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Keywords:

  • ostracodes;
  • Ordovician;
  • sexual dimorphism;
  • Baltoscandia.

Abstract

  1. Top of page
  2. Abstract
  3. Sexual dimorphism in the genus incisua
  4. Retention furrows in palaeozoic ostracodes
  5. Retention furrow in incisua?
  6. Discussion
  7. References

Abstract:  In Ordovician ostracodes (genus Incisua) a new kind of sexual dimorphism is described, which is the first example among this group with males being the heteromorphs. The valves of males are larger and less frequent than the females, and are characterized by having a furrow in the ventral part. The furrow may have functioned as a device to enable the two sexes to cling together during mating.

Sexual dimorphism expressed in the shell is a common feature among Ordovician ostracodes. Different kinds are known (Schallreuter 1997; Hinz-Schallreuter and Schallreuter 1998). Mostly, the dimorphic features refer to female valves (‘female’ dimorphism). The males are usually morphologically similar to the larva. There are only few examples where males develop their own special features (‘male’ dimorphism), which makes them distinct from both females and larva (‘trimorphism’); for example, in Uhakiella (Schallreuter 1997, fig. 13; Hinz-Schallreuter and Schallreuter 1998, fig. 62). Female dimorphism is much more common, and its taxonomic significance is much more important than that of males. Concerning velar dimorphism (Text-fig. 1), which is characteristic for the suborder Palaeocopa, male dimorphism has been documented in several palaeocope genera, e.g. in Tetrada krausei (Schallreuter 1988, fig. 2; 1989, fig. 3), Bubnoffiopsis and Hollinella (Schallreuter 1997, figs 23, 27; Hinz-Schallreuter and Schallreuter 1998, figs 65, 69), but is of minor significance only. Only the hamal or nodellid dimorphism of some Devonian forms, a ‘male’ dimorphism, has been considered an important enough feature for establishing a suborder of its own (Nodellocopina) (Becker 1968).

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Figure TEXT-FIG. 1..  Main types of velar dimorphism in Ordovician ostracodes. A–B, antral dimorphism in Distobolbina, female right valve (A, length 0·73 mm) and male left valve (B, length 0·79 mm). C–D, cruminal dimorphism in Fallaticella, female right valve (C, length 0·96 mm) and tecnomorphic left valve (D, length 0·88 mm). Upper Ordovician Öjlemyrflint glacial erratic boulders (geschiebes), Isle of Gotland (Baltic Sea).

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Sexual dimorphism has been observed previously in Lower Ordovician ostracodes: Hithis proximus, Billingen (Lower Arenig), and several species of the Volkhov Stage (Middle Arenig). Among pre-Ordovician ostracodes or ostracode-like crustaceans, it has been observed in phosphatocopes (Hinz-Schallreuter 2000).

Sexual dimorphism in the genus incisua

  1. Top of page
  2. Abstract
  3. Sexual dimorphism in the genus incisua
  4. Retention furrows in palaeozoic ostracodes
  5. Retention furrow in incisua?
  6. Discussion
  7. References

In 1949 Hessland established the two species Conchoides minuta (recte: minutus) and Conchoides ventroincisurata (recte: ventroincisuratus). Morphologically, the two species are very similar to each other, but the latter is characterized by a deep furrow in the ventral part of the valve. Schallreuter (1993) assigned them to the two newly established genera Miniconchoides and Incisua.

Considering the ventral indentation (‘score’) of Incisua as a retention mark, Tinn (2002, p. 24) and Tinn and Meidla (in Tinn 2002, p. IV, 2) united Incisua ventroincisurata (Hessland, 1949) and Miniconchoides minutus (Hessland, 1949), preferring the name Incisua ventroincisurata because the two species ‘were identified as morphologically identical, apart from the retention mark’ (in I. ventroincisurata), and placed the species within the Conchoprimitidae (recte Conchoprimitiidae) of the suborder Eridostraca. The typical members of this suborder are characterized by valve/shell retention and thus are sometimes multilamellate. In the Conchoprimitiidae retention is rather rare but often a weak retention furrow is present in the antero- and/or posterodorsal regions (Pl. 1, figs 34).

image

Figure EXPLANATION OF PLATE 1.  Figs 1–2. Pariconchoprimitia conchoides (Hadding, 1913). 1, juvenile left valve, GG-400-2295, with retained valve, lateral view, length 0·99 mm. 2, juvenile right valve, GG-302-2274, with retained valve partly broken away, lateral view, length 0·92 mm. Local geschiebe, Gis-29, Gislövshammer, Scania, Sweden, lower Upper Ordovician. Note the distance of the retained valve from the free margin ventrally, especially in 2. Figs 3–4. Conchoprimitia glauconitica (Kummerow 1924). 3, juvenile right valve, WMN A16-121, with distinct retention furrow posterodorsally, lateral view, length 1·25 mm, geschiebe Ahl-87-189, Ahlintel, Münsterland, Westphalia, Middle Ordovician. 4, carapace, WMN A16-105, with distinct retention furrows posterodorsally, posterior view, height 1·11 mm, geschiebe Ahl-88-47, Ahlintel, Münsterland, Westphalia, Middle Ordovician. Figs 5–9. Incisua ventroincisurata (Hessland, 1949) from geschiebes from Ahlintel, Münsterland, Westphalia, Middle Ordovician. 5–7, left and right male valves, WMN A16-138 and A16-137, lateral (5, 7) and ventral (6) views; lengths 0·73 and 0·71 mm, respectively; geschiebe Ahl-87-198. 8−9, female carapace, WMN A16-162, right lateral view, and right female valve, WMN A16-143, lateral view; lengths 0·77 and 0·76 mm, respectively; both geschiebe Ahl-1118. LV, left valve; RV, right valve; car., carapace; post., posterior; RF, retention furrow; B, buttress; geschiebe, glacial erratic boulder; WMN, Westphalian Museum of Natural History, Münster; GG, Institute of Geography and Geology, University of Greifswald.

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image

Figure TEXT-FIG. 4.. Incisua ventroincisurata (Hessland, 1949), size-dispersion diagram and valve length plotted against L:H ratio (shape).

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However, in 2003 Tinn and Meidla (in Tolmacheva et al. 2003) cited both species again as separate taxa. Miniconchoides minutus has been found in many more samples (13 from the Volkhov Stage in Putilovo quarry) than Incisua ventroincisurata (three samples, only in the upper part of the Volkhov Stage in the same quarry).

Retention furrows in palaeozoic ostracodes

  1. Top of page
  2. Abstract
  3. Sexual dimorphism in the genus incisua
  4. Retention furrows in palaeozoic ostracodes
  5. Retention furrow in incisua?
  6. Discussion
  7. References

Retention furrows are produced during ontogeny by formation of the next larger valve while retaining the previous valve. Such furrows could be produced only in ostracodes with morphologically simple valves and can be observed only in cases of temporary retention, i.e. where the retained valve that produced the furrow in the next larger valve was later lost. In general, retention furrows are rather rare among ostracodes. They are observed in, for example, Platybolbina psednaKesling, 1960 (pl. 5, fig. 2; pl. 8, figs 1–3), P. chalaziaKesling, 1960 (pl. 7, fig. 3), P. compsaKesling, 1960 (pl. 4, fig. 1) and P. dictyotaKesling, 1960 (pl. 5, fig. 1; pl. 7, fig. 1). Other examples are Tvaerenella caesuraSchallreuter, 1993 (pl. 60A, fig. 1) and Conchoprimitia. In a very few cases, a retention furrow may also be developed for pathological reasons as, for example, in Frostiella pliculata (Schallreuter 1995, pl. 2, fig. 3).

In the examples mentioned above the retention furrow is developed mainly in the central region of the valve. Contrary to these species, Conchoprimitia is characterized by a retention furrow that is relatively distinct only in the anterodorsal (Hessland 1949, pl. 1, figs 1a, 2a; pl. 2, fig. 12a) and/or posterodorsal (Pl. 1, figs 3–4; Schallreuter 1993, pl. 16A, fig. 1P; pl. 18B, fig. 2) region of the valve.

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Figure TEXT-FIG. 3..  Schematic drawing of the both dimorphs of Incisua; left valves.

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Retention furrow in incisua?

  1. Top of page
  2. Abstract
  3. Sexual dimorphism in the genus incisua
  4. Retention furrows in palaeozoic ostracodes
  5. Retention furrow in incisua?
  6. Discussion
  7. References

The occurrence of a Conchoprimitia-like retention furrow (mainly anterodorsally) in Miniconchoides minutus has been mentioned previously by Schallreuter (1993, p. 124), and similar slight depressions in the anterior and posterior regions were found in Incisua ventroincisurata by Tinn and Meidla (in Tinn 2002, p. IV,14; Tinn and Meidla 2004, text-fig. 5I). However, the latter authors also considered the deep ventral furrow of Incisua ventroincisurata to be a retention furrow (Incisua-type), which according to them, occurs in ‘both adult and young instars’. (The term ‘young instars’ probably refers to the last preadult instars.)

However, this ventral furrow cannot be regarded as a retention furrow for the following reasons:

1. Judging from their (Tinn and Meidla 2004) text-figure 5I, the ventral furrow does not form the direct continuation of the anterior retention furrow. In both Conchoprimitia and Pariconchoprimitia the distance between the free marginal rim and the retained valve is broader ventrally than anteriorly (Pl. 1, figs 1–2; Tinn and Meidla 2004, text-fig. 5B–C), which is not the case in Incisua (Tinn and Meidla 2004, text-fig. 5H–I). Furthermore, the ventral furrow of Incisua is much deeper than the dorsal retention furrows. Tinn and Meidla explained the presence of the ventral indentation (‘score’) in Incisua, but considered its absence in Conchoprimitia to be a result of the ‘distinct ventral morphologies’, i.e. by the ‘slightly bulbous mid-ventral area in the left valve’ of Incisua. Because the ventral surface in Incisua is distinctly steeper than in Conchoprimitia, the formation of a retention furrow would be more difficult.

2. The ventral furrow is divided by a vertical buttress into a longer anterior part and a short posterior part (Pl. 1, figs 5–7). Although the posterior indentation behind the buttress is not very distinct in many specimens, the abrupt termination of the anterior indentation would be difficult to explain. The free margin of the valve forms a contact plane as in Conchoprimitia and would not form such a discontinuous retention furrow.

3. The ventral furrow is placed very close to the free margin, whereas in Conchoprimitia species with retained valves the free margin of the retained instar is always placed a certain distance from the free margin of the larger valve (compare Pl. 1, figs 1–2 with figs 5–6 or Tinn and Meidla 2002, fig. IV5E, B, H). Note, in Plate 1, figure 4 the angle formed by the retention furrow with the free margin and the high position the furrow would occupy in the ventral regions, and in Plate 1, figure 6 the close position to the free margin of the furrows in Incisua.

4. The ventral furrow is much deeper than the dorsal retention furrows. However, it would be vice versa if it were part of a continuous furrow.

Class OSTRACODA Latreille, 1802 Order, suborder and family incertae sedis

Incisua ventroincisurata (Hessland, 1949) Plate 1, figures 5–9; Text-figures 2–4
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Figure TEXT-FIG. 2..  Schematic drawing of the main morphological features of Incisua.

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1993 Incisua ventroincisurata (Hessland, 1949); Schallreuter, p. 126, pl. 20B, fig. 1; pl. 21; pl. 25A, fig. 2; pl. 25B [q.v. for further synonymy].

1993 Miniconchoides minutus (Hessland, 1949); Schallreuter, p. 124, pl. 22A, fig. 3; pl. 24B, fig. 4; pls 26B, 27; pl. 28A, fig. 2; pl. 62B, fig. 4 [q.v. for further synonymy].

2002 Incisua ventroincisurata (Hessland, 1949); Tinn, pp. 13, 24, 26, 32, 34, 38, fig. 7; table 1 (p. 22), 3.

2002 Incisua ventroincisurata (Hessland, 1949); Tinn and Meidla in Tinn: pp. IV2, 4, 11–14; text-figs 2–3, 5E–I; tables 2–3; appendix 2.

2003 Incisua ventroincisurata; Tolmacheva et al., figs 2, 5 [faunal logs].

2003 Miniconchoides minutus; Tolmacheva et al., p. 35, figs 2, 5 [faunal logs].

2004 Incisua ventroincisurata (Hessland, 1949) = Miniconchoides minutus (Hessland, 1949); Tinn and Meidla: pp. 212–213, 215; text-figs 2–3, 5E–I; tables 2–3.

Holotype.  Carapace, Palaeontological Institution of the University of Uppsala (PIU), ar. os. 173, Hessland (1949, pl. 2, fig. 8). (Holotype of Conchoides minutus: PIU, ar. os. 200, Hessland 1949, pl. 3, fig. 3).

Type locality. Röjeråsvägen, Dalecarlia, Sweden; lower part of Hessland's stratum G (about 1·2 m above RI/G). (Holotype of Conchoides minutus: about 1·0 m above RI/G.)

Description.  Length up to 1·00 mm, left valve slightly larger than right valve (Pl. 1, fig. 8). Outline postplete, i.e. greatest height in posterior half. Both cardinal angles significantly larger than 90 degrees, anterior cardinal angle larger than posterior, giving valve a slight forward swing. Short and straight hinge-line, very small indistinct epicline dorsum, so that dorsal margin is very slightly convex. Anterior end of valve slightly tapering, posterior end broadly rounded. Greatest transverse convexity at mid-height, greatest convexity longitudinally in posterior half, making posterior area slightly bulbous. Ventral surface relatively steep. Sometimes weak rounded muscle spot slightly dorsal of centre in a very weak depression (Pl. 1, figs 5–6, 9). Ventral surface of one dimorph (presumably the male) with a distinct furrow that is divided posteroventrally by a buttress into two parts, a longer anterior indentation and a short posterior impression. Surface unornamented.

Remarks.  Within both Miniconchoides minutus and Incisua ventroincisurataHessland (1949, pp. 172–178) distinguished two types, which he considered as males and females, mainly differentiated by gibbosity. The suggested female is proportionally more gibbous than the assumed male. These differences, if they really exist, are considered here as mere intraspecific variation. Both species are now considered as one, which exhibits a special, hitherto unknown kind of sexual dimorphism. The specimens with the ventral furrow (Incisua ventroincisurata) are considered as heteromorphs, those without such a furrow (Miniconchoides minutus) as tecnomorphs.

The tecnomorphs are more common than the heteromorphs, which become larger and are not smaller than 0·69 mm (Text-fig. 1; Table 1). The instars (Text-figs 1, 4A) do not cluster into clear moult stages. Therefore, the heteromorphs either show a range of size variation in the adult stage or may also comprise preadults. The shape (L:H ratio) remains constant during ontogeny (Text-figs 1, 4B).

Table 1.   Number of specimens and size range of different collections of Incisua ventroincisurata.
AuthorNominal speciesNo. of specimensSize range [measured specimens]
  • *

    Only samples counted which contain also Incisua ventroincisurata.

HesslandConchoides minutus, ‘male and female type’15600·42–0·90 mm [30]
C. ventroincisuratus, ‘male and female type’250·74–1·00 mm [10]
Tolmacheva et al. (2003)Miniconchoides minutus50* 
Incisua ventroincisurata6 
this paperMiniconchoides minutus> 2000·33–0·81 mm [112]
Incisua ventroincisurata430·69–0·85 mm [13]

Discussion

  1. Top of page
  2. Abstract
  3. Sexual dimorphism in the genus incisua
  4. Retention furrows in palaeozoic ostracodes
  5. Retention furrow in incisua?
  6. Discussion
  7. References

In fossil taxa lacking comparable recent counterparts it is difficult to identify the sex of either dimorph. This was the reason why Jaanusson and Martinsson (1956, p. 402) introduced the terms heteromorph (nominally the ‘female’) and tecnomorph. As tecnomorphs they designated ‘specimens belonging to preadult instars as well as adult specimens, which are essentially similar to preadult instars, and which do not show the dimorphic characters of heteromorphs’.

The ratio between numbers of the two dimorphs may provide a clue for the sexual determination. In recent adult ostracode populations females generally outnumber males (Pokorný 1958, p. 101; 1965, p. 105), but not always (Hartmann 1966, p. 36).

When trying to determine the sex in the Silurian Beyrichia tuberculata authors of the nineteenth century compared the heteromorphs with all tecnomorphs (larvae and adults) and came to the conclusion that the sex with the pouch is less frequent and thus the male type. However, when comparing only adults, the sex with the pouch forms the majority and is therefore the female dimorph. That this interpretation is correct has been proven by other evidence, e.g. a pouch filled with the youngest instars in the Silurian Craspedobolbina clavata (Hessland 1949; Spjeldnaes 1951; Martinsson 1956), and in the Silurian Bolbiprimitia inaequalis (Martinsson 1962). With regard to Incisua, the dimorph with the ventral furrow is less frequent and therefore considered as male. Incisua is one of the few examples where the heteromorphs are of the male type and the first example among Ordovician ostracodes.

The type with the ventral furrow (male type) is slightly larger than the tecnomorphs. This is in accordance with many other Ordovician (and Silurian) ostracode species in which the males are larger (e.g. Hinz-Schallreuter and Schallreuter 1998, figs 59, 66, 77, 96).

The furrow in the valves of the males probably served as a means for the female to clamp onto the otherwise rather smooth male carapace/valves during the mating process. In this respect the mating behaviour is assumed to have been similar to Bicornucythere bisanensis in which mating takes place in a ventral to ventral position (Abe and Vannier 1991, fig. 6f).

According to Hartmann (1975, p. 635), little is known about the mating process of Recent ostracodes. It has been documented for some Podocopa where it does not seem to be a uniform process. In some cases, male and female copulate with their ventral margins being in contact with each other. In other cases the male meets the female dorsal margin with its ventral side (Hartmann 1975, p. 637).

Although copulatory habits of fossil species cannot be precisely determined, as noted by Kesling (1969, p. 273), the latter discussed mating positions in extinct ostracodes because certain positions were feasible while others were highly improbable. For velar dimorphic palaeocopes he reconstructed both ventral and dorsal copulatory positions. He did not deal with non-palaeocopes.

Acknowledgements.  We thank the reviewers, Mark Williams and Tonu Meidla, for critical comments and Mark Williams for improving the manuscript linguistically.

References

  1. Top of page
  2. Abstract
  3. Sexual dimorphism in the genus incisua
  4. Retention furrows in palaeozoic ostracodes
  5. Retention furrow in incisua?
  6. Discussion
  7. References
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  • HARTMANN, G. 1966. Dr. H. G. Bronns Klassen und Ordnungen des Tierreichs Fünfter Band: Arthropoda I. Abteilung: Crustacea 2. Buch, IV. Teil 1. Geest and Portig, Leipzig, pp. 1216.
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