Arianagnathus jafariani, Männik & Miller & Hairapetian, 2015

Männik, Peep, Miller, C. Giles & Hairapetian, Vachik, 2015, A new early Silurian prioniodontid conodont with three P elements from Iran and associated species, Acta Palaeontologica Polonica 60 (3), pp. 733-746 : 738-740

Arianagnathus jafariani sp. nov.

Figs. 3, 4.

1980 Ozarkodina n. sp. A. Helfrich, 1980; Helfrich 1980: 568, pl. 1: 13–15. Pa element .

2013 Gen. et sp. nov.A (aff. I.? sandersi Mabillard and Aldridge, 1983 ); Männik et al. 2013: 10, fig. 4: r–s. Pa and Pb elements.

Etymology: After late Professor Mohammad Ali Jafarian (Esfahan), in recognition of his studies on Late Devonian brachiopods and regional stratigraphy.

Holotype: NHMUK PM X 3626 , Pb element (Fig. 3E).

Type locality:A section located about 65 km north-west of Tabas, on the eastern side of the Dahaneh-e-Kolut Gorge in the Derenjal Mountains, East Central Iran; Hill B, uppermost Unit 11 (sample S19).

Type horizon: Pterospathodus amorphognathoides lennarti Conodont Zone, Telychian, Llandovery, Silurian.

Diagnosis.—Same as for genus.

Material.— 178 specimens: Pa element , 80; Pb, 16; Pc, 26; M, 26; Sc, 2; Sb 1, 10; Sb 2, 13; Sa, 5.

Description.— Pa element (Fig. 3A–D): Pastiniscaphate, very slightly S-shaped in upper view, with a stout low posteriorly inclined cusp situated close to posterior end of element. Anterior process long, slightly curved to outer side of element, and bearing up to 13 low denticles which thicken perpendicular to blade forming short ridges of icriodelliform aspect that get progressively wider from mid-blade towards termination of process. Height of anterior process decreases gradually away from cusp and more rapidly towards termination of process. Process can end abruptly or taper to a point. Posterior process short with 3–5 low, broad denticles that rapidly decrease in height distally. Basal cavity deep, thin-walled and open below entire element. Inner lateral flare of cavity lip located beneath cusp, usually bears distinct medial ridge that extends into lateral face of cusp (Fig. 3A 2, B 3, C 2, D 1). Outer lateral flare of cavity lip better developed and slightly to posterior of inner flare. Some specimens also bear a distinct medial ridge on outer flare extending into lateral face of first or second denticle on posterior process and not up side of cusp (Fig. 3B 1, B 2, C 4, D 3). Some larger specimens with outer flare developed into short process with indistinct distal denticles (Fig. 3D 3, D 4).

Pb element (Fig. 3E–I): Pastiniscaphate, sub-triangular to quadrangular in upper view with three main processes bearing rows of low denticles with main denticle row arranged sigmoidally. On dextral elements main row of denticles curve smoothly into an S-shape (Fig. 3E 2, F 1, G 2, I), on sinistral elements (Fig. 3H?) probably a reverse S-shape, although only specimen is incomplete. Denticles in main denticle row widen distally to short ridges of icriodelliform aspect. Cusp at junction point of processes, indistinct from denticles. Basal cavity deepest below the cusp and opens along the whole element. On larger specimens, more quadrangular in outline (Fig. 3F 1), margins of basal cavity become strongly flared and additional ridges, sometimes denticulated, appear on the outer side between the main rows of denticles (Fig. 3G 2).

Pc element (Fig. 4A, B): Pastinate with tall slightly curved cusp. Cusp triangular in transverse section, anterior and posterior edges sharp, inner face almost flat. All three processes short and pointed towards base with up to three low denticles. Distinct ridge continues up outer face of cusp from denticles on base of outer lateral process (Fig. 4A, B 1). Basal cavity deeply excavated and extends below entire element.

M element (Fig. 4C, E): Dolabrate with tall inwardly inclined and blade-like cusp. Anterior and posterior edges of cusp sharp, outer lateral face gently convex, inner lateral face gently convex near cusp tip but becoming increasingly convex downwards to produce an inwardly flared basal cavity margin. Margin is pinched at base of cusp; occasionally a weak ridge is developed at this point and extends to base of cusp. Anterior edge of cusp continues to short anticusp. Posterior process straight, inclined downwards and bearing up to four tall, discrete denticles. Denticle closest to cusp inclined inwards towards base of cusp and partly overlaps it on inner side (Fig. 4C 1, C 2, E 2). Basal cavity triangular and deep, extending below whole element.

Sc element (Fig. 4D): Bipennate with short posterior and inner lateral processes both probably bearing a single denticle. Inner lateral process slender and inwardly pointed; posterior process denticle better developed with broad base. Basal cavity deep beneath cusp and filled with well preserved basal body.

Sb 1 element (Fig. 4F, J): Asymmetrical tertiopedate with

Fig. 4. Prioniodontid conodont Arianagnathus jafariani gen. et sp. nov. from Llandovery (Silurian) of Derenjal Mountains, East Central Iran, Hill B (sam- → ple S19). A, B. Pc elements. A. Dextral element, NHMUK PM X 3630, in outer lateral view. B. Sinistral element, NHMUK PM X 3631, in outer lateral B 1), inner lateral (B 2), and lower (B 3) views. C, E. M elements. C. Dextral element, NHMUK PM X 3632, in lower (C 1), inner lateral (C 2), posterior (C 3), and outer lateral (C 4) views. E. Dextral element, NHMUK PM X 3634, in inner lateral (E 1) and lower (E 2) views. D. Sc element, NHMUK PM X 3633, in outer lateral (D 1), lower (D 2), and inner lateral (D 3) views. F, J. Sb 1 elements. F. Dextral element, NHMUK PM X 3635, in posterior (F 1), outer lateral F 2), and inner lateral (F 3) views. J. Sinistral element, NHMUK PM X 3639, in posterior (J 1) and outer lateral (J 2) views. G. Sa element, NHMUK PM X 3636, in lateral (G 1) and posterior (G 2) views. H. Dextral Sb 2 element, NHMUK PM X 3637, in posterior (H 1) and inner lateral (H 2) views. I. Dextral Sb 2 element, NHMUK PM X 3638, in posterior (I

1

), lower (I

2

), and outer lateral (I

3

) views. Scale bars 0.1 mm.

reclined cusp. Anterior face of cusp rounded, posterior face bears a medial ridge that extends to denticle row on long slightly downward and inwardly curved posterior process. The process bears up to six laterally compressed, irregular, erect denticles that are slightly fused at base. Cusp continues downwards to an adenticulate inner lateral process.A distinct high ridge is present on inner lateral side of cusp and continues along entire length of process. Steeply downwardly directed outer lateral process bears 1–4 denticles. Basal cavity full of basal body in all specimens available.

Sb 2 element (Fig. 4H, I): Asymmetrical tertiopedate with tall reclined cusp. Morphologically, Sb 2 element almost identical to Sb 1 element. Sb 2 element differs as both lateral processes are denticulated and bear up to three denticles. Inner lateral process of Sb 2 element curved slightly inwards rather than as continuation of cusp as on Sb 1 element.

Sa element (Fig. 4G): Alate and morphologically similar to the Sb 2 element. The main difference is that the Sa element is symmetrical. Both lateral processes bear up to three compressed denticles.

Remarks.— Arianagnathus jafariani gen. et sp. nov. displays features characteristic of several known apparatuses. As not- ed above, the apparatus of Arianagnathus is most similar to Notiodella . However, in Notiodella two P elements (P 1 and P 2) are pastinate and one (P 3) makellate (or modified pastinate) ( Aldridge et al. 2013) whereas in Arianagnathus only one P element (Pc) is pastinate, other two (Pa and Pb) are pastiniscaphate. One of the morphologically closest is also Icriognathus from the earliest Llandovery (Rhuddanian) of Estonia ( Männik 1992). However, the Pa element of Icriognathus has poorly developed parallel rows of three denticles on the anterior process ( Männik 1992: pl. 1: 2, 3, 7, 10, 12, 14, pl. 2). Undescribed collections (PM unpublished material) suggest that these become better developed in specimens from younger strata. The second P (Pb) element of Icriognathus is also pastinate (ambalodiform) with a distinct cusp Männik 1992: pl. 1: 1) and a third P element has not been found. The M and S elements of A. jafariani gen. et sp. nov. are also similar to elements in the Icriodella , Pranognathus and Pterospathodus apparatuses. The apparatus of Icriodella has a pastinate second P element with a tall stout cusp ( Sweet 1988: fig. 5.18) and is lacking a third P element . The Pranognathus Pa element has five well developed processes each of which bears a single row of denticles, Pb and Pc elements are pastinate the first one with three primary and one secondary process ( Männik and Aldridge 1989: text-fig. 5D–H, I, V–X) and the second with three primary processes ( Männik and Aldridge 1989: text-fig. 5J, K; Y, Z). Both elements have a distinct cusp. Pterospathodus has four pairs of P elements two of which (Pb 1 and Pb 2) are angulate, the Pa is carminate or pastinate and Pc pastinate with a distinct cusp ( Männik 1998). Helfrich (1980: pl. 1: 13–15) figured elements from Virginia and Maryland, United States that are morphologically quite similar to the Pa element of A. jafariani gen. et sp. nov. These elements identified by Helfrich (1980) as Ozarkodina n. sp. A occur together with Pterospathodus amorphognathoides Walliser, 1964 , i.e., in strata of roughly the same age as the Iranian material. Unfortunately, the illustrations of the elements of P. amorphognathoides in Helfrich (1980: pl. 1: 1–6, identified as Carniodus carnulus Walliser, 1964 ; pl. 2: 17–19) are of too poor quality to allow identification of its subspecies and further refinement of the age. The illustrations of Ozarkodina n. sp. A in Helfrich (1980) do not allow detailed comparison of these specimens with the Pa element of A. jafariani gen. et sp. nov. as only lateral views of them are illustrated.

Several elements of A. jafariani gen. et sp. nov. are very similar to elements in the apparatus of Icriodella? sandersi Mabillard and Aldridge (1983: 33 , pl. 2: 1–10). According to the original description by Mabillard and Aldridge (1983: 33), the apparatus of I.? sandersi contains a straight thin-walled broad-based P element with icriodelliform denticulation on its anterior process which is very similar to the Pa element of A. jafariani gen. et sp. nov. However, the Pa element in I.? sandersi is almost straight whereas that in A. jafariani gen. et sp. nov. is slightly S-shaped in upper view, its anterior process slightly curved to the outer side and posterior process curved to the inner side of the element. The anterior process of the Pa element of A. jafariani gen. et sp. nov. is also longer and usually bears more than 10 denticles instead of the 4 to 7 characteristic of the Pa element of I.? sandersi . Additionally, the M element of I.? sandersi lacks the characteristic denticle closest to the cusp of A. jafariani gen. et sp. nov. that is inclined inwards towards the base of the cusp and partly overlaps it on the inner side. In addition, our restudy of the 247 specimen collection of I.? sandersi made by Mabillard and Aldridge (1983) did not show any elements similar to the Pb element recognized in the A. jafariani gen. et sp. nov. apparatus.

The Pb element identified here is considered part of the A. jafariani gen. et sp. nov. apparatus mainly because of its great general morphological similarity to the Pa element : both have thin-walled wide and deep basal cavities and, most importantly, both elements bear rows of denticles that in distal parts of the processes are laterally thickened to short ridges of icriodelliform aspect. All three P elements occur in association with the other unusual elements figured here as part of the Arianagnathus apparatus and this, as well as the similar style of denticulation for two of them, is the main reason for suggesting the apparatus has three P elements.

Stratigraphic and geographic range.— Type locality and horizon only.