Saturday 21 March 2015

A potential Ameghinornithid Bird from the Early Oligocene of Egypt.

The Ameghinornithids are a group poorly understood and possibly flightless Birds known from the Eocene and possibly the Oligocene of Europe, and possibly the Palaeocene of China. They are thought to be related to the modern Cariamidae (Seriemas), as well as the extinct Bathornithidae and Idiornithidae. Like other Bird groups the bones of Ameghinornithids were hollow and light, which meant they had rather poor preservational potential, with the group being known largely from very fragmentary remains, making it hard to access their palaeobiology (the biology of the living animals).

In a paper published in the journal Palaeontologica Electronica in February 2015, Thomas Stidham of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences and Adam Smith of the National Evolutionary Synthesis Center and the North Carolina Museum of Natural Sciences describe a potential Ameghinornithid Bird from the Early Oligocene Jebel Qatrani Formation of the Fayum Depression in Egypt.

The Jebel Qatrani Formation has a calculated age of 33 million years, placing it within the Eocene-Oligocene Climatic Transition, an interval in which Avian and Mammalian faunas underwent considerable turnover, with many Early Cainozoic groups disappearing and being replaced by early members of modern groups. The Jebel Qatrani is interpreted to be of fluvial (riverine) origin, and has produced a variety of Bird fossils in the past, including Cuckoos (Cuculiformes), Falcons (Falconiformes), Cranes (Gruiformes), Shorebirds (Charadriiformes), Flamingos (Phoenicopteriformes), Cormorants (Pelecaniformes) and Herons (Ardeidae) and Storks (Ciconiiformes).

The specimen described by Stidham an Smith comprises the distal end of a right tibiotarsus. This lacks an ossified supratendinal bridge, a feature thought to have been present in the common ancestor of all modern Birds, but secondarily lost in certain groups, including the Ratites (Ostriches, Emus etc.), Owls, Hoatzin, and Hornbills, Oilbirds and some Parrots, as well as some extinct groups, including the Ameghinornithids. The specimen lacks a concave distal margin, a feature found in all known Owls, and is considered too large to have come from a Parrot or Oilbird; and is morphologically quite distinct from the bones of Ratites, Hoatzin and Hornbills.

The distal tibiotarsus of the Fayum Ameghinornithid-like Bird. (1) Lateral view. (2) Cranial view. (3) Medial view. (4) Caudal view. (5) Distal view. Abbreviations: ep, elongate pit; es, extensor sulcus; fs, flattened spot; g, groove; is, intercondylar sulcus; lc, lateral condyle; mc, medial condyle; me, medial epicondyle; mf, medial flange of the tibial cartilage articulation; r, ridge; s, sand/sediment grains. Stidham & Smith (2015).

As well as being potentially the first Ameghinornithid from Africa, this specimen is also possibly the youngest known member of the group. This conforms with our general understanding of palaeobiogeography in Europe at the beginning of the Oligocene, when European animals were beginning to appear in Africa and African animals were beginning to appear in Europe; the two continents were at this time still separated by a rapidly narrowing Tethys Ocean, and the exact details of how some groups were crossing this while others did not are unclear, but this is a consistent pattern across many groups. Seen in this light, it is likely that Ameghinornithids arose in Europe and/or Asia then crossed the Tethys into Africa at the beginning of the Oliocene. However Birds in general and Ameghinornithids in particular have a very poor fossil record, and Bird fossils are more-or-less unknown in the Palaeocene and Eocene of Africa, so it is also possible that this group arose in Africa (closer to the origin of their purported relatives, the flightless Seriemas of South America) and subsequently spread to Europe.

Maps showing the geographic range of fossil localities with ameghinornithid specimens (left panel; (1) Quercy, France; (2) Messel, Germany; (3) Geiseltal, Germany; (4) Fayum Depression, Egypt) and the location of the Fayum Depression in northern Egypt (right panel) where the ameghinornithid-like specimen was collected. Stidham & Smith (2015).

See also…

The Phorusrhacid ‘Terror Birds’ were large, carnivorous flightless Birds that appear to have held the role of top terrestrial predators in South America throughout much of the Cenozoic. They are known to have colonized...

The Palaeocene Epoch lasted from 65 to 55.8 million years ago. It is during this time that terrestrial faunas recovered from the mass-extinction at the end of the Cretaceous that wiped out the non-avian dinosaurs...

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