flajs, 1976). It
is the first site of shell deposition according to studies on the
recent Nautilus (Arnold, 1987; Arnold et al., 1987). This plate is
mineralized as an entire unit from an organic membrane template
(see Tanabe and Uchiyama, 1997).
Embryonic shell.The most apical part of the shell, adorally
marked by the nepionic constriction. It is interpreted as the pre-
hatching shell. In this part of the shell there is a reduction in
septal spacing and there are no sublethal injuries.
Juvenile shell.The posthatching interval of the shell located
orad of the nepionic constriction. This part of the shell often ex-
hibits numerous areas of healed shell damage and there is a
change in chamber height. There may be a change in external
ornamentation from the embryonic to the juvenile shell.
Nepionic constriction.A narrow or wide annular groove, of-
ten distinct and sharply limited furrow or ornamental break on
the shell surface. Sometimes the growth lines at the shell surface
are unconformable and sometimes the external ornament can
change as it crosses this feature. The nepionic constriction is in-
terpreted as a structure produced at the time of hatching (see e.g.,
Arnold et al., 1987 et cit.). Also, there can be changes in the
distance between the growth lines and/or transverse ornament.
Thus, this structure marks the ending of the embryonic shell that
is formed in the egg and the beginning of the juvenile, post-
hatching shell.
Initial chamber.The first, most apical chamber of the shell
that is sometimes called the protoconch. Within the first chamber
the first segment of the siphuncle appears. A bulged or ovate first
segment of the siphuncle is sometimes called the caecum. The
first segment of the siphuncle can show also a tubular shape. The
first septum of nautiloids is usually achoanitic to suborthochoan-
itic. Subsequent septal necks usually conform to the shapes seen
in the more mature stages of growth.
Specimens described herein are reposited in the Ohio Univer-
sity Zoological Collections (OUZC).
SYSTEMATIC PALEONTOLOGY
Order O
RTHOCERIDA
Kuhn, 1940
Superfamily P
SEUDORTHOCERATACEAE
Flower and Caster, 1935
(nom. transl. Sweet 1964 ex P
SEUDORTHOCERATIDAE
Flower
and Caster, 1935)
Family P
SEUDORTHOCERATIDAE
Flower and Caster, 1935
emend. Flower, 1939
Diagnosis.Cyrtochoanitic orthocones with slightly exogas-
tric, cyrtoconic apices with endosiphuncular deposits; the earliest
stages of siphuncle slender and orthochoanitic getting more ex-
panded in later segments; deposits appear as simple annuli or a
continuous parietal lining.
Included genera.More than 40 genera ranging from Early
Silurian to the Permian.
Discussion.Sweet (1964) followed Flower (1939), who sub-
divided the Pseudorthoceratidae by shape and dimension of the
endosiphuncular deposits into subfamilies. However, he consid-
ered the subfamily Dolorthoceratinae Flower, 1939 as a junior
synonym of Spyroceratinae Shimizu and Obata, 1935. But as dis-
cussed below, the outline and shape of the endosiphuncular de-
posits are considered here as homeomorphic traits of the Pseu-
dorthoceratidae; they vary considerably within the species. More-
over, species with very similar morphology and development of
endosiphuncular deposits show very different embryonic shell
forms. These differences in early ontogenetic growth stages must
562 JOURNAL OF PALEONTOLOGY, V. 78, NO. 3, 2004
be considered in the higher taxonomy. At present, the available
data are insufficient to establish a consistent subdivision of the
Pseudorthoceridae and we follow Balashov and Zhuravleva
(1962), who did not subdivide the family into subfamilies.
Based on the features seen in the Imo Pseudorthoceridae and
the overall lack of study of the early embryonic growth stages of
these strongly homeomorphic orthoconic nautiloid taxa, the entire
concept of the family requires revision. The family definition of
Flower and Caster (1935) is based on homeomorphic traits like
shape and extent of endosiphuncular deposits and outline of the
septal necks. The outline of the first segment of the siphuncle and
the external features of the embryonic shell must be considered
to more accurately define the family. The present investigation of
the Imo orthoconic nautiloids shows the inconsistencies of the
current systematic treatment and points out the necessity of using
embryonic characters.
Genus P
SEUDORTHOCERAS
Girty, 1911
Type species.Orthoceras knoxense McChesney, 1860.
Emended diagnosis.Pseudorthoceratidae with smooth shell in
nearly mature growth stages, early ontogeny ornamented by faint,
slightly sinuous growth lines; conch circular or slightly depressed
in cross section; apex blunt with slight exogastric curvature; first
segment of siphuncular tube (i.e., caecum) expanded into chamber
lumen; slender, slightly s-shaped embryonic shell with faint ci-
catrix at dorsal side of apex, and faint longitudinal striations; si-
phuncle, throughout entire ontogeny central to slightly eccentric;
septal neck at apical part orthochoanitic, later cyrtochoanitic sep-
tal necks; connecting ring subglobular; siphuncular and cameral
deposits in adapical shell parts; continuous or discontinuous an-
nulate siphuncular deposits, heavier on ventral side; thick mural
cameral deposits.
Discussion.With regard to the embryonic shell, the genus
Pseudorthoceras comprises a number of very similar morpholo-
gies. Miller et al. (1933), Ristedt (1971), and Sturgeon et al.
(1997) illustrate the apical parts of Pseudorthoceras knoxense
(McChesney, 1860); its earliest growth stages are slightly cyrto-
conic; the apex is blunt; the first segment of the siphuncle is ovate
and strongly bulged outward in the first chamber. Pseudorthocer-
as sp. from the Buckhorn Asphalt (Pennsylvanian, North Amer-
ica), figured by Ristedt (1971) and Blind (1988), shows a rela-
tively blunt apex with faint longitudinal striations. A very narrow
cicatrix is situated at the center of the apex of a slightly cyrtoconic
embryonic shell. The first segment of the specimen is bulged out-
ward and ovate. Balashov and Zhuravleva (1962) illustrated two
specimens that they assign to Pseudorthoceras; both have ovate,
first segments of the siphuncle, and the apices are straight and
blunt.
Dzik (1984), when considering the outline of the genus Pseu-
dorthoceras followed the original definition of Girty and treated
Uralorthoceras Shimansky 1954, Mooreoceras Miller, Dundar,
and Condra, 1933, and Euloxoceras Miller, Dunbar, and Condra,
1933 as subjective junior synonyms. The embryonic shell of Ur-
alorthoceras is described by Shimansky (1954) and that of Eu-
loxoceras is described below. The early growth stages of these
two genera are very similar to but are slightly different from the
apical part of Pseudorthoceras knoxense (see Miller et al., 1933;
Ristedt, 1971). The apex of Mooreoceras differs from that of
Pseudorthoceras knoxense in shape as well as in length of the
embryonic conch. Although the overall similarities of the three
genera support the idea of Dzik (1984) the concept of the genus
Pseudorthoceras needs a general revision. It is suggested here that
in order to develop a more realistic generic consistency, the di-
agnosis should include the caecum shape and the longitudinal
striation of the blunt apex as diagnostic characters crucial for the
definition of the genus.
P
SEUDORTHOCERAS KNOXENSE
(McChesney, 1860)
Figures 1.7, 4.7, 4.15, 5.9
Orthoceras knoxens M
C
C
HESNEY
, 1860, p. 69.
Pseudorthoceras knoxense M
ILLER
,D
UNBAR
and C
ONDRA
, 1933, p. 81–
85, pl. 1, figs. 4–9 (see for prior synonymy).
Pseudorthoceras knoxense G
ORDON
, 1965, p. 109–111, pl. 5, figs. 19, 20
(see for prior synonymy).
Pseudorthoceras knoxense R
ABITZ
, 1966, pl. 4, fig. 7a, b; pl. 5, figs. 1,
3a, b.
Pseudorthoceras knoxense D
AVIS AND
C
AMPBELL
, 1968, p. 279–284,
text-figs. 2, 3.
Pseudorthoceras knoxense F
ISCHER AND
T
EICHERT
, 1969, p. 8ff, text-
figs. 1, 2; pl. 2, fig. 4; pl. 3, figs. 1, 4; pl. 4, figs. 1, 4.
Pseudorthoceras knoxense R
ISTEDT
, 1971, p. 165, pl. 28, figs. 11, 12; pl.
33, fig. 3.
Pseudorthoceras knoxense D
ZIK
, 1984, p. 115, 125–126, text-fig. 49.84.
Pseudorthoceras knoxense E
DGECOMBE
, 1987, p. 87–88, pl. 1, figs. 1–4.
Pseudorthoceras knoxense G
RE
´GOIRE
, 1989, p. 78, text-figs. 8–10.
Pseudorthoceras knoxense M
APES
and B
OARDMAN
, 1992, figs. 1, 2.
Pseudorthoceras knoxense D
AVIS
and M
APES
, 1996, pl. 14, figs. 1, 2.
Pseudorthoceras knoxense S
TURGEON ET AL
., 1997, p. 23–24, pl. 1, figs.
1–23 (see for prior synonymy).
Description.Blunt embryonic shell slightly cyrtoconic, faint
longitudinal lirae; apical angle 33 degrees (mean, n511) (Fig.
6.15); first segment of siphuncle bulged outward into first cham-
ber (Fig. 5.9); septal neck of first septum short and orthochoanitic;
faint nepionic constriction at approximately 3 mm adoral of apex.
Material examined.Eighteen specimens with embryonic
shells are available from the Imo Formation. One specimen was
sectioned to expose the internal structures of the shell (OUZC
5035).
Occurrence.The Imo Formation (Chesterian) of Arkansas
(USA) is the earliest report of Pseudorthoceras knoxense. The
species occurs throughout the Upper Carboniferous and Permian
of North America and Europe.
Discussion.Pseudorthoceras knoxense is one of the most
common and by far the best known of the Pseudorthoceratidae of
the Carboniferous. The list of synonymy, which gives only the
most important citations in the focus of our investigation, shows
the long descriptive history of this species. Detailed descriptions
and a list of previous illustrations of the embryonic parts of the
shell of Pseudorthoceras knoxense are given in Ristedt (1971).
The species is illustrated herein for completeness of the descrip-
tion of the Imo fauna and for comparison with different species.
Genus R
ISTEDTOCERAS
new genus
Type species.Pseudorthoceras stonense Gordon, 1965.
Included species.Two species from the Mississippian Period
of North America, Ristedtoceras teriliratum n. sp. and Ristedto-
ceras stonense (Gordon, 1965).
Diagnosis.Pseudorthoceratidae with smooth or faintly lirate
shell; conch circular or slightly depressed in cross section; apex
acute with slight exogastric curvature, first segment of siphun-
cular tube tubular; slender embryonic shell with cicatrix at dorsal
side of apex, siphuncular and cameral deposits in adapical shell
parts; embryonic shell slightly s-shaped; siphuncle throughout en-
tire ontogeny slightly eccentric; cyrtochoanitic septal necks; con-
necting ring subcylindrical; continuous or discontinuous annulate
siphuncular deposits, heavier on ventral side; thick mural cameral
deposits.
Etymology.In honour of Heinrich Ristedt, who made impor-
tant contributions to the understanding of the early ontogeny of
several different mid Paleozoic cephalopod groups.
Discussion.The embryonic shell of Ristedtoceras teriliratum
563KRO
¨GER AND MAPESLOWER CARBONIFEROUS (CHESTERIAN) EMBRYONIC ORTHOCERATID NAUTILOIDS
F
IGURE
1—1,Mooroceras striatulum n. sp., nearly adult specimen, 30.95, OUZC 5019. 2, M. imoense n. sp., 32.15, OUZC 5020. 3, Euloxoceras
angustinus Gordon, 1965, 33.8, OUZC 5021. 4, Dolorthoceras tenuifilosum Gordon, 1965, a strong change in ornamentation is marking the adoral
edge of the embryonic shell, 34.7, OUCZ 5022. 5, E. angustinus, juvenile specimen, 31.2, OUZC 5023. 6, M. striatulum n. sp., 31.4, OUZC
5024. 7, Pseudorthoceras knoxense (McChesney, 1860), 33.8, OUZC 5025.
n. sp. from the Imo Formation, differs from that of Pseudortho-
ceras knoxense significantly in the development of the initial seg-
ment of the siphuncle and the shape of the apex. A nearly iden-
tical embryonic shell was previously described by Gordon (1965)
as ‘‘Pseudorthoceras stonense Gordon, 1965.’’ Considering the
original definition of Gordon (1965), P. stonense (Gordon, 1965)
belongs to Pseudorthoceras. Gordon (1965) emphasized the out-
line of the connecting ring-septal neck complex and the shape of
the endosiphuncular deposits in his diagnosis. These morphologic
features by themselves are insufficient for the definition of the
genus Pseudorthoceras (see above), therefore it is necessary to
assign ‘‘Pseudorthoceras stonense’ Gordon to the new genus.
R
ISTEDTOCERAS TERILIRATUM
new species
Figures 1.4, 5.34, 6.18–6.20
Diagnosis.Ristedtoceras n. gen. with smooth acute apex and
lirate shell, juvenile and mature shell with rounded lirae and nar-
row, deep striae.
Description.Embryonic shell: Apex of shell acute, apical an-
gle in first 3 mm of approximately 29 degrees, (n55), very
slightly cyrtoconic, smooth, no growth lines or remains of sculp-
ture visible (Fig. 6.18, 6.19); cicatrix embedded as an elongate
groove at dorsal part of apex (Fig. 6.20); rim of cicatrix smooth;
apical part of siphuncle does not touch internal shell area of ci-
catrix; first segment of siphuncle nearly tubular, diameter of ap-
proximately 0.2 mm (Fig. 5.3); at first septum achoanitic necks;
younger segments of siphuncle slightly bulged into chamber-vol-
ume, cyrtochoanitic septal necks; necks less curved but longer at
dorsal side of septa; length of embryonic shell at least 6 mm,
probably approximately 10–15 mm; nepionic constriction not ob-
served; distinct onset of faint lirate ornamentation at approxi-
mately 8 mm orad from apex (Fig. 2.4); endosiphuncular deposits
filling entire apical part of siphuncle; height of chambers approx-
imately 0.8 of the diameter of shell; significant reductionofcham-
ber height between first and second chamber (Fig. 5.3); chambers
filled with mural, endosiphuncular deposits.
Juvenile and mature shell: Ornamentation faintly lirate with
deep, narrow striae; circular cross section, slightly depressed in
mature specimens; expanding rate 7 degrees; chamber height 0.3–
0.4 of diameter of conch; siphuncle slightly eccentric;cyrtochoan-
itic septal necks; connecting ring subcylindrical; continuous or
discontinuous annulate siphuncular deposits, heavier on ventral
side; thick mural cameral deposits.
Etymology.From Latin ‘‘teres,’’ rounded, and ‘‘lira,’’ ridge.
Type.Holotype OUZC 5047 from Imo Formation of Peyton
Creek Road Cut (Arkansas, USA).
Other material examined.Forty-eight embryonic specimens
are available. Only three have more than two apical chambers.
One of these three specimens was sectioned in median section for
internal morphology (OUZC 5028–5029). Three juvenile to ma-
ture specimens (OUZC 5060–5062) were investigated.
Occurrence.Imo Formation of central and northern Arkansas.
Discussion.The apical parts of Ristedtoceras teriliratum in-
clude an asymmetric development of the endosiphuncular deposits
and cyrtochoanitic septal necks, a subconical first segment of the
siphuncle, and an acute apex that differs from known embryonic
shells of other genera assigned to the Pseudorthoceratidae. The
juvenile and mature Mitorthoceras perfilosum Gordon, 1965 and
Mitorthoceras crebiliratum (Girty, 1909) differ from Ristedtocer-
as teriliratum by their ornamentation. The ribs of M. perfilosum
are acute and the interribs are smooth depressions, whereas, the
ribs of the holotype of Ristedtoceras teriliratum are smooth and
its interribs are very thin and sharply developed. Additionally, the
only known embryonic shell of Mitorthoceras Gordon, 1960,
564 JOURNAL OF PALEONTOLOGY, V. 78, NO. 3, 2004
F
IGURE
21, Reticycloceras peytonse Gordon, 1965, 33.7, OUZC 5026.
2, Dolorthoceras tenuifilosum,37, OUZC 5027. 3, Euloxoceras an-
gustinus,37, OUZC 5021. 4, Ristedtoceras teriliratum n. sp., 33.5,
OUZC 5028. 5, Mooreoceras imoense n. sp., 32.5, OUZC 5020. 6, M.
striatulum n. sp., 32.2, OUZC 5024.
F
IGURE
3—Diagrams shows measured chamber height (in mm) of the
most apical chambers of Dolorthoceras tenuifilosum (n59), Reticy-
cloceras peytonense (n511), and Euloxoceras angustinus (n55).
The thick lines show the mean chamber height of the respective cham-
ber number that are numbered beginning from the apex. Dolorthoceras
Miller, 1931 emend. Flower, 1939 and Reticycloceras Gordon, 1960
have relatively long first chambers and this changes at the fifth chamber
where the conch begins to curve. In Euloxoceras Miller, Dunbar, and
Condra, 1933 the first chamber is short.
F
IGURE
4—Histogram showing the apical angle of the first 3 mm, of
Euloxoceras angustinus (peak at 27 degrees) and Pseudorthoceras
knoxense (peak at 33 degrees).
which is blunt and very short (approx. 2 mm), differs strongly
from that of Ristedtoceras n. gen., which is acute and approxi-
mately 10 mm in length. Ristedtoceras stonense differs from Ris-
tedtoceras teriliratum in its smooth shell, the higher apical angle,
and the stronger s-shaped embryonic shell.
Genus M
OOREOCERAS
Miller, Dunbar, and Condra, 1933 emend.
Flower, 1939 emend. Gordon, 1965
Type species.Mooreoceras normale Miller, Dunbar, and Con-
dra, 1933.
Included species.More than 35 species ranging from Upper
Devonian to Lower Permian.
Diagnosis.Pseudorthoceratidae with smooth shell; conch
slightly to strongly depressed, circular in young stages; apex
blunt, bullet-shaped; tip with slightly cyrtoconic exogastric cur-
vature; siphuncle slightly eccentric; septal necks strongly re-
curved, cyrtochoanitic; connecting ring bulged outward into
chambers, not in contact with adjacent septa outside of septal
necks; siphuncular deposits continuous on ventral side of siphun-
cle, generally discontinuous to subcontinuous, if present on dorsal
side; mural cameral deposits, heavier on ventral side of conch.
Discussion.When reviewing the various definitions of Moor-
eoceras and Pseudorthoceras (Miller et al., 1933; Flower, 1939;
Sweet, 1964; Gordon, 1965) it seems very difficult, if not impos-
sible, to differentiate between the two genera. The definitions in
565KRO
¨GER AND MAPESLOWER CARBONIFEROUS (CHESTERIAN) EMBRYONIC ORTHOCERATID NAUTILOIDS
F
IGURE
5—Median sections of the apical parts of the Pseudorthoceratidae from the Imo Formation. 1, Mooreoceras striatulum n. sp., 31.6, OUZC
5024. 2, M. imoense n. sp., 32.3, OUZC 5020. 3, Ristedtoceras teriliratum n. sp., 38.2, OUZC 5029. 4, Dolorthoceras tenuifilosum,36.3, OUZC
5030. 5, Reticycocleras peytonense,38.3, OUZC 5031. 6, Family, genus, and species indet., 310, OUZC 5032. 7, Mitorthoceras girtyi, Gordon,
1965, 314.3, OUZC 5033. 8, Euloxoceras angustinus,37.1, OUZC 5034. 9, Pseudorthoceras knoxense,38.3, OUZC 5035. The dotted areas
indicate endosiphuncular deposits, the finely hatched area indicates the endocameral deposits.
the Treatise of Invertebrate Paleontology (Sweet, 1964) and Gor-
don (1965) are based on minimal differences in conch shape (cir-
cular vs. depressed) and in the dimension of siphuncular deposits
(heavy vs. slight deposits; ventrally fusing vs. not fusing). These
morphological differences vary depending on the ontogenetic
stage and the interval of the conch preserved. Fortunately, when
Gordon (1965) emended Mooreoceras, he based his observation
on the apical region of Mooreoceras normale, the type species of
the genus, and linked for the first time the description of the apex
to juvenile and mature specimens of Mooreoceras. Below we
place Mooreoceras sp. B of Gordon (1965) in Mooreoceras im-
oense n. sp. and include a detailed investigation of the embryonic
shell of the genus. When the embryonic growth stages of Pseu-
dorthoceras and Mooreoceras are compared, it is evident that they
are different. Thus, the suppression by Dzik (1984) of Mooreo-
ceras as a subjective, junior synonym of Pseudorthoceras is in-
valid.
M
OOREOCERAS IMOENSE
new species
Figures 1.2, 2.5, 5.2, 6.14
Mooreoceras sp. B G
ORDON
, 1965, p. 114–115, pl. 8, figs. 21, 22, 26.
Diagnosis.Mooreoceras with a depressed, broadly elliptical
juvenile and adult shell; smooth, ornamented only with faint
growth lines; strong expansion of the conch (.13 degrees); cham-
ber height 0.18–0.35 of diameter of shell; siphuncle subcentral
towards the venter; connecting ring strongly bulged outward; an-
nular endosiphuncular and strong mural cameral deposits at apical
part of shell; apex very slightly cyrtoconic, blunt, with high apical
angle (43 degrees).
Description.Embryonic shell slightly cyrtoconic, apical angle
approximately 43 degrees, no growth lines at most apical part of
shell, adoral part of apical part of juvenile shell ornamented by
irregularly spaced growth lines; nepionic constriction not visible,
adoral end of embryonic shell may be marked by slight change
from coarse to slightly finer growth lines and faint U-shaped con-
striction at about 9 mm from apex (Fig. 2.5); cicatrix at center of
apex, nearly perpendicular to growth direction; flat cap-shaped
with central, elongated depression (see Fig. 6.14) and circumfer-
ential groove; apical part of siphuncle not touching internal shell
area of cicatrix; first segment of siphuncle a caecum with diameter
of approximately 1 mm (Fig. 5.2); achoanitic necks on first sep-
tum; younger segments of siphuncle strongly bulged into chamber
volume; strongly recurved cyrtochoanitic septal necks; endosi-
phuncular deposits fill entire apical part of siphuncle, later in on-
togeny a continuous lining and/ or a partial, discontinuous lining
with strongest development on ventral side; chamber height varies
between 0.18–0.35 of the diameter of shell, significant reduction
of chamber height at fourth chamber (Fig. 5.2); strong mural de-
posits.
Juvenile shell smooth, faint sinusoidal growth lines; cross sec-
tion broadly elliptical, depressed (lateral vs. dorsoventral diameter
ratio of 0.88); expansion rate 13 degrees; chamber height lower
than that of embryonic shell approximately 0.2 of diameter; sub-
central siphuncle approximately 0.1 of the diameter; connecting
566 JOURNAL OF PALEONTOLOGY, V. 78, NO. 3, 2004
F
IGURE
6—Embryonic shells of the Pseudorthoceratidae from the Imo Formation. 16, Reticycloceras peytonense, bar 2 mm, OUZC 5039. 2, 3, bar
0.5 mm, OUZC 5045. 4, bar 0.5 mm, OUZC 5039. 5, detail from embryonic shell surface, bar 0.2 mm, OUZC 5039. 6, bar 0.3 mm, OUZC 5039.
7, Pseudorthoceras knoxense, bar 2 mm, OUZC 5048. 8, 9, Euloxoceras angustinus, bar 1 mm, OUZC 5043. 10, 11, Mitorthoceras girtyi, bar 2
mm, OUZC 5033. 12, 13, family, genus, and species indet., bar 2 mm, OUZC 5032. 14, Mooreoceras imoense n. sp., bar 2 mm, OUZC 5042. 15,
Pseudorthoceras knoxense, bar 0.6 mm, OUZC 5048. 16, Dolorthoceras tenuifilosum, bar 0.5 mm, OUZC 5041. 17, D. tenuifilosum, bar 2 mm,
OUZC 5041. 1820, Ristedtoceras teriliratum n. sp. 18, 19, bar 2 mm, OUZC 5047; 20, bar 0.5 mm, OUZC 5047.
567KRO
¨GER AND MAPESLOWER CARBONIFEROUS (CHESTERIAN) EMBRYONIC ORTHOCERATID NAUTILOIDS
ring subcylindrical; mural and endosiphuncular deposits that de-
crease adorally, generally heavier at ventral side of conch; largest
specimen investigated 16 mm in lateral diameter (Fig. 1.2); its
dorsoventral diameter not preserved.
Etymology.From ‘‘Imo Formation.’
Type.Holotype, OUZC 5020 from Imo Formation of the Pey-
ton Creek Road Cut (Arkansas, USA); displaying the apical and
juvenile interval of the conch; length 21 mm, lateral diameter
8.5 m.
Other material examined.Three fragments of apical, juvenile,
and nearly mature parts of the shell (OUZC 5038, 5042); shell
recrystallized. Two specimens were sectioned in median and cross
section for internal analysis.
Occurrence.Imo Formation (Chesterian of Arkansas, USA).
Discussion.Mooreoceras bakeri Miller, Dunbar, and Condra,
1933, from the Virgilian and Missourian of Nebraska and Kansas,
resembles Mooreoceras imoense n. sp., but differs in size and
apical angle.
M
OOREOCERAS STRIATULUM
new species
Figures 1.1, 1.6, 2.6, 5.1
Diagnosis.Mooreoceras with a compressed, broadly elliptical
juvenile and mature shell; smooth, ornamented only with faint
growth lines and a slight longitudinal striation in juvenile shell
parts; strong expansion of conch (approx. 12 degrees); chamber
height 0.25 of the diameter of shell; siphuncle subcentral ventral;
connecting ring fusiform; annular endosiphuncular and strong
mural cameral deposits at apical part of shell; apex cyrtoconic,
blunt, with a large apical angel of 45 degrees.
Description.Embryonic shell smooth, cyrtoconic, apical an-
gle approximately 45 degrees (Fig. 5.1); cicatrix at center of apex,
nearly perpendicular to growth direction, flat cap with a central,
elongated depression and a circumferential groove; no growth
lines appear at most apical part of shell; nepionic constriction not
visible; faint change in ornamentation and smooth constriction at
approximately 3 mm from apex; significant lowering of chamber
height after third chamber; first minor healed shell injuries and
irregularities at approximately 10 mm; apical part of siphuncle
touches internal shell area of cicatrix; first segment of siphuncle
tubular with a diameter of approximately 0.5 mm; achoanitic
necks at first septum; younger segments of siphuncle bulged into
chamber volume; strongly recurved cyrtochoanitic septal necks;
endosiphuncular deposits filling entire apical part of siphuncle, in
more adoral parts of apical shell parietal deposits, at ventral side
only incipiently developed and only at very apicad parts; height
of chambers between 0.3–0.4 of shell diameter; heavy irregular
mural deposits.
Nearly mature shell: Two specimens of Mooreoceras with dor-
solateral diameters of 42 mm and 40 mm; compressed cross sec-
tion; smooth shell with slight annulations and straight growth
lines; the conch expands with apical angel of approximately 12
degrees (Fig. 1.1); chamber height 0.3 of diameter; subcentral
siphuncle approximately 0.07 of the shell diameter (2.5 mm at
shell diameter of 36 mm); the septal necks short cyrtochoanitic;
connecting ring not preserved; no cameral deposits.
Etymology.From ‘‘stria’’ (Latin) for furrow, hollow, or chan-
nel, referring to the sculpture of the species.
Type.Holotype, OUZC 5024 from Peyton Creek Road Cut
(Arkansas, USA); displaying the apical and juvenile interval of
the conch; 32 mm in length, 10 mm in dorsoventral diameter.
Other material examined.Two fragments of the phragmocone
of nearly mature specimens (OUZC 5019, 5037). All specimens
were sectioned in median section for internal analysis.
Occurrence.Imo Formation (Chesterian of Arkansas, USA).
Discussion.Mooreoceras striatulum n. sp. is unique because
of its cyrtoconic apex, its faint, longitudinal ornamentation and
its compressed cone. Mooreoceas striatulum n. sp. is the only
species of the genus with a compressed cone.
Genus R
ETICYCLOCERAS
Gordon, 1960
Type species.Reticycloceras croneisi Gordon, 1960.
Included species.Six genera, all are from the Mississippian.
Emended Diagnosis.Annulate orthocones with nonannulate
embryonic shell; juvenile shell ornamented by fine network of
transverse and longitudinal lirae to form a delicate network; lon-
gitudinal lirae die out at juvenile stage, but transverse lirae con-
tinue until mature stages; siphuncle central to slightly eccentric,
suborthochoanitic to cyrtochoanitic; connecting ring slightly con-
vex; continuous or discontinuous annulate siphuncular deposits,
heavier on ventral side; thick mural cameral deposits.
Discussion.Originally Gordon (1960) described siphuncular
deposits as continuous, but as shown below in Reticycloceras pey-
tonense Gordon, 1965, they occur as both discontinuous and con-
tinuous endosiphuncular deposits. This notion is important be-
cause the conventional taxonomic division (Flower, 1939; Sweet,
1964) of the Pseudorthoceratidae is based on the shape of the
endosiphuncular deposits. Dzik (1984) regarded Reticycloceras as
a junior, subjective synonym of Cycloceras McCoy, 1844, the
type genus of the Cycloceratidae Hyatt, 1900. Although the sim-
ilarities between nearly mature Reticycloceras and Cycloceras are
amazing, until the insufficiently well-known Cycloceras is better
understood, Dzik’s (1984) ideas remain suggestive but unproven.
To our knowledge no embryonic shell of Cycloceras is described.
The very distinctive shape of the cicatrix and embryonic shell in
Reticycloceras should allow the higher taxonomic status of this
genus to be identified with certainty when the embryonic shell of
Cycloceras is known.
R
ETICYCLOCERAS PEYTONENSE
Gordon, 1965
Figures 2.1, 5.5, 6.1–6.6
Reticycloceras peytonense G
ORDON
, 1965, p. 118, pl. 6, figs. 23, 24, 31.
Description.Embryonic shell: nearly straight, faintly s-
shaped, showing a remarkable variation in apical angle–mean: 30
degrees, minimum: 25, maximum: 32, standard derivation: 2, with
n536 (compare Fig. 6.1, 6.2), ornamented by distinct longitu-
dinal lirae, at its most adoral part by transverse lirae (Fig. 6.5),
faint nepionic constriction 10–12 mm adoral of apex at diameter
of approximately 2.5–3.5 mm; distinct cicatrix on dorsal side of
apex, raised against apex of embryonic shell (Fig. 6.1–6.6), some-
times making an acute elongation to tip of shell, oval plate that
bears cicatrix always placed dorsal, plate placement may vary
somewhat in its orientation at apex (Fig. 6.1–6.6), outside of reg-
ularly, thickened rim of cicatrix longitudinal lirae appear and ra-
diate adorally; apical part of siphuncle touches internal shell area
of cicatrix; first segment of siphuncle tubular with a diameter of
approximately 0.3 mm; achoanitic necks at first septum; younger
siphuncle segments slightly bulged into chamber volume; short
cyrtochoanitic septal necks at second septum; endosiphuncular de-
posits may fill entire apical part of siphuncle, sometimes creating
continuous lining, sometimes annulate, discontinuous lining,
greatest development on ventral side; chamber height between
0.3–0.5 of the diameter of shell, significant reduction of chamber
height between fourth and fifth chamber (Fig. 5.5); irregular, mu-
ral deposits.
Juvenile shell: Shell ornamentation underwent a pronounced
change during juvenile interval (Fig. 2.1), longitudinal lirae dis-
appear at approximately 18–20 mm from apex, at most adoral
part of embryonic shell; for the first time, transverse lirae appear
and increasingly develop into strong annulations with increasing
distance adorally, between annulations faint growth lines or lirae
present; central siphuncle approximately 0.1 of diameter.
568 JOURNAL OF PALEONTOLOGY, V. 78, NO. 3, 2004
Material examined.One hundred thirty-one specimens pre-
served juvenile and embryonic shell parts; four embryonic shells
were sectioned for observation of internal structures (OUZC
5027, 5031, 5039, 5045).
Occurrence.Imo Formation of central and northern Arkansas
(USA).
Genus D
OLORTHOCERAS
Miller, 1931 emend. Flower, 1939
Type species.Dolorthoceras circulare Miller, 1931.
Included species.More than 35 species, ranging from Lower
Devonian to Lower Permian.
Diagnosis.Smooth orthocones with faint growth lines; cir-
cular to broadly elliptical in cross section; siphuncle subcentral
ventral; recurved cyrtochoanitic sepal necks; connecting ring sub-
cylindrical; continuous to discontinuous annulate siphuncular de-
posits, heavier on ventral side; thick lamellar, mural cameral de-
posits.
Discussion.The embryonic shells of Dolorthoceras exhibit
little morphological variation at the species level: Dolorthoceras
stiliforme Shimansky, 1954 and Dolorthoceras siphocentrale
(Krotov, 1885) have straight, blunt apices; however, in Dolorth-
oceras stiliforme the first three chambers are very long and the
first siphuncular segment is in contact with the shell at the central
apex. The straight embryonic shell of Dolorthoceras patens Zhu-
ravleva, 1978 shows a more acute apex with a tubular first si-
phuncular segment centrally touching the inner surface of the api-
cal shell. Balashov and Zhuravleva (1962) illustrated two Upper
Devonian Dolorthoceras sp. which show a very similar part of
the embryonic shell as that illustrated by Zhuravleva (1978). A
remarkable decrease in chamber height is visible in these speci-
mens after the fourth and fifth chamber. Dolorthoceras tenuifilos-
um Gordon, 1965, from the Imo Formation, differs from the for-
merly illustrated species of Dolorthoceras only in minor details
(see below). Dzik (1984) regarded the genus as a junior, subjec-
tive synonym of Trematoceras Eichwald, 1851, but the embry-
onic shell of Trematoceras differs from that of Dolorthoceras in
its blunt shape, as well as in its lack of ornamentation and dorsal
position of the cicatrix. Therefore, Dzik’s suggestion seems highly
unlikely.
D
OLORTHOCERAS TENUIFILOSUM
Gordon, 1965
Figures 1.4, 2.2, 5.4, 6.16, 6.17
Dolorthoceras tenuifilosum G
ORDON
, 1965, p. 121, pl. 7, figs. 1–4, 7.
Description.Embryonic shell: straight, mean apical angle at
the first 3 mm of 24 degrees (minimum: 22 degrees, maximum:
25 degrees, standard derivation: 0.2 with n: 31; shell smooth but
shows very faint longitudinal notches, which subsequently fade
out adorally) (Fig. 2.2); faint nepionic constriction hardly visible,
smooth, sinusoidal growth lines and frequent minor, sublethal
shell injuries show that embryonic shell has a length between 12–
14 mm from apex (Fig. 1.4); cicatrix at center of apex (Fig. 6.16),
nearly perpendicular to growth direction, elongated depression
with an irregular rim (see Fig. 6.16), faint notches appear at this
rim and radiate adorally; apical part of siphuncle touches internal
shell area of cicatrix; first segment of siphuncle tubular with a
diameter of approximately 0.2 mm (Fig. 5.4); achoanitic necks at
first septum; younger segments of siphuncle slightly bulged into
chamber volume; strongly recurved cyrtochoanitic septal necks;
endosiphuncular deposits sometimes fill entire apical part of si-
phuncle, in some cases building a continuous lining and in others
an annulate, discontinuous lining with strongest development on
ventral side; chambers height between 0.4–0.5 of the diameter of
shell; significant reduction of chamber height between the fifth
and seventh chamber (Fig. 3); heavy mural deposits.
Juvenile shell: smooth with faint sinusoidal growth lines; ex-
pansion rate approximately 5 degrees (see Fig. 1.4); chamber
height approximately 0.2 of diameter; subcentral siphuncle ap-
proximately 0.1 of shell diameter; connecting ring subcylindrical;
mural and endosiphuncular deposits decrease adorally, heavier on
ventral side of conch.
Material examined.Two hundred thirty-three specimens pre-
served juvenile and embryonic shell parts; three embryonic shells
were sectioned for observation of internal structures (OUZC
5027, 5030, 5041).
Occurrence.Fayetteville and Imo Formations of northern and
central Arkansas (USA).
Genus M
ITORTHOCERAS
Gordon, 1960
Type species.Mitorthoceras perfilosum Gordon, 1960.
Included species.Twenty-two species; ranging from Lower
Devonian to Upper Carboniferous.
Diagnosis.Small orthocones ornamented with raised trans-
verse lirae; lirae very gently sinuous; septa saucer-shaped, at right
angles to growth axis; siphuncle cyrtochoanitic, subcentral; con-
necting ring subcylindrical, slightly constricted at septal necks;
annulosiphuncular deposits, strong, lamellar episeptal and weaker
hyposeptal deposits.
Discussion.Gordon (1960, p. 135) described the genus gen-
erally as ‘‘transversely lirate orthoceracones with dolorthoceroid
siphuncle.’’ Consequently, a great number of species that may
differ greatly in their early ontogeny have been designated as
Mitorthoceras. Presently, nothing is known about the early on-
togenetic growth stages of the type of Mitorthoceras. Since the
two reports by Gordon (1960, 1965), only two authors (Zhurav-
leva, 1978; Niko, 1996) have referred to the genus and defined
new species. Dzik (1984) suggested that Mitorthoceras is a sub-
jective, junior synonym of Michelinoceras Foerste, 1932. Gordon
(1960) included endosiphuncular deposits in the original defini-
tion of Mitorthoceras; however, Zhuravleva (1978) included lirate
species without endosiphuncular deposits. The differences be-
tween Michelinoceras and Mitorthoceras may be defined by the
absence or occurrence of endosiphuncular deposits. Alternatively
the shape of the siphuncular necks may be crucial. It is defined
for Michelinoceras by Sweet (1964) as orthochoanitic to suborth-
ochoanitic. Gordon (1960) defined the necks of Mitorthoceras as
cyrtochoanitic; however, the necks of the type specie are suborth-
ochoanitic, and he also included species with suborthochoanic
necks (see Gordon, 1965). We now understand the importance of
describing the apicad parts of the shell in the original diagnosis.
At the present time, because there is no information on the em-
bryonic shell of the type of Mitorthoceras, the synonymy pre-
sented by Dzik remains unsubstantiated. The description of M.
girtyi Gordon, 1965 (see below) shows that there are strong dif-
ferences between the early ontogenetic growth stages of Michel-
inoceras (see Ristedt, 1968) and at least one species designated
as Mitorthoceras.
M
ITORTHOCERAS GIRTYI
Gordon, 1965
Figures 4.11, 4.12, 5.7
Orthoceras indianum G
IRTY
, 1915, p. 126, pl. 5, fig. 7, 7a.
Mitorthoceras girtyi G
ORDON
, 1965, p. 123, pl. 7, figs. 15, 16.
Description.Embryonic shell: slightly cyrtoconic, faint trans-
verse lirae, strong change in ornamentation at approximately 1
mm adoral of the apex; apex blunt, smooth (Fig. 6.10, 6.11);
cicatrix a slightly raised line or narrow ridge with smooth irreg-
ular rim, at ventral side of the apex; apical part of siphuncle not
touching internal shell area of cicatrix in first chamber; first seg-
ment of siphuncle ovate with a diameter of 0.05 mm (Fig. 5.7);
first septum with achoanitic necks; younger segments of siphuncle
569KRO
¨GER AND MAPESLOWER CARBONIFEROUS (CHESTERIAN) EMBRYONIC ORTHOCERATID NAUTILOIDS
subcylindrical, suborthochoanitic septal necks; no endosiphuncu-
lar deposits; heavy mural deposits at first three chambers.
Juvenile shell: distinct transverse lirae, slightly imbricate to-
wards apex; expansion rate of juvenile conch low (7 degrees);
chamber height approximately 0.8 of diameter; subcentral siphun-
cle approximately 0.2 of shell diameter; connecting ring subcy-
lindrical; mural deposits decrease in size and volume adorally.
Material examined.One specimen composed of the early ju-
venile and embryonic shell parts (OUZC 5033); four specimens
preserved premature shell parts.
Occurrence.Fayetteville Shale, Batesville Sandstone, and
Imo Formation (Chesterian) of northern and central Arkansas
(USA).
Discussion.Mitorthoceras girtyi may eventually prove to be
conspecific with Mitorthoceras perfilosum; however, no early ju-
venile or embryonic part of Mitorthoceras perfilosum is now
known. The species differ mainly in shape and distribution of the
transverse lirae. As shown in Reticycloceras, the ornamentation
may change significantly during the ontogenesis.
Genus E
ULOXOCERAS
Miller, Dunbar, and Condra, 1933
Type species.Euloxoceras greenei Miller, Dunbar, and Con-
dra, 1933.
Included species.Thirteen species; ranging from Lower Car-
boniferous to Upper Carboniferous.
Diagnosis.Smooth orthocones; circular in juvenile growth
stages to vertically compressed in cross section; septa slightly
sinuous and oblique; siphuncle subcentral toward venter; cyrto-
choanitic sepal necks; connecting ring subcylindrical; continuous
to discontinuous annulate siphuncular deposits, heavier on ventral
side; thick mural cameral deposits.
Discussion.Dzik (1984) proposed that Euloxoceras is a sub-
jective, junior synonym of Pseudorthoceras probably based on
the general shape of the siphuncle and its internal deposits. More-
over he suggested that Euloxoceras sp. A. of Gordon (1965) may
be related to Plagiostomoceras Teichert and Glenister, 1952; how-
ever, this conclusion cannot be substantiated until the early growth
stages of the latter is known. The embryonic shell of Euloxoceras
has a unique external morphology, in that the ovate first segment
of the siphuncle and the shape of the connecting ring-siphuncular
neck complex resembles that of Pseudorthoceras. This suggests
there is a phylogenetic connection between the two genera. The
erection of Euloxoceras seems to be justified by the differences
in septal spacing and the unique external embryonic shell mor-
phology.
E
ULOXOCERAS ANGUSTINUS
Gordon, 1965
Figures 1.3, 1.5, 2.3, 5.8, 6.8, 6.9
Euloxoceras angustinus G
ORDON
, 1965, p. 127–128, pl. 8, figs. 1–5, 25.
Description.Embryonic shell slightly cyrtoconic; very faint
longitudinal lirae which subsequently become adorally obsoles-
cent; mean apical angle 27 degrees (n531) (Fig. 4); nepionic
constriction; smooth, sinusoidal growth lines and frequent minor,
sublethal shell injuries mark a length of embryonic shell of ap-
proximately 3 mm from apex (Fig. 1.3); cicatrix at dorsal side of
apex; cicatrix a raised, line or narrow ridge (Fig. 6.8) with smooth
irregular rim; faint longitudinal lirae appear at this rim and radiate
adorally; apical part of siphuncle not touching internal shell area
of cicatrix; first segment of siphuncle ovate with diameter ap-
proximately 0.2 mm (Fig. 5.8); achoanitic necks at the first sep-
tum; younger segments of siphuncle slightly bulged into chamber
volume; cyrtochoanitic septal necks; endosiphuncular deposits fill
entire apical part of siphuncle; first chamber significantly shorter
(Figs. 3, 5.8) than that of juvenile shell; heavy mural deposits in
apical chambers.
Juvenile shell smooth with faint sinusoidal growth lines and
longitudinal lirae that become obsolescent adorally; expansion
rate is significantly lower than that of embryonic shell (4 degrees)
(Fig. 1.5); chamber height approximately 0.3–0.4 of shell diam-
eter; subcentral siphuncle approximately 0.15 of the shell diam-
eter; connecting ring subcylindrical; mural deposits decrease ador-
ally and are generally heavier at ventral side of conch.
Material examined.Ninety-seven specimens preserved juve-
nile and embryonic shell parts; two embryonic shells were sec-
tioned to show internal features (OUZC 5023, 5034).
Occurrence.Pitkin Limestone and Imo Formation of northern
and central Arkansas (USA).
Discussion.The measurement of the apical angle shows that
the embryonic shell of Euloxoceras angustinus is significantly
smaller than the nearly identical embryonic shell of Pseudortho-
ceras knoxense, indicating that the apical angle may be a good
diagnostic feature for differentiation. Additionally, the protoconch
of Pseudorthoceras knoxense shows smoother ornamentation.
Family, genus, and species
INDETERMINATE
Figures 5.6, 6.12, 6.13
Description.Apical part straight; apex blunt with low (ap-
proximately 2 degrees) expansion rate of conch in subsequent
chambers (Fig. 6.12, 6.13); initial chamber caplike, significantly
shorter than subsequent chambers (first chamber: 0.2 mm, sub-
sequent chambers approximately 0.9 of shell diameter) (Fig. 5.6);
slightly expanded third chamber; shell shows no ornamentation;
cicatrix developed at center of apex; cicatrix a raised, caplike
structure with central, narrow ridge (Figs. 5.6, 6.12) and smooth
irregular rim, that forms a slight circular depression, resembling
cicatrix of Mooreoceras; apical part of siphuncle not touching
internal shell area of cicatrix in first chamber; first segment of
siphuncle ovate with a diameter of approximately 0.2 mm;
achoanitic necks at first septum; younger segments of siphuncle
subcircular; suborthochoanitic septal necks; endosiphuncular de-
posits not observed; cameral deposits fill entire chamber volume;
first two septa more or less perpendicular to the shell wall, septa
of later chambers at lower angles to the shell wall; subcentral
siphuncle approximately 0.2 of shell diameter.
Material examined.One specimen with the first five cham-
bers is present in the collection (maximum diameter: 1.3 mm;
length of the fragment 4.5 mm) (OUZC 5032). The specimen was
sectioned to show internal features.
Occurrence.Imo Formation of northern and central Arkansas
(USA).
Discussion.By the current definitions of Pseudorthoceratidae
and Orthoceratidae (Sweet 1964), it is impossible to assign this
specimen definitively to one of the two families. The specimen,
therefore, provides an excellent example of the inadequacy of the
valid higher taxonomy of the Orthoceratida. As shown in Kro¨ger
(2004), both Orthoceratidae and Pseudorthoceratidae display en-
dosiphuncular deposits. Additionally a lack of endosiphuncular
deposits has been observed in some classical members of the
Pseudorthoceratidae (as shown herein). Consequently this char-
acter cannot be a diagnostic morphologic structure of one of the
families as defined in Sweet (1964) and all later references. Also,
suborthochoanitic septal necks occur in members of the Ortho-
ceratidae (Bitaunioceras Shimizu and Obata, 1936, Trematoceras)
as well as in members of the Pseudorthoceratidae (Pseudortho-
ceras,Ristedtoceras n. gen.). Therefore, it cannot be a diagnostic
character for differentiation between the two families. With regard
to these morphologic features the specimen described herein lacks
all of the features that have been considered to have diagnostic
value and thus cannot be classified to the family level. But, if the
apical features are utilized, it is apparent that this specimen is a
unique taxon that constitutes a rarely observed or recovered part
570 JOURNAL OF PALEONTOLOGY, V. 78, NO. 3, 2004
of the orthoconic population in the Imo Formation. The shape of
the caecum and the cicatrix, embryonic shell ornament, and the
amount of curvature in the apex are features that can be used to
establish species level taxonomic relationships. These features
have not been studied for most of the genera assigned to the
Orthocerida. Thus, at the present time the taxonomic divisions at
the familial level in the Orthocerida are not clear and a meaning-
ful classification is not possible.
DISCUSSION
Embryonic morphology reflects species level.The investiga-
tion of the early ontogeny of gastropods strongly influenced the
systematic and phylogenetic investigations of molluscs in the last
several decades. The protoconch—the apical whorls of the shell
of the gastropods secreted prior to metamorphosis—is now con-
sidered to be a fundamental morphological character that has a
major impact on the taxonomy, and the study of the protoconch
has become an essential tool in taxonomic evaluation in that field
(Bandel, 1979, 1988). Moreover, since poecilogony (occurrence
of more than one mode of larval development in one species) was
discovered in marine gastropods (Bouchet, 1989), even the tax-
onomy at the species level is considered to be a complex problem.
Despite the problems created by poecilogony, embryologic shell
development can be used as part of species definitions (Solsona
and Martinell, 1999). The investigation of the Imo Pseudortho-
ceratidae shows that knowledge of gastropod taxonomy and em-
bryological shell characters are equally valid concepts and char-
acteristics that may be applied to the Orthocerida. Thus, specific
protoconch morphology is reflected at the species level. Each type
of protoconch can be integrated into existing taxonomic rasters,
such as those proposed by Miller et al. (1933) and Gordon (1965)
that, at the species level, are based on more mature growth stages
rather than embryological shells. Of the hundreds of embryologic
orthoconic nautiloids recovered from the Imo Formation, only one
specimen cannot be matched with more mature growth stages.
This single specimen (OUZC 5032) is unlike all the other em-
bryologic orthocones, but we do not believe at this time that this
single individual is sufficient to erect a new species.
Embryonic morphology confuses higher taxonomy.However,
at higher systematic levels the known characters of the embryonic
shell are far more difficult to fit within the current accepted clas-
sifications. There is no congruence with the subfamily level of
Flower (1939), nor that of the Treatise of Invertebrate Paleontol-
ogy (Sweet, 1964) nor that of Zhuravleva (1978), which are based
on characters like the shape of the septal neck, the shape and
dimension of siphuncular deposits, and the outline of the con-
necting ring. Crucial for the definition of the subfamilies is the
shape of the endosiphuncular deposits (see Flower, 1939; Sweet,
1964). These are morphologic features which in most species
change during the ontogeny. They are considered taxonomically
relevant only in nearly mature specimens.
Classically (i.e., when the last great monographs and the Trea-
tise of Invertebrate Paleontology were produced in the last cen-
tury), the adult characters of a given clade were generally con-
sidered to be significant for lower levels of the taxonomy and the
more juvenile characters were expected to reflect the higher levels
of the taxonomy. Thus, the assumption was that more juvenile
characters are present in the apical part of the shell, and more
adult features are in the orad part of the mature or nearly mature
shell. But, the deposition of carbonate in the siphuncle and in the
chambers of the apical shells is a sequential process that takes
place in more or less later stages of the ontogeny. Now we must
consider the shape of the siphuncular deposits as a feature of the
late ontogeny, even though in the classical logic this was a feature
of consequence only to lower level taxonomic elevations. Possibly
the ontogenetically late setting of morphologic structures in the
apical parts of the shell causes this logical inconsistency in the
higher taxonomy of the Pseudorthoceratidae.
Nearly 40 years have passed since the most widely accepted
systematic classification of the Orthocerida was established in the
Treatise of Invertebrate Palaeontology. The important discoveries
in the field of developmental biology since then and especially in
the last 10 years (including and deserving of special mention are
the discoveries in the early ontogenetic development of Recent
molluscs) have strongly influenced evolutionary theory (see e.g.,
Raff, 1996; Gould, 2002). One important aspect of these novelties
is the discovery that a morphologic change in a given species
takes place not simply due to accumulation of characters through
the ontogeny but also through complex mechanisms in nearly all
intervals of the ontogeny. We have to mention these fundamental
changes in evolutionary theory in order to better understand the
systematic (and phylogenetic) framework of the Orthocerida,
which has remained nearly unchanged and unrevised for so long.
The current investigation shows that species-level characteristic
morphologic features are present in the embryonic shells of taxa
that we have assigned to the Pseudorthoceratidae. The question
that remains is, which set of characters may reflect the higher
taxonomy of the problematic order Orthocerida? Kro¨ger (2004)
showed that endosiphuncular deposits occur in the Orthoceratidae
as well as in Pseudorthoceratidae and Gordon (1965) showed that
suborthochoanitic septal necks also occur in both families. With
this overlap of these critical morphological structures there needs
to be a reevaluation so that the new features can contribute to a
better recognition of taxa and a refined phylogenetic framework.
With regard to the generic level, there is a better congruence
between the characters of the embryonic shell and of quasi-mature
conch characters, which are used for the current taxonomic di-
vision. Nevertheless, the embryonic shells show that Ristedtocer-
as teriliratum n. sp. and R. stonense do not belong to Pseudorth-
oceras. Further investigation of the embryonic shells will proba-
bly yield other generic level distinctions. At the species level and
perhaps the generic level, the characters of the cicatrix and em-
bryonic shell may yield some useful information about the sys-
tematic hierarchy.
The value of the characters of the embryonic shellDimen-
sion.A common assumption is that ammonoids, bactritoids, and
coleoids have small embryonic shells and nautiloids have large
embryonic shells. But our investigation of the embryonic shells
from the Imo Formation shows that there are great differences in
the embryonic shell dimensions. Some nautiloids display embry-
onic shells that match the dimensions of the ammonoids (see Ta-
ble 1). According to Bandel and Boletzky (1979) and Landman
et al. (1996), two size classes can be distinguished within the
recent cephalopods: embryos greater than about 2 mm in size (;3
310
1
mm
3
, assuming a more or less circular shape of the em-
bryo) and smaller than 2 mm in size. The embryos of ammonoids
(ammonitellas) and most observed embryonic shells of the pseu-
dorthocerids (nautiltellas) fall into the latter category. Thus, there
are no sharp limits between the dimensions of ammonoid and Imo
orthoconic nautiloid embryo shell sizes. It may be that the di-
mension of the embryonic shell has some limited diagnostic value
at certain taxonomic levels (see for example the large embryonic
shells of some endocerids, Balashov, 1968; and actinocerids,Zhu-
ravleva and Balashov, 1981), but there is no general difference
between orthoconic nautiloids and ammonoids as stated e.g., in
Lehman (1990), Engeser (1996), and Keupp (2000).
Ornamentation.The ornamentation of the embryonic shell
shows a great variety, which reflected at the species level in the
case of Pseudorthoceratidae. There are species with fine longitu-
dinal lirae, coarse striae, and transverse ornamentation (see Table
2). But there is no visible connection between ornamentation and
other characters, like shape of the first segment of the siphuncle
571KRO
¨GER AND MAPESLOWER CARBONIFEROUS (CHESTERIAN) EMBRYONIC ORTHOCERATID NAUTILOIDS
or septal neck morphology. Thus, the ornamentation is a morpho-
logic feature that can be used in species or genera diagnosis
Shape and position of cicatrix.In Recent Nautilus the cicatrix
mimics the position of the early mantle tissue of the embryo (Ar-
nold, 1987). The cicatrix is therefore the earliest evidence of the
soft body morphology observable in fossil cephalopods. It is ex-
creted not like the later shell parts by accretion of the rim of the
shell but by a single excretion of a thin periostracum layer and a
succeeding secretion of a nacre plate under the periostracum(Tan-
abe and Uchiyama, 1997). Consequently, the cicatrix displays no
growth lines. But a number of features characterize any cicatrix:
1) a central slitlike median groove, which is the symmetric plane
between the two bilateral parts; 2) sometimes longitudinal lirae
perpendicular to the median groove; and 3) a subcircular or ovate
depression or constriction of the shell enclosing the cicatrix. The
cicatrices of the investigated pseudorthoceratids show significant
interspecific differences in these features. They are flat, platelike,
and subcircular in Mooreoceras (Fig. 6.14) and Mitorthoceras
(Fig. 6.10, 6.11), extremely elongate and cuplike in Reticyclocer-
as (Fig. 6.1–6.6), and flat and deeply embedded in the shell in
Dolorthoceras (Fig. 6.16). Intraspecific variations in shape are
visible in both Reticycloceras and Dolorthoceras. Therefore, the
morphologic structure of the cicatrix has diagnostic value at the
species and perhaps generic taxonomic levels (see Table 2).
With exception of Mitorthoceras, all of the observed Imo spe-
cies show a cicatrix on the dorsal side of the apex or at its center.
Shape of the first segment of the siphuncle.Within the Pseu-
dorthoceratidae of the Imo Formation, two different shapes of first
siphuncle segments occur. Ristedtoceras n. gen., Dolorthoceras,
and Reticycloceras show an elongated, tubular first siphuncular
segment (Fig. 5.3–5.5), which is attached adorally at achoanitic
or suborthochoanitic septal necks. In all three genera the first seg-
ment was filled by endosiphuncular deposits. Pseudorthoceras
knoxense,Euloxoceras,Mooreoceras, and Mitorthoceras show an
expanded, caecumlike, first siphuncular segment (Fig. 5.1, 5.2,
5.7–5.9), which is either filled by endosiphuncular deposits or
empty. The septal necks of the first septum are generally achoan-
itic. A tubular first siphuncular segment as seen in Dolorthoceras
and Reticycloceras has not been observed. The shape of the first
segment of the siphuncle appears to be a distinctive morphologic
feature with some taxonomic value (see Table 2). But it is not
clear at which taxonomic level these differences will prove most
useful. An investigation of nonpseudorthoceratids is necessary to
resolve this problem.
ACKNOWLEDGMENTS
We wish to thank T. Engeser for the use of part of his unpub-
lished compilation of nautiloid taxa. We thank J. Olesik for pro-
viding scanning electron microscope assistance at Ohio State Uni-
versity. N. H. Landman, S. Klofak, K. Sarg and A. Klaus of the
American Museum of Natural History, New York City, were very
helpful in their assistance and the use of the scanning electron
microscope; in addition, Landman provided valuable insights on
buoyancy and equilibrium problems in ancient cephalopods. We
also thank M. Dunn, Ohio University, Athens, who provided in-
valuable technical support for the electronic images used herein.
K. Bandel, University of Hamburg, Germany, is thanked for his
general support of this project. RHM was partially supported in
this research by a National Science Foundation Grant (EAR
0125479) and an Ohio University Research Challenge Grant. BK
was supported in this research by a Deutsche Forschungs–Ge-
meinschaft Grant.
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CCEPTED
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CTOBER
2003
... Discussion.⎯Kröger and Mapes (2004) described two new species (M. striatulum Kröger andMapes, 2004 andM. imoense Kröger andMapes, 2004) which originally had been assigned by Gordon (1965) to Mooreoceras sp.. But, as shown in Kröger and Mapes (2005), Mooreoceras must be abandoned. ...
... striatulum Kröger andMapes, 2004 andM. imoense Kröger andMapes, 2004) which originally had been assigned by Gordon (1965) to Mooreoceras sp.. But, as shown in Kröger and Mapes (2005), Mooreoceras must be abandoned. Both species exactly match the external characteristics of Campyloceras. ...
Article
New material from the Late Mississippian Fayetteville and Caney Formations of the south-central USA supports the taxonomic revision of the Mississippian actmoceroid cephalopods. New representatives of the previously poorly known Rayonnoceras solidiforme, Campyloceras striatulum. and Campyloceras imoense are described. Carbactinoceras procerum new species and Ehnoceras graffhami new genus and species are described, and the genus Campyloceras is emended. Character evaluation suggests the shape of the endosiphuncular deposits in the Actinoceratida and Pseudactinocerida are homoplastic because similar patterns of endosiphuncular deposits occur in distantly related nautiloid clades. Apex shape, however, is shown to have previously unrecognized potential for taxon discrimination. Cladistic analysis of 13 actmoceroid and pseudorthocerid nautiloids supports the splitting of the Orthocerida and the creation of a sister group to the Actinoceratida which is identical with the largely neglected Pseudorthocerida.
... As pointed out by Gordon (1965) and Kröger and Mapes (2005), Mitorthoceras was one of the common Carboniferous orthoconic cephalopods in North America. It was assigned to the superfamily Pseudorthoceridae of the ectocochleate order Orhocerida (Gordon 1960(Gordon , 1965Sweet 1964;Kröger & Mapes 2004;Kröger & Mapes 2005) but it is here included in the new coleoid order Tuborthocerida. Another common Carboniferus orthoconic genus was Pseudorthoceras. ...
Article
Full-text available
The Carboniferous genus Mitorthoceras from USA, previously assigned to orthocerids, is here restudied and assigned to the new coleoid order Tuborthocerida. Mitorthoceras has several orthocerid characteristics: longiconic shell with long body chamber; central or subcentral siphuncle containing endosiphuncular deposits; and a tubular, uni-layered connecting ring probably consisting of mixed calcareous and chitinous elements. Coleoid characteristics are: shell wall rich in a chitinous material, and lack of a nacreous layer. Characteristic for the new order is the unique structure of the outer layer of the shell wall; this layer is composed of numerous, narrow, transverse furrows, separated by distally expanding ridges and closed from the exterior by thin calcareous layers, transforming the furrows into tube-like structures. The apical end of the shell forms an acute tip. A similar tube-like furrow-layer probably occurs within the shell wall of the bactritoid-like coleoid Shimanskya that has a smooth shell surface.
... The proposal that the Pseudorthoceratida are the sister group of the Actinoceratida (Kröger and Mapes 2007, fig. 2) would imply that there was a substantial reduction in the size of the embryonic shell in Carboniferous pseudorthoceratids, where the diameter of the initial camerae does not exceed 2 mm and may be as little as 0.5 mm (Kröger and Mapes 2004) compared with that of their actinoceratid ancestors (approximately 8 mm in diameter in Actinoceras tennuifilum (Hall, 1843) [Flower 1940]). This was reversed in the Carbactinoceratidae (placed in the Pseudorthocerida by Kröger and Mapes (2007), where the conch diameter at the first septum reached 15 mm and 13 mm, respectively, in Rayonnoceras solidiforme Croneis and Carbactinoceras torleyi Schindewolf (Kröger and Mapes 2007, fig. ...
Article
High-level classification of the nautiloid cephalopods has been largely neglected since the publication of the Russian and American treatises in the early 1960s. Although there is broad general agreement amongst specialists regarding the status of nautiloid orders, there is no real consensus or consistent approach regarding higher ranks and an array of superorders utilising various morphological features has been proposed. With work now commencing on the revision of the Treatise Part K, there is an urgent need for a methodical and standardised approach to the high-level classification of the nautiloids. The scheme proposed here utilizes the form of muscle attachment scars as a diagnostic feature at subclass level; other features (including siphuncular structures and cameral deposits) are employed at ordinal level. We recognise five subclasses of nautiloid cephalopods (Plectronoceratia, Multiceratia, Tarphyceratia nov., Orthoceratia, Nautilia) and 18 orders including the Order Rioceratida nov. which contains the new family Bactroceratidae. This scheme has the advantage of relative simplicity (it avoids the use of superorders) and presents a balanced approach which reflects the considerable morphological diversity and phylogenetic longevity of the nautiloids in comparison with the ammonoid and coleoid cephalopods. To avoid potential confusion arising in the higher levels of nautiloid classification employed in the revision of the Treatise Part K, we propose herein to replace the suffix ‘-oidea’ at subclass level with the suffix ‘-ia’. Apart from removing ambiguity and clarifying the nomenclature, this approach also brings greater consistency and affinity with modern zoological classification schemes used for cephalopods. The original Treatise Part K adopted an ‘abbreviated’ form of name for nautiloid orders using the ending ‘-cerida’ rather than ‘-ceratida’ (e.g., Order Actinocerida rather than Actinoceratida). For the revision of Treatise Part K, we propose using the ‘full’ version of the ordinal names. This approach re-employs several order names in their original form, e.g., Ellesmeroceratida, Oncoceratida, and Tarphyceratida. For reasons of consistency, we also apply the same to ordinal names created since the original Treatise Part K; therefore, Order Bisonocerida becomes Bisonoceratida.
... Discussion.-Numerous specimens exhibiting wide morphological variations have been assigned to Pseudorthoceras knoxense, whose stratigraphic range is exceptionally long, with a range spanning the Chesterian (Late Mississippian, early Carboniferous; Kröger and Mapes, 2004) to Wordian (middle Permian; Gordon, 1964). showing unconformity between the Ada and Boggy Formations. ...
Article
Nine longiconic cephalopod species of Desmoinesian (Middle Pennsylvanian; upper Carboniferous) pseudorthoceratid orthocerids are described from the Buckhorn Asphalt Lagerstätte (Boggy Formation) in Southcentral Oklahoma, Midcontinent North America. The fauna consists of Pseudorthoceras knoxense (McChesney), Arbuckleoceras tricamerae (Smith), Bitaunioceras buckhornense (Smith), Cyrtothoracoceras? sp., Dolorthoceras boggyense sp. nov., Smithorthoceras unicamera (Smith), Sueroceras oklahomense (Smith), Sulphurnites taffi sp. nov. and Unklesbayoceras striatulum sp. nov. Bitaunioceras buckhornense represents the first Pennsylvanian and therefore the oldest record of this genus. Arbuckleoceras gen. nov. differs from a comparable genus, Shikhanoceras, in possessing a weak exogastric curvature with a circular cross section of the conch and in lacking a conspicuous inflation at the embryonic shell. Smithorthoceras gen. nov. resembles orthoceratids rather than pseudorthoceratids in characters of camerae and siphuncle; however it refers to the Pseudorthoceratidae by having endosiphuncular deposits. These similarities seem to be the result of convergent evolution. Endosiphuncular deposits in Sulphurnites gen. nov. initiate at apical and adoral junctions between septal neck and connection ring, whose characters are unique for pseudorthoceratids. Unklesbayoceras gen. nov. differs from Mitorthoceras in having the endogastric conch, longer camerae and a less eccentric siphuncle. Taxonomic status of these orthoceratids was uncertain in previous biogeochemical and morphological studies. Sediments in the Buckhorn Asphalt Lagerstätte were deposited in a tropical epeiric sea (the Midcontinent Sea). Small, restricted marine basins, like that in this Oklahoma occurrence, probably provided an orthocerid refuge habitat as indicated by the high diversity and provincialism in comparison with other Middle Pennsylvanian (= Moscovian) faunas in other regions of the world.
... The internal characters are few; only the shape of the septal neck and the shape of the connecting ring were subjected to significant variability during evolution. Surprisingly, the shapes of the apical shell portion of longicones are strongly differentiated down to the species level (Ristedt 1968; Krö ger & Mapes 2004). However, these apices have often been overlooked with regard to orthoceridan classification. ...
Article
Full-text available
The annulated orthoceridans of the Middle and Late Ordovician of Baltoscandia are described and their systematic frame is revised. The revision of these nautiloids, which are part of the Orthocerida and Pseudorthocerida, is based on the investigation of characters of the septal neck, the siphuncular tube, and the apex. An unequivocal terminology of these characters is suggested and applied. The shape of the septal neck and the siphuncular tube are described for the first time in Palaeodawsonoceras n. gen., Striatocycloceras n. gen., Dawsonoceras fenestratum Eichwald, 1860, and Gorbyoceras textumaraneum (Roemer, 1861). Ctenoceras sweeti n. sp. is erected. The apex of Dawsonoceras barrandei Horný, 1956 is figured and described for the first time. The distribution of the character states of the apex and the septal neck support the emendation of the families Orthoceratidae, Dawsonoceratidae, and Proteoceratidae. The analysis shows also that the families Kionoceratidae, and Leuroceratidae must be refused because they represent not natural groups. However, it is also shown that the present knowledge is not sufficient to establish an unequivocal classification of the Middle, and Late Ordovician annulate cephalopods. Die orthoceriden Cephalopoden des Mittleren bis Späten Ordoviziums im Baltoskandium werden beschrieben und revidiert. Die Revision dieser Cephalopoden, welche zu den Orthocerida und Pseudorthocerida gehören, stützt sich auf die Untersuchung der Apikalenden, der Septalduten und der Form der Siphonalröhre. Eine eindeutige Terminologie für diese Merkmale wird vorgeschlagen und angewandt. Die Form der Septalduten und der Siphonalröhre von Palaeodawsonoceras n. gen., Striatocycloceras n. gen., Dawsonoceras fenestratum, Eichwald, 1860 und Gorbyoceras textumaraneum (Roemer, 1861) wird erstmals beschrieben. Die Art Ctenoceras sweeti n. sp. wird aufgestellt. Der Apex von Dawsonoceras barrandei Horný, 1956 wird erstmals beschrieben und dargestellt. Die neu gefundenen Merkmale stützen eine Emendation der Familien Orthoceratidae, Dawsonoceratidae und Proteoceratidae. Es wird daher dafür plädiert, die Familien Kionoceratidae (Hyatt, 1900) und Leuroceratidae (Sweet, 1964) nicht mehr zu verwenden, da diese keine natürlichen Gruppen repräsentieren. Die Untersuchung zeigt aber auch, dass es derzeit noch nicht möglich ist die annulaten Cephalopoden des Mittleren und Oberen Ordoviziums zweifelsfrei zu klassifizieren. doi:10.1002/mmng.200600005
... Study of early growth stages has crucial significance for taxonomy and for the tracing of phylogenetic relationships amongst cephalopods (Schindewolf, 1933;Ristedt, 1968;Engesser, 1996;Kröger and Mapes, 2004). Unfortunately this thin-walled and fragile part of shell was frequently damaged or destroyed during the life of the animal or after their death. ...
... The presence of a cicatrix is assumed by a very character− istic oblique surface at the tip of the apex. Such a surface is also present in Pseudorthoceras Girty, 1911 (Kröger and Mapes 2004) and the living Nautilus. This surface displays the same dimension of the lentoid structure of the cicatrix (initial shell of the conch) in Nautilus (compare Tanabe and Uchiyama 1997). ...
Article
The quarry in the north Estonian village of Porkuni provides a succession of shallow-water limestones and cherts spanning the Ashgillian Normalograptus? extraordinarius graptolite Biozone. This interval comprises the initial pulse of the end-Ordovician extinction. The succession of Porkuni contains abundant and extraordinarily well-preserved fossils. 71 cephalopod specimens were extracted from these strata at Porkuni. Many of these specimens are fragments of juvenile shells or small adults. The embryonic shells of the cephalopods are usually preserved and provide insight into their early ontogeny. The faunal composition is considered as autochthonous and reflects a "palaeo-nursery" in a Hirnantian reef environment. The collected specimens represent twelve genera and four orders. Small oncoceridans and orthoceridans dominate the association. The rate of endemism is very high, since only two genera found in Porkuni, are known from outside Baltoscandia. The new genera Parvihebetoceras, Pomerantsoceras, Porkunioceras, and the new species Parvihebetoceras wahli, Pomerantsoceras tibia, Porkunioceras tuba, and Strandoceras orvikui are erected.
Article
Full-text available
An exhaustive study of existing data on the relationship between egg size and maximum size of embryonic shells in 42 species of extant cephalopods demonstrated that these values are approximately equal regardless of taxonomy and shell morphology. Egg size is also approximately equal to mantle length of hatchlings in 45 cephalopod species with rudimentary shells. Paired data on the size of the initial chamber versus embryonic shell in 235 species of Ammonoidea, and 1 Spirulida demonstrated that, although there is a positive relationship between these parameters in some taxa, initial chamber size cannot be used to predict egg size in extinct cephalopods; the size of the embryonic shell may be more appropriate for this task. The evolution of reproductive strategies in cephalopods in the geological past was marked by an increasing significance of small-egged taxa, as is also seen in simultaneously evolving fish taxa.
Article
Full-text available
An early Chesterian (early Carboniferous; Late Mississippian) nautiloid fauna is described from the Ruddell Shale Member of the Moorefield Formation in Arkansas, Midcontinent North America. It consists of the three orthocerids, Euloxoceras buffalowallowense sp. nov., Mitorthoceras pofilosunz Gordon and Moore-fieldoceras yochelsoni gen. et sp. nov., and the two nautilids, Tylonautilus sp. and Peripetoceras milleri sp. nov. Reexamination of M. perfilosum, the type species of the genus, provides grounds for emendation of the generic concept emphasizing the supracentral position of the siphuncle. Moorefieldoceras is differentiated from comparable genera by its relatively rapid shell expansion, prominent annulations with a distinct ventral sinus, and relatively large siphuncle diameter with a central to subcentral position.
Article
Four species of trigonoceratid nautilids from the late Chesterian (Early Carboniferous) strata in the Imo Formation of Arkansas, Midcontinent North America are described. A specimen of Aphelaeceras arkansanum Gordon, 1964 reveals the external juvenile whorl morphology. Based on the discontinuous points of its surface lirae, we suggest the possibility that the hatching shell of A. arkansanum is cyrtoconic with two septa and a length of approximately 4.9 mm. Species previously described or assigned to Aphelaeceras are reexamined. Epistroboceras lesliense sp. nov. differs from E. caneyense Niko and Mapes, 2004 and E. pitkinense Niko and Mapes, 2005 in its lower ratios of umbilical zone height per whorl height and lower siphuncular position ratios. Epistroboceras? sp. indet. is also identified in this fauna. A new genus, Imonautilus, is erected on the basis of I. meeki sp. nov. The most characteristic features of this genus are the Stroboceras-like juvenile whorl, the obsolescence of the adoral ventrolateral grooves at or near maturity, and the roundly inflated adoral venter. Based on early ontogenetic similarities, a close evolutionary relationship between Aphelaeceras and Imonautilus is suggested.
Article
Seven species of Lochkovian (Early Devonian) pseudorthoceratid cephalopods are described and figured from the D1 member of the Fukuji Formation in Gifu Prefecture, central Japan. The fauna includes two pseudorthoceratines: Fukujiceras kamiyai gen. et sp. nov. and an indeterminate genus and species of the subfamily; four spyroceratines: Spyroceras fukujiense sp. nov., Spyroceras melolineatum sp. nov., Hamadaites labyrinthus gen. et sp. nov. and Mitorthoceras ? kamitakarense sp. nov.; and a cayutoceratine: Buchanoceras sp. The fauna may represent warm-temperate water conditions.
Article
The Chesterian Imo Formation in north-central Arkansas contains an abundant and diverse bivalve fauna which is dominated in numbers by nuculoids. The pterioids, pholadomyoids, and nuculoids show the greatest, and nearly equal, species diversity. Ecologically, on a generic level, the fauna is dominated by infaunal, burrowing, suspension feeders with attached suspension feeders a close second. Among the 40 taxa identified, the following new taxa are proposed: Economolopsis gordoni n. gen. and sp.; Spathelopsis browni n. gen. and sp.; Paleyoldia angustia n. sp.; P. bruta n. sp.; Phestia corrugata n. sp.; P. wortheni n. sp.; P. obtusa n. sp.; Imoella obscura n. gen. and sp.; Prothyris ( Lophoprothyris ) crista n. subgen. and sp.; Ectogrammysia crassatis n. gen. and sp.; Asketomorpha grandis n. gen. and sp.; and Sphenotus pisinnus n. sp. On the basis of ammonoids, the upper portion of the Imo Formation falls within the Elvirian Stage, E 2 b–c undifferentiated. Foraminifera place the formation in Mamet zone 19. The bivalve fauna discussed herein was collected from the E 2 b–c portion of the Imo.
Chapter
A color pattern in an ammonoid was first reported and illustrated by d’Orbigny in 1842 (p. 185, Pl. 45, Fig. 4). He recognized that the pattern preserved on the shell of a specimen of Asteroceras stellare (at that time known as Ammonites stellaris) from the Lower Jurassic was a remnant of a biologically produced color pattern emplaced by the animal when it was alive. Furthermore, he recognized that these kinds of patterns were observable only on well-preserved specimens. These brief observations laid the groundwork for the now generally recognized conclusions that color patterns are scarce on ammonoids and that they have paleobiological significance. Since d’Orbigny’s time, there have been periodic reports of ammonoid color patterns (see Table I), and some workers have attempted to integrate this information into a greater understanding of the biology of ammonoids.
Chapter
The position of the Ammonoidea within the Cephalopoda is no longer in much dispute. Almost all those who study cephalopods agree that the Ammonoidea are more closely related to the Coleoidea than to the other ectocochleate cephalopods, i.e., the Nautiloidea (Jacobs and Landman, 1994). There also can be no serious doubts that the Ammonoidea were derived from bactritids (Jacobs and Landman, 1994). However, it is still worth the effort to state more precisely the position of the Ammonoidea within the Coleoidea clade (= Neocephalopoda) using a cladistic approach. Berthold and Engeser (1987) and Engeser (1990b) discussed the position of the Ammonoidea only in general.