Cephalic Remains

3.2.1 Radula

Morphology

In specimen PE 32521, a long portion of the radula (Figs. 6.2-4) and several dispersed arm-hooks (Figs. 6.5, 6.6) are preserved. The radula is located on the proostracum-like structure close to its margin. A few irregular ridges can be distinguished in front of the proostracum-like structure (Fig. 6.1A, B). Several arm hooks occur close to these ridges (Figs. 6.5, 6.6). It is probable that the ridges are impressions of the arms. To judge from the distribution of the arm hooks in front of the proostracum-like structure, the arms probably were short. Short arms also occur in the holotype of Jeletzkya douglassae (Johnson and Richardson, 1968: Figs. 1, 2). The radula is about 3.5 mm in length and 1.5 mm in width (Fig. 6.2). In front of the radula, there are two poorly preserved carbonaceous, triangular elements oriented transverse to the radula axis (Fig. 6.2). They may be poorly preserved remains of the jaws. About 30 transverse rows of the radula are visible; one side of the radula is better preserved than the other side. SEM examination shows that the radula has two marginal plates on each side; both are situated outside the marginal teeth, in contact with each other (right side of Fig. 6.3). The two marginal plates and the

Fig. 6.7 Saundersites illinoisiensis g. and sp. n, PE 32521. A. A fragment of the soft tissue remnant from the external surface of the shell showing traces of ink substance and from the shaly sediment around it. Each division of scale bar is 3 mm. B, D. Two types of crystals in the shaly sediment surrounding the soft tissue remnants, enlarged details of A. Each division of scale bar is 1.2 mm and 0.12 mm, respectively. C, E. Globular ultrastructure of the ink, enlarged detail of A. Each division of the scale bar is 0.3mm and 60 nm, respectively.

Fig. 6.7 Saundersites illinoisiensis g. and sp. n, PE 32521. A. A fragment of the soft tissue remnant from the external surface of the shell showing traces of ink substance and from the shaly sediment around it. Each division of scale bar is 3 mm. B, D. Two types of crystals in the shaly sediment surrounding the soft tissue remnants, enlarged details of A. Each division of scale bar is 1.2 mm and 0.12 mm, respectively. C, E. Globular ultrastructure of the ink, enlarged detail of A. Each division of the scale bar is 0.3mm and 60 nm, respectively.

Fig. 6.8 Saundersites illinoisiensis g. and sp. n, PE32521. General view of the fossilized soft tissue debris preserved on the shell surface. Each division of scale bar is 1-2 mm.

large marginal tooth are distinct. The marginal tooth has a broad, semilunar shape in cross section; its basal portion, about one-third of the total height of the tooth, is massive; the rest of the tooth is pointed and curved backward; its sides form an angle of about 20°. The ratio between the height and maximum diameter of the tooth is ca 1:1. The central (or rachidian) and lateral teeth are poorly preserved and cannot be described in detail. This is particularly the case with the inner lateral tooth of which only indistinct remains can be seen. The radula formula is C L1 L2 M MP1 MP2 (11 elements in total) (Fig. 6.4).

Comparison

In present-day molluscs a radula of 11 elements in each transverse row is known in the class Monoplacophora (Starobogatov, 1990a). This class is considered a possible ancestor of the Cephalopoda (Yochelson et al., 1973). Two or more marginal plates along the periphery of the radula are known in the class Polyplacophora, and there are 17 elements in each transverse row (Ivanov and Sirenko, 1990; Starobogatov, 1990a, b).

In addition to the present specimen, several radulae have been recovered from Paleozoic cephalopods: (1) in an unidentified Late Ordovician orthocone the radula has five teeth in each transverse row; the radula configuration is more similar to that of ammonoids and coleoids than to that of nautiloids (Gabbott, 1999); (2) in the

Silurian orthocerid Michelinoceras the radula is reported to have seven elements in each transverse row (Mehl, 1984), but this report has been questioned (Nixon, 1988: 113); (3) two separately preserved radulae with 13 elements in each transverse row have been found from the Late Carboniferous (Desmoinesian) Mazon Creek deposits; they are described as Paleocadmus herdinae (Solem and Richardson, 1975) and Paleocadmus pohli (Saunders and Richardson, 1979) and have the radula formula: C L1 L2 M1 MP1 M2 MP2; both Paleocadmus radulae have two marginal plates on each side, but they are separated by a second marginal tooth; the similarity of these two Carboniferous radulae with the 13-element radula in present-day Nautilus suggests that there is a strong evolutionary stability of the radula with 13 elements in each transverse row; (4) radulae with nine elements in each transverse row are reported in the following Carboniferous goniatites: Glaphyrites (Closs and Gordon, 1966), Cravenoceras (Tanabe and Mapes, 1995) and Girtyoceras (Doguzhaeva et al., 1997).

Present-day coleoids and fossil ammonoids have either seven or nine elements in each transverse radula row (see Nixon, 1988). Individuals of these taxa each have a single marginal plate on each side. The occurrence of radulae with two marginal plates that are located outside the marginal tooth makes the radula structure in the coleoid specimen under discussion unique in the class Cephalopoda.

Evolutionary Development of the Radula in Molluscs

In present-day molluscs, the number of elements in each transverse row in the rad-ula is different in different classes: Polyplacophora - 17, Monoplacophora - 11, Scaphopoda - 5, Cephalopoda - 13, 9, 7, or none, Gastropoda - variable (Ivanov and Sirenko, 1990; Ivanov and Starobogatov, 1990; Rossolimo, 1990). Starobogatov (1990a, b) concluded that radulae in monoplacophorans evolved through oligomer-ization and that the number of teeth was about 80 in Paleozoic genera instead of 11 in present-day monoplacophorans. The evolutionary reduction of the number of elements in the radula of monoplacophorans was believed by Starobogatov to have been caused by migration of these molluscs from the littoral zone, characterized by coarse substrate, to abyssal zones with soft substrate.

At present, it is uncertain whether the evolutionary transformation of radula elements in cephalopods occurred by oligomerization. The possible scenario is as follows: at early stages of coleoid evolution, the radula with 11 elements (seven teeth and four plates) was derived from the nautiloid radula with 13 elements (Nixon, 1988: Fig. 6.2e) by the elimination of the second marginal tooth. That resulted in a changed position of the two marginal plates to side by side, which is the case with the present specimen (PE 32521). At the next evolutionary stage, the two marginal plates fused into one plate, and as a consequence, the radula had nine elements per transverse row. Finally, the marginal plates became entirely eliminated, and a rad-ula with seven elements in each transverse row was formed.

One can also assume that the radula possessing 11 elements in each transverse row (like that in the studied specimen) was inherited from the monoplacophoran radula, and that the radula of the studied specimen is similar to the archaic radula of early cephalopods. In this case, the nautiloid type of radula, with 13 elements in each row, might have arisen from a radula with 11 elements by adding an additional marginal tooth between the two marginal plates on each side. The radula in present-day coleoids could have been formed by oligomerization of the two marginal plates.

3.2.2 Arm Hooks

Morphology

The hooks (Figs. 6.5, 6.6) are small (about 0.2-0.3 mm in length), shiny, hollow structures composed of black horny material. The shaft is short and thick. The distal part, extending from the maximum curvature to the tip, is well developed and long. The angle between the shaft and the distal part is about 90°. The cross section through the basal part is triangular.

Comparison

The arm hooks of the present specimen and the separately preserved hooks from the Mazon Creek area, illustrated by Saunders and Richardson (1979: Fig. 9c), show significant morphological differences. The separate hooks appear to be similar to the hooks seen in the holotype of Jeletzkya douglassae. They are bigger than those seen in the specimen studied here (ca 1.0 mm versus 0.2-0.3 mm, respectively). In addition, the shaft in each of the hooks of the holotype of J. douglassae and the hooks illustrated by Saunders and Richardson (1979: Fig. 9c) are thicker and swollen on the internal side, and the distal part is relatively shorter than those in the studied specimen.

The hooks of the present specimen show some similarity to the hooks of the Lower Jurassic "fossil teuthid" Loligosepia aalensis Zieten (Doguzhaeva and Mutvei, 2003: Fig. 1c, d, e). In both, the shafts are short and thick, and the distal parts are long. The approximately 90° angle between the shaft and distal part is also similar. Both the Carboniferous and the Jurassic hook-specimens have cross sections that are triangular, and the ratio of wall thickness to hook diameter is 1:3.

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