INTRODUCTION

In the first half of the last century, Kakhadze (1937) described an ammonite assemblage from the upper Bajocian of western Georgia, mainly from the historical area of Okriba. This work was primarily focussed on representatives of the genus Parkinsonia: a new subgenus Okribites was established, with two new species; representatives of the nominal subgenus were also characterized, and two new taxa, a species and a variety, were described in that subgenus.

In subsequent decades, the taxa of the species group established by Kakhadze were not revised using new material from the type or any other locality, and the name Okribites was treated by researchers as a genus (Arkell, 1957; Besnosov and Kutuzova, 1982; Krymholts et al., 1958) or as a junior subjective synonym of Parkinsonia (Énay, Howarth, 2019).

New collections from the Bajocian of Karachay-Cherkessia (Northern Caucasus) make it possible to clarify the diagnosis and relationships of two taxa described by Kakhadze from Transcaucasia as Parkinsonia djanelidzei Kakhadze and P. djanelidzei var. dertshiensis Kakhadze.

LOCALITY AND MATERIAL

The ammonites discussed in the article were collected during fieldwork in 2015–2020 from in an outcrop on the left bank of the Kyafar River (left tributary of the Bolshoi Zelenchuk River) 1.8 km upstream of its mouth, in an area more than 150 m long (locality 25; Fig. 1). Here, the outcrop shows compacted gray and dark gray mudstone-like clays, with scattered nodules 5–7 cm in diameter, sometimes forming interlayers. The apparent thickness of the outcrop (Fig. 2) is not less than 5 m, taking into account the bed dip (5° NNE). The locality yielded the ammonites Parkinsonia djanelidzei Kakhadze [M], P. dertshiensis Kakhadze [m], rare Rarecostites donezianus (Borissjak) [m], Lissoceras sp., and relatively numerous Dinolytoceras zhivagoi Besnosov. In addition, infrequent belemnite rostra Dicoelites aprilis Dzyuba, D. octobris Dzyuba, and others (Dzyuba et al., 2019; 2021) and rare bivalves come from this locality. Subvertically buried remains of horsetail plants (Naugolnykh and Mitta, 2016) are also found in this locality.

Fig. 1.
figure 1

Ammonite locality: (a) general scheme, the frame shows the outline of the detailed map; (b) map showing Locality 25.

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Fig. 2.
figure 2

The left bank of the Kyafar River, part of locality 25 (Parkinsonia parkinsoni Zone, Beds with Parkinsonia djanelidzei).

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Ammonites are represented by molds, sometimes with shell remains, but most often deformed or incompletely preserved. The interval of the section is identified as Beds with djanelidzei corresponding to the middle part of the Parkinsonia parkinsoni Chronozone (Mitta et al., 2017).

The studied specimens are kept in the Borissiak Paleontological Institute of the Russian Academy of Sciences (PIN RAS), coll. no. 5546.

DISCUSSION

Representatives of the family Parkinsoniidae (species of Parkinsonia and Rarecostites) are important for detailed biostratigraphy in a short list of ammonite taxa characteristic of the Beds with djanelidzei. An incompletely preserved specimen of Lissoceras (Haploceratoidea: Lissoceratidae) and Dinolytoceras (Lytoceratoidea: Lytoceratidae) common, but transient for the Upper Bajocian, are also not particularly significant for biostratigraphy.

It should be noted that Locality 25 is the only known section of the Upper Bajocian–Lower Bathonian in the Kuban–Urup interfluve (Fig. 1) where not a single specimen of Phylloceratoidea was found during all visits. Most likely, this is due to the shallowness of these deposits of coastal origin.

During the study of the site, only four specimens of Rarecostites donezianus were found (Mitta et al., 2017, pl. 1, fig. 4; here, fig. 3), represented only by microconchs of this dimorphic species (Mitta, 2017; Mitta and Bakaryukina, 2020). The rarity of the finds suggests that the diversification of Rarecostites (very numerous down the section, in the sherstyukovi and subarietis faunal horizons) had already ended in the Northern Caucasus by the time of accumulation of the beds under discussion.

Fig. 3.
figure 3

Rarecostites donezianus (Borissjak), microconch, specimen PIN, no. 5546/229: (a) lateral view, (b) ventral view; Karachay-Cherkessia, Zelenchuk District, left bank of the Kyafar River, Locality 25; Upper Bajocian, Parkinsoni Zone, Beds with Parkinsonia djanelidzei; coll. 2017. Scale bar 10 mm.

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Most important for study is the dimorphic pair belonging to the genus Parkinsonia—P. djanelidzei [M]/P. dertshiensis [m]. In addition to being important for biostratigraphy, these two species appear to be the first representatives of Parkinsonia s. str. in the Caucasus. At Locality 25, the number of finds of these taxa in a sufficient state of preservation for a confident identification exceeds two dozen, and the same number of fragments can be counted among them in open nomenclature.

In the same publication where P. (Parkinsonia) djanelidzei Kakhadze (Kakhadze, 1937, pp. 95, 140, 177) Footnote 1 and P. (P.) djanelidzei Kakhadze var. dertshiensis (ibid., pp. 98, 142, 179), the subgenus Okribites was first proposed, including the new species Parkinsonia (O.) okribensis Kakhadze (ibid., pp. 106, 146, 183) and P. (O.) djanelidzei Kakhadze (ibid., pp. 108, 148, 184). According to the International Code of Zoological Nomenclature (1999), identical species-group names established for different nominal taxa in initial combination with the same generic name are primary homonyms (ICZN Art. 57). According to the Principle of the First Reviser, I propose to recognize the name P. (Parkinsonia) djanelidzei as the senior primary homonym and valid, and the name P. (Okribites) djanelidzei as a junior primary homonym, permanently invalid.

The name Okribites Kakhadze was later treated both as a subgenus in the genus Parkinsonia (Arkell, 1957, p. L309), and as a generic name in its own right (Besnosov and Kutuzova, 1982, p. 48; Krymholts et al., 1958, p. 79; Topchishvili et al., 2006, p. 346). In the revised edition of “Treatise …” (Énay and Howarth, 2019, p. 15) it is provisionally treated as a junior synonym of Parkinsonia; I think that this is the most acceptable solution pending the study of additional material from the type locality.

Below the dimorphic pair P. djanelidzei [M]/ P. dertshiensis [m] is described.

  • SYSTEMATIC PALEONTOLOGY

  • Superfamily Perisphinctoidea Steinmann, 1890

  • Family Parkinsoniidae Buckman, 1920

  • Subfamily Parkinsoniinae Buckman, 1920

  • Genus Parkinsonia Bayle, 1878

  • Parkinsonia djanelidzei Kakhadze, 1937 [M]

  • Plate 7, figs. 1–4

Parkinsonia (Parkinsonia) djanelidzei: Kakhadze, 1937, pp. 95, 140, 177, text-fig. 4, pl. 7, figs. 1, 2.

Parkinsonia djanelidzei: Topchishvili et al., 2006, pl. 31, figs. 4, 5.

Parkinsonia djanelidzei [M]: Mitta et al., 2017, pl. 1, fig. 5.

non Parkinsonia (Okribites) djanelidzei: Kakhadze, 1937, pp. 108, 148, 184, pl. 8, fig. 3 (junior primary homonym; name invalid).

Lectotype. Specimen figured by Kakhadze (1937, pl. 7, fig. 2; Topchishvili et al., 2006, pl. 31, fig. 5); Georgia, Okriba area, vicinity Dzmuisi; upper Bajocian; coll. by I.R. Kakhadze; designated here.

figure a

Explanation of Plate 7

Figs. 1–5 . Parkinsonia djanelidzei Kakhadze [M]: (1) specimen PIN, no. 5546/227, lateral view; (2) specimen PIN, no. 5546/329, lateral view; (3) specimen PIN, no. 5546/332, lateral view; (4) specimen PIN, no. 5546/327: (4a) lateral view, (4b) ventral view; (5) specimen PIN, no. 5546/128: (5a) lateral view, (5b) ventral view; Karachay-Cherkessia, Zelenchuk District, left bank of the Kyafar River, Locality 25; upper Bajocian, Parkinsoni Zone, Beds with Parkinsonia djanelidzei; the author’s coll. 2015–2020. Scale bar 10 mm; asterisk (*) marks the beginning of the body chamber.

Description. The shells of adult specimens reach a diameter of 180 mm. With Dm up to 20 mm, whorl of medium width, with a rounded cross-section. Subsequently, the height of the whorl exceeds its width, and the section becomes ellipsoidal with the greatest width in the umbilical region. The umbilicus is wide and shallow; the umbilical wall is not high; the umbilical shoulder is rounded. The body chamber of young specimens occupies about one whorl; the apertural margin is simple, the apertural constriction is well expressed on the mold.

The ornamentation is represented by raised and subradial primary and thinner, prorsiradiate secondary ribs that break off on the venter to form a median groove (or strip, in adults). At the early stages, the ribs are bipartite, with weakly pronounced nodes at the branching point, which disappear at Dm = 30–35 mm. When Dm = 40–50 mm, intercalating ribs appear between the bipartite ribs; with the further growth of the shell, some intercalating ribs become connected to the branches of the bipartite ribs, forming tripartite ribs. The ribbing coefficient on an adult shell varies from 2.5 to 2.9. The spaces between the primary ribs noticeably widen with age. When Dm is about 100 mm, the ends of the branches, approaching the median groove alternately on each side, begin to be located opposite, and then gradually smooth out.

Dimensions in mm and ratiosFootnote 2:

Specimen no.

Dm

WH

WW

UW

WH/Dm

WW/Dm

UW/Dm

5546/327*

111

36

26.5

51

0.32

0.24

0.46

 

92

33

24.5

40

0.36

0.27

0.44

5546/227

102

31

26

44

0.3

0.25

0.43

5546/333

86

29

21

35

0.34

0.24

0.41

5546/332

52

16.5

15

23.5

0.32

0.29

0.45

Variability. The differences are manifested primarily in small variations in the shape of the cross-section (the greatest whorl width may be in the mid-flank) and in the ribbing coefficient.

Comparison. The alternation of bipartite and intercalating ribs on adult whorls, which then transform into tripartite ribs, readily distinguishes the described species from the rest of the Upper Bajocian species of Parkinsonia.

Remarks. The holotype has not been designated. The author of the species illustrated two of the five specimens of the type series. A more complete shell (Kakhadze, 1937, pl. 7, fig. 1) is distorted. Therefore, I have designated an incomplete specimen, showing the shape of the whorl section as the lectotype. Conclusions about the final size and shape of the aperture of the shell were made based on the unillustrated fragment of the body chamber (specimen PIN, no. 5546/335).

Material. Twelve relatively complete specimens and about 20 fragments of body chambers of various sizes and degrees of preservation; Karachay-Cherkessia, locality 25; Upper Bajocian, Djangura Formation, middle part of the Parkinsonia parkinsoni Zone.

  • Parkinsonia dertshiensis Kakhadze, 1937 [m]

  • Plate 8, figs. 1–5

Parkinsonia djanelidzei var. dertshiensis: Kakhadze, 1937, p. 98, 142, 179, pl. 6, fig. 2.

Parkinsonia djanelidzei [m]: Mitta et al., 2017, pl. 1, fig. 6.

Lectotype. Specimen illustrated by Kakhadze (1937, pl. 6, fig. 2); Georgia, Okriba area, vicinity of the village of Derchi; Upper Bajocian; coll. by A.I. Djanelidze; designated here.

figure b

Explanation of Plate 8

Figs. 1–5 . Parkinsonia dertshiensis Kakhadze [m]: (1) specimen PIN, no. 5546/142, body chamber: (1a) lateral view, (1b) ventral view; (2) specimen PIN, no. 5546/330: (2a) lateral view, (2b) apertural view, (2b) ventral view; (3) specimen PIN, no. 5546/331, young specimen: (3a) lateral view, (3b) apertural view; (4) specimen PIN, no. 5546/328, body chamber, lateral view; (5) specimen PIN, no. 5546/228: (5a) lateral view, (5b) ventral view; Karachay-Cherkessia, Zelenchuk District, left bank of the Kyafar River, Locality 25; upper Bajocian, Parkinsoni Zone, Beds with Parkinsonia djanelidzei; the author’s coll. 2015–2020. Scale bar 10 mm; asterisk (*) marks the beginning of the body chamber.

Description. The shells of adult specimens reach a diameter of about 100 mm. At Dm up to 20 mm, the whorls are of medium thickness, with a rounded section; the height of the whorl is only slightly greater than its width. Subsequent whorls (including the body chamber of adult specimens) are flattened, ellipsoidal in section, with the greatest width at mid-flank or closer to the umbilical margin. The umbilicus is wide and shallow. The umbilical wall, which falls steeply at young whorls, becomes flatter in the adult body chamber. Constrictions are irregular, very weakly expressed. The body chamber occupies 0.6 whorls, the aperture has a short lappets. At Dm = 20 mm, bipartite ribs are observed, slightly inclined forward. In the mid-venter, to which the branches of the ribs come alternately on each side, they break off, forming a smooth median stripe. At Dm about 50 mm, intercalating ribs appear among bipartite ones, increasing in number with age and sometimes connecting with bipartite ribs, forming tripartite ribs. Thereafter, in the adult body chamber, the ribbing coefficient increases from 2.0 to 2.5–2.7, and the gaps between the primary ribs noticeably increase.

Dimensions in mm and ratios:

Specimen no.

Dm

WH

WW

UW

WH/Dm

WW/Dm

UW/Dm

5546/142*

96

26.5

21

42

0.27

0.22

0.44

5546/328*

94

27.5

17

44

0.29

0.18

0.47

5546/330

67

20.5

16.5

29

0.31

0.25

0.43

5546/228

66.5

21

16

29

0.32

0.24

0.44

 

53.5

18.5

14

23

0.35

0.26

0.43

5546/331

33

11.5

9.8

14.5

0.35

0.3

0.44

Variability. Differences in shell shape are manifested in small variability in cross-sectional shape. Individual differences in the number of ribs on an adult shell are much more pronounced: the ratio of secondary to primary ribs on a half-whorl of the body chamber varies from 40/15 to 51/19.

Comparison. The described species is primarily distinguished from the macroconch P. djanelidzei Kakhadze by its small size and aperture with an auricle, as well as more compressed whorls. P. dertshiensis differs from other early representatives of the genus in the presence of intercalating and tripartite ribs, along with the bipartite ribs.

Remarks. No holotype of the species was designated. The author of the species, when establishing it, had two specimens; I have designated the only illustrated specimen as the lectotype. In each of the parts of Kakhadze’s work, each published in a different language, a different spelling of the name of the new “variety” (dertshyensis, dertshiensis, dertschiensis) is given. In the explanations for the photo table with the image of the lectotype, the spelling “dertshiensis” is used; in this paper, using the Principle of the First Reviser, I am choosing it as the correct spelling.

According to the similarity of the shell shape and the development of ornamentation and association, the species forms a dimorphic pair with P. djanelidzei.

Material. Ten specimens of varying degrees of preservation; Karachay-Cherkessia, Locality 25; Djangura Formation, Upper Bajocian, middle part of the Parkinsonia parkinsoni Zone.