What Lies Beneath: Sub-Articular Long Bone Shape Scaling in Eutherian Mammals and Saurischian Dinosaurs Suggests Different Locomotor Adaptations for Gigantism

Abstract

Eutherian mammals and saurischian dinosaurs both evolved lineages of huge terrestrial herbivores. Although significantly more saurischian dinosaurs were giants than eutherians, the long bones of both taxa scale similarly and suggest that locomotion was dynamically similar. However, articular cartilage is thin in eutherian mammals but thick in saurischian dinosaurs, differences that could have contributed to, or limited, how frequently gigantism evolved. Therefore, we tested the hypothesis that sub-articular bone, which supports the articular cartilage, changes shape in different ways between terrestrial mammals and dinosaurs with increasing size. Our sample consisted of giant mammal and reptile taxa (i.e., elephants, rhinos, sauropods) plus erect and non-erect outgroups with thin and thick articular cartilage. Our results show that eutherian mammal sub-articular shape becomes narrow with well-defined surface features as size increases. In contrast, this region in saurischian dinosaurs expands and remains gently convex with increasing size. Similar trends were observed in non-erect outgroup taxa (monotremes, alligators), showing that the trends we report are posture-independent. These differences support our hypothesis that sub-articular shape scales differently between eutherian mammals and saurischian dinosaurs. Our results show that articular cartilage thickness and sub-articular shape are correlated. In mammals, joints become ever more congruent and thinner with increasing size, whereas archosaur joints remained both congruent and thick, especially in sauropods. We suggest that gigantism occurs less frequently in mammals, in part, because joints composed of thin articular cartilage can only become so congruent before stress cannot be effectively alleviated. In contrast, frequent gigantism in saurischian dinosaurs may be explained, in part, by joints with thick articular cartilage that can deform across large areas with increasing load.

Three‐Dimensional Musculoskeletal Modeling of the Sauropodomorph Hind Limb: The Effect of Postural Change on Muscle Leverage

“…Left-side elements of Diamantinasaurus (pelvis) were mirrored in Rhinoceros to produce a complete right limb. Cartilage at joints was estimated to be approximately 10% of long bone length, as has been proposed in previous studies (Schwarz et al, 2007;Bonnan et al, 2010;Holliday et al, 2010;Mallison, 2011a;Bonnan et al, 2013).…”

Articular soft tissue anatomy of the archosaur hip joint: Structural homology and functional implications

“…Haines () and Maisano () referred to the mineralization center simply as “secondary center” and the surrounding tissue as “epiphyseal cartilage.” This scheme was followed by Holliday et al (), who distinguished between the cartilaginous epiphyses and its underlying bony metaphysis. In contrast, most paleontological studies on tetrapods lacking secondary centers of calcification (e.g., fossilized dinosaurs) tend to refer to the subchondral growth plate as an “epiphysis” or “epiphyseal surface” (Horner et al, ; de Ricqlès et al, ; Bonnan et al, ). These contrasting definitions complicate inferences of homology in soft tissue structures in sauropsid joints.…”

Late Jurassic–Early Cretaceous trackways of small-sized sauropods from China: New discoveries, ichnotaxonomy and sauropod manus morphology

“…The orientation of the manus prints is intriguing to consider in light of recent studies by Holliday et al (2010) and Bonnan et al (2010Bonnan et al ( , 2013 on archosaur articular cartilage. Unlike graviportal mammals in which a very thin layer of articular cartilage may constrain the range of movements to more stereotyped patterns to avoid joint injury , it is likely that sauropods retained centimeters of cartilage on their joints into adulthood (Bonnan et al, 2010Holliday et al, 2010).…”