The first known fossil Bird, Archaeopteryx lithographica, was described from the Late Jurassic Solnhofen Limestone of Bavaria in 1861, only two years after the publication of Darwin's On the origin of species by means of natural selection, and, showing a clear connection between modern Birds and the Theropod Dinosaurs, provided the first strong evidence in support of Darwin's theories after the publication of the book. Since that time many fossil Bird species have been described, but until the 2012, when Birds were described from the Tiaojishan Formation of eastern China, Archaeopteryx lithographica was the only Bird species known from the Jurassic, and remains to this day the only described Jurassic Bird species from outside of China.
In a paper published in the journal BMC Evolutionary Biology on 2 December 2017, Christian Foth of the Department of Geosciences at the Université de Fribourg, and the Staatliches Museum für Naturkunde Stuttgart, and Oliver Rauhut of the Bayerische Staatssammlung für Paläontologie und Geologie at Ludwig-Maximilians-University Munich, describe a second species of Bird from the Solnhofen Limestone.
The new species is described from a specimen first described as a Pterodactyl in 1857 under the name Pterodactylus crassipes. This specimen was identified as a Bird in 1970, and assigned to the species Archaeopteryx lithographica, and has since then been known as the 'Haarlem specimen', as it is kept in Teylers Museum in Haarlem, the Netherlands.
Overview of the 'Haarlem specimen'. Foth & Rauhut (2017).
The Haarlen specimen is not a complete Bird, like the more famous Berlin and London specimens, but rather a partial specimen comprising portions of a wing and hindlimb, which has prevented direct comparison to other specimens assigned to the species. Foth and Rauhut carried out a morphometric analysis of the bones that were preserved intact in order to compare the Haarlem specimen to better preserved examples of the species.
Morphometric analysis is a tool used by palaeontologists, archaeologists, anthropologists and forensic pathologists to analyse and compare specimens. It relies on taking numerous measurements of an object such as a bone or shell, and comparing both these measurements and ratios between measurements to those obtained from other specimens in order to establish relationships between them.
Using this methodology Foth and Rauhut found that almost all of the proportions of the Haarlem specimen fell outside those of other Archaeopteryx lithographica, specimens, strongly suggesting that it is a member of a new species. In order to test this further, they carried out a cladistic analysis (computerised analysis of relationships within the group based entirely upon shared common features rather than assumed relationships) including the Haarlem specimen, Archaeopteryx lithographica, and a selection of other early Birds and Maniraptoran Dinosaurs (the group of Therapods that includes Birds). This strongly suggested that the Haarlem specimen is more closely related to the Tiaojishan Birds Pedopenna, Eosinopteryx, and Anchiornis, than it is to Archaeopteryx.
On this basis Foth and Rauhut conclude that the Haarlem specimen represents a second species of Bird from the Solnhofen Limestone, and formally describe that species as Ostromia crassipes, where 'Ostromia' honours John Ostrom, who first recognised the specimen was a Bird rather than a Pterodactyl, and 'crassipes' is the original species name assigned to the specimen in 1857.
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