Dinosaur eggshells prove that dinosaurs weren’t molluscs.

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Chicken egg lit up using the candling method. Image from backyarchickens.com

This week saw the release of a new paper that has implications for dinosaur metabolism.

Dawson, R.R., Field, D.J., Hull, P.M., Zelenitsky, D.K., Therrien, F., Affek, H.P. 2020. Eggshell Geochemistry Reveals Ancestral Metabolic Thermoregulation in Dinosauria. Sci. Adv. 6:eaax9361. (Open Access)

The paper makes some pretty hefty claims regarding dinosaur metabolism, and as such, has received a fair share of media coverage touting this as the latest evidence for “warm-blooded” (i.e., automatic endothermic) dinosaurs.

Every time there is a major headline like that, I feel obliged to go back to the source to see what the media has likely overblown. In this case, media claims don’t seem that far off what was written in the actual paper, which is not necessarily good. Some of these claims do extend beyond the reach of the available evidence (e.g., there actually is no comparison with other contemporaneous reptiles of the region, weakening any arguments for metabolic thermoregulation).

It’s been an age since I’ve done one of these paper breakdowns, but I think this one warrants a more thorough analysis, especially given the implications of the interpretations.

Clumped isotopes and eggshells as proxies for body temperature

Um, not exactly the same thing. Go Topes?

Robin Dawson and colleagues built upon the work of Robert Eagle and others (2015). These authors had previously incorporated a process called a carbonate “clumped isotope” thermometer (Eagle et al. 2010). This method uses the natural tendency for Carbon 13 (13C) and Oxygen 18 (18O) isotopes to “clump” together in molecules. This is a natural phenomenon that is mostly dependent on the temperature at which the clumping occurs. Clumped isotopes are measured by recording the  amount of 47 CO2 (13C18O16O, a common variant [isotopologue] of CO2) in a substance and subtracting it from the amount of 47 CO2 one would expect from a randomized distribution of those isotopes in that sample, resulting in the typical notation form of Δ47, which we see used a lot in the Dawson et al. paper.

The appeal of this method over previous methods (e.g., Barrick and Showers 1996; Amiot et al. 2004) is that the clumping of 18O with 13C eliminates the biases that occur when not taking into account how the 18O was acquired (freshwater vs. marine. Montane vs. coastal streams etc.). By using the clumped method, you are basically just measuring temperature, not diet or water acquisition. That’s not to say that it is a perfect system (nothing is), only that it is the best one we have found so far.

Dawson et al. focused specifically on eggshells because, as they put it:

Since extant endotherms—especially large-bodied ones—are often “thermal mosaics,” with their extremities approaching environmental temperatures while retaining a warmer core…, temperature estimates from eggshells that developed near the center of a dinosaur’s body are likely to be more reliable than estimates from tooth enamel.

This makes sense, though this is hardly true for just automatic endotherms. Most large animals are thermal mosaics, regardless of thermophysiology. In fact, this concept was originally used by Barrick and Showers (1996) to argue for automatic endothermy in dinosaurs (their argument being that automatic endotherms are not that mosaic).

Using eggshells makes sense (as opposed to teeth) as the shelling of eggs happens in the uterus (shell gland), which is located deep in the pelvis, close to the body core in all amniotes. As such, it is more likely that eggshells will record core body temperature more accurately than tooth enamel. Dawson et al. argue similarly in their paper, citing Eagle et al.’s temperature estimates from their 2010 study, and comparing it to Eagle et al.’s 2015 estimates (though it’s unclear if the eggshells and teeth were really from the same locality, much less the same specimens).

So far so good. The methodology seems sound.

Now, Dawson et al. expanded on the work from Eagle and colleagues by looking at these clumped isotope thermometers in relation to the estimated temperatures of the environments that those dinosaurs were in. To do that, they needed to know what those temperatures were.

Measuring molluscs (and making assumptions)

To obtain information for the environments that the test dinosaurs lived in, Dawson et al. used clumped isotope information from molluscs that lived in the same area. These included the shells from freshwater gastropods (snails) and bivalves (clams).

This, unfortunately, is where things start to get a bit shaky. The choice of molluscs is never really explained. I suspect it is because, as invertebrates, their remains are plentiful enough that there is no worry of doing destructive sampling to obtain necessary isotope data. The other reason likely stems from an underlying assumption about molluscs, as discussed in the paper [emphasis mine]:

Here, we compare dinosaurian core body temperature estimates from fossil eggshells to paleoenvironmental temperatures by using Δ47 paleothermometry of co-occurring ectothermic fossil mollusks or other climatic paleotemperature estimates derived from published literature.

This probably seems small but make no mistake, this is an underlying assumption that is seen time and time again in the literature. All invertebrates are ectothermic, as are all fish and all squamates, turtles, crocodylians, etc. This is no different than the arguments made for ectothermic dinosaurs prior to the Dinosaur Renaissance. If it was wrong to assume the physiology of dinosaurs based solely on the fact that  “they were reptiles”, so too is it wrong to assume that all other non-dinosaurs of the Mesozoic would have a specific thermophysiology.

As I’ve mentioned before, uniformitarianism does not work for living things. Unless we see evidence for common descent of a trait (via the extant phylogenetic bracket or other ancestral character reconstruction techniques), all traits seen in extant life forms should be cautiously extended back in time. This goes doubly so for physiology and behaviour. There is another problem raised by this approach, but I’ll return to this later.

Paleolatitiude

Dawson et al. are the first to really look at potential body temperatures of dinosaurs from high paleolatitudes. This adds some important data points to ideas of dinosaur physiology, but also environmental temperatures at these near polar latitudes.

All the dinosaurs used in this study come from the Oldman Formation of Southern Alberta. At the time (Upper Campanian), this formation would have been around 55° North during the Mesozoic. By looking at more polar paleolatitudes, the authors aimed to better determine if dinosaur body temperature was metabolically induced vs. environmentally. The estimated mean annual temperature (MAT) at the time was 12–13°C, although the methods used to calculate MAT are more susceptible to systematic biasing (δ18Ophosphate), and the study cited (Amiot et al. 2004) only covered areas that may have encompassed the collection sites that the eggshell material came from (the authors also cite Spicer and Herman 2010, but that paper is really just about Alaska). Still, it’s a good starting proxy.

Dinosaur body temperatures

Dawson et al. were able to obtain the following body temperatures for their study dinosaurs:

  • Troodon formosus = 38° +/- 4°C | 27° +/- 4°C | 28° +/- 3°C
  • Maiasaura peeblesorum = 44° +/- 2°C
  • Romanian eggshell (Megaloolithus siruguei) = 36° +/- 1°C

And the following were the temperatures obtained from their molluscs:

  • Gastropod (W20) = 24.6° +/- 1.4°C
  • Bivalve (TMP 2009.149.5) = 27.5° +/- 1.6°C

Truly neat results (especially the range of values on T. formosus). It’s where the authors go from here that I find my largest gripes with the paper.

Faulty assumptions

Up until this point, the foundations for the paper have been fairly solid, as have their results regarding estimated body temperatures. However, there are areas along the way where the authors make assumptions that I do not feel are justified, and in some cases are just not accurate.

For instance, Dawson et al. argue that crocodylians such as Alligator, are poikilotherms that closely tracked the temperature of their environment.

Studies of American alligators from the RSWR [Rockefeller State Wildlife Refuge] in June and July 2001 show that alligator body temperatures closely follow environmental temperatures…

Figure 2 from Seebacher et al. 2003. Jagged line = air temp. Tiny dotted line = water temp. Colourized lines reflect different sized alligators

The justification for this comes from a citation of Seebacher et al. 2003. My issue with the Seebacher et al. reference is that it is not what Seebacher et al. found. Their paper specifically mentions how their study alligators were maintaining cooler temperatures than predicted by random movement in the summer and warmer than predicted in the winter. That is to say, they found distinct evidence for thermoregulatory behaviour in the Rockefeller State Wildlife Refuge (RSWR) alligators, albeit over a less strict range than other crocodylians such as Crocodylus johnstoni.

Another strange statement that the authors made happened early in the paper when discussing egg laying in reptiles. They argued that the reason for testing high paleolatitude species is because these are areas where daily temperature fluctuations should stay below the core body temperatures predicted for an ectothermic animal.

As Dawson et al. put it:

eggshell samples from the ectothermic-radiated tortoise (Astrochelys radiata) and bearded lizard (Pogona barbata) taken from the Los Angeles Zoo yielded temperatures within the range of living endotherms only because they came from a warm environment...

It seems like the authors are attempting to discount the thermoregulatory abilities of these animals here. The estimates for A. radiata (31–34°C) are consistent with the preferred temperatures of the animals, as are the likely estimated temperatures for Pogona barbata (34.6°C, Schauble and Grigg 1998). I say likely, because Eagle et al. 2015 (where Dawson et al. obtained their data) forgot to input the P. barbata data. Dawson et al. appear to be arguing that only animals from areas with warm temperatures can achieve body temperatures on par with automatic endotherms, as if bradymetabolic critters are strictly passive players on the thermal landscape. While there is a certain truth to this (bradymetabolic animals with the highest recorded body temperatures typically live in sweltering hot places), it is far from a hard and fast rule.

For instance, many lizard species in the genus Liolaemus inhabit the chilly peaks of the Andean mountains (3.5 km above sea level). Species here (e.g., L. alticolor and L. jamesi) have average recorded body temperatures of 29.1°C with a range between 23–34°C. These body temperatures are attained in spite of living in areas where the ambient air temperature varies from 14–28.5°C (Marquet et al. 1989). Similarly, we have the poster child for cold-weather lizards, Zootoca vivipara, which regulates its core temperature to 29.5–31.9°C across 1.2 km of elevation and across MAT ranges of at least 1.5–8.5°C (Gvozdik and Castilla 2001). In both cases, careful use of available microhabitats allows these bradymetabolic animals to maintain body temperatures that are substantially higher than their surroundings.

Beware of relying on MAT

Multiple times in their paper, Dawson et al. use the estimated mean annual temperature for a region as a stand-in for the hottest available temperature in that environment, but that’s not how MAT works. MAT is the average temperature of an area during the course of a year. Average temperature for the year can hide a lot of things, including very hot days / months and very cold ones. For example, the MAT for Cairo, Egypt is 21.3°C. This encompasses a range of temperatures from a chilly 7°C in January, to a sweltering 34.8°C in June (data from climate-data.org), and it masks extremely high daily temperatures such as the time Cairo hit 48°C in 1961. Same with extreme low temperatures. MAT really only gives us a rough idea of how warm or cool an area is in general. Once we drill down to ecologically and physiologically functional levels (i.e., microhabitats that can be exploited by life), MAT becomes largely unusable. 

Only automatic endotherms need apply

As mentioned, the results obtained by Dawson et al. make it clear that all the dinosaur specimens studied ran pretty hot (27–44°C). These high body temperature estimates led to the following interpretation:

This is within the range of living endothermic animals, such as birds and mammals (34° to 44°C)… and is warmer than the modeled MAT for the Haţeg Basin locality of ~30°C… The warmest Troodon (38° ± 4°C) and Maiasaura (44° ± 2°C) body temperature estimates are also within the range of living endothermic animals and substantially exceed the warmest environmental temperatures estimated from mollusk shells at the same location (25° ± 1°C and 28° ± 2°C; Fig. 5).

There is an underlying assumption here that just doesn’t make sense. The authors are arguing all temperatures between 34–44°C are strictly the realm of automatic endotherms, but that’s simply not the case. Aside from the animals already mentioned above, we have Komodo dragons (Varanus komodoensis) which run preferred body temps in the 34–40°C range (McNab and Auffenberg 1976), and many Southwestern desert lizards such as Uta stansburiana, which regularly maintain body temperatures of 37.2°C +/- 2.9°C (Goller et al. 2014) across their substantial range. Further, the high body temperature champions of the world are not automatic endotherms. They are invertebrates like the Pompeii tube worm (Alvinella pompejana), which appears capable of thriving at body temperatures near 80°C (Desbruyeres et al. 1998), and less crazily, ants of the genus, Cataglyphis, living in the Sahara desert, routinely operate at body temperatures over 50°C (Gehring and Wehner 1995).

Dawson et al. cite Clarke and Rothery 2008 (Scaling body temp in mammals and birds), but nowhere in there do those authors state that these ranges are an automatic endotherm thing. In fact, Clarke and Rothery pretty emphatically state that there is no definitive trend towards higher or lower temperatures in automatic endotherms. Birds show a negative correlation of body temperature with body mass (ratites have lower body temperatures than passerines), whereas marsupials show the exact opposite.

Returning to the Dawson et al. paper, the authors point out that their inferred dinosaur body temperatures exceed the highest recorded environmental temperatures from that area, as told by their mollusc data. The problem I have with this interpretation (aside from my initial misgivings of using molluscs) is that they are using freshwater animals, not terrestrial animals. Freshwater molluscs are living in bodies of water, which will both buffer temperature fluctuations throughout the day, but will also lower the maximum temperature that the molluscs will face. The highest temperature that Dawson et al. recorded from their mollusc data was the bivalve shell information at 27.5°C (range: 25.9–29.1°C). Depending on how clear the water was that housed those bivalves, the thermocline (change in temp with depth in water) could have been as shallow as 2 meters (6.56 ft), resulting in substantial drops in temperature (~10°C) fairly quickly.

For comparison, we can return once more to the Seebacher et al. (2003) chart, which showed water temperature in the summer getting as hot as 30°C at the surface. That’s pretty hot, but the air temperature during that same time frame was up to 60°C. The ability of water to absorb substantial amounts of heat will also mask the true warmest environmental temperatures that these dinosaurs would have experienced.

Is this evidence against gigantothermy?

Deeper in the discussion, Dawson et al. start focusing attention more towards the gigantothermy / inertial homeothermy hypothesis. It starts with a definition of gigantothermy that is for some reason linked to the rather recent work by James Gillooly et al. 2006

The concept of “gigantothermy” or “inertial homeothermy” is based on the link between low surface area–to–volume ratios and heat retention in animals and is modeled from the relationship between body mass and temperature in living crocodilians…

To be clear, that is not where gigantothermy came from. It was initially proposed by Frank Paladino back in 1990, and the concept was later tested in other large reptiles such as crocodiles (Seebacher et al. 1999). Gillooly et al. 2006 did test this hypothesis using their own mathematical model. They also found support for gigantothermy, but also noted that the largest dinosaurs in their sample size were approaching dangerously hot body temperatures (a toasty 48°C).

Can’t talk about gigantothermy without mentioning leatherbacks.

As Dawson et al. correctly point out, this particular model of Gillooly et al. was criticized pretty widely, but it wasn’t until Eva Griebeler’s 2013 paper that it was actually reassessed. Griebeler did find issues with the Gillooly et al., “ever hotter” model, and pointed them out, but she never actually questioned the gigantothermy hypothesis. As Griebeler wrote [emphasis mine]:

The decrease in body temperature with increasing body mass in sauropods, which is statistically supported by the fitted parabola (Figure 3), again strongly contradicts the hypothesis that the body mass of the largest dinosaurs was ultimately limited by body temperature. This is not to say that sauropods did not exhibit inertial homeothermy…but that they were able to efficiently cool themselves down

This hypothesis has seen further support with the paper released last year by Ruger Porter and Larry Witmer (2019), showing extensive cranial vasculature in sauropods that likely served as effective heat dumping mechanisms.

Media image showcasing potential thermal strategies in sauropods from Porter and Witmer 2019

Which means that when Dawson et al. stated the following regarding the potential dwarf sauropod eggshell material:

the similarity in body temperatures with that of giant sauropods (~35° to 38°C) …despite an at least 10-fold difference in body mass, would be inconsistent with an inertial homeothermy thermoregulatory model (dashed curve, Fig. 5)

Their statement is not really accurate. Extant, large reptiles do actively cool themselves to keep from overheating, especially when they live in hot environments. More likely, the data that Dawson et al. recovered from the Romanian eggshell may more likely suggest that sauropods as a group may have hovered closer to the mid-high 30°C range.

Dawson et al. later go on to bring in “mesothermy” as a potential alternative to automatic endothermy. My only real problem with this is that mesothermy is really just a repackaging of gigantothermy. The authors point out that mesothermic animals use high metabolic rates to heat their insides, but that is not what mesothermic animals do. As I discussed in T-U-R-T-L-E Power part 3, the initial studies on Dermochelys coriacea thought that the animals had higher resting metabolisms than a “regular” reptile. Later studies contested this and have shown the opposite (but with important caveats). Mesotherms / gigantotherms are capable of maintaining high body temperatures because of smart plumbing. Appropriate fat bodies in the right places and lots of countercurrent heat exchangers allow for parts or all of the bodies of a mesotherm to stay substantially warmer than their surroundings.

Dangerously hot Maiasaura and chilly Troodon

I certainly can’t leave this paper without also talking about the two strangest data points from this study. The authors found their Maiasaura peeblesorum eggshells were an incredibly toasty 44°C (range: 42–46°C), whereas their multiple Troodon formosus eggshells routinely brought in rather cool core temperatures (avg range: 28–38°C, raw range: 25–42°C). Dawson et al. went into a fair bit of detail on the T. formosus data, suggesting potential heterothermy or maybe even a more mesothermic lifestyle. That this is the second time a small theropod that was close to the lineage of birds has been found to have a relatively cool core body temperature is certainly interesting. As Eagle et. al. (2015) pointed out when they reported on their rather low body temperatures in an oviraptorosaur:

The results presented here are striking given what is considered a close relationship between Oviraptoridae and birds, suggesting the endothermy of modern birds was not present in at least this one species of non-avian theropod dinosaurs. This proposition is also congruent with interpretations based on histological analysis of pre-modern birds, where it is proposed that basal birds were also not fully endothermic in comparison with extant species and that full endothermic homoeothermy evolved later and after the evolution of feathers and flight…

That the T. formosus eggshells were also the best preserved makes it hard to argue for some weird effect of diagenetic alteration. It really does just seem that smaller dinosaurs had more variable body temperatures than larger dinosaurs. Filamentous integument may have evolved as a means of slowing the rate of temperature change within the body of these smaller dinosaurs. Essentially, McNab’s “Hot is Good” model for the evolution of automatic endothermy, just with a few more steps.

On the other side of the coin, Dawson et al. talk less about the M. peeblesorum data. 44°C is awful close to 45°, which is lethal for most animals. The M. peeblesorum had shown an 11.3% diagenetic alteration to the eggshells, which will affect the estimated temperature ranges. In the discussion, Dawson et al. do cover the potential range of values that this diagenetically altered eggshell could encompass, depending on where the recalcification occurred (deep underground vs. surface level). This produces a range of potential core body temperatures between 40–46°C. I suspect that the real body temperature is closer to that safer low end. Nonetheless, this is still remarkably hot. Exactly how this temperature was obtained is difficult to say. It’s possible that the animals were absorbing extensive solar radiation during the day while foraging, and then getting a double whammy from heat production via gut fermentation of all the vegetation eaten throughout the course of the day.

Of course, the automatic endotherm hypothesis remains open as well. It’s just not the only explanation.

So, to summarize all of this

  • The new paper from Robin Dawson and colleagues offers a fascinating new insight into dinosaur core body temperatures. The use of eggshells is a smart way to get close to the body core, even if it does limit one to just females (sexually selected body temperature differences remain a possibility).
  • Using molluscs as a proxy for environmental temperatures is a choice I can’t fully get behind as it relies on a uniformitarian view of living things with no real evidence to back it up. Further, the use of water-dwelling animals is going to significantly mask the warmest available temperatures in the terrestrial environment.
  • Mean Annual Temperature is not the same thing as—nor that great a proxy for—warmest temperature.
  • Using dinosaurs from more “temperate” latitudes is a nice touch, and one that really helps flesh out the range of temperatures in the Mesozoic.
  • Lots of weird assumptions about bradymetabolic critters in this paper. Some come from interpretations of previous papers that I don’t think are accurate.
  • Finding relatively (and in one case, extremely) high body temperatures in these dinosaurs adds further support that most dinosaurs were running in the 30–40°C zone.
  • These body temperatures do not discount a more bradymetabolic view of dinosaurs that incorporates gigantothermy / mesothermy without the need for boosting standard metabolic rates.
  • Once again, the dinosaurs that are closest to birds seem to show the lowest and most variable body temperatures. This is starting to be a consistent trend, suggesting that the automatic endothermy we see in birds is really a Avian synapomorphy.
  • The silent majority of Dinosauria (i.e., Ornithischia) continues to surprise us.

~ Jura

References

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