Pasternakevia podolica Selden and Drygant, 1987
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publication ID |
https://doi.org/ 10.4202/app.2008.0074 |
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persistent identifier |
https://treatment.plazi.org/id/1F1487D5-E304-FFC6-454A-78ACFC76FE9B |
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treatment provided by |
Felipe |
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scientific name |
Pasternakevia podolica Selden and Drygant, 1987 |
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Pasternakevia podolica Selden and Drygant, 1987
Figs. 1 View Fig , 3–5 View Fig View Fig View Fig .
Diagnosis.—The same as for the genus, by monotypy.
Material.—Specimen 1: part ISEA I−F/MP/2a/1499/08 (positive, convex cast; Fig. 3A View Fig ) and counterpart ISEA I−F/MP/2b/ 1499/08 (negative, concave cast; Fig 3B View Fig ); both parts housed in ISEA. Measurements (mm). Specimen 2: negative cast ISEA I−F/MP/3/1499/08 ( Fig. 4A View Fig 1 View Fig ), eight tergites of opisthosoma preserved. Specimen 3: part WNoZ/S/3/40 ( Fig. 4B View Fig 2) (positive), and counterpart (negative) ISEA I−F/MP/1/1499/08 ( Fig. 4B View Fig 3 View Fig ). In both part and counterpart a complete specimen of Baltoeurypterus tetragonophthalmus Fischer, 1839 is preserved ( Fig. 4B View Fig 1 View Fig ).
All specimens from Zalissia village, 12 km north of Kamenets Podilsky, Ukraine, Podolia; Ustye Suite series of the Bagovytsa horizon, lowermost Leintwardinian, Ludlow, Silurian.
Measurements (mm).—Specimen 1: Entire length of complete specimen 29.0; carapace 12.8 long and 16.1 wide; opisthosoma (part) 16.6 long. Specimen 2: length 15.2, greatest width 14.1. Specimen 3: entire length 34.1; carapace 15.2 long and 22.0 wide; opisthosoma 18.4 mm long. Description.—The first specimen comprises a carapace and the first nine tergites of an opisthosoma in positive (convex) part ISEA I−F/MP/2a/1499/08; in the negative ISEA I−F/MP/ 2b/1499/08, all 10 tergites and the base of the telson are preserved. This is the most complete specimen of the species and genus yet described. The body is preserved in a flexed attitude, the opisthosoma being arched in the region of tergites 2–3 and depressed around tergite 5. Length ratio of carapace to opisthosoma is 1.22, but this value is probably underestimated, as the opisthosoma is flexed, and thus shortened sagittally. Carapace is 1.3 times wider than long, and dis−
doi:10.4202/app.2008.0074
tinctly wider than the opisthosoma. Anterior and lateral portions are rounded; major central part strongly convex, separated from narrow crescent flat margin by inconspicuous ridge, which is less pronounced in front, central section ( Fig. 3C View Fig ). Surface of carapace seemingly smooth, without other ridges or grooves visible. Two small darkened spots are traceable on a ridge in the positive part, and their symmetrical position may suggest a pair of anterior eyes ( Fig. 3D View Fig ). However, no trace of lensar structures was detected within these spots or elsewhere on carapace (for further comment see the discussion chapter). Centrally situated on the anterior margin a round structure is detectable on both parts. The posterior border of the carapace (in contact with the opisthosoma) is slightly deflected anteroproximally from genal cornua, which are rounded, prominent, delimited by small incisions in lateroposterior margin of carapace.
The opisthosoma comprises 10 tergites, entirely preserved in negative cast ( Fig. 3B View Fig ; reconstruction Fig. 3C View Fig ). The first tergite (microtergite) is hidden under the carapace and only lateral portions are partially detectable (m on Fig. 3E View Fig ). The second tergite hypertrophied (almost twice length of the third, sagittally), strongly convex, ellipsoidal with lateral portions bent about 50 ° posteriorly; tergite 3 equally wide, but almost half as long. A posterior margin of tergite 2 has two small incisions in distal margin, matching the proximal margin of an adjacent tergite 3. Tergites 2 and 3 are flexed up exposing area of articulation preserved as two darker margins; the mid portion of this area is filled with sediment ( Fig. 3E View Fig ). Both outlines of these tergites combined form a complete regular ellipse; also their adjacent pleurae are directed to each other, suggesting close functional fit. The tergite 4 not much shorter than the preceding one, and twice the length of subsequent tergites. The axial region beyond tergites 2 and 3 gradually narrowing, except for tergites 5 and 6, which are of the same width (transversely). In the region of tergite 6 the body is flexed down, so that this tergite is seemingly narrowest and constricted medially, being partially hidden by the preceding one; the area of superimposition is detectable as darker strip. Distinct axial furrows reach tergite 8, but are less developed than in preceding segments; in the tergite 9 the furrows are still more shallow, and in tergite 10 not detectable. This last tergite is semicircular, forming an entity with its pleurae, and partially encloses the base of telson. The base is preserved in cross−section, exposing a raised core inside the base ( Fig. 3F View Fig ). The outline of the base is oval shaped, with an upper margin more arched, and laterally flanked by two small teeth directed posterolaterally, which are possibly remnants of lateral ridges flanking the telson, preserved in cross section. This appearance suggests that the telson was originally preserved in upturned position (plausible reconstruction of this position in Fig. 3G View Fig ). Opposite the base of telson, and in its direct prolongation, a widely triangular structure is detectable within the tergite 10 (s on Fig. 3F View Fig ). It might be identified as a portion of the posterior margin of tergite 10 compressed by the upturned telson. From this structure a brown triangle extends from over the entire tergite 10; the apex of the triangle almost reaches the middle of tergite 9 and is also detectable in the positive cast. This colored area may indicate a part of the system of muscles underneath connected with the telson. The initial width of the telson must have been about half that of last tergite. In spite of our efforts, no further portion of the telson was found.
Pleurae are well developed in all tergites 2–10, shortest in a second tergite (proportionally and also absolutely—only 0.2 times its width). Subsequent pleurae are longer (0.3–0.5 times width of axial region of respective tergite) and, beginning from tergite 4, directed posteriorly, with the angle of inclination growing gradually in the more posterior segments. Pleurae have sculpture: central portions are raised and delimited from thinner margins by sharp ridges. Basal portions adjacent to axial furrows appear more robust.
Specimen ISEA I−F/MP/3/1499/08 has only first eight opisthosomal tergites preserved in excellent condition ( Fig. 4A View Fig 1 View Fig ); the absence of the carapace suggests a molt. If so, it is noteworthy that the microtergite was left with opisthosoma after molting. Earlier views on the function of this small tergite held that it functionally belongs to a carapace (Eldredge 1974). The microtergite is completely exposed and has a shape of a narrow, crescentic strip devoid of pleurae and as wide (transversely) as axial region of tergite 2. The anterior margin of tergite 3 is hidden under tergite 2. The general shape of the opisthosoma is congruent with that in specimen ISEA I−F/MP/2a/1499/08. Also in this specimen the tergites 5 and 6 are narrowest (sagittally). Axial furrows are deep only in first four full tergites (2–5), gradually becoming more shallow in tergite 6, and almost flat in tergites 7 and 8. Furrows are convergent in tergites 3 and 4, parallel in tergite 5 and 6, and slightly convergent in two subsequent ones. Pleurae of tergites 4–9 pointed, more elongated than in specimen ISEA I−F/MP/2a/1499/08, reaching half of axial region width in tergites 5–8 (comparison of shapes in next section). Central regions of pleurae are more robust and delimited from thinner margins not only by ridges, but also by darker colour, especially so in tergites 5 and 6 ( Fig. 5A View Fig ).
The third specimen comprises a carapace and nine opisthosomal tergites in part WNoZ/S/3/40 and counterpart ISEA I−F/MP/1/1499/08. Both parts are in poor condition, showing initial stages of decomposition ( Fig. 4B View Fig 2, B 3 View Fig ). The carapace is only weakly connected with the opisthosoma; tergites are greatly flattened, so that axial furrows are barely detectable; also, the borders between tergites are partially obscured due to their superimposition. The counterpart (negative) has right pleurae of tergites 2–6 torn off. The carapace is large, ca. 1.4 times wider than long, ellipsoid, spatulate, without any superficial structures preserved. The details of the posterior margin are not preserved. The traces of the edge of the microtergite are poorly visible in both parts of opisthosoma (m on Fig. 4B View Fig 2, B 4 View Fig ). General appearance of the opisthosoma similar to that in both previous descriptions: tergite 2 hypertrophied, with two small incisions in the distal margin, and matching the proximal margin of an adjacent tergite 3; the tergite 3 equally wide, but almost half as long. Tergite 4 only little shorter than preceding, but conspicuously narrower; tergites 5 and 6 are narrowest (sagittally), the former seems biconcave (constricted medially); subsequent tergites are slightly longer. Axial furrows in part WNoZ/S/3/40 greatly flattened; better detectable in a counterpart ISEA I−F/MP/1/1499/08. A reconstruction ( Fig. 4B 4 View Fig ) is based on both parts.
Comparison of four specimens described.—The differences between the four known specimens of Pasternakevia podolica relate to the shape of the carapace and the pleurae. Carapaces of two specimens described here are conspicuously wider than the corresponding opisthosoma. Their length/ width ratio equals 1.3 and 1.4, respectively, when compared to 1.1 in the holotype ( Fig. 1 View Fig ). A poor state of preservation of the specimen WNoZ/S/3/40 (positive part) and ISEA I−F / MP/1 /1499/08 (negative) suggests that some of the extra width of this carapace might be gained in a result of partial decomposition and doi:10.4202/app.2008.0074
compression. On the other hand, the differences between the holotype and specimen ISEA I−F / MP/2 a/1499/08 are most probably genuine, considering a very good state of their preservation, especially of the posterior margin of carapaces .
Regarding the shape of pleurae, some discrepancy is observed in the original illustrations of the holotype. The reconstruction ( Selden and Drygant 1987: fig. 3d; redrawn herein as Fig. 1 View Fig ) is based on one photograph and shows very short pleurae, and axial region oval and relatively wide. On the other hand, the second photograph of the same specimen ( Selden and Drygant 1987: fig. 3b), made under low angle light, shows the axial region more narrow and the furrows parallel−sided in tergites 6 and 7; accordingly, the pleurae are longer. This latter view is also conveyed in the schematic drawing of Pasternakevia ( Anderson and Selden 1997: fig. 2F). In our opinion this discrepancy may be explained by the structure of the pleurae, in which, under compression, a “pseudofurrow” may appear between the robust basal portion, and the more delicate remainder. The “pseudofurrows” aligned form a row parallel and distal to axial furrows. It is conceivable that in the first photograph of the holotype the rows of pseudofurrows appeared more distinct than real axial furrows, and were pictured in the reconstruction. In our two first described specimens the pseudofurrows did not appear, probably because of relatively low compression applied to the specimens during fossilization. However, the borders between both portions of pleurae are to be detected (arrow in Fig. 5 View Fig ). The pseudofurrows were illustrated in nearly all “Synxiphosura” (and also are present in the schematic drawings in Anderson and Selden 1997: fig. 2). The most complex system of furrows is documented in the photograph of Cyamcephalus cf. C. loganensis , which shows even three rows of furrows ( Eldredge and Plotnick 1974: fig. 2). These authors regard the axial furrows as real structures, and the other furrows “may be (but in our opinion, probably are not) compressional features” ( Eldredge and Plotnick 1974: 8).
On comparing the pleurae of all specimens of Pasternakevia , we state that the pleurae of the holotype are comparable with those of specimen ISEA I−F / MP/2 a/1499/08 (the specimen WNoZ/S/3/40 is greatly flattened and hence not taken in consideration), while the pleurae of specimen ISEA I−F / MP/3 /1499/08 are distinctly more elongated ( Fig. 5A, B View Fig ). The axial regions of these specimens are in very good condition and retain much of their natural convexity; the pleurae are perfectly symmetric. Thus we assume that these differences are plausibly genuine. What is the primeval cause of this variation, remains to be explored .
Summarizing, on assumption that the characters of the carapace and pleurae were present in living specimens, three morphotypes may be discerned among the four specimens described: Morphotype 1: Holotype with smaller carapace and short pleurae; Morphotype 2: Specimen ISEA I−F / MP/2 a/1499/08 (and possibly WNoZ/S/3/40) with larger carapace and same short pleurae; Morphotype 3: Specimen ISEA I−F / MP/3 / 1499/08 with longer pleurae, and the carapace unknown.
The variation described may be due either to sexual dimorphism, or intra−specific variation, or might indicate the different species.
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