The Early Evolution of Penguin Body Size and Flipper Anatomy: Insights from the Discovery of the Largest-known Fossil Penguin

Largest-known fossil penguin provides insight into the early evolution of sphenisciform body size and flipper anatomy

Daniel T. Ksepka, Daniel J. Field, Tracy A. Heath, Walker Pett, Daniel B. Thomas, Simone Giovanardi, and Alan J.D. Tennyson

Summarized by Faris Al-Shamsi, a geology student at the University of South Florida, currently in his senior year of undergraduate studies. His passion for geology fuels his commitment to sharing scientific knowledge with others. Faris is currently working on a project to simplify a challenging scientific article for general audiences, reflecting his dedication to communicating complex ideas to diverse readerships. After graduation, he plans to pursue a career in geology and continue to promote scientific literacy among the public.

Hypothesis: The study investigates new fossils, including the recently discovered largest-ever penguin, named Kumimanu fordycei, found in New Zealand. Scientists used these fossils to clarify the evolutionary relationships between this new species and other know penguin species in the evolutionary history record in order to gain a better understanding of their evolutionary development. 

Data used: Researchers discovered penguin fossils in rocks from the late Paleocene Epoch (55.5-59.5 million years ago). They found various bones, including the humerus (upper arm bone) and wing bones. Researchers also used data sets of previously described fossil penguin species, created by scientists Bertelli and Giannini, which included 279 morphological characteristics to compare different species of penguins.

Methods: First, they created 3D digital replicas of the recently discovered bones using a handheld laser scanner and processing software, then finalized the 3D replicas using a software called Blender. Second, they conducted phylogenetic analyses by analyzing morphological characters between different samples of penguin bone to understand different species of penguins’ relationships and evolution over time. Two types of analysis were used: parsimony analysis, which seeks to find the simplest explanation of an evolutionary tree with the fewest evolutionary changes, and Bayesian analysis, which uses statistical methods to estimate an evolutionary tree. They used a large set of data created by scientists called Bertelli and Giannini with 279 characteristics to compare different types of penguins, and the scientists added data from the new fossils they discovered. The scientists modified existing characteristics which means they compared the physical traits of the new species with old species to determine how similar or different they are. Researchers used the length and proximal width (closest to the shoulder) of the humerus to estimate body mass of different penguin species using mathematical equations. 

Results: The study looked at the wing bones of ancient penguins and compared them to those of modern underwater diving birds like diving petrels and alcids (like puffins) through evolutionary tree analyses. Using the mathematical equations for determining size, researchers determined the new penguin fossils likely belonged to a giant, extinct penguin. The researchers estimated the body mass of the new species, Kumimanu fordycei, to be 148.0 kg (326 lbs) based on the length of its humerus, which measured 236 mm. Researchers found that some features of the wing bones in the ancient penguins are similar to those in fossil flying birds, which suggests that these early giant penguins may have kept some features that were once necessary for flying but would have been less efficient for swimming. Because modern day penguins are far smaller than these fossils, it indicates that smaller body sizes were likely selected for along the evolutionary pathway of these birds. 

Chart with the geologic timeline at the bottom ranging from Cretaceous (73 million years ago) to Pleistocene Epoch (nearly recent) and two penguin family trees on the left side: one on the top constructed using parsimonious comparison of physical traits representing the penguin Kumimanu fordycei as closely related to many other species, while the tree on the bottom constructed using complex statistical analysis and it represents the penguin Kumimanu fordycei sharing the same node (branch) with Kumimanu biceae species. The chart represents a drawing of three penguins on the right, starting from the left, penguin 3, which represent penguin Kumimanu fordycei is the largest one, second we have Petradyptes stonehousei, which is the second largest, and last the extant penguin Aptenodytes forsteri which is the smallest. They have the actual bones inside them that were found in fossils colored in white, while non-preserved bones are colored in gray. Crownward (advanced and closer to the tips of the evolutionary tree) penguins exist more than other species with a lifetime ranging from Paleocene Epoch (65 million years ago) to recent while other species lifetime range from Paleocene Epoch to Eocene Epoch (65 million years ago – 50 million years ago)
Figure 1 This figure shows the family tree of penguins with x-axis representing time in million years and the epochs. Family trees started from the Paleocene Epoch, which was about 60 million years ago. The figure is made up of two different kinds of trees. The first one is based on parsimony analysis which is the simplest explanation of a tree taking the fewest changes of the evolutionary changes, and the second one is based on Bayesian analysis which uses statistical methods. The figure also includes pictures of three different penguin species: 3. The largest known penguin Kumimanu fordycei 4. The second new species and genus Petradyptes stonehousei 5.the living penguin Aptenodytes forsteri. The white bones shown on the pictures of the ancient penguins are the actual bones that were preserved, while the gray bones used to complete the skeleton of the penguin even though they are not preserved just in order to show the difference in size between penguins 3,4, and 5 in physical traits.

Why is this study important? The discovery of the largest penguin humerus ever found, and the estimation of body mass based on this bone provides valuable insights into the evolution and growth patterns of penguins. Additionally, the study provides evidence that penguins reached their upper limit of body size early in their evolutionary history and experienced a decrease in size over time, which can provide insights into the impact of environmental factors such as climate change and competition for food resources on the evolution of organisms.

Broader Implications beyond this study: Body size in the fossil record can open a number of questions about how an animal lived. Researchers think penguins lost their flying capabilities before these larger penguins described here  evolved. This might provide a potential reason for the increase on body size: without the ability to fly, penguins faced fewer selection pressures to keep a smaller body size. Researchers also think penguins evolved in Zealandia, where the fossils from this study were located. The large body size of these particular penguins may have given them better control over their body temperatures (called thermoregulation) and allowed for them to disperse to other areas of the world by being able to swim greater distances. This gives researchers new hypotheses to test about how penguins reached and established populations in other continents. 

Citation: Ksepka, D., Field, D., Heath, T., Pett, W., Thomas, D., Giovanardi, S., & Tennyson,(2023). Largest-known fossil penguin provides insight into the early evolution of sphenisciform body size and flipper anatomy. Journal of Paleontology, 1–20. doi:10.1017/jpa.2022.88