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Read Ebook: Thoracic and Coracoid Arteries In Two Families of Birds Columbidae and Hirundinidae by Jenkinson Marion Anne
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Ebook has 72 lines and 9785 words, and 2 pages
Possibly the thoracic artery has undergone migration but apparent differences in its origin might well be due to differences in other vessels of the thoracic area. Additionally, there seems to be no reason to assume that the lateral position of the thoracic artery is the primitive one, or that the medial is the derived position, as is implied by the phrase "medial migration." Although the lateral site of attachment is predominant in the lower orders of birds, and the medial attachment is found primarily in Passeriformes, a fact which may indicate that type 1 is the more primitive, it must nevertheless be kept in mind that a sequence of a single morphological character does not necessarily represent the phylogenetic sequence of the character itself .
Also, a given arterial arrangement might be independently derived more than once. If such has been the case, similarities in arterial arrangements in different taxa would sometimes be "chance similarities," that is to say, "resemblance in characteristics developed in separate taxa by independent causes and without causal relationship involving the similarity as such" .
The particular arrangement of the arteries of the thoracic area also seems to be of limited value as a clue to taxonomic relationships. If the origin of any artery is determined by skeletal and muscular features, as I suggest, the artery perhaps ought not be considered as a separate character, but as part of a "character complex" that varies as a unit . The skeleton offers a potential fossil record for consideration. Changes in the skeleton and muscles, great enough to affect the blood vessels, would probably be detected more easily than would the resulting vascular changes. Also, I did not find as much individual variation in the skeleton and muscles in the area studied as I did in the vascular system. In other words, within the bounds established by the skeletal and muscular features, the artery still exhibited individual variation in exact origin.
SUMMARY
In most of the vessels studied there is a high degree of individual variation, but few interspecific differences were noticed within either family. Differences in the arteries of the thorax between the two families are described by discussing the resulting different origins of the thoracic artery. In swallows the thoracic artery arises from either the subclavian artery or the coracoid artery, whereas in pigeons it arises from the pectoral trunk. This difference in site of attachment seems to be a result of differences between the two families in muscular and skeletal elements of the thorax.
The particular site of attachment of the thoracic artery is of limited value as a taxonomic character. Several considerations influenced this conclusion. If the location of the artery is determined by skeletal and muscular elements, these associated structures must be considered taxonomically as a "character complex" . Even within the bounds established by the skeleton and muscles, the artery displays a high degree of individual variation in exact origin. A given arterial arrangement could have been derived independently many times. Because differences are defined relative to other likewise variable vessels, supposed similarities or differences in the one artery may be artifacts of the system of description.
My findings and interpretations do not support previous suggestions that the thoracic artery has undergone a mediad migration, and that the various sites of attachment of that vessel may come to represent various levels of evolution. The primitive site of attachment of the vessel is unknown, and it seems to me that it has not been sufficiently demonstrated that the vessel has undergone any "migration."
LITERATURE CITED
AMERICAN ORNITHOLOGISTS' UNION
ASHLEY, J. F.
BEDDARD, F. E.
BERGER, A. J.
BHADURI, J. L., and BISWAS, B.
BHADURI, J. L., BISWAS, B., and DAS, S. K.
COUES, E.
GLENNY, F. H.
HOWARD, H.
HOWELL, A. B.
HUDSON, G. E., and LANZILLOTTI, P. J.
MAYR, E.
MAYR, E., LINSLEY, E. G., and USINGER, R. L.
SIMPSON, G. G.
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