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Ankylosaurid acetabula (reversal??)




Dear All,
I would be interested in hearing more discussion of George's and Tracy's view that Ankylosauridae and Stegosauridae (sensu lato) are not sister groups. But that stegosaurs split in a separate clade in between ankylosaurs and cerapodans.
I was particularly intrigued when Pete mentioned that Maryanska and Osmolska (1984) suggested such a topology apparently based on the imperforate acetabulum found in many Ankylosaurs. I don't know the citation to that 1984 paper (or if I could get a hold of it in any case).
Can anyone briefly describe the variation of the acetabula (imperforate, semi-perforate, or perforate) in the various major clades within Ankylosauridae (including nodosaurs in my classification). Here's a partial classification of Ankylosauridae (the better known genera?) to provide some context (clade 2 are the nodosaurs):
Ankylosauridae
1 Minmi
2 Edmontonia
B Panoplosaurus
C Sauropelta
D Silvisaurus
3 Gastonia
B Hylaeosaurus
C Mymoorapelta
4 Shamosaurus
5 Ankylosaurus
6 Pinacosaurus
B Euplocephalus
7 Saichania
8 Tarchia
MY COMMENT: Since perforate acetabula have probably arisen in dinosaurs several (three or more) times, it seems to me that the imperforate (or semi-perforate) acetabula in ankylosaurs could possibly be retention of the plesiomorphic state (rather than a reversal back to it). If so, it could give important support for George's view that Ankylosaurs split off before a stegosaur-cerapodan clade (which apparently is pretty uniformly perforate).
-----Cheers, Ken Kinman
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