Isotopes provide key insights into dinosaur lives
Jack Wilkin (UK)
Isotopic geochemistry has a long history in the palaeosciences since Urey (1947) first suggested that 𝛿18O from fossil calcite could be used to estimate past temperatures. Stable isotope analysis of fossils has become an increasingly important method for gathering dietary, physiological and environmental/climatic information from extinct species in terrestrial and aquatic ecosystems. The benefits of these analyses come from the geochemical fingerprint that an environment leaves in bones, teeth and soft tissues.
Ongoing studies of living organisms have found that the stable isotope composition of several light (hydrogen, carbon, nitrogen, oxygen and sulphur) and even a few heavy (calcium and strontium) elements are useful tracers of ecological and physiological information, and many of these can be similarly applied to the study of dinosaurs.
Over the last few decades, stable isotopes have greatly expanded our understanding of dinosaur palaeobiology and diet. Thermoregulation in an animal is affected by metabolic rates. Therefore, by learning more about dinosaur thermoregulation, we can make an accurate interpretation of their metabolic strategies, life histories and even evolution. Thermoregulation – the internal body temperature of an animal – can be ascertained by directly measuring oxygen isotope ratios in their bones. Isotopes and other geochemical proxies can also help reconstruct dinosaur diets and food webs. Below, I will briefly discuss the applications of oxygen, carbon and calcium isotopes in dinosaur research.
Diagenesis
Before continuing, it is worth discussing the effects of diagenesis – the process by which fossils are formed. Diagenesis is the term that encompasses all the chemical changes that occur to a fossil after burial. Diagenetic processes include dissolution, recrystallisation and replacement by other minerals: all these processes can affect the isotopic ratios in a substance. An understanding of diagenetic processes is fundamentally crucial because meaningful data can only be collected from unaltered material – that is, containing the original atoms in the bone phosphate that was there when the animal was still alive.
Diagenesis tends to homogenise seasonal signals, as it resets the original isotope values to those of the surrounding sediment and pore fluids, and therefore alter and (in extreme cases) can completely erase any geochemical data. Therefore, researchers must assess the degree of alteration before continuing a geochemical analysis to ensure the data they collect is reliable.
The following list of screening techniques is adapted from Owocki et al. (2019).
- Comparison of the geochemical and isotopic composition of skeletal tissues with various levels of preservation potential, for example, enamel, dentine and bone.
- Analysis of the preservation of expected isotopic differences between related taxa with known ecological or physiological differences and seasonal cycles.
- Analysis of crystalline microstructure and composition as prerequisites to the preservation of original isotope values as recrystallisation deforms crystals in the fossils and alters their chemistry.
- Precipitation of secondary minerals through water-fossil interactions can result in elemental enrichment of Fe and Mn, as these replace Ca. Therefore, researchers apply ‘diagenetic cut-off points’ to samples with specimens with a Fe/Ca or Mn/Ca ratio over a given amount being omitted from further study. (See Ullmann and Korte, 2015 for a review of diagenetic screening techniques in belemnites – a group of prehistoric cephalopods – but the geochemical principles are similar.)
Dinosaur metabolism
The two oxygen isotopes we are interested in for calculating dinosaur body temperature is oxygen-18 and oxygen-16. The difference between the two oxygens is the number of neutrons in their nucleus, while the numbers of protons and electrons remain constant. This means that the isotopes are stable and so, unlike radioactive isotopes -such as carbon-14 – they do not decay over time, meaning that the oxygen isotope ratios (unless altered by diagenesis) will be the same as when the bone was formed during the Mesozoic.
The oxygen isotope ratio within a substance – whether it be bone, teeth, shells or wood – is temperature-dependent. This means that during the formation of bone phosphate [Ca10(PO4)6(OH)2], the ratio between 16O and 18O will vary depending on two factors. Firstly, the dinosaur’s body temperature and secondly, the 16O-18O of the water the atoms original precipitated from.
The ratio of 16O to 18O decreases as temperatures and evaporation rates increases. In general, a 𝛿18O increase of 1.0‰ is equivalent to approximately 4°C of cooling.
The following equation is used to calculate the 𝛿-value of oxygen:
If you know the 𝛿18O of the water and the bone, it is possible to calculate the body temperature using the following equation:
T°C=111.4-4.3(𝛿18Ophosphate– 𝛿18Owater)×1000.
An overview of the principles of determining dinosaur metabolism using oxygen isotopes is outlined by Showers et al. (2002).
Oxygen isotopes taken from dinosaur bones show little variation, implying a near-constant body temperature. A study published by Amiot et al. (2006) analysed Cretaceous dinosaurs from high latitudes for different taxonomic groups. They found that dinosaurs maintained a constant body temperature of 36 to 38°C, a range consistent with living endotherms, regardless of ambient temperature. This conclusion implies that high metabolic rates were commonplace among Cretaceous dinosaurs. The study tested the 𝛿18Owater and 𝛿18Ophosphate of sauropods, theropods, ornithopods and ceratopsians. Such a similar thermophysiology and endothermy was a common trait – synapomorphy – shared by all dinosaurs.
Dinosaur diets
Another application of isotopes in dinosaur research is determining the diet and food webs of these prehistoric animals. The carbon-isotope values of tooth enamel can potentially be used to infer predator-prey and trophic-level relationships for extinct vertebrates (see, for example, Kohn et al., 2005). One recently published example (Owocki et al.,2019) analysed oxygen and carbon isotopes from tooth enamel of Tarbosaurus bataar, a close relative of Tyrannosaurus but from Mongolia, and a sauropod (Nemegtosaurus) and hadrosaurs (Barsboldia and Saurolophus) from the Nemegt Formation (Maastrichtian, Upper Cretaceous).
Carbon (C) is the fourth most abundant element in the solar system and plays an essential role in biological systems. Carbon becomes incorporated into bones and teeth and so on byan animal consuming plants and it then passes along the food chain through predation when that animal is eaten.
The isotopic composition of C is expressed by the δ13C values, defined as:
When the δ13C values are positive, the C is enriched with 13C relative to the standard, while negative δ13C values imply depletion in 13C relative to the standard – the PDB (Pee-Dee belemnite).
The 𝛿13C values of herbivorous dinosaur tooth enamel suggested a closed canopy forest of Araucaria, which is supported by petrified wood found in multiple localities throughout the Nemegt Formation. By comparing the 𝛿13C signals of the different dinosaur fauna, Owocki et al. (2019) concluded that Tarbosaurus was the apex predator in its ecosystem. The isotopes show that the tarbosaurs were feeding on giant sauropods, although it is possible that were scavenging sauropod carcasses.
This method works because differentplant groups have different δ13C values due to the way they take in carbon during photosynthesis – a process called fractionation. As carbon moves through food webs, it slowly becomes enriched (therefore, the δ13C values increases), so the values need to be adjusted due to a process called trophic fractionation.
A study by Wings et al. (2014) compared 𝛿13C from theropod and sauropod enamel from the Jurassic Qigu and Shishugou Formations of western China and found that theropod δ13C was about 1.7‰ lower than the contemporaneous herbivores. Using Wings et al. (2014) results, Owocki et al. (2019) then adjusted their tarbosaur results accordingly. The researchers found that the carbon-isotope values overlapped both hadrosaur and sauropod δ13C values, providing clear evidence of a predator-prey relationship and carbon transfer. The results in both studies are consistent with what has been found in modern mammalian predators who have about 1.3-2.6‰ lower δ13C values in their enamel than their prey (Bocherens et al., 1995).
Spinosaurus
The mid-Cretaceous Gadoufaoua (Niger) and Kem Kem Beds (Morocco) have an overabundance of giant theropod dinosaurs, including Spinosaurus and Carcharodontosaurus – both of which are larger than Tyrannosaurus. Compared to modern and other Mesozoic continental ecosystems, in which herbivores represent most of the vertebrate biomass, predators are overrepresented in the mid-Cretaceous of North Africa. The reason for this has been debated in the literature, with most studies concluding that some theropods, particularly spinosaurs, fed on aquatic or semiaquatic food sources. A remarkable paper by Nizar Ibrahim et al. (2014), which described semiaquatic adaptations in Spinosaurus,helped confirm this hypothesis, which has subsequently been confirmed by a more recent description of a Spinosaurus tail, which shows that it was used for aquatic locomotion (Ibrahim et al., 2020)
The aquatic habit of Spinosaurus has also been supported by analysis of calcium isotopes (δ44/42Ca) of north African dinosaurs, crocodilians and fish enamel conducted by Hassler et al. (2018). By comparing the δ44/42Ca values of the Kem Kem and Gadoufaoua fauna, it is estimated that non-spinosaurid theropods fed mainly on herbivorous dinosaurs, with less than 30% of their diet potentially coming from fish. Spinosaurus, on the other hand, had the most depleted δ44/42Ca values, supporting a piscivorous diet. Fishes and other aquatic organisms tend to have negative δ44/42Ca values compared to terrestrial animals due to fractionation and as a result of the number of trophic levels. Therefore, animals that have a diet with a high percentage of seafood tend to have lower δ44/42Ca values compared to those who get food from terrestrial settings.
References
Amiot, R.et al. (2006) Oxygen isotopes from biogenic apatites suggest widespread endothermy in Cretaceous dinosaurs. Earth and Planetary Science Letters 246: 41–54.
Bocherens, H. et al. (1995) Trophic structure and climatic information from isotopic signatures in Pleistocene cave fauna of southern England. Journal of Archaeological Science 22 (2): 327-340.
Hassler, A. et al. (2018) Calcium isotopes offer clues on resource partitioning among Cretaceous predatory dinosaurs. Proceedings of the Royal Society B 285 (1876): http://doi.org/10.1098/rspb.2018.0197
Ibrahim, N. et al. (2020) Tail-propelled aquatic locomotion in a theropod dinosaur. Nature 581: 67-70.
Ibrahim, Nl.et al. (2014) Semiaquatic adaptations in a giant predatory dinosaur. Science 345 (6204): 1613-1616.
Owocki K. et al. (2019) Diet preferences and climate inferred from oxygen and carbon isotopes of tooth enamel of Tarbosaurus bataar (Nemegt Formation, Upper Cretaceous, Mongolia). Palaeogeography, Palaeoclimatology, Palaeoecology 537: 109-190.
Showers, W.J. et al. (2002)Isotopic analysis of dinosaur bones: A new pyrolysis technique provides direct evidence that some dinosaurs were warm-blooded. Analytical Chemistry 74: 142-150.
Ullmann, C.V. & Korte, C. (2015) Diagenetic alteration in low-Mg calcite from macrofossils: a review. Geological Quarterly 59(1):3-20.
Urey, H.C. (1947)The thermodynamic properties of isotopic substances. Journal of the Chemical Society 1947: 562-581.
Wings, O. et al. (2014) Dinosaur teeth from the Jurassic Qigu and Shishugou Formations of the Junggar Basin (Xinjiang/China) and their paleoecologic implications. Paläontologische Zeitschrift 89: 485–502.