Comparative Morphology Shell

3.1.1 Morphology

The shell length in specimens PE 32521 (Fig. 6.1A, B) and PE 20808 is 56 mm and 46 mm, respectively, and the maximum diameter is 12 mm and 14 mm, respectively. In both specimens the shell has a narrow triangular shape with an apical angle of approximately 12°-15°. The shells appear to be exposed from the ventral side.

Fig. 6.2 Saundersites illinoisiensis g. and sp. n, PE 32521. General view of a radula showing the approximate radula width (RW) and longitudinal rows of two marginal plates on each side (MP1, MP2) and marginal teeth (M). Scale bar is 1.25mm.

Four portions can be distinguished in each shell. The most adoral portion forms a plate on the apparent dorsal side of the shell. The length of this plate is about one-fifth of the total shell length. In PE 32521, the anterior and lateral margins of this plate are fractured, and the surface of the plate is uneven due to compression. In PE 20808, the adoral plate is well preserved and shows a slightly forward curved, broadly rounded anterior margin. Its surface shows growth lines parallel to the anterior margin. Because it is developed only on one side of the shell the anterior plate is here interpreted as a proostracum-like structure. The middle portion of the shell is here interpreted as the body chamber. In both specimens, it is three times longer than the proostracum-like structure. It has a rounded cross section that is slightly compressed in PE 32521. As in all other cephalopod shells from the Mazon Creek area (Richardson and Johnson, 1971; Saunders and Richardson, 1979), the shell material is not preserved. The third portion, immediately apical of the body chamber, can be clearly distinguished from the rest of the shell, because, in PE 32521, it is completely filled and, in PE 20808, partially filled with white calcite and has a sharp boundary toward the middle portion (the body chamber) of the shell. The length of this portion is about one-sixth of the total shell length. It is here interpreted as a phragmocone, although septa cannot be clearly distinguished. Because the phragmocone is short (one-sixth of the shell length only), it could have been truncated during the lifetime of the animal. The most apical portion of the shell is a narrow, short structure that in each specimen shows irregularly shaped longitudinal ridges. This portion is here interpreted as a partly calcified, partly organic rostrum. It is interesting to note that, in

Fig. 6.3 Saundersites illinoisiensis g. and sp. n, PE 32521. Enlarged detail of Fig. 2. The anterior portion of the radula shows two rows of marginal plates (MP1, MP2), a row of well exposed marginal teeth (M) that have a broad basal part, two rows of lateral teeth (L1, L2), which are partly preserved, and a row of central teeth (C) of which only the tips are exposed. Scale bar is 1.25 mm.

Fig. 6.3 Saundersites illinoisiensis g. and sp. n, PE 32521. Enlarged detail of Fig. 2. The anterior portion of the radula shows two rows of marginal plates (MP1, MP2), a row of well exposed marginal teeth (M) that have a broad basal part, two rows of lateral teeth (L1, L2), which are partly preserved, and a row of central teeth (C) of which only the tips are exposed. Scale bar is 1.25 mm.

Fig. 6.4 Saundersites illinoisiensis g. and sp. n., PE 32521. Model of a radula with 11 elements in each transverse row: on each side of the central tooth (C) there are two lateral teeth (L1, L2), a marginal tooth (M), and two marginal plates (MP1, MP2).

these two Mazon Creek specimens, the apical portion of the phragmocone and rostrum shows certain similarities to those in the Early Carboniferous coleoid Hematites Flower and Gordon, 1959 (Doguzhaeva et al., 2002a: Fig. 6.4; pl. 7, Figs. 1, 2). In specimens of this genus, the apical portion of the phragmocone that is totally

Fig. 6.5 Saundersites illinoisiensis g. and sp. n, PE 32521. Arm hooks (AH) preserved in front of the radula (R, left bottom corner). Two hooks with tips turned to the left show their general shape (right bottom corner). Other hooks, exposed in cross section, demonstrate that they are hollow and have a thin, originally organic wall. The hooks are arranged in pairs and in longitudinal rows. Each division of the scale bar is 0.5mm.

Fig. 6.5 Saundersites illinoisiensis g. and sp. n, PE 32521. Arm hooks (AH) preserved in front of the radula (R, left bottom corner). Two hooks with tips turned to the left show their general shape (right bottom corner). Other hooks, exposed in cross section, demonstrate that they are hollow and have a thin, originally organic wall. The hooks are arranged in pairs and in longitudinal rows. Each division of the scale bar is 0.5mm.

surrounded by the rostrum is almost always truncated, and the internal ultrastructure of the rostrum suggests that it was constructed of both organic and calcified material. In addition, the surface of the rostrum of specimens of this genus has longitudinal ridges that vary in size and spacing.

3.1.2 Comparison

From the Desmoinesian deposits of the Mazon Creek Lagerstätte, Saunders and Richardson (1979: Fig. 1d) illustrated an orthoconic shell and identified it as Bactrites sp. (specimen PE 25584). This shell is 52 mm long and ca 20 mm in diameter at the aperture. It has a long body chamber, seven short camerae in the phragmocone, a ventral marginal siphuncle, straight sutures, and an apical angle of about 20°. With the exception of this specimen, bactritoids have not been found in the Mazon Creek fauna. Assignment of this shell to Bactrites sp. is questioned herein, and its relationship to the studied specimen PE 32521 is left open. However, the overall lack of distinct morphological characteristics and the poor preservation of PE 32521 makes its taxonomic assignment to either the coleoids or bactritoids difficult, if not impossible.

Another comparable specimen of Desmoinesian age is the shell of the coleoid Donovaniconus oklahomensis Doguzhaeva et al., which also has a proportionally long, moderately breviconic body chamber, comparatively short phragmocone, and a short proostracum-like structure (Doguzhaeva et al., 2002b, 2003).

Fig. 6.6 Saundersites illinoisiensis g. and sp. n, PE 32521. Enlarged detail of Fig. 5 (right bottom corner). Five arm-hooks (AH) are arranged in rows. Scale bar is 0.2 mm.

Was this article helpful?

0 0

Post a comment