Astrolecignathus cf. milleri Over and Chatterton, 1987

Waid, Christopher B. T. & Cramer, Bradley D., 2017, Telychian (Llandovery, Silurian) conodonts from the LaPorte City Formation of eastern Iowa, USA (East-Central Iowa Basin) and their implications for global Telychian conodont biostratigraphic correlation, Palaeontologia Electronica 42 (6), pp. 1-37 : 12-19

Astrolecignathus cf. milleri Over and Chatterton, 1987

Figure 9.17

*1987 Astrolecignathus milleri ; Over and Chatterton, 1987, pl. 3, figs. 11-12, 15.

2007a Apsidognathus milleri Over and Chatterton ; Männik, fig. 6M.

2008 Apsidognathus milleri Over and Chatterton ; Männik, fig. 8L.

Description. Pa elements amorphognathiform with a slightly bowed blade with relatively evenly spaced denticles. Outer process has four aligned nodes that decrease in size away from the blade. Inner process contains three aligned nodes that are less developed than those on the outer process. A flat platform is developed on the outside of the blade and lateral processes. Outer-posterior part of the element is broken. The posterior process is slightly recurved and in oral view is deflected counter-clockwise from the anterior process. The basal cavity is widely expanded, and the basal cavity walls converge to creases under each process.

Remarks. This specimen is tentatively identified as Astrolecignathus milleri because the style of denticulation and nodes, orientation of lateral processes, and the presence of a wide, flat basal platform that extends under the entire element is similar to the holotype ( Over and Chatterton, 1987, plate 3, figure 11). The holotype specimen bears a second, posteriorly oriented process, but that part of the element is broken on our specimen. The antero-lateral process on our specimen is significantly shorter than on the holotype. But the other Pa element photographed in Over and Chatterton (1987, plate 3, figure 15) bears a smaller process, similar in size to the one on our specimen.

Because the platform lacks any significant ornamentation by nodes, ridges, or webbing, which are a ubiquitous characteristic of species classified as Apsidognathus (e.g., Aps. tuberculatus, Aps. ruginosus Mabillard and Aldridge, 1985), milleri should remain in Astrolecignathus , and should not be assigned to Apsidognathus . Furthermore, the processes on the Pa elements of species within Apsidognathus tend to radiate from a central point or ridge, whereas the processes of the Pa elements of Astrolecignathus radiate extend from a point about 1/3 above the most posterior point of the blade. There is also the possibility that the specimens identified as Apsidognathus milleri in Männik (2007a, 2008) are not the same species as the specimens named by Over and Chatterton (1987). The semi-circular arrangement of the denticles on the lateral processes of specimens in Männik (2007a, 2008) is significantly different from the holotype specimen in Over and Chatterton (1987), where the denticles on the holotype are oriented in completely straight rows.

Material examined. One broken Pa element.

Figures

FIGURE 9. Conodonts from the Garrison Core and DX5 outcrop. 1-8, Wurmiella? polinclinata polinclinata, S0 element, anterior view, GC 130.79–131.09 m, SUI 142286 (1); S1/2 element, posterior view, GC 115.51–115.79 m, SUI 142287 (2); S3/4 element, lateral view, GC 115.51–115.76 m, SUI 142288 (3); M element, lateral view, GC 130.20– 130.48 m, SUI 142289 (4); P2 element, lateral view, GC 130.20–130.48 m, SUI 142290 (5); P2 element, GC 130.79– 131.09 m, SUI 142291 (6); P1 element, lateral view, DX5 4.59–4.61 m, SUI 142292 (7); and P1 element, lateral view, DX5 3.49–3.59 m, SUI 142293 (8). 9-10, Aulacognathus angulatus, P1 element, oral view, GC 135.65–136.10 m, SUI 142294 (9) and S1/2? element, posterior view, GC 135.65–136.10 m, SUI 142295 (10). 11-14, Aulacognathus bullatus, P2 element, lateral view, DX5 1.34–1.54 m, SUI 142296 (11); P1 element, oral view, DX5 4.59–4.61 m, SUI 142297 (12); P1 element, oral view, DX5 0.67–0.74 m, SUI 142298 (13); and P1 element, oral view, GC 120.88– 121.13 m, SUI 142299 (14). 15-16, Aulacognathus kuehni, P1 element, oral view, GC 131.95–132.23 m, SUI 142300 (15) and juvenile P1 element, oral view, GC 131.95–132.23 m, SUI 142301 (16). 17, Astrolecignathus cf. milleri, Pa element, oral view, GC 121.92–121.88 m, SUI 142302. 18, Apsidognathus tuberculatus, Pa element, oral view, DX5 1.34–1.54 m, SUI 142303.