What a Fossil Foot Can Teach Us About Ancient Relatives of Cranes

By Grace Musser

Linked paper: A new species of Eogruidae (Aves: Gruiformes) from the Miocene of the Linxia Basin, Gansu, China: Evolutionary and climatic implications by G. Musser, Z. Li, and J.A. Clarke, The Auk: Ornithological Advances.

In ancient Chinese mythology, cranes were said to carry the souls of the dead to paradise. Today, a new crane-like fossil from Gansu, a province in northwestern China, allows us to begin to answer questions about the past lives of these birds.

The fossil, a single clawed foot with elongate toes, is from the Linxia Basin located along the northeastern margin of the Tibetan Plateau in Gansu, China. This powerful two-toed foot belonged to a member of a now-extinct bird family related to modern cranes, the Eogruidae, and it can help us begin to answer important questions about both living and extinct birds: what did the extinct ancestors of living cranes look like? What drove cranes to differentiate into a variety of species? How did cranes disperse around the world? And why does the Tibetan Plateau hold the one of the highest amounts of extant crane diversity in the world?

Eogruidae were some of the earliest relatives of living cranes and were endemic to Eurasia. They have been found as far east as Mongolia and as far west as the Balkan Peninsula and were present from the Eocene Epoch to the Pliocene Epoch (approximately 50 to 6 million years ago). We still don’t fully understand the evolutionary relationships among Eogruidae species, but members of the family fall into two major groups. “Eogruid-like” eogruids have three toes and were present from the Eocene to the Oligocene; “ergilornithid-like” eogruids have two toes and were alive from the Eocene to the Pliocene. Due to the loss of the second toe, scientists at first confused the “ergilornithid-like” feet with those of ostriches, and many researchers have suggested that the loss of this toe is an adaptation for running and may indicate that some eogruids were flightless. However, this has yet to be confirmed.

fossil bird foot embedded in a rock, against a black background
The fossil foot. Photo by Zhiheng Li.

We determined that the fossil foot from Gansu belonged to a member of a previously undescribed species, which we’ve named Sinoergilornis guangheensis. The genus name, Sinoergilornis, refers to the fact that this is the first eogruid fossil from west China and that its morphology is most similar to that of the “ergilornithid-like” group. The species name, guangheensis, refers to the location where it was found. The fossil is late Miocene in age (7-6.5 mya) and was recovered from the Linxia Basin, which is known mostly for mammal fossils. Specifically, S. guangheensis was recovered from the Liushu Formation, the only portion of the Basin that is known to contain bird fossils. Ostriches, a pheasant, vultures, and falcon-like birds have also been recovered there.

When I first visited the Linxia Basin, I was in awe of the stunning rainbow of yellow ochre, burnt sienna, and ivory sediments towering above me and the vista of emerald stepped rice fields it was set against. The fossil was similarly extraordinary due to its three-dimensional preservation and articulation of the elements. I first became interested in fossil cranes as a master’s student at Columbia University when I realized how little is known about crane evolution and how much learning about their evolutionary history can help us to better understand the evolutionary history of all birds, and it was so exciting to realize that the specimen represented a new species of Eogruidae and that it was the first to be found in west China.

Current fossil evidence suggests that the Eogruidae were largely limited to southeastern Mongolia during the Eocene and then diversified and dispersed westward during the Miocene. This burst of diversity and range expansion corresponds approximately with the beginning of monsoon-dominated climate patterns and the northwestern expansion of humid environments in China. The presence of Eogruidae in the late Miocene Liushu Formation also corresponds with northeastern Tibetan Plateau uplift and intensification of the East Asian monsoon around the late Miocene (~8 mya). Several studies have linked the uplift of the Tibetan Plateau and the intensification of the East Asian monsoon, but the exact relationship between them remains uncertain, as does how they might have affected ancient relatives of cranes.

Other climate shifts also highlight the uncertainty surrounding what might have caused the diversification and dispersal of Eogruidae and other extinct relatives of cranes. Eocene-Oligocene global cooling may have had a negative effect on crane-like birds around the world, including the Eogruidae. Crane-like fossils from the Eocene are abundant in Europe and North America, but only one Oligocene fossil has been found in Europe, and few eogruids have been found in Oligocene sediments.

Fossil evidence from Linxia suggests that large environmental changes have occurred since the late Miocene. We still don’t know why Eogruidae apparently died out in the early Pliocene, while other extinct birds more closely related to living cranes were not abundant in Asia until the Pliocene and Pleistocene. Recovery of more complete fossils and further analysis of evolutionary relationships within Eogruidae is needed to better understand how cranes and their extinct relatives dispersed around the world and to better estimate the primary causes of eogruid extinction and living crane diversification.

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