Pomerantsoceras, Kroger, 2007

Manda, Štěpán & Turek, Vojtěch, 2009, Minute Silurian oncocerid nautiloids with unusual colour patterns, Acta Palaeontologica Polonica 54 (3), pp. 503-512 : 507

Pomerantsoceras

Minute malformations of shell due to the damage of the apertural margin in a living individual are very frequent in Recent and fossil nautiloids and subsequent healing of the shell can often be traced on the shell surface. Sublethal and pathological damage to the shell expressed as anomalies in the growth of the shell and the malformed development of septa have been only rarely documented (e.g., Barrande 1866: pl. 118: 1; Strumbur 1960; Keupp and Mitta 2004; Kröger and Keupp 2004; Klug et al. 2008). The phragmocone is of crucial importance in determining the hydrostatic and hydrodynamic properties of the shell and overall mode of life of the animal. Because septal morphology, including the geometry of the mural ridges and sutures, provide mechanical limits to the depth at which a particular ectocochleate cephalopod could survive without the phragmocone imploding ( Hewitt and Westermann 1987) serious damage to the phragmocone was likely to have been lethal to the animal. Pomerantsoceras with its small shell was highly vulnerable and despite limited amount of material, four different kinds of malformations have been found.

The holotype of “ Pomerantsoceras castor ” (NM−L 571, Fig. 9C 1 ) displays a striking anomaly in the arrangement of septa in the adapertural part of the phragmocone, which was not documented on Barrande's illustrations (1866: pl. 184: 20–23). Abrupt change of course in the suture lines appears in the seventh septum (counted from the body chamber towards the apex). A broad lateral lobe changed into a parabolic lobe. During subsequent growth the septa returned to their normal growth pattern so that the course of last three sutures was not affected by this injury. The anomalous growth of septa in this part of the shell was caused by a sublethal crush to the body chamber in the mid−lateral region of aperture. It is indicated by mid−lateral longitudinal depression shallowing adaperturally. During subsequent shell growth and shifting the body adaperturally, the animal secreted new septa in a narrowed internal space resulting in their change of convexity. Due to exfoliation of the shell in this part of phragmocone, superficial manifestation of this injury could not be observed.

In addition to the anomalous growth of septa, a pair of pits has been observed on internal mould of the last phragmocone chamber in the same specimen. Another larger pit, partially filled with shell material (sparitic calcite), is recognisable on the right side of the shell in the adapical part of the body chamber (see Fig. 9C 2 ). No depression is indicated on the cross section of the shell of this single pit. These pits probably correspond to a marked local thickening of the shell wall. This phenomenon was described in detail by House (1960) and Chlupáč and Turek (1983) in Devonian goniatites (see also Klug 2002; Korn and Klug 2002); rather rarely it occurs in nautiloids (e.g., Stridsberg and Turek 1997). House (1960) explained the pitting as pearl−like growth mounds or deposits due to irritation of the mantle by foreign particles that penetrated between mantle tissue and the shell of the animal.

A distinct growth anomaly has also been observed in specimen CGU SM 318 ( Fig. 9A ). The adapical part of the internal mould of the body chamber is laterally folded to form a false rib. Adaperturally, the internal mould regained its normal smooth form. Due to the absence shell of the body−chamber, the character of injury causing this malformation cannot be determined.

A fragment of the adapical part of the body chamber (CGU SM 316, Fig. 9B ), preserved as an internal mould, shows marked elevation situated almost mid−ventrally. It is bordered on both sides by deep furrows. The straight course of this elevation is strikingly disrupted adaperturally probably due to damage of the shell in this part of the body chamber. Morphology of this structure markedly changed here. Instead of elevation there is a mid−ventral groove bordered by relatively wide elevated zones. The structure resembles conchal furrow, which may be single or double ( Teichert 1964). Conchal furrows, located in the mid−ventral part of the phragmocones and the body chambers of ectocochleate cephalopods, were recently discussed in details by Chirat and von Boletzky (2003) and Klug et al. (2008). According to Chirat and von Boletzky (2003: 167) it represents a taxonomically unimportant developmental by−product originating “from the inner part of the initial, calcified shell apex, in line with the ventral termination of the central linear depression of the cicatrix”. However, it should be noted that observed conchal furrow in all members of the family Oonoceratidae , to which Pomerantsoceras is here assigned, forms a single shallow depression. Finding the conchal furrow in only one specimen of Pomerantsoceras is not surprising as it corresponds to the low frequency of occurrence of this structure at nautiloids ( Chirat and von Boletzky 2003). However, it seems more probable that the specimen described here does not display the conchal furrow but rather a malformation caused by an injury inflicted to the mantle margin. This malformation caused the formation of an irregular trace (“Rippenscheitelung”; compare Hengsbach 1996; Keupp 2006) as known from Recent nautilids, ammonoids and bactritoids ( Klug 2007).

Figures

Fig. 9. Shell malformations in Pomerantsoceras pollux (Barrande, 1866). A. Specimen CGU SM 318 with a false rib at the base of body chamber, in lateral (A1) and ventral (A2) views. B. Specimen CGU SM 316, body chamber with conchal furrow−like malformation, in ventral view. C. Specimen NM−L 571, anomalous septal growth, in lateral view (C1), and detail of youngest phragmocone chamber with a pair of pits, in lateral view (C2). All figured specimens come from Butovice Na Břekvici Section, Colonograptus colonus Zone, Gorstian, Ludlow. Each specimen is coated with ammonium chloride.

Fig. 1. A. Distribution of Silurian rocks in the eastern part of the Prague Synform and position of named localities (see Kříž 1992, Röhlich 2007). B. Butovice, Kační Quarry−Na břekvici Section, stratigraphy, graptolite zones, lithology (adopted from Kříž 1992, Kříž et al. 1993) and range of Pomerantsoceras pollux. C. Lochkov, Nad ubikacemi Section, stratigraphy, graptolite zones, lithology and range of Pomerantsoceras pollux (Barrande, 1866). Abbreviations: C. lundgreni, Cyrtograptus lundgreni; C. colonus, Colonograptus colonus; N. kozlow., Neocuculograptus kozlowskii, M., Monograptus; P., Pristiograptus.

Fig. 2. Shell size of nautiloids: shell height (axis x), length of body chamber (axis x), and shell length (diameter of circle), Butovice Na břekvici Section, Ludlow, Gorstian, Colonograptus colonus Zone (A) and Lochkov Nad ubikacemi Section, Ludlow, Ludfordian, Monograptus latilobus Zone (B). Data from Barrande (1865–1877) and author's collection. Note that Pomerantsoceras in both assemblages is the smallest nautiloid. The largest present nautiloids, tarpyhcerids Uranoceras and Boionautilus are all closely related taxa, as suggested by Turek (2008).