Kiangyousteus yohii, LIU, 1955

Zhu, You-An & Zhu, Min, 2013, A redescription of Kiangyousteus yohii (Arthrodira: Eubrachythoraci) from the Middle Devonian of China, with remarks on the systematics of the Eubrachythoraci, Zoological Journal of the Linnean Society 169 (4), pp. 798-819 : 801-805

publication ID	https://doi.org/ 10.1111/zoj.12089
persistent identifier	https://treatment.plazi.org/id/936C297A-BE2D-F567-FEC4-FD74FE7AF90F
treatment provided by	Marcus
scientific name	Kiangyousteus yohii
status

Treatment

KIANGYOUSTEUS YOHII LIU, 1955

Holotype

IVPP V801, displaced plates in association, including a right anterior supragnathal ( IVPP V801.8 , Fig. 3 View Figure 3 ), a parasphenoid ( IVPP V801.1 , Figs 4 View Figure 4 , 5E View Figure 5 ), a median dorsal ( IVPP V801.4 , Fig. 6 View Figure 6 A−C), a submedian dorsal ( IVPP V801.6 , Fig. 6D, E View Figure 6 ), a right anterior lateral ( IVPP V801.2 , Fig. 7 View Figure 7 ), a right anterior ventrolateral ( IVPP V801.5 , Fig. 8A, B View Figure 8 ) and a right posterior dorsolateral ( IVPP V801.3 , Fig. 8C, D View Figure 8 ).

Type locality and horizon

Mount Guanwushan of Jiangyou, Sichuan Province, China; Guanwushan Formation, Givetian, Middle Devonian.

Emended diagnosis

Dunkleosteid species in which the parasphenoid bears an articulation with the anterior supragnathal; the median dorsal plate bears a dorsal branch of the main lateral line; and the anterior supragnathal bears an occlusal shelf posterior to the lateral cusp.

Morphological description

Head shield: Anterior supragnathal ( IVPP V801.8, Fig. 3 View Figure 3 ): The right anterior supragnathal, comprising anterior, lateral, and dorsal laminae (ala; Fig. 3C View Figure 3 . lla, dla; Fig. 3A, B View Figure 3 ), closely resembles that of Dunkleosteus and Gorgonichthys in shape ( Dunkle & Bungart, 1946). The lateral lamina possesses only one lateral cusp (lr.cusp; Fig. 3A, B View Figure 3 ; Dunkle & Bungart, 1946: fig. 1, anteroventral cusp). Two ridges or thickenings (cusp.th; Fig. 3B, C View Figure 3 ) reinforce the cusp on both labial and lingual sides. Posteriorly to the inner cusp ridge a depressed occlusal accommodation (pr.occ; Fig. 3B View Figure 3 ) for the cusp of infragnathal plate is visible. The arrangement differs from that of Dunkleosteus terrelli Newberry, 1873 , in which the occlusal accommodation for the infragnathal cusp lies anterior to the supragnathal cusp ( Stensiö, 1963: fig. 118a−c). The incised occlusal surface extends mesially to the inner surface of the anterior lamina.

The possible posterior process and the contact region to the parasphenoid on the posterior part of the dorsal lamina of the plate are missing. A depression (ch.eth; Fig. 3C View Figure 3 ) for the articulation with subnasal elements as defined in Dunkle & Bungart (1946) is present on the outer face, although obscure because of poor preservation. No tubercle or adsymphyseal denticle is present on the plate.

Parasphenoid ( IVPP V801.1, Fig. 4 View Figure 4 ): This plate, initially described by Liu (1955) as the pineal plate, was later identified by Obruchev (1964) as the parasphenoid. It is roughly pentagram-shaped, and can be divided into pre- and posthypophysial divisions by the level of the buccohypophysial foramen (f.bhy; Fig. 4A, B View Figure 4 ). Unlike most arthrodires, the prehypophysial division is short and heavily thickened (a.th; Fig. 4A View Figure 4 ), with two concave facets for the contact with anterior supragnathals (a.con; Fig. 4B View Figure 4 ) at its anterior extreme. Amongst the genus Dunkleosteus, Carr & Hlavin (2010) identified two contact types between the parasphenoid and supragnathal plates: transverse articular facets in D. terrelli and Dunkleosteus raveri Carr & Hlavin, 2010 , and thickened anterolateral contact face in Dunkleosteus amblyodoratus Carr & Hlavin, 2010 . Kiangyousteus shows transverse facets resembling those of D. terrelli and D. raveri . The septum between the contact facets forms the anterior process of the ‘pentagram’. The posthypophysial division is larger and thinner than the prehypophysial division, with two posterior processes of the ‘pentagram’ separated by a large gap. A similar posterior profile can also be seen on the parasphenoid of Dunkleosteus ( Carr & Hlavin, 2010) .

Dennis-Bryan (1995) reversed the pre- and posthypophysial regions of the parasphenoid of Kiangyousteus from the earlier interpretation by Denison (1978). Carr & Hlavin (2010) followed Denison (1978) regarding the orientation of the plate and suggested an alternative interpretation that the anterior part of the prehypophysial region was incomplete, according to which the restored parasphenoid might resemble the typical pachyosteomorph condition with a longer and thinner-built prehypophysial region. However, based on the comparison with the parasphenoid of D. terrelli , we consider that the prehypophysial region is short and almost complete.

The dorsal surface of the plate is coarse. The buccohypophysial thickening (bhy.th; Fig. 4A View Figure 4 ) bears a pituitary depression, which accommodates the paired buccohypophysial foramen. The right extremity of the thickening develops a rounded lateral (posterolateral) process (pl.pr; Fig. 4A View Figure 4 ) as defined by Gardiner & Miles (1990). The absence of process on the left counterpart and the asymmetry of the buccohypophysial thickening may be the result of post-mortem deformation or weathering, as the dorsal surface was exposed when discovered ( Liu, 1955). There is no sign of a median hypophysial vein foramen. Posterior to the buccohypophysial thickening is a second weak thickening (sec.th; Fig. 4A View Figure 4 ), forming two steps. No visible dorsal median groove is present.

The ventral surface is relatively plain and smooth, with only radiation textures visible. The transverse ventral groove (gr.a.com; Fig. 4B View Figure 4 ) is very shallow, disappearing near the mid-line. Two deep notches are visible at the lateral edges of the plate, giving the plate a pentagram-like outline. An inverted V-shaped prehypophysial ventral median crest (cr.m; Fig. 4B View Figure 4 ) is present. The anterior end of the crest, forming a downward process, does not reach the anterior edge of the plate, contrary to the conditions observed in Eastmanosteus pustulosus Eastman, 1897 , and Eastmanosteus calliaspis Dennis-Bryan, 1987 ( Fig. 5 View Figure 5 ). No tubercular or denticulate ornament is present on any part of the plate. Dennis-Bryan (1995) stated that the parasphenoid of placoderms is speciesspecific, lacking synapomorphies useful in phylogenetic analysis. Whereas the parasphenoid of Kiangyousteus bears specialized features such as the pentagram shape, the extremely shallow ventral groove, and the short inverted V-shaped median crest, it shows definitive similarities to the ‘dunkleosteid type’ parasphenoid as defined in Stensiö (1963) in possessing articular facets. As will be studied in the following phylogenetic analysis, certain characters of the parasphenoid can be useful for the investigation of eubrachythoracid phylogeny and the systematic position of Kiangyousteus .

Trunk shield: Median dorsal plate ( IVPP V801.4, Fig. 6A, B, C View Figure 6 ): The median dorsal plate ( MD) is shovelshaped and is arched transversely. The anterior border is emarginated, and forms two anterolateral horn-like processes together with the slightly concave lateral edges. The posterior border is rounded.

The right half of the plate is heavily deformed. A reconstruction based on the intact left half reveals that the median dorsal plate of Kiangyousteus is shorter and broader than former interpretations by Liu (1955) and Denison (1978). However, with a length/width ratio of 1.2, the median dorsal plate of Kiangyousteus is still coded as ‘long and narrow’ in the current data matrix (Appendix 1, character 26).

In comparison, most members of the Pachyosteomorphi have a short and broad median dorsal plate (length/breadth ratio <1), with the exception of E. calliaspis , whose median dorsal plate has a length/ breadth ratio of 1.5. Belosteus elegans Jaekel 1919 ( Stensiö, 1963) also has a narrow median dorsal plate, but in this particular case it is probably a derived state owing to the lateral compression of the entire body.

In visceral view, a well-developed ventral keel (kv; Fig. 6B View Figure 6 ) goes along the midline. Most of its rear part is missing. Judging from the fracture face of the missing carinal process (ff.pca; Fig. 6B View Figure 6 ), the keel bears a stout posterior carinal process at its end. The carinal process does not go beyond the posterior border of the median dorsal plate. Two ventral transverse thickenings ( MD.th; Fig. 6B View Figure 6 ) extend from the base of the carinal process to the lateral edges.

The dorsal surface of the plate is covered with densely distributed small tubercles. The dorsal branch of the main lateral sensory line (ld; Fig. 6A View Figure 6 ) is visible on the dorsal surface of the median dorsal plate as a shallow groove.

Anterior lateral plate ( IVPP V801. 2, Fig. 7 View Figure 7 ): The anterior lateral plate ( AL) is a thick triangular plate with an obstantic process (obst.pr; Fig. 7A, B View Figure 7 ). The large upper part restored by Liu (1955: fig. 4a), referring the plate to the type of Dinichthys (possibly Dunkleosteus here), does not exist. Nor does it resemble the near-equilateral triangular AL shape of coccosteomorphs, as the postbranchial embayment (em.pb; Fig. 7A View Figure 7 ) on the anterior border is clearly developed, and the anterior ventral part of the anterior lateral plate is moderately long.

In general shape, the anterior lateral plate of K. yohii can be compared to that of E. calliaspis ( Dennis-Bryan, 1987: fig. 22c, d), except that the former possesses an exceptionally developed postbranchial lamina (pbl; Fig. 7B, C View Figure 7 ), which extends down and beyond the anteroventral corner on the external surface of the anterior lateral plate. The extension of the postbranchial lamina on the anterior lateral plate indicates that the interolateral plate possesses a branchial lamina as well. A tubercular ornament is present both on the exposed surface of the plate and on the postbranchial lamina (lam.dent; Fig. 7C View Figure 7 ). The postbranchial lamina ornament would extend to the postbranchial lamina of the interolateral plate.

Anterior ventrolateral plate ( IVPP V801.5, Fig. 8A, B View Figure 8 ): This is an almost flat triangular plate, with a Y-shaped thickening ( AVL.th; Fig. 8B View Figure 8 ) on the anterior part of the inner surface. Anteriorly, there is an overlap area for the interolateral plate ( IL.oa; Fig. 8A View Figure 8 ). A moderate lateral embayment forms the subpectoral emargination (s.e; Fig. 8A View Figure 8 ). The existence of an overlap area along the embayment described by Liu (1955) is doubtful.

A tubercular ornament is present on the external surface of the plate. The tubercles in the middle are slightly larger than those along the margins, opposite to the ornament pattern of the median dorsal plate. The transverse ventral sensory line groove (vts; Fig. 8A View Figure 8 ) is present.

Posterior dorsolateral plate ( IVPP V801.3, Fig. 8C, D View Figure 8 ): The anterior part of the posterior dorsolateral plate ( PDL) is missing. The missing part was reconstructed with plaster on the current specimen before this study. The posterior margin of the plate is very long and slightly concave. A poorly preserved portion of the area overlapped dorsally by the median dorsal plate is clearly identifiable ( MD.oa, Fig. 8A View Figure 8 ), and shows the extension and angle of the contact between the two plates. The lower part of the plate is reinforced by an internal thickening. A developed socket-like overlap area ( PL.oa; Fig. 8A View Figure 8 ) is present for the posterior lateral plate. A tubercular ornament is present on the external surface of this plate.

Endoskeleton: Submedian dorsal plate ( IVPP V801.6, Fig. 6D, E View Figure 6 ): The submedian dorsal plate of Kiangyousteus differs from the oval-shaped submedian dorsal plate of Coccosteus cuspidatus Miller, 1841 ( Miles & Westoll, 1968: fig. 48) in its right trapezoid outline, resembling more that of E. calliaspis , although proportionally larger and sturdier. The plate is a perichondrally ossified element. It is triangular in cross-section and devoid of ornament, with thickenings visible along its anterior and ventral sides. The anterior face shows a fossa for the carinal process of the median dorsal keel (f.pca; Fig. 6D, E View Figure 6 ). An embayment for the neural arches (em.arc; Fig. 6E View Figure 6 ) is visible on the ventral face.