Repeated Evolution of Fused Thoracic Vertebrae in Songbirds

Abstract

Resumen.-La fusión de dos o más vértebras torácicas, independientemente del sinsacro, es un fenómeno que está más ampliamente distribuido entre los paseriformes que lo que se había documentado previamente. El hueso que se forma de este modo se llama notario. Hice una evaluación de esqueletos de aves paserinas y observé la presencia de un notario con vértebras completamente fusionadas en Leptopterus chabert, algunos Artamidae y Laniidae, Rhipidura leucophrys, Phainopepla nitens, los Remizidae incluyendo a Auriparus flaviceps, varios Alaudidae, Sturnus vulgaris y las especies de los géneros Toxostoma y Loxia. El mapeo de la evolución de este rasgo sobre un superárbol sugiere que un notario completamente fusionado ha evolucionado independientemente al menos 12 veces en los paseriformes oscinos y que notarios con fusión menos completa de las vértebras (por ejemplo, con sólo los procesos espinosos fusionados) están aún más ampliamente distribuidos filogenéticamente. La expresión fenotípica de un notario está fija en algunas especies y grupos taxonómicos mayores, pero varía entre individuos en otras especies. Ontogenéticamente, el notario completamente fusionado se forma cuando el ave es inmadura. El desarrollo evolutivo de los notarios probablemente depende de mutaciones que alteran los patrones de expresión de genes de transición (los genes Pax y Hox son candidatos probables) que controlan la diferenciación embriológica de las vértebras. Entre las implicaciones sistemáticas de este estudio se encuentran el apoyo adicional para la ubicación de Auriparus en la familia Remizidae y para la monofilia de un grupo de mímidos occidentales (Toxostoma spp.) con picos fuertemente decurvados.-862 - Abstract.-The fusion of two or more thoracic vertebrae, independent of the synsacrum, is more widespread in the Passeriformes than has previously been reported. The bone thus formed is known as a "notarium." I surveyed oscine passerine skeletons and found a notarium with fully fused vertebrae in Chabert's Vanga (Leptopterus chabert), certain woodswallows (Artamidae) and shrikes (Laniidae), the Willie Wagtail (Rhipiduridae, Rhipidura leucophrys), the Phainopepla (Bombycillidae, Phainopepla nitens), the penduline tits (Remizidae) including the Verdin (Auriparus flaviceps), various larks (Alaudidae), the Common Starling (Sturnus vulgaris), the sicklebilled thrashers (Toxostoma spp.), and the crossbills (Loxia spp.). Mapping of character evolution on a supertree suggests that a fully fused notarium has evolved independently at least 12 times in the oscine passerines and that notaria with less extensive fusion of the vertebrae (only the spinous processes fused, for example) are even more widespread phylogenetically. Phenotypic expression of a notarium is fixed in some species and higher taxonomic groups but varies within the species in others. Ontogenetically, the fully fused notarium forms when the bird is immature. The evolutionary development of notaria probably depends on mutations that alter expression patterns of transcription genes...

Evolution and function of anterior cervical vertebral fusion in tetrapods

“…Although extant beavers are all semi-aquatic, extinct taxa of this family had more diverse ecologies (e.g. Samuels & Van Valkenburgh, 2008, 2009. One species, Paleocastor fossor, was fossorial and an adult specimen of this species has a fused axis and third cervical vertebra, but fusion is not present in a juvenile (Peterson, 1905).…”

The third nearly complete passerine bird from the early Oligocene of Europe

“…At least four of the thoracic vertebrae bear ribs. In extant Passeriformes there is a large variation regarding the notarium: from fully ossified notarium, through less extensive fusion to no fused vertebrae (James 2009). The caudal vertebrae and the pygostyle are separated from the pelvis.…”

Multiple origins of secondary temporal fenestrae and orbitozygomatic junctions in birds

“…In summary, the repeated origins of STF and OZJ in neognathous birds (Figure ) can be accounted for in terms of a combination of proximity of two bony processes (PP and PZ) that repeatedly lead to either exaptive origins of their direct terminal junction or adaptive origins of their indirect junction via a cross‐bridge and the advantages conveyed by providing or strengthening the attachments of m. adductor mandibulae externus, in particular its pars media. The number (21‐23) of independent origins of STF and/or OZJ in Neognathae essentially equals the number (23) of independent origins of the notarium, except that the fusions of thoracic vertebrae evolved as many as 12 times among Passeriformes alone (James ). However, the vertebrae are contiguous and already connected by frequently ossifying tendons, which seems to make the origin of a notarium easier than the origin of an OZJ as this is contingent on the meeting of the two processes or crossing of the gap between them.…”

“…adductor mandibulae externus, in particular its pars media. The number (21-23) of independent origins of STF and/or OZJ in Neognathae essentially equals the number (23) of independent origins of the notarium, except that the fusions of thoracic vertebrae evolved as many as 12 times among Passeriformes alone (James 2009). However, the vertebrae are contiguous and already connected by frequently ossifying tendons, which seems to make the origin of a notarium easier than the origin of an OZJ as this is contingent on the meeting of the two processes or crossing of the gap between them.…”