Bayesian Morphological Clock Methods Resurrect Placoderm Monophyly and Reveal Rapid Early Evolution in Jawed...

Area of Science:

• Paleontology and Evolutionary Biology
• Vertebrate Phylogeny
• Gnathostome Evolution

Background:

• Understanding the phylogeny of early jawed vertebrates (gnathostomes) is crucial for tracing vertebrate evolution.
• Recent studies suggested placoderms, an extinct group of armored fish, are paraphyletic and basal to other jawed vertebrates.
• Resolving the root position of jawed vertebrates remains a challenge due to limited fossil evidence and analytical complexities.

Purpose of the Study:

• To revise and expand morphological data sets for phylogenetic analysis of early gnathostomes.
• To investigate the phylogenetic position of placoderms, specifically testing hypotheses of their paraphyly versus monophyly.
• To apply Bayesian tip-dated clock methods to infer evolutionary rates and improve phylogenetic resolution.

Main Methods:

• Expanded a morphological data set, incorporating autapomorphies and parsimony-informative traits for Bayesian tip-dated clock analyses.
• Re-analyzed phylogenetic relationships of early gnathostomes, addressing issues like character correlation and coding errors.
• Utilized simulations to compare parsimony and Bayesian clock methods, particularly with uninformative outgroups.

Main Results:

• Phylogenetic analyses revealed that both placoderm paraphyly and monophyly are equally parsimonious under standard methods.
• Bayesian tip-dated clock analyses strongly support placoderm monophyly, indicating more balanced evolutionary rates.
• Gnathostome evolution shows a rapid burst in the Silurian period, followed by a decline in the Early Devonian.

Conclusions:

• Placoderm monophyly is the favored hypothesis, resolving ambiguities in early jawed vertebrate phylogeny.
• Bayesian tip-dated clock methods are effective in overcoming limitations of parsimony analysis with incomplete data.
• The rapid Silurian diversification of gnathostomes, evidenced by this study, contributes to the difficulty in reconstructing their earliest evolutionary history.