On
the simplified tree of life from Frost et al., I’ve placed arrows indicating
the nature of the amphibian papilla. Blue arrows indicate taxa for which the papilla consists of one patch; red and Pink arrows indicate taxa for
which it comprises two patches. Branch 7 corresponds to caecilians,
branch 24 to salamanders and newts. The blue arrows to families in branch
7 are based on SEM studies by Jim White (James S. White) carried out while he
was in the laboratory of I.L. Baird. Jim also examined ambystomatid salamanders, as did we. The remaining
arrows in branch 24 are based on our results, as are all of the red and pink
arrows.
Red
arrows indicate taxa with two patches but no caudal
extension. Pink arrows indicate taxa in which
caudal extensions are found, and the thickness of the arrow indicates the
relative size of the extension (in area and number of hair
cells). We examined eight Eleutherodactylus species. In Frost et al., Those species are placed in
the Brachycephalidae branch, which has been
elaborated extensively by the 2008 monograph of Hedges, Duellman
and Heinicke (Zootaxa 1737:1-182). In four of those eight Eleutherodactylus species, the
caudal extensions contained between 55 and 65% of the total AP hair cells. In the one leptodactylid
species and eight ranid species, the caudal
extensions contained between 45 and 55% of the total AP hair cells. In the hylids, dendrobatid, rhacophorid and hyperoliid that we examined, caudal extensions contained 35
to 45% of the total AP hair cells. In
the sooglossid, and microhylids
that we examined, caudal extensions contained 25 to 35% of the total AP hair
cells. Most of the bufonids
fell in the same range (25 to 35%), but a few fell outside. In the context of this tree, the high degree
of emphasis on the caudal extension (indicated by the thickest arrows) is not a
widely shared trait. For example, it seems to have arisen independently
in the Ranoides (branch 108) and Hyloides
(branch 314). Within the Hyloides, it seems to
have arisen independently in the Leptodactylidae and
the Brachycephalidae (the Terrarana
of Hedges et al.).
What
we must infer from this result is a high degree of plasticity in the caudal
extension. It evidently is a structure easily shaped by selective
pressures. The trick will be to identify those pressures. With respect to reverse engineering, we need
to provide strong inferences regarding the selective advantages of the caudal
extension. Old data from the Capranica and Lewis Labs will be helpful in that
regard.