The osteohistology of gorgonopsian therapsids and implications for Permo-Triassic theriodont growth.

During the Late Permian, saber-toothed gorgonopsian therapsids were the dominant terrestrial predators, playing crucial roles as apex predators alongside therocephalian therapsids within Permian terrestrial ecosystems. The entire gorgonopsian clade went extinct during the Permo-Triassic mass extinction, leaving other therapsids to continue into the Triassic. Gorgonopsians have not been well studied, particularly in terms of their growth patterns, with only a few genera having undergone osteohistological analysis. In this study, I present a thorough osteohistological examination of the most extensive collection of gorgonopsian specimens to date, spanning a diverse range of limb bones sourced from various species. The osteohistological analysis of gorgonopsian specimens reveals a trend of rapid growth characterized by a highly vascularized woven-parallel complex. The abundance of growth marks and variable zone widths suggests a growth trajectory that could indicate longer lifespans and slower growth rates when compared to Early Triassic therapsids. The high vascularity, coupled with the observed growth patterns, implies that gorgonopsians experienced rapid growth during favorable conditions. However, the multiple growth marks indicate that they likely had the capacity for longer lifespans and more gradual maturation than their Early Triassic counterparts. Additionally, their ability to reach later ontogenetic stages supports the hypothesis that favorable environmental conditions facilitated larger body sizes. In contrast, Early Triassic therapsids primarily consisted of juveniles or individuals who reached reproductive maturity within a year, likely indicative of harsher conditions that contributed to higher mortality rates at younger ages. The onset of decreased growth rates, usually indicative of reproductive maturity, occurred later in gorgonopsians compared to Early Triassic therapsids and may have contributed to their decline, as the heightened juvenile mortality rates during the PTME would have limited the gorgonopsians' ability to reproduce effectively.