Circularva, Shcherbakov et Ponomarenko, 2023, Shcherbakov et Ponomarenko, 2023

Makarov, K. V., 2024, On the systematic position of Circularva reichardti Shcherbakov et Ponomarenko, 2023, Russian Entomological Journal 33 (1), pp. 39-42 : 39-41

Results

All information concerning the morphology of Circularva [ Shcherbakov, Ponomarenko, 2023] is grouped in three obvious categories:

(a) Structures that are clearly observed and can be accurately interpreted. These are the number and shape of the dorsal sclerites, the number and location of ocelli on each side of the head, the sculpture of the integument (rows of tubercles at the posterior margin of tergites), and the pleurites of the abdominal segments;

(b) Poorly distinguishable, unclear structures that cannot be accurately interpreted based on the visible organization. These are the several processes/appendages on the segments, the setae on tubercles in the posterior part of the tergites, and the abdominal sclerites. Their interpretation is based on the principle of taxonomic analogy, i.e. such that can be considered as spiracular gills, setiferous tubercles, sternites only if the fossil does belong to Coleoptera ;

(c) Expected structures that are not visible on the impressions. First of all, such are the appendages of the head and body. Nothing is known yet as to how many and which antennae this creature may have had, nor how many legs could have been involved.

Since we do not know the set and structure of the appendages and legs of Circularva , we can confidently attribute it only to the Arthropoda, among which it could potentially have been related to both crustaceans and myriapods, as well as to hexapods ( Figs 1–4 ). Let me consider these three hypotheses one by one.

Crustaceans are highly diverse in terms of the number of segments and their combinations into tagmata [ Schram, Koenemann, 2021]. If this hypothesis is accepted, a complete coincidence of the segmentation of Circularva is to be found with one of the tagmosis variants of Isopoda, in which the thoracic region consists of seven segments, the abdominal region includes five free segments, and the sixth abdominal segment is merged with the telson ( Fig. 2 ). Among the isopods, there are animals with an almost homonomous segmentation of the thoracic region, yet heteronomous segmentation with varying segment sizes is likewise very common [ Brandt, Poore, 2003]. The wide sternal region of Circularva is similar to that of Isopoda. The appendages of Isopoda are diverse and often specialized, this giving room to interpreting the unclearly distinguishable structures. Fossils of Isopoda have been known since the Carboniferous [ Schram, Koenemann, 2021], this failing to contradict the age of Circularva .

Oniscomorph forms are common among millipedes, or Diplopoda [ Koch, 2015]. Modern Sphaerotheriida have 11 free trunk segments (segment XII is merged with the telson) and a telson [ Wesener, 2015]. This resembles the tagmosis of Circularva (see Fig. 3 ), but requires at least two assumptions: (a) a weakly bound area at the base of the head must be interpreted as a collum, and (b) segment XII has not yet become fused with the telson in that primitive putative millipede. Millipedes of the superorder Oniscomorpha have well-developed pleurites [ Blower, 1985], and their fossil representatives (Amynilyspedida) are known from the Upper Carboniferous [ Edgecombe, 2015]. The number of trunk segments in the fossil Oniscomorpha ranged from 13 to 15, supporting the diplopod hypothesis. However, in Diplopoda the sternites are usually narrow, which is not consistent with the impression. Besides this, Amynilyspedida showed characteristic spines on the tergites [ Lheritier et al., 2023], these being absent from Circularva . Thus, the “diplopod” hypothesis is acceptable, but has obvious disadvantages compared to the “isopod” hypothesis.

The absence of wings, simple ocelli on the head, slightly enlarged first segments (interpreted as thoracic segments) and a ten-segmented abdomen ( Fig. 4 ), all this suggests that Circularva is a Holometabola larva, probably Coleoptera . It seems important that the size differences between the thoracic and abdominal segments are very small: starting with the metathorax, their length is approximately the same and half the length of the prothorax (measured from the photograph in the original description). The rather narrow abdominal segments V–VII (0.3–0.4 the length of the prothorax) look strongly deformed in the photographs of the impression, while the following segments VIII–IX already show the normal length, which is half the prothorax length. Consequently, only thoracic segments I and II differ in size from the other segments and this trait is also common among Isopoda.

As shown above, the number of segments in Circularva satisfies the interpretation in favour of the “diplopod” hypothesis and corresponds very well to the “isopod” hypothesis. In addition, Circularva has several features that fail to fit well with the morphology of Coleoptera larvae and, more generally, of Holometabola larvae in general. Firstly, they lack molting sutures, which are usually clearly visible on impressions, especially on the head, since the sutures often diverge when the head capsule is deformed. Secondly, Circularva had an unusually wide abdominal segment X. In Holometabola larvae, this segment usually assumes a locomotor function and has the shape of a tube, more or less strongly bent ventrally. Among the larvae of Coleoptera , no forms with such a wide segment X are known [ Böving, Craighead, 1931; Costa et al., 1988; Lawrence et al., 1991, 2011]. Wide tergites in insect larvae are associated with the presence of dorsoventral and dorsopleural muscles. In Ectognatha, these muscle groups in segment X are greatly reduced. In the Zygentoma, two muscle pairs remain [ Rousset, 1973] and in the Ephemeroptera, only one muscle pair is present [ Birket-Smith, 1971], whereas in all Holometabola larvae whose anatomy has been studied, segment X comprises only longitudinal muscles [ Kemner, 1913; Snodgrass, 1931; Berrios-Ortiz, Selander, 1979; Beutel, Hornschemeyer, 2002; Wipfler et al., 2012; Yavorskaya et al., 2015]. Therefore, the attribution of Circularva to Coleoptera , based on the structure of the last segment, is extremely unlikely.

To summarize, I believe that Circularva could not have been a coleopteran larva. Therefore, discussing its placement in the suborder Myxophaga and its similarities to the family Torridincolidae is meaningless. Most likely, that was a crustacean, but its attribution to Isopoda still remains unclear. Therefore, it seems rational to consider Circularva Shcherbakov et Ponomarenko, 2023 as an Eumalacostraca incertae sedis.

Figures

Figs 1–4. Circularva reichardti and interpretations of its morphology: 1 — Circularva reichardti Shcherbakov et Ponomarenko, 2023, drawing from the original description, left half, unclear structures not shown; 2–4 — putative tagmosis of Circularva due to hypotheses about its belonging to crustaceans, millipedes and coleopterans, respectively. Рис. 1–4. Circularva reichardti и интерпретации её строениЯ: 1 — Circularva reichardti Shcherbakov et Ponomarenko, 2023, рисунок иЗ первоописаниЯ, леваЯ половина, неЯсные структуры не покаЗаны; 2–4 — предполагаемый тагмоЗис Circularva в рамках гипотеЗ о её принадлежности к ракообраЗным, многоножкам и насекомым, соответственно.