The family Lamellorthoceratidae is a small group of Early to Middle Devonian non-Ammonoid Cephalopods possessing orthoconic shells characterized by abundant cameral deposits composed of more or less closely spaced sets of radial lamellae. They have been exclusively known, along with other Cephalopod taxa, from the warm-waters of low to middle paleolatitudinal regions, including Morocco, Algeria, Germany, Russia, North America, Turkey, and Japan. Other Cephalopod groups with restricted known palaeolatitudinal distribution are the Actinoceratids, Discosorids, and Ascoceratids, although for the last group, new findings from the Paraná Basin of Brazil have recently shown that their distribution pattern should be revised. In contrast, the cold water settings of the Early to Middle Devonian austral circumpolar region known as the Malvinokaffric Realm has been claimed to have an extremely poor Cephalopod record. This major Devonian paleobiogeographic unit corresponds to southwestern Gondwanan marine basins and is recognized in the modern regions of South Africa, Ghana, Antarctica, and southern South America. It is certainly characterised by a high level of supra-generic endemic taxa (notably Trilobites) and a scarcity (or absence) of some typical Palaeozoic groups such as Stromatoporoids, Graptolites, Conodonts, and Goniatites.
In a paper published in the journal Acta Palaeontologica Polonica on 2 April 2020, Marcela Cichowolski of the Instituto de Estudios Andinos 'Don Pablo Groeber' and the Facultad de Ciencias Exactas y Naturales at the Universidad de Buenos Aires, and Juan Rustán of the Centro de Investigaciones en Ciencias de la Tierra and Centro de Investigaciones Paleobiológicas at the Universidad Nacional de Córdoba, report the first record of Lamellorthoceratids from the Devonian of Argentina.
Cichowolski and Rustán's discovery is significant with regard to the alleged scarcity of Malvinokaffric Cephalopods. This scarcity was recently challenged by the first records of Bactritoid Cephalopodss from the Devonian of South America.
The Andean region of southern South America, the area from which these specimens were recorded corresponds to the Argentine Precordillera Basin in west-central Argentina. The studied specimens come from the Lower Devonian Talacasto Formation, which is widely exposed in San Juan Province, and with isolated outcrops in the northern La Rioja Province. The succession is composed of intensely bioturbated greenish- gray mudstone with intercalated beds of fine-grained sandstone. It typically consists of dark argillaceous horizons basally (black to greenish mudstone and shale), passing upwards into sand-rich horizons. This unit corresponds to a muddy shelf depositional system developed during a high stand system tract. In San Juan Province, this unit increases in thickness from the south, where it is 300 m thick (in the Talacasto section), to more than 1000 m in the north. It overlies the mainly late Silurian shelf deposits of the Los Espejos Formation, and underlies the turbiditic deposits of the Early to, probably, Middle Devonian Punta Negra Formation. The Talacasto Formation has yielded the bulk of Devonian fossils described from Argentina. As in other closely related Early to Middle Devonian Malvinokaffric basins, Conodonts and Graptolites are absent and Goniatites are extremely rare. Thus, an early Lochkovian to Emsian age has been proposed for this unit based on Brachiopod and palynological data.
Location map of the fossil localities and middle Palaeozoic outcrops (A), and stratigraphic sections with fossil occurrences indicated (B). Cichowolski & Rustán (2020).
Cichowolski and Rustán report three Lamellorthoceratid specimens, all of which are assigned to the genus Arthrophyllum, but not to specific level. CEGH-UNC 27426 is an incomplete phragmocone preserved within a nodule, from the Quebrada de los Algarrobos locality, which is Pragian in age (410.8-407.6 million years old); CEGH-UNC 27427 and CEGH-UNC 27428 are moulds of incomplete phragmocones from the Loma de los Piojos locality, which is Pragian or Emsian in age (410.8-393.3 million years old).
CEGH-UNC 27426 is an incomplete phragmocone with seven complete chambers preserved. In order to orient the specimens Cichowolski and Rustán assumed 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 about 0.4 mm). The septal depth is about 0.28 mm. The shell surface does not show ornamentation, neither does the external mould, although the most external layer is probably not preserved. The siphuncle diameter is 2 mm in a section of 11.4 mm (ratio 0.17). Its position is nearly central. In the longitudinal polished section, the septal necks appear to be orthochoanitic, with a length of about 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. 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. 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. 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. 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.
The Lamellorthoceratid Cephalopod Arthrophyllum sp. from the Lower Devonian Talacasto Formation in the Precordillera Basin, Argentina. (A) CEGHUNC 27426, general view of the longitudinal section of the specimen (A₁), details showing a closer view of the lamellae and siphuncle structure (A₂)–(A₅), external view (A₆), specimen before cutting in posterior view (A₇). Note the lamellar deposits where the septum is removed. (B) CEGH-UNC 27427, specimen in lateral view (B₁), posterior views with different orientations (B₂), (B₃). (C) CEGH-UNC 27428, lateral views of the internal mould with different orientations (C₁)–(C₄), internal mould in posterior view (C₄), external mould in anterior view, showing the lamellar deposits inside the apicalmost chamber (C₅), external mould in lateral view with the same lamellar deposits in the posterior part (C₆). Cichowolski & Rustán (2020).
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. The chamber filling consists of radial lamellae, which are recrystallised. They are straighter near the center and more sinuous towards the margins. The siphuncle is not distinguishable in posterior view, and Cichowolski and Rustán 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. One part is an external mould of some chambers, on which the sutures are visible. 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. The external-most part of the chamber space is empty, maybe due to lamellar dissolution. 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 about 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. 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 Cichowolski and Rustán 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. 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 about 5 mm, with similar measurement for the width and the siphuncle diameter being 1.2 mm
Cichowolski and Rustán consider that CEGH-UNC 27427 and CEGHUNC 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. 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.
An eurytopic (demersal and pelagic) inferred mode of life of adult Arthrophyllum, following an initial planktonic stage after hatching, would satisfactorily explain the observed widespread geographic distribution, that now includes the cold waters of the southwestern Gondwanan basins. The Devonian faunas of Argentina have been described as typically Malvinokaffric (endemic to Southwestern Gondwana), although palaeobiogeographic information from different groups offers contrasting insights. The Lamellorthoceratids were unexpected in this scenario, because they were previously known only from warm waters regions. In addition, they were extremely scarce in faunal associations from this region and time, since our few specimens represent the current total of records of the group known, in spite of the fact that their thick cameral deposits probably enhanced their chances of preservation. Such a poor fossil record along with the absence of specific studies could have biased their previously known distribution pattern toward better studied regions of low palaeolatitudes. In this regard, an analogous case might be the Bactritid Cephalopods, recently reported from the same units in Argentina, which, although more abundant, also were unknown from high Gondwanan palaeolatitudes. With the increasing knowledge of the Devonian austral Cephalopod faunas, the distribution pattern of several other groups may need to be reevaluated.
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