New Late Cretaceous titanosaur sauropod dinosaur egg clutches from lower Narmada valley, India: Palaeobiology and taphonomy

Abstract

The Upper Cretaceous (Maastrichtian) Lameta Formation is well-known for its osteological and oological remains of sauropods from the eastern and western parts of the Narmada Valley, central India. The newly documented ninety-two titanosaur clutches from Dhar District (Madhya Pradesh State, central India) add further to this extensive data. Previously parataxonomy of these titanosaur clutches was carried out with a few brief reports on palaeobiological and taphonomic aspects. The quantitative data collected from the new clutches (this study) opens avenues to additionally understand more about titanosaur palaeobiology and to qualitatively understand preservation and taphonomical aspects of their egg clutches. Herein, we document 256 eggs and three clutch patterns (viz. circular, combination, linear) that are assignable to six oospecies. The high oospecies diversity points to a possible high diversity in titanosaur taxa in the Indian sub-continent though it is not reflected in titanosaurid body fossils. All the macro- and micro-structures helped in understanding egg deformation and preservation from a taphonomic point of view. Additionally, a pathologic egg documented from the study area helped in understanding the reproductive biology of titanosaurs, such as the possibility of segmented oviduct and sequential laying of eggs by titanosaurs. In addition, we made an attempt to infer aspects such as egg burial, absence of parental care, colonial nesting behavior. All the egg clutches were observed within sandy limestone and calcareous sandstone lithologies that occur in scattered outcrops with rocks showing floating siliciclastic grains in a micritic groundmass. Further, the presence of ferruginous sandstone in the Jamniapura and Padlya regions (Dhar District, central India) is indicative of a possible alluvial/fluvial setting. The presence of grainy intraclastic fabric, alveolar-septal fabrics, brecciation and shrinkage cracks observed in the clutch-bearing rocks are indicative of a low energy-low gradient palustrine depositional condition in a fluvial/alluvial setting. Finally, we envisage that a few egg clutches of this area were laid close to lake/pond margins while most were laid away from the lake/pond margins, and thus, were hatched.

Fig 1. Map of India showing the distribution of Late Cretaceous dinosaur eggs and nesting sites (modified after Mankar and Srivastava [25]; outline of India—courtesy of the University of Texas Libraries, The University of Texas at Austin).

Fig 2. Map of the study area displaying the location of investigated dinosaur clutches (courtesy of the University of Texas Libraries, The University of Texas at Austin).

Fig 3

Stratigraphic subdivisions of the Lameta Formation in the type section at Jabalpur (A) (modified after Tandon et al. [21]), Rahioli (B) (modified after Srivastava et al. [4]), and Padlya, Dhar District, M.P. (C) (modified after Dhiman et al. [51]). (D) Field photograph showing the Sandy Limestone of the Lameta Formation overlying the Nodular Limestone of the Bagh Group within Dinosaur Fossil National Park (DFNP), near Padlya, Dhar District, M.P. The Nodular Limestone shows lensoid character while the sandy limestone has massive appearance.

Fig 4. Field photographs showing outcrop characteristics.

(A) Outcrop near Dholiya Raipuriya, M.P. showing variation in the calcareous content expressed in terms of the patchy colour changes; also note the blocky character of these sandy limestone outcrops because of the joints. (B) Outcrops of red-coloured ferruginous sandstones at Padlya, M.P.; also note the variation in colour most likely due to differences in iron oxide content. (C) Red and grey-coloured rocks occurring in close association with each other within DFNP at Padlya, Dhar District, M.P. Note the prominent blocky nature of the outcrops.

Fig 5. Microstructural details of the eggshells.

(A) Stereoscopic binocular microscope photomicrograph of the tangential image of eggshell from clutch P32 showing compactituberculate ornamentation in the form of dense nodes separated by pore spaces (see arrow) (Scale bar: 2 mm). (B) Radial thin section of eggshell oospecies Megaloolithus cylindricus from clutch A3 showing long and cylindrical shell units with few areas of diagenetic alteration and vertical tubocanaliculate pore canals (see white arrow). The growth lines are limited to individual shell units (after Dhiman et al. [15]). (C) Radial thin section of M. jabalpurensis eggshell from clutch P11 showing fan-shaped shell units and arching growth lines. (D) Highly altered radial thin section of the oospecies M. dhoridungriensis from clutch P47 showing conical shell units which are broader in the upper part. (E) Radial thin section of eggshell representing oospecies F. mohabeyi from clutch P22 showing fused shell units and growth lines merging with each other (after Dhiman et al. [15]). (F) Radial thin section of F. baghensis eggshells from clutch J9 showing fused shell units and swollen basal end units. (G) Radial thin section of F. padiyalensis eggshells from clutch A5 showing long shell units fusing with each other in the lower parts (Scale bar from (B) to (G): 1000 μm). (H) SEM photograph of tangential surface of eggshell from clutch DR8 showing resorption craters (see arrow).

Fig 6. Field photographs showing sedimentary features in the clutch-bearing outcrops.

(A) The outcrop from Dholiya Raipuriya shows a characteristic brecciated nodular structure. Some nodules show autobrecciation represented as brecciated fragments with a gap in between them which indicates that the nodules fitted with each other before the disruption. The autobrecciation indicates the non-transported character of the nodules. The areas where zig-saw fit does not exist between nodules indicate their rotation and translocation after shrinkage and collapse (after Dhiman et al. [15]). (B) Intraclast collapse breccias from Dholiya Raipuriya showing variably spaced sub-angular, sub-rounded, and elliptical shaped coarse-grained brecciated clasts in a carbonate matrix. (C) In outcrops at Padlya, the chert exists in association with brecciated nodular limestone where the light grey zones show shrinkage characteristics while the dark grey zones are matrix-rich areas. The brecciated nodules also show zig-saw fit at some places while at other areas the zig-saw fit has collapsed.

Fig 7. Photomicrographs of thin sections of the dinosaur egg-bearing lithological units.

(A) Sandy limestone showing variably sized and shaped quartz grains (q) floating in the calcitic groundmass (m) (Scale bar: 200 μm). (B) Calcareous sandstone with moderate to poorly sorted quartz grains showing angular and sub-rounded grains floating in carbonate matrix; the grains also show elongated shapes. (C) Ferruginous sandstone with a bimodal distribution of quartz grains; elongated grains are present along with round shapes; the iron cement can be seen in the form of reddish to brownish-coloured stains around the quartz grains and pore filling cement. (D) Spar calcite rim and sparry calcite groundmass; the quartz grains are surrounded by corona rims of microspar and in between them dark-coloured, finer grained micrite can be seen (Scale bar in (B) to (D): 1000μm).

Fig 8. Circular clutch type observed in the investigated areas.

(A) Sketch of the circular clutch type (modified after Moratalla et al. [64]). (B) Field photograph of circular type clutch showing eggs with sediment gaps from clutch P35 from Padlya, M.P.

Fig 9. Schematic diagram of clutch P1 from Padlya, M.P. showing evidence for 20 eggs.

The eggs i to xii show closely grouped eggs while other eggs in the clutch (xviii to xx) are spaced at a distance from these grouped eggs.

Fig 10. Combination type clutch documented from the investigated area.

(A) Sketch of tightly grouped eggs (modified after Vila et al. [24]). (B) Field photograph of clutch P1 showing tightly grouped eggs i-xii in plan view with very little to no spacing between them.

Fig 11. Field photographs of linearly arranged eggs in clutch DR8 from Dholiya Raipuriya.

The sketch of such an arrangement is shown in the inset. The eggshell spread starts with a compressed unhatched egg (Box A) which is followed on the right side by concentrically arranged eggshells (Box B) with more eggshells after a gap of around 5 cm to the right (Box C). After another gap of 20 cm, more concentrically arranged eggshells are found (Box D). It appears that around four eggs were laid side by side with respect to each other. The eggshell deposits are concentrically arranged while no such concentric eggshells could be seen in the compressed egg. This indicates that except for this unhatched compressed egg (Box A) other eggs may have hatched, because, if post-burial compression was the reason behind the breaking of eggs, it would have affected all the eggs. The sediment gaps indicate that the eggs were buried in a pit with sediment gaps in between them.

Fig 12. Field photographs showing egg material variably preserved.

(A) Fragmented and morphed egg outlines of the eggs from clutch DR9. (B) Egg bottom surface preserved as partially crushed egg bottom in clutch DR4. (C) Eggshells associated closely with each other in the clutch P20. (D) Weathered surface obliterating the eggshell material (see arrow).

Fig 13. Field photographs showing hatching window and shell fragment pile.

(A) Hatching window from clutch P14 present as the topmost eroded portion of the egg and egg fragment piles occurring inside the egg. (B) Egg outline in clutch DR2 with fallen eggshells inside (see arrows). (C) Curved eggshell layer from clutch P20 which is either a remnant of hatching window or due to erosion. (D) Half-preserved egg outline from clutch P5 with little to no eggshells around, indicating it to be either hatching window or erosional remnant. The arrow shows the gap. (E) Egg from clutch P25 with double bottom (see arrow).

Fig 14. Field photographs of eggs and egg outlines showing various features.

(A) Completely unhatched egg from the clutch P43. (B) Almost fully intact circular outline of egg possibly indicating it to be unhatched and no loose eggshells are found in the clutch P6. (C) Compressed egg from clutch DR10 showing hatching window (arrow showing gap) and few eggshells collected just around the hatching window (circled) which possibly represent the remnants of hatching window. (D) Egg from clutch P26 showing curved outline. (E) Deformed egg from clutch P30 showing egg surfaces slipping past each other.

Fig 15. Pathological eggs documented from the Lameta Formation.

(A) Multi-shelled egg pathology as shown by double eggshell layers from the clutch P20. (B) Ovum-in-ovo pathology as revealed by gap in between two complete eggshell layers from the clutch P7 (after Dhiman et al. [51]).

Fig 16. Photomicrographs of sandy limestone showing alveolar-septal fabric (accompanied by sketches).

(A) The alveolar fabric from Jamniapura, Dhar District consists of a tubular structure filled with microspar and the lower part shows quartz-rich zones, the in-between dark zones inside the tubular structure could be residual micrite. (B) The alveolar fabric from Dholiya Raipuriya, Dhar District shows wispy and vermiform structures consisting of micro-spar replaced micrite channels and networks irregularly spaced, with micrite groundmass as dark patches (Scale bar: 1000 μm).

Fig 17. A block diagram showing the interpreted depositional environment of the Lameta Formation in the study areas.

It is inferred that some of the clutches were laid close to the banks of the aquatic bodies (lakes/ponds) while others were deposited away from the lakes or ponds. The clutches laid close to the margins were prone to frequent submergence by water and thus got buried under sediment and remained unhatched, while the clutches laid away from the margins could hatch and hence showed more broken eggshells.