PAN-PODOCNEMIDIDAE Joyce, Praham & Gauthier 2004

Gen. et sp. indet.

Comparative description.

LPRP-USP 0052 (figure 2 A) is elliptic (0,5 width/length ratio) with a missing pole. Its main axis is 5,1 cm long, whereas the minor axis range from 2,9 to 2,2 cm due to compression. Although taphonomicaly distorted, the elongation of LPRP-USP 0052 still matches that of chelids such as Elseya sp., Chelodina sp. and Hydromedusa maximilliani (Winkler 2006). This differs from the rounded shape of almost all described turtle fossil eggs (Azevedo et al. 2000; Jackson et al. 2008, table 1; Knell et al. 2011), except for those from the Jurassic of England (Hirsch 1996; Bray & Hirsch 1998), United States (Bray & Hirsch 1998) and China (Wang et al. 2013). The outer surface of the egg shows folded areas that suggest that the eggshell was flexible before its fossilization (figure 2 A). A different type of sediment fills the egg from its damaged portion compared to that surrounding the egg, suggesting that it was buried and fossilized without the pole, thus helping keeping its biological shape intact. The missing pole may also indicate that the egg had hatched, which is independently supported by CT analysis not revealing any embryonic remains. The WDS analysis of the eggshell indicate calcium as the main component of the eggshell crystalline structure, suggesting that diagenetic modifications had been minimal.

The egg outer surface is smooth, differing from the undulated and rough surfaces of turtle eggs from the Jurassic of the United States (Bray & Hirsch 1998) and the Cretaceous of Brazil (Azevedo et al. 2000). The eggshell is relatively thin (145–160 µm thick), including the biomineralized layer and an additional cuticle layer of about 7 µm thick (figures 2 B, D–E). Turtle fossil eggshells with a similar thickness are inferred to have either flexible or rigid eggs, e.g. Testudooflexoolithus bathonicae (Hirsh 1996; Bray & Hirsch 1998), Testudoolithus hirschi (Kohring 1999), eggs from the Jurassic of Colorado (Bray & Hirsch 1998), Haininchelys curiosa (Schleich et al. 1988) and Testudinarum ovum (Schleich & Kästle 1988; Schleich et al. 1988). Hirsch (1983) noticed that, along with to the degree of rigidity, eggshell thickness may be ecological and biological indicators. The sea turtle Lepidochelys kempi lays highly pliable eggs, with eggshells about 40 µm thick; the tortoise Geochelone elephantopus lays rigid-shelled eggs, with eggshells about 400 µm thick; the fresh-water turtle Chelydra serpentina has moderately flexible eggs, with 110 µm thick eggshells, although other fresh-water turtles also have rigid eggshells. The Chelydra serpentina condition best compares to that of LPRP-USP 0052, which is congruent with the host freshwater deposits. In addition, the thickness of the additional cuticle layer of LPRP-USP 0052 resembles that of the other freshwater taxon Erymnochelys madagascariensis (Winkler 2006). The functionality of the cuticle layer in turtles is rarely mentioned, however, analogous cuticle structures in bird eggs are directly related to nesting in wet conditions, as described in Megapodiidae, Podicipedidae and Phoenicopteridae, and are thought to preclude the blocking of pores apertures by foreign material to faciliate gas exchange and limit chemical erosion from microorganisms in the soil (Board 1981; Board et al. 1982; Board & Sparks 1991; Booth & Thompson 1991).

The outlines of the shell basic units are not easily seen on the outer surface of LPRP-USP 0052 (figure 2 F). Pore openings are very sparse (figure 2 F), a condition also present in the eggs of the extant pleurodires Podocnemis unifilis (Winkler & Sánchez-Villagra 2006), Hydromedusa maximiliani, Phrynops hilarii and Acanthochelys spixii (Winkler 2006). The pores apertures are typical of those seen in podocnemidid eggs, but also of some chelids (Winkler 2006). The diameters range from 76 to 95 µm (figure 2 F and G), differing from the podocnemidids Podocnemis unifilis (pore openings about 27,7 µm wide) and Bairdemys (pores openings about 170 to 200 µm wide, Winkler & Sánchez-Villagra 2006). In radial view, the shell units display the characteristic acicular crystallographic pattern of aragonitic calcium carbonate crystals (figure 2 E), which is considered a Testudines apomorphy (Young 1950; Hirsch 1983, 1996; Packard & Packard 1988; Winkler 2006). The metastable calcium carbonate crystals project radially from the large primary spherites (figure 2 E) differing from the condition in chelids Elseya sp. and Chelus fimbriatus and the podocnemidids Peltocephalus dumerliana and Erymnochelys madagascariensis, with no visible spherites (Winkler 2006). Most shell units of LPRP-USP 0052 are roughly triangular, without marked borders, but more columnar unities are also present. This diversity of shapes results in relatively loosely abutting shell units (figure 2 B, C and E). According to Winkler & Sánchez-Villagra (2006), this allows “caverns” (large inter-units spaces) among shell units in some portions of the eggshell (figure 2, E), which are absent from rigid-shelled eggs, such as those of the podocnemidid Bairdemys (Winkler & Sánchez-Villagra 2006). On the contrary, the semi-flexible egg of Podocnemis unifilis (Foote 1978) has a mix of areas with and without “caverns” (Winkler & Sánchez-Villagra 2006). Finally, shell units of LPRP-USP 0052 are, in average slightly higher than wide (high/width ratio of 1,1–1,2). This is also notable in flexible turtle fossil eggs, as such Testudooflexoolithus bathonicae (Hirsh 1996; Bray & Hirsch 1998) and Testudooflexoolithus agassizi (Hirsch 1996).