Phylogenetic relationships of psammosteid heterostracans (Pteraspidiformes), Devonian jawless vertebrates

Abstract

Psammosteid heterostracans are a group (suborder Psammosteoidei) of Devonian-age jawless vertebrates, which is included in the order Pteraspidiformes. The whole group of psammosteids is represented by numerous species (more than 40); their phylogenetic relationships are still poorly known and deserve further study. Classical researchers of the psammosteids, such as D. Obruchev, E. Mark-Kurik and L. Halstead Tarlo, had different views on the phylogeny of the group (e.g. origins and evolution of Psammosteus). To check the modern hypotheses of psammosteid origins from various Pteraspidiformes and to clarify psammosteid interrelationships, the most complete phylogeny of this group (38 ingroup taxa + juvenile Drepanapsis) is presented here. Different methods of data analysis were used to explore the psammosteid data set, including equally weighted characters versus implied weighting. According to the results of the phylogenetic analysis, the monophyletic status of the group and their early development from the Pteraspidiformes are supported. The diagnoses and interrelationships of many taxa are clarified. Two new genera are proposed (Vladimirolepis gen. nov. and Elgaia gen. nov.), and two subfamilies are erected (Placosteinae subfam nov. and Psammosteinae Traquair).

The last pteraspids (Vertebrata, Heterostraci): new material from the Middle Devonian of Alberta and Idaho

“…These late-appearing taxa all have elongated branchial plates with terminal branchial openings, and the cornual plates are reduced or lost. These trends are also seen in the Psammosteoidei and continue in that lineage to the extinction of the Pteraspidiformes in the Late Devonian (Glinskiy, 2017). The similarity among these features in the pteraspids and psammosteids caused the original Middle Devonian pteraspids to be identified as psammosteids, however, this new analysis confirms previous studies in placing the Psammosteoidei in a basal position within the Pteraspidiformes.…”

“…minima Pernègre and Goujet, 2007, were used to retain the family Gigantaspididae. These taxa were also used in the recent, more comprehensive analysis of the psammosteids (Glinskiy, 2017), in which Drepanaspis gemuendenensis Schlüter, 1887 was selected as a basal and relatively complete taxon to represent the Psammosteoidei. Extensive recoding of the character states for Drepanaspis was required because the published matrix (Pernègre and Elliott, 2008) showed a considerable number of errors.…”

“…Although heterostracans are a numerous and taxonomically diverse group, little phylogenetic analysis has been carried out on them until recently. Currently the Cyathaspididae (Lundgren and Blom, 2013), the Psammosteoidei (Glinskiy, 2017), and the Pteraspidiformes (Pernègre and Elliott, 2008; Randle and Sansom, 2016, 2017) have been analyzed. The first computerized analysis of the Pteraspidiformes was published by Ilyes and Elliott (1994), but was limited to a small number of taxa from the western USA.…”

A Phylogeny for Heterostraci (stem-gnathostomes)

“…Also demonstrated to belong within Pteraspidiformes are the Psammosteidae. Indeed when included within phylogenetic analyses of Pteraspidiformes they have been found positioned between the Anchipteraspididae and non-Anchipteraspididae Pteraspidiformes (Pernègre & Elliott, 2008), in a highly derived position within the Pteraspidiformes (Pernègre, 2002, Glinskiy 2017) or nested somewhere in between (Randle & Sansom, 2016; 2017).…”

“…Many key groups such as the tessellate heterostracans, Amphiaspididae and Traquairaspididae have yet to be subjected to a cladistic analysis, with previous analyses focusing on smaller clades within the Heterostraci i.e. Pteraspidiformes and Cyathaspididae (Ilyes & Elliott, 1994; Pernègre, 2002; Pernègre & Goujet, 2007; Pernègre & Elliott, 2008; Lundgren & Blom, 2013; Randle & Sansom, 2016, 2017) or Psammosteoidei (Glinskiy 2017).…”