19 January 2016
Where Are Your Bird Parts?
Anchiornis lacks a lot of parts of birds that are diagnostically avian. The most telling lack is in the chest. A requirement for powered flight is to develop muscles that can manage to power one's wings such that they provide thrust and lift. Bats, birds, and insects (and pterodactyls before) all manage this in different ways. Birds solved this problem by placing the muscles for both the power and recovery strokes of their flight on the chest and extending the sternum ventrally such that the bone resembles the keel of a ship. This keeled carinate sternum and the resulting pectoralis and supracoracoideus muscles (responsible for the down and upstroke respectively) are enormous and the center of gravity lies, in most birds, in the middle of the sternum between the chest muscles. This is important because Anchiornis lacks that sternum morphology, the associated muscle morphology and,more importantly, may represent the basal most characteristic of the avian sternum. That is the hypothesis of Zheng, et al. 2014 that investigates the absence of the sternum in both Anchiornis and Sapeornis (an Early Cretaceous basal avian from China). This study contemplates other anatomical studies investigating the flight capabilities of Anchiornis. The lack of a sternum and the associated musculature negates the abilities of Anchiornis to conduct powered flight. The feathers corroborate this hypothesis. Feathers are the subject of Longrich, et al. 2012. Longrich, et al. concludes that both Archaeopteryx and Anchiornis possessed similar feather configurations. The two configurations of the feathers consisted of multiple layers of nearly symmetrical feathers. Powered flight feathers are asymmetrical and neatly layered so that they create a thin layer of feathers rather than a multilayered arrangement. The feathers of both Anchiornis and Archaeopteryx, according to this study, possess limited flight capabilities. These are most likely limited to gliding and controlled descent in respect to this study.