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| Rumporostralis xikengensis (Pan & Wang, 1980)
in Shan, Zhu, Zhao, et al., 2020
Artwork: Xiaocong Guo.
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Abstract
Galeaspids are an endemic clade of jawless stem-gnathostomes known as ostracoderms. Their existence illuminates how specific characteristics developed in jawed vertebrates. Sinogaleaspids are of particular interest among the galeaspids but their monophyly is controversial because little is known about Sinogaleaspis xikengensis. Newly discovered sinogaleaspids from the Lower Silurian of Jiangxi, China provide a wealth of data and diagnostic features used to establish the new genus, Rumporostralis gen. nov., for Sinogaleaspis xikengensis. A morphological study showed that the sensory canal system of sinogaleaspids had mosaic features similar to those of three known galeaspids. There are 3–8 pairs of transverse canals in the Sinogaleaspidae, which suggests that the sensory canal system of galeaspid probably had a grid distribution with transverse canals arranged throughout the cephalic division. Phylogenetic analysis of Galeaspida supports the monophyly of the Sinogaleaspidae, consisting of Sinogaleaspis, Rumporostralis, and Anjiaspis. However, Shuyu and Meishanaspis form another monophyletic group, Shuyuidae fam. nov., which is outside all other eugaleaspidiforms. We propose a cladistically-based classification of Galeaspida based on our analysis.
Order Eugaleaspiformes (Liu, 1965) Liu, 1980
Family Shuyuidae fam. nov.
Differential diagnosis. Shuyuidae differs from other families of Eugaleaspiformes in the splayed posterior supraorbital canals and absence of U-shaped median dorsal canals.
Type genus. Shuyu Gai et al., 2011
Remarks. Newly discovered sinogaleaspid material provides a wealth of data and reliable diagnostic features to erect the new genus Rumporostralis gen. nov. for ‘Sinogaleaspis’ xikengensis. Our phylogenetic analysis of Galeaspida shows that Shuyu and Meishanaspis are not included in Sinogaleaspidae and form another monophyletic group. A new family, Shuyuidae fam. nov., was created for Shuyu and Meishanaspis. Shuyuidae is positioned in the new cladogram as the sister to all other Eugaleaspididiformes with synapomorphies including a subtriangular head-shield and longitudinal oval or wedge-shaped median dorsal opening.
Family Sinogaleaspidae Pan & Wang, 1980
Differential diagnosis. Sinogaleaspidae differs from other families of Eugaleaspiformes in the V-shaped posterior supraorbital canals and more than 2 pairs of median transverse canals (autapomorphy).
Type genus. Sinogaleaspis. Pan & Wang, 1980
Remarks. The amended Sinogaleaspidae, including Sinogaleaspis shankouensis, Rumporostralis xikengensis, (Figs. 3, 4) R. shipanensis (Fig. 5), and Anjiaspis reticularis is monophyletic with synapomorphy (U-shaped median dorsal canals) and autapomorphy (more than 2 pairs of median transverse canals).
Genus Rumporostralis gen. nov.
Etymology. Rumpo latin, state of being dehiscent or split; rostralis, Latin, snout, in referring to the rostral margin of the head-shield split by the anterior end of median dorsal opening.
Type species. Rumporostralis xikengensis (Pan & Wang, 1980)
Differential diagnosis. Rumporostralis differs from other Eugaleaspiformes by an unclosed rostral margin of the head-shield, indicating autapomorphy.
Remarks. The genus including R. xikengensis and R. shipanensis is uniquely characterized by the unclosed rostral margin.
Holotype. A nearly complete head-shield GMC V1753A, B
Referred specimens. A nearly complete head-shield IVPP V25136.1, three incomplete head-shields IVPP V25136.2–4.
Type locality and horizon. Xikeng village and Shipan reservoir, Taiyangsheng Town, Xiushui County, Jiangxi Province, China; Xikeng formation, Telychian, Llandovery, Silurian.
Differential diagnosis. R. xikengensis differs from the other species R. shipanensis in the small size of the head-shield.
Rumporostralis shipanensis gen. et sp. nov.
Holotype. An incomplete head-shield IVPP V26114.1a, b
Type locality and horizon. Shipan reservoir, Taiyangsheng Town, Xiushui County, Jiangxi Province, China; Xikeng formation, Telychian, Llandovery, Silurian.
Differential diagnosis. R. shipanensis differs from the type species R. xikengensis by the large size of the head-shield.
Systematic paleontology
Subclass Galeaspida Tarlo, 1967Order Eugaleaspiformes (Liu, 1965) Liu, 1980
Family Shuyuidae fam. nov.
Differential diagnosis. Shuyuidae differs from other families of Eugaleaspiformes in the splayed posterior supraorbital canals and absence of U-shaped median dorsal canals.
Type genus. Shuyu Gai et al., 2011
Referred genera. Meishanaspis
Remarks. Newly discovered sinogaleaspid material provides a wealth of data and reliable diagnostic features to erect the new genus Rumporostralis gen. nov. for ‘Sinogaleaspis’ xikengensis. Our phylogenetic analysis of Galeaspida shows that Shuyu and Meishanaspis are not included in Sinogaleaspidae and form another monophyletic group. A new family, Shuyuidae fam. nov., was created for Shuyu and Meishanaspis. Shuyuidae is positioned in the new cladogram as the sister to all other Eugaleaspididiformes with synapomorphies including a subtriangular head-shield and longitudinal oval or wedge-shaped median dorsal opening.
Family Sinogaleaspidae Pan & Wang, 1980
Differential diagnosis. Sinogaleaspidae differs from other families of Eugaleaspiformes in the V-shaped posterior supraorbital canals and more than 2 pairs of median transverse canals (autapomorphy).
Type genus. Sinogaleaspis. Pan & Wang, 1980
Referred genera. Rumporostralis gen. nov., Anjiaspis
Remarks. The amended Sinogaleaspidae, including Sinogaleaspis shankouensis, Rumporostralis xikengensis, (Figs. 3, 4) R. shipanensis (Fig. 5), and Anjiaspis reticularis is monophyletic with synapomorphy (U-shaped median dorsal canals) and autapomorphy (more than 2 pairs of median transverse canals).
Genus Rumporostralis gen. nov.
Etymology. Rumpo latin, state of being dehiscent or split; rostralis, Latin, snout, in referring to the rostral margin of the head-shield split by the anterior end of median dorsal opening.
Type species. Rumporostralis xikengensis (Pan & Wang, 1980)
Referred species. Rumporostralis shipanensis gen. et sp. nov.
Differential diagnosis. Rumporostralis differs from other Eugaleaspiformes by an unclosed rostral margin of the head-shield, indicating autapomorphy.
Remarks. The genus including R. xikengensis and R. shipanensis is uniquely characterized by the unclosed rostral margin.
Holotype. A nearly complete head-shield GMC V1753A, B
Referred specimens. A nearly complete head-shield IVPP V25136.1, three incomplete head-shields IVPP V25136.2–4.
Type locality and horizon. Xikeng village and Shipan reservoir, Taiyangsheng Town, Xiushui County, Jiangxi Province, China; Xikeng formation, Telychian, Llandovery, Silurian.
Differential diagnosis. R. xikengensis differs from the other species R. shipanensis in the small size of the head-shield.
Rumporostralis shipanensis gen. et sp. nov.
Holotype. An incomplete head-shield IVPP V26114.1a, b
Type locality and horizon. Shipan reservoir, Taiyangsheng Town, Xiushui County, Jiangxi Province, China; Xikeng formation, Telychian, Llandovery, Silurian.
Differential diagnosis. R. shipanensis differs from the type species R. xikengensis by the large size of the head-shield.
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| Figure 7: Life restoration of Sinogaleaspis shankouensis (left) and Rumporostralis xikengensis (right) in a fresh river.
Artwork: Xiaocong Guo.
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Conclusion:
The newly discovered sinogaleaspids from the Lower Silurian in Jiangxi, China provides a wealth of new data and reliable diagnostic features to assign the new genus, Rumporostralis gen. nov., to ‘Sinogaleaspis’ xikengensis. This in-depth morphological study determined that the sensory canal system of sinogaleaspids exhibits the mosaic features of three known galeaspid patterns. The presence of 3-8 pairs of transverse canals in Sinogaleaspidae suggests that the sensory canal system of galeaspids probably displayed a grid distribution with transverse canals arranged throughout the cephalic division. An extended phylogenetic analysis of Galeaspida corroborates the monophyly of Sinogaleaspidae, which consists of Sinogaleaspis, Rumporostralis, and Anjiaspis. Shuyu and Meishanaspis were excluded from Sinogaleaspidae to form the monophyletic group Shuyuidae fam. nov., which is the sister of all other Eugaleaspididiformes. We propose a cladistically-based classification of the Galeaspida.
Xianren Shan, Min Zhu, Wenjin Zhao, Zhaohui Pan, Pingli Wang and Zhikun Gai. 2020. A New Genus of Sinogaleaspids (Galeaspida, stem-Gnathostomata) from the Silurian Period in Jiangxi, China. PeerJ. 8:e9008. DOI: 10.7717/peerj.9008