Ontogeny and evolutionary significance of a new acrotretide brachiopod genus from Cambrian Series 2 of South China Author Zhang, Zhiliang State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments, Early Life Institute, Department of Geology, Northwest University, Xi’an 710069, China; & Department of Biological Sciences, Macquarie University, North Ryde, NSW 2109, Australia; & Institute of Earth Sciences, Palaeobiology, Uppsala University, SE- 752 36 Uppsala, Sweden Author Holmer, Lars E. State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments, Early Life Institute, Department of Geology, Northwest University, Xi’an 710069, China; & Institute of Earth Sciences, Palaeobiology, Uppsala University, SE- 752 36 Uppsala, Sweden Author Chen, Feiyang State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments, Early Life Institute, Department of Geology, Northwest University, Xi’an 710069, China; & Department of Biological Sciences, Macquarie University, North Ryde, NSW 2109, Australia; Author Brock, Glenn A. State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments, Early Life Institute, Department of Geology, Northwest University, Xi’an 710069, China; & Department of Biological Sciences, Macquarie University, North Ryde, NSW 2109, Australia; text Journal of Systematic Palaeontology 2020 2020-08-07 18 19 1569 1588 http://dx.doi.org/10.1080/14772019.2020.1794991 journal article 292855 10.1080/14772019.2020.1794991 886e6be0-29c4-42fa-93f6-7ba938cb2b85 1478-0941 10932614 2C95FB13-7B15-43A2-B37C-AAA1B74A2D1C Palaeotreta zhujiahensis ( Li & Holmer, 2004 ) ( Figs 8 , 9 ) 2004 Eohadrotreta zhujiahensis Li & Holmer : 208, figs 14,15. 2016 Eohadrotreta zhujiahensis Z.-F. Zhang et al .: 342, fig. 6. 2018a Eohadrotreta ? zhujiahensis Zhang et al . : 187–197, figs 7–10. 2018b Eohadrotreta ? zhujiahensis Zhang et al . : 4–8, figs 2a, 4. Holotype . Li & Holmer (2004) , NIGP1351 77, conjoined shell (W = 1167 M m). Type locality. Guojiaba Formation (uppermost) at the Fucheng section of Zhenba County, south-eastern Shaanxi , South China . Cambrian Series 2. Paratype . ELI-AJH 8-2-1 CE-03, ventral valve ( Fig. 8I–N, L = 975 M m, W = 1220 M m) from the upper Shuijingtuo Formation at the Aijiahe section of Yichang , South China . Other material. A total of 23 conjoined valves, 124 ventral and 115 dorsal valves from the middle and upper parts of the Shuijingtuo Formation at the Aijiahe section (30 Ǫ 44 , 55.2 ,, N, 111 Ǫ 03 , 58.5 ,, E) of Yichang, western Hubei , South China . Description. Shell ventribiconvex, transverse oval in outline with slightly straightened posterior margin ( Fig. 8 ). 1.3 M m hemispherical pits evenly distributed on the whole metamorphic shell surface without overlapping ( Fig. 9N ), while post-metamorphic shell covered by finely circular growth lines and drape structures ( Fig. 8I ). Shell structure consists of thin-lamella (2 M m) primary layer and thin-lamina (5–10 M m) secondary columnar layers ( Fig. 9O, P ). Ventral valve sub-circular, on average 83% as long as wide with maximum width at the posterior half of valve. It is convex, with a low conical shape ( Fig. 8I–N ), on average 28% as deep as long, with a maximum height almost at mid-valve. Metamorphic shell pronounced at the apex ( Fig. 8B, I ), occupying 31% of the valve length. Pseudointerarea weakly developed, almost catacline, divided by a very short intertrough, which is on average about 5% of the length and 11% of the width of the valve ( Fig. 8H, M ). Apical process weakly developed, occupying on average 30% of valve length, close to pedicle foramen. Pedicle foramen is relatively large, about 90 M m in diameter, enclosed and located directly outside the metamorphic shell until valve reaches about 650 M m in length. Growth lines distinctively developed at the posterior margin of the metamorphic shell ( Fig. 9D, E, L, M ). Cardinal muscle scars and vascula lateralia weakly impressed. Table 1. Average dimensions and ratios of ventral and dorsal valves of Palaeotreta shannanensis gen. et sp. nov. from the Cambrian Series 2 Shuijingtuo Formation.

V	L	W	H	Lm	Lms	Wms	La	Li	Wi	Lc	Wc	Lf	Wf	Lv	L/W	H/L
N	15	13	14	13	9	9	8	16	16	7	7	16	16	4	13	14
Mean	998	1130	208	561	142	199	235	8	70	146	423	45	47	417	88.64%	21.06%
Min	707	750	120	410	101	161	147	0	40	77	258	28	25	274	82.32%	16.97%
Max	1319	1533	282	778	179	240	311	25	99	249	587	68	68	566	94.67%	26.00%
Median	1009	1190	213	538	145	195	221	0	72	129	417	43	48	413	88.07%	21.13%
SD	200	258	38	134	28	26	64	10	15	61	115	10	13	129	3.95%	2.56%
V	Lm/L	Lms/L	La/L	Li/L	Wi/W	Li/Wi	Lf/Li	Wf/Wi	Lf/Wf	Lc/L	Wc/W
N	13	10	9	6	12	7	7	15	16	7	7
Mean	56.07%	12.44%	20.44%	1.55%	6.13%	24.71%	255.64%	66.07%	101.43%	13.86%	36.18%
Min	48.76%	0.00%	0.00%	0.96%	3.34%	18.87%	172.73%	41.77%	47.06%	9.63%	31.64%
Max	64.82%	18.42%	29.19%	2.09%	10.20%	33.33%	409.09%	95.77%	135.71%	20.82%	44.10%
Median	55.63%	13.51%	22.42%	1.56%	5.65%	22.00%	228.57%	65.06%	112.67%	12.54%	34.40%
SD	4.03%	5.01%	8.31%	0.39%	2.01%	5.63%	75.97%	16.68%	27.55%	4.01%	5.09%
D	L	W	H	Lm	Lms	Wms	Ls	Lp	Wp	Lc	Wc	Lg	Wg	L/W	H/L
N	6	6	6	6	5	5	4	4	4	3	3	4	4	6	6
Mean	975	1127	180	537	144	212	602	59	425	172	548	58	189	86.8%	18.5%
Min	890	996	146	490	118	198	354	57	392	145	485	57	156	82.2%	14.8%
Max	1135	1311	225	604	185	230	743	62	465	221	599	59	218	91.3%	22.4%
Median	961	1103	176	521	142	209	655	59	422	150	561	58	190	86.0%	17.7%
SD	92	127	30	52	27	12	171	2	39	43	58	1	32	3.4%	3.2%
D	Lm/L	Lms/L	Lms/Wms	Ls/L	Lp/L	Wp/W	Lg/Lp	Lg/Wg	Wg/Wp	Lc/L	Wc/W
N	6	5	5	4	4	4	4	4	4	3	3
Mean	55.1%	14.7%	67.5%	59.0%	5.8%	36.3%	98.4%	31.4%	44.6%	16.6%	44.5%
Min	49.5%	10.6%	55.8%	38.0%	5.2%	29.9%	93.5%	26.8%	34.5%	13.2%	39.6%
Max	59.1%	18.7%	80.4%	75.1%	6.3%	38.8%	100.0%	37.2%	55.6%	22.3%	48.2%
Median	55.6%	14.1%	68.6%	61.4%	5.9%	38.2%	100.0%	30.9%	44.2%	14.4%	45.7%
SD	3.5%	3.2%	10.0%	15.9%	0.5%	4.3%	3.2%	5.3%	8.7%	5.0%	4.4%

All measurements are in µm. Abbreviations: D, dorsal valve; V, ventral valve; L, H, W , length, height and width of valve where not specified; La , length of ventral apical process; Lc , Wc , length and width of cardinal muscle scars; Lf , Wf , length and width of pedicle foramen; Lg , Wg , length and width of dorsal median groove; Li , Wi , length and width of ventral intertrough; Lm , length at maximum width; Lms , Wms , length and width of metamorphic shell; Lp , Wp , length and width of dorsal pseudointerarea; Ls , length of dorsal median septum; Lv , length of vascula lateralia. Figure 8. Ontogenetic development of ventral valve of Palaeotreta zhujiahensis from the Shuijingtuo Formation of western Hubei. A–F, ventral valves demonstrating pedicle foramen forming stage (T1); A, oblique dorsal view of a very small conjoined specimen showing unrestricted pedicle notch, ELI-AJH 8-1-2-B AF12; B, lateral view of a small conjoined specimen, ELI-AJH 8-2-3 AD2-07; C, posterior view, box indicates the area shown in Figure 9E, ELI-AJH S05 AG07; D, interior view of C; E, oblique view of a larger juvenile, ELI-AJH 8-2-3 AC-27; F, posterior view of E, note ‘U’-shaped pedicle notch, box indicates the area shown in Figure 9D; G, lateral view, indicating pedicle foramen-enclosing stage (T2), showing enclosed pedicle foramen outside of the metamorphic shell, ELI-AJH 8-2-D AD2-12; H–N, intertrough-increasing stage (T3); H, posterior view, ELI-AJH S05 AF-16; I–N, adult with short intertrough, ELI-AJH 8-2-1 CE-03; I, exterior view; J, interior view; K, oblique lateral view; L, lateral view; M, posterior view, note the posterior migration of enclosed pedicle foramen, outside the metamorphic shell; N, oblique anterior view. Dorsal valve transversely oval, on average 82% as long as wide, with maximum width almost at mid-valve (see details in Z.-L. Zhang et al . 2018a ) ( Fig. 8A ). Slightly convex ( Fig. 8A, B ), on average 17% as deep as long. Pseudointerarea small, orthocline, occupying about 7% of valve length and 38% of valve width. Median groove subtriangular, short, on average 44% of pseudointerarea width. Median buttress moderately developed, fading anteriorly. Median septum vestigial, only developed in adult valve, extending anteriorly at mid-valve. Cardinal muscle scars gently impressed, occupying 22% of the length and 51% of the width of the valve. Remarks. Based on their similar morphology, Palaeotreta zhujiahensis was originally considered to represent a second species of Eohadrotreta by Li & Holmer (2004) . However, Z.-L. Zhang et al . (2018a) demonstrated significant differences in ontogenetic growth between these two species. New material collected from western Hubei demonstrates that P. zhujiahensis has a lower ventral valve, straightened posterior margin, relatively larger pedicle foramen, late enclosure of the pedicle foramen, smaller ventral pseudointerarea, much shorter intertrough, weakly developed growth lines, apical process and median septum, thinner secondary layers and more weakly impressed cardinal muscle scars than those of E. zhenbaensis . Furthermore, the most characteristic feature of P. zhujiahensis is that the pedicle foramen is located directly outside of the metamorphic shell, which fits closely with the diagnosis of the new genus Palaeotreta . The valve shape and ontogenetic development of the pedicle foramen in P. zhujiahensis can be compared with that of P. shannanensis ( Figs 6 , 9 ). However, the former has a catacline inclination of the ventral pseudointerarea and a relatively longer intertrough, which is two times longer than that of P. shannanensis . Figure 9. Ontogenetic development of pedicle foramen of Palaeotreta zhujiahensis from the Shuijingtuo Formation of western Hubei. A, enlarged pedicle notch of Figure 8A; B, juvenile with unrestricted pedicle notch, showing raised propareas (arrows), ELI-AJH 8-2-3 AC-11; C, posterior view of Figure 8B; D, ‘U’-shaped pedicle notch, note the growth of propareas at the posterior margin of metamorphic shell (arrow); E, F, ‘U’-shaped pedicle foramen is soon to be enclosed, note the growth of propareas (arrow); G, enclosed pedicle foramen with short intertrough, ELI-AJH 8-2-D AD2-12; H, larger adult showing pedicle foramen outside the metamorphic shell, ELI-AJH 8-2-3 CD2-02; I, enlargement of propareas growing at the posterior margin of metamorphic shell and lateral sides of pedicle foramen of G; J, lateral view of I, note propareas growth (arrow); K, posterior view, showing pedicle foramen mostly located outside the metamorphic shell, box indicates the area shown in L, ELI-AJH 8-2-3 AC-22; L, enlarged view showing propareas growth (arrow); M, enlargement of propareas at the posterior margin of metamorphic shell, note pedicle foramen (arrow), ELI-AJH 8-2-1 AE-09; N, pitting structures on metamorphic shell, ELI-AJH 8-2-1 AE-09; O, P, enlarged secondary columnar layer, ELI-AJH 8-2-1 AE-09, ELI-AJH 8-2-1 CE-03. Figure 10. Ontogenetic scheme of Palaeotreta and Eohadrotreta from Cambrian Series 2 of South China, demonstrating transitions of important characters during different ontogenetic stages (modified from Z.-L. Zhang et al . 2018a , Claybourn et al . 2020 ). The shell structure of P. zhujiahensis is comparable with that of P. shannanensis . Both have a very thin primary layer about 2 M m thick, but P. shannanensis has relatively thinner columnar layers. The thickness of columns in P. zhujiahensis is variable in different shell regions, ranging from 5 M m to 10 M m ( Fig. 9O, P ), which is quite short compared to the columns in E. zhenbaensis ( Zhang et al . 2017 , fig. 5E, H). At the Aijiahe section, P. zhujiahensis co-occurs with E. zhenbaensis at the middle part of the Shuijingtuo Formation. Compared to the biostratigraphy with southern Shaanxi (Z.-F. Zhang et al . 2016), P. zhujiahensis is slightly younger than P. shannanensis . P. zhujiahensis is the second species discovered in southern Shaanxi and western Hubei (after E. zhenbaensis ), but E. zhenbaensis has a much wider palaeogeographical distribution (Z.-L. Zhang et al . 2017 ).