Intertrappean beds of Kislapuri, Dindori District, Madhya Pradesh.

Late Cretaceous (Maastrichtian).

Ten isolated eggshell fragments (DUGF/70-79).

The eggshell fragments are brown or amber in color. The external surface of the eggshells is either covered by sub-circular pits encircled by ridges as in a golf ball but not as closely placed (Fig. 2 and Fig. 3) or a combination of sub-circular pits and elongated grooves bounded by longitudinal ridges (Fig. 2.1E). The individual pits range in diameter between 200–260 μm. Visible pore canals are generally absent, but some eggshell fragments display craters or pore canals filled with secondary deposits within the coarse sub-circular pits (Fig. 3.1A–C). These pores have a diameter of approximately 55 μm. The external surface of DUGF/75 is also rough and flaky in appearance (Fig. 3.1B–C). Fractured radial sections under Scanning Electron Microscope (SEM) exhibit a single layer of shell units, which are in the form of inverted triangles. The shell units are composed of acicular radiating crystallites that originate from nucleation centers at the inner surface and radiate outwards (Fig. 2 and Fig. 3). The acicular structure of the eggshell in radial section is suggestive of its aragonite composition. However, this needs to be confirmed by X-ray diffraction analysis. The shell units are tightly packed at the broad, outer zone and are separated from adjacent units at the inner one-third of the height and crystals of adjoining shell units are tightly interlocked. Because of this reason, no pore canals are visible in the radial section. Growth lines in the form of horizontal banding are visible in thin sections of these eggshell fragments (Fig. 3.2). The nucleation centres from which the acicular crystallites radiate are visible at the inner base of the fractured radial surface in the form of a depression (Fig. 2.2B–C). The shell thickness varies from 370–450 μm and is slightly variable in different eggshell fragments. The height and width of individual shell units range from 370–450 μm and 285–350 μm, respectively. The height to width ratio of the shell units is 1.28:1. The shell unit bases are tightly packed on the inner surface. The inner surface of these eggshells has nucleation centers or primary spherites in the form of spherical depressions (130–150 μm in diameter) with a central core and radiating crystals at the periphery (Fig. 2 and Fig. 3). Many irregular pores occur at the junction of two or three such craters (Fig. 2 and Fig. 3).

The Kisalpuri chelonian eggshells cannot be assigned to sea turtles because they have shell units that are wider than high, flexible and loosely arranged. They also cannot be assigned to chelydrid and emydid turtles because they lay eggs with flexible shell units with considerable space between them and are as high as wide. The presence of acicular radiating crystallites and primary spherites in the presently described eggshells point to their testudoid affinity. The presence of interdigitating crystallites of adjacent shell units without much gap between nucleation sites further suggests that these eggshell fragments belong to a rigid-shelled chelonian egg. The eggshells from Kisalpuri with interlocking shell units higher than wide and having acicular crystallites indicate these eggshell fragments belong to the oofamily Testudoolithidae.

In India, except for two reports, one each from the Lameta Formation (Mohabey, 1998) and intertrappean beds of Duddukuru (Bajpai et al., 1997), majority of fossil eggshell studies were of dinosaur eggshells. Mohabey (1998) referred a partial nest of elliptical egg and eggshell fragments from the Upper Cretaceous Lameta Formation of Pavna, in Chandrapur District, Maharashtra to a testudoid. The eggshells from Pavna have a thickness of 800 μm and a smooth external surface. Their height to width ratio is 3.5:1.0 The eggshells from Kisalpuri differ from those described by Mohabey (1998) in being very thin (370–450 μm) and having a pitted and ridged external surface ornamentation and a height/width ratio of 1.28:1. It has been pointed out that the thickness of the eggshell as measured from the figure of Mohabey (1998, fig. 10G) is only 500 μm (Jackson et al., 2008) and our measurements from Mohabay's (1998) figures confirm this. The highly diagenetically altered shell units obscuring acicular aragonitic structure and the observation of radiating mammillary layer on the inner surface have led to the conclusion that these Lameta eggshells are unlikely those of chelonians (Jackson et al., 2008 and Kohring, 1999). From the figures of Mohabey (1998, fig. 10G–H), distinct inverted triangular shell units characteristic of testudoid basic type are not discernible. Rather, radial structure in the basal caps restricted to the lower one-fifth of the shell height and faint horizontal banding visible in rest of the shell unit are reminiscent of the radial structure of avian eggshells. Therefore, the testudoid identification of Pavna eggs and eggshells needs to be confirmed with additional well-preserved material.

The eggshells from Kisalpuri resemble the turtle eggshells described by Bajpai et al. (1997) from the intertrappean beds of Duddukuru, West Godavari District Andhra Pradesh in having shell units higher than wide (1.28:1.0), lacking intervening spaces between them and in the presence of sub-spherical depressed central cores with radiating needle-like crystals on the internal surface. However, the outer surface of the eggshells is smooth in those of Duddukuru as compared to the coarsely pitted or/and ridged ornamentation in Kisalpuri eggshells. Moreover, the small and fine crystallites present between adjacent shell units at their inner one-fifth of the shell unit height in Duddukuru eggshells are not very prominent in those of Kisalpuri. The Kisalpuri eggshells (370–450 μm) are also comparatively thicker than those of Duddukuru (190–260 μm). In this respect, they are more close to eggshells described from the Cretaceous of Mongolia under Testudinovum (Mikhailov, 1991 and Mikhailov, 1997), which is now regarded as nomen nudum (Lawver and Jackson, 2014). In the Mongolian eggshells, the thickness ranges between 300–400 μm (Table 1). In shell thickness and height/width ratio, Kisalpuri eggshells though not very similar, are closer to those of an unidentified chelonian (400–430 μm) from the Early Cretaceous of Japan (Isaji et al., 2006), and the eggshells of Emydoolithus laiyangensis Wang et al., 2013 (400-500 μm) described from the Late Cretaceous of China (Wang et al., 2013) (Table 1). However, in all these taxa, the outer surface of the eggshells is either smooth or undulating as compared to the pitted and ridged ornamentation of Kisalpuri specimens. Other Cretaceous taxa, such as Testudoolithus jiangi (Fang et al., 2003) Jackson et al., 2008 from the Early Cretaceous of China and Testudoolithus sp. from Upper Cretaceous (Campanian) Fruitland Formation, USA (Tanaka et al., 2011) have thicker eggshells and greater height to width ratio than the present specimens. Eggs from a gravid turtle from Upper Cretaceous (Campanian) Kaiparowits Formation, USA (Knell et al., 2011) have shells thinner than those of the new Indian eggshells and have relatively greater height to width ratio. Haininchelys curiosa Schleich et al., 1988 from the Palaeocene of Belgium with a shell thickness of 250–300 μm and height to width ratio of 1.2:1–2.3:1 compares well with the present specimens. (Table 1). But unlike Kisalpuri eggshells, those of Haininchelys are characterized by funnel-like pores and rounded external surface of shell units.

Although the shell structure of present eggshells, compares well with that of the oogenus Testudoolithus Hirsch, 1996, all known rigid testudoid eggs and eggshells have smooth external surface as compared to coarsely pitted and ridged ornamentation in the eggshells of Kisalpuri. In this respect, they appear to represent a new genus or species, but we refrain from erecting a new taxon in view of limited number of specimens at our disposal.

Oofamily Krokolithidae Kohring and Hirsch, 1996

Incertae sedis

(Fig. 4 and Fig. 5)

Intertrappean Beds of Kisalpuri, Dindori District, Madhya Pradesh.

Late Cretaceous (Maastrichtian).

Three isolated eggshell fragments (DUGF/80-82).

The height and width of an individual shell unit vary from 425–480 μm and 400–420 μm, respectively. The height to width ratio is 1.06–1.14:1.0. The external surface of the eggshells differs in its appearance in different crocodilian species. It is known to vary from fairly smooth to dimpled, coarse or heavily eroded and flaky (Hirsch, 1985). In the described specimens, the external surface is either roughly undulating with sub-circular to oval pits, some of which may represent dissolution pits (Fig. 4.1A) or with coarse sub-circular pits bounded by ridges (Fig. 5.1A). Under SEM, fractured radial section reveals discrete, inverted triangular, radiating crystalline wedges that form a tabulate layer, with pores between the shell units (Fig. 4 and Fig. 5). Inverted funnel-like pore openings extending from the base to outer surface of the shell unit are visible in the radial section (Fig. 4 and Fig. 5). The pore openings have an average diameter of 185 μm. The pore canals are continuous from the internal to external surface (Fig. 4 and Fig. 5) and hence interlocking between adjacent shell units is weak. These wedged shell units rise from basal plate groups beneath them, which are loose aggregates of platy, calcitic crystallites (Fig. 4 and Fig. 5). Radial striations are observed rising from the basal plate groups and passing through the upper end of the shell units (Fig. 4 and Fig. 5). The interstices between the crystalline wedges form an acute-angled triangle. The basal plate groups are surrounded by large irregular interstices (Fig. 4.1C). On the inner surface, the coalescing basal plate groups are separated by elongated depressions some of which may represent pore openings. The basal plate groups bear a number of circular, crater-like depressions with ring-like or stepped layers (Fig. 4.1E). Sometimes these craters-like depressions occur in twins and are surrounded by inwardly directed microcrystals (Fig. 4.1D). Beneath these craters, the basal plate groups exhibit horizontally laid crystals oriented parallel to the outer surface (Fig. 4.1E). The basal plate groups are separated by a distance of about 130–160 μm. The three structural layers identified in crocodilian eggshells (Ferguson, 1982 and Moreno-Azanza et al., 2014), viz., the inner layer (IL) or the basal plate groups, the middle layer (ML) growing parallel to the shell surface like a book-like tabular structure, and the densely calcified outer layer (OL) are not always identifiable in fossil crocodiloid eggshells (Moreno-Azanza et al., 2014). In the present specimens, IL is evident, but it is difficult to distinguish between ML and OL as prominent accretion lines with tabular book-like structure extending from the top of basal plate groups to the outer surface are not visible.

Crocodilian eggshells are easily distinguished from those of turtles, which are generally diagnosed by their spherulitic shell units consisting of needle-like radiating aragonitic crystals. The typical radial section of the Kisalpuri eggshells clearly demonstrates that these specimens belong to the crocodiloid type of basic eggshell organization (Mikhailov, 1997). The external surface of these eggshells exhibits the same general ornamentation of other crocodiloid eggs such as in K. wilsoni, K. helleri and Bauruoolithus fragilis and in extant Crocodylus johnstoni Krefft, 1873 and C. porosus Schneider, 1801, and has trapezoidal eggshell units and basal knobs. In shell thickness, the Kisalpuri eggshells fall within the range of variation for fossil crocodiloid eggshells (290 μm–700 μm) and eggshells of extant crocodiles (400 μm–530 μm) (Hirsch, 1983) (Table 2). Though DUGF/81 with coarse pitted external surface ornamentation appears distinct from DUGF/80, in which the outer surface is undulating, in fractured radial section both have similar looking shell units separated by inverted funnel-shaped pore canals.

Ferguson (1982) identified five layers, viz., the inner membrane layer, the mammillary layer, the organic layer, the honeycomb layer, and the outer calcified layer in the eggshells of Alligator mississippiensis. However, the single-layered eggshell structure corresponding to crocodiloid basic type of Mikhailov (1997) has been widely followed in describing fossil eggshells until now. More recently, Moreno-Azanza et al. (2014) revisited the taxonomic status of eggshell fragments from the Uppermost Cretaceous of Pyrenees, Spain attributed to Megaloolithidae (López-Martínez, 2003) and classified them as Krokolithidae indet. In this work, the authors confirmed that the microstructure and ultrastructure of crocodylomorph eggshell contains several structural layers as was envisaged by Ferguson (1982). According to Moreno-Azanza et al. (2014), the calcified part of crocodiloid eggshell consists at least of three layers, namely the inner layer (IL) with basal knobs made up of poorly ordered aggregates of calcite microcrystals and protein fibers (basal plate groups of Hirsch, 1985), the middle layer comprising an aggregate of prismatic calcite crystals interwoven with protein fibers and growing parallel to the shell surface like a book-like tabular structure (corresponding to organic and honeycomb layers of Ferguson, 1982), and the densely calcified outer layer lacking organic matter and reminiscent of tabular ultrastructure of theropod eggshells. As discussed above, it is not easy to identify all these structural layers in fossil crocodiloid eggshells. Only the inner layer (IL) is identifiable in the studied sample and as the horizontal accretion lines are not preserved, the distinction between the middle and outer layers is not possible.

The present eggshells, particularly DUGF/80, differ from most of the known crocodiloid eggs and eggshells in which wedges were found to be interlocked in a continuous layer. In contrast, the crystalline wedges of Kisalpuri eggshells do not show interlocking. The eggshells from the intertrappean beds show crystalline wedges without interlocking from the base to near the external surface, in which it differs from almost all the described fossil taxa and may warrant their placement in a new oogenus. However, Mikhailov et al. (1996) suggested that oogenera should be based on egg shape and differences in structural morphotypes, outer surface ornamentation and pore system. Moreover, we cannot rule out the possibility that the lack of interlocking shell units may be an artifact of dissolution causing enlargement of pore canals as the available sample size is too small.

A few Cretaceous records of crocodiloid eggs and eggshells are known from the Upper Cretaceous (Campanian) Fruitland Formation (Tanaka et al., 2011), the Two Medicine Formation (Jackson and Varricchio, 2010), the Lower Cretaceous Glen Rose Formation (Rogers, 2000) and the Upper Cretaceous Adamantina Formation, Brazil (Oliveira et al., 2011). The crocodiloid eggshells from the Glen Rose Formation and the Two Medicine Formation have relatively thicker shells than those of Kisalpuri. Moreover, the eggshells from the Two Medicine Formation have smooth outer surfaces and lack pore canals both in radial section and on the exposed shell surface. In Krokolithidae eggshells from the Fruitland Formation, the lower end of eggshell thickness range (450–800 μm) is comparable to those described here. However, they differ in having reticulate to nodose surface ornamentation, horizontal accretionary lines and a step-like erosional pattern. The new eggshells from central India also differ from Bauruoolithus fragilis from the Upper Cretaceous of Brazil, as the shell is much thinner (150–250 μm), shell units are broader, and the interstices between the crystalline wedges are proportionately smaller, forming an obtuse angle in the Brazilian oospecies. Among all known fossil crocodilian eggshells, those of Krokolithes wilsoni from the Eocene De Beque Formation, Colorado, USA (Hirsch, 1985) have a shell thickness (0.36–0.45) which is the closest to that of the Kisalpuri eggshells (Table 2). But in the former, the external surface is heavily degraded and bears stepped erosion craters.

In the Indian subcontinent, there are two reports of fossil crocodilian eggshells from the Siwalik deposits and one from the intertrappean beds of Bombay (Singh et al., 1998). The eggshells from Kisalpuri are slightly thicker (480–420 μm) than those described from the intertrappean beds of Bombay (350 μm). However, Singh et al. (1998) mentioned the presence of discrete mammillae on the internal surface, which are not generally observed in crocodiloid eggshells. They also mentioned that the average thickness of individual shell units is only 75 μm, which is smaller than that of the Kisalpuri eggshells. Moreover, the inverted triangular wedge-like crystallites are not very clear from the photomicrographs of Singh et al. (1998; Plate 2a-c); rather they appear as cylindrical spheroliths. In view of these morphological inconsistencies, we consider them doubtful crocodiloid type eggshells. Patnaik and Schleich (1993) described smooth crocodiloid eggshell fragments from the Pliocene Saketi Formation, near Moginand, in Himachal Pradesh, which are comparatively thicker than those of Kisalpuri with a thickness range of 1.9–6.6 mm. These eggshells were assigned to Gavialis cf. G. gangeticus Gmelin, 1789 and Crocodylus cf. C. palustris Lesson, 1831 just based on the thickness range of eggshells. The lone egg reported from the Upper Miocene Chinji Formation in Pakistan has a shell thickness ranging from 180–760 μm (Panadès et al., 2009). No pore canals are present in the radial section of Chinji eggshells as in the present specimens but have well developed accretion lines. In these characters, it differs from the eggshells from Kisalpuri referred to Krokolithidae. Based on the size dimensions of the egg and shell thickness range, the Chinji egg has been compared with those of Gavialis cf. G. gangeticus.