Systematic Paleontology

The following features of the morphology and preservation in the specimens under discussion were analyzed to elucidate their systematic assignment: (1) division of the shell into a proostracum-like structure, body chamber, phragmocone, and rostrum; (2) presence of fossilized soft-tissues covering the shell; (3) presence of the arm hooks, (4) presence of ink, and (5) a unique radula structure.

Fig. 6.11A, B Austrotrachyceras sp., NHMW2005z/0006/0001, Upper Triassic, Lower Carnian, Lower Austria, locality Schindelberg near Lunz. A. Fossilized mantle tissues preserved

Subclass COLEOIDEA Bather, 1888

Superorder is unknown

Order DONOVANICONIDA, ord. n.

Diagnosis. Small to medium-sized, about 30-60 mm in length, phragmocone-bearing coleoids each with long body chamber, relatively short and broad proostracum-like structure at the aperture, ink sac, arm hooks, and small rostrum or sheath; siphuncle narrow, ventral; connecting rings thin, organic.

Comparison. The erection of a new order is required because no existing coleoid order can accommodate all of the characteristics of a new genus being described from the Lower Carboniferous Bear Gulch Formation in Montana (Mapes, personal communication, 2007, and submitted) and two described genera, one from the Upper Carboniferous Mazon Creek locality (Westphalian D), Francis Creek Shale, in Illinois (described herein), and the other from the Wewoka Formation in Oklahoma (Doguzhaeva et al., 2002b). This order better accommodates the family Donovaniconidae that was provisionally placed in the order Phragmoteuthida (Doguzhaeva et al., 2003). Members of the new order Donovaniconida have a relatively long body chamber, a relatively short rostrum, and a short broad proostracum-like structure.

Comparison of all the presently recognized Upper Paleozoic coleoid orders are in general as follows: Members belonging to the order Hematitida are readily distinguished from the Donovaniconida by having a very short body chamber and breviconic phragmocone, a massive, short rostrum that covers the entire phragmo-cone, and being without a proostracum-like structure. The order Aulacocerida has a longiconic phragmocone, a proostracum-like structure, and a relatively long, massive rostrum on more mature specimens and in the new order the phragmocone is very short and the rostrum is short and weakly developed. The order Spirulida in the Carboniferous can be separated from the new order by the presence of a longi-conic phragmocone, and the lack of a proostracum-like structure. The order Phragmoteuthida differs from the new order by having an exceptionally long, broad tripartite proostracum, no enclosed body chamber, and a short phragmocone with closely spaced chambers. The new order can be separated from the order Octopoda because the latter lacks an internal shell.

Discussion. The geologic range of the new order is presently confined to the Carboniferous.

Fig. 6.11A, B (continued) in the body chamber. B. Globular ultrastructure of possible ink dispersed in soft tissues from the body chamber. Each division of scale bar is: A - 0.12mm; B -0.15flm. C. Unnamed phragmocone-bearing coleoid, Missourian, Middle Pennsylvanian, Nebraska, USA, University of Iowa Paleontology collections, SUI62497. Globular ultrastructure of ink preserved within the ink sac. Each division of scale bar is 0.3mm. D. Loligo forbesi Steenstrup, 1856, globular ultrastructure of dried Recent ink. Each division of the scale bar is 0.12 mm.

Family DONOVANICONIDAE Doguzhaeva et al., 2003 Genus SAUNDERSITES, gen. n.

Derivation of name. In honor of W. Bruce Saunders for his valuable contributions to the study of fossil cephalopods and present-day Nautilus. Type species. Saundersites illinoisiensis gen. and sp. n.

Diagnosis. Orthocone about 60 mm in length and 12 mm in diameter, apical angle of about 12°-15°; length ratio of proostracum-like structure/body chamber/phrag-mocone together with rostrum is ca. 3:9:5; rostrum irregularly calcified; arms apparently short, with arm-hooks arranged in double rows; radula formula C:L1: L2:M:MP1:MP2.

Comparision. The closest coleoid taxon in the Upper Paleozoic to Saundersites is Donovaniconus from time equivalent beds located in the southern part of the American mid-continent in Oklahoma. These two taxa differ in the apical angle of the phragmocone; Donovaniconus has an apical angle of about 20° and Saundersites has an apical angle of about 15° degrees. Another difference is that the phragmocone of Donovaniconus makes up about one-third (33%) of the conch length, whereas Saundersites has a phragmocone that makes up about 40% of the conch length.

Discussion. Preservational differences of Saundersites and Donovaniconus prevent additional comparisons. In Saundersites the radula and arm hooks are preserved; ink is present as a dispersed material. With the exception of the dispersed ink, these structures are not preserved in Donovaniconus. However, Donovaniconus does preserve the shell ultrastructure, shell ornament, cameral spacing, ink in an ink sac, and siphuncular details. These features are not well preserved in either Donovaniconus or Saundersites. In both genera the proostracum-like structure is short and broad, and it can be determined that this feature is longer in Saundersites than in Donovaniconus. An ultrastructural comparison of this structure cannot be made because in Saundersites this structure is preserved as an impression, while on Donovaniconus the presence of this structure was identified by the shape of the growth lines on shell fragments and by the shape of a partly complete aperture.

Saundersites illinoisiensis, sp. n.

Holotype. Specimen PE 32521.

Paratype. Specimen PE 20808.

Type locality. Mazon Creek, northwestern Illinois, USA.

Type Horizon. Upper Carboniferous, Francis Creek Shale (Westphalian D), Middle Pennsylvanian, Desmoinesian.

Derivation of name. Named for the state of Illinois.

Description. The holotype is an orthocone 56 mm long and a short cephalic area 8 mm long. Apical angle of the shell is about 15°. Body chamber is long (ca. 30 mm in length and 12 mm in diameter). Proostracum-like structure is short (ca. 1/3 body chamber length) and broad (ca. 1/2 circumference length). Short phragmocone

(apparently truncated) and short rostrum are about 1/2 body chamber length together. Rostrum is irregularly mineralized. The radula has 11 elements and two marginal plates, on each side, located outside the marginal tooth (Figs. 6.2, 6.3). The arm hooks are arranged in double rows. The hooks are about 0.2-0.3 mm in length. They show a short and thick shaft. A distal part, extending from the maximum curvature to the tip, is long. The angle between the shaft and the distal part is about 90° (Fig. 6.4). The paratype shows a well-preserved proostracum-like structure at the aperture. The shell has a similar size and shape as that in the holotype.

Was this article helpful?

+1 0

Post a comment