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Abstract Complex structures, like the vertebrate skull, are composed of numerous elements or traits that must develop and evolve in a coordinated manner to achieve multiple functions. The strength of association among phenotypic traits (i.e., integration), and their organization into highly-correlated, semi-independent subunits termed modules, is a result of the pleiotropic and genetic correlations that generate traits. As such, patterns of integration and modularity are thought to be key factors constraining or facilitating the evolution of phenotypic disparity by influencing the patterns of variation upon which selection can act. It is often hypothesized that selection can reshape patterns of integration, parceling single structures into multiple modules or merging ancestrally semi-independent traits into a strongly correlated unit. However, evolutionary shifts in patterns of trait integration are seldom assessed in a unified quantitative framework. Here, we quantify patterns of evolutionary integration among regions of the archosaur skull to investigate whether patterns of cranial integration are conserved or variable across this diverse group. Using high-dimensional geometric morphometric data from 3D surface scans and computed tomography scans of modern birds (n = 352), fossil non-avian dinosaurs (n = 27), and modern and fossil mesoeucrocodylians (n = 38), we demonstrate that some aspects of cranial integration are conserved across these taxonomic groups, despite their major differences in cranial form, function, and development. All three groups are highly modular and consistently exhibit high integration within the occipital region. However, there are also substantial divergences in correlation patterns. Birds uniquely exhibit high correlation between the pterygoid and quadrate, components of the cranial kinesis apparatus, whereas the non-avian dinosaur quadrate is more closely associated with the jugal and quadratojugal. Mesoeucrocodylians exhibit a slightly more integrated facial skeleton overall than the other grades. Overall, patterns of trait integration are shown to be stable among archosaurs, which is surprising given the cranial diversity exhibited by the clade. At the same time, evolutionary innovations such as cranial kinesis that reorganize the structure and function of complex traits can result in modifications of trait correlations and modularity.
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This content will become publicly available on October 25, 2026
New craniodental materials of Falcarius utahensis (Theropoda: Therizinosauria) reveal patterns of intraspecific variation and cranial evolution in early coelurosaurians
Abstract Despite documented ecomorphological shifts toward an herbivorous diet in several coelurosaurian lineages, the evolutionary tempo and mode of these changes remain poorly understood, hampered by sparse cranial materials for early representatives of major clades. This is particularly true for Therizinosauria, with representative crania best known for the late‐divergingErlikosaurus andrewsiand the early taxonJianchangosaurus yixianensis. Here we describe a series of new cranial bones ofFalcarius utahensis, the geologically oldest therizinosaurian from the Early Cretaceous Cedar Mountain Formation, Utah, United States. This new material provides the most complete understanding of the skull to date forFalcariusand frames the pattern and timing of cranial evolution in therizinosaurians and early coelurosaurians. Previously unknown elements include a well‐preserved maxilla, jugal, parietals, squamosal, laterosphenoids, and pterygoid. Computed tomography data differentiate the incisiform rostral dentary dentition from possible premaxillary teeth, the first in a therizinosaurian. Notable features include a primitive morphology of the jugal and frontoparietal complex shared with other early diverging taxa (e.g., tyrannosauroids,Incisivosaurus,Ornitholestes, Fukuivenator), and a large maxillary fenestra, convergent with troodontids. Additional specimens of previously known elements confirm their taxonomic utility and provide insight into intraspecific variation. Following patterns of other archosaurs, variable traits relate to the prominence of ridges and contours (likely associated with musculature) and the proportions of pneumatic features, whereas invariant traits correspond to the topology of bony contacts and major cranial nerves. Early, integrated evolution of the rostrum and adductor complex characterized early therizinosaurians, which was further modified alongside reduced paranasal complexity in later therizinosaurids.
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- Award ID(s):
- 1925973
- PAR ID:
- 10649993
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- The Anatomical Record
- ISSN:
- 1932-8486
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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