Nk. Dulvy et Jd. Reynolds, EVOLUTIONARY TRANSITIONS AMONG EGG-LAYING, LIVE-BEARING AND MATERNAL INPUTS IN SHARKS AND RAYS, Proceedings - Royal Society. Biological Sciences, 264(1386), 1997, pp. 1309-1315
Sharks and rays are thought to have a large number of independent orig
ins of live-bearing. We examined evolutionary transitions to live-bear
ing and maternal input to embryos in this subclass by optimizing repro
ductive characters onto a composite phylogeny. Egg-laying (40% of all
species) is the likely ancestral reproductive mode for this clade, and
there is evidence that live-bearing has evolved independently 9-10 ti
mes and maternal input 4-5 times. Most transitions (12-15) have been t
oward live-bearing with provisioning limited to yolk. These have occur
red from egg-laying ancestors or live-bearing taxa that provide matern
al input to embryos. Only 2-3 transitions have occurred in the other d
irection, i.e. away from yolk-only live-bearing. Egg-laying has evolve
d from live-bearing ancestors in skates, Rajidae (25% of all species)
and possibly in the zebra shark, Stegostoma fasciata. Thus, although t
here has been an overall trend toward the evolution of live-bearing in
elasmobranchs, the evolution of additional maternal input has been ex
tremely labile.