Geometric and Developmental Perspectives on the Evolution of the Skull and Internal Carotid Circulation in Turtles
Internal carotid circulation is arguably one of the most intensively analyzed morphological characters in turtle systematics and, thus, it is critical for understanding turtle phylogeny. I used landmark-based geometric morphometrics to facilitate a quantitative analysis of variation in turtle skull shape and osteological correlates of the internal carotid circulation. The analysis indicates that the position of the foramen posterior canalis carotici interni differs among eucryptodires, paracryptodires, and pleurodires, but remains relatively stable within these lineages. This supports the hypothesis that the position of the foramen posterior canalis carotici interni is a reliable character for identifying higher turtle taxa. Results from the geometric morphometric analysis are consistent with two of the three traits recently proposed to affect patterns of internal carotid circulation. However, these three traits do not fully explain six different patterns of internal carotid circulation in turtles. I identify the association between the internal carotid artery and the palatine branch of the facial nerve (CN VII) as a developmental constraint in turtles. Using this association as a reference point, embryological observations reported in the literature suggest morphogenetic processes that may govern different patterns of internal carotid circulation. When mapped on a phylogenetic tree, some characters transform more than once independently. The evolution of internal carotid circulation in turtles may have been even more complex than reconstructed in this paper, because of uncertain relationships among basal eucryptodires and because of a mosaic of character states in that critical part of the tree.
KeywordsCranial arteries Cranial foramina Cranial nerves Cryptodira Eucryptodira Pleurodira
A project like this would not have been possible without the precise, consistent, and well-illustrated descriptive works by Eugene Gaffney (American Museum of Natural History) throughout his career. Ren Hirayama (Waseda University) encouraged me in the after-hours party at an annual meeting of the Society of Vertebrate Paleontology in 2008 to do a turtle project as a reprieve from my immersion in dinosaurs, which eventually led to this paper. Heather Jamniczky (University of Calgary) sent her publications and Lara Shychoski (University of Alberta) lent her copy of Zelditch et al. (2004). Don Brinkman and Dennis Braman (Royal Tyrrell Museum of Palaeontology) provided logistical support when the preliminary version of this project was presented in the Gaffney Turtle Symposium. I thank the editors for the additional time they provided to put my thoughts together for this project. Jim Gardner (Royal Tyrrell Museum of Palaeontology) handled the manuscript. Reviews by Larry Powell (University of Calgary), Juliana Sterli (Museo Paleontológico Egidio Feruglio), and an anonymous person substantially improved the paper. Rich Palmer and Richard Fox (both University of Alberta) carefully critiqued and edited earlier versions of the manuscript. I thank Kesia Andressen for her medical support. Financial assistance came from the Alberta Ingenuity Fund and from Junichi and Kanae Miyashita.
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