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Cephalopoda is the only class of molluscs in which virtually all its modern representatives have a pair of powerful jaws. There is little doubt that jaws have contributed to the evolutionary success of cephalopods, but their origin still remains a mystery. Though cephalopods appeared at the end of the Cambrian, the oldest unequivocal jaws have been reported to date from the Late Devonian, though they were initially interpreted as phyllopod crustaceans of the suborder Discinocarina. After their relation with ammonoids was proven, they were considered as opercula, and only later their mandibular nature was recognized and widely accepted. Finds of discinocarins from Silurian deposits are still considered as opercula of ammonoid ancestors ‐ nautiloids of the order Orthocerida. However, according to modern ideas, there is no place within their soft body for the location of such large opercula. Moreover, the repeated appearance of very similar structures in the same evolutionary line at least twice, but in different places of the body and for different purposes seems highly improbable. A new hypothesis is proposed herein, in which the Silurian fossils, earlier assigned to Discinocarina, are not specialized opercula, but protective shields, to defend orthocerids not from the predators, but from their own prey. The chitinous plates around the mouth likely appeared in the Silurian orthocerids for protection from such damage and later, during Silurian and Devonian, most likely gradually evolved into the jaws.
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Early Palaeozoic Discinocarina: a key to the appearance of
cephalopod jaws
ALEKSANDR A. MIRONENKO
Mironenko, A. A. 2020: Early Palaeozoic Discinocarina: a key to the appearance of
cephalopod jaws. Lethaia, https://doi.org/10.1111/let.12414.
Cephalopoda is the only class of molluscs in which virtually all its modern representa-
tives have a pair of powerful jaws. There is little doubt that jaws have contributed to
the evolutionary success of cephalopods, but their origin still remains a mystery.
Though cephalopods appeared at the end of the Cambrian, the oldest unequivocal jaws
have been reported to date from the Late Devonian, though they were initially inter-
preted as phyllopod crustaceans of the suborder Discinocarina. After their relation with
ammonoids was proven, they were considered as opercula, and only later their
mandibular nature was recognized and widely accepted. Finds of discinocarins from
Silurian deposits are still considered as opercula of ammonoid ancestors nautiloids of
the order Orthocerida. However, according to modern ideas, there is no place within
their soft body for the location of such large opercula. Moreover, the repeated appear-
ance of very similar structures in the same evolutionary line at least twice, but in differ-
ent places of the body and for different purposes seems highly improbable. A new
hypothesis is proposed herein, in which the Silurian fossils, earlier assigned to Dis-
cinocarina, are not specialized opercula, but protective shields, to defend orthocerids
not from the predators, but from their own prey. The chitinous plates around the
mouth likely appeared in the Silurian orthocerids for protection from such damage and
later, during Silurian and Devonian, most likely gradually evolved into the
jaws. Anaptychi, aptychi, Aptychopsis, Cephalopoda, Discinocarina, jaw apparatus.
Aleksandr A. Mironenko [paleometro@gmail.com], Geological Institute of Russian
Academy of Sciences, Pyzhevski Lane 7/1 Moscow 119017, Russia; manuscript received
on 24/06/2020; manuscript accepted on 31/10/2020.
Virtually all modern cephalopods have a superbly
developed jaw apparatus, consisting of the upper and
lower jaws and the radula enclosed between them.
Whereas the radula is a typical feature for the Mol-
lusca (Vinther et al. 2017), the pair of dorsal and
ventral jaws of cephalopods is a unique structure
among molluscs. Using a pair of jaws, cephalopods
are capable of catching, holding and killing their
prey. Each jaw of living cephalopods (coleoids and
nautilids) and of extinct ammonoids consists of two
lamellae the outer and the inner, whose sizes greatly
vary among taxa (see Tanabe et al. 2015). Nautilids,
starting from the Middle Triassic,and some of the
Jurassic and Cretaceous ammonoids have additional
calcareous elements in the jaws (Klug 2001; Riegraf
& Moosleitner 2010; Tanabe et al. 2015; Mironenko
& Gulyaev 2018). Nevertheless, in general, the jaws
of coleoids, nautilids and ammonoids share the same
general structural plan. Such a fundamental similar-
ity indicates that the last common ancestor of all
these cephalopod lineages either already had the jaw
apparatus, organized in the same way, or at least had
some sort of protojaws, the structure of which con-
tained a general planof the prospective jaws.
However, the origin of the cephalopod jaws still
remains a mystery. Boletzky (2007) suggested that
the upper jaw of cephalopods may be homologous to
the unpaired upper jaw of some gastropods, whereas
the lower jaw somehow could have arisen later. It
was also suggested that the cephalopod jaws could
have emerged from some kind of precursor or anla-
genof jaws which was present in the common
ancestor of cephalopods, monoplacophorans and
gastropods (Klug et al. 2016, 2017). These assump-
tions are based on the supposed homology of the
jaws of molluscs of various classes. However,
although some gastropods and monoplacophorans
have a peculiar kind of solid mandibular elements
(Lemche & Wingstrand 1959; Gittenberger & Gitten-
berger 2005; Vortsepneva et al. 2013, 2014; Vortsep-
neva & Tzetlin 2014; Mikhlina et al. 2015), these
structures are completely different from jaws of
cephalopods. The mandibular elements of gastropods
consist of a single plate above the radula (Vortsep-
neva et al. 2013); two plates both on the dorsal side
of the buccal cavity (Vortsepneva et al. 2014; Mikh-
lina et al. 2015); or lateral paired serrated blades
which are located by the sides of the radula (Gitten-
berger & Gittenberger 2005). Moreover, various taxa
of modern gastropods have a dissimilar structure of
the mandibles (Vortsepneva & Tzetlin 2014; Mikh-
lina et al. 2015). Their jaws vary not only in shape
DOI 10.1111/let.12414 ©2020 Lethaia Foundation
and location, but also in mechanism of formation
during ontogenesis, which suggests that the jawlike
structures had appeared in different evolutionary
lines of gastropods independently (Vortsepneva &
Tzetlin 2014). Therefore, it is highly unlikely that the
common ancestor of gastropods and cephalopods
had some sort of jaws or protojaws, inherited by
both these groups of molluscs (see also Dzik 1981).
The problem of the origin of cephalopod jaws is
complicated by the fact that although cephalopods
are known since the end of the Cambrian, the oldest
unequivocal jaws of cephalopods are from the Upper
Devonian, attributed to ammonoids (Klug et al.
2016, 2017). In addition, ndings of separate cepha-
lopod radulae without any traces of jaws are
described from the Silurian (Mehl 1984) and Ordovi-
cian (Gabbott 1999). It is quite strange since jaws are
more robust structures than radula and usually they
are much better preserved in fossil state (Kruta et al.
2015). Therefore, isolated radulae specimens without
any remnants of jaws raise doubts over the presence
of jaws in cephalopods in preDevonian times.
Finds of peculiar solid structures from the Silurian,
associated with cephalopod shells have been
described. The most famous among them is Apty-
chopsis, which is widely considered as belonging to
the orthocerid nautiloids (Turek 1978; Holland et al.
1978; Dzik 1981; Stridsberg 1981, 1984; Zakharov &
Lominadze 1983; Klug et al. 2015, 2017). The func-
tions of Aptychopsis remains controversial: most
researchers consider it as a protective operculum,
which could have closed the shell opening in case of
danger (Turek 1978; Holland et al. 1978; Stridsberg
1981, 1984), whereas others assumed that it could
have served as primitive jaws of orthocerids (Laufeld
et al. 1975; Dzik 1981; Zakharov & Lominadze 1983).
It should be noted, however, that the second hypoth-
esis is now mostly consider outdated, the last article
in which the Apthychopsis was considered as the part
of jaw apparatus was published almost 40 years ago
(Zakharov & Lominadze 1983). In more recent pub-
lications it is referred to exclusively as an operculum
(Stridsberg 1984; Edgecombe & Chatterton 1987;
Holland 1987a, 1987b; Frye & Feldmann 1991; Kise-
lev 1992; Holland 1996; Manda 2007; Manda &
Turek 2018).
Nevertheless, the Aptychopsis is not a oneofa
kind structure, but a member of parataxonomic sub-
order Discinocarina, which were originally consid-
ered as phyllopod crustacean shells (Salter 1863;
Woodward 1866, 1882a, 1882b; Barrande 1872;
Nicholson 1872; Packard 1882; Novák 1892), but
later many of them were recognized as cephalopod
aptychi and anaptychi, i.e. lower jaws (Ruedemann
1916; Brooks & Caster 1956; Rolfe 1969; Frye &
Feldmann 1991; Goolaerts et al. 2017). However,
only Devonian and younger genera of discinocarins
are now interpreted as anaptychi, since ammonoids
appeared in the late Early Devonian. Most Silurian
discinocarins, such as Peltocaris and Discinocaris,
have not been mentioned in the literature since the
1980s, although they had always been classied
together with Aptychopsis for many years. Particular
attention to Aptychopsis in modern literature is likely
explained by the fact that it is the only member of
Discinocarina that was found directly at the aperture
of the orthocerid shells (Turek 1978; Holland et al.
1978). However, for a century before these nds, and
a short time after them, researchers considered both
Aptychopsis,Peltocaris, and Discinocaris as members
of the same group of fossils, regardless of whether
they were interpreted as crustaceans, cephalopod
opercula or mandibular elements (Barrande 1872;
Clarke 1902; Ruedemann 1916; Rolfe 1969; Holland
et al. 1978).
In this paper, Silurian discinocarins are reconsid-
ered, including Aptychopsis in the context of new
data on cephalopod anatomy and evolution, using
a collection of discinocarins (mainly Aptychopsis)
in the Czech National Museum in Prague, Czech
Republic (part of which have never been published)
and literature devoted to the discinocarins. A care-
ful approach to all this data allows us to conclude
that the previous assumptions about Aptychopsis
and its relativesof Discinocarina were overlooked
and these structures represent the key to under-
standing the origin and early evolution of cephalo-
pod jaws.
Taxonomic remarks
Fossils belonging to the suborder Discinocarina
(Clarke in Zittel 1900) originally were included into
the order Phyllopoda (Salter 1863; Woodward 1866,
1882a, 1882b; Barrande 1872; Nicholson 1872; Pack-
ard 1882; Novák 1892; Clarke in Zittel 1900). How-
ever, later they were excluded from the crustaceans
(Brooks & Caster 1956; Rolfe 1969). Since all dis-
cinocarins are currently considered as hard struc-
tures from the apertural region of cephalopods
(Brooks & Caster 1956; Rolfe 1969; Holland et al.
1978; Frye & Feldmann 1991; Goolaerts et al. 2017),
Discinocarina should be considered now as a
parataxonomic group, similar to the formal genera of
the Mesozoic aptychi (see Tanabe et al. 2015). In this
paper, the name Discinocarina is used not in a taxo-
nomic sense, but as a historical umbrella term. The
same applies to the genera included in this suborder:
they are considered not as real taxa, but as
2A. A. Mironenko LETHAIA 10.1111/let.12414
conventional names of various morphotypes of solid
structures of Palaeozoic cephalopods.
Results
Discinocarina: stratigraphy, variety, and
structure
The question of age of the oldest representatives of
Discinocarina has until recently, remained open. The
genera Peltocaris (Salter 1863) (Fig. 1) and Dis-
cinocaris (Woodward 1866) (Figs 2, 3) were both
described in 19th century from Moffat Shales in
Dumfriesshire, Scotland. Both Salter (1863) and
Woodward (1866) attributed the layers, in which
these fossils were found, to the Llandeilo ags. Since
Llandeilo is currently considered as middle Ordovi-
cian, the stratigraphical distribution of Peltocaris and
Discinocaris was often assumed to be started from
the middle Ordovician (e.g. Rolfe 1969). However,
Lapworth (1878) provided a stratigraphical scheme
of Moffat Series for the Dob's Linn locality, which
later, in 1984, was designated the OrdovicianSilurian
boundary stratotype (Williams 1988). In this scheme,
the ndings of both Peltocaris and Discinocaris are
shown only in the upper part of Moffat series, in
Birkhill Shales. According to Lapworth's zonation,
based on graptolites, Discinocaris appeared in Mono-
graptus gregarius Zone and Peltocaris slightly later in
Rastrites maximus Zone. The OrdovicianSilurian
boundary is now placed at the base of the Acumina-
tus Zone in Birkhill shales, which is below the rst
discinocarin ndings (Williams 1988). Therefore, the
oldest known discinocarins from Moffat shales, rst
Discinocaris, then Peltocaris, appear in palaeontologi-
cal record in the lower Silurian, in the Rhuddanian
age of the Llandovery epoch. In the same Rhudda-
nian age they appear in Bohemia fossil record (Turek
1978). Therefore, whereas Ordovician origin of Dis-
cinocarina cannot be excluded, the oldest ndings of
these fossils are known from the lower Silurian.
Both Peltocaris (Fig. 1) and Discinocaris (Figs 2, 3)
represent circular or elliptical structures which were
originally organic, likely chitinous, without any cal-
citic elements. Peltocaris consists of three parts: two
large and symmetrical plates, which are considered
as ventral, and one small plate, likely dorsal. Whereas
large ventral plates in Peltocaris connect to each
other along the straight edges, the small dorsal plate
has a rounded edge and the corresponding notch in
the large plates is also semicircular (Salter 1863,
g. 1; Nicholson 1872, g. 102d; Woodward 1866,
pl.XXV, g. 6; Jones & Woodward 18881899,
Plate XVI, g. 20).
Discinocaris consists of only two elements: its ven-
tral part is a single wide plate, very similar to the
ammonoid anaptychi (Woodward 1866, pl.XXV,
gs 4,7; Packard 1882, g. 8; Jones & Woodward
18881899, Plate XVI, g. 6). The small dorsal plate
and the corresponding notch in the ventral plate in
Discinocaris have usually been described as subtrian-
gular, with straight edges, in contrast to their semi-
circular shape in Peltocaris (Woodward 1866;
Packard 1882; Jones & Woodward 18881899;
Novák 1892). In both Discinocaris and Peltocaris the
dorsal plate was very weakly attached to the ventral
part, so most of the known specimens of these genera
were found without this element (Jones & Wood-
ward 18881899; Turek 1978; see also Figs 13).
Careful examination of specimens from the Sil-
urian of Bohemia demonstrates that there is a transi-
tional form between the typical Discinocaris and
Peltocaris that combines the characteristics of both
taxa. The specimen NM L 27959 (holotype of the
species Discinocaris dusliana Novák 1892) is origi-
nally depicted with a subtriangular notch, typical for
the genus Discinocaris in the sketch in Novák (1892,
g. 1), whereas the examination of this specimen
shows that the notch is clearly rounded (Fig. 2). Such
a shape of a notch is a characteristic of Peltocaris,
however, Peltocaris has a pair of ventral plates not a
single plate, such as in this specimen. Another speci-
men NM L 59568 of the same species but poorly pre-
served, has also a wide rounded notch (Fig. 3A).
Apparently, these differences in the shape of the
notch were mentioned by Novák (1892, p. 149) in his
reference to a different slope of the nuchal suturein
the description of a new species. The difference in
the shape of the notch is clearly visible when com-
pared with a typical Discinocaris browniana (Fig. 3
B).
In the Telychian age of the Early Silurian (see
Turek 1978) the most famous genus of Discinocarina
Aptychopsis appeared (Figs 46). Its shape is very
similar to the shape of Peltocaris: it consists of three
plates, two large ventral and one small dorsal, but the
shape of the dorsal plate is not semicircular, as it is in
Peltocaris and Discinocaris dusliana, but triangular,
similar to that in Discinocaris browniana, with a
prominent rostrum (see Kiselev 1992, table 2, gs 3,
5). The notch in the ventral part of Aptychopsis has a
corresponding subtriangular shape. There is another
important difference between Aptychopsis and other
discinocarins: the outer surface of its organic plates is
covered with a thick layer of calcite. Therefore, Apty-
chopsis resembles aptychi of Jurassic and Cretaceous
ammonoids, whose surface is also covered with cal-
cite (see Tanabe et al. 2015). Probably due to these
calcitic layers, specimens of Aptychopsis are well
LETHAIA 10.1111/let.12414 Discinocarina and cephalopod jaws 3
preserved and well studied (Barrande 1872; Clarke in
Zittel 1900; Clarke 1902; Turek 1978; Holland et al.
1978; Stridsberg 1981, 1984; Kiselev 1992). However,
nds of whole Aptychopsis specimens are very rare,
most often individual ventral valves or incomplete
specimens without a dorsal plate (Turek 1978; Dzik
1981). The shape of Aptychopsis varies from almost
round to elliptical, elongated in a dorsoventral direc-
tion (see Turek 1978, g. 11), but extremely wide
specimens are also known (see Kiselev 1992, table 2,
g. 3).
Several specimens of Aptychopsis were found at
the apertural parts of the shells of nautiloid order
Orthocerida, these nds conrmed they belonged to
AB
Fig. 1. Silurian Peltocaris sp., Bohemia, collection housed in the National Museum in Prague, Czech Republic. A. specimen NM L
59569, B. NM L 59567. Scale bars 5 mm.
A
C
B
Fig. 2. Discinocaris dusliana, specimen NM L 27959, described by Novák (1892) from the Aeronian (middle Llandovery) of Bohemia. Col-
lection housed in the National Museum in Prague, Czech Republic. A, general view. B, cracks in the outer edge appeared due to attening
of the specimen. C, subcircular dorsal edge. Scale bars: A, 5 mm; B, C, 2.5 mm.
4A. A. Mironenko LETHAIA 10.1111/let.12414
these cephalopods (Turek 1978; Holland et al. 1978).
It should be noted that at least in one case, a speci-
men found in situ, the dorsal plate is absent (Holland
et al. 1978, g. 1A), which indicates its extremely
weak connection with the ventral plates.
Tripartite discinocarins such as Aptychopsis and
Peltocaris apparently disappeared at the beginning of
Pridoli epoch (Turek 1978). Aptychopsis permiana
Riabinin 1921, described from Permian strata, really
is a crustacean (see Riabinin 1921, table 4, g. 4),
whereas Triassic Aspidocaris triassica Reuss 1867
(synonymized with Discinocaris by Rolfe 1969) is an
ammonoid anaptychus (Radwanski & Summesberger
2001). However, there are numerous fossils from
AB
Fig. 3. A, Discinocaris dusliana, specimen NM L 59568. B, Discinocaris browniana, specimen NM L 59566. Collection housed in the
National Museum in Prague, Czech Republic. Scale bars, 5 mm; B, 2.5 mm.
AB
CD
Fig. 4. Complete Aptychopsis specimens, collection housed in the National Museum in Prague, Czech Republic. A, NM L 11263. B, NM L
59560. C, NM L 59562. D, NM L13890. Scale bars for A and B, 2.5 mm; C, 1 cm; D, 5 mm.
LETHAIA 10.1111/let.12414 Discinocarina and cephalopod jaws 5
Upper Devonian strata that were also placed into the
Discinocarina by many researchers, such as genera
Ellipsocaris,Cardiocaris,Spathiocaris,Pholadocaris,
Lisgocaris and several others (Clarke 1882; Wood-
ward 1882a, 1882b; Straelen & Schmitz 1934). These
bodies represent oval, initially organic plates with a
semicircular notch, resembling Peltocaris, but are
most often more elongated in the dorsoventral
direction (Fig. 7). In some cases, these fossils occur
in a folded state (Clarke 1882: g. 3) and in this type
of preservation they are practically indistinguishable
from the aptychi of Mesozoic ammonoids, with the
only difference being that the Devonian discinocarins
did not have a calcite layer, which was present in
aptychi and Aptychopsis. Finds of dorsal plates have
never been described for them.
On the basis of the structure of the ventral part
discinocarins are divided into two families: Dis-
cinocaridae Woodward and Peltocaridae Salter
(Clarke in Zittel 1900; Straelen & Schmitz 1934; Rolfe
1969). The rst includes genera with an undivided
ventral part: Silurian Discinocaris and Devonian
Ellipsocaris,Cardiocaris,Spathiocaris and Pholado-
caris, the second includes genera Peltocaris and
Aptychopsis, whose ventral part is separated into two
symmetrical valves (Straelen & Schmitz 1934).
Therefore, in general, it can be concluded that dis-
cinocarins existed in the Silurian and Devonian, and
in the Silurian, they all had a dorsal plate, whereas
for the Devonian specimens such a plate is unknown.
Most of the discinocarins were completely organic
and only the Aptychopsis had an outer calcitic layer.
The evolution of opinions on the nature of
Discinocarina
Salter was rst discuss discinocarins in 1863 when
described the new genus Peltocaris, with the type spe-
cies Peltocaris aptychoides. The species was named
due to the similarity of its ventral plates with ammo-
noid aptychi, which have been known at least from
the beginning of the 18th century and at that time
were considered as opercula of ammonoids (see
Trauth 1927 for review). Discinocaris was rst
described by Woodward (1866), followed by the
description of Aptychopsis by Barrande (1872). After
10 years, the number of genera of the small Palaeo-
zoic phyllopod crustaceans (as these structures were
interpreted at that time) reached eight: such genera
as Ellipsocaris,Cardiocaris,Spathiocaris,Lisgocaris
and Pholadocaris were described (Clarke 1882;
Woodward 1882a, 1882b). The study of these Phyl-
lopodstook place in parallel with the study of apty-
chi and anaptychi of ammonoids. However, in 1885
Woodward described a nd of the anaptychus at the
aperture of the Devonian ammonoid shell, and this
anaptychus was nearly identical with the specimens
of the genus Cardiocaris, previously described as
phyllopod crustacean (Woodward 1885). Based on
that fact, it was suggested that the genera Cardiocaris
and Pholadocaris are actually ammonoid anaptychi,
whereas the remaining genera were still considered
phyllopod crustaceans (Woodward 1885; Jones &
Woodward 18881899).
This view was not accepted by all researchers. For
example, Holzapfel (1899) and Scalia (1922) believed
that all these genera had no relation with ammonoids
and are phyllopods. Clarke, who introduced the sub-
order Discinocarina (Clarke in Zittel 1900), agreed
with Woodward and assumed that some of the Devo-
nian discinocarins were most likely related with
ammonoids. At the same time, he noted that a para-
doxical situation had appeared: on the one hand, the
Devonian genera are clearly of the same nature as the
more ancient discinocarins, but on the other hand
there are no ammonoids in the preDevonian strata
and therefore they could not be considered as
A
B
Fig. 5. Rostrum of the dorsal plate and growth lines of ventral
plates of Aptychopsis. A, NM L 59560, scale bar 2 mm (see also
Fig. 4B). B, NM L 59562, scale bar 3 mm (see also Fig. 4C).
6A. A. Mironenko LETHAIA 10.1111/let.12414
ammonoid anaptychi (see Clarke 1902). As a result,
he continued to consider a more ancient genera such
as Discinocaris and Peltocaris as phyllopods, pointing
out the inexplicable similarity of their ventral plates
with ammonoid anaptychi (Clarke in Zittel 1900;
Clarke 1902).
Ruedemann (1916) proposed a simple solution to
this paradox in his opinion, all these genera are
anaptychi, which likely originated not in ammonoids,
but in their ancestors. Therefore, Ruedemann was
the rst who suggested that these structures could
have occurred in cephalopods before the appearance
of ammonoids. However, Ruedemann considered
anaptychi not as jaws, but as supporting plates for
the retractor muscles of the hyponome.
Ruedemann's assumption went unnoticed for
some time by most researchers and as a result dis-
cinocarins, including even the Devonian genera, con-
tinued to be considered as crustaceans (Straelen &
Schmitz 1934). Brooks & Caster (1956) probably
independently came to the same conclusion: accord-
ing to their opinion all discinocarins are anaptychi of
various cephalopods, not only of ammonoids (they
considered anaptychi as opercula). Krestovnikov
(1960) was probably the last to consider dis-
cinocarins as crustaceans. Rolfe, in the Treatise on
Invertebrate Paleontology (1969) who noted Ruede-
mann's (1916) suggestion that the aptychi could have
existed in the Silurian cephalopods has been largely
overlooked. Rolfe also considered all Devonian dis-
cinocarins as ammonoid anaptychi and the three
more ancient genera formally as incertae sedis, but he
noted that they are most likely anaptychi of nau-
tiloids. Rolfe (1969) also noted that although dis-
cinocarins have never been found along with
cephalopod shells, most likely is due to postmortem
separation.
A new era in the study of these structures began in
the 1970s. Bergström in Laufeld et al. 1975 was the
rst to recognize the mandibular nature of Silurian
AB
DE
C
Fig. 6. Incomplete Aptychopsis specimens and a separate dorsal plate, collection housed in the National Museum in Prague, Czech Repub-
lic. A, B, specimen NM L 59564. A, general view. B, separate dorsal plate. C, specimen NM L 59565 with injured ventral valve. D, specimen
NM L 59563. E, specimen NM L 59561. Scale bars: A, 1 cm; B, C, 3 mm; D, E, 5 mm.
LETHAIA 10.1111/let.12414 Discinocarina and cephalopod jaws 7
discinocarin genus Aptychopsis. The quote should be
given in its entirety: It may be concluded that Apty-
chopsis prima exhibits several features typical of
cephalopod (ammonoid) aptychi. Lehmann (1970,
1972) showed that the ammonoid aptychus probably
functioned as the lower jaw of the animal. The lower
jaw is commonly so large that a correspondence in
shape with the cephalopod shell seems explainable.
The median plate gured by Barrande is less easy to
explain. One possibility is that it represents the upper
jaw.(Laufeld et al. 1975, p. 218).
The most important discovery to determine the
nature of discinocarins was made in 1978. In that
year, Turek (1978) rst described nds of Aptychop-
sis from the Silurian of Bohemia in the apertural
parts of the orthocerid shells. Similar nds in the Sil-
urian of Sweden were also published (Holland et al.
1978). Unfortunately, all these shells were completely
attened, but the diameter of their apertures was
commensurate with the diameter of the Aptychopsis
specimens, found in them. Based on these nds
Turek (1978) assumed that Aptychopsis and Pelto-
caris are orthocerid opercula, which were likely
located on the hood above the head (since the hood
is present in modern Nautilus, see Turek 1978,
g. 8). He interpreted the genus Discinocaris as prob-
ably a phyllopod. Holland et al. (1978) did not agree
on the distinct nature of Discinocaris and suggested
that all three genera are opercula of orthoconic
cephalopods. According to their opinion, Aptychopsis
A
DF
E
BC
Fig. 7. Devonian (Frasnian) anaptychi from the Domanik Formation of Timan, Russia. Collection housed in the Geological institute of
Russian Academy of Sciences. AC, Spathiocaris, scale bars 3 mm. D, E, Cardiocaris, scale bars 3 mm. F, Ellipsocaris, scale bar 5 mm. A,
GIN MPC 6/1. B, GIN MPC 6/2. C, GIN MPC 6/3. D, GIN MPC 6/4. E, GIN MPC 6/5. F, GIN MPC 6/6.
8A. A. Mironenko LETHAIA 10.1111/let.12414
and Peltocaris can be compared with aptychi and
may conveniently be termed paraptychi, while Dis-
cinocaris is similar to anaptychi and may be called a
paranaptychus (Holland et al. 1978). Though by this
time many researchers had accepted a hypothesis of
the mandibular nature of aptychi, Holland and his
coauthors assumed that if not all, but some of apty-
chi could have served as opercula, then discinocarins
were homologous with such aptychi, but not with the
mandibular ones.
Subsequently, Aptychopsis was considered as a jaw
apparatus in just two publications (Dzik 1981;
Zakharov & Lominadze 1983), whereas in the vast
majority of articles this structure is considered exclu-
sively as an operculum (Stridsberg 1981, 1984; Edge-
combe & Chatterton 1987; Holland 1987a, 1987b,
1996; Frye & Feldmann 1991; Kiselev 1992; Manda
2007; Klug et al. 2017). Even the consensus about
aptychi and anaptychi now widely considered as
lower jaws of ammonoids has not changed the situ-
ation, and the current view on Aptychopsis as an
opercula is absolutely predominant. Only Lehmann
et al. (2015) referred to Aptychopsis as being inter-
preted as jaws and Klug et al. (2015) mentioned that
Aptychopsis could have been homologous with later
cephalopod beaks.
Due to the fact that it was Aptychopsis plates that
were found in the apertural part of the orthocerid
shells, this genus began to be considered separate
from all other discinocarins. Discinocaris and Pelto-
caris appeared to be overshadowed by Aptychopsis
and almost disappeared from palaeontological litera-
ture in recent decades, only Page et al. (2008) men-
tioned them as structures whose afnities remain
uncertain, and Manda & Turek (2018) briey men-
tioned them as cephalopod opercula. Devonian gen-
era of discinocarins such as Ellipsocaris and
Spathiocaris are currently widely regarded as the
lower jaws of ammonoids (Frye & Feldmann 1991;
Goolaerts et al. 2017) but their relationship with gen-
era such as Peltocaris and Discinocaris has not been
mentioned since Rolfe (1969).
Modern views of cephalopod evolution
An understanding of the origin of various subclasses
and orders of Cephalopoda is directly related to solv-
ing the mystery of the appearance of the jaws of these
molluscs. Beyond the shadow of a doubt, two large
cephalopod subclasses extinct Ammonoidea and
extant Coleoidea, (the last survived KPg extinction
and widespread in modern seas), evolved from a
small nautiloid order Bactritida in the Devonian and
Carboniferous, respectively (Kröger et al. 2011; Klug
et al. 2015, 2019). Bactritids, in turn, earlier had
evolved from the Orthocerida (Kröger & Mapes
2007).
The question of the origin of the order Nautilida is
much more complicated. Until recently, the most
popular hypothesis was Nautilida evolved in the Car-
boniferous from the order Oncocerida (e.g. Flower
1955). The lineages of orthocerids, which gave rise to
ammonoids and coleoids, and of oncocerids diverged
at the dawn of cephalopod history, in the Late Cam-
brian (Kröger et al. 2011). Therefore, if both nautilids
and descendants of orthocerids have nearly identical
jaws, we can expect that their last common ancestor
should also have had jaws or at least their prototype.
If this ancestor lived in the Late Cambrian, the
cephalopod jaws should have appeared to that time.
However, not all palaeontologists have agreed with
the hypothesis of the origin of nautilids from onco-
cerids. Dzik (1981, 1984) pointed out the fundamen-
tal differences between oncocerids and nautilids in
the structure of muscle attachment areas (so called
muscle scars), shell shape and ontogeny, and in the
position of the siphuncle. He argued that the ances-
tors of Nautilida can be found among orthocerids.
This prediction was conrmed several decades later,
when molecular clock data showed that the nautilids
and coleoids diverged in the Silurian (or probably in
the Ordovician considering measure of inaccuracy),
but not in the Cambrian (Kröger et al. 2011). The
embryonic development of nautilids and coleoids is
also very similar, despite the huge differences in the
anatomy of adult animals (Shigeno et al. 2008, 2010).
All these data led to the rejection of the old hypothe-
sis, according to which Nautilida evolved from
Oncocerida, and now orthocerids are commonly
believed to be nautilid ancestors, as well as the ances-
tors of both ammonoids and coleoids (Kröger et al.
2011; see Fig. 8 herein).
The new hypothesis is not without weaknesses.
The main problem that Nautilida has a more basal
type of embryonic shell than Orthocerida. In nau-
tilids, as well as in other earlier nautiloid orders, the
embryonic shell has a scarlike structure called cica-
trix, while in orthocerids, ammonoids and coleoids,
the cicatrix is absent, but they have a hemispherical
initial chamber a protoconch (Engeser 1996). The
protoconch is widely considered as a more derived
evolutionary feature (Engeser 1996; Kröger 2006)
and, on its basis, it was even proposed to combine all
protoconchbearing cephalopods into a new infra-
class Neocephalopoda, in contrast with the cicatrix
bearing Palcephalopoda (see King & Evans 2019).
Therefore, the origin of cicatrixbearing Nautilida
from protoconchbearing Orthocerida seems unli-
kely. This problem, however, can be easily solved if
we assume the origin of nautilids from the order
LETHAIA 10.1111/let.12414 Discinocarina and cephalopod jaws 9
Pseudorthocerida. Pseudorthocerids are so similar to
orthocerids that for a long time they were included
into Orthocerida in the rank of superfamily or even a
family (e.g. Dzik 1984). Later they were separated
into a different order due to the presence of a cicatrix
instead of a protoconch on their embryonic shells
(see King & Evans 2019). The degree of relationship
between Orthocerida and Pseudorthocerida is not
completely clear (Niko et al. 2019), but it is logical to
assume that the pseudorthocerids, with their more
ancient type of embryonic conch, are the ancestors of
both orthocerids (with protoconch) and nautilids in
which the cicatrix remains. In such a case, everything
falls into place. Although the oldest known nautilids
to date belong to the suborder Lechritrochoceratina
(Dzik & Korn 1992; Manda & Turek 2019), which
was originally placed in the order Barrandeocerida
(Sweet 1964; Turek 1975), it seems possible that the
roots of lechritrochoceratins also lie inside the pseu-
dorthocerids.
The recently developed and widely accepted
hypothesis that orthocerids (sensu lato, including
Pseudorthocerida) were the ancestors of the nautilids
is extremely important for understanding the evolu-
tion of the cephalopod jaw apparatus. Namely, that
the jaws are known not in two different lineages of
cephalopods, which diverged in the Late Cambrian,
but only in one single branch of cephalopod evolu-
tion tree: descendants of orthocerids (Fig. 8). It also
should be noted that the Silurian Aptychopsis, a solid
structure which resembles the aptychustype jaws of
Mesozoic ammonoids, belonged precisely to ortho-
cerids (Turek 1978).
Discussion
Evidence against the opercular hypothesis of
discinocarins
Cephalopod embryogenesis and anatomy of early nau-
tiloids. Modern Nautilus has a peculiar leathery
opercula called the hood, which is located above the
animals head and closes the shell aperture when the
Fig. 8. Discinocarina distribution and cephalopod phylogeny. Stars mark nds of Discinocarina in the Silurian and Devonian: D Dis-
cinocaris,PPeltocaris,AAptychopsis,Ajtaxa which are currently considered as ammonoid jaws. Red point marks a presumable
point of divergence of Nautilida (see Kröger et al. 2011), in which the jaws (or at least protojaws) mast have already existed. In the phylo-
genetic scheme, line 1 marks lineages in which the jaws are known: Ammonoidea, Nautilida and Coleoidea (are not shown, they appeared
later, in the Carboniferous); 2 lineages, in which the jaws mast have been present according to phylogenetic bracketing; 3 lineage (Sil-
urian and Ordovician Orhocerida +Pseudorthocerida), in which some sort of jaws or protojaws likely existed; 4 lineages, from which
the jaws have never been reported. For the phylogenetic scheme the Ordovician and Cambrian are not in the same scale with Silurian and
Devonian.
10 A. A. Mironenko LETHAIA 10.1111/let.12414
mollusc is withdrawn into the body chamber.
Researchers who interpreted Aptychopsis exclusively
as an operculum believed that it was located on the
hood of ancient nautiloids (Holland et al. 1978;
Turek 1978; Stridsberg 1984). In the same way, both
the aptychi of the Mesozoic, and anaptychi of Palaeo-
zoic, ammonoids were also thought to have been
located on the surface of the hood as mentioned in
the studies published up to the middle of 20th cen-
tury (Keyserling 1846; Owen 1878; Woodward 1885;
Schindewolf 1958, abb.6), but now they are recog-
nized as the lower jaws. Since the Nautilus has a lot
of features that at rst glance looked basal (external
shell, pinhole eyes, double set of gills, etc.) for a long
time it was considered as a living fossilwhose anat-
omy changed very little from the Early Palaeozoic.
Due to this, many of the anatomical characteristics of
Nautilus were extended to all ancient nautiloids. The
protective hood was also widely considered as a very
ancient structure that existed in basal cephalopods,
including orthocerids and ammonoids (Schindewolf
1958; Turek 1978; Bandel 1988). Therefore, it seemed
logical to previous researchers that in some ancient
cephalopods the surface of the hood could have been
covered with mineralized plates aptychi or Apty-
chopsis.
However, recently it was shown that during the
embryonic development, the Nautilus hood forms
from two dorsal arm pairs together with ocular tis-
sue and part of the collar/funnel complex (Shigeno
et al. 2008, 2010). At the embryonic stage of devel-
opment both coleoids and nautilids have ve pairs
of arms. The Nautilus hood includes two pairs of
arms (almost a hundred tentacles of this animal
are formed by splitting the remaining six arms),
whereas the coleoids retain the original 10 arms in
the adult state (except for octopods, one pair of
their arms is reduced). If the ancestors of the
coleoids had a hood, which later was reduced dur-
ing the internalization of the shell, the original
number of arms could hardly have been preserved
(see discussion in Mironenko 2015). Therefore, pre-
sently the hood does not look like an apomorphic
character of cephalopods, but a secondarily derived
structure, which likely appeared only in Nautilida
(Shigeno et al. 2008). Embryos of both coleoids
and nautilids have a collar protrusion in the ante-
rior part of the body, which in Nautilus is also
included in the hood (Shigeno et al. 2008, 2010).
However, this protohoodis too small in order to
have protective functions and could have only
served to attach the collar to the shell above the
animal's head (Mironenko 2015). Therefore, the
Aptychopsisbearing orthocerids (which are widely
considered as ancestors of coleoids) most likely had
no hood. Therefore, Aptychopsis and other dis-
cinocarins can not be not regarded as part of the
hood complex.
Dzik (1981) suggested that Aptychopsis is a
primitive jaw apparatus which evolved from a sin-
gle operculum plate, homologous to the opercula of
gastropods. Gastropod operculum is located on the
foot and Dzik assumed that the reduction of the
foot in ancient cephalopods could have placed the
operculum close to the mouth. However, according
to modern data, not tissues around the mouth but
the arm crown was formed from the ancestral foot
(Shigeno et al. 2008, 2010). Judging by the same
number of basal arms in embryos of coleoids and
nautilids, in the last common ancestor of these
cephalopod lineages the foot had already been
transformed into an arm crown. Due to this, the
presence of a foot, large enough to carry an oper-
cula, in the Silurian orthocerids looks completely
improbable. Therefore, ancient nautiloids, according
to modern data on their anatomy, had neither a
hood nor another area near the head which could
have been large enough to accommodate dis-
cinocarins as opercula.
The structure of discinocarins is not similar to the
structure of mollusc opercula. Gastropod opercula,
although sometimes having a very complex structure
(e.g. Kaim & Sztajner 2005), always consist of one
element (Yochelson & Linsley 1972; Dzik 1981; Rohr
& Frýda 2001). Silurian Discinocarina, which are
interpreted as cephalopod opercula, consist of three
(Peltocaris and Aptychopsis) or two (Discinocaris)
separate valves. Similar compound opercula are not
known in any other molluscs. Turek (1978) showed
that the adjacent edges of Aptychopsis valves become
thinner towards the apex (the junction point of all
three plates, see Turek 1978, gs 13). Proponents of
the opercular hypothesis assumed that the com-
pound structure of Aptychopsis was needed to reduce
the area of the opercula in an open state, when the
animal was active (Turek 1978, g. 8; Holland et al.
1978; Stridsberg 1984). However, if all Aptychopsis
plates were constantly connected to each other, as
was assumed according to opercular hypothesis, the
possibility of Aptychopsis changing its shape was
extremely small. Moreover, the genus Discinocaris
has a single ventral plate whose shape could not
changed, but the small dorsal subtriangular element
also is present in this type of discinocarins this fact
cannot be explained in terms of the opercular
hypothesis. Additionally, this hypothesis cannot
explain how such a structure could have arisen, or
LETHAIA 10.1111/let.12414 Discinocarina and cephalopod jaws 11
why the protective opercula needed to be so thin in
its central part, since this slimness undoubtedly
increased its vulnerability.
Extreme similarity of discinocarins and cephalopod
jaws. A third argument against the opercular
hypothesis is that it seems unlikely that practically
the same structures in cephalopods could have
appeared at least twice in different places of the body
and for fundamentally different purposes. This was
perfectly expressed by Clarke (1902) who said about
discinocarins and anaptychi that «objects of so simi-
lar a character would a priori be of similar nature»
(although he considered them as crustacean shells).
Many researchers have noted a fundamental similar-
ity of Discinocaris,Peltocaris and Aptychopsis on the
one hand and aptychi and anaptychi on the other
(Salter 1863; Barrande 1872; Ruedemann 1916; Rolfe
1969; Laufeld et al. 1975; Holland et al. 1978; Turek
1978; Zakharov & Lominadze 1983). This similarity
is also reected in the names Aptychopsisand Pelto-
caris aptychoides.
Even researchers who considered these structures
as opercula, noted their similarity with ammonoid
aptychi and anaptychi (Holland et al. 1978; Turek
1978). Turek (1978) mentioned that Aptychopsis and
aptychi are similar not only in general shape (except
for the presence of a dorsal valve in Aptychopsis) but
also in mode of growth. Forty years ago there was no
unequivocal opinion on the nature of the aptychi
and anaptychi among researchers, the hypothesis
about the mandibular function of these structures
was still young and not all scholars shared it. How-
ever, to date, the mandibular nature of the aptychi
and anaptychi is not in doubt, although these struc-
tures could have additional functions, including a
protective one (see Parent & Westermann 2016).
Holland et al. (1978) assumed that some ammonoid
aptychi, such as Laevaptychi of aspidoceratid ammo-
nites, are probably homologous to Aptychopsis and
could have functioned as opercula, whereas other
types of aptychi likely have a different nature and
could have functioned as jaws. However, to date,
scholars are sure that Laevaptychi are also lower
jaws, since they have been recently found together
with ammonite radula (Keupp et al. 2016). There-
fore, it is obvious now, that if Aptychopsis is homolo-
gous to Laevaptychi (Holland et al. 1978), it is
consequently homologous to ammonite jaws, as was
previously assumed by several researchers (Laufeld
et al. 1975; Zakharov & Lominadze 1983).
Summary. Summarizing these points, it is clear
that the consideration of the Aptychopsis and other
Silurian discinocarins (Peltocaris and Discinocaris)as
specialized opercula presents some aws. Firstly, the
ancient nautiloids most likely had no hood on which
such an operculum could have been located. Sec-
ondly, a twoor threecomponent structure has never
been observed in mollusc opercula and there are no
reasons for the appearance of such opercula. More-
over, assuming that the Silurian discinocarins are
opercula, it would force us to accept that almost
identical structures in the same animals must have
arisen twice on different parts of the body and for
different purposes.
But if these structures are not specialized opercula,
then what are they? As mentioned above, the Meso-
zoic aptychi and both Palaeozoic and Mesozoic anap-
tychi, which were previously considered as opercula,
are now widely interpreted as lower jaws. The second
hypothesis about the nature of Aptychopsis and its
relatives, rejected by Turek (1978) and Stridsberg
(1984), and now almost forgotten, suggests that they
are elements of the cephalopod jaw apparatus (Lau-
feld et al. 1975; Dzik 1981; Zakharov & Lominadze
1983). Are there any known facts to support this
hypothesis and what do discinocarins and cephalo-
pod jaws have in common?
Evidence in favour of the mandibular nature of
discinocarins
The similarity of the Silurian discinocarins and
ammonoid jaws. The idea proposed by Clarke
(1902) that Objects of so similar a character would a
priori be of similar naturesimultaneously disproves
the hypothesis of the opercula and conrms the rela-
tion of discinocarins with cephalopod jaws. As men-
tioned above, a striking similarity between
Discinocaris,Peltocaris and Aptychopsis on the one
hand and aptychi and anaptychi on the other was
noted not only by supporters of the mandibular
hypothesis, but also by researchers who considered
discinocarins as crustacean shells (Salter 1863; Bar-
rande 1872) or as opercula (Holland et al. 1978;
Turek 1978). One can raise an objection that Dis-
cinocaris,Peltocaris and Aptychopsis, in contrast with
Devonian discinocarins (considered as ammonoid
anaptychi) and Mesozoic ammonoid lower jaws have
a dorsal subtriangular plate. However, most of the
specimens of Silurian discinocarins were found with-
out this plate (Turek 1978; Holland et al. 1978; Dzik
1981; Kiselev 1992). Researchers noted, that in Apty-
chopsis this plate was more loosely connected with
the two ventral plates than they were joined to each
other (Jones & Woodward 1894; Turek 1978). The
same goes for the other two Silurian discinocarin
genera: most specimens of Peltocaris and Discinocaris
were found without a dorsal plate.
12 A. A. Mironenko LETHAIA 10.1111/let.12414
A careful examination of the dorsal plate of the
Aptychopsis shows that the concentric growth lines
which are characteristic of the ventral plates, are
barely visible on its surface. Instead of a concentric
ornament, it is covered with longitudinal lines
stretching from the rostrum to the outer edge (Turek
1978, g. 7; Figs 4B,C, 5). This difference suggests
that the dorsal plate, regardless of being a part of the
unied structure of Aptychopsis, must have grown in
a different way in comparison with the ventral plates.
Therefore, the soft tissues, which formed the dorsal
and ventral parts of Aptychopsis, were not identical.
It can be assumed that the weakening of this con-
nection is an evolutionary trend and postSilurian
discinocarins could also have had a dorsal plate, but
its connection with the ventral elements was very
weak and always collapsed after the death of the ani-
mal. Given that not a single nd of a separated dorsal
plate of both Discinocaris,Peltocaris and Aptychopsis
has been described (although we know that these
plates exist), it is not surprising that such ndings
are also not known for later discinocarins. Most
likely these plates, being separated from the ventral
elements after the death of cephalopods, are simply
not noticed due to the small size and absence of char-
acteristic shape (a minute subtriangular element can
easily be confused with an unidentiable piece of
some fossil). Another interesting fact is that Meek &
Hayden (1864), who were the rst to recognized
mandibular nature of aptychi (although their correct
idea was not appreciated by contemporaries),
described the rst nding of an ammonite upper jaw
as a third piece of aptychi(Meek & Hayden 1864, p.
120).
Location of beccublast cells and mandibular mus-
cles. Hard cephalopod jaws are secreted by a thin
layer of cells, named beccublast cells or beccublasts
(Dilly & Nixon 1976; Tanabe 2012; Nixon 2015; Tan-
abe et al. 2015). These cells also attach mandibular
muscles to the jaws. The beccublasts are described
from the jaw apparatus of both modern coleoids and
nautilids (Dilly & Nixon 1976; Tanabe 2012; Tanabe
et al. 2015) as well as their imprints are found in
ammonoid jaws (Tanabe & Fukuda 1983, 1999;
Doguzhaeva et al. 1997; Tanabe et al. 2001). In all
known cases of both modern and fossil jaws, bec-
cublast cells are always located only on the inner sur-
face of the jaws, between the outer and inner
lamellae (see Tanabe et al. 2015, g 1).
In modern cephalopods, the jaw apparatus (buccal
mass) is surrounded by a buccal membrane and only
the anterior tips of the jaws protrude from it (Uyeno
& Kier 2005). In such an arrangement of the cepha-
lopod jaws (mostly inside the soft tissues),
beccublasts could have also covered the rear parts of
the outer surface of the jaws, but in reality this have
never been observed. Therefore, it cannot be
excluded that the location of beccublasts exclusively
on the inner surfaces of jaws, which is difcult to
explain if we take into account only the modern
structure of the jaws, is a legacy of the time when the
surrounding buccal membrane around the jaw appa-
ratus had not yet formed and outer surfaces of jaws
were devoid of soft tissues and exposed.
A similar external arrangement of the jaws without
coverage by soft tissue was previously assumed for
the Mesozoic aptychi, the surface of which in some
cases bears tubercles, spines and even the possible
traces of colour pattern (Keupp 2000; Tanabe et al.
2015). It should be noted that such an important dif-
ference between aptychi, whose surface was most
likely exposed, and modern cephalopod jaws, sur-
rounded by soft tissues, does not prevent researchers
from considering aptychi as lower jaws.
Both jaws are a united structure in the embryo of
Sepia ofcinalis.Embryogenesis is often a kind of
window into the past, through which we can see the
sequence of occurrence and development of various
organs of the animal. In 2007 Boletzky described a
very interesting fact: in the early stages of embryonic
development in modern cuttlesh Sepia ofcinalis,
the front tips of the upper and lower jaw are con-
nected by an organic membrane. That is, in fact, both
jaws at this stage of embryogenesis represent a single
structure. They have approximately the same size
and grow at the same rate, and this does not t well
with the hypothesis proposed by Boletzky (2007)
about the nonsimultaneous appearance of both jaws
(i.e. the upper jaw is homologous to the jaws of gas-
tropods, whereas the lower jaw appeared later and
only in Cephalopoda). However, the fact that both
jaws are a single structure at the embryonic stage
brings to mind Aptychopsis,Peltocaris and Dis-
cinocaris in which the upper dorsal plate was also
connected to the lower (or a pair of lower) plates by
aexible ligament. Stridsberg (1984) believed that
the connection of the dorsal plate with the ventral
part of these discinocarins disproves the jaw hypoth-
esis, since the upper jaw in cephalopods is not con-
nected to the lower. However, as Boletzky (2007) has
shown, this is not the case in the early stages of
embryogenesis.
Discinocarins and jaws are known to exist in the same
evolutionary line of cephalopods. Jaws are still not
found in any group of cephalopods apart from
descendants of orthocerids (sensu lato). Of course,
the absence of evidence is not evidence of absence,
LETHAIA 10.1111/let.12414 Discinocarina and cephalopod jaws 13
but a fact is a fact both discinocarins and jaws are
characteristics of only one evolutionary lineage of
cephalopods. Moreover, it was the representatives of
this one lineage that survived up to present day. It
can be assumed that only in orthocerids did the abil-
ity to secrete nonshell solid parts (in addition to the
radula) arise for some reason. This is a possible
explanation for the existence of both opercula and
jaws in the same lineage. However, assuming that
Aptychopsis and its relativesare just opercula, we
are forced to accept the idea, that at the Silurian
Devonian boundary cephalopod opercula disap-
peared without a trace, whereas jaws (anaptychi)
appeared slightly later (in Late Devonian) out of
nowhere. This looks extremely doubtful and less
likely than a smooth evolutionary transition between
these structures.
A new hypothesis about the possible occurrence
of Discinocarina and cephalopod jaws
Therefore, taking into account modern ideas about
the anatomy and evolution of cephalopods, the most
likely explanation of the nature of the Silurian dis-
cinocarins is that they are part of a primitive jaw
apparatus of orthocerid nautiloids. However, the
facts that previously forced researchers to consider
them as opercula should not be ignored. Aptychopsis
is very large in comparison with the shell size of its
hosts: its diameter is from 77% (Zakharov & Lomi-
nadze 1983) to 105% (Stridsberg 1984) of the diame-
ter of the shell aperture. Although it is not possible to
accurately measure the ratio of its diameter to the
aperture of attened shell, due to the fact that the
degree of shell atness is unknown, it must be admit-
ted that such a size is large and suggests some protec-
tive functions. Turek (1978) and Stridsberg (1984)
argued that the dorsal plate of Aptychopsis could
have not functioned as an upper jaw since it was con-
nected with ventral plates. Although this connection
was not very strong, there is no doubt that the dorsal
plate could not have really moved far enough to bite
or hold the prey, as the upper jaw does.
Proposed here is a new hypothesis which can
explain and link together all observed facts such as
the appearance and imminent disappearance of dis-
cinocarins, and the occurrence of the jaw apparatus.
According to this discinocarin plates appeared as
protective shields, which defended orthocerids not
from the predators, but from their own prey. The
small earliest cephalopods probably only fed on small
planctonic organisms (Servais et al. 2016). Some of
their descendants, such as large orthoconic Endo-
cerida, even with an increase in size most likely
retained planktonophagy (Mironenko 2020).
However, most cephalopods become predators and
the larger they became, the larger and more active
prey they could have hunted. Modern cephalopods
equipped with powerful jaws, can quickly kill their
prey. However, if the ancient cephalopods had been
armed with only the radula, the process of killing
prey could have been long and difcult. During this
time, the victims resisting or simply attempting to
escape could have damaged the soft tissues of the
predator with its jaws, limbs or hard elements of the
protective covering, such as spines or tubercles. The
formation of a solid chitinous covering on the tissues
around the mouth and radula could likely have
brought signicant benets to the nautiloids, protect-
ing their anterior part of the head from such injuries
(Fig. 9).
Since the protective coating covered the soft tis-
sues around the mouth, it should have had an
opening for the radula. Most likely the composite
structure of discinocarins and the mobility of their
A
B
Fig. 9. Hypothetical reconstruction of Silurian Orthocerida with
Aptychopsis as protojaws. A, general view of the cephalic region
of an orthocerid in an attacking position. The Aptychopsis is
working as a protective shield, the dorsal plate is displaced to
open the mouth with a radula on a short proboscis. The number
of arms (ten) is based on Shigeno et al. 2008, 2010. The presence
of welldeveloped eyes in orthocerids is based on the molecular
study of Nautilus eyes (Ogura et al. 2013). B, various views of
Orthocerida with Aptychopsis. A small formation on the top of
the orthocerid's head is an anterior part of a collar. In modern
Nautilus it is a part of the protective hood (Shigeno et al. 2008),
but in ancient cephalopods most likely it served to connect the
head to the shell and to support collar folds (see Mironenko
2015). (Andrey Atuchin, based on the sketch by the author).
14 A. A. Mironenko LETHAIA 10.1111/let.12414
dorsal elements is related with this purpose. It can
be assumed that the dorsal plate could have risen
forward and upward, or shifted up for a short dis-
tance releasing the radula, which was probably
located on some sort of proboscis (Fig. 9). The ven-
tral plates (or a single plate), most likely, had more
limited mobility, however, they could have also
slightly displaced downward for opening the radula.
The extension of the radula when feeding is typical
for many modern molluscs. Anyway, protective
shields could have allowed the mollusc to kill its
prey using the radula without any damage. In the
case of active resistance of the prey, the predator
could have completely defended itself by closing
the valves of the protective shield, while continuing
to hold the prey with arms or tentacles. When clos-
ing the valves, small pieces of victim's tissues could
have been accidentally pinched off thus the evo-
lution of the jaw apparatus began.
Interpretation of discinocarins as external protec-
tive shields on the anterior part of the nautiloid head
suggests a rather large head size relative to the diam-
eter of the shell aperture. However, this does not
mean that the head was exactly equal to the size of
the aperture or to the size of a attened discinocarin
shields. Firstly, all discinocarins were more or less
convex and their diameter in fossil state is obviously
larger than it was during the lifetime of their host
animals, this can be clearly seen from the ruptures of
the outer edge of the Discinocaris ventral plate (see
Novák 1892, g. 1; Frye & Feldmann 1991, gs
9.1,9.2; see Figs 2A,B, 3B). The convex shape of dis-
cinocarins, which resembles the shape of a visor from
a medieval knights armour could have helped to
deect strokes of defending prey. Secondly, the size
of the head could have increased in such dis-
cinocerinabearing nautiloids so that the protective
shields could have covered the largest area, in order
to put the eyes and bases of arms at a maximum dis-
tance from the danger zone.
Appearing originally as protective structures
which defended nautiloids from their prey, dis-
cinocarin shields in different evolutionary lines could
have acquired various additional functions, depend-
ing on feeding strategies and environmental condi-
tions. It cannot be excluded that they also could have
been used for protection against predators, i.e. as
opercula, but it was a secondary additional function,
not the main and only one, as was previously
assumed. A serious injury of a ventral plate of one of
the Aptychopsis specimens stored in the National
Museum in Prague (Fig. 6C) does not mean that this
damage must have occurred when Aptychopsis was
used for defence against a predator, since the injuries
of lower jaws are known from the modern Nautilus,
even though they do not use their jaws as protective
shields (Kruta & Landman 2008).
In those orthocerids whose protective shields were
strong enough, their edges could have accidentally
pinched off small pieces of the preys soft tissues.
Undoubtedly, such biting off of pieces of food could
have made the process of feeding and killing prey
faster and more efcient. This was the beginning of
the evolution of the jaws in cephalopods. A recent
analysis of the time of appearance of the jaws in vari-
ous groups of vertebrates and invertebrates showed
that different animals almost simultaneously (on a
geological time scale) acquired jaws during the Sil-
urian and Devonian (Klug et al. 2017). Conversion of
protective structures, which likely originally defended
orthocerids from their dangerous prey, into the
upper and lower jaws, also happened in the Silurian
and it ts well into the described picture of the Sil-
uroDevonian Jaw Armament (Klug et al. 2017). The
genera Discinocaris and Peltocaris, which appeared in
the Lower Silurian, are apparently the very same jaw
precursors, which theoretically should have existed
(Klug et al. 2017).
All researchers who considered Aptychopsis as
jaws (Laufeld et al. 1975; Dzik 1981; Zakharov &
Lominadze 1983) believed that the dorsal plate was
the upper jaw. Zakharov & Lominadze (1983) noted
the presence of an elongated rostrum on the dorsal
plate of the Aptychopsis, which was ignored by sup-
porters of the opercular hypothesis, but clearly visible
in many specimens of this genus (Kiselev 1992, table
2, g. 3; Fig. 5). However, opponents of this theory
pointed out that since the dorsal plate was not fully
separated from the ventral valves of Aptychopsis,it
could have not functioned as the upper jaws of mod-
ern cephalopods do (Stridsberg 1984). According to
the new hypothesis, proposed herein, the dorsal plate
is not an upper jaw as it is now, but a precursor of
the upper jaw, which means the structure must have
originally appeared as the protective shield above the
radula. It likely gradually evolved into an upper jaw
by increasing mobility and decreasing its connection
with ventral plates.
As the biting ability of discinocarin elements
increased (primarily due to the increasing mobility of
the dorsal plate), the killing process became faster
and more efcient, and as a result, the original pro-
tective function of the shields decreased. It should be
noted that this process could have occurred in paral-
lel in different discinocarinabearing lineages of
cephalopods. This is indirectly conrmed by the
diversity of the Silurian discinocarins and by the
mixing of their characters in different taxa. Taking
into account the fact that the Silurian Aptychopsis
with its pointed edges of the valves could have likely
LETHAIA 10.1111/let.12414 Discinocarina and cephalopod jaws 15
been used for both purposes i.e. as a defence and in
the process of feeding, we can assume that in Cepha-
lopoda the jaws appeared earlier than in vertebrates
(see Klug et al. 2017).
Stridsberg (1981) drew a parallel between Apty-
chopsis and restricted apertures (masks) of some Sil-
urian oncocerid nautiloids. Aptychopsis and masks
are found in different phylogenetic lineages of nau-
tiloids: narrow apertures are typical for representa-
tives of the Oncocerida, Discosorida and
Tarphycerida, in which neither jaws nor discinocarin
elements have ever been found, whereas for ortho-
cerids open wide apertures are typical. Stridsberg
(1981) believed that both Aptychopsis and narrowed
oncocerid apertures served the same purpose: for
protection from predators which he noted was essen-
tial for cephalopods during the Silurian. It is worth
noting that at the same time, the trend of the protec-
tion of the cephalic part of the body was also present
in vertebrates: the Silurian was the heyday of Placo-
dermi, a paraphyletic vertebrate group, whose ante-
rior end was also protected by armour.
However, it cannot be ruled out that in those
days the dangerous prey was a more important fac-
tor than predators and that the narrow apertures of
the oncocerids could have protected their bearers
from their own victims, as likely did the dis-
cinocarin plates in orthocerids. Although some
modern gastropods use narrowed apertures to pro-
tect themselves against predators, in these cases the
protrusions of the aperture are thick and robust. In
oncocerids, the protrusions at the apertural edges
were thin, their thickness does not exceed the thick-
ness of the shell wall. Drilled holes of some boring
predators are known on these protrusions (Strids-
berg 1985, g. 23F), and it is unlikely that such thin
structures could have protected molluscs against a
serious predator. The shape of the closedapertures
of many oncocerid genera of the family Hemiphrag-
moceratidae resembles Aptychopsis or Peltocaris:
they have wide ventral areas by the sides of the cen-
tral opening, which can be compared with ventral
plates of discinocarins, and a small dorsal protru-
sion, which is similar to dorsal plate (Barrande
1865, pl. 73, gs 7,11,14,19; Stridsberg 1981, gs 6
DF, 7; Stridsberg 1985, g. 38DF). Therefore, it is
logical to assume that both discinocarins and
restricted apertures protected the animals from
resisting prey. But if the chitin plates, due to their
mobility, quickly had begun to take part in the feed-
ing process and later had evolved into the jaws, the
using of shell protrusions for protection of the ante-
rior part of the body would have turned out to be a
dead end, since the jaws likely never appeared in
nautiloids, which used this method.
The disappearance of Aptychopsislike dis-
cinocarins with a dorsal subtriangular element from
the fossil record after the Silurian is likely caused by
an increase in the mandibular function of protojaws
and by the separation of the dorsal plate, which
began to evolve into an upper jaw. The jaws began to
gradually be surrounded by the soft tissues and
ceased to be used for protection. However, in differ-
ent evolutionary lineages, this process could have
proceeded at different rates, and the aptychi, which
were found in the Carboniferous (Harper 1989) can
also be elements of the jaw apparatus of some ortho-
cerids, which retained an archaic structure.
Whereas aptychi of Mesozoic ammonites are often
regarded as strange and peculiar deviation during the
evolution of cephalopod jaws, it seems possible that
they represent a partial «reincarnation»of the basal
type of the jaw apparatus. The revival of the protec-
tive function in the aptychi can be explained by the
assumption that the jaws remained in an external
position in the earliest stages of ontogenesis in
ammonoids (see Mironenko & Rogov 2016).
Stratigraphical distance between the Silurian
and Devonian Discinocarina
Whereas discinocarin fossils in general are quite
numerous in both Silurian and Upper Devonian lay-
ers, there is stratigraphical gap of more than 35 mil-
lion years between the last Silurian discinocarins
(Aptychopsis) and their Upper Devonian relatives,
which are currently considered as ammonoid lower
jaws (Fig. 8). Klug et al. (2016) indicate that the earli-
est ammonoid jaws are known from the Frasnian
deposits. However, Frye & Feldmann (1991) noted
that Clarke in 1882 described the ndings of dis-
cinocarins not only from Frasnian, but also from the
Givetian (Middle Devonian). The genus Lisgocaris
was described by Clarke (1882) from the Devonian
deposits of Hamilton Group in western New York,
which contain small crustaceans Estheria pulex.
According to modern concepts, these layers really
belong to the Givetian stage of the Middle Devonian
(see Jones & Olempska 2013). Therefore, the oldest
ndings of the Devonian Discinocarina (and at the
same time of the oldest ammonoid jaws) are known
to date from the Givetian, but not the Frasnian.
This fact, however, does not signicantly reduce
the stratigraphical distance between the latest Sil-
urian and earliest Devonian cephalopod mandibular
elements. Is it possible that this lack of ndings indi-
cates the absence of any relation between the pre
Devonian discinocarins and Devonian ammonoid
jaws? If the answer is yes, we must assume that
practically identical jaws in cephalopods have arisen
16 A. A. Mironenko LETHAIA 10.1111/let.12414
at least twice (separately in orthocerids and ammo-
noids) or even three times (in orthocerids, nautilids
and ammonoids). Theoretically, this cannot be com-
pletely ruled out. In the course of the evolution of
cephalopods, calcareous elements of their jaw appa-
ratus evolved repeatedly and independently. Calcitic
pointed tips of jaws (rhyncholites and con-
chorhynchs) independently appeared in Triassic nau-
tilids (Riegraf & Moosleitner 2010) and Jurassic
ammonoids, moreover, likely they evolved indepen-
dently and at different times in Jurassic ammonoid
suborders Phylloceratina and Lytoceratina (Miro-
nenko & Gulyaev 2018). A calcitic layer on the sur-
face also independently arose in the jaws of Jurassic
ammonites and in Aptychopsis. Therefore, the inde-
pendent emergence of similar structures in cephalo-
pods is possible. However, for the Silurian and
Upper Devonian discinocarins, it seems unlikely.
Since for today, the jaws are known in three evolu-
tionary lineages of ammonoids, two of which
diverged in the Early Devonian (Emsian), there is lit-
tle doubt that at the time of this divergence the
ammonoids already had jaws (Klug et al. 2016).
Thus, during the Emsian, Eifelian and the most part
of the Givetian, the ammonoids most likely already
had jaws and the absence of their ndings was likely
due to taphonomic reasons. Therefore, it is logical to
assume that the lack of cephalopod mandibular ele-
ments in the entire Lower and most part of the Mid-
dle Devonian is either an artefact of preservation, or
a result of insufcient information about the locali-
ties of these ages. In any case, this absence of ndings
cannot disprove the common origin of the Devonian
and earlier Discinocarina.
Possible future tests of the new hypothesis
If the hypothesis formulated in this article is correct,
we can assume that sooner or later the traces or
imprints of the radula will be found on the inner sur-
face of discinocarins, similar to how the radula was
found on the aptychi of the Late Jurassic ammonite
Aspidoceras (Keupp et al. 2016). As far as can be con-
cluded using an analogy with Jurassic aptychi, the
hard plates of Aptychopsis are best suited to preserve
the radula. It can also be assumed that traces of wear
or minute scratches will be found on the edges of the
third dorsal plate of Aptychopsis,Peltocaris or Dis-
cinocaris or on adjacent edges of its ventral plates.
Such damage could have been produced by the
radula or hard particles of food, such as scratches,
which were found on the calcitic elements of the jaws
of the Cretaceous ammonoids (Mironenko & Rogov
2018). Possible ndings of beccublast cells on the
inner side of discinocarins can also support this
hypothesis.
Potential ndings of the moderntype jaws in the
Ordovician or Lower Silurian deposits, especially
among nautiloids which are not related with ortho-
cerids can seriously cast doubt on this hypothesis.
However, the discovery of such jaws in the Middle or
Late Silurian strata or in the orthocerid nautiloids
(sensu lato) seems although unlikely, but possible,
since in different lineages the discinocarinas could
have evolved into moderntype jaws at different
rates.
Conclusions
The consideration of Aptychopsis as a specialized
orthocerid operculum does not t with modern data
on the anatomy and evolution of cephalopods and
should be rejected. Moreover, Aptychopsis is not a
oneofakind structure but one of the members of
the suborder Discinocarina which also includes Sil-
urian Discinocaris and Peltocaris and several Devo-
nian taxa such as Spathiocaris,Pholadocaris,
Ellipsocaris,Cardiocaris, etc.
Originally described as crustaceans, discinocarins
are now considered as hard elements of the cephalo-
pod body. Devonian discinocarins are widely consid-
ered as anaptychi (lower jaws) of ammonoids.
However, assuming that preDevonian discinocarins
are specialized opecula, we are forced to accept the
idea, that at the Silurian Devonian boundary
cephalopod opercula disappeared without a trace,
whereas jaws slightly later appeared out of nowhere.
Taking into account the obvious similarities of the
Silurian and Devonian discinocarins, this assumption
seems unlikely. Instead, a smooth evolutionary tran-
sition between preDevonian Discinocaris,Peltocaris
and Aptychopsis, and Devonian anaptychi seems far
more possible.
It can be assumed that the mystery of the origin of
cephalopod jaws and the question of the origin and
functions of discinocarin plates are not separate, but
one single issue. The new hypothesis proposed herein
explains the occurrence of protojaws (discinocarin
plates) by the need to protect the anterior part of the
head of ancient nautiloids not from predators, but
from actively resisting prey. The process of killing
prey using only the radula could have been long and
difcult. During this time, the victims resisting or
simply attempting to escape could have damaged the
soft tissues of the predator and the development of
organic plates (likely chitinous) around the mouth
and radula could have prevented such injuries. At
LETHAIA 10.1111/let.12414 Discinocarina and cephalopod jaws 17
least one of these protective plates must have been
more or less mobile to open the mouth and radula.
Due to this mobility, the protective plates, being
quite thick, accidentally began to pinch off pieces of
prey tissue. From this moment, the evolution of the
jaw apparatus of cephalopods began. It is possible
that it occurred in parallel in different orthocerid lin-
eages, which already had protective plates. It is likely
that in some cases discinocarins could have also pro-
tected cephalopods from predators, but this was an
additional secondary function, not the main one. As
the protective plates gradually turned into primitive
jaws, hunting efciency and the speed of killing prey
increased, and the need to protect the tissues around
the mouth decreased.
Therefore it seems most likely that cephalopod
jaws arose from the external protective structures
(but not the opercula), which were previously
included into the suborder Discinocarina. Of course,
other hypotheses may appear in future, however, the
consideration of very similar structures, clearly hav-
ing a common origin and nature in some cases as
opercula and in others as the lower jaws is an
anachronism and should be rejected, and all dis-
cinocarin taxa, regardless of age, should be consid-
ered together as objects of the same nature.
If jaws really evolved in only one group of the nau-
tiloids (Orthocerida sensu lato), it is likely that the
other Early Palaeozoic cephalopods were jawless (re-
sembling the jawless sh, Agnata). During the Sil-
urian, cephalopods likely went through the
placodermstage of their evolution, protecting the
front end of the body with chitinous plates or cover-
ing it with protrusions of the shell aperture. How-
ever, the second path (using the shell protrusions)
turned out to be a dead endand it is possible that
the extinction of all nonorthocerid naitiloids in the
Late Palaeozoic was caused by the competition with
jawbearing cephalopods.
Acknowledgements. I am very grateful to Vojtech Turek
(National Museum, Praha, Czech Republic) for his invaluable
help in accessing the collections of Czech National Museum in
Prague, to Mikhail Rogov (GIN RAN, Moscow, Russia) for com-
ments on the draft version of the manuscript and to Sergei Oklad-
nikov (Ukhta, Komi Republic, Russia) and Andrey Fedyaevsky
(SaintPetersburg, Russia) for donation Devonian specimens for
this study. Accurate reconstructions of discinocarinbearing
Orthocerida were drawn by Andrey Atuchin (Kemerovo, Russia).
I thank Irina Smurova (Moscow) for preparing photos to this
study. I am also very grateful to reviewers Kenneth De Baets
(FriedrichAlexanderUniversitat ErlangenNurnberg, Germany)
and Rene Hoffmann (RuhrUniversitat Bochum, Germany) and
the editor Peter Doyle (London South Bank University, UK) for
their very useful comments, advice and criticism, which helped to
greatly improve the manuscript. Special thanks to Steve Ford
(Vancouver, Canada) who assisted me in improving the English.
The research was carried out following the plans of the scientic
research of the Geological Institute of RAS.
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20 A. A. Mironenko LETHAIA 10.1111/let.12414
... In extant cephalopods, it consists of upper and lower jaw elements, both of which are accommodated together with a radula within a globular shaped muscular organ, the buccal mass (Nixon, 1996;Tanabe et al., 2015). Calcified and horny remains of cephalopod jaws have been reported from the middle Palaeozoic and younger marine deposits (see also Mironenko, 2020a). In most cases, they occur as isolated specimens, together with conchs of ammonoids and nautiloids and belemnoid rostra, but are rarely found in situ within the body chambers of ammonoid conchs whose taxonomic relationships are known. ...
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Jaws of ammonites which inhabited the Panboreal Superrealm during the Jurassic and Cretaceous are poorly known in comparison to those of Tethyan ammonoid faunas. This paucity may be explained by limited thickness, or even absence of an outer calcitic layer, in lower jaw elements (aptychi) of Boreal ammonites. Here we describe, for the first time, the jaws (both lower and upper) of ammonites of the Boreal family Polyptychitidae, of the Early Cretaceous (Valanginian) age. Polyptychitid lower jaws are of the aptychus type, but have an unusual pointed and convex shape. However, lower jaws of Late Jurassic ancestors of polyptychitids (Craspeditidae) as well as Middle Jurassic cardioceratids (Pseudocadoceras) have a near-identical shape, as do previously described aptychi of the Late Cretaceous genera Neogastroplites and Placenticeras (Hoplitoidea). The close resemblance of lower jaws of evolutionarily distant ammonites may be linked to a similar lifestyle, but more data are needed to substantiate this. Upper jaws of polyptychitid are closely similar to previously described upper jaws of Jurassic ammonites, which indicates the conservatism of this part of the jaw apparatus. Together with shells and jaws of the Valanginian ammonites described herein, jaws of coleoids (likely belemnites) as well as arm hooks (onychites) have been found.
... Although isolated findings of aptychi-like structures are described from Paleozoic deposits (Closs et al., 1964;Thompson et al., 1980;Yochelson, 1983;Harper, 1989), most of these specimens are most likely shells of bivalves, and some of them (Harper, 1989) are likely relict Aptychopsis of Orthocerids (see Turek, 1978 andMironenko, 2021a). Undoubted aptychi arose in the Early Jurassic (Toarcian) among ammonites of the family Hildoceratidae (Engeser and Keupp, 2002). ...
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Comments are provided on a published paper on Middle Jurassic Laevaptychus from central Mexico [C.Esquivel-Macías, P.Zell, J.A.Moreno-Bedmar and K.Flores-Castro, Giant Middle Jurassic (Bathonian) cf. Laevaptychus sp. of the Aztlán section, Hidalgo State, central Mexico, Journal of South American Earth Sciences, 110, 103302]. This article describes an interesting finding of large-sized ammonite lower jaws (aptychi referred to Laevaptychus paragenus), claimed as the largest Jurassic aptychi ever known. However, the age of these specimens was erroneously defined due to misidentification of an associated ammonite specimen as Bathonian Procerites. Although poorly preserved, this ammonite shows typical features of the Kimmeridgian genus Idoceras. The Kimmeridgian age of these occurrences is in agreement with findings of Laevaptychus, as this is one of few aptychi formal genera, which belongs to a single ammonite family (Aspidoceratidae). Aspidoceratids appeared in the late Callovian and during the evolution of this lineage maximum sizes of adult specimens and the relative whorl height gradually increased up to Kimmeridgian - Tithonian; only prior to their extinction in early Berriasian, aspidoceratids became uncommon and smaller in size. Laevaptychi are thick-valved aptychi, which have high preservation potential while compared with other aptychi of Jurassic ammonites and their host shells. Giant laevaptychi reported in previous publications (the largest of which reaches 35 cm in length) are briefly reviewed. In adult aspidoceratids the maximum length of aptychi is slightly less than the maximum whorl height. Thus, taking into account the size of the largest aspidoceratid ammonites (up to 85 cm in diameter), the estimated length of the largest laevaptychi can be expected to be ∼35–40 cm, which is close to their known record.
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The taxonomic affinities of fossils from the Frasnian succession of Belgium previously described as phyllopod and phyllocarid crustacean shields are discussed. The rediscovery of the holotype of Ellipsocaris dewalquei, the type species of the genus Ellipsocaris Woodward in Dewalque, 1882, allows to end the discussion on the taxonomic assignation of the genus Ellipsocaris. It is removed from the phyllopod crustaceans as interpreted originally and considered here as an ammonoid anaptychus. Furthermore, it is considered to be a junior synonym of the genus Sidetes Giebel, 1847. Similarly, Van Straelen’s (1933) lower to middle Frasnian record Spathiocaris chagrinensis Ruedemann, 1916, is also an ammonoid anaptychus. Although ammonoids can be relatively frequent in some Frasnian horizons of Belgium, anaptychi remain particularly scarce and the attribution to the present material to peculiar ammonoid species is not possible.
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The family Lechritrochoceratidae Flower, 1950 includes the earliest cephalopods acquiring a lateral pair of retractor muscles—a diagnostic character of the Nautilida. The embryonic shell of Silurian lechritrochoceratids is minute, with a diameter of 2–4 mm, a length of 4.5–7.5 mm and consists of three growth stages: (1) elliptical, dorsoventrally elongated and flattened cicatrix with a central bar, (2) a short, rapidly expanding shell part and (3) a gradually expanding, very slightly curved shell. The sculpture consists of longitudinal lirae, and faintly distinct growth lines or ridges. A ventral lobe formed by ridges corresponds in its course to post-hatching ribs. This lobe indicates the early development of the hyponome in the egg capsule. The hatching is indicated by the appearance of ribs and sometimes also by a change in shell curvature. The juvenile stage is characterised by the prolongation of the body chamber, a change in the shape of the cross section, increasing expansion rate, and by a deepening of the hyponomic sinus. The juveniles were probably demersal with the aperture oriented more or less towards the sea floor. The dorsoventrally elongated cicatrix with a central bar and a depression corresponding to the caecum position and the appearance of longitudinal lirae near the cicatrix margin support systematic position of the lechritrochoceratids among the Nautilida. The hemispherical apex and the very slightly curved embryonic shell, ribbed juvenile shell and narrow annular elevation around the body chamber base, however, call for the definition of a new suborder Lechritrochoceratina.
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The Lower Setul Limestone in the Langkawi Islands of Peninsular Malaysia is well known for its continuous succession for most of the Ordovician Period. It is part of an extensive carbonate platform in the West Malaya (=Sibumasu) Block that is considered to have been in low-latitude, northern Gondwana during the early Palaeozoic. Highly endemic cephalopods belonging to the two orders Orthocerida and Pseudorthocerida occur in the late Floian–early Sandbian (late Early–early Late Ordovician) sections of this limestone. Based on some 170 shells collected, 10 species out of six genera were identified: the four orthocerids, Malayorthoceras gracilentum gen. et sp. nov., Tofangoceras kedahense sp. nov., Tofangoceras staiti sp. nov. and Tofangoceras rayense sp. nov.; and the six pseudorthocerids, Andamanoceras densiseptum gen. et sp. nov., Langgunites mucronulatus gen. et sp. nov., Shanthaiceras amplum gen. et sp. nov., Sibumasuoceras langkawiense (Kobayashi, 1959 Kobayashi, T. 1959. On some Ordovician fossils from northern Malaya and her adjacence. Journal of Faculty of Science, the University of Tokyo, Section II, 11, 387–407 + pls 24‒27. [Google Scholar]), Sibumasuoceras kilimense sp. nov. and Sibumasuoceras scrivenori sp. nov. Tofangoceras, a typical Darriwilian–Sandbian element of the North China Platform, was found also