Hesperornis isn’t a celebrity fossil, but it used to be. Soon after paleontologist O.C. Marsh named the toothed bird in 1872, the Cretaceous avian had the dual distinction of being a wonderful example of evolutionary change – that birds truly had their origin among ancient reptiles – and a target for conservatives who saw the description of such fossils as a waste of government funding. (“Birds with teeth” Capitol Hill politicians incredulously grumped.) But even though Hesperornis has lost its status among the public, paleontologists have not forgotten this diving bird. Researchers are still striving to understand how Hesperornis made its living along a seaway that once washed over the middle of North America. Among the remaining questions – did these toothy birds stay put their entire lives, or did the migrate to follow seasonal warmth?

Bones of Hesperornis have been found in rocks between 83 and 72 million years old from Arkansas to the Arctic. That’s quite a range, and the distribution of their fossils roughly map out the extent of the Western Interior Seaway – a stretch of warm, shallow ocean that once connected the Arctic Ocean with the Gulf of Mexico. Conditions varied significantly along this marine corridor. While sea temperatures in the prehistoric Gulf of Mexico were a comfy 75ºF or so, those in the Late Cretaceous Arctic ranged from about 28 to 46ºF. The presence of Hesperornis bones so far north means they tolerated cool temperatures as well as warm ones, but did it’s been unclear whether they did so year round.

View Images Where the samples of Hesperornis in the study were found, shown in relation to Late Cretaceous North America. From Wilson and Chin, 2014.

To investigate these competing ideas, paleontologists Laura Wilson of the Sternberg Museum of Natural History and Karen Chin of the University of Colorado looked inside the bones of Hesperornis from Kansas and the Arctic. That’s because bones can act as records of major events in a vertebrate’s life. During a particularly difficult time of an animal’s life when resources are scarce – such as an Arctic winter or a stressful migration – an animal’s bone growth may slow or stop in response, often leaving a marker known as a line of arrested growth (or LAG).

Wilson and Chin didn’t just look at fossil bones. While Hesperornis has most often been compared to loons based on appearance alone, the paleontologists decided to look at the internal bone structure of some modern day penguins to see what patterns overwintering and migration might leave in bone. In particular, they studied sections of bone from gentoo penguins – which stay put through the winter – as well as Adélie and chinstrap penguins, which migrate. If the habits of these penguins were recorded in their bones, then they could be used as proxies for prehistory.

View Images The bone structure of a Hesperornis tibiotarsus from Kansas, showing the major regions of bone and the transitions from fast, messy bone growth to slower, more organized growth towards the outside. From Wilson and Chin, 2014.

Unlike the carefully-prepared bone sections themselves, though, conclusions from fossil bones are not always clear cut. Wilson and Chin did not find any LAGS or other signs of “hurry up and wait” growth in the bones of Hesperornis. The bird bones showed a pattern of rapid early growth that only slowed towards adulthood, culminating in an outer bone layer called an OCL in the Kansas Hesperornis samples that marks adulthood. (The Arctic sample lacked an OCL but the details of its bone show that it was very close to completing its skeletal growth when it perished.) The modern penguin bones didn’t show any signs of withstanding harsh winters or undertaking long journeys, either. How birds grow explains this confounding twist.

Baby Adélie, chinstrap, and gentoo penguins grow fast. They go from hatchlings to adults within a year. That’s why there aren’t any lines or signs of stopping in their bones, Wilson and Chin point out. It’s too rapid for any environmental triggers to leave such marks. And it looks like the same was true of Hesperornis. Whether born in prehistoric Kansas or the Arctic, Hesperornis chicks achieved their full skeletal development within one year. That means that they were done growing by time the first winter set in. Regardless of whether the Arctic individuals stayed put or started swimming back down in the latitudes, their bones could no longer record the stresses of migration or withstanding the winter.

But there was one curious clue. In the gentoo penguin – the species that doesn’t migrate to escape winter – Wilson and Chin found evidence of even quicker growth rates than the other penguins. This may be because the only way for these penguins to survive is to become adults before the too-short summer is over and winter sets in again. There’s more pressure on them to get to adult size than in migratory species, and the signs of this extra-rapid growth differ from both the other penguins in the study and Hesperornis. Wilson and Chin note that more work needs to be done on living penguins, but perhaps there are still subtle details that will allow paleontologists to piece together the life history of the toothed birds that once swam through the sea that split America.

[For another take on this paper, read Andy Farke’s post here.]