Wednesday, February 7, 2018

Who Ate Those Seeds? Not Hongshanornis!

In 2011, a study found clusters of seeds preserved in the body cavity of several Cretaceous birds. This shed light not only on the dietary habits of these birds, but also on their digestive anatomy, as the clustering of the seeds suggested the presence of a crop, which is a pouch at the end of the esophagus that serves as a temporary food store in many modern birds. Most of the Cretaceous specimens were identified as Sapeornis, a bizarre basal avialan that I've previously written about. One of them, however, was assigned to Hongshanornis, a smaller avialan more closely related to modern birds.

Some things about this supposed Hongshanornis did not add up though. Hongshanornis had slender hindlimbs and long toes, which have been suggested as evidence that it foraged on the margins of water, similar to plovers and sandpipers today. Though it is perhaps not inconceivable that Hongshanornis ate seeds occasionally, its apparent wading habits are not generally characteristic of seed-eating birds.

A new study reevaluates the specimen and confirms that it's not a specimen of Hongshanornis at all. The authors assign it to a new species, Eogranivora edentulata, which translates appropriately to "toothless dawn seed-eater". Like Hongshanornis, Eogranivora was a euornithine, the broad group including modern birds and all other avialans more closely related to them than to the enantiornithines or "opposite birds". However, Eogranivora differed from Hongshanornis in several significant ways.

The holotype of Eogranivora, from Zheng et al. (2018).

To start off, Eogranivora didn't have any teeth, as suggested by its species name, whereas Hongshanornis, like most other non-neornithine theropods, did. One might wonder how the authors of the 2011 paper managed to mistake a toothless specimen for a toothed taxon in the first place. This is almost certainly because the teeth of Hongshanornis were minuscule, to the point where Hongshanornis was initially thought to have been truly toothless. Only detailed reexamination of its holotype revealed that Hongshanornis did have tooth sockets, and it took an entirely new specimen for preserved teeth to be found at all. Indeed, the 2011 paper outright follows the original claim that Hongshanornis was toothless, despite having been submitted for review after the paper identifying tooth sockets in Hongshanornis was published...

There are some more obvious discrepancies between Eogranivora and Hongshanornis, such as the fact that Eogranivora did not have the slim hindlimbs of Hongshanornis. On the contrary, the feet of Eogranivora were relatively short and stocky, with stubby toes. Whatever it was doing, it almost certainly wasn't wading like Hongshanornis may have been. Each of its feet also lacked a hallux, the typically backward-pointing "perching toe" of most short-tailed avialans.

Photograph and schematic of the foot of Eogranivora, from Zheng et al. (2018)

The absence of halluces in Eogranivora is notable because this is a common feature of modern birds that spend much of their time on the ground. Unlike many ground-dwelling birds, however, the short feet of Eogranivora don't look particularly suited to fast running. Its wings, on the other hand, were still fairly well-developed. I am reminded of the Pallas's sandgrouse, a modern bird that has lost its halluces and primarily forages for seeds on the ground, but is still a powerful flier and not a specialized runner.

Eogranivora and Hongshanornis drawn to scale. The approximate lengths of the flight feathers are known in both taxa. I've been attempting actual measurement-based restorations of fossil birds lately, and though I may be no Emily Willoughby or Matt Martyniuk, I find that these drawings do help give me a better sense of the proportions and possible life appearance of these animals, especially regarding species that have rarely or never been properly restored by others in the past.

Eogranivora represents the first direct evidence of herbivory in a Mesozoic euornithine. It also lends support to the hypothesis that tooth loss was correlated with herbivorous diets in Mesozoic avialans (and perhaps theropods more generally), including the ancestors of modern birds. Granted, other Mesozoic avialans that preserve direct evidence of seed-eating (Jeholornis and Sapeornis) had teeth, but they did exhibit partial tooth loss. (Jeholornis lacked teeth in at least the front of the upper jaw, whereas Sapeornis lacked teeth in the lower jaw as an adult.)

Welcome to our ever-growing roster of Mesozoic birds, Eogranivora. You may not be Hongshanornis, but being a toothless, seed-eating, ground-dwelling Mesozoic euornithine that is not a runner is still pretty cool in my book.

Reference: Zheng, X., J.K. O'Connor, X. Wang, Y. Wang, and Z. Zhou. 2018. Reinterpretation of a previously described Jehol bird clarifies early trophic evolution in the Ornithuromorpha. Proceedings of the Royal Society B 285: 20172494. doi: 10.1098/rspb.2017.2494