The Echoes of Toothed Birds

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.

The family tree of hesperornithiform birds created by Bell and Chiappe, 2015.
The family tree of hesperornithiform birds created by Bell and Chiappe, 2015.

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.

Reference:

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

6 thoughts on “The Echoes of Toothed Birds

  1. I have similar feelings about grebes!

    I’d like to point out that two of the genera included in this paper have some of the laziest names ever imparted onto fossil animals:

    Asiahesperornis (“Hesperornis from Asia”); and
    Canadaga (“Canadian bird”)

    Makes Utahceratops seem like Agathaumus by comparison!

  2. I love it. But the burning question about hesperornis are: (a) did they walk on land? If so, how? and (b) did they lay eggs (probably, but how)?

  3. Llewelly–
    Walking bipedally like typical birds may have been beyond them (I take it the problem you see is how they would be able to balance given the extreme rearward position of the hind legs), but this might not have precluded nesting and egg-laying ashore. After all, seals can’t really walk, and even a small seal is larger in body mass (and so, a priori, would have more trouble moving around on shore) than a large hesperornithid.

    Eggs I don’t understand. Live-bearing seems to evolve over and over all over the lepidosaur family tree (lizards! snakes! plesiosaurs!) but as far as I know has never been observed in archosaurs or turtles. This is really weird. Given the dangers of the run from nest to beach for hatchling sea-turtles, you’d think that IF a mutation led to a sea turtle’s retention of eggs to maturity and delivery of swimming young at sea, there would be immense selective pressure for it. But in all the time since the Cretaceous that the critters have been swimming around and the females laboriously beaching themselves to lay eggs just above the tide line, it doesn’t seem to have happened.

  4. Not an entirely related question here, but does anyone have anything on the evolution of ‘bone spines’ in birds? Several species here seem to have weird sliver-like bone fragments along their legs and even spinal columns.

  5. On bipedality in hesperornithiformes:

    I imagine that grebes and loons provide the best analogue. Loons can’t actually walk on land–they shove themselves forward on their bellies with their hindlimbs, which, like grebes, point back rather than down. Grebes are not as constrained but avoid terrestrial locomotion whenever possible. Loons nest close to shore so they don’t have to walk very far, but most grebes actually nest on the water, building floating vegetation nests. The chicks of loons and grebes can swim after hatching.

    The trick is that loons (and most grebes) can fly between marine and freshwater waterways, and both nest in calm fresh waters. But hesperornithiformes were oceanic birds that couldn’t fly. I assume they found their way to coastal areas to nest, like most seabirds do.

    As for whether they laid eggs–I have to assume they did. No known bird (or, indeed, archosaur) is viviparous. Immediately, one wonders what metriorhynchid crocodilians were doing…

  6. Archosaurs and turtles depend on their eggshells to construct their skeletons. Of course they haven’t developed vivipary.

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