Wu et al.—Chinese Permian–Triassic chonetid taxonomy


1297


pl. 4, figs. 16–19), original designation as Waagenites soochowensis quadrata, and Fusichonetes nayongensis (Liao, 1980a) copied from Liao (1980a, pl. 4, figs. 7–9) and from Xinmin section: (1) dorsal valve internal mold, CUG26123, bs: brachial scar, ms: median septum; (2) ventral valve internal mold, CUG25824; (3) dorsal valve external mold, CUG25224; (4) ventral valve external mold, CUG25524. All scale bars=2 mm.


costellae with deep interspaces. Although no internal features were illustrated by Liao in Zhao et al. (1981), he did indicate that the dorsal interior of Fusichonetes possessed a cardinal process that is bilobate internally and quadrilobate externally. Interestingly, however, of the two Fusichonetes species recog- nized by Liao, only the type species, F. nayongensis, most consistently demonstrates the external morphological features that Liao used to define Fusichonetes. The second species, Fusichonetes pygmaea (see Liao, 1979, fig. 14; 1980a, pl. 4, figs. 4–6; 1980b, pl. 1, figs. 5, 6; 1981, pl. 8, figs. 7, 8), is in fact characterized by a subquadrate outline, cardinal extremity with an angle ~75–90°, and coarse costellae with a few bifurcations. When Chen et al. (2000) proposed Tethyochonetes, based


on Waagenites soochowensis quadrata Zhan, they emphasized both external features as well as internal characteristics. Among the external features, Chen et al. (2000) considered the com- plexity of costellae (i.e, whether costellae are simple and undi- vided, or increase by bifurcation), the shell width/length ratio and the cross-section shape of the interspace between costellae


Figure 1. Type species of Tethyochonetes Chen et al., 2000 and Fusichonetes Liao, 1981. Tethyochonetes quadrata (Zhan, 1979), copied from Zhan (1979,


as the main characteristics to distinguish the two genera, sup- plemented by differences in internal structures. In comparing Tethyochonetes with Fusichonetes, Chen et al. (2000) noted that the former had a ventral median septum, an internally bilobate and externally trilobate cardinal process, a dorsal median sep- tum, and lateral septa and brachial scars. But, it should be pointed out that most of these features that supposedly are diagnostic of Tethyochonetes were unknown in Fusichonetes when it was first formally proposed by Liao in Zhao et al. (1981) as he only had limited amount of internal material to study (this is evident from the fact that he figured no internal shell char- acteristics, nor mentioned any internal features, except for a brief comment on the cardinal process). Furthermore, even though Chen et al. (2000) mentioned that Tethyochonetes pos- sessed a cardinal process that is bilobate internally and trilobate externally, they did not illustrate the trilobate aspect. But, when examining the illustrations of a well-preserved dorsal interior of Tethyochonetes cheni figured by Zhang et al. (2013, fig. 9J), we found that it clearly has an internally bilobate and externally quadrilobate cardinal process, which is the same as what Liao in


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