The dinosaurian femoral head experienced a morphogenetic shift from torsion to growth along the avian stem

Abstract

Significant evolutionary shifts in locomotor behaviour often involve comparatively subtle anatomical transitions. For dinosaurian and avian evolution, medial overhang of the proximal femur has been central to discussions. However, there is an apparent conflict with regard to the evolutionary origin of the dinosaurian femoral head, with neontological and palaeontological data suggesting seemingly incongruent hypotheses. To reconcile this, we reconstructed the evolutionary history of morphogenesis of the proximal end of the femur from early archosaurs to crown birds. Embryological comparison of living archosaurs (crocodylians and birds) suggests the acquisition of the greater overhang of the femoral head in dinosaurs results from additional growth of the proximal end in the medial-ward direction. On the other hand, the fossil record suggests that this overhang was acquired by torsion of the proximal end, which projected in a more rostral direction ancestrally. We reconcile this apparent conflict by inferring that the medial overhang of the dinosaur femoral head was initially acquired by torsion, which was then superseded by mediad growth. Details of anatomical shifts in fossil forms support this hypothesis, and their biomechanical implications are congruent with the general consensus regarding broader morpho-functional evolution on the avian stem.

Macroevolutionary patterns in the pelvis, stylopodium and zeugopodium of megalosauroid theropod dinosaurs and their importance for locomotor function

“…A more medially oriented femoral head should have contributed to a more parasagittal gait and support (e.g. [ 46 , 83 , 91 , 92 ]).

Figure 17 Evolutionary history of character 301 (femur, head orientation) and the ancestral state reconstruction.

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“…10: 230481 more medially oriented femoral head should have contributed to a more parasagittal gait and support (e.g. [46,83,91,92]). The orientation of the femoral head (character 302) is ventromedial (302[0]), again in most early theropods (e.g.…”

The evolution of femoral morphology in giant non-avian theropod dinosaurs

“…A lower epiphyseal offset reflects improved protraction/retraction motions of the femur, which is associated with more parasagittal limb posture (Carrano 2000; Hutchinson and Gatesy 2000), commonly associated with the archosaurian erect posture and, more relevant here, the evolution of more knee-based locomotion toward birds (Farlow et al 2000; Hutchinson and Allen 2009). Yet although a shift in epiphyseal offset evolved within theropods toward crown-group birds (Egawa et al 2022), it also convergently appeared several times within other dinosaur clades (e.g., Ornithischia and Sauropodomorpha; Carrano 2000; Hutchinson 2001). Therefore, our observations further characterize the convergent nature of femoral epiphyseal offset by demonstrating its occurrence along non-coelurosaurian lineages, which diverged earlier than the avian lineage within Theropoda.…”

“…Inevitably, these changes of body mass are associated with changes in the morphology of hindlimb bones (Gregory 1912;Biewener 1983Biewener , 1989Carrano 1998Carrano , 2001Campione and Evans 2012;Mallet et al 2019;Etienne et al 2021). Archosaurian femoral morphology was highly constrained by several biological factors such as variations of body size/mass, shifts in locomotor habits, variation from a more sprawling to erect posture, and a shift from a more hip-to knee-driven gait, as demonstrated through morphometrics, biomechanics, microanatomy, and embryology (Charig 1972;Bakker and Galton 1974;Parrish 1986;Gatesy 1990;Christiansen 1998;Carrano 1999;Farlow et al 2000;Hutchinson and Gatesy 2000;Maidment and Barrett 2012;Bishop et al 2018a,b;Allen et al 2021;Egawa et al 2022;Lefebvre et al 2022). The resulting shifts in the functional morphology and biomechanics of the femur have been investigated at the scale of Archosauria as a whole, but also between several dinosaurian clades and in the context of major faunal turnover across biological crises (Kubo and Kubo 2012;Sookias et al 2012;Bishop et al 2018aBishop et al ,b, 2020Cuff et al 2022;Pintore et al 2022b).…”

Forelimb muscle activation patterns in American alligators: Insights into the evolution of limb posture and powered flight in archosaurs

“…First, a shift from a more abducted ‘sprawling’ limb posture to a more adducted ‘erect’ limb posture occurred during the Triassic (Charig, 1972 ). Multiple archosaur subgroups underwent skeletal transformations associated with such postural shifts, including fenestration of the acetabulum, development of a strong supraacetabular crest, and medial deflection of the femoral head (Bonaparte, 1984 ; Carrano, 2000 ; Charig, 1972 ; Egawa et al., 2022 ; Griffin et al., 2022 ; Hutchinson, 2001a , 2001b , 2006 ; Hutchinson & Gatesy, 2000 ; Iijima & Kobayashi, 2014 ; Parrish, 1986 ). Although this anatomical evidence suggested that the transition to erect limb posture may have occurred gradually, fossil trackways suggest that the shift toward more erect limb posture might have occurred rather abruptly in the Early Triassic (Kubo & Benton, 2009 ).…”