Acanthotretella decaius, Hu & Zhang & Holmer & Skovsted, 2010

Hu, Shixue, Zhang, Zhifei, Holmer, Lars E. & Skovsted, Christian B., 2010, Soft-part preservation in a linguliform brachiopod from the lower Cambrian Wulongqing Formation (Guanshan Fauna) of Yunnan, South China, Acta Palaeontologica Polonica 55 (3), pp. 495-505 : 497-499

?Order Siphonotretida Kuhn, 1949

Remarks.—The presence of hollow spines has been considered to represent the most important unique character of the

Order Siphonotretida ( Holmer and Popov 2000) . However, Williams et al. (2004) showed that some of the oldest wellestablished siphonotretides from the upper middle Cambrian are completely imperforated, whilst other Cambrian–Ordovician siphonotretides are perforated, but lack spines. Recently, Holmer and Caron (2006) described a soft−shelled siphonotretide−like brachiopod, Acanthotretella spinosa from the middle Cambrian Burgess Shale, whose shell surface is covered by long and slender “spine”−like setae. The setal spines may be considered as potentially homologous with the inferred non−mineralised structures that would emerge from the pores of the siphonotretides Helmersenia and Gorchakovia ( Williams et al. 2004) , and they can also be compared closely with the exceptionally preserved setal structures emerging from stunted spines in siphonotretides from Iran ( Popov et al. 2009). However, Acanthotretella was originally referred to the “stem group Brachiopoda ” in view of the lack of a mineralised shell and enigmatic combination of characters. The new older record of an Acanthotretella with a better−preserved and possibly more mineralised shell could indicate that the “soft−shelled” nature of the middle Cambrian A. spinosa might be a secondary loss; however, it is also possible that this is due to differences in preservation between Guanshan and the Burgess Shale faunas. Here we provisionally assign Acanthotretella to the Siphonotretida in view of the presence of a small circular apical foramen, internal pedicle tube and an elongate, large subtriangular pseudointerarea lacking flexure lines, which is also typical of the Siphonotretida .

?Superfamily Siphonotretoidea Kutorga, 1848

Genus Acanthotretella Holmer and Caron, 2006

Type species: Acanthotretella spinosa Holmer and Caron, 2006 . Fossil ridge between Wapta Mountain and Mount Field; Emerald Oncolite Shale Member and Walcott Quarry Shale Member (Greater Phyllopod bed), Burgess Formation .

Emended diagnosis.—Shell mostly organic with variable amount of phosphatic mineral component, inequivalved, biconvex or ventri−biconvex; ventral valve probably apsacline, with elongate and thin pedicle with a central coelomic region, emerging through a short tube with a small circular apical foramen; ventral pseudointerarea elongate, triangular or V−shaped,extending posteriorly far beyond the hinge line; setae variably developed elongate, spine−like; visceral region of both valves, short, triangular, not extending to mid−valve.

Remarks.—As mentioned above, the genus Acanthotretella was first described from the Burgess Shale. The shell valves were strongly flattened and evidently torn apart in some doi:10.4202/app.2009.1106

specimens. Exterior of both valves was covered by long, spine−like and shell−penetrating setae. By contrast, the new material dealt with here exhibits an invariable shell contour, which might imply a higher degree of shell mineralisation. But this proposal is uncertain because of the difference in preservation between the Burgess Shale and Guanshan fauna. In the new Chinese specimen there is convincing evidence of spine−like setae comparable to those seen in the Canadian species ( Holmer and Caron 2006). However, the pedicle differ somewhat between Burgess Shale and Guanshan species: it could be tightly folded and was thus more flexible in the middle Cambrian Acanthotretella .