Pteropelta huberti, Hints & Eriksson, 2010

Hints, Olle & Eriksson, Mats E., 2010, Ordovician polychaeturid polychaetes: Taxonomy, distribution and palaeoecology, Acta Palaeontologica Polonica 55 (2), pp. 309-320 : 313-316

Pteropelta huberti sp. nov.

Fig. 2J–AI.

1998 Polychaetura sp. A ; Hints 1998: 498–499, figs. 12E–G.

2000 Pteropelta sp. A ; Hints 2000: pl. 2: 1, 2.

Etymology: Named in honour of Hubert Szaniawski in recognition of his work on fossil scolecodont−bearing polychaetes.

Type material: Holotype: left MI, GIT 592−1 , Fig. 2X; paratypes: GIT 592−2 to 592−26 .

Type locality: Lelle D−102 borehole, central Estonia .

Type horizon: Vormsi Regional Stage (Katian).

Material.—Several hundred right and left MI, ca 50 basal plates, and seven pairs of carriers from Laeva 18, Lelle D−102, Rapla and Orjaku drill cores and Viru mine section.

Diagnosis.—Posterior maxillae of Pteropelta huberti are most doi:10.4202/app.2009.0086

similar to those of P. kielanae . The main differences concern (i) outline of the shank, which is sub−triangular in P. huberti , but sub−rectangular in P. kielanae ; (ii) inner wing in left MI, which is longer in P. huberti and with more slanting anterior margin; (iii) dentary in left MI, where the largest denticle is the second or third in P. huberti instead of fourth or fifth as in P. kielanae . Pteropelta huberti differs from P. gracilis in having almost straight posterior margin in left MI and straight or slightly convex anterior margin in carriers. P. huberti commonly differs from other species of Pteropelta also in having two anteriormost denticles in both MI transversally extended.

Description.—Right MI: L = 0.3–1.2 mm, L/W ratio = 2.0– 3.0. Anterior half of jaw considerably wider than posterior half. Outer−lateral margin is almost straight or with a small incurvature in the middle. Sub−rounded, more or less clubshaped ramus extends to 0.43–0.55 of jaw length from anterior. Bight more or less anteriorly concave; in smaller specimen it may be almost transverse and perpendicular to length axis. Outer margin along the shank is directed posteriorly in the first half of the shank, then it bends continuously and is directed towards the end of the dentary. Posteriormost termination generally developed into a small tubercle. Dentary comprises 11–17 sub−conical denticles, first three (particularly first two) of which may be prolonged transversally into short ridges. Denticles rather tightly packed and decrease gently in size particularly posterior of midlength where they are also more slanted. Posteriormost part of dentary commonly slightly dextrally bent. Inner margin sub−parallel to dentary and inner face is steep. Anteriorly inner face widens slightly, forming a more or less distinct inner wing that extends for approximately 0.9 of jaw length. Anteriormost termination of inner wing forms a more or less well−developed cove, anterior of which the inner face is very narrow in dorsal view. In ventral view the myocoele is almost gaping with a cover extending for 0.05–0.07 of jaw length.

Left MI: L = 0.33–1.1 mm, L/W ratio = 2.0–3.1. Sub−rectangular jaw with its widest part usually just anterior of midlength. Outer margin runs most often postero−laterally for 0.4–0.5 of jaw length, where it bends and continues towards the denticulated ridge, and then continues sub−parallel to dentary until it meets the posterior margin. Sometimes a small bight, similar in extension to that in right MI, can be seen. Below the ramus the edge of the outer face is thinner and less sclerotised which sometimes gives the outer margin a ragged, semi−translucent appearance. Such specimens in particular resemble the morphology of the ramus and bight of the right MI. Long, almost rectangular inner wing reaching to 0.70–0.75 of jaw length from posterior with an inner margin sub−parallel to dentary. Relatively wide, almost straight and transverse posterior margin forms 0.35–0.50 of jaw width. Dentary consists of 10–15 denticles, three first ones may be prolonged transversally like in right MI. If not prolonged, the first two are smaller than the third denticle. Dentary particularly elevated on a ridge in posterior half of jaw. In ventral view, myocoele is almost gaping, cover extends to approximately 0.05 of jaw length. Small denticle pits visible in the narrow furrow that corresponds to the dentary and which widens anteriorly.

Basal plate: Sub−trapezoidal element approximately half as long as right MI. L = 0.35–0.8 mm, L/W ratio varies from 2.0 to 3.3. Inner margin is longer than outer margin. In dorsal view, posteriormost termination usually has posteriorly−directed, narrow and blunt extension. Dentary houses 10–18

Fig. 2. Elements of jaw apparatuses of Ordovician polychaeturid polychaete Pteropelta kielanae ( Hints, 1998) , Pteropelta sp. A and Pteropelta huberti sp. nov. Ą All in dorsal view except U, W and AI which are in ventral view. A–C. Pteropelta kielanae ( Hints, 1998) A. Left MI, GIT 592−27, sample OM−169, Metsküla F−198 borehole, 35.12 m, Estonia, Haljala Stage, Sandbian. B. Right MI, GIT 592−28, sample OM−176, Metsküla F−198 borehole, 24.13 m, Estonia, Keila Stage, Sandbian. C. Basal plate, GIT 159−41, sample OM−135, Apraksin Bor borehole, 144.25 m, St. Petersburg Region, Russia, Haljala Stage, Sandbian.

D–I. Pteropelta sp. A . D. Left MI, GIT 592−29 , sample M−4416, Vasalemma 758 borehole, 8.65 m, Estonia, Keila Stage, Katian. E. Left MI, LO 10478 , sample 04E1−3, Stormyr−2 borehole, 213.65 m, Estonia, Pirgu Stage, Katian. F. Left MI, GIT 592−30 , Båticke−3 borehole, 313.6 m, Sweden, Pirgu Stage , Katian.

G. Right MI, GIT 592−31 , Båticke−3 borehole, 313.6 m, Sweden, Pirgu Stage , Katian. H. Basal plate, GIT 592−32 , Båticke−3 borehole, 313.6 m, Sweden, Pirgu Stage , Katian. I. Right MI, GIT 592−33 , sample OM98−43, Saku quarry, Estonia, Oandu Stage , Katian. J –AI. Pteropelta huberti sp. nov. J. Left MI, GIT 592−2 , sample OM03−44, Viru mine, Estonia, Uhaku Stage , Darriwilian. K. Right MI, GIT 592−3 , sample OM03−44, Viru mine, Estonia, Uhaku Stage , Darriwilian. L. Left MI, GIT 592−4 , sample OM03−49, Viru mine, Estonia, Uhaku Stage , Darriwilian. M. Left MI, GIT 592−5 , sample OM03−49, Viru mine, Estonia, Uhaku Stage , Darriwilian. N. Right MI, GIT 592−6 , sample OM03−43, Viru mine, Estonia, Uhaku Stage , Darriwilian. O. Basal plate, GIT 592−7 , sample OM03−43, Viru mine, Estonia, Uhaku Stage , Darriwilian. P. Left MI, GIT 592−8 , sample M96−78, Orjaku borehole, 107.4 m, Estonia, Rakvere or Nabala Stage , Katian. Q. Right MI, GIT 592−9 , sample M96−78, Orjaku borehole, 107.4 m, Estonia, Rakvere or Nabala Stage , Katian. R. Right MI, GIT 592−10 , sample M96−40, Laeva 18 borehole, 228.08 m, Estonia, Nabala Stage , Katian. S. Left MI, GIT 592−11 , sample D−102/49, Lelle D−102 borehole, 146.18 m, Estonia, Vormsi Stage , Katian. T. Left MI, GIT 592−12 , sample M96−34, Laeva 18 borehole, 221.35 m, Estonia, Vormsi Stage , Katian. U. Left MI, GIT 592−13 , sample M96−35, Laeva 18 borehole, 223.4 m, Estonia, Vormsi Stage , Katian. V. Left MI, GIT 592−14 , sample D−102/49, Lelle D−102 borehole, 146.18 m, Estonia, Vormsi Stage , Katian. W. Right MI, GIT 592−15 , sample M96−35, Laeva 18 borehole, 223.4 m, Estonia, Vormsi Stage, Katian. X. Left MI, holotype, GIT 592−1 , sample D−102/49, Lelle D−102 borehole, 146.18 m, Estonia, Vormsi Stage, Katian. Y. Basal plate, GIT 592−16 , sample D−102/49, Lelle D−102 borehole, 146.18 m, Estonia, Vormsi Stage, Katian. Z. Left MI, GIT 592−17 , Båticke−3 borehole, 315.0 m, Sweden, Pirgu Stage, Katian. AA. Right MI, GIT 592−18 , sample D−102/49, Lelle D−102 borehole, 146.18 m, Estonia, Vormsi Stage, Katian. AB. Right MI, GIT 592−19 , sample D−102/49, Lelle D−102 borehole, 146.18 m, Estonia, Vormsi Stage, Katian. AC. Right MI, GIT 592−20 , sample D−102/49, Lelle D−102 borehole, 146.18 m, Estonia, Vormsi Stage, Katian. AD. Right MI, GIT 592−21 , sample D−102/49, Lelle D−102 borehole, 146.18 m, Estonia, Vormsi Stage, Katian. AE. Carriers, GIT 592−22 , sample OM97−209, Laeva 18 borehole, 222.4 m, Estonia, Vormsi Stage, Katian. AF. Carriers, GIT 592−23 , sample OM97−209, Laeva 18 borehole, 222.4 m, Estonia, Vormsi Stage, Katian. AG. Carriers, GIT 592−24 , sample OM97−209, Laeva 18 borehole, 222.4 m, Estonia, Vormsi Stage, Katian. AH. Basal plate, GIT 592−25 , sample D−102/49, Lelle D−102 borehole, 146.18 m, Estonia, Vormsi Stage, Katian. AI. Basal plate, GIT 592−26 , sample M96−35, Laeva 18 borehole, 223.4 m, Estonia, Vormsi Stage, Katian. SEM micrographs, scale bars 100 µm .

doi:10.4202/app.2009.0086

subtriangular denticles of which the anteriormost generally is the largest one. In ventral view, cover comprises 0.5–0.7 of jaw width.

Carriers: Fused carriers are about 2.2 times longer than wide. The shaft of a single carrier has two posteriorly directed branches, the proximal one being 3 times longer than the distal one. Anterior margin of carriers are nearly straight.

Variability.—The outline, especially the length/width ratio varies considerably in the first maxillae. The anteriormost denticles of these elements, moreover, may be developed into short transverse ridges. In the basal plate, two types of denticulation occur: (i) all denticles are relatively large and of nearly equal size and shape, and (ii) anteriormost denticles are smaller and/or more slender than the remaining ones.

Remarks.—The development of the anteriormost denticles in the MI into transverse ridges is unique for Pteropelta huberti among the polychaeturids, but similar denticles are not uncommon for Ordovician scolecodonts. Two examples, where such ridges are developed in extreme form are seen in the jaws of Rhytiprion magnus Kielan−Jaworowska, 1966 (cf. Bergman et al. 2003) and Kaljoprion laevaensis Hints, 2008 . Moreover, the long−ranging Tretoprion astae Hints, 1999 and a species of Pistoprion ( Kielan−Jaworowska 1966: pl. 7: 1; Hints 2000: pl. 1: 2) possess similar ridges in their maxillae. Thus taxa having different types of jaw apparatuses (placognath and labidognath) could develop such denticles. Only in the case of P. huberti , however, the lack or presence of this feature can be considered as falling within the range of the intraspecific variability. It is noteworthy that the ridges in P. huberti are well developed only in specimens occurring in the uppermost part of its stratigraphical range (Vormsi and Pirgu stages, upper Katian), where it co−occurs with R. magnus , K. laevaensis , and T. astae . It is therefore likely that the development of such denticles in P. huberti had functional advantage, perhaps for certain types of diet or burrowing.

Stratigraphic and geographic range.— Pteropelta huberti ranges from the Uhaku to Pirgu regional stages (Darriwilian to Katian) in Baltoscandia and Poland, being most characteristic of the Nabala and Vormsi stages (middle Katian).