Erraticodon patu Cooper, 1981

Zhen, Yong Yi, Percival, Ian G. & Webby, Barry D., 2003, Early Ordovician Conodonts from Far Western New South Wales, Australia, Records of the Australian Museum 55 (2), pp. 169-220 : 195-198

Erraticodon patu Cooper, 1981

Figs. 16A–K , 17A–O

Erraticodon patu Cooper, 1981: 166 , pl. 32, figs. 1–6, 8. Erraticodon patu .– Nicoll, 1990, fig. 2.1.

? Erraticodon patu .–Albanesi & Vaccari, 1994: 137, pl. 1, figs. 11–16.

? Erraticodon patu .– Lehnert, 1995: 88, pl. 10, partim only fig. 11.

Material. 314 specimens (48 Pa, 29 Pb, 44 M, 29 Sa, 50 Sb, 59 Sc, 55 Sd) from the Tabita Formation at Mount Arrowsmith, and eight specimens (1 Pb, 1 M, 1 Sa, 1 Sb, 2 Sc, 2 Sd) from unnamed dolomitic limestone unit at Koonenberry Gap .

Diagnosis. A species of Erraticodon with a septimembrate ramiform-ramiform apparatus, consisting of digyrate Pa and Pb elements, makellate M, alate Sa, bipennate Sb and Sc, and tertiopedate Sd elements ; all hyaline with a prominent cusp, and two or three denticulate processes bearing discrete peg-like denticles; cusp bearing two or three thin, flangelike costae which extend basally to form the upper margin of the processes; basal cavity shallow, extending as shallow grooves towards the tips of the processes.

Description. Pa element digyrate with a less prominent cusp than the Pb element; the inner lateral process extending horizontally, with a straight basal margin which is normal to the axis of the cusp; the outer lateral process extending downwards at an angle of about 130° to the cusp, with a gently arched basal margin. The cusp varies from straight and erect ( Fig. 16A,B ) to prominently curved posteriorly ( Fig. 16C ); in the latter variation, denticles on the lateral processes also curve posteriorly; cusp and denticles have broad anterior and posterior faces and a flange-like costa on each lateral face; in cross section, cusp and denticles vary from rounded near the base to anteroposteriorly compressed towards the tips; shallow basal cavity represented by a small pit beneath the cusp, extending as a faint basal groove underneath each of the lateral processes. Pb element is a modified digyrate element with a lateral process on each side, bearing up to five denticles, and a shorter anterior process generally bearing three denticles; cusp erect ( Fig. 16E ), rounded or water drop-like in cross section near the base ( Fig. 16G ), with a sharp anterior margin, broadly rounded posterior face ( Fig. 16D,G,H ), and a thin flange-like lateral costa on each side which extend downwards to form the upper margin of the lateral process; the anterior process inner laterally curved, in the same plane as the outer lateral process, and forming an angle of 60– 70° with the inner lateral process in upper view ( Fig. 16G,H ); denticles on the lateral processes more or less rounded in cross section near the base, but towards tips they become anteroposteriorly compressed with sharp lateral margins marked by a flange-like costa, and broad anterior and posterior faces; denticles on the anterior process compressed laterally with sharp anterior and posterior margins, marked by a flange-like costa extending from the tip of the cusp, along its anterior margin and the upper margin of the anterior process, to the distal end of the process ( Fig. 16J,K ). M element makellate with a shorter inner lateral process bearing one or two denticles, and an outer lateral process bearing three to five denticles ( Fig. 17A,B ); cusp robust, anteroposteriorly compressed, and slightly curved posteriorly, with broad anterior and posterior faces, and sharp lateral margins; basal margin straight; basal buttress weakly developed. Sa element alate, symmetrical (or nearly so) bearing a posterior process and a lateral process on each side ( Fig. 17F,G ); cusp more or less rounded in cross section with broad anterior margin, a sharp posterior costa and a median costa on each lateral face; costae extending downwards to form the upper margins of the processes; lateral processes bearing two or three anteroposteriorly compressed denticles; posterior process bearing three or more, smaller, laterally compressed denticles; the posterior process of all Sa elements studied from western New South Wales is broken. Sb element bipennate, with a posterior process bearing three to five denticles and a downward extending and strongly inner laterally curved anterior process ( Fig. 17K–O ); cusp robust, posteriorly inclined, with posterior and anterior costa; anterior costa and anterior process strongly curved inner-laterally so that in lateral view, they appear to be more inner lateral than anterior in position ( Fig. 17K ). Sc element bipennate with a strongly downwardly extended anterior process bearing three or more denticles and a posterior process bearing three or more denticles ( Fig. 17C–E ); cusp suberect, laterally compressed with sharp anterior and posterior margins; thin anterior margin sharply curved inner-laterally; basal margin strongly arched with the two processes nearly normal to each other. Sd element tertiopedate, asymmetrical, with a short inner lateral process bearing two or three denticles, and a longer outer lateral process bearing three to six denticles ( Fig. 17H–J ); two lateral processes extending downwards forming an angle of about 60 to 70°; posterior process bearing two or more denticles; cusp erect, oval in cross section with broad anterior margin, a posterior costa, and a mid-lateral costa on each side; costae extending downwards to mark the upper margins of the processes.

Remarks. Specimens from the Tabita Formation are identical with the type material from the Horn Valley Siltstone. The majority of the Mount Arrowsmith elements bear a basal attachment, and similarly, many of the Horn Valley specimens also have well preserved attachment cones (Nicoll, 2002, pers. comm.). Cooper (1981) originally described the Pb element as having three processes— anterior, posterior and a lateral. However, Sweet (1988) correctly indicated that the Pb element is actually a modified digyrate element, with a lateral process on each side and a strongly curved anterior process. This morphology is also clearly illustrated in the holotype of the species ( Cooper, 1981, pl. 32, fig. 6). Close examination of specimens of the Pb element from Mount Arrowsmith has confirmed that these lack a posterior process.

Cooper (1981) described the M element as neoprioniodiform. Subsequently, Nicoll (1990) introduced the term makellate for the element occupying the M position, and suggested that the M element of E. patu was a typical makellate element. In the Mount Arrowsmith specimens, however, the anticusp is actually an antero-inner lateral process bearing one or two denticles. The basal buttress on the posterior face is generally weak ( Fig. 17B ).

The stratigraphically older E. patu is readily distinguished from the type species, E. balticus , from the Middle Ordovician by having a distinct Pb element with a denticulate anterior process, M element with a denticulate inner lateral process, and by absence of an accentuated denticle on the posterior process in the Sa, Sb and Sc elements .

The Pb element of E. tangshanensis Yang & Xu , in An et al., 1983 is a modified digyrate form (originally termed plectospathognathiform, but including both Pb and Sd elements ) which resembles the same element in E. patu , but the elements of E. tangshanensis have much more robust denticles and some S elements bear an accentuated denticle on the posterior process. The alate Sa element of this Chinese species has a short lateral process, represented by a single stout denticle, on each side, while the M element is more comparable with that of E. balticus in lacking an inner lateral process.

Figures

Fig. 16. Erraticodon patu Cooper, 1981: A,B, Pa element, AMF120370, M/A11-3, A, posterobasal view, B, posterior view; C, Pa element, AMF120371, Y4–6, posterior view; D, Pb element, AMF120372, M/A4, postero-upper view; E, Pb element, AMF120373, Y4–5, antero-outer lateral view; F, Pb element, AMF120374, Y4–7, upper, anterior view; G, Pb element, AMF120375, Y4–7, upper, posterior view; H, Pb element, AMF120376, M/A4, upper, posterior view; I, Pa element, AMF120296, Y4–6, anterior view; J,K, Pb element, AMF124209, M/A7, J, antero-upper view, K, postero-upper view. Scale bars 100 µm.

Fig. 17. Erraticodon patu Cooper, 1981: A, M element,AMF120378, Y4–6, anterior view; B, M element,AMF120379,Y4–6, posterior view; C, Sc element, AMF120380, Y4–6, inner lateral view; D, Sc element, AMF120381, M/A11-6, outer lateral view; E, Sc element, AMF120382, Y4–6, outer lateral view; F, Sa element, AMF120383, M/A11-5, anterior view; G, Sa element, AMF120384, Y4–4, posterior view; H, Sd element, AMF120385, Y4–8, anterior view; I, Sd element,AMF120386, M/A11-3, posterior view; J, Sd element, AMF120387, Y4–7, posterior view; K, Sb element, AMF120388, Y4–7, inner lateral view; L, Sb element, AMF120389, Y4–7, outer lateral view; M, Sb element, AMF120377, Y4–6, posterobasal view; N,O, Sb element, AMF120390, M/A11-5, N, postero-inner lateral view, O, antero-inner lateral view. Scale bars 100 µm.