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Bony-fish-like scales in a Silurian maxillate placoderm
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Bony-fish-like scales in a Silurian maxillate placoderm
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Journal Article
Bony-fish-like scales in a Silurian maxillate placoderm
2023
Overview
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 fish
Entelognathus
combines 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 of
Entelognathus
. 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 in
Entelognathus
of 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 origin and early evolution of large scales in bony fishes and small scales in cartilaginous fishes are unclear. Here, the authors report a 425-million-year-old fish,
Entelognathus
, with a mosaic of scale and fin spine characters.
