Arthrophyllum sp.
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publication ID |
https://doi.org/ 10.4202/app.00699.2019 |
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persistent identifier |
https://treatment.plazi.org/id/EE17DB0F-F07A-FFF3-FFBC-4C29BB2349B1 |
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treatment provided by |
Felipe |
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scientific name |
Arthrophyllum sp. |
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Fig. 2 View Fig .
Material.— Three specimens: CEGH-UNC 27426 , an incomplete phragmocone preserved within a nodule, from the Quebrada de los Algarrobos locality, Pragian; CEGH-UNC 27427 and CEGH-UNC 27428 , moulds of incomplete phragmocones from the Loma de los Piojos locality, Pragian or Emsian .
Description.—CEGH-UNC 27426 is an incomplete phragmocone with seven complete chambers preserved ( Fig. 2A View Fig ). In order to orient the specimens we assume the side with cameral deposits is the ventral side. This specimen is 40 mm long, with a dorsoventral apical diameter of 11.4 mm and a lateral apical diameter of 9.3 mm, indicating a slightly compressed cross section (compression ratio 0.8). Adorally, the dorsoventral diameter is 13.2 mm and the lateral diameter is 11.7 mm compression ratio 0.88). In the dorsoventral plane, therefore, the expansion rate is 0.045, and the apical angle is 2.6°. The lateral expansion rate is 0.06 and the apical angle in that plane is 3.4°. The length of the chambers varies between 5 and 6 mm (cameral depth of ~0.4). The septal depth is ~0.28. The shell surface does not show ornamentation, neither does the external mould, although the most external layer is probably not preserved ( Fig. 2A View Fig 6). The siphuncle diameter is 2 mm in a section of 11.4 mm (ratio 0.17). Its position is nearly central Fig. 2A View Fig 1 View Fig , A 2 View Fig ). In the longitudinal polished section, the septal necks appear to be orthochoanitic ( Fig. 2A View Fig 3), with a length of ~ 1.6 mm within a chamber length of 5.4 mm (ratio 0.3). On one side of the section, the connecting rings are preserved, whereas on the other side they are not present ( Fig. 2A View Fig 1 –A View Fig 3). On the side where the connecting rings are preserved, the camerae are filled with very sinuous lamellar deposits along the length of the specimen. In the adapical camerae, the deposits are denser, while in the adoral camerae, the deposits are more open, with spaces being visible between the lamellas ( Fig. 2A View Fig 1 View Fig ). However, on the opposite side of the conch, lamellar deposits are seen to be developed at the adapical end, where the partial removal of the septum facilitates their visibility within the lumen of the camera ( Fig. 2A View Fig 7). In the other chambers, cameral deposits are present but much less developed, thinly covering the septa as epi- and hyposeptal deposits, as well as occasionally covering the septal necks as epichoanitic deposits ( Fig. 2A View Fig 3). The counterpart of the longitudinal section is a sagital section does not include the siphuncle. The ventral side of the chambers consists of insipient lamellar deposits that develop from the shell margin into the chamber lumen towards the siphuncle.
CEGH-UNC 27427 is a small phragmocone fragment, consisting of one chamber and part of a second chamber, slightly compacted due to taphonomic processes, and without the shell wall ( Fig. 2B View Fig ). The chamber filling consists of radial lamellae, which are recrystallized. They are straighter near the center and more sinuous towards the margins ( Fig. 2B 2 View Fig , B 3). The siphuncle is not distinguishable in posterior view, and we cannot identify dorsal and ventral sides. The fragment is 9.3 mm long, 7.4 mm wide and 5.4 mm high adapically. Adorally the fragment is covered and impossible to be measured. The length of the only complete chamber is 5.7 mm.
CEGH-UNC 27428 is broken into three parts of a fragmentary phragmocone ( Fig. 2C View Fig ). One part is an external mould of some chambers, on which the sutures are visible ( Fig. 2C View Fig 6). The external mould is included in a rock fragment, and contains a partial internal mould of a chamber in its apicalmost part, represented by the infill of the interlamellar spaces and of the siphuncle ( Fig. 2C View Fig 5, C 6). The external-most part of the chamber space is empty, maybe due to lamellar dissolution ( Fig. 2C View Fig 5). The external mould is 13.2 mm long and the apical part has a diameter of 4.7 mm. The sutures are straight and separated by ~ 3.5 mm. The infill in the apical part is 2.8 mm wide and shows the lamellar deposits lining the cameral surfaces of the siphuncle as well as the infill of the siphuncle itself. The siphuncle diameter is 0.8 mm (ratio with the conch diameter of 0.17) and is located more or less centrally.
The other two parts consist of an isolated chamber and one fragment of some (probably three) chambers that remain intact and are preserved without the external wall, the lamellar deposits are visible filling the entire space, with the siphuncle preserved in the middle ( Fig. 2C View Fig 1 –C View Fig 3). The larger portion consists of three chambers that represent the most adorally preserved part of the phragmocone. This fragment is 15 mm long and has been compacted in some parts (especially adorally). Therefore the diameters are not precise, and we cannot distinguish between the ventral and dorsal side (in any of the three fragments). The cross section appears to be almost circular. The apical diameter is ca. 5 mm, with a siphuncle diameter of 0.76 mm (0.15). The length of the chambers varies between 4 and 4.5 mm. The shape of the lamellar deposits is straight in the middle part of the chamber (adjacent to the siphuncle) and become more sinuous towards the shell wall, a trait that can be observed externally in this specimen. Although it is not possible to measure the apical angle accurately due to the compaction of the specimen, it is very low. The last part of this specimen to be described is an isolated chamber (that fits between the external mould and the previously described part). This is also preserved with the cameral deposits filling the entire space and without the shell wall. That infill is broken through the siphuncle, forming a “half-chamber”. Its length is ~ 5 mm, with similar measurement for the width and the siphuncle diameter being 1.2 mm (the relation with the cameral width being 0.2).
Remarks.—We consider that CEGH-UNC 27427 and CEGH-UNC 27428 probably represent the most adapical region of the conch, since lamellae entirely fill each chamber. By contrast, the most adoral chambers are usually partially devoid of cameral deposits ( Bandel and Stanley 1989). CEGH-UNC 27426 corresponds to a more adoral region of the phragmocone, since lamellae occupy the whole of the apical-most chambers but only the ventral part in the remaining chambers.
It is beyond the scope of this work to discuss the taxonomic composition of the family, or to evaluate the validity of previously described genera. Although we only have three incomplete specimens, two of which are deficiently preserved, we have assigned them to Arthrophyllum because the observed characters conform the concept of this genus according to e.g., Kröger (2008). We prefer to leave the material in open nomenclature because of the incompleteness and scarcity of these conchs. As mentioned in previous studies, the species assigned to Arthrophyllum show a high degree of internal variability in different respects, especially in the apical angle, shape of the cross section, chamber lengths, siphuncle position and shape of the lamellae. High variability was also documented for several traits through ontogeny (e.g., Babin 1966; Bandel and Stanley 1989).
Our specimens differ from Arthrophyllum vermiculare reported by Kröger (2008) and Pohle and Klug (2018) from the Pragian and Emsian of Morocco in having a lower apical angle and maybe longer phragmocone chambers. The growth pattern of lamellae is, however, similar, with a different development between dorsum and venter. It is also evident from one of their figures ( Pohle and Klug 2018: fig. 10T) that the lamellae are present in adapical chambers but absent in adoral ones. A. gracile was considered to be a synonym of A. vermiculare by Kröger (2008), who suggested that it represented a different growth stage. He did not, however, comment on the species A. crassus (the type species), the type material of which is apparently lost ( Zhuravleva and Doguzhaeva 2004).
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