Powered flight, a pivotal adaptation in vertebrate evolution, evolved independently three times. Pterosaurs evolved powered flight first in the late Triassic (~220 million years ago) and achieved the largest body size of any powered fliers. Powered flight evolved in birds by the Late Jurassic (~160 million years ago) and in bats by the Eocene (~52 million years ago). Few quantitative comparisons among these three clades exist for key structural attributes or functional parameters. Surveys of vertebrate flight tend to present analyses of the three clades into separate sections, and only comparing basic traits like aspect ratio, body mass, and wing loading. When they do discuss vertebrate fliers as a whole, the discussion turns to a theoretical volant animal instead of drawing comparisons between clades. Complicating such comparisons is their divergent wing structure. My research focuses on the wing skeleton—its overall shape including integumental structures and the shape of the bony anchor (sternum) for the main muscles that power the wing. I aim to bring to light additional fundamental properties of vertebrate wings based in biological data, particularly those relating to possible constraints on bat maximum body size.
For my doctoral thesis, I am beginning with birds:
Does waterbird wing shape correlate to its ecology and how do phylogenetic considerations influence this relationship?
Do avian sterna converge on a particular shape for a particular foraging strategy?
How do skeletal air-sacs affect avian wing structure?
Chapter | 01
Waterbird wing ecomorphology
co-authors: Drs. Paul Sereno and Mark Westneat
Left: A rail with broad, rounded wings. Right: An albatross with narrow, pointed wings. Illustrations: Stephanie Baumgart.
Wing shape plays a critical role in flight function in birds and other powered fliers and is correlated with flight performance, migratory distance, and the biomechanics of generating lift during flight. Avian wing shape and flight mechanics have also been shown to be associated with general foraging behavior and habitat choice. To further explore the relationship between avian wing shape, biomechanics and ecology, we focused on 'waterbirds,' a diverse array of birds united by their coastal and aquatic habitats. We aim to determine if wing shape in this functionally and ecologically diverse assemblage of birds is correlated with various functional and ecological traits.
Article available here!
Sterna from bird specimens at the Field Museum of Natural History. Photographs: Stephanie Baumgart.
Chapter | 02
Avian sternum ecomorphology
co-author: Dr. Leon Claessens
The avian sternum anchors the main muscles powering flight and is highly disparate in morphology. For instance, some birds feature long, narrow sternal plates with deep keels, others have almost square sternal plates and shallow keels, and some have very reduced or non-existent keels. Little work has focused on the relationship between the complex sternum shape and a bird’s ecomorphology. Here, we aim to examine relationships between sternal form and function using three-dimensional (3D) geometric morphometrics on a sample of sterna across Aves.
CT scan of a vulture humerus, showing the spongy bone inside (purple). Rendering: Stephanie Baumgart.
Chapter | 03
Disparity in skeletal pneumaticity
Most birds have some bones filled with air instead of marrow. These air-filled (pneumatic) bones help redistribute the bird's body mass and lighten the bones, making it easier and more efficient to fly. It is known from the literature that gliding and soaring birds tend to have the majority of their bones filled with air, while more agile flapping birds tend to have some vertebrae and their humerus filled with air. Diving birds reduce the air in their bones, preferring marrow or thicker-walled bones to decrease buoyancy. I aim to use CT scans of a sampling of birds across a variety of body masses and behaviors to begin quantifying skeletal pneumaticity in wing bones of birds. Ultimately, this data would be compared back to wing bones of pterosaurs, because they too have air-filled bones but grew to a much larger size than birds. More extreme skeletal pneumaticity might have enabled these animals to grow to much larger body sizes.
Fun Side Project
Ontogeny of the anuran urostyle
Co-authors: Gayani Senevirathne (lead author), Nathaniel Shubin, James Hanken, Neil Shubin
Left: cleared and stained adult Xenopus, bone is pink, cartilage is blue; IL = ilium, UR = urostyle, HL = hindlimb. Right: CT rendering of the pelvic region of adult Xenopus; CX = coccyx, RV = renal vein, DA = dorsa aorta, IA = iliac artery, HY = hypochord. Images from Figures 1A and 5C in Senevirathne et al. 2020.
Anurans (frogs and toads) have a distinctive skeleton - adults fuse their tail bones to make what is called a urostyle, part of a unique hip structure which enhances an anuran's ability to jump. The urostyle is composed of two parts, fused tail bones and a hypochord, a structure unique to anurans. As part of her Ph.D. thesis, Gayani Senevirathne set out to study the developmental mechanisms behind the formation of the anuran urostyle, and I was recruited to help CT scan a series of tadpoles at different growth stages in metamorphosis and visualize the change in blood vessel structure in the pelvic region as the tadpole turns into a frog.
Check out the paper here!
Senevirathne, G., Baumgart, S., Shubin, N., Hanken, J., & Shubin, N. H. (2020). Ontogeny of the anuran urostyle and the developmental context of evolutionary novelty. Proceedings of the National Academy of Sciences, 117(6), 3034-3044.
Part of a scan of a common basilisk (Basiliscus basiliscus, FMNH 68188). Lizards often release part of their tail when fleeing predators, but the tail grows back later. This picture shows the regrown tail in red. Notice it's one long cylindrical structure compared to the many vertebral units in the original tail. (Limbs digitally removed for visualization.) Link to scan here.
Fun Side Project
openVertebrate (oVert) Thematic Collections Network
For the past few years, I have been working with the oVert team (lead PI: Dr. David Blackburn) to CT scan as many vertebrate genera as possible for free access through MorphoSource.org. Many museums and universities across the country are engaged with this NSF-funded project, and so far, we have CT scanned over 4300 vertebrate genera.