Palaeotreta zhujiahensis ( Li & Holmer, 2004 )

Zhang, Zhiliang, Holmer, Lars E., Chen, Feiyang & Brock, Glenn A., 2020, Ontogeny and evolutionary significance of a new acrotretide brachiopod genus from Cambrian Series 2 of South China, Journal of Systematic Palaeontology 18 (19), pp. 1569-1588 : 1578-1582

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.

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 .

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).

Figures

Figure 6. Ontogenetic development of pedicle foramen of Palaeotreta shannanensis gen. et sp. nov. from the Shuijingtuo Formation of southern Shaanxi. A, B, juvenile with unrestricted pedicle notch, note raised propareas (arrows), ELI-XYB S4-3 AV-09; C, D, juvenile with unrestricted pedicle notch, ELI-XYB S4-3 AV-17; E–G, semicircular pedicle foramen soon to be enclosed, ELI-XYB S4-3 AV-05; H, raised propareas (arrows), ELI-XYB S4-3 AV-11; I–L, pedicle foramen, just enclosed with very short intertrough, XYB S4-3 AU-07; M, N, adult with enclosed pedicle foramen, showing the successive growth of propareas at the posterior margin of the metamorphic shell (arrow), ELI-XYB S4-3 AU-01; O, enclosed pedicle foramen is mostly outside the metamorphic shell, ELI- XYB S4-3 AV-07.

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 S05AF-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.

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.