Pore-canal network (“wrinkles”) in ammonoid shell wall (Cephalopoda)

ABSTRACT The shell wall in ammonoids contains a pore-canal network similar to that detected recently in the shell wall of the Ordovician nautiloid Orthoceras. In the Jurassic monomorphic ammonoids Quenstedtoceras, Cosmoceras, Lissoceras and the Cretaceous heteromorphic ammonoid Ptychoceras investigated in this study vertical canals arranged in parallel rows, are partially preserved, although no horizontal canals are preserved as in Orthoceras. In Quenstedtoceras and Ptychoceras the vertical canals extend not only through the inner prismatic layer, as in Orthoceras, but also through the nacreous layer. Therefore both layers were secreted on the body-shell attachment band behind the apertural margin. In the juvenile shells, only the very thin outer prismatic layer formed the apertural margin. As in Orthoceras, the canals had walls of calcified organic fibres that were partially or entirely dissolved during diagenesis. The pore-canal network in the shell wall of Quenstedtoceras and Ptychoceras contributed to achieving neutral buoyancy. As indicated by the high numbers of mini-pores in the connecting rings, the siphuncle was employed for adjusting buoyancy during vertical migrations. The “wrinkles”, “furrows”, “pits”, “dots”, “ridges”, “Ritzsteifen” and “Runselschicht” in the “wrinkle layer”, previously described in the ammonoid shell walls, are remnants of the pore-canal network.


Introduction
The shell wall in ammonoids consists of the same three calcareous layers as that in the extant Nautilus: the outer prismatic layer, the nacreous layer and the inner prismatic layer.These layers in ammonoids have different thicknesses in different taxa and during different ontogenetic stages.In mature shells, the layers are secondarily thickened at the shell margin.In juvenile shells, it is therefore not possible to reconstruct the growth of the layers and the thickness of the apertural margin.Juvenile animals which die during crises have their shells preserved in the sediment.The shells can be sectioned and structurally studied when they are covered by the sediment (Mutvei 2014).

Material and methods
The pore-canal network was studied in the following ammonoids: two specimens of Quenstedtoceras sp.(NRM Mo199802, NRM Mo199803) and two specimens of Cosmoceras sp.(NRM Mo 199804, NRM Mo199805) from the Callovian, Lukow, Poland; one specimen of Lissoceras erato D'Orbigny from the upper Oxfordian, Raueacien Sakahara, Madagascar (NRM Mo 199801).The previously described pores in the shell wall of the Cretaceous ammonoid Ptychoceras from the Caucasus (Doguzhaeva & Mutvei 1989, fig.14:2) are re-interpreted.The Callovian ammonoids were collected by the author from a clay quarry at the brick factory in Lukow, eastern Poland, close to the town Kielce.The pore-canals in ammonoids were compared with similar canals in the Ordovician nautiloid Orthoceras sp.(NRM Mo199806), Carboniferous coleoids Shimanskya postremus Miller (Mutvei et al. 2012, Fig. 4) and Mitorthoceras perfilosum Gordon (Mutvei & Mapes 2019, Fig. 1(A, Fig. 3A, 7C) The studied material is housed in the invertebrate palaeontology collection of the Swedish Museum of Natural History, Stockholm (NRM).The shell structure was studied using incident light with a Wild Photomakroskop M400 at the Swedish Museum of Natural History, Stockholm.

Previously described pore-canal network in the Ordovician nautiloid Orthoceras
The pore-canal network in ammonoids is similar to that in the Ordovician nautiloid Orthoceras (Fig. 1) (Mutvei 2022, Fig 9A).The shell in Orthoceras consists of equally thick inner prismatic and nacreous layers.The pore-canal network occurs in the inner prismatic layer.It consists of horizontal and vertical canals (Fig. 1A, C).The vertical canals are arranged in parallel rows, and the canals in each row open into one of the horizontal canals (Fig. 1).The vertical canals extend through the inner prismatic layer and open into the body chamber.All canals have thin walls of calcified organic fibres (Mutvei 2022, Figs.5-8C)

Pore-canals in Quenstedtoceras
As in Orthoceras, the walls of the horizontal and vertical porecanals in ammonoids are partially or entirely dissolved during diagenesis, hence the canals have not been recognized in earlier studies.In the first studied specimen of Quenstedtoceras (NRM Mo 199802) the inner prismatic and nacreous layers in the shell wall are separated from each other by a thick layer of pyrite.Because the pyrite fills parts of the vertical pore-canals, these structures can be distinguished on the vertical section of the shell wall (Figs.2(A, B), Fig. 3A).The vertical canals have a diameter of 0.05 mm, penetrate the entire nacreous and inner prismatic layers and open into the body chamber (Fig. 9A).The very thin, outer prismatic layer of the shell wall (Kulicki 1996) is not preserved (Fig. 9(A, B)).
In the second specimen (NRM Mo 199803) the vertical pore-canals are preserved as numerous whitish spots on the vertical section of the nacreous and inner prismatic layers (Fig. 3B).As in Orthoceras (Mutvei 2022, Fig. 2A, Fig 5C,  Figs 6-8), the canal walls consist of calcified organic fibres, and the whitish spots are remnants of partially dissolved canal walls.
The horizontal pore-canals are not preserved in studied specimens.However, Kulicki (1979, pp. 121, 125, Fig. 9) described "wrinkles" in the dorsal shell wall both in Quenstedtoceras and Cosmoceras.Further Radtke & Keupp (2017), pp.88-89, fig.21) noted that the dorsal shell wall in Quenstedtoceras is composed of the "wrinkle layer and the dorsal inner prismatic layer".Because the "wrinkles" are interpreted as sections of the walls of the horizontal pore-canals (see below), the horizontal canals in Quenstedtoceras occurred at least in the dorsal shell wall (Fig. 9B).

Pore-canals in Cosmoceras and Lissoceras
In the first specimen of Cosmoceras sp. (Fig. 4) (NRM Mo 199804) the vertical pore-canals have contrasting preservation in the nacreous and inner prismatic layers.In the inner prismatic layer several canals are filled with pyrite and preserved, whereas in the nacreous layer the canal walls are partially dissolved and the canals can be recognized as whitish spots (Fig. 4).In a second specimen (Fig. 5) (NRM Mo 199805) the vertical pore-canals are filled with pyrite and preserved in the dorsal shell wall.The diameter of the canals is about 0.05 mm.
In the specimen of Lissoceras orata (NRM M0 199801) the vertical pore-canals are exposed in the horizontally broken dorsal shell wall (Fig. 6A).The canals are numerous and situated closely together.The canals have diameters of about 0.05 mm, and densities of about 100 canals per one square mm.Several canals have empty cavities and thin whitish walls (Fig. 6B).
Growth of the shell wall in Nautilus, Orthoceras, Quenstedtoceras and Ptychoceras

Nautilus
The shell wall in extant Nautilus is composed of the outer spherulitic-prismatic, nacreous and inner prismatic layers.The inner prismatic layer is very thin and traversed by short and narrow, vertical pores (Doguzhaeva & Mutvei 1986, Fig. 9B;Mutvei & Doguzhaeva 1997, pls. 7, 8, text-Fig. 9, Fig. 10;Mutvei 2022, Fig. 1A).The pores contain small, finger-shaped extensions from the mantle epithelium that are attached to the inner shell surface on the body-shell attachment band behind the apertural margin (Stenzel 1964;  10).During the shell growth the epithelial extensions are detached from the shell wall leaving behind the pores in the inner prismatic layer.The body moves forward and the finger-shaped extensions become attached on a new body-shell attachment band.Thus, the secretion of the thin inner prismatic layer takes place periodically on the inner shell surface, whereas the spherulitic-prismatic and nacreous layers are secreted at the shell aperture.The apertural margin in juvenile shells is, therefore, thicker and mechanically stronger than that in Orthoceras, and much stronger than that in Quenstedtoceras and Ptychoceras.

Orthoceras
In contrast to Nautilus the inner prismatic layer of the shell wall of Orthoceras is as thick as the nacreous layer.These layers are separated from each other by a thin intermediate sub-layer (Mutvei 2022, Fig. 11 A, C).In juvenile shells the inner prismatic layer and the intermediate sub-layer were secreted on the bodyshell attachment band behind the apertural margin, whereas the nacreous layer was secreted at the shell aperture.The apertural margin was thinner than that in Nautilus but thicker than that in the ammonoids Quenstethoceras and Ptychoceras.

Quenstedtoceras
As described in the present paper, the inner prismatic and nacreous layers of the shell wall are both traversed by numerous vertical pore-canals (Figs.2-3, Fig. 9(A, B)).The horizontal canals are not preserved, but as indicated by the occurrence of "wrinkles", they existed at least in the dorsal shell wall (Kulicki 1979;Radtke & Keupp 2017).Because the pore-canals occur in the inner prismatic and nacreous layers, both layers were secreted on the body-shell attachment band behind the apertural margin.Only the very thin outer prismatic layer was secreted at the shell aperture.The apertural margin in juvenile shells was therefore very thin and had a long growth zone (Mutvei 2014, Figs 4,5).

Ptychoceras
The structure of the shell wall in Ptychoceras is similar to that in Quenstedtoceras.Doguzhaeva & Mutvei (1989, p. 111, text-fig. 11B, pl. 14:2, pl.15:1, 2) noted that in Ptychoceras "all calcareous layers of the shell wall are traversed by numerous pores of about one millimicron in diameter" that are "arranged in rows".These authors also found that the apertural shell margin is "very thin" and "characterized  3(A-D) the shell margin "is very thin and almost never preserved intact"; in adult shells it forms first a thick varix, followed by "a constriction ending in a very thin lip".
Re-interpretation of "wrinkle layer", "Runzelschicht", "Rizsteifen", "pits" and "dots" in the shell walls of ammonoids All of the structures discussed here are exposed in the broken shell wall, but cannot be distinguished in the vertical sections of the shell wall (see below).Walliser (1970, p. 115) described the wrinkle layer ("Runzelschicht") in goniatites as "ridges and tubercles" on the surface of the inner prismatic layer of the shell wall; the internal moulds of these elements are covered by "furrows and pits" ("Rizstreifung").According to House (1971, p. 23), the wrinkle layer is a "finely ridged structure on the surface of certain goniatite shells".Tozer (1972) described the wrinkle layer in several Triassic ammonoids and also defined the "Runzelshicht" ("wrinkle layer ") as composed of "small ridges and furrows, resembling human fingerprints" while the "Rizstreifen" are visible as "small pits".Bayer (1974) described the "wrinkle layer" ("Runzelschicht") in ammonoids as a "wavelike structure with hollow spaces" in the inner prismatic layer.Korn (1985) described "Runzelschicht" and "Rizstrifen" in glymenid ammonoids.Bucher et al. (2003) described "radial lira" on the surface of the nacreous layer in the Jurassic ammonoid Calliphylloceras.Keupp (2008, p. 437)   described "well developed dotted lines" on the moulds of the body chamber in the Cretaceous ammonoid Desmoceras, and interpreted them as "Rizstreifen".The "dotted lines" originated from "small ridges on the inner prismatic layer as the result of an incomplete mineralization".(Korn et al. 2014) described a "coarse wrinkle layer" in the early Carboniferous Maxigoniatites.This layer consists of vertical lamellae in the ventro-lateral sides of the body chamber.The adjacent lamellae are separated by wide interspaces.
The "wrinkles" are here interpreted on the basis of similar structures in the shell wall of Orthoceras, Shimanskya and Mitorthoceras.In a shell of Orthoceras, studied here, a row of "wrinkles" is distinguished on a vertical section of the shell wall (Fig. 1A).The "wrinkles" consist of triangular structures that have similar size and shape as the "wrinkles" in ammonoids.The triangular structures in Orthoceras are situated in the same layer as the horizontal pore-canals (Fig. 1B).The "wrinkles" are here interpreted as remnants of vertically sectioned walls of the horizontal pore-canals (Fig. 9D).Because the canal walls were cut at different angles, the shape of the "wrinkles" is variable (Kulicki et al. 2001, Fig. 13).This interpretation is supported by the shape of the "wrinkles" in similarly cut walls of the horizontal canals in the coleoids Shimanskya (Mutvei et al. 2012, Fig. 4) and Mitothoceras (Mutvei & Mapes 2019, Figs. 1, 3A).In Shimanskya, the horizontal canals are filled with fine-grained material and form "calcareous rods" (Fig. 7A, B).The oblique sections of the canal walls have triangular shape, similar to the "wrinkles" in ammonoids (Fig. 7C).In Mitorthoceras, the "tube-like furrows" cover the outer shell surface and have similar shape and arrangement as the horizontal pore-canals (Fig. 8A, D).The thin calcareous walls of adjacent "tube-like furrows" are in contact with each other and form expanding ridges ("lirae") on the horizontal sections (Fig. 8B, C).The ridges have shapes similar to the "wrinkles" in ammonoids.

Discussion
The pore-canal network in ammonoids is poorly preserved, but can be reconstructed by analogy with the similar but better preserved pore-canal network in the Ordovician nautiloid Orthoceras (Mutvei 2022), and in the Carboniferous coleoids Shimanskya and Mitorthocera (Mutvei et al. 2012;Mutvei & Mapes 2019).It consists of parallel rows of vertical canals each of which opens into one horizontal canal (Fig. 9B, C).As in Orthoceras (Mutvei 2022, Fig. 7A-E), the canals in Quenstedtoceras and Ptychoceras had thin walls of calcified organic fibres that were partially or entirely dissolved during diagenesis (Figs.3B, Fig. 4, Fig. 9D).The pore-canals have therefore not been detected previously.
Because the pore-canal network does not exist in extant Nautilus or in other molluscs, it is difficult to explain its origin.As in Orthoceras and Mitortheras (Mutvei 2022, Fig. 11D; Mutvei & Mapes 2019, Fig. 4B), the walls of the horizontal canals in ammonoids were secreted by epithelial ridges on the mantle surface at the body-shell attachment band, behind the apertural margin (Fig. 9E).The mantle epithelium then formed vertical, finger-like extensions that secreted the walls of the vertical pore-canals and extended through the nacreous and inner prismatic layers (Fig. 9C).
As described above, the shell wall in Quenstedtoceras and Ptychoceras had a very thin outer prismatic layer (Kulicki et al. 2015) that in juvenile shells was secreted at the shell aperture.In order to protect the very thin apertural margin, and the horizontal canals in the outermost part of the nacreous layer (Fig. 9B), the shell must have been covered by the mantle as in endocochleates.Further evidence that the shell was internal is given by the mode of truncation of the initial part of the phragmocone in three species of Ptychoceras (Doguzhaeva & Mutvei 1989, 1993, 2015).The additional layers on the shell surface in several ammonoids also indicate that the shell was covered by the mantle (Doguzhaeva & Mutvei 1993, 2015).Ammonoids that have the pore-canals only in the inner prismatic layer, as known to occur in the ceratid Phyllocladiscites ascheshbokensis Shev (Doguzhaeva & Mutvei 1986, Figs. 3B, Fig. 4A, B) and in goniatids (Walliser 1970. p.115), were probably ectocochleates.The apertural margin in the juvenile shells of these ammonoids probably consisted both of the outer prismatic and nacreous layers, and was thicker and mechanically stronger than that in the endocochleates.
Quenstdtoceras and Ptychoceras were probably less adapted for jet-powered swimming than the extant Nautilus.It is generally assumed that Extant NautilusNautilus has the following requirements for jet-powered swimming: (1) the shell aperture has a hyponomic sinus; (2) the body contains a large mantle cavity; (3) contractions of two powerful retractor muscles create the jet-propulsions.In contrast to Nautilus, Quenstedtoceras and Ptychoceras did not have these requirements.(1) instead of the hyponomic sinus the shell aperture has a prominent ventral keel; (2) the long and narrow body had no place for a large mantle cavity; and (3) the small, dorsally attached shell muscles could only retract the body into the body chamber.The previously unknown pore-canal network described here decreased the hydrostatic stability of the shell.Because the shell was covered by the mantle, it is highly probable that the body was equipped with fins (Doguzhaeva & Mutvei 1993, Fig. 4a).
At present, it is not possible to calculate the buoyancy and mode of life of ammonoids by using 3D models based on tomography and other methods because the pore-canals have a still unstudied, highly variable density in different taxa (compare Figs. 4 and 5); it remains unknown which ammonoids had the canals both in the nacreous and inner prismatic layers or only in the inner prismatic layer, and whether the canals contained gas or some other substance.It is also not possible to calculate depth limits of ammonoids because the structure and mechanical strength of the shell wall and siphuncle differs from that in extant Nautilus.
The pore-canal network in ammonoids likely had an important function for the animal because it existed in the majority of shelled cephalopods for more than 350 Myrs, without major changes.Its function probably was to aid achievement of neutral buoyancy.Extant Nautilus lacks the porecanal network in the shell wall and neutral buoyancy is achieved by regulating the volumes of liquid and gas in the shell chambers through the connecting rings of the siphuncle by osmosis (Ward 1987;Ward & Greenwald 1982;Dunstan et al. 2011).The connecting rings have a simple fibrous structure and are not perforated by pores (Mutvei et al. 2010).As a contrast to Nautilus, the ammonoids Quenstedtoceras and Ptychoceras have the pore-canal network in the shell wall to aid regulations of neutral buoyancy.The connecting rings are perforated by large number of micro-pores, 0.1 mµ in diameter (Obata et al. 1980;Mutvei et al. 2004;Mutvei & Dunca 2007;Doguzhaeva et al. 2010).Quenstedtoceras and Ptychoceras were not adapted for Nautilus-like jet-powered swimming but the structure of their connecting rings indicates that they were adapted for vertical migrations by using rapid buoyancy changes.The pore-canal network in the shell wall was, therefore needed to obtain the neutral buoyancy.The increase of buoyancy would be small if the canals contained low-density liquid or some other low-density substance.Only if the canals contained gas, could neutral buoyancy be obtained.This would explain why the vertical canals were connected to horizontal canals to create a circulatory system, why the canals had walls of calcified organic fibres and why the canals were important for the majority of shelled cephalopods during their evolution.

Conclusions
(1) Both Quentdtoceras and Ptychoceras were endocochleates and had their shells covered by the mantle.(2) They were not adapted to Nautilus-like jet-powered swimming.
(3) They were vertical migrants that re0gulated buoyancy by rapid changes in the volumes of gas and liquid in the shell necessary for vertical migrations.(5) Unlike modern Nautilus, the majority chambers.(4) The pore-canal network facilitated neutral buoyancy, of shelled cephalopods had a pore canal network in their shell wall.

Figure 2 .
Figure 2. A, B. Quenstedtoceras sp.Specimen no.Mo 199802.A. Oblique vertical section of the shell wall; note the vertical pore-canals that are filled with pyrite in the inner prismatic and nacreous layers; the outer prismatic layer is not preserved.B. Section of the same figure at higher magnification.

Figure 3 .
Figure 3. A, B. Quenstedtoceras sp. A. Section of the Fig.1A in higher magnification.B. Specimen no.Mo 199803.Oblique vertical section of the shell wall; vertical porecanals with partially dissolved walls appearing as dark spots.

Figure 4 .
Figure 4. Cosmoceras sp.Specimen no.Mo 199804.Horizontal section of the shell wall; note the vertical pore-canals with pyritic walls in the inner prismatic layer; in the nacreous layer the canal walls are partially dissolved and appear as spots.

Figure 5
Figure 5. A, B. Cosmoceras sp.Specimen no.Mo 199805.A. Vertical section of the dorsal shell wall to show the vertical pore-canals; the cavities of the canals are filled with pyrite.B. Vertical pore-canals at higher magnification.

Figure 6 .
Figure 6.A, B. Lissoceras erato D´Orbingy.Specimen no.Mo 199801.A. Numerous vertical pore-canals exposed in the horizontally broken dorsal shell wall.B. Porecanals at higher magnification; note the high density of the canals.

Figure 7 .
Figure 7. A, B, C. Carboniferous coleoid Shimanskya postremus Miller: A. Outer surfaces of the horizontal pore-canals.Same specimen as illustrated by Mutvei et al. 2012, fig.3b.B.Vertical section of the shell wall to show horizontal pore-canals ("rods").C. Longitudinal section of the shell wall to show triangular sections of the walls of the horizontal canals; note the similarity between the "wrinkles" and the triangular sections.Same specimen as illustrated byMutvei et al. 2012, fig.4b, c.

Figure 8 .
Figure 8. A-D.Carboniferous coleoid Mitorthoceras perfilosum Gordon: A. Shell surface with horizontal pore-canals ("tube-like furrows").Same specimen as illustrated by Mutvei & Mapes 2019, Fig. 1A.B, C. Horizontal pore-canals in longitudinal vertical sections of the shell wall; note that the walls of the adjacent canals form "wrinkle"like structures.Same specimen as illustrated by Mutvei & Mapes 2019, Fig. 3 A, B, C. D. Horizontal pore-canals on the shell surface; note that the canals have thin calcareous walls and empty cavities.Same specimen as illustrated by Mutvei & Mapes 2019, Fig. 7C.

Figure 9 .
Figure 9. Schematic representation of the pore-canal network in ammonoids.A. Shell wall with vertical pore-canals in Quenstedtoceras sp.B. Pore-canal network in the dorsal shell wall of endocochleate ammonoids; note that the vertical canals extend through the nacreous and inner prismatic layers, and that the outer prismatic layer is extremely thin.C. Horizontal and vertical pore-canals in the shell wall of the body chamber in the living animal.D. Remnants of the horizontal and vertical pore canals in the shell wall after diagenesis; note the origin of "wrinkles" and "Ritzstreifen".E. Secretion of walls of the horizontal pore-canals by epithelial ridges.