Date Published: April 5, 2017
Publisher: Public Library of Science
Author(s): Valéria Vaškaninová, Per E. Ahlberg, Matt Friedman.
The taxonomy of Early Devonian placoderm material from the Lochkovian and Pragian of the Prague basin, previously attributed to the genera Radotina and Holopetalichthys, is revised. The Pragian species Radotina tesselata Gross 1958 shares detailed similarities with the holotype of the Lochkovian Radotina kosorensis Gross 1950, which is also the holotype of the genus; the assignation of both species to Radotina is supported. However, the Lochkovian material previously attributed to Radotina kosorensis also contains two unrecognised taxa, distinguishable from Radotina at the generic level: these are here named Tlamaspis and Sudaspis. The disputed genus Holopetalichthys, synonymised with Radotina by some previous authors, is shown to be valid. Furthermore, whereas Radotina, Tlamaspis and Sudaspis can all be assigned to the group Acanthothoracii, on the basis of several features including possession of a projecting prenasal region of the endocranium, Holopetalichthys lacks such a region and is probably not an acanthothoracid. Skull roof patterns and other aspects of morphology vary greatly between these taxa. Radotina has a substantially tesselated skull roof, whereas the skull roofs of Tlamaspis and Holopetalichthys appear to lack tesserae altogether. Tlamaspis has an extremely elongated facial region and appears to lack a premedian plate. Sudaspis has a long prenasal region, but unlike Tlamaspis the postnasal face is not elongated. Past descriptions of the braincase of ‘Radotina’ and the skull roofs of ‘Radotina’ and ‘Holopetalichthys’ incorporate data from more than one taxon, giving rise to spurious characterisations including an apparently extreme degree of skull roof variability. These descriptions should all be disregarded.
The early evolution of vertebrates has recently become a major research topic in vertebrate biology [1–8]. One of the main areas of interest is the gnathostome (jawed vertebrate) stem group, which is important from both evolutionary and phylogenetic perspectives. In evolutionary terms, the gnathostome stem group encompasses the origin of jaws and associated major changes in facial architecture [3,5,7]; in phylogenetic terms, it is a segment of the vertebrate tree whose content and topology has long been the subject of debate [9,10,11,1,2,3,4,6,7]. A key development in the understanding of this stem group has been the recognition that the placoderms (armoured jawed fishes of Silurian to Devonian age), which until recently were regarded as a clade branching off the gnathostome stem group, probably form a paraphyletic segment of that stem group [2,7,8] (but see ). Some groups of placoderms appear to be very primitive and close to jawless vertebrates  whereas others possess what were previously regarded as osteichthyan autapomorphies (notably a maxilla, premaxilla and dentary) and are probably close to the gnathostome crown-group node [6, 13].
The majority of specimens considered in this paper belong to the Národní Muzeum (NM) in Prague (the collections of the Czech Geological Survey and Charles University contain no acanthothoracid material or specimens relevant to this study). Additional specimens from the collections of the Natural History Museum, London (NHM) and Humboldt Museum für Naturkunde, Berlin (HMN) are also discussed. The specimen numbers are listed in the Systematic palaeontology section below. No permits were required for the described study, which complied with all relevant regulations.
The Černá rokle (= Black Gorge) quarries are situated in the south-eastern part of the Prague Basin in the Radotín Valley (Fig 1A1). Several quarries were operating there from the 19th century until the 1960s. Perner  noted a major increase in fossil fish discoveries at the beginning of the 20th century when closer contacts were established between the quarry workers and the local fossil collectors. The rocks were being quarried and processed by hand to make paving setts, and the workers were instructed and well trained in detecting precious finds. Those were then sold to the collectors, recruited from the local intellectual elite. The largest collections were amassed by A. Schubert (postmaster), F. J. Pecka (local teacher), W. Kolář (clerk) and R. Růžička (engineer); the collection of the late professor I. Chlupáč also contained a few finds acquired in his youth. The majority of these collections were donated to the NM, over the course of several decades. Crucially, important material was donated after Walter Gross had finished his studies of the NM placoderm collections in 1959, and is described here for the first time. After the quarrying ceased only collecting in debris was possible at the locality.
Class Placodermi M’Coy, 1848 
This paper has significantly changed the view of the systematic structure of the Prague Basin Lower Devonian (Lochkovian and Pragian) placoderm fauna, finally resolving the long prevailing confusion about the variable character states in individual taxa. Four genera (Radotina, Tlamaspis, Sudaspis and Holopetalichthys) have been established and provided with unambiguous diagnoses based on well documented unique characters. The species name Radotina kosorensis Gross, 1950 is now limited to a few specimens that share unique characters with the holotype. Radotina tesselata Gross, 1958 remains a single specimen species. Radotina has a skull roof with large tesselated areas separating the macromeric bones. Tlamaspis is characterised by a very long facial region and a dermal skull roof lacking tesserae. Sudaspis has a long prenasal region with a distinctive, stepped lateral profile. The skull roof is essentially macromeric, including large and prominent paranuchals. We consider all the previously published attempts to reconstruct or interpret the “radotinid” material, apart from references to holotypes, as invalid because they inadvertently combine information from more than one taxon.