Podoliacanthus zychi, Voichyshyn & Szaniawski, 2012

Voichyshyn, Victor & Szaniawski, Hubert, 2012, Acanthodian jaw bones from Lower Devonian marine deposits of Podolia, Ukraine, Acta Palaeontologica Polonica 57 (4), pp. 879-896 : 886-890

Podoliacanthus zychi sp. nov.

Figs. 3, 4, 5A–F, H, 6A, B, D, G, H, 7A, B, 8B.

1999 Acanthodii indet.: Blom 1999: 73, fig. 41K.

Etymology: In honour of the late Dr. Władyslaw Zych (1899–1981), famous researcher of Early Devonian agnathan fauna of Podolia.

Holotype: ZPAL P.14/13.3, jaw bone in two pieces, 5.1 mm long.

Type locality: Left bank of the Dniester River, east vicinity of Ivanie Zolote, Zalishchyky rayon, Ternopil oblast’ (Podolia), Ukraine; Fig.2C.

Type horizon: Upper part of Ivanie Horizon of the Tyver Series, 3–4 m below the boundary with the Dniester Series, Lower Devonian.

Material.—Except for the holotype, 26 near−complete jaw bones or their fragments: ZPAL P.14/1.1, probable juvenile jaw bone with partly preserved posterior end, 1.7 mm long, with 6 or more teeth; ZPAL P.14/2.2, jaw bone with poorly preserved posterior end, 4.5 mm long, with 9 teeth from the beginning of lateral tooth row; ZPAL P.14/3.1, fragment of rather anterior jaw bone part, 1.5 mm long, with 3 lightly worn teeth; ZPAL P.14/3.3, fragment of central to anterior part of jaw bone, 3.9 mm long, with 6 worn or broken teeth; ZPAL P.14/4.2, fragment, 1.3 mm long, with 3 teeth; ZPAL P.14/6.1, fragment of anterior part of adult jaw bone, 2.9 mm long with 2 teeth, probably the largest ones; ZPAL P.14/6.5, fragment 0.6 mm long with 2 teeth; ZPAL P.14/7.2, jaw bone with preserved posterior end and broken anterior part, 2.9 mm long, with 5 teeth; ZPAL P.14/7.5, fragment of central part of jaw bone, 2.5 mm long, with 6 very worn or broken teeth; ZPAL P.14/7.6, fragment of central part of jaw bone, 2.1 mm long with 5 worn teeth, ZPAL P.14/7.10, fragment, 1 mm long, with 2 moderately worn teeth; ZPAL P.14/8.3, fragment of possible juvenile jaw bone, 1.3 mm long, with 4 teeth; ZPAL P.14/9.2, fragment 2 mm long, with 3 teeth; ZPAL P.14/9.3, fragment of jaw bone in two pieces, together 3.1 mm long, with 5 teeth and half of the 6th one; ZPAL P.14/9.5, straight jaw bone with partly preserved posterior end, about 2.5 mm long, with 7 or more teeth (posterior part is dirty and unavailable for observation); ZPAL P.14/9.11, jaw bone with almost preserved posterior end, 2.6 mm long, with 6 teeth; ZPAL P.14/11.2, fragment, 1.2 mm long, with 3 teeth of, possibly, juvenile individual; ZPAL P.14/11.11, straight jaw bone fragment with partly broken posterior end, 2.3 mm long, with 5 teeth and sixth one broken off; ZPAL P.14/12.4, posterior part of jaw bone, 1.7 mm long, with probably 4 teeth; ZPAL P.14/12.6, fragment, 1.5 mm long, with 3 or more worn and broken teeth; ZPAL P.14/12.7, fragment, 1.4 mm long, with 3 preserved teeth; ZPAL P.14/13.1, partial jaw bone with broken posterior and anterior ends, 5 mm long, with 7 more or less preserved teeth; ZPAL P.14/13.4, fragment, 2.2 mm long, with 4 worn or broken teeth; ZPAL P.14/13.5, posterior half of jaw bone, 2.9 mm long, with 7 teeth from the beginning of lateral row; ZPAL P.14/13.6, jaw bone posterior half, about 2.1 mm long, with 5 teeth; ZPAL P.14/13.8, fragment, 2.6 mm long, with 5 teeth.

Diagnosis.— Podoliacanthus species with elongate slender jaw bones. The teeth of the lateral tooth row have one accessory denticle on their medial side. Pores of the vascular canal system on jaw bone medial side are very rare or lacking. The inter−tooth pits are deep and bear pores of vascular canals. The posterior part of the jaw bone has moderately developed postero−lateral and shorter postero−medial processes.

Description.—Teeth of the lateral tooth row increase in size anteriorly, in usual as a rule for the ischnacanthiforms manner. The anterior edge, preserved only in the holotype, is smooth and arcuate in shape ( Fig. 3G). The foremost tooth is the largest in the lateral tooth row ( Fig. 3D). The teeth are conical in shape, gradually tapering to the tip, which is worn or broken in many cases. The upper third of each tooth is slightly inclined posteriorly ( Figs. 4F, 6B, D). Their width to height ratio is about 0.6–0.65. The tooth surface is smooth, but one denticle is attached medially near the tooth base or about 1/5 to 1/4 of the tooth height ( Figs. 3F, 4B, E, F, 5A, B, 6A, B, G, H). There is an isthmus between each tooth and its side denticle ( Figs. 5A, 6G, H). The isthmus crest sometimes is extended up the tooth side as a small short ridge ( Figs. 4E, 5A). The teeth have a variably expressed narrow posterior flange ( Figs. 3D, F, 4F, 6A, G), but it is often poorly preserved. The anterior flange on teeth in the lateral tooth row seems to be absent in this species, although there are two specimens in the material possessing probable remains of the flange, ZPAL P.14/3.1 and 9.2 ( Figs. 4E, 5A, 6B, D). These specimens are somewhat similar to Podoliacanthus sp. 1 (see below). The medial side denticles are about the same shape and proportions as the teeth. Their height averages a third to a quarter of the tooth height. Both the teeth ( Figs. 4A, B 1, F, 6G) and side denticles ( Fig. 3G) are hollow in their lower half or third. In one specimen, ZPAL P.14/3.3, the tip of the largest tooth appears worn and is crooked medially and posteriorly ( Fig. 5B).

The teeth are clearly separate in occlusal view from the medial side denticles, and are subcircular−triangular in cross section ( Figs. 4E, 6G, 8B). The medial side denticles are circular−oval in cross section ( Fig. 3G). In cross section, the joined tooth and side denticle is an elongate−triangular shape, and often is stretched diagonally in postero−anterior direction with the side denticle situated antero−medially ( Fig. 4B 1, C, E). This displacement of the side denticles may be observed along the entire lateral tooth row ( Fig. 4C), but sometimes it is more obvious on the anterior teeth than in the posterior (smaller and older) ones ( Fig. 4E). At the same time, the opposite trend in denticle position can be observed in the jaw bone of the supposed juvenile individual ( Figs. 4A, 8B). Possibly these opposite trends reflect upper or lower position of jaw bones.

The inter−tooth denticles usually are poorly preserved ( Figs. 3D, 4F, 6D). They form a continuous wall−like group between the teeth ( Figs. 4D, 6B). Sometimes these groups lack the posterior denticle ( Fig. 4B 2, C) and, in such case, together with the tooth (“main cusp”) placed anteriorly, may be considered as joined “multicuspidate tooth” in the classical understanding (see above, Material, methods and terminology). If this proves to be true, such a “tooth” in the described species consists of a “main cusp” and only posterior “secondary cusps”. The number of the inter−tooth denticles ranges from 3–4 in the supposed juvenile jaw bone ( Fig. 8B) up to 6–7 in mature jaws ( Fig. 6A, B). The denticles of the inter−tooth groups slightly increase in size in the anterior direction ( Figs. 4C, 6A, B).

The inter−tooth pits are comparatively deep and clearly defined ( Figs. 3A, F, 4A, 5E, F, 6G, 8B). Fairly large pores of vascular canals are present on the inter−tooth pit surface. They concentrate mainly along the perimeter of the pit, especially at the base of the teeth ( Figs. 3F, G, 4E, 6G, 8B).

http://dx.doi.org/10.4202/app.2011.0079

The jaw bone base (underside) forms a long trough for holding cartilage, as is typical for ischnacanthiform acanthodians ( Figs. 5A, C, D, H, 6A, 7A, B). Some of the longer specimens show a certain longitudinal curvature with a slightly convex lateral jaw bone side and concave medial side ( Figs. 3C, 4A, B 1, 5B), in other cases the jaw bone seems to be straight ( Figs. 4D, 5C, 7B). Pores of vascular canals on the jaw bone medial side are very rare or lacking ( Figs. 3D, E, 4B 2, C, 5B, C, E, F).

The lingual ridge of the jaw bone bears a tooth row or field which consists of denticle groups. The denticle group is composed of one fairly large central denticle and two, or sometimes more, denticles of smaller size ( Figs. 3F, 4A, 5F, H, 6A, H, 8B). Each group of denticles has a cavernous common base ( Figs. 4E, 5A) surrounded by several pores of vascular canals, analogous with the pores in the inter−tooth pits ( Fig. 3F). The arrangement of the groups and denticles within these groups is often irregular ( Figs. 3F, 6H, 8B), although some appear aligned postero−lingual. In the holotype, the lingual tooth row reaches as far as inter−tooth pit between the two last teeth ( Fig. 3A, B, D). In some cases, the denticle groups are situated on both the lingual ridge and the lingual slope, expanding the lingual row to a lingual field ( Figs. 5D, 6H).

The posterior part of the jaw bone has moderately developed postero−medial and postero−lateral processes ( Figs. 3B 5E, F, 7A, B). The former is usually smaller than the latter. The posterior concavity between them is more or less expressed ( Figs. 5F, 7A, B), depending on development of the posterior projection ( Figs. 3A, B, 5E). Distal parts of the postero−lateral and postero−medial processes in the holotype are slightly bent downward ( Fig. 3B, E).

Reconstruction of the complete holotype reveals at least 12 teeth in the lateral tooth row and the entire jaw is about 5.1 mm long ( Fig. 3A). The largest specimen of Podoliacanthus zychi gen. et sp. nov., ZPAL P.14/3.3 ( Fig. 5B), is massive and thick in cross section relative to all other specimens. We estimate the original length of this jaw bone as about 7 mm. Remarks.—Specimen ZPAL P.14/1.1 ( Figs. 4A, 5H, 8B) is 1.75 mm long, broken anteriorly, with up to five teeth from the beginning of the lateral tooth row. There are 3–4 fairly large inter−tooth denticles; the main denticles of the denticle groups in the lingual field reach almost the size of the teeth in the lateral tooth row. Estimated total length of the jaw bone is about 3 mm, or even less. Despite being much smaller in size than the others, we regard this jaw bone as from a juvenile individual rather than another species .

The jaw bone fragment MGUH VP 3617 ( Blom 1999: fig. 41K) from the Upper Silurian or Lower Devonian of North Greenland is about 1.5 mm long and shows the posterior end of the lateral and lingual tooth rows, with at least two of the lateral teeth. Blom (1999) thought that the specimen was figured in postero−lateral view, but we suppose it is rather in postero−medial view. Each of the two worn teeth has one medial side denticle. The lingual tooth row on the jaw bone fragment comprises two denticle groups, recognized by Blom (1999: 73) as “two rows of two larger, separated, three−cusped teeth”, of Podoliacanthus − type, namely a large central denticle and two smaller ones, all arranged in−line.

http://dx.doi.org/10.4202/app.2011.0079

The denticles of the lingual row are of comparable size to the teeth of the lateral row, similar to the probable juvenile Podoliacanthus jaw bone (cf. Figs. 4A, 5H). These features are characteristic for Podoliacanthus zychi sp. nov., and the estimated size of the specimen is within the range for the species. The difference is in the weakly developed inter−tooth pit, but this could result from the position of this fragment along the jaw, as inter−tooth pits are not strongly developed near the origin of the main tooth row. Also the pores of the vascular canals are not observed in the Greenland specimen, neither on the inter−tooth pit surface nor around denticle groups, possibly caused by their different preservation.

Stratigraphic and geographic range.—Ivanie Zolote, Zalishchyky and Ustechko; upper part of the Ivanie Member of the Tyver Series (from 20 to 2 m below the boundary with the Dniester Series; Late Lochkovian); Monument locality, central Hall Land, North Greenland, Chester Bjerg Formation (Pridoli or Lochkovian).