Saturday 17 February 2024

A possible crown-group Bird from the Late Cretaceous Lance Formation of Wyoming.

The origins and evolution of Mesozoic Birds are now well understood, but the emergence and development of the crown group Birds (a crown group contains all living members of a group, their most recent common ancestor, and everything descended from that ancestor) remains largely clouded in mystery. Most living Bird groups have a very poor fossil record, if they have a fossil record at all, despite Birds being the most diverse group of flying Vertebrates alive today, with more than 10 000 species. Fossils, where known, tend to be extremely fragmentary in nature, with most phylogenies of the group based entirely upon genetic data. This is particularly frustrating as the living Birds are the only group of Dinosaurs to have survived the End Cretaceous Extinction, something which has been taken to imply they had some quality missing in all other Avian and non-Avian Dinosaur groups. However, while crown group Birds are known to arisen before the End of the Cretaceous, they appear to have been at best a minor component of the Cretaceous Fauna, with few-or-no specimens found even in deposits which have produced numerous fossils of extinct Mesozoic Bird groups.

In a paper published in the journal BMC Ecology and Evolution on 9 February 2024, Chase Doran Brownstein of the Department of Ecology and Evolutionary Biology at Yale University and the Stamford Museum and Nature Center, describes a possible crown group Bird from the End Cretaceous Lance Formation of Wyoming.

The specimen, YPM VP 59473, comprises partial skeleton consisting of the complete left quadrate, portions of the skull roof a partially articulated, though very poorly preserved, cervical series, a fragment of the synsacrum, the left humerus, the articulated left radius and ulna, partial left tibiotarsus, and a partial pes. The material is largely disarticulated, but all of the bones are from a young juvenile and no duplicate bones are present, supporting the idea that they came from a single Animal.

Preservation of YPM VP 59473. The blocks containing all bones of the holotype (except for the humerus, tibiotarsus, synsacrum fragment, and large distal pedal phalanx) are shown under light microscopy (a), (e), (h) and with multiple x-ray views of the largest (b), (c), (d), second largest (f), (g), and smallest (i) blocks as rendered in VGStudio, showing the relative placement of bones in the matrix blocks. Brownstein (2024).

Despite the extremely fragmentary nature of the material, Brownstein feels confident in assigning the specimen to the Galloanserae, the group which includes the living Land and Water Fowl, and one of the three groups of Neornithine Birds thought to have diverged before the End of the Cretaceous, with the Palaeognaths and the Neoaves. This diagnosis is on the basis of the clear separation of the otic and squamosal capitula on the quadrate, the presence of a subcapitular tuberculum below the squamosal capitulum on the quadrate, the expansion of the ventral condyles and pterygoid condyle on the quadrate, the humeral head being dorsally offset from the rest of the proximal margin of the humerus, tricipital fossa being deeply excavated, and the dorsal tubercle of the humerus being large and offset from the rest of the proximal margin, all of which traits are typical of Galloanserine Birds, but absent in the various Mesozoic Avian stem groups.

Forelimb of YPM VP 59473. Humerus in (a) posterior, (b) anterior, (c) lateral, and (d) medial views. In (a) and (b), both CT scans and colour images are shown. Radius in (e) anterior, (f) posterior, (g) lateral, (h) medial, and (i) distal views. Ulna in (j) posterior, (k) anterior, (l) lateral, and (m) medial views. Brownstein (2024).

While the presence of a Galloanserine Bird in an End Cretaceous deposit is not unexpected, the presence of the specimen in the Lance Formation is significant in two ways. 

Firstly, because the deposit is from the Northern Hemisphere; phylogenetic studies of Birds based upon genetic data have found that the earliest diverging members of many groups have Southern Hemisphere distributions, which has led to speculation that the Neornithine Birds might have had a Southern Hemisphere origin, and the establishment of YPM VP 59473 adds to a growing body of data which contradicts that, suggesting that Neornithine Birds already had a global distribution in the Late Mesozoic. 

Secondly, unlike other deposits which have yielded Mesozoic Neornithine Birds, the fossils of the Lance Formation are thought to have been buried in situ, rather than being an accumulation deposit. This is important because the deposit has also produced toothed stem-Birds from at least four major clades, as well as Eudromaeosaurian, Alvarezsaurid, Troodontid, and potentially ‘four-winged’ Microraptorine Dinosaurs, all of which are thought to have been ecologically close to Birds. This is significant, as it suggests that the Neornithine Birds were not occupying some ecological niche which protected them from the impacts of the End Cretaceous Extinction, but instead were part of a community of ecologically similar Animals living in similar environments. This undermines the idea that Neornithine Birds were able to survive the End Cretaceous Extinction because they were in some way special, supporting the alternative hypothesis that they survived due to simple luck an important but sometimes overlooked factor in evolutionary biology.

The ecological and temporal origins of living Birds. Left side of the diagram shows the temporal and spatial range extensions and records of key small-bodied non-Avian Theropod clades found in the Lance Formation assemblage, and cladogram at right shows the major clades of stem and crown Birds that survive to or past the End  Cretaceous extinction, with ecologically relevant features that have been considered important to differential Avian survival through that event noted along branches. All clades shown on tree are unambiguously represented in the Lance Formation assemblage, except Neoaves and Paleognathae. Bird illustrations by John Gould. Brownstein (2024).

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