Were the synapsids primitively endotherms? A palaeohistological approach using phylogenetic eigenvector maps

Area of Science:

• Paleophysiology
• Evolutionary Biology
• Paleontology

Background:

• The evolutionary origins and timing of mammalian endothermy remain unclear.
• Understanding thermometabolic regimes in extinct vertebrates is crucial for tracing evolutionary pathways.

Purpose of the Study:

• To infer resting metabolic rates (RMRs) and thermometabolic regimes (endothermy/ectothermy) for extinct synapsids.
• To reconstruct the phylogenetic and temporal acquisition of endothermy in vertebrates.

Main Methods:

• Palaeohistology and phylogenetic eigenvector maps (PEMs) were used to estimate RMRs in extinct synapsids.
• Respirometry data from extant tetrapods were integrated with inferred data for ancestral state reconstruction.
• Parsimony optimization was employed to determine the independent evolution of endothermy.

Main Results:

• Anomodonts (e.g., Lystrosaurus) and early therapsids showed high RMRs indicative of endothermy.
• Early diverging synapsids like Dimetrodon exhibited low RMRs, suggesting ectothermy.
• Maximum-likelihood reconstruction indicated primitive endothermy in Anomodontia and Mammalia.

Conclusions:

• Endothermy likely arose independently in Archosauromorpha, Sauropterygia, and Therapsida.
• The study provides evidence for early endothermy within the lineage leading to mammals.
• This research refines our understanding of thermoregulation evolution in deep time.