Life and reproduction of titanosaurians: Isotopic hallmark of mid-palaeolatitude eggshells and its significance for body temperature, diet, and nesting
Graphical abstract
Introduction
Reproduction is an essential trait of vertebrates, and the reproductive strategy adopted by a given species can be decisive for its success or decline when facing environmental stress and/or inter-taxa competition. In the fossil record, eggs provide undisputed evidence of breeding activity and represent key snapshots of the animal's life, contrasting with the vast expanse of the geological timescale. Like other archosaurians, titanosaurian dinosaurs were oviparous and laid eggs (Chiappe et al., 1998), which, as in other amniotes, provided protection and a stable micro-environment that contained all the essential nutrients for the embryo to develop in a terrestrial realm (Nys and Guyot, 2011). Titanosaurian eggs are known from several nesting sites worldwide, usually in association with fluvial deposits that may have been lithified. Nesting sites have been found in palaeolatitudes between 10° and 45° (e.g., Folinsbee et al., 1970; Erben et al., 1979; Sarkar et al., 1991; Chiappe et al., 2003; Cojan et al., 2003; Kim et al., 2009; Bojar et al., 2010; Fiorelli et al., 2012; Hechenleitner et al., 2016b, Hechenleitner et al., 2020). They are an invaluable record for this clade, especially when skeletal remains are poorly preserved or absent. Resistance of the eggshells to diagenetic alteration is mainly due to a dense microstructure low in organic matter content (about 2 vol% vs. >17 vol% in dentine and ~25 vol% in bone; Hincke et al., 2012; Wang and Cerling, 1994). Additionally, the nesting strategy adopted by titanosaurians, which consists of burying the eggs into the substrate (Hechenleitner et al., 2015), plays a key protective role against transport, breakage and weathering.
Vertebrate hard tissues (including eggshells, teeth and bones) crystallise from body fluids that are derived from ingested water and food. Therefore, their oxygen (δ18O) and carbon (δ13C) stable isotope compositions largely reflect environmental cycles and climatic conditions (DeNiro and Epstein, 1976, DeNiro and Epstein, 1978; Luz et al., 1984; Luz and Kolodny, 1985; Cerling and Harris, 1999; Kohn and Cerling, 2002; Koch, 2007), and more generally the palaeoenvironment in which the animals used to live. Since they form within a short period of time (days to weeks) (e.g., Tütken, 2011) in the distal part of the oviduct (Nys and Gautron, 2007), fossil eggs are of prime interest as they represent very specific environmental conditions prevailing during the reproduction period. The isotopic compositions of eggshells thus reflect the body temperature, diet and water assimilated by the female just before and during the production of the eggs. However, the use of the isotopic compositions measured in fossil remains as reliable palaeoenvironmental indicators requires careful evaluation of the preservation state of their mineralogical, chemical, and isotopic compositions. As alteration of isotopic compositions can be cryptic, i.e., without unequivocal changes in the primary mineralogy and biogenic structure of the fossil material (e.g., Araújo et al., 2013), reliable indicators are increasingly needed in this field. In this context, we propose here a new definition of the δ13C and δ18O isotopic compositions characteristic of well-preserved titanosaurian eggshells that may be used as a hallmark in future work.
Several isotopic studies on titanosaurian eggshells from Europe and Asia have been carried out over the last decades to determine the conditions best suited for the reproduction of this clade in South America, Europe and Asia (Folinsbee et al., 1970; Erben et al., 1979; Sarkar et al., 1991; Riera et al., 2013; Eagle et al., 2015). However, none has focused so far on the nesting sites known from NW Argentina (Fiorelli et al., 2012; Hechenleitner et al., 2016b, Hechenleitner et al., 2020). This study presents the stable isotope compositions of titanosaurian eggshells, as well as that of a single tooth and a bone, discovered in three nesting sites of La Rioja Province (NW Argentina). After a careful determination of the preservation state of the fossils, and hence, of their potential as palaeoenvironmental and physiological proxies, we used the best-preserved specimens to re-evaluate the thermoregulation strategies of titanosaurians and the palaeoenvironmental conditions prevailing during nesting and incubation. Comparison of our results with previous studies from other mid-palaeolatitude nesting sites worldwide indicates that the conditions required for the breeding of titanosaurians in NW Argentina were very similar to those in other sites. We also speculate that limiting food resources for a sizable population of very large animals was likely a controlling factor for sedentariness, which further supports the idea that titanosaurians were breeding migrants.
Section snippets
Nesting sites
Three nesting sites – Sanagasta, Tama, and Quebrada de Santo Domingo – all located in La Rioja Province, NW Argentina (Fig. 1), have been investigated in detail. For a general description of the regional geology, the reader is referred to previous work (Ciccioli et al., 2005; Ezpeleta et al., 2006; Grellet-Tinner and Fiorelli, 2010; Fiorelli et al., 2012; Hechenleitner et al., 2016b; Limarino et al., 2016; Basilici et al., 2017; Hechenleitner et al., 2020).
The nesting site of Sanagasta is
Analytical techniques
The degree of preservation of the fossil remains, crucial for the interpretation of isotopic compositions, was evaluated using a combination of optical and in situ chemical analyses. The sample preparation followed a well-established procedure during which the fossil specimens were mounted in epoxy resin or cut in thin sections, stepwise diamond-powder polished down to a grade of 0.1 μm, and carbon coated to prevent charging. The samples intended for conventional and clumped stable isotope mass
Fossil preservation
Thin section optical microscopy of the eggshells from Sanagasta, Tama and QSD Sur confirms poor preservation and massive recrystallisation (mainly silicates with minor secondary calcite), as observed macroscopically. The eggshells show extensive alteration features, with only occasional and partial preservation of biogenic structures. In contrast, the eggshells from QSD Norte show obvious biogenic features typical for the clade and indicative of good preservation. Those include a mono-layered
Associated abiogenic geological samples as comparative tools
A common way of identifying potential disturbances of the biogenic isotopic composition is to compare the data provided by the best-preserved fossils with the results obtained from the matrix and the other associated abiogenic materials (carbonate rocks and nodules, hydrothermal calcite and quartz). Fractionation processes involved in the formation of biogenic vs. abiogenic material may be different (physiological vs. physicochemical). Also, the crystallisation temperature and the source of
Conclusion
Based on an extensive isotopic database compiled from this and other studies, an isotopic hallmark (δ13CVPDB = −15 to −11‰; δ18OVSMOW = 27 to 33‰) has been proposed to identify the well-preserved eggshells produced by mid-palaeolatitude (23° to 45°) titanosaurians. This hallmark may serve as a geochemical reference to select eggshell samples best suited for reconstructing the palaeoenvironment in which titanosaurians used to reproduce, especially when the alteration is cryptic or when
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
Acknowledgements
Many thanks to Dr. Benita Putlitz for measuring the oxygen stable isotope compositions of the silicate samples. Many thanks to Madalina Jaggi and Stewart Bishop for their work and support at the ETH lab, and to Sergio de la Vega and Tonino Bustamante from CRILAR. Thank you to the Secretaría de Cultura de La Rioja, Municipalidad de Sanagasta and Tama, and Gobierno de La Rioja, to the Grupo Roggio and all Quebrada de Santo Domingo supports and funds. This work was supported by the Jurassic
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Current affiliation: Laboratorio de Paleontología de Vertebrados, IDEAN (CONICET), Departamento de Ciencias Geológicas, FCEN, Universidad de Buenos Aires, Intendente Güiraldes 2160, Ciudad Universitaria – Pabellón II, C1428EGA Ciudad Autónoma de Buenos Aires, Argentina.