Developmental strategies underlying gigantism and miniaturization in non-avialan theropod dinosaurs

Abstract

In amniotes, the predominant developmental strategy underlying body size evolution is thought to be adjustments to the rate of growth rather than its duration. However, most theoretical and experimental studies supporting this axiom focus on pairwise comparisons and/or lack an explicit phylogenetic framework. We present the first large-scale phylogenetic comparative analysis examining developmental strategies underlying the evolution of body size, focusing on non-avialan theropod dinosaurs. We reconstruct ancestral states of growth rate and body mass in a taxonomically rich dataset, finding that contrary to expectations, changes in the rate and duration of growth played nearly equal roles in the evolution of the vast body size disparity present in non-avialan theropods—and perhaps that of amniotes in general.

Osteohistological insight into the growth dynamics of early dinosaurs and their contemporaries

“…Femoral histologies are described below for Herrerasaurus , Sanjuansaurus , Eodromaeus , Eoraptor , and Chromogisaurus . These histological descriptions are the first for Sanjuansaurus [ 87 – 88 ], Eodromaeus [ 83 ], Eoraptor [ 60 , 82 , 84 ], and Chromogisaurus [ 81 , 82 ], and supplement existing histological descriptions of the humerus and tibia of Herrerasaurus [ 17 , 18 , 147 ]. Our sampled Herrerasaurus femur (PVSJ 614) is from a similarly sized individual to those previously studied histologically ( Table 1 ) [ 17 , 18 , 76 , 147 ].…”

“…These histological descriptions are the first for Sanjuansaurus [ 87 – 88 ], Eodromaeus [ 83 ], Eoraptor [ 60 , 82 , 84 ], and Chromogisaurus [ 81 , 82 ], and supplement existing histological descriptions of the humerus and tibia of Herrerasaurus [ 17 , 18 , 147 ]. Our sampled Herrerasaurus femur (PVSJ 614) is from a similarly sized individual to those previously studied histologically ( Table 1 ) [ 17 , 18 , 76 , 147 ]. The Eoraptor femur in our sample is derived from a referred specimen (PVSJ 559) that is slightly larger than the adult holotype ( Table 1 ) [ 84 ].…”

“…In other studied Herrerasaurus specimens vascular networks tend to be plexiform, with an abundance of circular vascular canals instead of the laminar-reticular pattern that dominates femur PVSJ 614. Moreover, thin layers of avascular endosteal bone line the medullary cavity to form an IFS in other sampled specimens, and secondary osteon formation is limited or absent [ 17 , 147 ]. In contrast, PVSJ 614 exhibits a distinctive pattern of intensive secondary remodeling, with both endosteal lamellar bone forming the IFS and secondary osteons that are dense in the deep cortex.…”

“…Our sample includes a thin periosteal zone of avascular PFB/LFB, at least on the posterior surface of the element, that is similar to that observed in other sampled specimens. A similarly sized Herrerasaurus tibia exhibits an EFS, and also lacks cortical growth marks [ 147 ], but other sampled specimens lack definitive EFS, and thus were still actively growing when they died.…”

Testing heterochrony: Connecting skull shape ontogeny and evolution of feeding adaptations in baleen whales

“…Allometric trajectories are a powerful tool for understanding heterochrony as they can be recorded during the growth of a single organism or across species without data on raw or relative developmental timing (McKinney, 1988). A clear instance of evolutionary allometry is the gigantism or extreme increase in body size in sauropods (herbivorous non-avian dinosaurs), achieved through peramorphosis and probably connected to shifts in global climate and resource availability (D'Emic et al, 2023). On the other hand, a typical case of pedomorphosis in evolutionary allometry is the dwarfism or reduction of body size, as well as brain size, in many mammalian insular species, such as hippos, elephants, and humans, which was likely caused by the lack of predators and decrease in diet quality (Herridge & Lister, 2012).…”