Abstract
BackgroundYunnanolepidoids constitute either the most basal consecutive segments or the most primitive clade of antiarchs, a highly diversified jawed vertebrate group from the Silurian and Early Devonian periods. Although the general morphology of yunnanolepidoids is well established, their endocranial features remain largely unclear, thus hindering our further understanding of antiarch evolution, and early gnathostome evolution. Phymolepis cuifengshanensis, a yunnanolepidoid from the Early Devonian of southwestern China, is re-described in detail to reveal the information on endocranial anatomy and additional morphological data of head and trunk shields.MethodsWe scanned the material of P. cuifengshanensis using high-resolution computed tomography and generated virtual restorations to show the internal morphology of its dermal shield. The dorsal aspect of endocranium in P. cuifengshanensis was therefore inferred. The phylogenetic analysis of antiarchs was conducted based on a revised and expanded dataset that incorporates 10 new cranial characters.ResultsThe lateroventral fossa of trunk shield and Chang’s apparatus are three-dimensionally restored in P. cuifengshanensis. The canal that is positioned just anterior to the internal cavity of Chang’s apparatus probably corresponds to the rostrocaudal canal of euantiarchs. The endocranial morphology of P. cuifengshanensis corroborates a general pattern for yunnanolepidoids with additional characters distinguishing them from sinolepids and euantiarchs, such as a developed cranio-spinal process, an elongated endolymphatic duct, and a long occipital portion.DiscussionIn light of new data from Phymolepis and Yunnanolepis, we summarized the morphology on the visceral surface of head shield in antiarchs, and formulated an additional 10 characters for the phylogenetic analysis. These cranial characters exhibit a high degree of morphological disparity between major subgroups of antiarchs, and highlight the endocranial character evolution in antiarchs.
Highlights
The lateroventral fossa of trunk shield and Chang’s apparatus are three37 dimensionally restored in P. cuifengshanensis
The endocranial morphology of P. cuifengshanensis corroborates a general pattern for yunnanolepidoids with additional characters distinguishing them from sinolepids and euantiarchs, such as a developed cranio-spinal process, an elongated endolymphatic duct, and a long occipital portion
In light of new data from Phymolepis and Yunnanolepis, we summarized the morphology on the visceral surface of head shield in antiarchs, and formulated additional ten characters for the phylogenetic analysis
Summary
One of the most diverse and widespread fish groups during the Middle Paleozoic, have been resolved at the base of the diversification of jawed vertebrates in most of recent phylogenetic studies (Brazeau, 2009; Davis, Finarelli & Coates, 2012; Zhu et al, 2012; Giles, Rücklin & Donoghue, 2013; Zhu et al, 2013; Dupret et al, 2014; Zhu, 2014; Giles, Friedman & Brazeau, 2015; Long et al, 2015; Qiao et al, 2016; Zhu et al., 2016). Yunnanolepidoids are endemic antiarchs discovered from the South China and Indochina blocks (Liu, 1983; Pan & Dineley, 1988; Tông-Dzuy, Janvier & Phuong, 1996; Wang, Qu & Zhu, 2010) They are considered to be the most primitive antiarchs because of the absence of characteristic dermal brachial process (Chang, 1978; Zhang, 1978; Zhang, 1980; Young, 1981a; Long, 1983; Janvier, 1995; Zhu & Janvier, 1996; Carr, Johanson & Ritchie, 2009; Zhu et al, 2012), their monophyly has not yet reached a consensus (Janvier & Pan, 1982; Young & Zhang, 1996). Yunnanolepidoids represent the oldest known antiarchs, even though the date of the oldest yunnanolepidoid Shimenolepis (Wang, 1991) has been revised to Late Ludlow recently (Zhao et al, 2016)
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