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A new genus of Eugaleaspidiformes (Galeaspida, Agnatha), Jiangxialepis gen. nov., is described from the Llandovery (Silurian) Fentou Formation from Wuhan, Hubei, South China. The new genus belongs ...

Micro-CT provides a means of nondestructively investigating the internal structure of organisms with high spatial resolution and it has been applied to address a number of palaeontological problems that would be undesirable by destructive means. This approach has been applied successfully to characterize the cranial anatomy of Shuyu, a 428 million-year-old galeaspid (jawless stem-gnathostome) from the Silurian of Changxing, Zhejiang Province, China. Here, we use the synchrotron X-ray tomographic microscopy (SRXTM) to further describe the circulatory system of the head of Shuyu. Our results indicate that the circulatory system of galeaspids exhibits a mosaic of primitive vertebrate and derived gnathostome characters, including a number of derived gnathostome characters that are absent from osteostracans — the group conventionally interpreted as the sister lineage of jawed vertebrates. Our study provides a rich source of information that can be used to infer and reconstruct the early evolutionary history of the vertebrate cardiovascular system.

Osteostraci and Galeaspida are stem-gnathostomes, occupying a key phylogenetic position for resolving the nature of the jawless ancestor from which jawed vertebrates evolved more than 400 million years ago. Both groups are characterized by the presence of rigid headshields that share a number of common morphological traits, in some cases hindering the resolution of their interrelationships and the exact nature of their affinities with jawed vertebrates. Here, we explore the morphological and functional diversity of osteostracan and galeaspid headshields using an innovative approach that combines geometric morphometrics and computational fluid dynamics, thereby constraining the underlying factors that promoted the evolution of their similar morphologies and informing on the ecological scenario under which jawed vertebrates emerged. Phylomorphospace, Mantel analysis and Stayton metrics demonstrate a high degree of homoplasy. Computational fluid dynamics reveals similar hydrodynamic performance among morphologically convergent species, indicating the independent acquisition of the same morphofunctional traits and, potentially, equivalent lifestyles. This confirms that a number of the characters typically used to infer the evolutionary relationships among galeaspids, osteostracans and jawed vertebrates are convergent in nature, potentially obscuring the understanding of the assembly of the gnathostome bodyplan.Ultimately, our results reveal that while the jawless relatives of the earliest jawed vertebrates were ecologically diverse, widespread convergence on the same hydrodynamic adaptations suggests they had reached the limits of their potential ecological diversity—overcome by jawed vertebrates and their later innovations.

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.

The 438-370-million-year-old galeaspids, diversified armoured jawless vertebrates ('ostracoderms') from China and northern Vietnam, were assumed to have a benthic feeding habit in a coastal, marine environment. Here, we describe two new genera of galeaspid fishes, Platylomaspis gen. nov. and Nanningaspis gen. nov. from the Middle Palaeozoic of China. The two new forms are characterized by a rostral process and strikingly broad ventral rim, and clustered with Gumuaspis to form a new family, Gumuaspidae, which represents the most primitive clade of Polybranchiaspiformes. They extend the earliest occurrence of Polybranchiaspiformes backward about 19 million years, and expand its geographical distribution from southern China and northern Vietnam to the Tarim Basin, northwestern China. The new taxa exhibit many morphological convergences with modern rays, and might specify a new kind of lifestyle of galeaspids, the half burrowing habit. Probably benefiting from the new lifestyle, the Gumuaspidae has become the longest lasting galeaspid family. The new findings demonstrated that the demersal galeaspids had developed three different kinds of lifestyles: semi-infaunal benthic (half buried), epibenthic, and suprabenthic (nektonic) habits to accommodate to differentiated ecological niches, and reached the peak of their diversity by the Pragian of the Early Devonian.

We present the first parsimony analysis of the agnathan subclass Galeaspida based on the analysis of 53 morphological characters. Three most parsimonious cladograms (126 steps in length; CI = 0.508; RI = 0.801) were discovered. An amended classification of the Galeaspida is proposed corresponding to the present analysis. Our results suggest that hanyangaspids, xiushuiaspids and day on-gaspids from the Llandovery-Wenlock of Silurian are basal galeaspids. Within the remaining galeaspids, three major monophyletic groups (the Eugaleaspidiformes, the Polybranchiaspidiformes and the Huananaspidiformes) are well supported. It is shown that the dorsal fenestrae of the headshield evolved twice within the Galeaspida, one in the polybranchiaspidiform lineage, and the other in the huananaspidiform lineage (nested within the Huananaspidae). The chronological distribution of galeaspids highlights two radiations of the group, one for basal galeaspids and eugaleaspids in the Telychian (Llandovery) of Silurian, and the other for polybranchiaspidiforms and huananaspidiforms in the Lochkovian of Early Devonian.

Little is known regarding the internal anatomy of thelodonts, with most of the available information coming from a single specimen, the holotype of Turinia pagei (Powrie). Previous descriptions have led to many partial, or conflicting interpretations. Herein, we describe fully the anatomy of T. pagei based on the holotype and additional material. T. pagei possessed a branchial system composed of eight pairs of gills, a buccal/nasohypophyseal region lined with minute denticles, comparable to buccopharyngeal denticles of sharks, and possessed a stomach which is preserved by sediment infill in the holotype specimen. Contrast between the petrological character of the gut infill and the sediment in which the animal is preserved suggests that the gut was infilled in vivo and that T. pagei was probably a deposit feeder. Phylogenetic analysis resolves T. pagei and the Galeaspida as sister-taxa, comprising a sister-group to the Osteostraci plus jawed vertebrates. In contrast to the view that has prevailed hitherto, thelodonts with a dorsoventrally compressed cross-sectional profile comprise a monophyletic group, of which T. pagei is the least derived member. The furcacaudiforms are resolved as an unnatural group, one taxon being the sister taxon to the ‘conventional’ thelodont clade, and the other, the sister taxon to this clade plus galeaspids, osteostracans and jawed vertebrates. The analysis agrees with the earlier view that thelodonts lack distinct synapomorphies, but we argue that distinct synapomorphies are not a requisite of monophyly.

International audience; A new species of the huananaspidiform galeaspid genus Sanqiaspis is reported from the Lochkovian Si Ka Formation of northern Vietnam and is somewhat older than the previously recorded occurrences of this genus, from the Pragian Posongchong Formation of southern China. This species provides new information about the anatomy of the head shield in the Sanqiaspididae and notably provides evidence of a complete endoskeletal and dermal postbranchial wall, like in osteostracans. The various types of vertebrate assemblages defined in the Lower Devonian of China are discussed, by comparison to the vertebrate faunas hitherto recorded from the Lower Devonian of Vietnam, and it is suggested that their composition is strongly influenced by environmental factors.

The phylogenetic relationships of two monophyletic groups of fossil jawless craniates, the Osteostraci and Galeaspida, are discussed on the basis of new observations. Maximum congruence of derived character distribution implies that the Galeaspida are the sister-group of the Osteostraci + Gnathostomata. This implies that the dorsal naso-hypophysial opening is not a synapomorphy of the Cephalaspidomorphi (Osteostraci + Anaspida + Petromyzontida), but is likely to have appeared independently, at least in lampreys and osteostracans. An outline of the phylogeny of the Galeaspida is proposed on the basis of the published material.

SummaryThe Devonian System of China occurs in the Tianshan–Hingan* Geosyncline of North China, the Yangtze Paraplatform of South China and in the geosynclinal areas of West China. Devonian formations are entirely absent from the Sino-Korean Paraplatform and the Tarim Platform, but along the border of the Tarim Basin and the slopes of Qilianshan (Chilienshan) are continental sequences rich in vertebrate and plant fossils. On the basis of differences in biostratigraphic characteristics the Devonian is divisible into 8 regions: 1. Junggar–Hingan (Dzungar–Khingan), 2. South Tianshan (South Tienshan), 3. Qilianshan (Chilienshan), 4. Longmenshan (Lungmenshan), 5. South China, 6. South-eastern China, 7. West Sichuan–North Xizang (West Szechuan-North Tibet), 8. Himalaya–West Yunnan.Using both lithology and fossil content, five principal facies (types) may be further differentiated: 1. Hingan type (eugeosynclinal facies), 2. Baoxin type (miogeosynclinal facies), 3. Xiangzhou type (platform near-shore facies), 4. Nandan type (platformal off-shore facies), 5. Cuifengshan (Chuifeng shan) type (continental facies).The Devonian of South China occurs in various facies, but has a well-defined principal stratotype. It has been divided by Chinese geologists into eight stages, two in the Upper Devonian, two in the Middle Devonian and four in the Lower Devonian.The Devonian sequences of the geosynclinal regions of West China are characterized by facies which contain graptolites and tentaculites, and have largely been affected by regional metamorphism. The Devonian biota of the Junggar–Hingan Region is essentially an endemic one, but it is associated with a few European and North American elements. Marine pyroclastics are the principal rocks. General stratigraphical columns of those regions are given with a correlation with other parts of China. The continental Devonian of China is unique especially in the light of its vertebrate content. Red sandstones are widespread in the Lower Devonian, and intercalated with argillaceous limestone and mudstone. This sequence is rich in endemic East Asiatic faunal elements including agnatha (Galeaspida, Polybranchiaspida), Antiarchi, etc. Middle and Upper Devonian lithostratigraphic units consist principally of light coloured quartzose sandstone formations in Southeastern China, with some red beds in South and North China, while their fauna and flora are cosmopolitan in character. Their exact geological ages vary, however, as clearly shown in the South China region, especially towards the top of the Upper Devonian (with Asterolepis) and in the early Middle Devonian (with Bothriolepis).

A number of polybranchiaspids (Agnatha) from the Devonian of China have parts of their soft anatomy, including the central nervous and vascular systems, extremely well preserved as mineral replicas. This has made possible direct comparisons with the internal anatomy of the living agnathans, the hagfish and lampreys, as well as the fossil heterostracans and cephalaspids. The polybranchiaspids do not seem to be related to either the hagfish or the heterostracans, but instead represent a separate grouping, the Galeaspida, allied to the lampreys and cephalaspids.

Racheboeuf, Patrick, Phuong, Ta Hoa, Hung, Nguyen Huu, Feist, Monique, Janvier, Philippe (2006): Brachiopods, crustaceans, vertebrates, and charophytes from the Devonian Ly Hoa, Nam Can and Dong Tho formations of Central Vietnam. Geodiversitas 28 (1): 5-36, DOI: http://doi.org/10.5281/zenodo.5376089

The fossil record of early vertebrates has been influential in elucidating the evolutionary assembly of the gnathostome bodyplan. Understanding of the timing and tempo of vertebrate innovations remains, however, mired in a literal reading of the fossil record. Early jawless vertebrates (ostracoderms) exhibit restriction to shallow-water environments. The distribution of their stratigraphic occurrences therefore reflects not only flux in diversity, but also secular variation in facies representation of the rock record. Using stratigraphic, phylogenetic and palaeoenvironmental data, we assessed the veracity of the fossil records of the jawless relatives of jawed vertebrates (Osteostraci, Galeaspida, Thelodonti, Heterostraci). Non-random models of fossil recovery potential using Palaeozoic sea-level changes were used to calculate confidence intervals of clade origins. These intervals extend the timescale for possible origins into the Upper Ordovician; these estimates ameliorate the long ghost lineages inferred for Osteostraci, Galeaspida and Heterostraci, given their known stratigraphic occurrences and stem–gnathostome phylogeny. Diversity changes through the Silurian and Devonian were found to lie within the expected limits predicted from estimates of fossil record quality indicating that it is geological, rather than biological factors, that are responsible for shifts in diversity. Environmental restriction also appears to belie ostracoderm extinction and demise rather than competition with jawed vertebrates.

Racheboeuf, Patrick, Phuong, Ta Hoa, Hung, Nguyen Huu, Feist, Monique, Janvier, Philippe (2006): Brachiopods, crustaceans, vertebrates, and charophytes from the Devonian Ly Hoa, Nam Can and Dong Tho formations of Central Vietnam. Geodiversitas 28 (1): 5-36, DOI: http://doi.org/10.5281/zenodo.5376089, {"references":["CHANG M. M. 1991. - Head skeleton and shoulder girdle of Youngolepis, in CHANG M. M., LIU Y. H. & ZHANG G. R. (eds), Early Vertebrates and Related Problems of Evolutionary Biology. Science Press, Beijing: 355-378.","CHANG M. M. & ZHU M. 1993. - A new Middle Devonian osteolepidid from Qujing, Yunnan. Memoirs of the Association of Australasian Palaeontologists 15: 183-198.","FEIST M. & GRAMBAST- FESSARD N. 1985. - Remarques sur la nature et la systematique des Charophytes paleozoiques. 110e Congres national des Societes savantes, Montpellier, 1985 5: 65-75.","FEIST M. & FEIST R. 1997. - Oldest record of a bisexual plant. 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