Testing hypotheses of element loss and instability in the apparatus composition of complex conodonts: articulated skeletons ofHindeodus

Abstract

Knowledge of the conodont skeleton, in terms of the morphology of the elements and the positions they occupy, provides the foundation for understanding of homology, taxonomy and evolutionary relationships in conodonts. This knowledge also underpins analyses of conodont functional morphology and feeding. Direct evidence of skeletal anatomy and apparatus architecture comes from natural assemblages: fossils that preserve together the articulated remains of the conodont apparatus, either collapsed onto a bedding plane or as clusters of elements in which juxtaposed and overlapping elements have been fused together by diagenetic minerals. Here we describe six clusters of the biostratigraphically important conodont Hindeodus parvus from the Lower Triassic Shangsi section, Sichuan Province, South China. Five of these clusters represent the partial remains of articulated skeletons, providing direct evidence of the number and arrangement of elements in the apparatus. Combined with data from previously published natural assemblages this provides a test of the hypothesis that Triassic conodonts had a reduced dentition. Hindeodus parvus possessed a complete raptorial array of two M and nine S elements (unpaired S0; symmetrically paired S1, S2, S3, S4); the paired P1 locations were occupied by carminiscaphate elements, but the apparatus lacked P2 elements. This is consistent with broader evidence for a particularly high degree of integration and constraint operating on the S–M array of morphologically complex conodonts, leading to conserved architecture of the array over a period of more than 250 million years. The loss of elements from the P domain implies a change in food processing ability and, given the predominance of data from P elements in conodont taxonomy and biostratigraphy, the hypothesis of element loss from the P domain has significant implications for the broader understanding of conodont diversity and evolutionary patterns.

Comments on: Testing hypotheses of element loss and instability in the apparatus composition of complex conodonts (Zhang et al.)

“…Images of discrete elements in Zhang et al . () include views of five different elements, but not of the problematic S 1 /P 2 element.…”

“…This proposed reconstruction leaves the apparatus without P 2 elements, which is interpreted by the authors to be an example of element loss caused by changes in food capture (Zhang et al . , p. 11). Agematsu et al .…”

“…Zhang et al . () discussed the apparatus composition of the Early Triassic conodont Hindeodus parvus on the basis of six fused clusters of elements recovered from Shangsi in South China. They used the evidence from these to suggest that the previous reconstruction of the apparatus of H. parvus by Agematsu et al .…”

“…The most striking difference between the reconstructions proposed by Zhang et al . (, fig. 10) and Agematsu et al .…”

“…Herein, we question the new reconstruction of Zhang et al . () on the basis of three main lines of evidence: (1) X‐ray CT scans of additional natural assemblages of Hindeodus parvus from Funabuseyama in Japan (Agematsu et al . ), which allow the recognition of 15 elements in the apparatus; (2) a previously unpublished reconstruction of the multielement apparatus of Lower Triassic Hindeodus based on discrete elements from the Lower Triassic of Heping in South China, which illustrates the morphology of both the P 2 and S 1 elements of this genus; and (3) a review of all previous reconstructions of Hindeodus , from the Mississippian to the Early Triassic, which demonstrates the striking conservatism of the apparatus composition of this genus throughout more than 100 myr.…”