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Unusual pectoral apparatus in a predatory dinosaur resolves avian wishbone homology.

ABSTRACT: The furcula is a distinctive element of the pectoral skeleton in birds, which strengthens the shoulder region to withstand the rigor of flight. Although its origin among theropod dinosaurs is now well-supported, the homology of the furcula relative to the elements of the tetrapod pectoral girdle (i.e., interclavicle vs clavicles) remains controversial. Here, we report the identification of the furcula in the birdlike theropod Halszkaraptor escuilliei. The bone is unique among furculae in non-avian dinosaurs in bearing a visceral articular facet in the hypocleideal end firmly joined to and overlapped by the sternal plates, a topographical pattern that supports the primary homology of the furcula with the interclavicle. The transformation of the interclavicle into the furcula in early theropods is correlated to the loss of the clavicles, and reinforced the interconnection between the contralateral scapulocoracoids, while relaxing the bridge between the scapulocoracoids with the sternum. The function of the forelimbs in theropod ancestors shifted from being a component of the locomotory quadrupedal module to an independent module specialized to grasping. The later evolution of novel locomotory modules among maniraptoran theropods, involving the forelimbs, drove the re-acquisition of a tighter connection between the scapulocoracoids and the interclavicle with the sternal complex.

SUBMITTER: Cau A

PROVIDER: S-EPMC8289867 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The availability of multiple avian genome sequence assemblies greatly improves our ability to define overall genome organization and reconstruct evolutionary changes. In birds, this has previously been impeded by a near intractable karyotype and relied almost exclusively on comparative molecular cytogenetics of only the largest chromosomes. Here, novel whole genome sequence information from 21 avian genome sequences (most newly assembled) made available on an interactive browser (Evolution Highway) was analyzed.Focusing on the six best-assembled genomes allowed us to assemble a putative karyotype of the dinosaur ancestor for each chromosome. Reconstructing evolutionary events that led to each species' genome organization, we determined that the fastest rate of change occurred in the zebra finch and budgerigar, consistent with rapid speciation events in the Passeriformes and Psittaciformes. Intra- and interchromosomal changes were explained most parsimoniously by a series of inversions and translocations respectively, with breakpoint reuse being commonplace. Analyzing chicken and zebra finch, we found little evidence to support the hypothesis of an association of evolutionary breakpoint regions with recombination hotspots but some evidence to support the hypothesis that microchromosomes largely represent conserved blocks of synteny in the majority of the 21 species analyzed. All but one species showed the expected number of microchromosomal rearrangements predicted by the haploid chromosome count. Ostrich, however, appeared to retain an overall karyotype structure of 2n=80 despite undergoing a large number (26) of hitherto un-described interchromosomal changes.Results suggest that mechanisms exist to preserve a static overall avian karyotype/genomic structure, including the microchromosomes, with widespread interchromosomal change occurring rarely (e.g., in ostrich and budgerigar lineages). Of the species analyzed, the chicken lineage appeared to have undergone the fewest changes compared to the dinosaur ancestor.

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Unusual pectoral apparatus in a predatory dinosaur resolves avian wishbone homology.

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