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Morphometric analysis of stem-group mollusks from the northern Yangtze Craton, China

Published online by Cambridge University Press:  27 May 2022

Yanchun Pang Open the ORCID record for Yanchun Pang [Opens in a new window] ,
Michael Steiner ,
Ben Yang Open the ORCID record for Ben Yang [Opens in a new window] ,
Mingcai Hou ,
James G. Ogg ,
Wenhu Ji ,
Qiang Hu ,
Siyu Liang ,
Mengshao Zhang  and
Yangjian He
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Yanchun Pang*
Affiliation:
Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China <1490340577@qq.com> <374755906@qq.com> <823494913@qq.com> <2466019744@qq.com>
Michael Steiner
Affiliation:
College of Earth Science and Engineering, Shandong University of Science and Technology, Qianwangang Road 579, Qingdao 266590, China Department of Earth Sciences, Freie Universität Berlin, Malteserstrasse 74-100, Haus D, Berlin 12249, Germany
Ben Yang
Affiliation:
MNR Key Laboratory of Stratigraphy and Palaeontology, Institute of Geology, Chinese Academy of Geological Sciences, Baiwanzhuang street 26, Beijing 100037, PR China
Mingcai Hou
Affiliation:
State Key Laboratory of Oil Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China
James G. Ogg
Affiliation:
Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China <1490340577@qq.com> <374755906@qq.com> <823494913@qq.com> <2466019744@qq.com> State Key Laboratory of Oil Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China Deprtment of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, Indiana 47907 USA
Wenhu Ji
Affiliation:
Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China <1490340577@qq.com> <374755906@qq.com> <823494913@qq.com> <2466019744@qq.com>
Qiang Hu
Affiliation:
Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China <1490340577@qq.com> <374755906@qq.com> <823494913@qq.com> <2466019744@qq.com>
Siyu Liang
Affiliation:
College of Earth Science, Chengdu University of Technology, Chengdu 610059, China
Mengshao Zhang
Affiliation:
Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China <1490340577@qq.com> <374755906@qq.com> <823494913@qq.com> <2466019744@qq.com>
Yangjian He
Affiliation:
Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China <1490340577@qq.com> <374755906@qq.com> <823494913@qq.com> <2466019744@qq.com>
Li Lin
Affiliation:
Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China <1490340577@qq.com> <374755906@qq.com> <823494913@qq.com> <2466019744@qq.com>
*
*Corresponding author
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Abstract

Cap-shaped skeletal fossils are the earliest undisputed body fossils of mollusks appearing in the basal Cambrian. A study on the morphometry of cap-shaped fossils from the Nanjiang area (North Sichuan, China) is undertaken to understand the origin and evolution of the early mollusks. The distribution of these fossil cap-shaped mollusks indicates a stepwise increase in their diversity during the early Cambrian. Maikhanella Zhegallo in Voronin et al., 1982 co-occuring with the spinose sclerites of siphogonuchitids, is regarded as the earliest scleritized mollusk. It is followed by other maikhanellids, e.g., Purella Missarzhevsky, 1974 and Yunnanopleura Yu, 1987, which co-occur with the earliest univalved helcionellids, e.g., Igorella Missarzhevsky in Rozanov et al., 1969. Cluster analysis of their morphometric characteristics shows that the Maikhanella group is similar to the Purella and Yunnanopleura groups, but is less comparable with univalved helcionellids. The maikhanellids are interpreted as representatives of the stem group Aculifera, although it remains uncertain if one or two larger cap-shaped shell plates were present on the elongate slug-like body, comparable to those of Halkieria Poulsen, 1967 or Orthrozanclus Conway Morris and Caron, 2007. Maikhanellids are characterized by the prominent protrusions or scales on the cap-shaped shell plates arranged in a concentric pattern around the shell apex. Evolutionarily, the protrusions or scales are reduced in younger strata, whereas subsequently a typically concentric ornament developed, the cap-shaped shell plates developed higher profiles, and the apical region became increasingly bare of scales. Meanwhile, the cap-shaped shell plates gradually evolved into a helcionellid-like appearance with an anteroposteriorly inclined apex. The morphological evolution of the earliest sclerotized mollusks reflects biotic evolution and environmental adaption among the stem-group mollusks during the early Cambrian.

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Journal of Paleontology , Volume 96 , Issue 5 , September 2022 , pp. 1024 - 1036
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Copyright © The Author(s), 2022. Published by Cambridge University Press on behalf of The Paleontological Society

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