Placental ontogeny of the Australian scincid lizardsNiveoscincus coventryiandPseudemoia spenceri

Abstract

Models for the evolution of placentation among squamate reptiles have been strongly influenced by early work on one lineage of Australian scincid lizards: one of three lineages that include species with specialized allantoplacentation. The discovery of two types of complex allantoplacentae among species currently assigned to the Eugongylus species group led Weekes ([1935] Proc. Zool. Soc. Lond., 2:625-645) to propose that a graded evolutionary sequence was exemplified by the morphological series of allantoplacental types that existed among Australian skinks. One of the model's strengths is the incorporation of testable inferences of the relationship between placental structure and function. However, descriptions of placental structure for some species are incomplete and subsequent taxonomic revisions have resulted in confusion concerning the species studied. One of the ambiguities is the identity of Lygosoma weekesae (Kinghorn [1929] Proc. Linn. Soc. N.S.W., 54:32-33), one of two species for which the most specialized type of allantoplacenta was defined. The distinguishing characteristic of this placental type is the presence of a structure known as a placentome. Subsequent to the original species description, Lygosoma weekesae was named a synonym for Pseudemoia spenceri (Rawlinson [1974] Mem. Natn. Mus. Vict., 35:87-96), yet the placental description (Weekes [1929] Proc. Linn. Soc. N.S.W., 54:34-60) is thought to be of Niveoscincus coventryi (Rawlinson [1975] Mem. Natn. Mus. Vic., 36:1-16). We describe placental ontogeny of N. coventryi and P. spenceri as a contribution to study of comparative placentation among lygosomatine skinks. We conclude that the placental description for L. weekesae could not have been N. coventryi, because a placentome is not a component of the allantoplacenta of N. coventryi. Further, the allantoplacenta of this species does not conform to previously defined categories for Eugongylus group skinks. The allantoplacenta of P. spenceri contains a placentome that is structurally congruous with the original description of placentation for L. weekesae (Weekes [1929] Proc. Linn. Soc. N.S.W., 54:34-60). Comparison of extraembryonic membrane and placental structure among the four viviparous and one oviparous species of Eugongylus group skinks for which data are available shows that each species exhibits characters that distinguish it from other species, while additional characters are shared among all, or some, species. Based on a recent phylogenetic hypothesis, the distribution of allantoplacental types among these species does not support Weekes' ([1935] Proc. Zool. Soc. Lond., 2:625-645) model of a graded sequence for the evolution of squamate placentation.