Bony-fish-like scales in a Silurian maxillate placoderm

Abstract

Major groups of jawed vertebrates exhibit contrasting conditions of dermal plates and scales. But the transition between these conditions remains unclear due to rare information on taxa occupying key phylogenetic positions. The 425-million-year-old fishEntelognathuscombines an unusual mosaic of characters typically associated with jawed stem gnathostomes or crown gnathostomes. However, only the anterior part of the exoskeleton was previously known for this very crownward member of the gnathostome stem. Here, we report a near-complete post-thoracic exoskeleton ofEntelognathus. Strikingly, its scales are large and some are rhomboid, bearing distinctive peg-and-socket articulations; this combination was previously only known in osteichthyans and considered a synapomorphy of that group. The presence inEntelognathusof an anal fin spine, previously only found in some stem chondrichthyans, further illustrates that many characters previously thought to be restricted to specific lineages within the gnathostome crown likely arose before the common ancestor of living jawed vertebrates.

The Paleozoic assembly of the holocephalian body plan far preceded post-Cretaceous radiations into the ocean depths

“…We analyzed the morphological matrix using a maximum parsimony approach in PAUP* v4.0a [50]. We set the maxillate 'placoderm' stem-gnathostome †Entelognathus primordialis [51,52] as the outgroup and ran a heuristic search with TBR branch swapping and 1000 addition sequence replicates holding 5 trees from each replicate (Fig. S1).…”