Ostrich evolution revealed: Fossil finds shed new light on the evolution of the African ratite

By Gerald Mayr, Senckenberg Research Institute and Natural History Museum in Frankfurt

Linked paper: Extinct crane-like birds (Eogruidae and Ergilornithidae) from the Cenozoic of Central Asia are indeed ostrich precursors by Gerald Mayr & Nikita Zelenkov, Ornithology.

Until now, the early evolutionary history of ostriches was largely unknown. Together with my colleague Nikita Zelenkov from the Russian Academy of Sciences, we examined bird fossils from Central Asia that had previously been considered relatives of the cranes. Our research now shows that these birds are instead early representatives of the ostrich lineage. The reinterpretation of the fossil findings reveals that early ancestors of these birds exhibit an unexpected mosaic of traits. According to our study, published today in the journal Ornithology, ostriches originated in Asia – today, these large, flightless birds are exclusively restricted to Africa. 

While most birds have four toes, in the ostrich that number has been reduced to two over the course of its evolution. This foot structure is unique among modern birds and helps the African ratite achieve its high running speed, among other things. Species of the Ergilornithidae, a group of birds that was widespread in Asia about 35 to 10 million years ago, also have only two toes. Because of this feature, there was earlier speculation that these birds might be related to ostriches. However, this hypothesis was not widely accepted, and the Ergilornithidae were assigned to crane-like birds. 

Fossil finds of the medium-sized and very long-legged Ergilornithidae are comparatively common at some Asian sites – especially in Mongolia. Until now, however, only fossil leg bones were known. Dr. Zelenkov and I examined additional material of these birds and the closely related Eogruidae from a collection in Moscow. This also includes the first known skull remnant of these animals. The finds date back to the 1970s – but have remained undescribed until our investigation. We were able to show on the basis of distinct skeletal features that the Eogruidae and Ergilornithidae are indeed Asian ostriches! 

Skull details. (A) Reconstruction of the eogruid/ergilornithid skull (dorsal view) from the late Eocene of Khoer Dzan in Mongolia (PIN 3110-170) by digitally mirroring the better-preserved left side of the specimen. (B) Skull of Palaeotis weigelti from the middle Eocene of the Geisel Valley in Germany (GMH 4362); dorsal view. (C) Dorsal view of the caudal portion of the skull of the extant Struthio camelus (Struthionidae); the colored area on the right site indicates the size of the nasal gland. (D) Dorsal view of the caudal portion of the skull of the extant Aramus guarauna (Aramidae). (E) Fossil skull (PIN 3110-170) in lateral view. (F), (G) Lateral view of the caudal portion of the skull of (F) S. camelus and (G) A. guarauna. The arrows in (F) and (G) denote the caudal ends of the fossae for the nasal glands. (H)‒(K), Left otic region in ventral view of (H), (I) the fossil skull (PIN 3110-170), (J) S. camelus (subadult individual), and (K) A. gurauna. The arrows in (A) and (C) denote enlarged details of the fossae glandularum nasales; the dotted lines in (B)‒(D) indicate the position of the cranial breakage line of the fossil skull; the turquoise dotted lines in (H), (J), and (K) outline the shapes of the articular surfaces for the otic head of the quadrate. Abbreviations: cqo = cotyla quadratica otici; cqs = cotyla quadratica squamosi; fgn = fossa glandulae nasalis; fos = fossa for lateral portion of otic process of quadrate; ftp = fossa temporalis; ios = interorbital section of skull; nfh = nasofrontal hinge; pop = processus postorbitalis; rec = recess on pila otica; rtd = recessus tympanicus dorsalis; smt = processus suprameaticus; sup = processus supraorbitalis; zyg = processus zygomaticus. Scale bars = 10 mm.

The oldest undisputed finds of these birds come from the Miocene of Africa and are about 20 million years old. Based on our findings, we conclude that the ancestors of today’s ostriches were widespread across the Northern Hemisphere in the early Cenozoic – about 40 million years ago. According to our study, the birds evolved into the modern, two-toed ostriches in the steppes of Central Asia and then colonized Africa about 20 million years ago, where they are still native today. Another important result of this study is that early ancestors of ostriches differed significantly from their modern relatives in several characteristics.

Interestingly, the bones of the Eogruidae and Ergilornithidae resemble those of the extinct, nearly one-meter-tall avian genus Palaeotis, which was found in the Messel Pit, a fossil site in Hesse. Its relationships were also controversial until now, although a relationship with ostriches had been discussed. The reinterpretation of these finds provides us with a new and coherent picture of the evolutionary history of ostriches!

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