Vaskaninova and Ahlberg 2017
studied the placoderm Radotina (similar to Romundina, Fig. 1) and described the state of the art regarding the phylogenetic placement of acanthodians and placoderms (Fig. 1) along with the origin of jaws (clade Gnathostomata, Fig. 2).
From their introduction:
“The early evolution of vertebrates has recently become a major research topic in vertebrate biology. 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; in phylogenetic terms, it is a segment of the vertebrate tree whose content and topology has long been the subject of debate.”
Recently the large reptile tree (LRT, 1565 taxa) nested the whale shark (genus: Rhincodon) and catfish (genus: Clarias) + placoderms (genus: Entelognathus) adding a novel hypothesis (Fig. 2) to that debate.
“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.”
The LRT recovers jawed placoderms, jawed sharks and other jawed fish at the base of their respective clades. Reduced and ventral jaws, as in sturgeons (genus: Pseudoscaphirhynchus), rays (genus: Rhinobatos) and placoderms (genus: Qilinyu) are derived, not basal taxa).
“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.”
As mentioned above, but reordered.
“Associated with this reinterpretation of the placoderms has been the recognition of homologies between the macromeric dermal skeleton of placoderms and osteichthyans, previously regarded by most workers as independently evolved.”
The LRT supports the ‘homologous’ interpretation.
“Another Palaeozoic vertebrate group, the acanthodians (ªspiny sharksº), which were previously seen as stem osteichthyans or as a multiply paraphyletic array of stem gnathostomes, stem osteichthyans and stem chondrichthyans, are in the most recent analyses assigned in their entirety to the chondrichthyan stem group.”
The LRT nests acanthodians (genus: Ischnacanthus) between certain ray fins (genus: Cheirolepis) and other ray fins (genus: Tachinocephalus) and notes that other fins turn into spines in basal tetraodontiforms (genus: Plectocretacicus) and sticklebacks (genus: Gasterosteus).
“This leaves the upper part of the gnathostome stem group occupied entirely by placoderms, and gives rise to the idea of a ªplacoderm-osteichthyan continuumº, where osteichthyans essentially continued the gradual development of the placoderm bauplan whereas chondrichthyans departed more radically from it, inter alia by losing their perichondral and macromeric dermal bones.”
The LRT does not recover this hypothesis of relationships. Chondrichthyans (sharks, rays, chimaera and kin) have a sister group origin parallel to catfish + placoderms originating with a Silurian sister to the whale shark (genus: Rhincodon) arising from the jawless outgroup taxon, Thelodus.
“Detailed studies of primitive placoderms therefore have the potential to provide crucial information about the early evolutionary steps on the path leading to our own body plan.”
Key to understanding the origin of jaws is the inclusion of whale sharks and catfish, two taxa that have been traditionally excluded.
VasÏkaninová V, Ahlberg PE 2017. Unique diversity of acanthothoracid placoderms (basal jawed vertebrates) in the Early Devonian of the Prague Basin, Czech Republic: A new look at Radotina and Holopetalichthys. PLoS ONE 12(4): e0174794. https://doi.org/10.1371/journal.pone.0174794