Why don't penguins fly if their ancestors did?

Today is World Penguin Day . These amazing birds have aroused the curiosity of naturalists since they were first observed by Europeans traveling on Vasco da Gama’s expedition in the late 16th century . Those sailors gave them the name of silly birds, because although the beak was a telltale feature of their status as a bird, their clumsy gait and their inability to fly made them question their capabilities.

Penguins are unique to the southern hemisphere and are the only birds that are adapted to wing-propelled diving—other birds dive by propelling themselves with their legs. They are obligate marine animals, that is, they need to spend part of their lives in the sea. They feed exclusively on marine animals, and use the water to move with greater speed and ease than on land, with adaptations similar to those of seals and sea lions.

A history of more than 60 million years

The fossil remains of penguins can be traced back to about 60 million years ago, during the Paleocene period. The distribution of the fossils suggests that, from the origin, they had a strong link with cold and temperate waters . The evolution of this group of birds runs parallel to the changing patterns of currents in the southern hemisphere.

Penguins, and in parallel cetaceans and pinnipeds, burst forth with an evolutionary radiation event, filling ecological niches vacated by the late Cretaceous mass extinction of earlier large marine reptiles and other seabirds.

The oldest known fossil species, considered a member of the sphenisciformes group — Sphenisciformes , a term used to designate penguins — is Muriwaimanu tuatahi . Its wings already show some early adaptations for powered diving, although it retains more primitive features that it shares with flying birds. Their wing feathers were probably still distributed and specialized like those of their ancestors, capable of flight. In addition, it had longer and more developed hind legs than those of modern penguins, from which it is inferred that they used them, in addition to their wings, to propel themselves when diving.

From those first prehistoric penguins to the present ones, we find a wide variety of fossil remains that describe how natural selection favored the presence of certain traits.

The wings change their shape and the distribution of their feathers , from wings inherited from flying animals, and therefore, almost totally adapted to flight and with some capacity for diving, to wings totally adapted to diving. The hind legs gradually adopted a more posterior position , optimal for maintaining a streamlined shape.

This forces the animal to maintain an upright resting posture on land. To maintain such a precarious balance, the center of gravity must be low; consequently, the upper part of the hind limbs ends up hidden inside the body, with the legs folded, and walking in a crouch , giving the appearance of short and clumsy legs. However, when entering the water and extending the legs, with them the body is extended, which becomes spindle-shaped .

It is estimated that in the late Oligocene, between 23 and 27 million years ago, the general appearance of the penguins was practically definitive; this is shown by the fossils of the Kairuku genus, whose body plan is almost identical to that of modern penguins , and only changes in some specific aspects related to the shape of the tail or beak.

It was a long and complex evolutionary process, which resulted in extraordinary birds, fully adapted to aquatic life. But they were not the only ones in the history of evolution .

Cretaceous toothed seabirds

Although the penguin has the title of undisputed winner among birds in the race to conquer the sea, there are other groups with adaptations that bring them closer to the aquatic habitat.

Ducks, seagulls, cormorants, grebes or flamingos are some examples. However, before, long before these groups appeared, in the middle of the Cretaceous period, another group of birds occupied the same habitat that penguins occupy today, except that, unlike these, they were distributed throughout the northern hemisphere: the group Hesperornithes, birds with long necks, unable to fly and adapted to aquatic life.

They are the only known group of marine dinosaurs from the Mesozoic . On the one hand, all birds belong to the group of dinosaurs. And it is noteworthy that other famous marine animals of the time, such as Mosasaurus or Ichthyosaurus , were not, in fact, dinosaurs.

Like modern penguins, the legs were placed in a very backward position and, at rest, kept them flexed. To propel themselves with them, they placed them on the sides of the body, like oars, and secondarily they helped themselves with their wings. Their bones were solid, thanks to which they could avoid floating in the water.

They had a mouth full of small, sharp teeth with which they hunted fish. The surprising thing about this fact is that the closest evolutionary ancestors of the Hesperornithes were toothless, so that trait must have evolved secondarily.

In any case, all aspects of similarity between the Hesperornithes and any modern bird — cormorants, grebes, loons or penguins — are evolutionary traits acquired independently, that is, by evolutionary convergence , and do not correspond to any evolutionary relationship. . In fact, the Hesperornithes were a very specialized group, unrelated to modern birds.

REFERENCES:

 

Cracraft, J. 1982. Phylogenetic Relationships and Monophyly of Loons, Grebes, and Hesperornithiform Birds, with Comments on the Early History of Birds.

Systematic Zoology, 31(1), 35. DOI: 10.2307/2413412

Fordyce, R. E. et al. 2012. Penguin history and new fossil material from New Zealand. En L. S. Davis et al., Penguin Biology (pp. 419-443). Elsevier.

Ksepka, D. T. et al. 2012. New fossil penguins (Aves, Sphenisciformes) from the

Oligocene of New Zealand reveal the skeletal plan of stem penguins. Journalof Vertebrate Paleontology, 32(2), 235-254. DOI: 10.1080/02724634.2012.652051

Mayr, G. et al. 2020. First Complete Wing of a Stem Group Sphenisciform from the Paleocene of New Zealand Sheds Light on the Evolution of the Penguin Flipper. Diversity, 12(2), 46. DOI: 10.3390/d12020046