Reappraisal of the Silurian placodermSilurolepisand insights into the dermal neck joint evolution

Abstract

Silurolepis platydorsalis , a Silurian jawed vertebrate originally identified as an antiarch, is here redescribed as a maxillate placoderm close to Qilinyu and is anteroposteriorly reversed as opposed to the original description. The cuboid trunk shield possesses three longitudinal cristae, obstanic grooves on the trunk shield and three median dorsal plates, all uniquely shared with Qilinyu . Further preparation reveals the morphology of the dermal neck joint, with slot-shaped articular fossae on the trunk shield similar to Qilinyu and antiarchs. However, new tomographic data reveal that Qilinyu uniquely bears a dual articulation between the skull roof and trunk shield, which does not fit into the traditional ‘ginglymoid’ and ‘reverse ginglymoid’ categories. An extended comparison in early jawed vertebrates confirms that a sliding-type dermal neck joint is widely distributed and other types are elaborated in different lineages by developing various laminae. Nine new characters related to the dermal neck joint are proposed for a new phylogenetic analysis, in which Silurolepis forms a clade with Qilinyu . The current phylogenetic framework conflicts with the parsimonious evolution of dermal neck joints in suggesting that the shared trunk shield characters between antiarchs and Qilinyu are independently acquired, and the sliding-type joint in Entelognathus is reversely evolved from the dual articulation in Qilinyu .

A new Silurian fish close to the common ancestor of modern gnathostomes

“…In arthrodires, in which group the organization of the dentition is best known, rows of non-shedding denticles or teeth radiate from the growth center of the dermal gnathal plates, with the serially larger new denticles positioned at the end of the rows. 9,25,27 Similar radiated dental rows seem to be present in the mandible of Bianchengichthys, but there are only two rows aligned along the mandible, whereas in arthrodires, multiple rows radiate from one center. Recent investigation suggests that dermal marginal jaw bones are present in at least three ''acanthothoracid'' taxa, which show significantly diverse organization of dentition.…”

“…To investigate the phylogenetic position of Bianchengichthys micros, we conducted a phylogenetic analysis based on the dataset consisting of 381 characters and 110 taxa in total. The data matrix was mainly sourced from Zhu et al, 9 which was in turn mainly based on the dataset from Zhu et al 7 The codings of Rodotina, Kosoraspis, Tlamaspis, and CPW.9 are in large part sourced directly from the matrix used in Va skaninová et al 29 Two new characters (character 380 and character 381) were added in accordance with the observation and comparison in this paper. The full character list is given in Data S1.…”

“…[1][2][3][4] However, the understanding of this early diversification is often hampered by the patchy nature of the Silurian fossil record, 5 with the articulated specimens of jawed vertebrates only known in isolated localities, most notably Qujing, Yunnan, China. [6][7][8][9] Here, we report a new Silurian maxillate placoderm, Bianchengichthys micros, from the Ludlow of Chongqing, with a near-complete dermatoskeleton preserved in articulation. Although geographically separated, the new taxon resembles the previously reported Qilinyu in possessing a unique combination of dermatoskeletal characters.…”

Endocranial morphology of the petalichthyid placodermEllopetalichthys scheiifrom the Middle Devonian of Arctic Canada, with remarks on the inner ear and neck joint morphology of placoderms

“…We suggest that the craniothoracic joints should be broken into multiple characters, rather than a single multi-state character (Zhu et al 2019). A para-articular process may unite arthrodires and petalichthyids; ptyctodontids could join these groups if the presence of a process could be more robustly documented.…”

“…The craniothoracic neck joint of placoderms carries important phylogenetic information (Goujet 1984b, Goujet and Young 1995, Zhu et al 2019. The occipital region and neck joint morphology of petalichthyids exhibits several, likely apomorphic, conditions.…”