{Note: I intend to append a short glossary to the end of this text in order to explain
technical terms [in boldface] for the non-systematics geeks.}
An aspect of what is known as the "tetrapod problem" is the focus of this study. Specifically, I am examining a longstanding controversy—the relationships among the amniotes. Early attempts to classify organisms into natural groups (in a non-evolutionary context) provided a hypothesis of (turtle,(mammal,((snake,lizard),(crocodilian,(bird)))). Recent studies have recognized that the turtle and mammal positions on this traditional hypothesis are reversed, and this modified version has gained wide acceptance among systematists. However, Gardiner published a paper in the Zoological Journal of the Linnean Society in 1982 challenging this hypothesis. His unorthodox view gained support among some (notably Løvtrup [1985]) and was dismissed out of hand by others (notably Devillers and de Ricqlés [1982]). Six years later, Gauthier and his collaborators published a rebuttal in Cladistics, using a considerably larger data matrix and including fossil taxa (an idea to which Gardiner was adamantly opposed). Since then, a number of investigators have attempted to address this issue using a variety of molecular sequence data, but the last comprehensive review was undertaken by Eernisse and Kluge in 1993. My thesis updates and supplements this work by adding 28S ribosomal DNA data for a representative bird and crocodilian, as I seek a definitive answer for Kemp's (1988) question: Haemothermia or Archosauria? The 28S data and the combined data matrix favor the Archosauria hypothesis.
As it turns out, this isn't quite the whole story. First, a bit of background is in order: the number of hypotheses regarding amniote interrelationships is virtually equivalent to the number of possible permutations of relationship among those groups. However, there are two major controversies surrounding a lgenerally agreed-upon framework (but see Hedges [1999]): 1) which came first, the mammals or the turtles?; and 2) are birds extant sister taxa to mammals or crocodilians? Of these, my main focus was the archosaur-haematotherm debate. I utilized approximately 4,200 bp of Wood Thrush 28S rDNA and 5,000 bp of American Alligator 28S rDNA for my analyses. These data were combined with 5,167 bases of Homo, Rattus, Mus, Xenopus, and Latimeria 28S data courtesy of Zardoya and Meyer (R. Zardoya, pers. comm., Zardoya and Meyer [1996]). I also used approximately 3,000 bases of 28S data from the Red-Tailed Pipe Snake collected previously by my major professor (Brian Crother) and his collaborator (Mary White) as a test of the position of the lepidosaurs (assumed by my analyses to be monophyletic).
I used Latimeria and Xenopus as a paraphyletic outgroup in my analyses. My extant-only analyses posited a sister-group relationship between birds and crocodilians, with the lepidosaurs (RT Pipe Snake) as sisters to this group. This supported the archosaur hypothesis. I also combined the 28S data with Eernisse and Kluge's datafile (Doug Eernisse, pers. comm.) to produce a unified file with data from proteins, 18S rDNA, 28S rDNA, and morphology relevant to the resolution of these relationships. The combined data also supported the archosaur group.
After my thesis work was completed, I returned to the lab and collected an additional 900 bp of Wood Thrush rDNA. When combined with the other data, these data actually made the bird relationship more ambiguous. I ended up with a trichotomy among the lepidosaur, bird, and crocodilian groups in my 28S analysis. This result supports neither hypothesis. However, the combined analysis continues to support the archosaur grouping, and, hopefully, as more taxa with 28S data are added to the matrix, this ambiguity may be resolved soon.
©1999 Brian R. Warren