NOTHOSAURID EMBRYOS FROM THE MIDDLE TRIASSIC OF NORTHERN ITALY: AN INSIGHT INTO THE VIVIPARITY OF NOTHOSAURS?

Reproductive strategies of Mesozoic aquatic reptiles are well documented only for the ichthyosaurs, which were undoubtedly viviparous since the Middle Triassic (Deeming et al., 1993; Dal Sasso and Pinna, 1996). Clear and direct evidence, such as embryos within the body of the mother, are lacking for sauropterygians. Tiny specimens of the nothosaurid Neusticosaurus (Sander, 1988) from the Middle Triassic of Monte San Giorgio (Canton Ticino, Switzerland) and of Keichousaurus (Lin and Rieppel, 1998) from China, have been interpreted tentatively as embryos, suggesting that at least some sauropterygians may have been viviparous. Recently, four additional nothosaurid embryos were found in the Monte San Giorgio area, at the Ca’ del Frate site (Viggiù, Varese, Northern Italy), and occurring in the Kalkschieferzone (Meridekalk, late Ladinian). These examples strongly support the hypothesis that nothosaurs were viviparous, particularly since three of the four embryos are clustered together in close association, excluding the possibility that they derive from isolated eggs washed into the coastal basin prior to fossilization. The fossiliferous level of the Kalkschieferzone is the youngest of five belonging to the famous paleontological area of Monte S. Giorgio-Besano-Viggiù, which has yielded vertebrates of great scientific interest since 1850 (Bürgin et al., 1989). The depositional environment of the Kalkschieferzone was a shallow complex lagoon, which formed the most protected area of a deep gulf opening toward the east (Tintori and Lombardo, 1999). Emergent land was present to the NNE, only a few kilometers away from the sites. The influence of fresh water could have been important periodically, as indicated by the presence in several levels (usually lacking vertebrates) of conchostrachans, plant remains, and insects (Tintori, 1990b; Lombardo, 1999; Krzeminski and Lombardo, 2001). To date, at least sixteen species of fishes (Lombardo, 1997, 2001) are known, along with one genus of reptiles, Lariosaurus (Tintori and Renesto, 1990; Renesto, 1993), which is represented by the holotype of L. valceresii (Tintori and Renesto, 1990), another adult specimen currently under study by one of us (SR), a juvenile (which has been referred to as Lariosaurus sp. Renesto, 1993), and the four possible embryos here described. Three of the newly discovered specimens are preserved clustered together on the same slab, while the fourth is on another slab. All four are articulated and nearly complete, lacking only those parts that were not ossified or were lost during recovery. In both cases, however, the specimens lie on a mass-mortality surface of the small fish Prohalecites, which forms more than 95% of the vertebrate remains in this fossiliferous level (Tintori, 1990a). The attribution of the four specimens to the genus Lariosaurus is based on observable characters of the skeleton. Moreover, there are no other nothosaur species in this level of the Kalkschieferzone, a situation similar to that of the pachypleurosauroids from the older fossiliferous levels of the Grenzbitumenzone and of the Meride Limestone, where only one species is present in each level (Sander, 1989). All specimens are exposed in ventral view. Three specimens (MCSNIO [Civico Museoinsubrico di Storia Naturale, Induno Olona] 701–3; Figs. 1, 2) lie close to one another and are similar in size; the fourth (MCSNIO 704; Figs. 1 and 3) lies on a different level, but is nearly identical both in shape and size to the other three. Specimens MCSNIO 701 and MCSNIO 702 are curled together, with the head of one lying in close proximity to the tail of the other. Specimen MCSNIO 703 lies with its skull close to the vertebral column of specimen MCSNIO 702 (Fig. 2), but has a straight neck and dorsal region, with only the tail curled. The overall morphology, along with all identifiable skeletal features, supports the identification of these specimens as nothosaurs. Nothosauria are a group of sauropterygian reptiles that thrived in the Middle Triassic (Rieppel and Hagdorn, 1997). In specimens MCSNIO 703 and MCSNIO 704 the cervical vertebrae number 22, and in specimen MCSNIO 702 at least four pairs of sacral ribs are detectable. This latter feature has been considered an autapomorphy of the genus Lariosaurus (Rieppel and Lin, 1995), a nothosaur of middle to large size (Rieppel, 1998) known mainly from Triassic outcrops of the Western Tethys; in addition, the number of cervical vertebrae is consistent with this attribution. Therefore, it seems reasonable to consider that the four specimens are immature individuals belonging to Lariosaurus and that their early growth stage is indicated both by absolute size and by relative proportions of different portions of the body. Even if identification of these specimens as L. valceresii seems reasonable, the lack of diagnostic characters prevents a precise species attribution; thus, they are considered as Lariosaurus sp. indeterminate.