Heads or tails: staged diversification in vertebrate evolutionary radiations

Area of Science:

• Evolutionary biology
• Paleontology
• Comparative anatomy

Background:

• Adaptive radiations are key drivers of biodiversity, characterized by rapid morphological divergence and species diversification.
• The prevailing 'early burst' model suggests an initial rapid phase of divergence followed by stasis.
• A vertebrate-specific model posits sequential diversification: postcranial traits first, then cranial traits.

Purpose of the Study:

• To empirically test existing models of adaptive radiation in actinopterygian fishes.
• To investigate the temporal sequence of cranial versus postcranial morphological diversification.
• To determine if vertebrate-specific models of radiation apply to fish evolution.

Main Methods:

• Comparative analysis of morphological diversification patterns in two major actinopterygian radiations.
• Application of model-fitting exercises to evolutionary rates.
• Use of the divergence order test to assess the sequence of trait diversification.

Main Results:

• Actinopterygian radiations exhibit distinct stages, with cranial diversification preceding postcranial diversification.
• Early actinopterygians and acanthomorph teleosts showed immediate cranial dispersal post-extinction/origin.
• Significant postcranial variation emerged only after cranial diversification had commenced.

Conclusions:

• The findings challenge the universality of the 'early burst' and 'general vertebrate' models of adaptive radiation.
• The evolutionary patterns are better explained by an Ornstein-Uhlenbeck model, indicating constant rates with a central tendency.
• The study highlights unique diversification dynamics in fish radiations, questioning generalized models.