The characters of Palaeozoic jawed vertebrates

Area of Science:

• Paleontology
• Vertebrate Zoology
• Evolutionary Biology

Background:

• Fossil discoveries from the Silurian and Devonian periods are challenging existing views on jawed vertebrate (gnathostome) origins.
• A clear cladistic framework for early gnathostomes, particularly relating fossils to major lineages like osteichthyans (bony fishes and tetrapods) and chondrichthyans (sharks, batoids, chimaeras), is lacking.

Purpose of the Study:

• To critically review the systematics and character data of early gnathostomes.
• To establish a robust cladistic framework using a hierarchy of synapomorphies for stem gnathostomes and the stem groups of osteichthyans and chondrichthyans.
• To re-evaluate the phylogenetic positions of problematic fossil groups, Acanthodii and Placodermi.

Main Methods:

• Systematic review of early gnathostome fossils and their character distributions.
• Development of a hierarchical scheme of synapomorphies (shared derived traits) for phylogenetic analysis.
• Cladistic analysis to test the monophyly of putative groups and evaluate conflicting evidence.

Main Results:

• Character lists supporting group monophyly often overstate evidence and lack cladistic support.
• A synapomorphy hierarchy provides a more falsifiable framework for understanding evolutionary relationships.
• Placoderms are interpreted as a paraphyletic assemblage of stem-group gnathostomes.
• Acanthodians have two plausible phylogenetic placements: solely on the chondrichthyan stem, or distributed across both chondrichthyan and osteichthyan stems.

Conclusions:

• The proposed synapomorphy scheme offers a more rigorous approach to understanding early gnathostome evolution.
• Placoderms represent an early, diverse radiation of jawed vertebrate ancestors.
• The phylogenetic position of acanthodians remains uncertain, highlighting areas for future research in vertebrate origins.