Contrasting life history in the diminutive Dimetrodon species from North America and Germany.

The sphenacodontid Dimetrodon is best known from lower Permian marginal-marine deposits of North America (NA), where it occupied apex predator roles in aquatic-based food chains. Dimetrodon teutonis, the smallest documented species and only known representative of this genus outside NA, comes from the fully terrestrial and resource-limited ecosystem of the upper Asselian Bromacker locality, Germany. To evaluate its growth strategy and diminutive nature, we present the first osteohistological analysis of D. teutonis. We also reassess NA material from four localities spanning distinct depositional settings. In contrast to the thick and highly vascularized woven to parallel-fibered bone cortices of most NA species, including the small-bodied D. natalis, D. teutonis exhibits thin cortices composed of poorly vascularized parallel-fibered bone. The presence of an external fundamental system indicates somatic maturity and confirms its diminutive nature. These data suggest extended and slow growth throughout ontogeny, likely linked to seasonal resource limitation and low predation pressure in the unique Bromacker ecosystem. In contrast, NA diminutive species inhabiting lowland ecosystems grew rapidly and matured early, in response to competition and high predation pressure. These contrasting life histories in small Dimetrodon species reflect differing ecological pressures and illustrate diverging evolutionary pathways to dwarfism in this iconic genus.