Wednesday, 29 July 2015

The Diet of Gastornis - Popular TV vs. Peer-Reviewed Literature

The Diet of Gastornis

Popular TV vs. Peer-Reviewed Literature

My introduction to the large flightless bird known as Gastornis occurs in the 2001 BBC documentary Walking with Beasts, a 6-part miniseries that looks at "snapshots" of life, and especially mammal life, during the Cenozoic era (the geological era that begins following the extinction of the dinosaurs). The first episode takes place in the warmer years of the early Eocene epoch, some 49 million years ago.

Walking with Beasts characterised Gastornis as a predator, not unlike the "terror birds" that would later dominate South America. Walking with Beasts represented this belief by featuring a Gastornis chasing several mammals belonging to a progenitor species of modern horses. Video of one of the chase sequences (this one ending in success for Gastornis) below the fold:

Gastornis hunts down a Propalaeotherium. (Video credit BBC)

However, nearly fifteen years later, it appears that this view is no longer correct. From Wikipedia, to National Geographic, to other sources besides, it appears to me that the evidence is now suggesting Gastornis was in fact an herbivore.

The above sources mention, in whole or in part, four key features supporting Gastornis' herbivory:
  1. Gastornis is missing a common beak feature of flesh-eating birds, the hooked tip. As described at Prehistoric Wildlife:
    the beak of Gastornis is also notable for not having a hooked tip,‭ ‬a feature that is common in meat eating birds as it greatly helps to hook into and tear off strips of flesh in the absence of teeth.
  2. Gastornis is also missing talons in its feet, another common feature of carnivorous birds. Prehistoric Wildlife reports:
    The feet of Gastornis are also noted as not having‭ ‬curved talons which could hook into and tear into bodies,‭ ‬another feature commonly seen‭ ‬in meat eating birds,‭ ‬but again absent in Gastornis.
  3. Gastornis' reconstructed beak musculature is more consistent with eating hard seeds and the like. From National Geographic's blog:
    Through dissections of modern birds ranging from Darwin’s finches to Eurasian sparrowhawks, Angst and colleagues studied the anatomy and connection points of the external adductor muscle in modern herbivorous and carnivorous birds. This is a major muscle that powers bird bites, and the herbivorous, seed-cracking birds typically had wider muscles with increased space for attachment on the lower jaw. That fits they way they feed. Much more power is needed to bust open hard fruits than to tear soft flesh.

    The actual muscles of Gastornis rotted away over 40 million years ago, but the bird’s lower jaw shows a wide space for the external adductor muscle to attach.
  4. Finally, chemical analysis of Gastornis fossils found that chemical signatures of calcium and CO2 support an herbivorous diet far more than a carnivorous one. From Wikipedia:
    Studies of the calcium isotopes in the bones of specimens of Gastornis by Thomas Tutken and colleagues showed no evidence that it had meat in its diet.
    And from National Geographic:
    Part of what makes carbon isotopes useful in paleontology is that they can be tied back to different types of plants that photosynthesize in different ways. This detail is what led Angst and coauthors to throw out the idea that Gastornis sliced small mammals. If the big bird was a carnivore, the researchers found, then the carbon isotope signatures inside it’s bones would indicate that it ripped open prey that, in turn, relied upon C4 plants – grasses and other plants that rely on a distinct form of carbon fixation. The snag is that plants didn’t evolve that C4 method of photosynthesis until about 14 million years after Gastornis lived. The chemical trace didn’t match up with the ecology of the time.

    When Angst and coauthors looked at the carbon isotope through the lens of Gastornis being an herbivore, however, the signature was a better match and was comparable to those of herbivorous mammals living at the same time. The bird’s carbon isotope profile was that of an avian that crushed seeds and crunched thick-skinned fruit.

(Italics in all quotes above are original.)

A key paper discussing the matter, referred to prominently by all three sources above, is Angst et al (2014). The "preview" of the paper at the publisher's site (which shows the first few pages) shows that in the scientific literature there was a robust debate about Gastornis' diet for many years, certainly many years prior to the production of Walking with Beasts.

All this goes to show that it's important to remember that production teams of documentary television aren't necessarily making scientific accuracy their top priority. A scientifically-accurate Walking with Beasts would have noted that it wasn't clear whether Gastornis was an apex predator or not, based on the literature of the time. (A reboot of the series today would pick some other predator to chase miniature horse-progenitors around, and show off Gastornis the gentle giant.)

Of course, while recent findings point very clearly towards herbivory, that's not to say that Gastornis didn't occasionally eat meat. Carnivorous and herbivorous animals alike display opportunistic omnivory from time to time. As the National Geographic blog explains, "Gastornis may very well have snagged the occasional unwary mammal or tried carrion."

If you want to keep open the idea of Gastornis being a scary creature, well, lots of herbivores are ill-tempered and violent. As one of the larger animals of the early Eocene, it wouldn't surprise me if Gastornis relied on its size and aggressiveness to keep predators at bay. So you still wouldn't want to run into one of the things, and if you did, it would probably still have you on the run.

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