Evolutionary Trends in Hind Limb Morphology of Titanosauriformes (Dinosauria, Sauropoda) Reveal Wide-Gauge Posture as an Exaptation for Gigantism

핵심 개념

The acquisition of wide-gauge hind limb posture in titanosaurian sauropods was an exaptation that facilitated the evolution of gigantism, rather than being directly driven by increases in body size.

초록

The study analyzes the evolution of hind limb morphology in titanosauriform sauropods using 3D geometric morphometrics. The results show that the trend toward a more arched and robust hind limb posture, known as the "wide-gauge" stance, persisted in deeply nested titanosaurs. However, this morphological trend was not directly correlated with increases in body size.

The analysis reveals a high degree of morphological convergence between different titanosaurian subclades, suggesting that the wide-gauge posture was an exaptation that enabled the adaptation to a range of ecological niches, rather than being driven solely by the evolution of gigantism.

While the largest titanosaurs exhibit the most extreme hind limb morphologies, smaller titanosaurs also convergently evolved similar robust and arched limb features. This indicates that the wide-gauge posture was established early in titanosaurian evolution and was then co-opted for different biomechanical functions, including potential rearing capabilities, rather than being directly linked to increases in body size.

The study also finds a phylogenetic trend toward decreasing hind limb size across titanosaurian evolution, despite the persistence of the wide-gauge morphology. This suggests that the acquisition of large body size was not the primary driver of the hind limb changes observed in this group.

통계

"The first six PCs accounted for 78.3% of the cumulative morphological variation."
"There is a trend in hind limb centroid size distribution from large, non-titanosaurian macronarians in the Early Cretaceous to small, deeply nested lithostrotian titanosaurs in the Late Cretaceous."
"Pagel's lambda estimation shows a significative phylogenetic signal in log-transformed hind limb centroid size (λ = 0.982), and PC1 (λ = 0.715), PC3 (λ = 0.760), PC5 (λ = 0.778) and PC6 (λ = 0.697)."

인용구

"The wide stance may enable enhanced lateral stability during locomotion allowing them to adapt to a range of new geographic and ecological niches."
"Despite some titanosaurian subclades becoming some of the largest vertebrates, there is a phyletic body size decrease in Macronaria."
"The acquisition of this particular morphology is correlated with the development of gigantism within Titanosauria (Carrano, 2005; Lefebvre et al., 2022; Ullmann et al., 2017), at least when analysing early branching members of Lithostrotia."

핵심 통찰 요약

Evolution of hind limb morphology of Titanosauriformes (Dinosauria, Sauropoda) analyzed via 3D Geometric Morphometrics reveals wide-gauge posture as an exaptation for gigantism

by Paramo Blazq... 게시일 www.biorxiv.org 10-06-2023

https://www.biorxiv.org/content/10.1101/2023.10.04.560839v2

더 깊은 질문

How might the wide-gauge hind limb posture have facilitated the exploitation of different ecological niches by titanosaurian sauropods?

The wide-gauge hind limb posture in titanosaurian sauropods likely played a crucial role in facilitating the exploitation of various ecological niches. This posture, characterized by an increasingly arched limb morphology, provided these massive dinosaurs with enhanced stability during locomotion. The wider stance allowed for better weight distribution and balance, especially in large-bodied species, which would have been essential for navigating diverse terrains and environments.
With a wide-gauge posture, titanosaurian sauropods could have effectively moved through different types of landscapes, including uneven terrain, dense vegetation, and possibly even shallow water bodies. This adaptation would have enabled them to access a wider range of food sources, such as high vegetation for browsing or aquatic plants for feeding. The increased stability provided by the wide-gauge hind limb posture may have also been advantageous for activities like rearing up on their hind limbs to reach higher vegetation or for defensive purposes against predators.
Furthermore, the wide-gauge posture could have been beneficial for energy efficiency during locomotion. By spreading their weight over a larger area, titanosaurian sauropods may have reduced the energy expenditure required for movement, allowing them to cover greater distances or sustain prolonged periods of activity. This would have been advantageous for foraging, migration, or other ecological behaviors.
Overall, the wide-gauge hind limb posture likely allowed titanosaurian sauropods to adapt to a wide range of ecological niches, enhancing their ability to thrive in diverse environments and exploit various resources for survival.

How might the wide-gauge hind limb posture have facilitated the exploitation of different ecological niches by titanosaurian sauropods?

The evolution of gigantism in titanosaurs was likely influenced by a combination of morphological and physiological adaptations beyond just hind limb structure. Gigantism in sauropods, including titanosaurs, required a suite of characteristics to support their massive body sizes and unique lifestyle.
One key adaptation contributing to gigantism in titanosaurs is their efficient respiratory system. These dinosaurs likely had avian-like air sacs that facilitated efficient respiration and oxygen exchange, allowing their large bodies to be adequately supplied with oxygen. This adaptation would have been crucial for sustaining the high metabolic rates necessary for supporting such massive body sizes.
Additionally, the long necks of titanosaurs played a significant role in their evolution of gigantism. The elongated necks allowed them to reach high vegetation for feeding, reducing competition for resources with other herbivores and enabling them to access a plentiful food source. The evolution of a specialized feeding mechanism, such as dental adaptations for processing tough plant material or a unique swallowing strategy, would have also been important for supporting their large body sizes.
Furthermore, titanosaurs likely had adaptations in their circulatory system to support their immense size. Efficient blood circulation, possibly aided by adaptations like a four-chambered heart or specialized vascular structures, would have been necessary to ensure that oxygen and nutrients were effectively distributed throughout their massive bodies.
Overall, the evolution of gigantism in titanosaurs was likely a complex interplay of various morphological and physiological adaptations beyond just their hind limb structure. These adaptations would have been essential for supporting their massive body sizes, high metabolic rates, specialized feeding habits, and overall ecological success.

Could the observed trend toward decreasing body size in lithostrotian titanosaurs be related to factors such as environmental changes or resource availability during the Late Cretaceous?

The observed trend toward decreasing body size in lithostrotian titanosaurs could indeed be influenced by various factors, including environmental changes and resource availability during the Late Cretaceous. Several hypotheses could explain this trend:

Resource Competition: As titanosaurs evolved and diversified, competition for resources within ecosystems may have increased. Smaller body sizes could have been advantageous in environments where resources were limited or where competition for food was intense. Smaller titanosaurs may have been able to exploit different niches or food sources that were not accessible to larger species.

Environmental Changes: Shifts in climate or habitat conditions during the Late Cretaceous could have also played a role in the decreasing body size trend. Changes in vegetation types, availability of food sources, or habitat fragmentation may have favored smaller-bodied titanosaurs that were more adaptable to these environmental changes.

Predation Pressure: Increased predation pressure from evolving predators could have also influenced the body size of lithostrotian titanosaurs. Smaller body sizes may have provided a survival advantage against predators, allowing individuals to be more agile, faster, or better able to evade or defend against predators.

Energetic Efficiency: Maintaining a large body size requires significant energy expenditure. Smaller titanosaurs may have been more energetically efficient, requiring less food intake and metabolic energy to sustain their bodies. In environments where resources were scarce or energy demands were high, smaller body sizes may have been more advantageous.

Evolutionary Constraints: Evolutionary constraints within the lithostrotian lineage may have also played a role in the decreasing body size trend. Genetic factors, developmental constraints, or trade-offs between different traits could have limited the ability of lithostrotian titanosaurs to maintain or increase their body sizes over time.

Overall, the trend toward decreasing body size in lithostrotian titanosaurs is likely a complex outcome of multiple interacting factors, including environmental changes, resource availability, competition, predation pressure, and evolutionary constraints. Further research and analysis of fossil evidence, paleoenvironmental data, and ecological interactions may provide more insights into the drivers behind this observed trend.