Robert L. Carroll, famous for his encyclopedic Vertebrate Paleontology and Evolution (1988) has produced another encyclopedic volume focusing on the basal tetrapods arising from fish and producing frogs, salamanders, caecelians and, our favorite subject: reptiles (including birds and mammals) in The Rise of the Amphibians – 365 Million Years of Evolution (2009).
Carroll takes us from the earliest Cambrian fossils up through the key Devonian fish taxa that ultimately developed limbs, then on through all the Carboniferous and later basal tetrapods (amphibians) that once dominated the shallow waters. Skeletal drawings are richly provided. The text is easy to read.
Carroll’s (1970) hypotheses on early amniote development are landmarks. According to the present large tree, his concept of a tiny tetrapod as the first amniote egg layer has been confirmed with the transition from a small Gephyrostegus species to the only slightly larger Cephalerpeton (both known from late surviving specimens). Earlier Carroll (1970) considered Gephyrostegus close to the ancestry of amniotes, but he dismisses that hypothesis here because, “they lack many definitive features [of reptiles] and the best-known genus only occurs long after the appearance of unquestioned amniotes.” Evidently he didn’t consider the possibility of a long ghost-lineage in this case.
No Cladistic Analysis Here
Carroll (2009) does not employ or present any novel cladistic analysis. Neither does he point out which of the several Carboniferous amniotes he surveys in chapter 7 is the best candidate for the title of “most primitive reptile.” Rather he introduces us to Hylonomus and Paleothyris and provides a list of the “Distinctive Characters of Early Tetrapods” (although I’m sure he meant to title the list the “Distinctive Characters of Early Amniotes.”)
Carroll reports, “Westlothiana has many features of the skeleton that are closer to those of early amniotes than any other Lower Carboniferous tetrapods, but the more primitive palate and tarsus and the great elongation of the trunk relative to the limbs suggest that it belongs to an earlier and divergent lineage.” In the large tree Westlothiana nested at the base of the new Archosauromorpha as a phylogenetic descendant of a sister to Cephalerpeton, despite its several autapomorphies and reversals. Yes, it was a “backslider.” It happens.
In chapter 7 Carroll (2009) also provides several illustrations of skeletal elements, stages in the development of the amniote egg of a chicken, a sequence illustrating the process of amniote entrapment in hollow tree trunks and several classic diadectid, pelycosaur and Petrolacosaurus skeletons. I’m glad he put diadectids into this chapter. He describes them as “…large animals also thought to be close to early amniotes…” so he doesn’t take a stand on which side of the amniote egg they belong. Here they nest with amniotes.
For anyone embarking on a study of the non-amniote tetrapods, here’s a great start and a thorough reference volume. This book should be on every college library shelf.
Carroll RL 1970. The Ancestry of Reptiles. Philosophical Transactions of the Royal Society London B 257:267–308. online pdf
Carroll RL 2009. The Rise of the Amphibians 365 Million Years of Evolution. The Johns Hopkins University Press. 360 pp.