Contrasting macroevolutionary patterns in pelagic tetrapods across the Triassic-Jurassic transition

Area of Science:

• Paleontology
• Macroevolutionary Biology
• Mesozoic Marine Ecosystems

Background:

• Ichthyosauria and Eosauropterygia, marine predators, co-existed throughout the Mesozoic Era.
• Previous studies suggested a macroevolutionary bottleneck across the Triassic-Jurassic (T/J) transition for these groups.
• Existing analyses often relied on limited morphological and temporal data.

Purpose of the Study:

• To comprehensively compare the macroevolutionary patterns of ichthyosaurian and eosauropterygian morphology and body size.
• To investigate the Middle Triassic to Early Jurassic interval for evolutionary trends.
• To re-evaluate the hypothesis of a T/J bottleneck using detailed data.

Main Methods:

• Comparative analysis of ichthyosaurian and eosauropterygian morphology, body size, and ecomorphospace.
• Examination of craniodental phenotypes and fin shape evolution.
• Temporal analysis across the Middle Triassic to Early Jurassic.

Main Results:

• Eosauropterygian ecomorphospace shows a strong phylogenetic signal with distinct craniodental clades, indicating leaps into new feeding ecologies.
• Ichthyosaurian diversification lacks clear trends, with overlapping craniodental morphologies between Triassic and Early Jurassic forms.
• No support for an abrupt T/J bottleneck; turnover events appear earlier, linked to rapid sea-level falls.

Conclusions:

• Ichthyosauria and Eosauropterygia exhibit contrasting macroevolutionary patterns, challenging previous bottleneck hypotheses.
• Eosauropterygian evolution is characterized by distinct phylogenetic leaps, while ichthyosaurian diversification is more continuous.
• Significant marine reptile turnover likely occurred before the T/J transition, influenced by environmental changes like sea-level fluctuations.