Hindeodus parvus (Kozur and Pjatakova, 1976)

Huang, Jinyuan, Hu, Shixue, Li, Jun, Zhou, Changyong, Zhang, Qiyue, Wen, Wen, Min, Xiao, Jiang, Haishui, Martínez-Pérez, Carlos & Zhang, Kexin, 2024, Natural assemblages of the earliest Triassic conodont HIndeodUS parVUS from the Shangsi section, Sichuan province, Southwest China, Swiss Journal of Palaeontology (42) 143 (1), pp. 1-15 : 6-11

Hindeodus parvus apparatus composition

None of the two studied conodont natural assemblages in our collection contain the remains of all count the 15 elements of a complete Hindeodus apparatus. However, the P 1 and P 2 elements are paired, along with a complete symmetrical pair of S 3 and S 4 elements, and the S 0 element is symmetrical by itself. Tis indicates that our

(See figure on next page.)

Fig. 3 Conodont natural assemblage of H. parvus . Collection number 2023SSCNA1 counterpart,the sample comes from the Bed 31. A–B, photographs are took by stereomicroscope under the different angle incident lights. C–D, SEM photograph of the whole conodont natural assemblage, and its corresponding outline drawing, the solid lines represent the edges of the solid conodont elements, while the dashed lines represent the edges of the mouldic conodont elements. E, the pair of P 2 elements. F, the M element. All scale bars represent 200 μm.The color coding is as described in Fig. 2

feeding apparatus also has a symmetrical architecture, similar to other ozarkodinid apparatuses. Terefore, theS 1, S 2, and M elements should also be paired. Based on the characteristic of symmetrical architecture in the feeding apparatus, our data indicates that the apparatus of H. parvus is composed by 15 elements. It includes an alateS 0 element and pairs of S 1, S 2,S 3,S 4, M, P 2, and P 1 elements ( Fig. 6 ).

Te S 0 element is an alate form, bilaterally symmetrical, crown shape, bearing a long cusp, but lacking a ‘posterior’ process, six small denticles on each ‘lateral’ process ( Fig. 2G). Although we cannot confirm the development of a ‘posterior’ process in the assemblage, this unique S 0 element shape is known from other natural assemblages and from discrete elements assigned to Hindeodus (Agematsu et al., 2015, 2017; Li et al., 1989; Zhang et al., 2017), providing strong evidence to our assignment.

TeS 1 and S 2 elements both have a similar shape. Te elements are laterally compressed and have a long cusp with two unequal long processes. Te denticles are delicate except for the distalmost two denticles on the long ‘posterior’ process (oriented caudally) and one denticle is large on the relatively ‘short lateral’ process. It is hard to tell the difference between S 1 and S 2 elements in our assemblages, however it seems that the ‘short lateral’ process is shorter in the S 1 than in the S 2 element

( Fig. 5F ).

BipennateS 3 and S 4 elements. Both are laterally compressed with a long and robust cusp. Te laterally flexed ‘anterior’ process is shorter than the ‘posterior’ process, bearing 2–4 denticles, the most distal of which is very large. Te longer ‘posterior’ process is straight and bears 7–8 denticles, with the most distal one of which are enlarged.

Te M element is dolabrate in shape. It has a long, robust, sharp cusp with a relatively long denticulate ‘posterior’ process and a weak anticusp at on the ‘under side’. Tere is marked gap between the main cusp and the first of the denticles. Te first denticle is longer than other delicate ‘posterior’ denticles. Te ‘posterior’ process has a gentle incurvature. Of the M elements, many elements are described as the “makellate” forms in previous studies. However, according to the definition of the “makelliform” in Sweet (1988, P. 24–25), the “makelliform” elements include various pick-shaped forms ( Sweet, 1988), such as, arched dolabrate, bipennate, breviform digyrate ramiform elements or geniculate coniform types. Terefore, we use the more specific term ‘dolobrate’ here.

TeP 2 elements are angulate with a robust cusp, long or short ‘anterior’ and ‘posterior’ processes carrying several denticles of varying sizes. Te sizes of the ‘anterior’ and ‘posterior’ processes vary among different the specimens. In specimen 2023SSCNA1, the ‘posterior’ process is approximately twice the length of the ‘anterior’ process ( Fig. 3E ), whereas in specimen 2023SSCNA2, the length of the ‘anterior’ and ‘posterior’ processes are similar ( Fig. 4E ). Not all denticles are the same size on the processes, in the sinistral P 2 element of specimen 2023SSCNA1, the second denticle is the largest of the ‘anterior’ process ( Fig. 3E ). Near the ‘anterior’ and ‘posterior’ ends of the ‘posterior’ process of the dextral P 2 element ( Fig. 3E ), the denticles are relatively large and long, while others are smaller. However, there is a pattern where some smaller second-order denticles intercalate between the bigger first-order denticles on the ‘posterior’ process of the dextral P 2 element. Tis pattern differs from the sinistral P 2 element in this assemblage with no second-order denticles ( Fig. 3E ). About the P 2 elements in specimen 2023SSCNA2 ( Fig. 4E ), the second and third denticles in the middle are the largest among the five denticles on the ‘anterior’ process. On the posterior process, the first and third denticles are larger than the other two.

Te P 1 elements are carminiscaphate with a large upright cusp ( Fig. 4C , 5E ), followed by 6 small denticles arranged with triangular gaps between them. Te ‘posterior’ edge is high, descending abruptly, nearly upright. Te basal cavity is ‘laterally’ enlarged in the ‘posterior’ half, with a smooth ‘upper surface’.

Figures

Fig. 3 (See legend on previous page.)

Fig. 6 The composition of the H.parvus apparatus based on conodont natural assemblages from the Shangsi section. Color coding follows Fig. 2

Fig. 5 Conodont natural assemblage of H. parvus. Collection number 2023SSCNA2 counterpart,the sample comes from the Bed 31. A–B, photographs were taking by stereomicroscope under the different angle incident lights. C–D, SEM photograph of the whole conodont natural assemblage, and its corresponding outline drawing, the solid black lines represent the edges of the conodont elements,while the dashed lines represent the edges of the mouldic conodont elements.E, the P1 element. F, the enlarged S-M elements. All scale bars represent 400 μm. The color coding is as described in Fig. 2

Fig. 4 Conodont natural assemblage of H. parvus. Collection number 2023SSCNA2 part, the sample comes from the Bed 31. A–B, photographs were taking by stereomicroscope under the different angle incident lights.C–D, SEM photograph of the whole conodont natural assemblage, and its corresponding outline drawing,the solid lines represent the edges of the solid conodont elements, while the dashed lines represent the edges of the mouldic conodont elements. E, the enlarged S-M elements and paired P2 elements. All scale bars represent 500 μm. The color coding is as described in Fig. 2