Agematsu et al.—Apparatus reconstruction of Hindeodus parvus


(Kozur and Pjatakova, 1976), H. changxingensis Wang, 1995, H. postparvus Kozur, 1989, and H. sosioensis Kozur, 1996 (e.g., Sweet, 1977, 1988; von Bitter and Plint-Geberl, 1982; von Bitter and Plint, 1987; Kozur, 1996). From the cladistic analysis of Donoghue et al. (2008), the


genus Hindeodus currently belongs to the superfamily Polygnathacea, suborder Ozarkodinina, order Ozarkodinida. That analysis used the premise that the multielement recon- struction of this genus proposed by Sweet (1977) and others is strongly reliable. In this study, the apparatus of the earliest Triassic conodont H. parvus is reconstructed using synchrotron radiation micro-tomography (SR–μCT). Our conclusion sup- ports Donoghue and his colleagues’ premise and provides a taxonomic framework for the genus.


Materials and methods


Agematsu et al. (2015) described natural assemblages of H. parvus and H. typicalis from siliceous claystones of the Hashikadani Formation, which forms part of the Mino Terrane, a Jurassic accretionary complex in central Japan. Their study section (3.8m thick) consists of uppermost Permian chert and lowermost Triassic black claystone; the latter contains con- odonts (Fig. 1). Because all specimens are preserved as impressions and some additional elements may be hidden under the visible molds, the composition of the assemblages cannot be completely observed under binocular or scanning electron microscopes. According to Agematsu et al. (2015), the assem- blages of both species comprise at most 13 elements, including pairs of P1,P2, and M elements, as well as a single S0 element with two digyrate and four bipennate elements making up the S array. A pair of S1 elements was not recognized. In this study, the multielement apparatus of H. parvus is reconstructed using SR–μCT. SR–μCT measurements were performed at experimental


hutch 1 of BL20B2 in the synchrotron radiation facility, SPring- 8, Hyogo, Japan (Goto et al., 2001). Claystone samples con- taining natural conodont assemblages were cut into small pieces ~10–20mm high, 5–10mm wide, and 2–5mm thick. The measurement conditions of the mirco-tomography were as fol- lows: X-ray energy was 15–35 keV; number of projections was 1800 for 180 degrees; exposure time for one projection was 200 msec; total scan time of one CT measurement was ~8 minutes; effective pixel size was 2.76 μm; distance from the sample to detector was set to 70 mm. The CT reconstruction was done with convolution back


projection method (Uesugi et al., 2010) after phase retrieval (Paganin et al., 2002). Three-dimensional images were built from the stacked 2D images with volume rendering using the open-source visualization software Drishti (Limaye, 2012). Nineteen samples were analyzed, of which two yielded


well-preserved fossil images. Figure 2 illustrates specimens of the ventral and dorsal parts of the assemblage, which originally constituted one natural assemblage. The images include the claystone-air interface and distinctly show the outline of each element. This fossil was recovered from a horizon 40cm above the base of the claystone strata, which is correlated with the lower Induan (Sano et al., 2010).


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Figure 1. Locality and horizon of the conodont natural assemblage: (1) locality map of the study section; (2) lithologic column of the study section indicating the sample horizon.


Repository and institutional abbreviation.—Two specimens described herein are deposited at the Doctoral Program in Earth Evolution Sciences, University of Tsukuba, Japan, with the prefix EESUT.


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