Xianguangia sinica Chen & Erdtmann, 1991

Zhao, Yang, Hou, Xian-guang & Cong, Pei-yun, 2023, Tentacular nature of the ‘ column’ of the Cambrian diploblastic Xianguangia sinica, Journal of Systematic Palaeontology (2215787) 21 (1), pp. 1-15 : 3-7

publication ID	https://doi.org/ 10.1080/14772019.2023.2215787
DOI	https://doi.org/10.5281/zenodo.10980168
persistent identifier	https://treatment.plazi.org/id/775187CE-0A45-FFAF-FC3F-78F5E353F8EE
treatment provided by	Felipe
scientific name	Xianguangia sinica Chen & Erdtmann, 1991
status

Treatment

( Figs 1–3 View Figure 1 View Figure 2 View Figure 3 )

$1991 Xianguangia sinica Chen & Erdtmann ; Chen & Erdtmann: 64–65, 73–74, pl. 3, figs 1–4.

1996 Xianguangia sinica Chen & Erdtmann ; Chen, Zhou, Zhu & Yeh: 93–94, figs 96–99.

1997 Xianguangia sinica Chen & Erdtmann ; Chen & Zhou: 30, fig. 24.

v.1999 Xianguangia sinica Chen & Erdtmann ; Hou, Bergstrom, Wang, Feng & Chen: 50–51, figs 54–56.

1999 Xianguangia sinica Chen & Erdtmann ; Luo, Hu, Chen, Zhang & Tao: 86, pl. 22, fig. 1.

2002 Xianguangia sinica Chen & Erdtmann ; Chen, Luo, Hu, Yin, Jiang, Wu, Li & Chen: 8, 39, pl. 21, fig. 4.

v.2003 Xianguangia sinica Chen & Erdtmann ; Hou & Bergstrom: 62, 64, fig. 8(2).

2004 Xianguangia sinica Chen & Erdtmann ; Chen: 164– 165, figs 246–247.

v.2004 Xianguangia sinica Chen & Erdtmann ; Hou, Aldridge, Bergstrom̈, Siveter, Siveter & Feng: 54–55, fig. 9.1.

2014 Xianguangia sinica Chen & Erdtmann ; Lei, Han, Ou & Wan: 968–970, fig. 3.

2017 Xianguangia sinica Chen & Erdtmann ; Ou, Han, Zhang, Shu, Sun & Mayer: 8835–8839, figs 1–3, S1–S3.

v.2017 Xianguangia sinica Chen & Erdtmann ; Hou, Siveter, Siveter, Aldridge, Cong, Gabbott, Ma, Purnell & Williams: 80–81, fig. 11.4.

v.2019 Xianguangia sinica Chen & Erdtmann ; Zhao, Vinther, Parry, Wei, Green, Pisani, Hou, Edgecombe & Cong: 1113–1114, fig. 1.

Holotype. NIGPAS 108506 ( Chen & Erdtmann, 1991, pp. 73–74).

Figured material. YKLP 13477 (Fig. 2D), YKLP 13478 (Fig. 1A–D), YKLP 13496 (Fig. 2A–C, E–G), YKLP 13829 (Fig. 1E, F), YNGIP 90001 (Fig. 3A–C), YNGIP 90002 (Fig. 3D), YNGIP 90003 (Fig. 3E).

Other material. YKLP 13429, YKLP 13473, YKLP 13474, YKLP 13475, YKLP 13476, YKLP 13830, YNGIP 90004 to YNGIP 90018.

Amended diagnosis (amended after Ou et al., 2017, pp. 8835–8836). Solitary, polypoid metazoan, with a calyx and a whorl of 18 tentacle-sheath complexes surrounding an oral region. Calyx with a digestive tract inside. Underside of the calyx smooth and convex with a prominent, circular pit. Putative oral region with a circumferential constriction, separating the tentacle-sheath complex from the calyx. Tentacle comprising a proximal tentacle rod and a distal flexible portion. Tentacle rod lightly sclerotized, bearing serially arrayed paired dark stains and enveloped by a single sheath externally. Both proximal and distal parts of the tentacle biserially flanked by flexible, ciliated pinnules. Cilia large, long and thick, densely arrayed along either side of each pinnule.

Locality. Yu’ anshan Member of the Chiungchussu Formation, lower Cambrian (Series 2, Stage 3, ca. 518 Ma), corresponding to the Eoredlichia –Wutingaspis trilobite Biozone ( Babcock & Zhang, 2001). Fossil specimens were collected from the classic Maotianshan section, and the sites included Ercai, Jianshan, Mafang and Erjie, southern Kunming, Yunnan Province, China.

Description

Gross morphology. Generally, most known specimens of Xianguangia sinica are laterally preserved, displaying a well-defined, distally tapering ‘column’ in the middle part of the body ( Fig. 1A, B, E View Figure 1 ). A circumferential constriction (‘Cc’, Fig. 1E View Figure 1 ) delimits the ‘column’ and the calyx. A few longitudinal ridges (‘ T 1– T 8’, Fig. 1A, E View Figure 1 ) in the ‘column’ are nearly parallel to one another.

Tentacle-sheath complex. One laterally preserved specimen displays a Xianguangia - type calyx that is cylindrical in shape (ca. 19.2 mm height and 20.9 mm wide) and a displaced ‘column’ part ( Fig. 2A–C View Figure 2 ). Although the ‘column’ is not directly connected with the calyx as it was normally preserved, it shares the same structural composition and gross shape with the undetached ‘column’. The displaced ‘column’ part preserved 12 longsword-shaped structures (‘ T1 – T12 ’, Fig. 2C View Figure 2 ) that are splayed out around the top of the calyx ( Fig. 2A–C View Figure 2 ). Each longsword-shaped structure has a clear, constant edge, further indicating that it is a separate unit. These units are ca. 29.5 mm long and a maximum of 3.4 mm wide (at the basal end). The same longsword-shaped structures, which are delimited by longitudinal deep folds that are formed by the tight contact between adjacent ones, can also be identified in these classical specimens preserved in lateral view (‘ T1 – T8 ’, Fig. 1A, B, E View Figure 1 ). These longsword-shaped structures are here termed tentacle-sheath complexes. A total of 18 tentacle-sheath complexes can be identified in a specimen compacted along the oral-aboral axis ( Fig. 2D View Figure 2 ) .

Two furrows with slight relief (‘Fu’, Figs 1E View Figure 1 , 2D, E View Figure 2 ) that converge distally can be discerned on the tentacle-sheath complex under low-angle light. Occasionally, a shallow longitudinal groove extends along the central section of the tentacle-sheath complex ( Fig. 2E View Figure 2 ). These observations together indicate that the complex was likely a three-dimensional wedge-shaped projection when alive. The sheath (‘Os’, Fig. 1A–C View Figure 1 ) is smooth on the surface and fits closely together with tentacles due to compression. Each sheath is wider than the tentacle ( Fig. 1A–C View Figure 1 ), and likely surrounds a tentacle that was preserved either as slight relief ( Fig. 2E View Figure 2 ) or as red-brown or brown-yellow in colour ( Fig. 1A–C View Figure 1 ). The tentacle tapers distally to form a flexible portion at 65–75% of the whole tentacle length (herein termed distal tentacle, ‘Dt’, Fig. 1A, E, F View Figure 1 ), which is arrayed and spread at the end of the sheaths, while the proximal portion (herein named tentacle rod, ‘Ter’, Fig. 1B–D View Figure 1 ) of the tentacle is stiff and robust, suggesting light sclerotization.

Dark stains. Paired dark stains are serially arrayed along the tentacle rod, and distinctly identified in the distal region of T 10 and T 11 with teardrop-shaped relief under low-angle light (‘Ds’, Fig. 2E, F View Figure 2 ). The size of the stains is variable (up to 0.35 mm wide and 0.83 mm long in T 11) and gradually decreases distally. A median dark line extends longitudinally between paired dark stains ( Fig. 2F, G View Figure 2 ), but no branches connecting with the stains are identified.

When the tentacle-sheath complexes are closed to form a tapering ‘column’, dark stains are discernable as well ( Figs 1A, B, E View Figure 1 , 3A View Figure 3 ). In YKLP 13478, the inner side of the tentacle rod was exposed when the specimen was split into part and counterpart, showing about six to eight well-defined rows of paired stains longitudinally extending to the base of the ‘column’ ( Fig. 1A, B View Figure 1 ). Dark stains appear to be present only in the tentacle rod ( Fig. 1A, E, F View Figure 1 ). These stains have a teardrop-like shape generally with an obvious dark outline, but also vary in size.

Pinnules. Pinnules (‘Pin’, Figs 1 View Figure 1 , 3A View Figure 3 ) are fringed serially on the distal tentacle ( Fig. 1E, F View Figure 1 ) and the proximal tentacle rod ( Fig. 1A–D View Figure 1 ). They were preserved as irregular dark patches, sometimes fading slightly, probably due to decay and/or weathering. In YKLP 13478, each pinnule can reach ca. 1.8 mm long and 56 µm wide. The space between adjacent pinnules is ca. 71– 123 µm. Overall, pinnules project to the upper right corner when they appear in the left half part and vice versa ( Figs 1A–D View Figure 1 , 3A View Figure 3 ), suggesting that they subtend to the oral-aboral axis when alive.

Dark patches. Finger-like dark patches are frequently preserved in the lower middle region of the ‘column’ (‘Dp’, Fig. 3 View Figure 3 ). This structure differs from the surrounding region with its definite shape with dark colour and sometimes seems to extend downwards to the oral surface of the calyx ( Fig. 3A–C View Figure 3 ), with the colour fading or turning to red-brown. The patches contain a few longitudinal lines in the most cases ( Fig. 3B–E View Figure 3 ) and are occasionally infilled with sediment ( Fig. 3E View Figure 3 ). The number of patches varies from two to three. When present as three, the two lateral patches sometimes incline towards the middle patch that is near vertical ( Fig. 3D, E View Figure 3 ).

Remarks. The rough surface of the column-like body part of Xianguangia sinica appears to have parallel, ‘longitudinal ridges’ observed in laterally preserved specimens, which were generally regarded as evidence for the presence of mesenteries ( Chen & Erdtmann, 1991; Ou et al., 2017). The present study illustrates that the ‘longitudinal ridges’ are actually discrete tentacle-sheath complexes, which have the ability to splay out freely. The assumed presence of mesenteries and digestive tract inside the ‘column’ is accordingly rejected. Instead, these structures are more likely to appear in the calyx ( Zhao et al., 2019). The organic material preserved in the lower central part of the ‘column’ ( Ou et al., 2017) is likely caused by the decay of the three dark patches, which likely derive from the oral surface. Paired dark stains are serially arrayed along the inner side of each tentacle rod, which is less likely to be a taphonomic artefact because of their regular arrangement and constant topological position across specimens. Given that dark stains and ciliated pinnules can be obscured by the tentacle-sheath complexes when specimens were preserved in lateral view, they have not been readily discerned in previous material of X. sinica . The preserved morphology of the stains and the pinnules is depicted here when tentacle-sheath complexes were decayed away or removed by splitting. It is therefore inappropriate to erect the present material as a new species because it has the same body composition and gross morphology as X. sinica .