1298


Journal of Paleontology 91(6):1296–1305


Figure 2. (1) Paleogeographical map of South China during the Permian−Triassic transition (from Feng et al., 1997; Yin et al., 2014), also showing the location of the Xinmin section (marked by a star). (2) Stratigraphical column of the Xinmin section, showing the lithology of the Talung Formation and the lower part of Daye Formation, as well as the stratigraphic distribution of Fusichonetes, as revised here (including species that would have been assigned to Tethyochonets prior to this study). (3) Global paleogeographical distribution of Fusichonetes and Tethyochonetes according to previous literature (basal map after Blakey, 2008).


Zhao et al. (1981) had described for the dorsal interior of Fusichonetes. As for the supposed significance of a ventralmedian septum,


brachial scars, and a dorsal median septum in distinguishing Tethyochonetes from Fusichonetes, they are equally dubious. This is because from the specimens we have recently collected from the Xinmin section in South China, the ventral interior of Fusichonetes possesses a median septum, and its dorsal interior has well-developed brachial scars and a median septum (Fig. 1). Although until nowno lateral septa are known fromFusichonetes, this does not necessarily mean it was originally absent in this genus, because even in some species of Tethyochonetes (e.g., Zhang et al., 2013, fig. 5O; He et al., 2014, fig. 4K, N) where lateral septa are supposed to exist, they are in fact absent, and presumably not preserved. With these observations, it suggests


that the two genera not only resemble each other strongly in external morphology, they also share very similar internal structures. Recently, we collected 1150 well-preserved rugosochonetid


specimens from a stratigraphic section in South China that straddles the Permian-Triassic boundary (Fig. 2.2, also see below for more details about the section). Our initial inspection of these specimens suggested that this collection, from a single section, seemed to represent a continuum of morphological variations from Fusichonetes (as one end member) to Tethyochonetes (at the other end), and that it was not possible to distinguish them with great confidence. The inability to separate them was due to the fact that there are specimens in the collection that clearly represent transi- tional forms between typical Fusichonetes characterized by a trans- versely quadrate outline to typical Tethyochonetes characterized by a


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