A juvenile bird with possible crown-group affinities from a dinosaur-rich Cretaceous ecosystem in North America (1 Viewer)

[Fred Ruhe]

Chase Doran Brownstein, 2024

A juvenile bird with possible crown-group affinities from a dinosaur-rich Cretaceous ecosystem in North America

BMC Ecology and Evolution 24, Article number: 20 (2024)

Abstract and free pdf: A juvenile bird with possible crown-group affinities from a dinosaur-rich Cretaceous ecosystem in North America - BMC Ecology and Evolution

Background
Living birds comprise the most speciose and anatomically diverse clade of flying vertebrates, but their poor early fossil record and the lack of resolution around the relationships of the major clades have greatly obscured extant avian origins.

Results
Here, I describe a Late Cretaceous bird from North America based on a fragmentary skeleton that includes cranial material and portions of the forelimb, hindlimb, and foot and is identified as a juvenile based on bone surface texture. Several features unite this specimen with crown Aves, but its juvenile status precludes the recognition of a distinct taxon. The North American provenance of the specimen supports a cosmopolitan distribution of early crown birds, clashes with the hypothesized southern hemisphere origins of living birds, and demonstrates that crown birds and their closest relatives coexisted with non-avian dinosaurs that independently converged on avian skeletal anatomy, such as the alvarezsaurids and dromaeosaurids.

Conclusions
By revealing the ecological and biogeographic context of Cretaceous birds within or near the crown clade, the Lance Formation specimen provides new insights into the contingent nature of crown avian survival through the Cretaceous-Paleogene mass extinction and the subsequent origins of living bird diversity.

Enjoy,

Fred

 

[Fred Ruhe]

Systematic paleontology

Avialae Gauthier 1986.
Neornithes Gadow 1892.
?Neognathae Pycraft 1900.
?Galloanserae Sclater 1880.
?Galloanserae indet.

Description
Yale Peabody Museum Vertebrate Paleontology Collections (YPM VP) 59473, a 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 partial pes. The specimen (Figs. 1, 2, 3, 4 and 5) is
preserved in course-grained sediment that also included a small non-avian theropod tooth (identifiable based on the presence of serrations, and plausibly assignable to Tyrannosauroidea or Dromaeosauridae; Supplementary Text III). The bones are all closely associated or articulated (radius and ulna) in three small (< 5 cm) long blocks of conglomerate. Because no bones overlap and all show the same ontogenetic indicators (striated bone texture), they most likely come from a single individual. All bones except for the humerus, tibiotarsus, and a large partial pedal phalanx are embedded in matrix. Matrix portions also still cling to spots on these prepped-out bones. The quadrate was embedded in what appears to be an iron or bone-flake rich region of sediment in the block, and so its external surfaces appear less well preserved on the CT scan render than other bones.

Locality and horizon
Niobrara County, Wyoming, United States, North America. The fossil was collected from the Lance Formation by the J.B. Hatcher expedition, 1890–1904. The Lance Formation is a Maastrichtian-age unit that crops out across the basin and range region of the western U.S. Previously described avian fossils from the YPM collections recovered from the Hatcher expedition to Niobrara County are assumed to come from the upper portion of the Lance Formation, making them up to 0.65 million years older than the Cretaceous-Paleogene boundary ( [19]). Consequently, the age of YPM VP 59473 can be constrained to between 66.02 and 67.0 million years ago.

Referral
Crown avian and galloanserine affinities for YPM VP 59473 are supported by: clear separation of the otic and squamosal capitula on the quadrate (Fig. 2d); presence of a subcapitular tuberculum below the squamosal capitulum on the quadrate (Fig. 2a); expansion of the ventral condyles and pterygoid condyle on the quadrate (Fig. 2a-c, e); humeral head dorsally offset from the rest of the proximal margin of the humerus (Fig. 3a, b); tricipital fossa is deeply excavated (Fig. 3a, b); dorsal tubercle of the humerus is large and offset from the rest of the proximal margin (Fig. 3a,

Ontogenetic assessment
Following the comprehensive review of osteological indicators of ontogenetic status presented in [20], I conducted a survey of YPM VP 59473 which revealed its juvenile status (Fig. 5). Features unambiguously indicating ontogenetic immaturity in birds present in this specimen include the presence of unfinished, heavily striated bone surfaces [20–22]. Histological sections were not made due to the fragmentary nature of the specimen. Among the characters used to assign YPM VP 59473 to crown Aves and to Galloanserae, several deserve comment because they appear to change during ontogeny in stem and crown birds: the shape of the humeral head, development of the dorsal tubercle, and the depth of the tricipital fossa. In both juvenile and adult stem birds (for example, †Archaeorhynchus and †Ichthyornis), these features are weakly developed [23–25]. In crown birds, the dorsal tubercle and the humeral head become progressively more prominent with ontogeny (for example, Phalacrocorax capillatus; Fig. 3A in [22]. Thus, even accounting for the juvenile status of the specimen, the morphology of YPM VP 59643 is inconsistent with the hypothesis that it is a juvenile stem bird.

Fred


Fig. 1 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