Prioniodus honghuayuanensis, Y Y Zhen, 2005

Y Y Zhen, 2005, Revision of Two Prioniodontid Species (Conodonta) from the Early Ordovician Honghuayuan Formation of Guizhou, South China, Records of the Australian Museum 57, pp. 303-320 : 312-319

Prioniodus honghuayuanensis n.sp.

Figs 6–8

Baltoniodus communis (Ethington & Clark) .– An, 1981: pl. 4, figs 20–23, 25, 27–29,?24.

Baltoniodus communis (Ethington & Clark) .–An & Ding, 1985: p. 8, 9, pl. 1, figs 1–5, 8,?9.

Baltoniodus ? communis (Ethington & Clark) .– An, 1987: p. 125, 126, pl. 19, figs 1–11.

Baltoniodus communis (Ethington & Clark) .–Ding et al. in Wang, 1993: p. 161, pl. 23, figs 1–12.

Etymology. After Honghuayuan Village, where the type section of the Honghuayuan Formation is exposed on the hill slope located to the southeast of the village.

Material. 435 specimens, including holotype ( AMF126760 , from sample AFI993 ) and 19 paratypes ( AMF126761 – AMF126779 ), from eight samples ( Table 1) .

Diagnosis. Species of Prioniodus consisting of a septimembrate apparatus, including pastinate Pa and Pb elements with denticulate posterior and outer lateral process and adenticulate (or with rudimentary denticles) anterior process which curves distally outer laterally; makellate M element with low but long inner lateral and outer lateral processes; alate triform Sa, triform asymmetrical (modified quadriramate) Sb, bipennate (modified quadriramate) Sc, and quadriramate Sd elements; all S elements with a proclined cusp, a long denticulate posterior process, and adenticulate (or rudimentarily denticulated) lateral and anterior processes.

Description. All elements are small, mostly albid, thin and fragile; the long denticulate posterior process of the P and S elements is broken in most specimens. Pastinate P elements bear a robust, laterally compressed cusp, with a sharp costa along its anterior and posterior margins, and with a prominent costa on the outer lateral face ( Fig. 6). Surface of the cusp is ornamented with fine striation, which are best developed posterior to the outer lateral process on the outer lateral face ( Fig. 6P). The posterior process is laterally compressed as a long blade with closely spaced, basally confluent, and more or less equal-sized denticles along its upper margin. The anticusp-like anterior process extends downward with its anterior margin strongly curved outwards and occasionally with confluent and small rudimentary denticles along its edge ( Fig. 6F). The outer lateral costa extends downward to merge with the upper margin of the outer lateral process, which is long and bearing small, closely spaced denticles ( Fig. 6B,M,N). The outer lateral process that extends downward laterally outward with an angle slightly>90° (Pb, Fig. 6O) or nearly normal (Pa, Fig. 6D,G) to the posterior process is often broken in the present collection. Basal cavity is triangular in shape extending underneath the three processes as narrow grooves ( Fig. 6E,F,Q,O). The Pa element has a suberect cusp with anterior process curved laterally outward and with the outer lateral process extending nearly normal to the posterior process in the upper view ( Fig. 6D,G). The Pb element resembles the Pa, but with a reclined cusp and an innerlaterally bent posterior process ( Fig. 6K–S).

The M element has a robust and strongly outer laterally reclined cusp, and adenticulate, low and long outer lateral and inner lateral processes ( Fig. 7A–D). The cusp is sightly curved posteriorly and anteroposteriorly compressed, with a weak costa on the broad anterior face, and a broad carina on the posterior face.The outer lateral process has gently arched upper and basal margins, while the inner lateral process has nearly straight upper and basal margins. Basal cavity is relatively narrow, without prominent basal buttress ( Fig. 7B,C).

The Sa element is triform and symmetrical with a proclined cusp, a long denticulate posterior process, and an anticusplike lateral process on each side ( Fig. 7E–S). The cusp is triangular in cross section with a broad anterior face ( Fig. 7M, O), a sharp blade-like costa along the posterior margin, and a sharp costa along the anterolateral corner of each side ( Fig. 7J, S). The posterior process is long, straight and strongly compressed laterally bearing small, closely spaced denticles ( Fig. 7P–R). The blade-like costa on each side is gently curved posteriorly and extends basally to merge with the upper margin of the lateral process, which often bears rudimentary denticles ( Fig. 7J,R). The basal cavity is an isosceles triangle in outline with moderate depth ( Fig. 7E,J).

[ Fig.6 caption continued] … AMF126762 , (K) inner lateral view ( IY63005 ), (L) outer lateral view ( IY63006 ), (M) upper view ( IY63007 ), (N) anterior view ( IY63004 ), (O) basal view ( IY50035 ), (P) close up of outer lateral view, showing the fine striae in the area posterior to the outer lateral costa ( IY50036 ); (Q–S) Pb element, paratype , AMF126763 , (Q) basal view ( IY50027 ), ( R) inner lateral view ( IY63011 ), (S) outer lateral view ( IY50028 ). All from sample AFI993 ; scale bars 100 µm unless otherwise indicated .

[ Fig.8 caption continued] … AMF126774 , AFI993 , (I) inner lateral view ( IY63021 ), (J) outer lateral view ( IY51026 ), (K) basal view ( IY51025 ); (L–P) Sc element, paratype, AMF126775 , AFI997 , (L) basal view ( IY51050 ), (M) inner lateral view ( IY51049 ), (N) outer lateral view ( IY63028 ), (O) upper view ( IY51051 ), (P) close up of the upper view showing the cross section of the cusp (IY51052); (Q) Sc element, paratype, AMF126776 , AFI993 , outer lateral view ( IY51022 ). ( R – T) Sd element, paratype, AMF126777 , AFI993 , ( R) outer lateral view ( IY63025 ), (S) inner lateral view ( IY51037 ), ( T) posterior view ( IY51036 ); (U–X) Sd element, paratype, AMF126778 , AFI993 , (U) inner lateral view ( IY63023 ), ( V) outer lateral view ( IY51033 ), (W) posterior view ( IY51035 ), (X) postero-outer lateral view ( IY51034 ); (Y–AA) Sd element, paratype, AMF126779 , AFI993 , (Y) postero-inner lateral view ( IY51032 ), (Z) outer lateral view ( IY63022 ), (AA) inner lateral view ( IY51031 ) . Scale bars 100 µm unless otherwise indicated.

The Sb element is strongly asymmetrical with a proclined and laterally compressed cusp, which bears a sharp costa along the anterior and posterior margins, and the third costa on the outer lateral face ( Fig. 8A–H). The anterior costa is inner laterally curved and extends basally to merge with the upper margin of the anticusp-like downwardly extending adenticulate anterior process ( Fig. 8A,E,G). The long posterior process is laterally compressed as a high blade bearing small, closely spaced denticles of similar sizes along its upper margin. The costa on the outer lateral face extends downward to merge with the upper margin of the downward extended, shorter, adenticulate outer lateral process. The inner lateral face is less convex, and bears a broad carina ( Fig. 8A,B,E,G).

The Sc element is bipennate with a broad carina on each lateral face (more prominent on the inner lateral face), a short, adenticulate anterior process, and a long, laterally compressed, denticulate posterior process ( Fig. 8I–Q). The cusp is proclined ( Fig. 8Q), and laterally compressed with a sharp blade-like costa along its posterior and anterior margins ( Fig. 8 O,P), which extends basally to merge respectively into the upper margin of the anterior process and the posterior process ( Fig. 8I,M). The anterior process is anticusp-like, and curved laterally inward ( Fig. 8I,M). The posterior process bears small, closely spaced denticles of more or less similar sizes ( Fig. 8Q). The basal cavity is biconvex in outline, and extends as a narrow groove underneath the anterior and posterior processes ( Fig. 8K,L).

The Sd element is quadriramate and asymmetrical, with an anterior process, a posterior process, and a lateral process on each side ( Fig. 8R –AA). The cusp is proclined with a sharp blade-like costa along the anterior and posterior margins and on each lateral side ( Fig. 8T,W,Y,AA).Anterior costa is inner laterally curved ( Fig. 8S,U,Y), and extends basally to merge into the upper margin of the anticusp-like anterior process which is adenticulate or has small, fused rudimentary denticles along its distal edge ( Fig. 8Y). As in the other S elements, the posterior process is long, laterally compressed, and blade-like bearing small, closely spaced denticles of similar sizes. The area defined by the anterior costa and the outer lateral costa is more broadly convex ( Fig. 8R,V). The lateral costa on each side extends basally to merge respectively with the upper margin of the short inner and outer lateral processes. Both lateral processes extend downward and often bear small rudimentary denticles along their distal edges ( Fig. 8S–U,W,X,AA).

Remarks. Although P elements of the new species show some resemblance to those of Oepikodus communis (Ethington & Clark, 1964) , they differ in having denticulate outer lateral processes, and an anterior process which bears rudimentary denticles along its distal edge. The anterior process of P. honghuayuanensis extends strongly downward as an anticusp, and curves strongly outer laterally ( Fig. 6F,J,Q). The Pb element of O. pincallyensis Zhen in Zhen at al., 2003 has denticulate anterior, outer lateral and posterior processes, but the anterior process of the Pa element is adenticulate. However, the P elements of O. pincallyensis are distinctively curved inner laterally (Zhen et al., 2003, fig. 19E,K) rather than outer laterally as in P. honghuayuanensis ( Fig. 6D,M). Furthermore, the S elements of the new species can be easily distinguished from those of O. communis , O. pincallyensis and other species of Oepikodus . The Sa element is triform and alate with a wide anterior face ( Fig. 7M,O) and the open basal cavity is an isosceles triangle in outline ( Fig. 7E,J). The Sb element can be described as asymmetrical triform (or modified quadriramate of some authors) with a strongly developed costa on the outer lateral face, which extends basally as a short but prominent adenticulate outer lateral process. The quadriramate Sd element of P. honghuayuanensis differs from that of O. communis in having more strongly developed costae, in having more prominent lateral processes, and in tending to develop rudimentary denticles along the distal edge of the anterior and lateral processes ( Fig. 8S–U,W–Y,AA).

Oepikodus communis was originally proposed as a form species based on pastinate elements (Ethington & Clark, 1964). The type material shows a long denticulate posterior process with small, closely spaced denticles, and adenticulate anterior and outer lateral processes (Ethington & Clark, 1964, pl. 114, figs 6, 14, text-fig. 2F). Subsequently, Ethington & Clark (1982) revised O. communis in multielement taxonomy by accommodating another three form species which were reported in association with the type material of the form species Gothodus communis Ethington & Clark, 1964 in the El Paso Formation of Texas. These include Oepikodus equidentatus Ethington & Clark, 1964 with typical quadriramate ramiform elements (including symmetrical and asymmetrical), Subcordylodus sp. aff. S. delicatus (Branson & Mehl) with modified quadriramate ramiform elements (bearing only a weakly developed broad carina on the lateral faces), and Oistodus longiramis Lindström, 1955 comprising a makellate element with a long and slender outer lateral process. Ethington & Clark (1982) recognized the symmetry transition among the ramiform elements of four morphotypes, symmetrical quadriramate element, asymmetrical quadriramate element, modified quadriramate element with costa on one side, and modified quadriramate element without lateral costa. Therefore, they indicated a seximembrate apparatus for the species, although no formal notation was made. Repetski (1982) also suggested a seximembrate apparatus for O. communis , including prioniodiform, falodiform and four types of ramiform (belodiform, tetraprioniodiform, hibbardelliform, and cordylodiform) elements. Nicoll & Ethington (2004) defined Oepikodus as consisting of a septimembrate apparatus of a geniculate makellate M, four types of quadriramate or modified quadriramate S, and two types of pastinate P elements.

We also interpret Oepikodus communis as consisting of a septimembrate apparatus. The pastinate Pa element is represented by the holotype (Ethington & Clark, 1964, pl. 114, fig. 6) with a suberect cusp, and the pastinate Pb element by the other figured specimen of G. communis (Ethington & Clark, 1964, pl. 114, fig. 14) with a reclined cusp. The Sa and Sd elements are represented by the form species O. equidentatus Ethington & Clark (1964, pl. 113, figs 6, 8, 10, 11, 14). The Sa element is symmetrical or nearly symmetrical with a straight anterior margin, while the Sd element is markedly asymmetrical with a curved anterior margin (e. g. Ethington & Clark, 1964, pl. 113, fig. 14). The Sc element of O. communis is represented by those described and illustrated by Ethington & Clark (1964, p. 701, pl. 115, figs 1, 5, 7, 10) as Subcordylodus sp. aff. S. delicatus (Branson & Mehl, 1933) . The makellate M element of O. communis is represented by those described and illustrated by Ethington & Clark (1964, p. 693, pl. 114, figs 2, 7) as Oistodus longiramis Lindström, 1955 . The asymmetrical Sb element is a modified quadriramate element with a more prominent costa on the outer lateral face (Albanesi in Albanesi et al., 1998, pl. 6, fig. 22).

Oepikodus intermedius Serpagli, 1974 , from the San Juan Formation of the Argentine Precordillera, is regarded herein as conspecific with O. communis ; it was originally described as consisting of a trimembrate apparatus (prioniodiform, oistodiform and oepikodiform). In more recent revisions of the San Juan faunas, both Lehnert (1995) and Albanesi (in Albanesi et al., 1998) did not recognize symmetrical Sa elements in any of the three species ( O. communis , O. intermedius and O. evae ) they ascribed to Oepikodus .

Lindström (in Ziegler, 1975) considered the form species Gothodus microdentatus van Wamel, 1974 from the Glauconitic Limestone of Sweden as part of O. communis species apparatus. However, not only have the wellrecognized elements of O. communis apparatus not been recorded in association with this form species in Sweden, but also the triform, asymmetrical G. microdentatus shows rather different morphology in comparison with the ramiform elements of O. communis from North America. Based on the original description and illustrations ( van Wamel, 1974), the form species G. microdentatus more likely represents the Sb element of a multi-element species of Prioniodus rather than of Oepikodus .

Oepikodus communis is widely distributed in North America (Ethington & Clark, 1964, 1982; Repetski, 1982; Stouge, 1982; Stouge & Bagnoli, 1988; Johnston & Barnes, 2000), Argentine Precordillera ( Serpagli, 1974; Lehnert, 1995; Albanesi in Albanesi et al., 1998), Australia ( McTavish, 1973; Zhen et al., 2003), and Greenland ( Smith, 1991). However, as the result of inclusion of a triform alate Sa element in this species apparatus by An and other Chinese authors ( An, 1981, 1987; An et al., 1985; Ding et al. in Wang, 1993), the occurrence of O. communis in South China needs to be re-examined.

Specimens referable to the Pa, Pb, Sa, Sb, Sc and Sd elements of the present new species were first recorded from the top of the “Honghuayuan Formation” of Hexian, Anhui Province as Baltoniodus communis ( An, 1981, 1987; An & Ding, 1985). An (1987) noted the abundant occurrence (up to 80% in some samples) of this species at the top of the Honghuayuan Formation in Guizhou, Hunan, Hubei and Anhui provinces, and correlated this interval at the top of the Honghuayuan Formation with the communis Zone of North America and with the elegans Zone of Baltoscandia. Although he recognized the occurrence of triform alate element as part of this species apparatus, An (1981, 1987) included this material into his rather broad interpretation of O. communis (Ethington & Clark, 1964) . However, as the triform Sa element occurring in the material from South China had not been recognized at the type locality of O. communis in North America, An (1981, 1987) doubtfully assigned O. communis to Baltoniodus rather than Oepikodus .

Specimens previously referred to as aff. Oepikodus ? minutus ( McTavish, 1973) from the upper Wah Wah Formation, and the overlying Juab and Kanosh formations in the Ibex area of Utah (Ethington & Clark, 1982) somewhat resemble P. honghuayuanensis . Prof. R. Ethington kindly provided sixty specimens of this Utah species from nine samples for comparative study. Stratigraphically the Ibex species (designated herein as Prioniodus sp. ; Fig. 9A–P) occurs at a slightly younger level (basal Whiterockian, victoriae graptolite Zone) than the new species from South China. Similarities include the prioniodiform Pb element of Prioniodus sp. ( Fig. 9C–E) that bears a denticulate outer lateral process with anterior margin curved outer laterally towards outer lateral process, and the ramiform S series that includes alate triform Sa ( Fig. 9H–K), asymmetrical triform Sb ( Fig. 9M,N), bipennate Sc ( Fig. 9 O,P), and quadriramate Sd ( Fig. 9F,G) elements. However, the associated M element of the Ibex species shows a much shorter inner lateral process, and the Pa element has an adenticulate and weakly developed outer lateral process and a more or less straight anterior process. Bagnoli & Stouge (1997) referred this Ibex species to Gothodus Lindström, 1955 . Based on the recent revision of Gothodus (Bagnoli & Stouge, 1997, and Stouge & Bagnoli, 1999), it has prioniodiform P elements comparable with those of Oepikodus and Prioniodus , but the S elements are more related to Phragmodus Branson & Mehl, 1933 . However, most other conodont authors regard it as a junior synonym of Prioniodus (e.g., Bergström in Clark et al., 1981) or Baltoniodus ( Bagnoli et al., 1988) .

The new species from the Honghuayuan Formation shows some resemblance to P. elegans documented from Sweden by van Wamel (1974, p. 87–89, pl. 6, figs 1–6) and by Löfgren (1978, pl. 9, figs 1–6), but Swedish material of P. elegans has the M element with a denticulate inner lateral process and the P and S elements with better developed denticles on the lateral and anterior processes. Prioniodus elegans described from the Leningrad region of Russia and Tulubäcken of Sweden by Bergström (1988, pl. 3, figs 33– 38), and from western Newfoundland by Stouge & Bagnoli (1988, pl. 13, figs 1–9) and Johnston & Barnes (2000, pl. 3, figs 1, 5–7, 11, pl. 16, fig. 17) has an even stronger development of denticulation with P elements bearing a longer, fully denticulated anterior process, and with S elements displaying well-denticulated anterior and lateral processes. The multi-element species definition of P. elegans followed herein conforms to the concept of Bergström (1971, 1981 in Clark et al., 1981), van Wamel (1974), and Stouge & Bagnoli (1988). Stouge & Bagnoli (1988, p. 134) noted that this species displayed a large degree of morphological variation and might be polymorphic. Some of the material of P. elegans documented by Bergström (1988, pl. 3, figs 33–37) came from Popowka to the south of the Leningrad region, from where Pander’s original type material (now lost) was collected. In comparison with those figured specimens of P. elegans , the P elements of P. honghuayuanensis display a more prominent cusp and smaller and less developed anterior and outer lateral processes ( Fig. 6H,L), the P and S elements show less development of denticles on the anterior and lateral processes ( Figs 6–8), and the M element lacks denticles on the inner lateral process ( Fig. 7A–D).Also, the anterior process of the P elements in P. elegans is distinctively curved inner laterally ( Bergström, 1988, pl. 3, fig. 33) rather than outer laterally ( Fig. 6D,F,G) as in P. honghuayuanensis . In South China, P. elegans has only been recorded from the Jinshan Formation (biostratigraphically coeval with the Honghuayuan Formation) in Zhejiang Province of southeast China ( An, 1987). It was also recorded from sub-surface core samples of the Tarim Basin, northwest China ( Zhao et al., 2000).

One specimen from the very top of the Honghuayuan Formation at the Honghuayuan Section (AFI999, Fig. 2) is similar to the Pb element of P. honghuayuanensis , but is stellate in outline with an additional sharp, blade-like costa on the inner side of the cusp, which extends basally into a short process with a few small rudimentary denticles. It is tentatively referred to herein as a Pc element of P. honghuayuanensis ( Fig. 9Q–U).

ACKNOWLEDGMENTS. Fieldwork byYYZ in Guizhou in late 2000 was undertaken with the support of the Australian Academy of Sciences and the Academia Sinica (visiting grant). Professors Zhiyi Zhou and Jiayu Rong from Nanjing Institute of Geology and Palaeontology, Academia Sinica kindly provided financial assistance and made arrangement for the shipment of the conodont samples to Australia (NSFC projects 40272001, and Major Basic Research Projects of MST, China G2000077703). Further collecting in Tongzi was undertaken by JBL in the following years (2001 to 2002) with the support of the Special Funds for Major State Basic Research Project (G200077700) of P.R. China, and these samples were processed at the Palaeontology Laboratory of the Geology Department, Peking University. Y.Y. Zhen’s study of the conodont fauna was partially funded by a Sydney Grammar School Science Fellowship. Prof. R. Ethington kindly provided material of a Prioniodus species from the Wah Wah, Juab and Kanosh formations of the Ibex area, Utah for comparative study. Gary Dargan (Geological Survey of New South Wales) assisted with acid leaching, residue separation and other laboratory work. Scanning electron microscope photographs were prepared in the Electron Microscope Unit of the Australian Museum. Reviews by R, Ethington and G. Albanesi assisted clarification of our ideas. IGP publishes with permission of the Director,Geological Survey of NSW.