Nisusia from the Cambrian Jialao Formation in Jianhe, Guizhou

Nisusia Walcott 1905 is one of the early representatives of articulate brachiopods. This genus is widely distributed in the world. However, most reports are from the last century and the classification of many species needs further research for lack of high quality illustrations and unequivocal descriptions. Nisusia has also been reported from many places in China, but little in-depth research has been done on its classification and evolutionary trend. In 2019, Holmer et al. further studied the classification and phylogeny of Nisusioidea and proposed a phylogenetic tree for the superfamily, which contains four genera. It is noteworthy that Holmer et al. revised N. guizhouensis, erected based on specimens from the Kaili Formation in Jianhe, as Nisusia? due to missing shell spine decoration, a character initially observed in Nisusia specimens, therefore, more fossil evidence is needed for further study to confirm this assignment.Recently, many Nisusia specimens have been collected from the Cambrian Jialao Formation in Jianhe, Guizhou. Two species (N. guizhouensis and N. granosa) and one indeterminate species (Nisusia sp.) are recognized. However, the distinct spines on these shells are different from those on the shells of N. guizhouensis reported from the underlying Kaili and Tsinghsutung formations. In order to further clarify if these differences are intraspecific or interspecific, quantitative morphological analysis was conducted for the specimens from the Jialao Formation and its underlying Kaili and Tsinghsutung formations. First, the tpsDig232 program was used to obtain landmark point information. Then, the data was imported into PAST software for multivariate analysis and TPS analysis. Landmarks and semi-landmarks were taken on each contour of Nisusia after getting the complete fossil photos.The PCA results of Nisusia guizhouensis from the Tsinghsutung, Kaili and Jialao formations show that there is a large overlapping area of morphological features in the outline, indicating that the ventral and dorsal valves have little morphological differences. Therefore, these ventral and dorsal valves belong to the same species. The CVA results show that the dorsal valves of N. guizhouensis from these three stratigraphic units are dispersed, indicating that there are subtle morphological differences of N. guizhouensis from different strata. The TPS analysis shows that the overall morphological changes of ventral and dorsal valves from the Tsinghsutung, Kaili and Jialao formations are consistent. The cardinal extremities of ventral and dorsal shells expand outwards from the Tsinghsutung Formation to the Jialao Formation, while the anterior of N. guizhouensis valves shrink inward and the visceral cavity also becomes smaller. The variation of ventral valves is slightly larger than that of dorsal valves; the variation of N. guizhouensis is more significant from the Kaili Formation to the Jialao Formation than that from the Tsinghsutung Formation to the Kaili Formation.Results from the quantitative morphological analysis of Nisusia guizhouensis show little change from the Tsinghsutung Formation to the Kaili Formation, while there is a relatively significant change from the Kaili Formation to the Jialao Formation. These trends may be resulted from environmental changes. The lithology of the Tsinghsutung and Kaili formations is mainly mudstone, and the paleoenvironment is interpreted as a deep-sea with weak hydrodynamic conditions. However, the water gradually became shallow from the middle to the top part of the Kaili Formation. The overlying Jialao Formation is mainly composed of siltstone and sandstone, and the sea may become shallower with strong hydrodynamic conditions. Most energy needs of the brachiopods are provided by organic matter filtered out of water flows. The visceral cavity serves as the place where filter feeding takes place: a larger visceral cavity means access to more food. Comparing specimens of N. guizhouensis of the same size, the visceral cavity of those from the Jialao Formation is the smallest. This may be because Nisusia, as a benthic animal, lived in an environment with shallower water and stronger hydrodynamic environment of Jialao Formation and they may get enough food relying on a stronger current instead of pumping more water into the visceral cavity. In addition, the specimens of the Jialao Formation are smaller, and the shell surface has obvious spine decoration with sparser and thicker ribs. Shell spines are important fixation organs of benthic brachiopods. The sparser and thicker radiation increases the efficiency of drainage in water flow. This study not only supplements more data to the biological assemblage of the Jialao Formation in Jianhe, Guizhou, but also provides new information on the systematics and the adaptive evolution of the brachiopods.

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