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Iberomesornis : Bird Transitional Fossil

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"The first avian species found at Las Hoyas was Iberomesornis romerali, a sparrow-sized small animal (Figure 2) whose mass has been estimated (6) between 15 and 20 g. The specimen is almost complete, lacking the skull and the anterior cervical vertebrae. Iberomesornis is characterised by a singular combination of traits, including some primitive ones, present in the non avian theropods, like a sacrum composed of five vertebrae, tarsal not fused to the tibia or to the metatarsals, and lacking any evidence of metatarsal fusion. Along with these symplesiomorfic features, Iberomesornis shares with modem birds a number of evolutionary novelties: a derived, modem avian furcula, with a low interclavicular angle, strut-like coracoids, and a series of free caudal vertebrae (eight in number) and a large pygostyle composed by the fusion of 10-15 vertebrae (7-9) (Figure 3). This combination of primitive and derived traits led Sanz et al., 1988 to propose a phylogenetic hypothesis for this Las Hoyas bird, placing it in an intermediate position between Archaeopteryx and modem birds."


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Iberomesornis romerali
Describer : Sanz & Bonaparte, 1992

Time : Cretaceous Early Barremian

Classification : Saurischia Theropoda Tetanurae Coelurosauria Maniraptora Avialae Ornithothoraces Enantiornithes

Fossilsite : Calizas de Huérgina Formation, Cuence, Spain


Postcranial skeleton. A basal member of Ornithothoraces that shares the general features of that group, which includes this genus, Neornithes and all theur common descendants. Iberomesornis has an INTERMEDIATE PHYLOGENETIC POSITION between Archaeopteryx and Ornithurae due to a combination of primitive and derived characters (Sanz and Bonaparte, 1992).

Primitive characters include the structure of the pelvis and hindlimb which is similar to that of Archaeopteryx. Postulated synamorphies (Chiappe, 1995) include a strutlike coracoid, a sharply pointed scapular end, an ulnar shaft twice as thick as the radial shaft, and a pogystyle. Iberomesornis romerali had a wingspan of about 20 centimetres. It was a true bird as it had feathered wings and was capable of powered flight rather than simply gliding. However, it had retained the reptilian features of teeth in its beak and a claw on its wing. It was probably omnivorous, feeding on anything it was big enough to tackle.

This ornithothoracine bird appears to have a scapula that exhibits the typical non−avian di nosaur orientation, as the glenoid is anteroventral to the ribcage (Sanz and Bonaparte. 1992). However, the head and neck have detached, and the scapula is displaced, having been dragged along with the proximal end of the humerus, which has broken off and rotated ventrally. The scapula was originally in the typical ornithothoracine position. (Senter, 2006)

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Added: Mar-31-2010 
By: lpankhurst
Tags: Transitional, Fossils, Evolution
Views: 7730 | Comments: 14 | Votes: 1 | Favorites: 0 | Shared: 1 | Updates: 0 | Times used in channels: 1
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