Bony-fish-like scales in a Silurian maxillate placoderm

Area of Science:

• Paleontology
• Evolutionary Biology
• Vertebrate Zoology

Background:

• Jawed vertebrates display diverse dermal armor (plates and scales).
• The evolutionary transition between these armor types is poorly understood.
• Key fossil taxa with intermediate characteristics are rare.

Purpose of the Study:

• To investigate the phylogenetic position and characteristics of Entelognathus.
• To clarify the evolutionary origins of jawed vertebrate dermal armor and fin structures.
• To understand the early evolution of gnathostomes.

Main Methods:

• Analysis of a newly discovered, near-complete post-thoracic exoskeleton of Entelognathus.
• Comparative morphological analysis of Entelognathus's scales and fin spines with extant and extinct vertebrates.
• Phylogenetic analysis incorporating Entelognathus's unique character combination.

Main Results:

• Entelognathus possesses large, rhomboid scales with peg-and-socket articulations, previously thought unique to osteichthyans.
• The presence of an anal fin spine in Entelognathus, found in some stem chondrichthyans, is reported.
• This mosaic of features challenges previous assumptions about character distribution in jawed vertebrates.

Conclusions:

• Many gnathostome characters, including scale types and fin spine presence, likely predated the common ancestor of living jawed vertebrates.
• Entelognathus provides crucial evidence for the early evolution of key vertebrate traits.
• The study refines our understanding of the evolutionary history of dermal armor and skeletal features in jawed vertebrates.