Friday 25 April 2014

An Enatiornithine Bird from the Early Cretaceous Jehol Biota.

Birds derived from small Theropod Dinosaur ancestors in the Middle Jurassic, and appear to have reached similar levels of diversity in the Cretaceous to that seen today. However the dominant group of Birds in the Cretaceous were the Enatiornithines, a group of toothed Birds related to but not ancestral to modern Birds (Ornithuromorpha), which died out in the end Cretaceous extinction event.

In a paper published in the Journal of Vertebrate Paleontology on 8 January 2013, a team of scientists led by Jingmai O’Connor of the Dinosaur Institute of the Natural History Museum of Los Angeles County and the Key Laboratory of Evolutionary Systematics of Vertebrates of the Institute of Vertebrate Paleontology and Paleoanthroplogy of the Chinese Academy of Sciences, describe a new species of Enatiornithine Bird from the Early Cretaceous Yixian Formation in Jianchang County in Liaoning Province, China. The Yixian Formation is one of the deposits that makes up the Jehol Biota, a fossil lagerstätte in which numerous Vertebrate, Invertebrate and Plant fossils are preserved in exquisite detail in lithographic limestones, providing one of the best preserved examples of a Mesozoic ecosystem known.

The new species is named Sulcavis geeorum, where ‘Sulcavis’ is a combination of ‘sulcus’, meaning ‘groove’ and ‘avis’, meaning ‘bird’, and ‘geeorum’ honours the Gee family of La Cañada, California, ‘for their generous contributions in support of Mesozoic bird research’. Sulcavis geeorum is a ~20 cm Enatiornithine Bird preserved in ventral view on a single slab. The skeleton is almost complete and there are poorly preserved feathers around the head and upper part of the body.

(Top) Photograph of the single known specimen of Sulcavis geeorum. (Bottom) Camera lucida drawing of the single known specimen of Sulcavis geeorum. Light gray indicates areas of poor preservation; dark gray indicates matrix. Abbreviations: ce, cervical vertebrae; co, coracoid; fe, femur; fi, fibula; fu, furcula; h, humerus; il, ilium; mc, metacarpal; mc I, alular digit; mc II, major digit; mc III, minor digit; mt, metatarsals; py, pygostyle; r, radius; s, synsacrum; sc, scapula; ti, tibiotarsus; u, ulna; ul, ulnare. O’Connor et al. (2013).

Sulcavis geeorum has notably large and robust teeth, with distinctive grooves on them that give the Bird its generic name. These suggest that it was adapted to a durophagous diet (diet of tough food that needed to be broken up, such as bones, nuts or shellfish). Modern Birds rely on gastroliths (swallowed stones) to break up hard foods in a muscular organ called the gizzard before it reaches their stomach, but no Enatiornithine Bird has ever been found to have had such structures. Gastroliths have been found in Ornithuromorph Birds from the Cretaceous, as well as Sapeornithiform Birds (another extinct Mesozoic group) and several types of Dinosaur, so their absence in Enatiornithine Birds is unlikely to be preservational, particularly given the large number of these Birds known from the Jehol Biota, where feathers and soft tissues are often preserved.

Detail of skull of Sulcavis geeorum. (A) Proximal half of rostrum; (B) entire skull, right lateral view; (C), interpretative drawing. Light gray indicates areas of poor preservation; dark gray indicates matrix. Abbreviations: ang?, angular?; bo?, basioccipital; d, dentary; eo, exoccipital; fr, frontal; fm, foramen magnum; ju, postorbital process of the jugal; max, maxilla; n, nasal; oc, occipital condyle; pa, parietal; pao, paraoccipital process; pm, premaxilla; po, postorbital; scl, scleral ossicle; so, supraoccipital; sp?, splenial; sur?, surangular. O’Connor et al. (2013).

This lack of gastroliths in Enatiornithine Birds suggests that they were obliged to retain their teeth in order to work food, despite the fact that teeth are both heavy (a problem for a small flying animal) and metabolically expensive. Teeth appear to have been lost rapidly among the earliest Ornithuromorph Birds, suggesting that they no longer suffered this constraint. This may have given the Ornithuromorph Birds an evolutionary advantage over the Enatiornithine Birds, explaining while the one group was able to survive into the Cenozoic while the other perished.

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