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Journal of Paleontology 91(6):1220–1227
pairs of P1,P2,S2,S3, and S4; single S0; dextral S1; and parts of paired M elements. The latter mainly comprises a ramiform cluster, including sinistral–dextral pair of M, sinistral S1, and parts of S2,S3, and S4 elements. Carminiscaphate P1 bears the largest distal denticle on an
‘anterior’ process. That denticle is twice as high as the other five, accompanied by accessory node on ‘posterior’ end of unit. Basal cavity expands ‘laterally,’ occupying ‘posterior’ two-thirds of element. P2 angulate, with long robust cusp and short ‘anterior’ process bearing large denticle. Relatively long ‘posterior’ process carrying at least a number of short denticles twists distally; its ‘inner’ surface faces medial side. Small basal cavity opens under cusp. Makellate M with long slender cusp, very short ‘inner lateral’ process, and denticulate ‘outer lateral’ process with relatively long denticle adjoining cusp. Unit bends sharply ‘posteriorly’ just ‘outside’ cusp. S0 symmetrical alate lacking ‘posterior’ process; both
‘lateral’ processes high and short. Digyrate elements in S1–2 positions possess long robust cusp and denticulate ‘inner’ and ‘outer lateral’ processes, although distal parts of ‘outer lateral’ processes of both S1 elements not preserved in thismaterial. ‘Inner lateral’ process higher and shorter than ‘outer lateral’ one; the latter twists at joint and its ‘posterior’ surface turns ‘upward’. Bipennate elements in S3–4 positions characterized by long slender cusp, denticulate long ‘posterior’ process, and short ‘anterior’ process bearing relatively long ‘antero-terminal’ denticle.
Materials.—Two clusters constituting one natural assemblage: EESUT-ag0005, 0006.
Remarks.—On the basis of the morphology of the P1 and S1–2 elements, the specimens are identified as H. parvus. According to Kozur (1996), the S1–2 element of this species is distinguished from those of H. typicalis and H. latidentatus (Kozur, Mostler, and Rahimi-Yazd, 1975) by the bending position of the unit: the S1–2 elements of the latter two species bend ‘posteriorly’ just ‘outside’ a cusp; in contrast, those of H. parvus bend ‘poster- iorly’ just ‘inside’ a cusp. Because both the sinistral and dextral S1 elements in the present specimens are broken just ‘inside’ the cusp, these cracks indicate the bending position. This feature of the S1–2 elements is consistent with H. parvus.
Results and discussion
Natural assemblage of Hindeodus parvus.—The SR–μCT ima- ges clearly show that an apparatus of H. parvus is composed of 15 elements settled in sinistral–dextral pairs in the P1,P2,M,S1, S2,S3, and S4 positions and a single S0 position probably on the median plane (Figs. 3, 4; Supplemental Data 1). The element structure is in agreement with the definition of Hindeodus by Sweet (1977). The SR–μCT images also clarify the three- dimensionalmicroarchitecture of the natural assemblage in terms of the general location and direction of elements in the apparatus. A crown-shaped alate element without a ‘posterior’
process, the S0 element, lies on the rostralmost side of the specimens. The element has the ‘upper’ side turned rostrally and the ‘anterior’ surface ventrally. It is probable that the element is on the median plane and its ‘anterior’ surface was oriented in the ventral or rostral direction in the living conodont body.
The central parts of the specimens are made up of ramiform clusters, consisting of juxtaposed elements in the S1–S4 and M positions. The so-called S array, which is the main part of the ramiform cluster, comprises two pairs of digyrate S1–2 elements on the inside and two pairs of bipennate S3–4 on the outside. Although a sinistral S1 element is out of place, the ‘inner lateral’ and ‘outer lateral’ processes of the other digyrate elements point to the rostral and caudal directions of the apparatus, respec- tively. Their ‘posterior’ surfaces face ventrally. The bipennate elements have their cusp and a terminal denticle on an ‘anterior’ process pointing in the rostro-inner direction, and with a ‘posterior’ process extending caudally. The ‘inner’ surfaces of these elements are turned to the ventral side of the specimen. Two makellate elements, located in the M positions, lie on the dorso-sinistral and -dextral sides of the S array with cusps pointing to the rostro-inner side and the ‘outer lateral’ process at the caudal side. The ‘posterior’ side faces ventrally. It is quite likely that the ‘outer lateral’ processes of the S1–2 and M elements and the ‘posterior’ ones of the S3–4 elements were arranged approximately parallel to the rostro-caudal axis in the living conodont body. In addition, all elements in the ramiform clusters seem to have their concave surfaces directed ventrally or inwardly as a whole. A pair of angulate elements in the P2 position is placed
between the caudal half of the sinistral and dextral S arrays. The ‘inner’ sides face ventrally. The ‘upper’ or ‘inner lateral’ surfaces appear to be opposite each other across the median plane. In the caudalmost part, there are P1 positions in which two carminiscaphate elements are disposed. Although the posture of the P1 elements in the living conodont body is unclear, the ‘upper’ surfaces at least appear to face each other. This arrangement and direction of elements is also
supported by the natural-assemblage specimens analyzed by Agematsu et al. (2015). This reconstruction is nearly identical to a well-known apparatus model of ozarkodinids including 4P-2M-9S elements (Purnell and Donoghue, 1997; Purnell et al., 2000).
Terms for orientation of S1–2 elements.—Elements disposed in the S1–2 and S3–4 positions have been labeled Sb and Sc, respectively, in previous studies. Sweet (1988) stated that the Sb element was the most diagnostic feature of the Hindeodus apparatus. According to Kozur (1996), the morphological dif- ferences of some Hindeodus Sb elements are sufficient to help with identification of species. However, the terms for orientation of the digyrate elements have been confused, and thus need to be summarized here. Most papers reporting Hindeodus S1–2 elements and their
equivalents have described the two processes of the element as the ‘anterior’ and ‘posterior’ ones, whereas Sweet (1977) defined the Sb element as a digyrate form with two ‘lateral’ processes. The present study follows the usage of Sweet and describes these processes as ‘lateral.’ An additional problem is that many previous studies have stated that the S1–2 elements of Hindeodus are characterized by an ‘upwardly’ bent process (e.g., Sweet, 1988). This character is sometimes difficult to discriminate. The LE element of Ellisonia teicherti Sweet, 1970a, which is included in some Hindeodus species as part of the ramiform complexes, is equivalent to the S1–2 elements.
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