| Tsidiiyazhi abini |
Ksepka, Stidham & Williamson, 2017
Molecular (DNA) studies suggest that birds radiated rapidly in the wake of the Cretaceous–Paleogene mass extinction (66 Ma), diversifying into nearly all the major groups we recognize today. However, fossil evidence for this pattern has been difficult to find because of the poor fossilization potential of small, delicate-boned birds. We report a tiny species of bird from ∼62.5 million-year-old rocks in New Mexico. Tsidiiyazhi abini (Navajo for “little morning bird”) is an ancient species of mousebird (Coliiformes). The fossil provides evidence that many groups of birds arose just a few million years after the mass extinction and had already begun evolving specializations of the foot for different ecological roles.
Evidence is accumulating for a rapid diversification of birds following the K–Pg extinction. Recent molecular divergence dating studies suggest that birds radiated explosively during the first few million years of the Paleocene; however, fossils from this interval remain poorly represented, hindering our understanding of morphological and ecological specialization in early neoavian birds. Here we report a small fossil bird from the Nacimiento Formation of New Mexico, constrained to 62.221–62.517 Ma. This partial skeleton represents the oldest arboreal crown group bird known. Phylogenetic analyses recovered Tsidiiyazhi abini gen. et sp. nov. as a member of the Sandcoleidae, an extinct basal clade of stem mousebirds (Coliiformes). The discovery of Tsidiiyazhi pushes the minimum divergence ages of as many as nine additional major neoavian lineages into the earliest Paleocene, compressing the duration of the proposed explosive post–K–Pg radiation of modern birds into a very narrow temporal window parallel to that suggested for placental mammals. Simultaneously, Tsidiiyazhi provides evidence for the rapid morphological (and likely ecological) diversification of crown birds. Features of the foot indicate semizygodactyly (the ability to facultatively reverse the fourth pedal digit), and the arcuate arrangement of the pedal trochleae bears a striking resemblance to the conformation in owls (Strigiformes). Inclusion of fossil taxa and branch length estimates impacts ancestral state reconstructions, revealing support for the independent evolution of semizygodactyly in Coliiformes, Leptosomiformes, and Strigiformes, none of which is closely related to extant clades exhibiting full zygodactyly.
Keywords: aves, phylogeny, morphology, fossil, evolution
|Fossil bones of Tsidiiyazhi abini, a 62.5 million-year-old fossil representing the oldest arboreal species of crown bird. |
photos: Kate Dzikiewicz
Aves Linnaeus, 1758
Coliiformes Murie, 1872 cf.
Sandcoleidae Houde and Olson, 1992
Tsidiiyazhi abini, gen. et sp. nov
Etymology. The genus and species names are derived from the Navajo (Diné Bizaad) language, reflecting the discovery of the fossil within ancestral Navajo lands. The genus name is derived from the words “tsidii” for “bird” and “yazhi” for “little,” in reference to the fossil’s small size. The specific epithet is derived from the Navajo word “abini” for “morning,” referencing the early Paleocene age of the taxon. Pronunciation using International Phonetic Alphabet phonetic symbols is as follows: /tsɪdi:jæʒi:/ /′ɔbɪnɪ/.
Type Locality and Horizon. NMMNH locality L-6898 comprises a relatively thin (<10 cm) muddy siltstone bed of restricted areal extent, representing an exposure of the Ojo Encino Member of the Nacimiento Formation. The site is located on the West Flank of Torreon Wash within the San Juan Basin, in Sandoval County, New Mexico. ....
Diagnosis. Tsidiiyazhi abini is differentiated from all other Coliiformes by the following apomorphies: (i) tubercle on the medial face of cranial end of the scapula, (ii) strongly developed triangular protuberance at the apex of impressio m. sternocoracoidei of the coracoid, (iii) medially displaced distal exit of canalis extensorius of the tibiotarsus, and (iv) arcuate arrangement of the metatarsal trochleae.
|Tsidiiyazhi abini life reconstruction by Sean Murtha.|
Daniel T. Ksepka, Thomas A. Stidham and Thomas E. Williamson. 2017. Early Paleocene Landbird Supports Rapid Phylogenetic and Morphological Diversification of Crown Birds after the K–Pg Mass Extinction. Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.1700188114
New species of ancient bird discovered in New Mexico phy.so/418976380 @physorg_com