Whenever I spot a grebe, I try to imagine the bird with teeth. This is another symptom of a chronically fossiliferous mind. You see, these snake-necked swimming birds have traditionally been taken as a proxy for a specific sort of ancient avian called hesperornithiforms. Best known by, as you might guess, Hesperornis – these toothed birds pioneered the diving lifestyle between 113 and 66 million years ago. Today’s grebes and loons look like edentulous echos of these lost Cretaceous days, or, as beautifully expressed by ornithologist William Beebe:
When in the depth of the winter, a full hundred miles from the nearest land, one sees a loon in the path of the steamer, listens to its weird, maniacal laughter, and sees it slowly sink downward through the green waters, it truly seems a hint of the bird-life of long-past ages.
No surprise, then, that paleontologists past sometimes considered loons and grebes to be the descendants of Hesperornis. Their bones carry the same superficial similarity. But this is an evolutionary ruse known as convergence. Loons and grebes are copycats that independently took up the same lifestyle as the toothed birds that dove after fish in warm Cretaceous seas and sometimes wound up in the stomachs of monstrous marine reptiles.
It takes an encyclopedic knowledge of anatomy to spot the differences, but the bones don’t lie. For example, as pointed out by Natural History Museum of Los Angeles paleontologists Alyssa Bell and Luis Chiappe in their new paper on Hesperornis and kin, loons, grebes, and the toothed birds all evolved different skeletal scaffolds for increasing their swimming power. Grebes have an expansion of the tibia for strong propulsive muscles, while loons split the space between a flange on their tibia and a kneecap. In the extinct toothed birds, however, the muscles mainly appear to have attached to a large, robust kneecap. Different anatomical solutions to the same mechanical problem.
From differences such as these, paleontologists have been able to discern that the hesperornithiformes are perched just outside the “modern” bird group – Neornithes to specialists – on the avian family tree. So far, so good. But as Bell and Chiappe point out in their paper, very little work has been done on sorting out the relationships of the various diving birds that snatched fish with snaggly jaws among the Northern Hemishphere’s Cretaceous waterways. With that in mind, the researchers cataloged 207 skeletal characteristics from 272 hesperornithiform specimens representing 18 different taxa to see who was related to whom.
After the various lineages grew or were pruned, Bell and Chiappe found that all the hesperornithiform birds created one group, and under this canopy there were several major branches. This is what allowed Bell and Chiappe to see how these birds became better and better adapted to lives spent at sea. Over time, the researchers point out, the birds show increasing specializations for better diving – such as a close connection of bone in the lower leg that allowed Hesperornis and other skilled swimmers to hold their toes close together during the recovery phase of a swim stroke, reducing drag and allowing them to get their feet in position for the next sweep faster.
Not that the story is one of straight-line progress from little flappy birds to flightless divers. Looking across the lineages, Bell and Chiappe found that these toothed birds evolved large body size at least three different times – in species of Brodavis and Pasquiaornis, as well as Hesperornis and its closest relatives. This might be a sign that each of these lineages were independently becoming adapted to being better divers. Bigger body size in diving animals, Bell and Chiappe point out, is related to larger lung capacity and the ability to better oxygenate the blood while paddling down deep. That means that even these toothy birds were copying each other before loons and grebes could continue the trend. In evolution, as with fashion and film, what’s old can be made new again.
Bell, A., Chiappe, L. 2015. A species-level phylogeny of the Cretaceous Hesperornithiformes (Aves: Ornithuromorpha): implications for body size evolution amongst the earliest diving birds. Journal of Systematic Paleontology. doi: 10.1080/14772019.2015.1036141